Ofcs - John M Senior

J(

10"1

= 0 30 1 Ra ylei~'"

The a tte n ua tio n due to Eq. (3 . n w tlere :

Att en uation

scatte ring in dB k,,,- ' m.l y be obta ined from

10 109,ol1 /l... .... 1 = 10109 ,0 3 32 2

=

_ 5 .2 d B km - ' A t a w BW'le nijth of 1.0 0 pm :

1.8 9 5 "

1O -~

YR - -- - - ""-

10-"

.,... 1.8 9 5 x 10- ' m- '

Using Eq. 13 .5): .(. ~m

= exp 1- 1.89 5 )( 10 ..... )( 10» = e xp (-0 . 18 9 5 )

0. 8 2 7 and Ea . (3 11 : A u en uat ian = 10 IOIl ,o 1.209 = 0.8 d B krn At

a

I

w av ele"\I t h M 1.3 0 j.l m :

1.89 5 x I O-bli fR=

2.8 56 )( 10 -

24

0 6 6 4)(1 0-

Using Eq (3 .5 ):

"'"

e xp l-Q.6 6 4 )( 10- " )( 10 ' 1 '" 0 ,9 3 6

a nd EQ. (3 , 1) :

Atte n uatio n = 10 109,0 1.069 = 0,3 dB

~m- '

The theoretical attenuation due to Rayleigh scattering in silica at wavelengths of 0.63. 1.00 and 1.30 11JTI-. from example 3.2. is 5.2. 0.8 and 0,) dB km- l relpective1y. These theoretical moults are in reasonable .,reemenl

TRANSM ISSION CHARACTERISTICS OF OPTICAL FIBERS

71

with experimental work. For instance the lowest reported value for Rayleigh scattering in silica at a wa velengt h o f O.6328 1JlJ1 is 3.9 d B km" IRef. LI t Ho wever. values of 4.8 d B k m " ! Ref. 121a nd 5.4 d B km-' IRef. 131ha ve also been reported . Th e predicted atten uation d ue to Rayleigh sca ttering against wa velength is indicated by a broken line on the att enuation characteristics shown in Figs. 3.1 and 3.3.

3 .4.2

Mis Scattering

Linear sca ttering may also occur at inho mogeneities which a rc compa rable in size to the guided wa velength . T hese result from the nonpcrfect cylind rical st ructure of the waveguide and may be caused by fi ber imperfection s such as irreg ularities in the core-cladding interface. core-clad ding refractive index differences a long the fiber length. diameter fl uctu at ions, strains and bubbles. W hen the scattering inhomogeneity size is greater than ))10, the scattered intensity which ha s an angular dependence can be very large. The scattering created b y such inhomogeneities is mainly in the forwa rd direc tion a nd is called Mie scattering. Depending upon the tiber material, de sign and manufacture Mie scattering can cause significan t lo sses. The inhomo geneities may be red uced by : (a ) remo ving imperfectio ns due to the glass manufacturing process ; (b) carefully controlled extrusio n a nd coat ing o f the fi ber; (c) increasing the fi ber g uida nce by increasing the relati ve refractive index difference. By these means it is possible to reduce Mie scattering to insignifi cant levels.

3.6

NONLINEAR SCATIERING LOSSES

Optical wa veguides do not always behave a.s completely linear channels .....hose increase in outp ut o ptical power is d irectly proportiona l to the input o ptical power. Several non linear effects occur, which in the ca se o f scatt ering cause disproportion ate attenuatio n, usually at high optical power levels. Th is non linear scattering causes th e optical power from one mode to be transferred in either the forward or backward direction to the same, or othe r modes, at a different frequency. It depend s critically upon the optical power density wit hin the fi ber and hence only becomes significant above threshold p ower levels. T he mo st important types of no nlinear scattering within optica l fibers a re stimulated Brillouin a nd Raman scattering, both o f which are usually only obser ved at high optical power d ensities in lo ng single mod e fi bers. These &Cattering mechanisms in fact give o ptical gain but with a shift in frequency lhua contributin, to attenuation for light transmission at a specific wa velength.

..

•

OPTICAL FIBER COMMUNICATIONS' PRINCIPLES AND PRACTICE

72

However, it may be noted that such nonlinear phenomena can also be used to give optical amplification in the context of integrated optical techniques (see Section 11.8.4). 3.5.1

Stimulated Brillouin Scattering

Brillouin scattering may be regarded as the modulation of light through thermal molecular vibrations within the fiber. The scattered light appears as upper and lower sidebands which are separated from the incident light by the modulation frequency. The incident photon in this scattering process produces a phonon" of acoustic frequency as-well as a scattered photon. This produces an optical frequency shift which varies with the scattering angle because the frequency of the sound wave varies with acoustic wavelength. The frequency shin is a maximum in the hack ward direction reducing to zero in the forward direction making Brillouin scattering a mainly backward process. As indicated previously, Brillouin scattering is only significant above a threshold power density. Assuming the polarization state of the transmitted light is not maintained (see Section 3.12), it may be shown IRef. 161 that the threshold power P E is given by: (3.6)

where d and A are the fiber core diameter and the operating wavelength respectively, both measured in micrometers, Clem is the fiber attenuation in decibels per kilometer and v is the source bandwidth (i.c. injection laser) in gigahertz. The expression given in Eq. (3.6) allows the determination of the threshold optical power which must be launched into a mono mode optical fiber before Brillouin scattering occurs (sec example 3.3). 3.5.2

Stimulated Raman Scattering

Stimulated Raman scattering is similar to stimulated Brillouin scattering except that a high frequency optical phonon rather than an acoustic phonon is generated in the scattering process. Also. Raman scattering occurs in the forward direction and may have an optical power threshold of up to three orders of magnitude higher than the Brillouin threshold in a particular fiber. Using the same criteria as those specified for the Brillouin scattering threshold given in Eq. (3.6), it may be shown [Ref. 16] that the threshold optical power for stimulated Raman scattering P R in a long single mode fiber is given by:

* The phonon

is a quantum of an elastic wave in a crystallattice. When the clastic wave has a frcquencyj. the quantized unit of the phonon has energy /if joule" where h i~ Planck's constant.

73

TRA NSMISSION CH ARACTERI STICS OF OPTICA L FIB ERS

PI/. = 5.9

X

lO- l d l Mr.t R wan s

(3.7)

where d. }. a nd «.sa a rc a s specifi ed fo r Eq. (3.6).

A IOfl9 sing le mode op lic1l l l ibe . has a... enenceuco o f 0 .5 ri b km - ' w h.m ope ri'll;"'Il .1 ' a w a....e le" 9 ttl of 1.3 IJ.m Th~ l iber core d iam " t ",r is 6 IIJTl and tile 1
PB

4 .4 =

~

1 0-~0'1FIl,:B V

44 x ' O -~ x 6 1 ~ 1.3 1 ~ 0 5 x 0 .6

80 .3 mW T il e th resho ld o ptica l pow er f o r st imu lat ed Ram al) scatt erin g mClY be o btJ i.wd f ro m Eq. (3. 71, w here:

P R . 5 .9 )( 1 0 - ldl }.q.cIB '" 5 .9 x 10 - 2 X 6 2 X 1 3 x 0 .5

= 1 38 W

In exam ple 3.3. the Brillo uin threshold occurs a t an optical power level of a ro und 80 mw whilst the Ra ma n threshold is approximately 17 t imes larger. 11 is therefore appa rent thai the losses introduced by nonlinear scattering may be avoided by usc of a suitable optical signal level [i.e. wor king belo w the t hreshold optical pow ers). However, it must be noted that thc Brillo uin threshold has been reported IRef. 171as occurring at optic al power!'> as low as LO mW in single mode fibers. Nevertheless. this is still a high p ower level fo r optical communications and may be easily avoided. Brillouin a nd Ra man scattering are not usually observed in multimode fibers becau se their relatively large core diameters make the threshold optical power levels extremely high. Moreover it should be noted that the threshold optical powers for both these scattering mechanisms may be increased by suitable adjustment o r the other param eters in Eqs. (3.6) a nd (J .7). In this context. ope ration at the lo ngest possible wavelength is advantageous altho ugh this may be offset by t he reduced fi ber attenuation (fro m Rayleigh scattering and material a bsorption) normally obtained.

3.8

FIBER BEND LOSS

Optic. 1 fibers suffer radiation losses at bends or curves 00 their paths. This is due to the eoerlY in tbe evanescent field at the bend exceeding the velocity of

7.

OPTICAL FIBER CO M MUNICATIONS: PRINCI PLES AND PRACTICE ( bdd llll:

Fig. 3 .4

,

An mustra non 01 the I
this , the e nergy conta ine d in Ihis p a rt of t he mode is radia le d away.

light in the cladding an d hence the guidance mech anism is inhibited, which causes light energy to be r ad iated from the fiber. An illu stration of this situation is sho wn in Fig. 3.4. T he part of the mode which j ~ on t he o utside of the bend is required to travel faster than that on the imide so that a wavefront perpendicular to the dir ectio n or propagation is mainta ined . Hence part o f the mode in the cladding need s to travel fa ster than the vetocuy o f light in tha t medium. As this is not po ssible. the energy associated with this part o f the mode is lost thro ugh rad iat ion. T he loss can genera lly be represented b~. . a rad iatio n attenuat io n coefficient which h as the fo rm IRef. 19 1:

where R is the rad ius o f curvature o f the fi ber bend and f l _ ' 1 arc constants wh ich a re independent of R. F urthermore, large bending lo sse s lend to occu r at a critica l radius o f c urvature R, which may be estimated from IRe f. 10 1:

(3.8)

It may be o bserved fro m the expression given in Eq. (3.8) th at po ssible bend ing lo sses ma y be red uced by: (a) d esigning fi bers with large relative refractive index differences ; (b) operating at the shorte st wavele ngt h possible. Both these factors therefore have lhe effect o f reducing the critical bending radius a s illustrated in the fo llo wing example.

..","

~

~

TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS

75

Example 3.4 Two step index fibers have the following characteristics: {a) A core refractive index of 1.500 with a relative refractive index difference of 0.2% and an operating wavelength of 1.5511m. (b) A core refractive index the same as lal but a relative refractive index difference of 3% and an operating wavelength of 0,82 urn. Estimate the critical radiu~ of curvature at which Iarqe bending losses occur in both cases. Solution: [a} The relative refractive index difference 8. is qiven by Eq 12 91 as'

,

n', _ n' 2c', Hence = n~

nj

2"'tJ~ = 2,250

-

0,004 x 2.250

_ 2,241 Using Eq. 13.81 for the critical R

r~d;us

3njA e

::::;

of ClJfvalurc:

3 x 2,250 x 1,55 x 10-" --.-

- - - _..._._-_. __.._---_..

411111; - n~I]'"

41110.009)''-' - 975 11m

(hi Again, from Eq. 12,9):

ni

=

nj -

2"'n; =

2.250

0,06

x

2.250

= 2,115

Substituting into Eq. 13,81:

R

e

3 x 2.250 x 0.82 x 10-" ~_

..

4Jt X 10.1351 3 , 2 - 9 urn

Example 3.4 shows that the critical radius of curvature for guided modes can be made extremely small (e.g. 9 11m), although this may be in conflict with the preferred design and operational characteristics. Nevertheless for most practical purposes. the critical radius of curvature is sufficiently small (even when considering case (a) which characterizes a long wavelength single mode fiber, it is approximately 1 mm) to avoid severe attenuation of the guided modets) at fiber bends. However, modes propagating close to cutoff, which arc no longer fully guided within the fiber core, may radiate at substantially larger radii of curvature. Thus it is essential that sharp bends, with a radius of

76

OPTICAL FIBER COMMUNICATIONS' PRINCIPLES AND PRACTICE

curvature approaching the critical radius, are avoided when optical fiber cables are installed. Finally, it is important that microscopic bends with radii of curvature approximating to the fiber radius are not produced in the fiber cabling process. These so-called microbends, which can cause significant losses from cabled fiber. arc discussed further in Section 4.6.2.

3.7

DISPERSION

Dispersion of the transmitted optical signal causes distortion for both digital and analog transmission along optical fibers. When considering the major implementation of optical fiber transmission which involves some form of digital modulation, then dispersion mechanisms within the fiber cause broadening of the transmitted light pulses as they travel along the channel. The phenomenon is illustrated in Fig. 3.5 where it may be observed that each pulse broadens and overlaps with its neighbors. eventually becoming indistinguishable at the receiver input. The effect is known as intersymbol interference (ISI). Thus an increasing number of errors may be encountered on the digital optical channel as the lSI becomes more pronounced. The error rate is also a function of the signal attenuation on the link and the subsequent signal to noise ratio (SNR) at the receiver. This factor is not pursued further here but is considered in detail in Section 10.6.3. However, signal dispersion alone limits the maximum possible bandwidth attainable with a particular opticalfiber to the point where individual symbols can no longer be distinguished. For no overlapping of light pulses down on an optical fiber link the digital bit rate B T must be less than the reciprocal of the broadened (through dispersion) pulse duration (21l Hence: 1

BT~­

2,

(3.9)

This assumes that the pulse broadening due to dispersion on the channel is r which dictates the input pulse duration which is also r. Hence Eq. (3.9) gives a conservative estimate of the maximum bit rate that may be obtained on an optical fiber link as l/2t. . Another more accurate estimate of the maximum bit rate for an optical channel with dispersion may be obtained by considering the light pulses at the output to have a Gaussian shape with an rms width of G. Unlike the relationship given in Eq. (3.9), this analysis allows for the existence of a certain amount of signal overlap on the channel, whilst avoiding any SNR penalty which occurs when intersymbol interference becomes pronounced. The maximum bit rate is given approximately by (see Appendix D): 0.2

B T (max) ~-bit s'

o

(3.10)

TRAN SM ISSION CHARACTERI STICS OF OPTICAL FIBERS

71

o

T"o ., i: !

""'l,I,'~~"

c

,.../r~--

" ,)

,' ", pIi Ll" '"

Fig.3.5

An ill"Slr al io n usin g the 0 '9;la l b~t pa tt em 10 \1 of the broa den ing of ligh t pu lses as the y a re t re nsrmtt ed a lon g a fibe r: (al fibe r il1 p<J I; (bt fiber o utp ut a t a dista nc e L,: icl f,h er Cll,lIP'Jl al a distanc e L. >L."

It must be noted th at certain source s [Refs. 25. 26 \ give the con stant term in the numerat or of Eq. (3. 10) as 0 .25. However, we la ke the sligh tly more con . servauve estimate given, fo llo w ing O lshansky IRef. 91 and Gambling et 01. IRd . 27J. Equation (3. 10) giv es a reasonably good approximation for other p ulse shapes which may o ccu r o n t he ch annel resulting from the various dispersive mechanism s withi n the fiber. Also (I" may be ass umed to represent th e rms im pulse res pon se for the channel as discussed furth er in Section 3.9.1. The conversion of bit rate 10 bandw idth in hertz depend s on the digital coding format used, For metall ic conductors wh en a non return to zero code is em ployed , the binary one level is held for t he whole bit period t . In this case there are two bit periods in o ne wavelength (i.e. twobits per second per hertz), II illus tr ated in Fig. 3.6(a ), Hence the maximum bandwidth B is one half the muimum dati rate or

..

Brlmu) - 2B •

(3. 11 )

78

OPTICAL FIBEA COMM UNICATIONS' PRINCIPLES AND PRACTICE

'"

---

, , , ,

-

, , , Fi g . 3 .6

,

----,,

,, •,,

,

•, ,, , , , - -!

,

c

, , ,,

,

, , ,

----

,

•,

•

,, , ,,

-

,, , •,,

,, ,

u

---,,

,,

, , , 11M ...' k ,n

,

•,

,, ,, ,

ScI,ema t ;c; ilhrstr ation 0 1 t he relat io n ships of t he en rete 10 w aveleng t h fo r d ig ital code s: (8) no n re tu rn to l ew (N RZ); (bl re tu rn to zero (RZ).

However, when a return to zero code is considered as show n in F ig. 3.6(b), the binary one level is held for only part (usuall y half) the bit period . For thi s signalling scheme t he data rate is eq ual to the bandwidth in hertz (i.e. on e bit pe r second pe r hertz) and thu s B J = B. The bandwidth B for metall ic cond uctors is also usually defined by the electrical 3 d B point s (i.e. the freq uencies at which the electrical power has dropped 10 one ha lf of it" constant maximum va lue). However, when the 3 d B o ptical band width o f a fi ber is considered it is significantly la rger th an the correspond ing J d lJ electrica l bandwidth for the rea sons discussed in Sectio n 7.4.3. Hence. when the hm itationv in the bandwid th o f a fi ber d ue to dispersion are stated {i.e. optical bandwid th B"'f'I )' it is usua lly with re gard to a retu rn to zero code where the bandwidth in hert z is considered equal to t he digital bit r ate. Within the context of dispersion the bandwidth s expressed in this cha pter will follow this general criterion unless otherwise stated. However, a v is made clear in Sect ion 7.4.3. wh en electro -optical device!' and o ptical fiber system s arc considered it is more usual to state the electrical 3 d B ha nd width, this being the more useful mea surement when interfacing an optica l fi ber link to electrica l terminal eq uipment. Unfo rt una tely the terms of bandwidth measu rement a re not always made clear and the read er must be warned tl1ft t this omission may lead l O verne confusion wh en specifying components and materials for optical fiber com munication systems. Figure 3.7 shows the th ree co mmon optical fiber structures, multimode step index, multimode grad ed index and single mode step index, whilst diagram matically illustrating the respective pulse broadening a ssociated with each fiber type. It may be ob served that th e multimod e step index fiber exhibits , the greatest dispersion of a tran sm itted light pu lse and that the rn ultimode graded inde x fiber gives a considera bly improved performance. F inally. the single mode tiber gives the minimum pulse broedening and thus is capable of • -,

-.

79

TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS

the greatest transmission bandwidths which arc currently in the gigahertz range, whereas transmission via multimode step index fiber is usually limited to bandwidths of a few tens of megahertz. However, the amount of pulse broadening is dependent upon the distance the pulse travels within the fiber and hence for a given optical fiber link the restriction on usable bandwidth is dictated by the distance between regenerative repeaters (i.e. the distance the light pulse travels before it is reconstituted). Thus the measurement of the dispersive properties of a particular fiber is usually stated as the pulse broadening in time over a unit length of the fiber (i.e. ns km"). Hence, the number of optical signal pulses 'which may be transmitted in a given period, and therefore the information-carrying capacity of the fiber is restricted by the amount of pulse dispersion per unit length. In the absence of mode coupling or filtering, the pulse broadening increases linearly with fiber length and thus the bandwidth is inversely proportional to distance. This leads to the adoption of a more useful parameter for the information-carrying

Multimooc ,101' ;[1(1",

,

,,!x,

,

""' /'L

CI"hli"g COT<'

'lL

R,f"otlvc' ;,~k,

Gulput pul se

In"u' 1"11;0

"V)

,m,,!

L"--.... ,

Multimodo grodcd ;Lldcx lib,',

,

"' ntr)

.J

'llL~

' "1

1\ "

Slnlll. mode ,tol> Index m""

r--_--,Am, It(fl

PtIl.3.7

"lJL,

Schemetlc diagram showing a multimode step index fiber, multimode graded Index fiber and single mode step index fiber, and illustrating the pulse broadenIng due to Int"model dispersion In eech fiber type,

... ".',

80

QPnCAL FI BER COM MU NICAT IONS: PRINCIPLES AND PRACTICE

capacity o r all optical fi ber which is known as the bandwidth- length product (i.e. B",p' x L). The typical best bandwidth-length products for the three fi bers shown in Fig. 3.7. are 20 MH z km. 1 G ha km and 100 G Hz km for multimodc step index, multimodc graded index and ..ingle mode step index fibers respectively. ullmple 3.5 A mu ltimo de g rad ed ind e x fi be r e d l,bils tota l pu lse b ro «de n;n o of 0 .1 d ' s ' aoce of 15 km . Es tima te :

~~

over a

la l the maximu m p ossib le b a ndwid th on the link a s su ming no rotc rsvmbot tmerterence: lb) the p u lse dispersion p er umt len91h :

Ie ) t he

bandw idlh~ le ngt h

p roduct lo r 1he liber.

S Olution: (a) The m axi m lJ m pc s stble op tica l band w id th w hich is e quival ent to tI,e ma xlrnu m possib le bit rat e (fo r ret urn t o zero pulses } essum in 9 no l SI may he ob t ai ned fro m Eq, (3 ,9 ], wh ere · ,

1

B"f)t = B T =-~--~ 2t

i

I

.

-

0 ,2 x1 0 -

5 MH z

0

(bl The di sper sion p er \J ni t len gt h m a y be acq ui red s im pl v bv c1ivifiil1 l.J the rora t d ;sp elsion b v th e tota l le ng th of the fi b er 0 . 1 X lO --lI

<1ispe rsl o n _

6 .67 ns km -

I

15 Ic) The b andwidth-Ie ngl h p roouCI may be obtainedin rwlJ ways. Fi ,stly by s implv m u ltiplying II>" ma. iml.lm banc!w idl" for Ina f iber linl:. by its le'lg' " He nce : B"... L = 5 MHz

~

15 I:.m

75 MHl km

Alterna tive ly it may O'l obta ined from the d is pers,on I "'" unit le l19 th using EQ. 13 .9 1 where : 1

:---::-:::---::-0 = 2 " 6.67

x to" 9

7 5 MHl km

In order to appreciate the reasons for the different amounts of pulse broadening within the variou s type s of optical tiber. it is: necessa ry to consider the dispersive mechani sms involved. These include material dispersion, waveguide dispersion, intcrmodal dispersion and profile dispersion which are considered in the follo wing sections.

-

3 .8

INTRAMODAL DISPERSION

Intramodal or chromatic dispersion may occur in all types of optical fi ber and "

TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS

81

results from the finite spectral linewidth of the optical source. Since optical sources do not emit just a single frequency but a band of frequencies (in the case of the injection laser corresponding to only a fraction of a per cent of the center frequency, whereas for the LED it is likely to be a significant percentage), then there may be propagation delay differences between the different spectral components of the transmitted signal. This causes broadening of each transmitted mode and hence intramodal dispersion. The delay differences may be caused by the dispersive properties of the waveguide material (material dispersion) and also guidance effects within the fiber structure (waveguide dispersion). 3.8.1

Material Dispersion

Pulse broadening due to material dispersion results from the different group velocities of the various spectral components taunchedsinto the fiber from the optical source. It occurs when the phase velocity of a plane wave propagating in the dielectric medium varies nonlinearly with wavelength, and a material is said to exhibit material dispersion when the second differential of the refractive index with respect to wavelength is not zero (i.e. d 2 n/d;J.,,1 -=t- 0). The pulse spread due to material dispersion may be obtained by considering the group delay 1:g in the optical fiber which is the reciprocal of the group velocity r~ defined by Eqs. (2.37) and (2.40). Hence the group delay is given by: 1:"

=~!=~ (n] dm

C

c

_A

dn]) dA

(3.12)

where 11 1 is the refractive index of the core material. The pulse delay r., due to material dispersion in a fiber of length L is therefore: 1:

m=!:..c (n1-A dn]) d1.

(3.13)

For a source with rms spectral width a~ and a mean wavelength A, the rms pulse broadening due to material dispersion am may be obtained from the expansion of Eq. (3.13) in a Taylor series about A where:

+ ...

(3.14)

As the first term in Eq. (3.14) usually dominates. especially for sources operating over the 0.8-0.9 urn wavelength range. then: (3.15)

82

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Hence the pulse spread may be evaluated by considering the dependence of Tm on A. where from Eg. (3.13):

dA

c

(3.16)

Therefore substituting the expression obtained in Eq. (3.16) into Eq. (3.15), the rms pulse broadening due to material dispersion is given by: d 2n] om "'-' - '- A "cc'-I oIL C

d}..2

(3.17)

• The material dispersion for opticalfibers is sometimes quoted as a value for 1)'}(d 2 n j /u'A' )1or simply Id 2 n l/d),,,lI. However, it may be given in terms of a material dispersion parameter M which is defined as: (3.18)

and which

IS

often expressed in units of ps nm"! km".

Example 3.6 A glass fiber exhibits material dispersion given by I FlcI'n,,'d)..?11 of 0,025, Dercmunn the material dispersion parameter at a wavelength of 0,85 urn. and estimate the rrns pulse broadening per kilometer for a (Jood LED ~()urce with ~n rms spectral width of 20 nm M this wavelength. Solutioll. The material rJispf1rsion parameter may be obtained from Eq. 13.181' 'A d'n

1

M - - - •_ c dF c'A

(j2n 1 }.'--

d'A1

0.025 --

2,998 \ 105 x 850 98,1 ps nm- 1 km- 1 The rms pulse broadening is given by Eq, 13.17) as:

TRAN SMISSION CHARACTERISTICS Of OPTICAL FIBERS Th erefor e

.3

te l ms 01 t he m ate rial d i, pe rs io n pa ra mete r M de fined by EQ. (3 .1 8 1;

In

Henc e. th'" m1 S pulse broadentrH;j pe r Io;;rometer d ue to ma terial dispersion: <'1 m

t1 kml

=

20 )( I " 9 8 1 x 10- 12 = 1.9 6 ns km- '

Figure 3.8 shows the variation of the mat erial dispersion parameter At with wavelength for pure silica [Ref 28 1. It may be observed that the material dispersion tends to zero in the longer wavelength region a round 1. 3 urn (for pure silica). This provides an additional incentive (other than low attenuation) for operation at longer wavelengths where the material dispersion may be minimized. Also the use of an injection lase r with a narrow spectral width rather than an LED as the optica l source leads to a substa ntial reduction in the pulse broadening due to mate rial dispersion, even in the shorter wavelength r egion.

11' '''';0] ~1 .....-mP"

J'I!l",n,''''

1;00 "",,-I ,_., ..., )

R.:1'l0ll pI

'" '""

"" lI"l>lr ",.",,;.1

J''''''';PD

"f-- - --"'...."'""-- - - -so

',."""' o. ~ ''''o', 1 0 U "", 1.4 1';--;';:--!;,--7' .. l .~ ~" "

' OC0-;'

",• • , 1,,,.,,,1, I"",,)

fig . 3 .8

T he m ateriil t dispe rsio n p aram et er for smca as a l unCl ion of wa velengt h. Re produ ced with pe rm ,ssi
11. c . 176. 19 7 5 .

Estim ate t he rms pu lse bro ad e ning p er kilo me ter lor th.. fibe r in exa m p le 3. 6 w h.." the optic al s o urce us ed is an inje ctio n l (l ~ p. r w ith a relative ec e cna r w id th r1 ~ il. of 0.00 12 at a we ve length of 0.85 11m. S olution: The rms s pe ctra l wid th mily be c btaln e d fro m the re lative spectral w idth by:

0), '" a .OD ln .. 0 .00 12 x 0, 8 5 )(

to-'

= t .0 2 nm

' TIl. rml pu l.. b rOl de 'llng In te rlTlS of t he met e ' ;11dispers ion paramet e r fo llow ing lX. mpl. 3.8 II S1 hrtf't by:

OPTICAL FIBER COMMUN ICATIONS : PRINCI PLES AND PRACTICE

84

Om ::::- 0 lLM

T herefo re t h e rms pu lse b roadeni ng p er k ilomet er du e to m a ,~ri
Om :::: ' .0 2 " 1 )( 98.1 )( 10 - n

=

0 .10 ns I<m - '

Hen ce , in t hi s exam ple t he rm s pulse b roade ning is reooc ec by a facIo. o f arou nd

20

u,e. equivale nt to t he recccee , m5 spf>(:l ral widt n of Ih'" mjltct;on Ies er sou rce]

comp ared w il h t hat ob t ain ed Wil h me LED sou rce of Il ll.lml.llC 3. 6 .

3 .8.2

Waveguide Dlaperaion

The waveguiding of the fi ber may also crea te intr amodal dispersion. This results from the va riation in gro up velocity with wa velength fo r a particular mod e. Considering th e ray theo ry approach it is equiva lent 10 the angle between th e ray and the fiber axis varying with wavelength which subsequently leads to a variation in the transmission times for the ray s, and hence dispc rsion Fo r a single mode whose propagation constant is ~. the fi ber exhibits v waveguide dispersio n when (d2 ~)/(dj,} ) -=I=- O. Multimod e fibers, where th e majorit y of modes propagate far fro m cutoff. are a lmost free of waveguide dispersion an d it is generally negligible compared with material dispersion (::::0. 1-0.2 ns km I ) [Ref. 281. Ho wever, with single mode fibers where the effec ts o f the different d ispersion mecha nisms a rc not easy to separa te, waveg uide d ispersion ma y be significant (see Sectio n 3. 10.2).

3.9

INTERMODAL DISPERSION

Pulse broadening due to intermodal d ispersion {sornerirncs referred to simply as modal o r mode dispersion) results from the pro pagatio n d elay d ifferences between modes within a muhimode fiber. As the different model' which constitute a pulse in a rnultimode fi ber travel along the channel a t d ifferent group velocit ies. t he pulse width at the o utput is d ependent upor. the transmissio n times o f th e slowest a nd fastest modes. Thi s dispersio n mechanism creates the fundamental difference in the ove ra ll dispersio n for the three types of fiber shown in Fig. 3.7. T hus muhimode step index fibers exhibi t a large amount of intermodal dispersion which gives the greatest p ulse b ro adening. However, intermodal dispersion in rnultimcdc fi bers may be reduced by adoption of an optimum refractive index profile which is provided by the near parabolic profile of most graded index fibers. Hence the overall pulse broadening in multimode graded index fibers is far less than that obtained in multimode step index fibers (ty pically by a fact or of 100). Thu s graded index fibers used with a multi mo de so urc e give a tremendou s bandwidth advantage o ver multimode step index fi bers. Under purely single mode o pera tion there is no intermodal di spersion and tberetcre pulse broademng i5 solely due to the intr. modal dilperaion '.

TRANSM ISSION CHARACTERISTICS OF OPTICAL FIBERS

85

mechanisms. In theory this is the ca se with single mode step index fibers where only a single mode is allowed to propagate. Hence t hey exhibit the least pulse broadening and have the greatest possible bandwidths, but may only be usefully operated with ~in g !e mode sources. In o rder 10 o btai n a simple comparison for intermodal pu lse broadening between rnultimod e step index a nd multimode graded index fi bers it is useful 10 consider the geomet ric optics picture for the two types of tiber.

3.9.1

Multlmode Step Inde x Fiber

Using the ray theory model, the fastest and slowest modes propagating in the ste p index tiber may be represented by the axial ra y an d the extreme meridio nal ray (which is incident
distance

(3.1 9)

,

velocit y

where n l is the refractive index of the core and c is the velocity o f light in a vacuum. The extreme meridional ray exhibits the maxi mu m delay time TM u where :

Ln,

(3.20)

c cos II

"'ir

"J". I) f---- 7":-- - - - - - - - -- - - - ,-... , i,) ,."

.... a..

CQ"' \" , I

".tt', tlklfl by tI'a IX', I !lY ,nd I n elttre me me ridiOt1 . 1 ray in a perfect multlmod. IUI p Incl• • fItI.r,

Th.

88

OPTICAL FIBER COMMUNICATIO NS: PRINCIPLES A ND PRACTICE

U<;ing Snell's law o f refraction at the core-cladding interface following Eq. ( 2.2) ,

n,

sin Q< - - " "-'- cos

n,

e

(3.2 1)

where 111 is the refr act ive index o f the cladding. F urthermore, substituting into Eq . 0 .20) for em gives :

e

(3.22)

The del ay difference ST, between the extreme me ridional ray and the axi al ray may be obta ined by subtracting Eq. (3.19) from Eq. (3.22). Hence:

ST, =

T~l ".

.

- 7 M ill

L l1 j"

L il t

('n2

c

= -- - -

(3.23)

en,

when 6.

«I

(3.24)

where ~ is the relative refractive index difference. However , when A -e I. then from the dcfininon given by Eq . (2.9). the rel ative refractive index difference may a lso be given ap proximately by :

(3 .25) Hence rearranging F..q . (3.23):

(3.26) Also substituting for .1. from Eq . (2.10) gives: (3.27) where N A is the numerical aperture for the fiber. The approximate expressions for the d elay diffe rence given in Eq . (3.26) and (3.27) a re usually employed to esti mate the maximum pulse broadening in time due to intermodal dispersion in multimode step index fibers. It rnU Ii{ be noted that this simple an alys is only

TRAN SMISSION CHARA.CTERISTICS OF OPTICAL FIBERS

87

considers pulse broadening due to mcridional rays and totally ignores skew ra ys with ac cepta nce angles e:>< > e. (see Section 2.2.4). Again considering the perfect step inde x fiber, a no ther useful q uantify with regard to intcrmodal dispersion on a n optical fiber link is the rms pulse broadening resulting from this dispersio n mecha nism along the fi ber. When the optical input to the fiber is a pulse p,(I) of unit area . as; illustrated in Fig. 3.10, then IRef. 3 IJ : (3.28) It may be noted that p ;(t) has a consta nt amplitude o f Il l)]: o ver the ra nge

The rms pulse broadening at the fi ber output du e to intermodal dispersion for the multimode step index fiber 0 , (i.e. th e stan dard deviation) may be given in terms of the varian ce as (see Appendix B):

0;

(3. 29)

where M I is the first temporal moment which is equiv alent to the mean value of th e pulse and ,'"f! the second tem poral moment is equivalent to th e mean squ are value of the pulse. Hence:

J'•"

M. =

tp,(t)dr

(3.30)

/2 p ;(t) dr

(3.31)

and

M! =

r

~

.l· .on.,l1fW<:

-'er, --.-+---,

I

c

", , Pig. :1.10

T""' (II

Arl lllullrlllDrl of I t>. lig 'lt inOl,ll tCllhe ml,llt imode step inde.. fiber consisting of . n lelI.1 1lU1i. or rKt.~ u ·. r NnlOtlon with \I n;! lrea.

.'

•

.'

ss

OPTICAL FIBER COMMU NICATIONS: PR INCIPLES A ND PRACTICE

T he mean value All for the unit input pulse o f Fig_3. 10 is zero, a nd assuming this is mainta ined fo r the o utput pulse, then fro m Eqs. (3.29) a nd (3.31): cr. = M ] =

J
(3.3 2)

r p,{t) dt

~

Integrating over the limits o f the input pulse (Fig. 3.1 0) and subs tituting for p ,(t ) in Eq. (3.3 2) o ver this range gives:

-

I

sr,

[~ ]"" ~ ~ 3

-5J ,/2

(o r, )' 3 2

(3.33)

Hence substituting from Eq . (3.26) for oT, gives:

o -

Ln, 6.

•- lV) '

L(NA)2 - 7 -,;: --

(3.34)

4V 3 ""

Equation (3 .34) allows estimatio n of the rms im pul se respons e o f a multimode step indcKfiber i f it is assu med that intermodal dispersio n dominates and

there is a uniform distribution of light rays over the ra nge 0 (.

e"

9~ .

The

p ulse broader.ing is direct ly proportio nal to the rela tive refractive index difference !!J. and the length of the fiber L. Thc lat ter emphasizes the bandwidth- length trade-off that exists. especially with muhimode step index fi be rs, and which inhibits their u se for wideband long haul (betwee n repe aters) sy ste ms. F urthermore, the pulse b roadening is reduced by reduction of the rela tive refractive ind ex difference !!J. for the fi ber. This suggest s that wea kly guiding fibers (see Secnon 2.3.6) with small !!J. are best for lo w dispersion transmission. However. a s may be see n from Eq. (3.34) this is a lso subject to a trade-orr as a reduction in !!J. red uces the acceptance angle a~ and the NA, lhu s wo rse ning the la unch conditio ns.

I "

,

"

I I, 1

E1Il8mpie 3 .8

A 6 km optical lirok cons is ts of mu ltimode ste p ind ex fibe r w ith a c o re re fra ct ive ind ex of 1.5 and a re lat ive refra ctive inde x differe nce 01 1%. Estlm e te : (81 t he del ay differe nce betwe e n t he s lowe s t and fast est mod", al th e fibe r outp ut :

lb ) the rms p ulse broad en in g d ue to inte rmod al d ispers io n on th e li f1 k: ic l the ma ximum bit rate tn a t ma y be obta ined witt-o ut s ubSlal'1 , ia l e reoes o n the link a ss u ming onlv ;n termod a l d ispersion; !d l th e bandw id th--Ie ng lh p roduc t c ooe spond in g to ic ).

S olulio,,; (81 The delay diH8't1nce is g ive'! b y Eq. 13 .2 61 u :

.., .",

:...

TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS Ln ,!'1

6T, ~ - = c

6" 1Q3

89

x 1.5)( 001

2.99 8:-: 10-

= 300 n5.

lbl The rms pu lse bro ad eni ng dU6 10 lm e rm oo e r d is f)Crsion m ay be obtemed from EQ. (3. 341 w here :

Ln ,!i.

1

6 )( 10 3 x 1,5:-:0,0 1

2";3 c

2";3

2, 9 98 x 10'

, r -r-r-: I

8 6.7

os

lei The m ax imu m bit rate may be estima ted in two ....e vs. Firs tly, to a c t eln Idea of t he ma ~i m lJ m b it ral e w he n assu m ing eo pu lse ove rl ap EQ. 13

I

8[

,

(max) = - ~ -

21

9J m ay be usee W hef C:

600 x 10

26Ts =

t

1.7 Mtlit s - '

A lt ern ative ly an im prov ed esti m at e may be obt ained lI sing t he calcu lat ed rrns pul se br o ad ening i n Eq . (3 , 10) w here :

8,

0 .2 02 [m ax) "'-= -~---:-O O~ 8 6 .7 :-: 10- t

= 2 ,3 M b't

9-'

ld l U siny t he m os t accura te eSl imate <:I f t he m a"im u m hit rate " o m k t and assu m ing ret urn to zero p ulses, t he b':lI'Iowidth -length produ ct is

2 .3 MHl

Bop, X L

l<

6 km = 13 8 M Hz

krn

lnterrnodal di spersion may be reduced by propagation mechanisms with in practica l fibers. For instance, there is different ial attenua tion o r tile vario us modes in a step index fiber. T his is d ue to the greater fieJ d penetration of the higher order modes into the cladding of the waveguide. These slower modes t herefore exhibit larger losses at any core-cladding irregula rities which tends to co ncentrate the transm itted optical power into the raster lower order modes. Thus the d itTerential a ttenua tio n of modes red uces intermodal pulse broadening o n a multimode o ptical link . Another mechanism which reduces intermodal pulse b roadening in non perfect (l.e. practical) m ultimodc fibers is the mode coupling or mixing discussed in Section 2.3.7. The coupling bet ween guided modes tran sfers optical power from the slower to the ra ster modes and vice versa. Hence with stronl coupling the optical power tends to be tra nsmi tted at an average speed , which is It mean of the various propagating modes. This red uces the intermodal d ilpcrsion on the link and makes it advantageo us to en courage mode c:ouplln. within muitimode fibers.

..

•

90

OPTICAL FIBER COM M UNICATIONS: PRINC IP LES AND PRACTICE

T he expressio n for delay d ifference given in Eq. (3.26) fo r a perfect step index fiber may be mod ilied for the fi ber wit h mode co upling among all guided modes to [Ref. 32 J:

(3.35) where L c is a c haracteristic length for the fiber which is inversely propo rtio nal to tbc coupling strength. Hence the delay difference increases at a slower r ate pro portional to (L L c) f instead of the d irect proportion ality to L given in Eq. (3.26). Ho wever. the most successful tec hniq ue fo r reduc ing intermodal dis persia n in m ultimodc fibers is by grad ing the co re refractive index to follo w a near parabolic profile. This has the effect of equalizing the transmissio n times of the various modes as discussed in the fo llowing section.

3 .9.2

Multimode Graded Index Fiber

Intcrrnodal dispers ion in multimode fibers is mini mized with the use of graded index libers. Hence mu ltimode graded index fiber s show substantial bandwidth im pro veme nt o ver multimode step index fibe rs. T he reason for the im pro ved perfo rma nce o f graded ind ex fibers may be ooscrved by co nsidering the ray d iag ra m for a graded index fiber shown in Fig. 3. 11. The fi ber shown has a parabolic inde x profile with a maxi mum a t the core axi s as illustra ted in Fig. 3. l l(a). Analytically, the index profile i!i given by Eq. (2.76) with 0 =2 as :

nCr) = " . (J - 2.:\(rl af r =

nl (l

- 2.6.)! =

nz

r


r ;)

(core)

(J.36)

a (claddi ng)

F igure 3. I I(b) shows several meridio nal ra y path s withi n the fiber core. It may be o b served that apart fro m the axial ray the meridional ra ys follow sinusoida l traject ories of d ifferent path lengths which result fro m the index

•

R. f,·,,' i...

"

'"

l"~,· , Ilt , )

--

R IJ. 3 .1 1

-

------

~~~ =f ~ (lao"I".

' " ' parobo llc

A multimooe Oll d ed inaell fi ber: 1.1 lbl mendional ,• v patl", wahl" t'll f.oe, cor• .

.... iol

'0'

retr.ctlve Ind Oi profile;

TRANS MI SSION CHARACTERISTICS OF OPTICAL FIBERS

91

grad ing as was discussed in Sec tion 2.5. However, following Eq . (2.40) the local grou p velocity is inversely proportional to the local refractive index and therefo re the longer sinusoidal paths are compensated for by higher speeds in the lower index medium aw ay from the axis. Hence there is an equalization of the tra nsmission limes of the vario us trajectories towards the transm ission time of the axial ray which tra vels exclusively in the high index region at the core axis. and at the slowest speed . As these vario us ray paths may be considered to represent the different mode s propagating in the liber, then the graded profile reduces the d isparity in the mode transit times. T he d ramatic improvement in multimod e tiber ba ndwidth ac hieved with a pa rabo lic o r near parabolic refractive index profile is highlighted by consideration of the red uced delay difference between the fastest and slo west modes for th is gr aded index fiber oT~ . Using a ray theory approac h the delay difference is given by (Ref. 331:

(NAY

(3.37)

As in th e step index. case Eq . (2. 10) is used for conversion bet ween the two expressions shown. However. a mo re rigorou s a nalys is using electromagnetic mod e theory gives a n absolute temporal width a t the fi ber o utput of (Ref. 34, 35 1: (3.38) wh ich correspo nds to an increase in transm ission time for the slowest mode of 6 1/ 8 over the fastest mode. T he expression given in Eq. (3.38) does not restrict the bandwidth to pulses with time slots corresponding to oT8 as 70'*. rel="nofollow"> of the optical power is concentrated in the first half of the interval. Hence the rm s pulse broadening is a useful parameter for assessment o f intermoda l dispersion in munimode graded index fibers. It may be shown [Ref. 35 1that the rms pulse broadening of a near parabolic index profile graded index fiber 0"1 is reduced compared to the similar broad ening for the corresponding step ind ex fiber 0 , (l,e. with the same relative refractive index difference) following:

a

~

= g

0

D '

(3 .39)

where D is a constant between 4 and 10 depend ing on the precise evaluatio n and the exact optimum prol1le cho sen. The ben minimum theoretical intermodal pulse broadening for a graded index nbCf with an optimum characteristic refractive index profile for the core •

92 {l""

OPTICAL FIBE R COM M UNICATIONS: PRINCIPLES A ND PR ACTICE

of IRefs. 35. 361 : 12,1.

~ ~ 2 - ~

s

(3.40)

is given by combining Eqs. (3.26) a nd (3.39) a s [Refs. 27. 361: Lf/ 1 t12

" . ~ ~2;;OVc,.3-C

(3.4 1)

Example 3 .9 Com pare t he rm s pulse brolId enir'lg per ki lom eler d ue 10 inter rno dal d isilersion for the mutlimo de s te p ind e>! f ib e' 01 e xa mple 3 ,8 w ilh the corresponding rm s p ulse broadening l o r an o p tim u m near Pdrabolic p rofile g rarlfld indel< f iber w it h the sam e core al
0. 11

km l

86,7

-'---,-- = - '" L

14.4 ns km - '

•

U sing Eq. 13 .4 1I, me rm s pu lse broade ning per kilo m eter for t he corr espo nd ing graded in del< ilbet is :

= 14 .4 ps km-"

I

Ii

j

Hence. from example 3.9. the theoretical impro vement factor o f the graded ind ex fiber in relation to intermodal pulse broadening is lOOO. However, this level of improvement is not usually achiev-ed in practice due to d iffi culties in co ntrolling the refractive index p rofile radially o ver long lengt hs of fiber . Any deviatio n in th e refractive ind ex profile from the optimu m results in increased intermodal pulse broadening. This may be observed from the curve sho wn in Fig 3. 12 which gives the variation in lntermodat pulse b roadening (oT.) as a function of the c haracteristic refractive index profile a for typical graded index fibers (where A = 1%). The curve disp lays a sh arp minimum at a characteristic refr active index profile slightl y less than 2 (u = 1.98). Th is corresponds to the optimum value of a in order to minimize intermodal d ispersion. F urthermo re, the extreme sensitivity of the intermodal pulse broadening to slight variations in a from t his optimum value is evide nt. T hus at present imp rovement (actors for practical graded index fi bers over correspond ing step index fi bers with regard to intermodal dispersion are around 100 [Ref. 341. Another import ant factor in the determination of the optimu m refractive

TRANSM ISSION CHARACTERI STICS OF OPTICAL FIBERS

93

'00

lo-!';;-- - -t,-----:"-- ....,,,--• .• - -co 3.0 "

FIV.3. 12

The i nt ermc dal pu lse b ro adening o Tg f or graded inde x f ibe rs having A = 1%. versu s the charact eristi c refra ct ive ind ex profile (1,

index pro fi le for a graded index fiber is the dispersion incurred due to the difference in refractive index between the fiber core and cl adding. It results fro m a variation in the refra ctive index profile with op tical wavelengt h in the grad ed fi ber and is often given by a pro file dispersion parameter ~ /dA . Th us the o ptimized profile at a given wavelength is not necessarily optimized at anothe r wavelength. As a ll o ptical fiber sources (e.g. injection lasers and light emitting diodes) ha ve a finite spectral width. the profile shape must be altered to compensate for this dispers ion mechanism . Moreo ver the minim um overall dispersion for graded index fiber is also limited by the other intr amod al dispersion mechanisms (i.e. material and waveguide dispersion]. The se: give temp oral pulse: bro adening of aro und 0 .08 and I ns km"! with injection lasers and light emitting diodes respectively. Therefo re practical pulse broadening values for graded index fibers lie in the ra nge 0.2-1 ns km-" . This gives bandwidthlength product.s of between 0.5 and 2.5 GHz km when using lasers a nd o ptimum profile fibe r.

3 .10 3.10.1

OVERALL FIBER DISPERSION Multimode Fib.r.

T he overall dispersion in multimode fibers comprises both intramod al and intermodal terms . The total rms pulse broadening O'T is given (see Appendix C)

by: Or = (~ + o;;)t

(3.42)

whert 0c Q the inuamodal or chromltic broadening and e , is the intermodal •

94

OPTICAL FIBER

CO M M UN I CATlO N S ~

PRINCIPLES AND PRACTICE

broade ning caused by dela y differe nces between the modes (i.e. 0 . for multimode step index fiber and 0 , for multimode graded index fi ber). The intramod al term 0c consists of pulse broadening due to both mat erial and waveguide dispersion. However, s ince waveguide dispersion is generally negligible compared with mat erial dispersion in multimcde fi bers. [hen 0, ~ 0 ",.

I

II I

,I

El
A multi mo de st e p index libe r h a s a n ume rica l a perlllm 01 0 ,3 a nd a co ra .efraI'1 i" a iJ1de ~ of 1.4 5 . The ma le ria l d ispersio n pa ra mete r '01 Ihe fiber is 250 ps nm- 1 km • w hic h rne kes me tena t d ;spe~ ion th e to ta lly do m ina tin9 ;nll amoo.,1 d is p.." ion me cha n ism. Est ima te (a) Ine tota l , ms ptJls e b'08rJe" ing PC' ~i ~me l "' r w he n lhe t iber is u sed w it h a n LED sou rce of rm 'l spectra l w id t h 5 0 ....m and Ib! the cor' espo ndi ng ba ndwidth -l e ng ll1 prod uc t lo r '''e fiber. S olution: (i1) Th e r ms p ulse broa d emnp p er kuo m eter du e to mat eriel disp ersion m ay be ob t a ined fr o m Eq. 13 . 17). wh ere

G).U.

(1 m

dI n , ( 1 km l :::;- - - - - .:
c

5 0. 1

X

2 50 ps l<m - '


=

Th e rm s pu lse b ro arj en in g per k ilo me t er d ue to hvterm oda l dis persion f o r th e st ep in d ex f ibe r is g iven by Eq. (3 .3 4 ) as .

L (NA IZ e, ( 1 km. :::;- -,---;;:--

10' x 0 .09

4y'3 n1c

,I

4 y'3 x 1.4 5.2.9 9 8 . 10" = 2 9 9 ns ~ - .

The to t al r m s pulse bro ad ening pe r kilo m ete r m ay be o bt ained u sing Eq. 13 .42 1, whe re (Ie "= Om as t he wav egu i(IA d isp ersion is n eg ligible and (I" = G. f o r t ile m ul ti · mode st ep index fi ber Hence :

CIT = td'm ~ ~ I i _ (1 2. 5· ... 29 . 9 ·l ~ -= 3 2 .4 ns km - ' In

(b ) T he bandw id t h-1 eng t h prod uct rn ev be esti m at ed fr o m t he rel at io nship given Eq. 13. 10 1 w here:

02 BOPl " L = -

er

02

.. ~

=

32.4 )( 10-= 6 .2 MHI

3.10.2

~m

Single Mode Fibe,..

The pulse bro aden ing in single mode fibers is solely d ue to intramodal dispersion a s only a single mode is allowed 10 propagate. Hence the bandwidth is limited by th e finit e spectral width of the source. Unlike the situatio n in multimode fibers, the mech anisms giving intramodal dispersion in single mode fibers tend to be interrelated in a complex ma nner. The transit timc or .roup delay f • .. / -

-.-

TRANSMISSION CHARACTERISTICS Of OPTICAL FIBERS

95

for a light pulse propagating along a unit length of fiber may be given as [R,f. 371 ,

(3.43) where c is the velocity of light in a vacuum, ~ is the propagation constant for a mode within the fiber co re of refracti ve index n l and k is the propagation conscant for the mode in a vacuu m. The fi ber exhibits intramodal dispersio n when ~ varies nonlinea rly with wavelength. From Eq. (2.71) ~ may be expressed in terms of the relative refractive index difference .6. and the normalized propagation constant bas:

0 .44) Th e rrns pulse broadening ca used by intramodal dispersion down a fi ber of length L is given by the derivative of the group delay with re spect to wavelength as IRef. 27 J: Total rms pulse broadening =

dt, 01 L -

dl,

(3.45)

where 0 1 is the source rms spectra l linewidth cen tered at a wavelength J... When Eq. (3.44) is substituted into Eq. (3 .4.5), detailed calculation of the first and second derivatives with respect to k gives the dependence of the pulse broadening on th e fiber material's properties and the nor malized propagation constant b. This gives rise to three interrelated effects which involve com plic ated cross-product terms. However. the fina l expression may be sepa rated into th ree composite dispersion components in such a way thai one of the effects dominates each term IRef. 381. The dominating effects are: (a) the material dispersion parameter defined by J../cld 1 n/d).,11 where fI = n l or for the core or cladding respectively; (b) the waveguide dispersion parameter defined as Vd ] (b V )/d v! where V is the normalized frequency fo r the fiber; (c) a profile dispersion para meter which is proportional to d6./dA.

"1

This situation is different from multimode fi bers where th e majo rity of modes propagate far fro m cutoff and hence most of the power is transmitted in the fiber core. In the multimode case the composite dispersio n components may be siniplifted and sepa rated into two intramodal terms which depend on either material or wevegulde dispersion as was discussed in Section 3.8. Also, .lpeciaIly whee conaideril1J step index multimode fi bers. the effect of profile

..

96

OPTICA L FI BER COMMU NICATION S: PRINCIPLES AND PRA CTICE

dispersion is negligible. Ho wever, al thoug h m at eria l and waveguide d ispersio n tend to be do mi na nt in single mode fibers. the composite p rofile dispersion term c annot be ignored. figure 3.8 shows the materia l di spersion paramete r for pure silica plotted against w avelength. It may be o bserved that t his characteristic goes th rough zer o at a wavelength of 1.27 urn. T his zero m aterial dispersion (Z MD) point can be shifted anyw here in the wavelength range 1.2-1 .4 u rn by the addition of suita ble do pants [Ref 39 1. For instance. the ZMD point shifts from 1.271J.m to a ppro xima tely 1.31 11m as the GeO: dopant concentra tion is inc reased from to 15%. Ho wever, the Z~t D point (al though of great interes t in single mode fiber'» doc!' rel="nofollow"> not represent a poi nt o f zero pulse broadening s ince the p ulse dispersion is influenced by bo th wa veguide and prof iledispersion. With .zero materia l d ispersion the pulse ...prcading is dictated by the waveguide pa rameter V d ~(b V )/d V I which is illustrated in Fig. 3. 13 as a fun ctio n of normalizcd Irequcncy fo r the LP o, m ode. It may be seen that in the sin gle mode region whe re the no rmalized frequ ency is less than 2.405 (see Section 2.4.2) the waveguide dispersion is always posit ive and has a maxim um at V = 1. 1S. In t hi ... case the waveg uide dispersion ge es to zero o utside the true sing le mode re gion at V = 3.0. H owever, a change in the fiber parameters (such as core radius) or in th e o perating wavelength al ters the no rmal ized frequency an d there fore the wa veguide d ispersion. T he total fiber dis persion which depends on both the fi ber m ateria l composit io n and d imensions m ay be m inim ized by trad ing o lT m at erial and wa veguide d ispersio n w hilst limiting the profile dispersion (i.e. restricting the variation in refractive index with wavelen gth). The wa ve length at which the first order dispersion is zero Ao may be selected in the range 1.3-2 um by careful control of the core diameter and profile ( Ref. 38 1. This is illustrated in Fig. 3. 14 w hich

o

l\-"'qllidc panuDCl«

Vd'l bY) d V'

"

:

I

I !

r. J',orm. ljz<XI r" '(I" ' '' e;'

v

The w ave guide parameter lId'!b Vj!dV' as a function 01 the oorm 81i18d trequencv V for tne LPo. mode, Reproduced with perml" siof'l from W . A , Gambling, A. H. Hartog af'ld C. M . Ragda1e
flg.3.'3

•

~

-

~;< ," ....-..

-,

.,

TRANSMI SSION CHARACTERISTICS OF OPTICAL FIBERS

To!.1 in".. d.1

J""",_ k'"

{ roo . ... ~ '

~'I

:1l

1,1>

I .K

.... ~k"'''' ("", I

Flg.3.14

The t ota l f irst order int rtim o dal disper sion as a tcn cnon of wa vel engt h for sing le mo de' fib ers w ith co re d iam ete rs of 4 , 5 and 6 1Jm . Reproduced w i l h p erm ission from W. A. Gambling. A . H. Ha rt og and C M . Ragdale, The R, di o find Etec tron: Eng, . 51 . p . 3 13, 1981 .

shows the to tal fi rst order dispersion as a function of wavelength fo r three single mode fibers with core diamet er s of 4 . 5 and 6 urn. The effect Dr the inter action of material and wav eguide dispersion on An is de mo nstrated in the dispersion against waveleng th characteristics fo r a singl e mode silica co re fibe r sh own in Fi g. 3. 15. It m ay be noted tha t the ZMD point occurs at a wavelength of 1.27 um b ut that the infl uence of wa veguide dispersio n shifts the total dispersion minim um towards the lo nger wa velength giving a ~ of 1.32 urn, T he wavelength at whic h the First order dis persion is zero ~) m ay be extended to wavelengths beyond 1. 55 /lID by a combination of th ree tech . n iq ues. These a re: (a) lo wering the norma lized freq uency ( V value) fo r the fiber; (b) incre asing the relative refractive index difference t1 for the fiber; (c) su itable doping o f the silica with germanium. This should allow bandw idth- length products for single mode fibers in ex cess of 100 G Hz km [Ref 401 at the slight disad vantage (If increased anen uanon d ue to R ayleigh scattering within the duped silica. H owever, it must be noted that although there Is zero firs t order di spersion at higher order chromatic effects im pose limitations on the possible band width for single mode fibers. At pnsent these give 8 fund amental lower limit to pul se sprea ding in silica-based Obert of. for example, around 2.~O x 10-1 ps nm- ' km' ' in fused silica at a

' {j,

I.

•

98

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

IJ['I',]'
'1"

"m-'j

.IIo'"i,,[ J i'I',,>100

... \ TQ"lJi,pcrsion

,,"" ....

,,//

-~ '.\...->-,

/

/

",

1

/

\

10' -

\ " ....1 I .."

....

..

.-..

.

"

'.

Wa\',~uid' d"pmioll

\

I

I

U

10' \.

"-\ \

I

I I I

,, ::

,

:: ....

I I '.0

Fig.3.15

-\

1.5

:',0

The pulse dispersion as a function of wavelength in 11 km single mode fiber showing the major contributing dispersion mechanisms (dashed and dotted curves] and the overall total dispersion (solid curve) Reproduced with permission from J, I. Yamada. M. Saruwalari. K, Asatanf. H. Tsuchiva. A. Kawana. K. Sugiyama and 1. Kimura, 'High speed optical pulse transmission at 1,29 urn wavelength using low-loss single-mode fibers', fEEE J. Quanwm Hectron., QE14, p. 791,1978. Copvriqht v 1980, IEEE.

wavelength of 1.273 urn 1Ref. 411. These secondary effects such as birefringence arising from ellipticity or mechanical stress in the fiber core are considered further in Section 3.12. However, they may cause dispersion, especially in the case of mechanical stress of between 2 and 40 ps km-! . If mechanical stress is avoided pulse dispersion around the lower limit may be obtained in the longer wavelength region (i.e. 1.3-1.7 urn). By contrast the minimum pulse spread at a wavelength of 0.85 urn is around 100 ps nm-' km" 1Ref. 331.

3.11

MODAL NOISE

The intermodal dispersion properties of multimode optical fibers (see Section 3.9) create another phenomenon which affects the transmitted signals on the optical channel. It is exhibited within the speckle patterns observed in multi-

TRANSMI SSION CH ARACTERISTICS OF OPTICAL FIBERS

99

mode tiber as fl uctuatio ns which ha ve characteristic times longer than the reso lution time o f the detecto r. a nd is known as mod al o r speckle noise. The speckle patterns are formed by the interference of the modes from a coherent source when the coherence time of the source in greater tha n the inte rmodal dispersion time 5T within the fiber. The coherence time for a source with uncorrelated source frequency width 5f is sim ply II&/. Hence. modal noise occurs when:

0/ > -

I

oT

(3.4 0)

Disturbances alon g the fi ber such as vibra tions. discontinuities, connectors, splices and source/detector coupling may cause fl uctua tions in the speckle patte rns and hence modal noise. It is generated whe n the correla tion between two or more modes wh ich gives the o riginal interference is d ifferentially delayed by these disturbances. The conditions which give rise to mo dal noise are therefore speci fied as : (a) a coherent source with a narrow spectral width and long coherence length (propagation velocity mu ltiplied by the coh erence time); (b) d isturbances along the fibe r which give differential mode delay or modal a nd spatial filtering ; (c ) pha se correlation between the mod es. Measurem ents (Ref. 46( o f rms signal to modal noise ratio using good narrow linewidth injection lasers sho w large signal to noi se ratio penalties under the previously men tioned cond itions. T he measurements were carr ied out by mjsalii-Ding connectors to create d isturbances. They ga ve carrier to noise ratios reduced by around 10 dB when the attenuation at each connecto r was 20 dB d ue to substa ntial axial misalignment. Mod al noise may be avoided by remo ving one of th e conditions (they must all be present) which give rise to this degradation. Hence mod al noise free tran smi ssion may be obtained by :

(a) T he use of a broad spectr um source in ord er to elimina te the modal in terference effects. This may be achieved by either 0) increasing the width of the single longitudinal mode and hence decreasing its coherence time or (2) by increasing the n um ber of longitudinal modes a nd averaging out of the interference patterns [Ref. 47 1. (b) In co nj unction with (aX2) it is fo und that fi bers with large numerical apertu res support the tran smission of a large number of modes giving a greate r number of speckles. and hence reduce the mod al noise generating efTect of individual speckles [Ref. 4 8J. (e) T he use of single mode liber which does nOI suppo rt th e tran smission of different modes and thus there is no intermodal interference. Cd) The nmO\'al diu urb ancel along the fiber. This has been investigated

or

•

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

100

with regard to connector design lRef. 491 in order to reduce the shift in speckle pattern induced by mechanical vibration and fiber misalignment. Hence, modal noise may be prevented on an optical fiber link through suitable choice of the system components. However, this may not always be possible and then certain levels of modal noise must be tolerated. This tends to be the case on high quality analog optical fiber links where multimcde injection lasers are frequently used. Analog transmission is also more susceptible to modal noise due to the higher optical power levels required at the receiver when quantum noise effects are considered (see Section 9.2.5). Therefore it is important that modal noise is taken into account within the design considerations for these systems.

3.12

POLARIZATION

Cylindrical optical fibers do not generally maintain the polarization state of the light input for more than a few meters, and hence for most applications involving optical fiber transmission some form of intensity modulation (see Section 7.5) of the optical source is utilized. The optical signal is thus detected by a photodiode which is insensitive to optical polarization or phase of the light wave within the fiber. Nevertheless. recently systems and applications have been investigated 1Ref. 521 (see Section 10.8) which do require the polarization states of the input light to be maintained over considerable distances, and fibers have been designed for this purpose. These fibers are single mode, and the maintenance of the polarization state is described in terms of a phenomenon known as modal birefringence. 3.12.1

Modal Birefringence

Single mode fibers with nominal circular symmetry about the core axis allow the propagation of two nearly degenerate modes with orthogonal polarizations. They are therefore bimodal supporting HEll and HEYI modes where the principal axes x and yare determined by the symmetry elements of the fiber cross section. Thus the Fiber behaves as a birefringent medium due to the difference in the effective refractive indices and hence phase velocities for these two orthogonally polarized modes. The modes therefore have different propagation constants ~x and ~y which are dictated by the anisotropy of the fiber cross section. When the fiber cross section is independent of the fiber length L in the z direction then the modal birefringence B F for the fiber is given by [Ref. 53],

(3.47)

TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS

101

where A. is the optical wavelength. Light polarized along one of the principal axes will retain its polarization for all L. The difference in phase velocities causes the fiber to exhibit a linear retardation 4I(z) which depends on the fiber length L in the z direction and is given by IRef. 531: (3.48) assuming that the phase coherence of the two mode components is maintained. The phase coherence of the two mode components is achieved when the delay between the two transit times is less than the coherence time of the source. As indicated in Section 3.11 the coherence time for the source is equal to the reciprocal of the uncorrelated source frequency width (lIof). It may be shown [Ref 54J that birefringent coherence is maintained over a length of fiber Lbc- (i.e. coherence length) when: (3.49) where c is the velocity of light in a vacuum and 6A. is the source lincwutth. However, when phase coherence is maintained (i.e. over the coherence length) Eq. 3.48 leads to a polarization state which is generally elliptical but which varies periodically along the fiber. This situation is illustrated in Fig. 3.16(a) [Ref 531 where the incident linear polarization which is at 45° with respect to the x axis becomes circular polarization at 41...,... n12, and linear again at c1l = n. The process continues through another circular polarization at cJi = 3nl2 before returning to the initial linear polarization at c1l = In. The characteristic length L s corresponding to this process is known as the beat length. It is given by: (3.50) Substituting for SF from Eq. (3.47) gives: L. ~

2.

-;;c-"'-;,-, (~.

-

~.)

(3.51)

It may be noted that Eq. (3.51) may be obtained directly from Eq. (3.48) where: (3.52) Typical single mode fibers are found to have beat lengths of a few centimeters [Ref. 55], and the effect may be observed directly within a Fiber via Ray1eiih scattering with use of a suitable visible source (e.g. He-Ne laser) [Ref. :561. It appears BS a series of bright and dark bands with a period

102

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Fig.3.16

An illustration of the beat length in a single mode optical fiber [Ref. 531: {a) the polarization states against illizI; Iblthe light intensity distribution over the beat length within the fiber,

corresponding to the beat length as shown in Fig. 3.16(b). The modal birefringence B 1 may be determined from these observations of beat length. Example 3.11

The beat length in a single mode optical fiber is 9 ern when light from an injection laser with a spectral linewidth of 1 nm and a peak wavelength of 0.9 urn is launched into it. Determine the modal birefringence and estimate the coherence length in this situation. In addition, calculate the difference between the propagation constants for the two orthogonal modes and check the result, Solution: To find the modal birefringence Eq. {3.501 may be used where: A SF

0.9 x 10-" 1 x 10-'

~~=

LB

0.09

Knowing SF' Eq. (3.491 may be used to obtain the coherence length:

1.2 L be ~--

BFIiJ..

0.81 X 10- 12 =

'·C',

81 m

10'x 10 g

The difference between the propagation constant for the two orthogonal modes may be obtained from Eq, {3.51) where,

13. -13 y

271

211

=- ~ - - =

La

69.8

0.09

The result may be Checked by UIln; Eq, 13.47) whirl:

_..... _..

_~ ~

TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS 211B F

211 X

~

103

10-0

69.8

In a nonperfect tiber various perturbations along the fiber length such as strain or variations in the fiber geometry and composition lead to coupling of energy from one polarization to the other. These perturbations are difficult to eradicate as they may easily occur in the fiber manufacture and cabling. The energy transfer is at a maximum when the perturbations have a period A, corresponding to the beat length, and defined by [Ref. 52J: ). A~-

B,

(3.53)

However, the cross-polarizing effect may be minimized when the period of the perturbations is less than a cutoff period A..: (around 1 mm). Hence polarization-maintaining fibers may be designed by either: (a) High (large) birefringence: the maximization of the modal birefringence, which following Eq. (3.50), may be achieved by reducing the beat length L H to around 1 mm or less; or (b) Low (small) birefringence: the minimization of the polarization coupling perturbations with a period of A. This may be achieved by increasing A..: giving a large beat length of around 50 m or more.. Example 3.12

Two polarization-maintaining fibers operating at a wavelength of 1 ,3 lim have beat lengths of 0.7 mm and 80 m. Determine the modal birefringence in eJch case and comment on the results. Solution: Using Eq. (3.50~, the modal birefringence is giverl by:

Hence, for a beat length of 0.7 mm:

1.3 x BF

=

1O-~

0.7 X 1O~3

=1,86xlO- 3

This typifies a high birefringence fiber. For e beat length of 80 m: 1,3 x 10-' BF=

=1.63x10----ll

80

whIch IndlCltll 8' low birefringence fiber.

104

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES ANO PRACTICE

Techniques are being developed in order to produce both high and low birefringence fibers in-order to facilitate coherent optical fiber communication systems. Fibers may be made highly birefringent by deliberately inducing large asymmetric radial stress. This may be achieved through thermal stress by using materials with widely different expansion coefficients coupled with an asymmetrical elliptical structure. A linear polarization state has been maintained over a kilometer of fiber with an extinction ratio of 30 dB using this technique !Ref. 571. Further investigation of the optimal cross section geometry for high birefringence has suggested [Ref. 57] fiber core cross sections shaped as a bow tie (bow tie fiber). To design low birefringence fibers it is necessary to reduce the possible perturbations within the fiber during manufacture. Therefore extreme care must bc taken when jacketing and winding these fibers in order to reduce bands or twists that may contribute to birefringence. It is also necessary to use materials which minimize the thermal effects that may create birefringence. One technique used to minimize the temperature dependence of birefringence which has proved successful is to spin the fiber preform during manufacture [Ref 581. This method, which reduces the linear retardation within the fiber, has produced fibers with no birefringent properties and variations in output polarization result only from fiber packaging. However, even with these low birefringence spun fibers some form of polarization controller IRef. 591 is necessary to stabilize the polarization state within the fiber.

PROBLEMS 3.1

The mean optical power launched into an optical fiber link is 1.5 mW and the tiber has an attenuation of 0.5 dB km I. Determine the maximum possible link length without repeaters (assuming losslcss connectors) when the minimum mean optical power level required at the detector is 211W.

3.2

The numerical input/output mean optical power ratio in a I km length of optical fiber is found to be 2.5. Calculate the received mean optical power when a mean optical power of I mW is launched into a 5 km length of the tiber (assuming no joints or connectors).

3.3

A 15 km optical fiber link uses fiber with a loss of 1.5 dB km I. The tiber is jointed every kilometer with connectors which given an attenuation of 0.8 dB each. Determine the minimum mean optical power which must be launched into the fiber in order to maintain a mean optical power level of OJ jJ W at the detector.

3.4

Discuss absorption losses in optical fibers comparing and contrasting the intrinsic and extrinsic absorption mechanisms.

TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS

3.5

'05

Briefly describe linear scattering losses in optical fibers with regard to: (a) Rayleigh scattering; (b) Mie scattering. The photoclastic coefficient and the refractive index for silica arc 0.286 and 1.46 respectively. Silica has an isothermal compressibility of 7 x 10- 11 m' N- 1 and an estimated fictive temperature of 1400 K. Determine the theoretical attenuation in decibels per kilometer due to the fundamental Rayleigh scattering in silica at optical wavelengths of 0.85 and 1.55 lim. Boltzmann's constant is 1.381 x 10-23 J K.

3.6

A K 20-Si0 2 glass core optical fiber has an attenuation resulting from Rayleigh scattering 01'0.46 dB km I at a wavelength of I urn. The glass has an estimated fictive temperature of 758 K, isothermal compressibility of 8.4 x 10- 11 m' N- 1, and a photoelastic coefficient of 0.245. Determine from theoretical considerations the refractive index of thc glass.

3.7

Compare stimulated Brillouin and stimulated Raman scattering in optical fibers, and indicate the way in which they may be avoided in optical fiber communications. The threshold optical powers for stimulated Brillouin and Raman scattering in a long 8 IJ.m core diameter single mode fiber are found to be 190 mW and 1.70 W respectively when using an injection laser source with a bandwidth of I GHz. Calculate the operating wavelength of the laser and the attenuation in decibels per kilometer of the fiber at this wavelength.

3.8

The threshold optical power for stimulated Brillouin scattering at a wavelength of 0.85 IJ.m in a long single mode fiber using an injection laser source with a bandwidth ofSOO MHz is 127 mW. The fiber has an attenuation of2 dB km- I at this wav elength. Determine the threshold optical power for stimulated Raman scattering within the fiber at a wavelength 01'0.911n1 assuming the fiber attenuation is reduced to I.S dB km I at this wavelength.

3.9

Explain what is meant by the critical bending radius for an optical fiber. A single mode step index fiber has a critical bending radius of 2 mm when illuminated with light at a wavelength of 1.30 urn. Calculate the relative refractive index difference for the fiber.

3.10

A graded index fiber has a refractive index at the core axis or 1.46 with a cladding refractive index of 1.45. The critical radius of curvature which allows large bending tosses to occur is S4 urn when the fiber is transmitting light of a particular wavelength. Determine the wavelength of the transmitted light.

3.' 1

(a) A multimode step index fiber gives a total pulse broadening or 95 ns over a 5 km length. Estimate the bandwidth-length product for the fiber when a non return to zero digital code is used. (b) A single mode step index fiber has a bandwidth-length product of 10 GHz km, Estimate the rms pulse broadening over a 40 km digital optical link without repeaters conslsttng of the fiber. and using a return to zero code.

106

3.12

O PTICAL FIBE R COM MUNICATIONS : PRINCIP LES AND PRACTICE

An 8 km optical fiber link witho ut repeaters uses multimode grad ed index fiber which has a bandw klth-lengrh product of -400 MH z km. E stima te : (al the total pulse broadening o n the link : (bl the rm s p ulse broadening on the link.

It may be a ssu med that a return to zero code is used . 3.13

Brie fly expl ain th e reasons for pulse br oadening d ue to material dt-oer sion in optical fibers . The group d elay T~ in an optical fiber is given by :

~ .:

T "

(n,-",",) dJ.

where c is me velocity of light in a vacuum, n , is rhe core refracti ve index a nd ). is th e wa vele ngth o f the transmit ted light. Dcnve an expression for the rm s pulse broadening due 10 mater ial dispersion in an optical fiber and define the m at erial dispcrslun para meter. T he material dispersion param eter for a gla ss tiber is 20 ps n m " ! km " at a wavelength of I.S urn. Estimate the pulse broadening due to m aterial dispersion within the fiber when light is launched from an injection la ser source with a peak wa velength of 1.5 J1D1 and an rm s spectra l wid th of 2 nm into a 30 km length of the tiber. optical fiber ddined by 1d 2 " lId). 21is 4. 0 x bro ad ening per kilo meter due 10 mal erial disis illuminated with all LE O source with a pea k rms spectral wid th o f 45 nrn.

3.14

The mater ial di spe rsion in an 10-2 IJ1T1 2. Estimate the pu lse persion wit hin the fiber when it wavcjength of 0.9 11JT1 and a n

3.15

Describe t he mechanism of inrermodal dispersio n in a roultimode step index fibe r. Show th at the total bro aden ing of a light pulse oT, du e to interm odal dispersion in a mu ltimodc step index tibe r may be given by :

where L is the fibcr lengt h, NA is the numer ic al apert ure o f the fil:>er , n I is the co re refractive index and c iii the velocity o f light in a vac uum. A muhim ode step index fiber has a nume rical ape rture of 0 .2 a nd a core refractive index o f 1.47. Estimate the ba ndw id th- length prod uct for the fiber assuming only intermodal dispersion and a ret urn to zero code whe n: (a) there is no mode coupling between t he guided modes; (b) mode co upling between the guided modes gives a ch aracteristic length eq uiva lent to 0.6 o f the act ual fiber length . 3 .16

U sing the rela tion for (iT, gi\'en in problerr. 3.15. derive an ekpression for th e rms pulse broadening due to lntermoda l di spersion in a mufl imode step inde x fiber. C ompare th is expression with a similar expression whic h may be o bt ained for an optimum near para bolic profLie I raded index. fiber.

TRAN SMISSION CHA RACTERISTICS OF OPTICAL FIBERS

101

Estimate the bandwidth-length product for the step index fiber specificd in problem 3. 15 eon r.idcring thc rms pulr.e broad enin g. d ue to interrnodal dispersion within the fiber and comment on the result. Indicate the poMibk improvemen t in the band widrtr- Icngrh product when an optimum near pa raboli..: profile graded index fibe r with the same relative refractive index difference and core axis refractive ind ex is used . In both ca ses a ssume only intermodal dis per~ iOD .....ithin the fiber and the U ~ o f a return to zero code.

3 .17

An 11 km o ptical fiber link consisting o f o ptimum nea r parabolic pro r:J e grade d inde x fi ber exhibits rms intermodal pulse bro adening of 346 ps over its length . If the fiber ha s a relative refractive index difference of 1.5 %, e stimate Ihe core SKis refractive index. Hence determine the numerical ape rture fo r the fiber .

3 .18

A multtmode, optimu m near par abolic profile graded index fiber ha s a material dispersion par amet er of 30 ps nm I km- ' when used with a good LED so urce o f r rns spec tral width 25 nm. T he fiber hac a numerical ape rture o r O.4 and a core axis refr active index o f 1.48. E stimate the tot al rms pulse broadening per kilomtter within the fiber assuming waveguide dispersion 10 be negligible. Hence estimate the b and width-length prod uct for the fiber .

3 .19

A m ultimoot step indelt fiber ha s a relative refractive index difference of 1% and a core refractive index o f 1.46. The maximum optical band....-idth that may be o btained with a pan icul ar source o n a 4.5 km link is 3.1 MHz. (3) Determine the r ms pulse broadeni ng per kilometer resulting from intram od al di<;rersiu n mecha nisms. (b) Assuming waveguide dispersion may be ignored , estimate the rms spectral width o f the source used, if Ihe material dispersion parameter for the fibe r at th e operating wa velength is 90 ps nm' km'.

3 .20

Discu ss dispersion mecha nisms with regard 10 single mode libers indicating the dominating effect s. Hence describe how intramodal disper sion may be minimized within the single-mode region.

3.21

Describe the phenome non of modal noise in optical fi bers and suggest how it may be e volced.

3 .22

Explain what is meant by: (a) mod al birefringence;

(b) the beat length; in single mode fiber s. The difference between the propagation constants for the IWO on hogonal modes in a single mode fibe r is 250. It is illuminated with light of peak wavelength 1.55 um fro m an injection laser so urce .....ith a spcctr allinewldth of 0 .8 nm . E."timate the coherence length within the fiber . 3 .23

A single mode fiber maintain s b irefr ingent coherence over a length of 100 km wh en it is illuminated with an injection laser so urce with a spectrallin ewidth of 1.5 nm IlItd a peak. wa velength of 1.32 urn. Esti ma te th e beat len gth within the fiber and co mment on the !"e§uIL

108

OPTICA L FIBER COMMUNICATIONS : PRINCIPLES AND PRACTICE

Answer. to Numerical Problems

3.' 3 .2 3.3

3." 3.8 3.7 3 .8

3."

3 ,10 3 ,11

3 ,12

57.5 km

10.0 uw 703 ..,W 1.5 7 dB km- l , 0 . 14 d B km- l l.49 1.50 IJ.m, 0.30 dB km- l 2.4 W 0.1 7%

0.86

)1m

(a) 13.2 M H7 k m : (b) (a ) 10 ns: (b) 4 ns

soo

ps

3 .13 3.14 3 ,15 3.16

3.17 3.18 3.19 3.22 3 .23

1.2 ns

5,.1 ns km- l (al 11.0 M Hz k m; (b) 14. 2 MHz km 15.3 M Hz km ; improvement to 10 .9 GHI. km 1.45, 0 .25 774 ps km ", 258 MH z km (a) 2.8 2 n s km- l : (b) 3 1 n m 48.6 m 113.6 m

REFERENCES 1

2 3

4 5

8

7

8 9 10 11

12

13 14 1'" 11

F. P. Kapro n, D. B. K ed an d R. D. Maure r, ' R aciarinn los ses in optical waveguides ', App l. Phys. L eu., 10, pp . 4 23--425, 1970 . T. Kim ur a, ' Single-mode digital transmission tech nology' , Proc. IEE E, 68(10), pp. 1263-1268, 1980. T. Miya, Y. T erarn una , Y, Hosak a and T. Miyasbita , 'Ultimate low-loss singlemode fibre at l.5 5 IJ.ffi ', S lectron. L ett., 15(4), pp. 106- 10 8, 1979. P. C. Schu lt z, ' Preparation of very low los s optical w aveguides', J . A m. Ceram. SOl'. , S2(4), pp. 383-385, 1973. H . Osanai, T. Shioda, T . Mo rivam a, S. A rak i, M. Horiguchi, T . lzawa and H . T akata, ' Effect of do pa nts o n transmission loss of low O H -coment optical fibre:;.', Electron. L eu .. 12(2 1), pp. 54 9-550 , 1976. D . B. Ked . K . D . Ma urer and P. C . Schultz, 'On the ultima te lo wer limit o f attenuatio n in glass optical wavegukl es", Appl. Phys. L en.; 21( 7), pp- ] 07-309. 1973. A . R . T ynes, A. O . Peerso n and D . L. Bisbee, ' Loss mech emsms a nd measurements in clad glass fibers a nd b ulk. glas s' , J. Opt. Sue. ,·h l., 61, pp . 143-153. 1971 . K . J . Beales and C . R . D ay. 'A review of gla ss fibre ! for o ptical comm unica. don s', Phy s. Chen. Glan es, 21(1 ), pp. 5- 2 1, 1980 . R. O lshansk y, ' Propagatjon in gla:;.s optical waveguides', Rn·. Mod. Phys., S 1(2), pp, 341-367, 1979. R . ht. G agliardi and S. Ka rp, Opucat CommunicatiOrfs, Jo hn Wiley, 1976. J . Schroeder, R. Mo hr, P. B. Macedo and C . J. Mont rose, ' Rayleigh and Brillouin scattering in. K ) Q-SiO : glass($', J. A m. Cerarn. Soc.; $6, pp. 5 10-5 14, 1973. R . D . Maurer, 'Glass fibers for o ptical com munications' , Proc. IE E E , 61, pp . 4 52--4 62, 1973. D . A. Pinnow, T. C. R ich, F . w. D sterma yer Jr and M . D iD omenico J r, ' f undamental optical attenuation limits in the liquid and gla ssy Slate with applica tion to fiber optical waveguide ma terials', App. Phys. Leu., 21, pp . 527-29. 1973. E . A. 1. Marcatili , 'O bjectives o f ea rly fiber s : evolution o f fiber types' , in S. E . Miller and A. G. Chynoweth {Eds.], Optical Fiber Telecommunications, pp. 1- 35, Ac ade mic Press, 19 79. O. G loge, ' Propagation effects in epdeal fibers', IEEE T ran s. Mtcro wctJe Theo ry re«, MIT·2), pp. 106-120 , J975. R . H . Stolen., 'N onlinc.& rity in fiber tr&ntmin jon', Proc. IEEE, '1( 10), pp. 1232-123&, 1980.

TRAN SM ISSIO N CHARACTER ISTICS OF OPTICAL FIBERS

17

18

19

20 21

22

23 24

25 26

27

28

29 30

31

32 33

34 3& 36

37 38

38

40

109

R. H. Stolen, ' Nonlinear prope rt ie s o r optical fibers', in S. E. Miller and A. G . Fib r Telecommunications; pp. 125- 150 . Academic C hynoweth (Ed~ . ), Press, 1979. '{. O hmori. Y. Sasaki a nd T. Eda hiro, ' Fi bre-length de pende nce ()( critical power (or stim ulated Ramen sca uermg", J::1fflron. u «. 17( 17), 1'1'. 593-594. 198 1. M. M. R am say lind G . A. Hoc kham, ' Pr opegatjon in optical fibre waveguides", in C. P. San dbank [Ed.I, Opt~ol Ftbr.. Communicw ion Systef7U, pp. 25--4 I, J ohn Wiley, 1980 . H. F. Wdf, 'Optical wa \'eguid cs' , in H. F. Wol f (Ed.). Handbook of Fibf!r Optics Theory und Applications, PI'. 43- 15 2, Gran ada, 19 79. A . W. Synde r, ' Lea ky -ray theo ry o f opt ical wavegu ides of circutar cross section'. Appl. Phys. (Gennan.L). 4(4 ), pp. 273 -298, 1974. T. Li, ' Structures, para mete rs and trans mission pro perties of oprical flbcrs', Proc. I£ £ E, 68{ 1O), pp. 1175- 1180 , 1980. P. Baues, 'The anatomy or a fiber optic link' , Comrot En/:.. pp. 46-49. August 1979. S. D . Peesonick, ' Receive r de sign for digital fiber optic cumm unicauon systems, Part 1 an d II', Bell S yst, Tech. J ., sz, pp. 843- 886, 1973 I. P. K aminow. D. Marc use and H. M. Presby, ' Muhimod e fi ber bandwidth: theory and practice' , Proc. IEEE, 68( 10), pp. 1209-12 13, 1980. M . J. Adams, D. N . Payne, F. M . Siaden and A. H . Hartog, 'Optim um operating wavelength for ch ro matic equalisation in multi mode optica l fihres', Electron. Leu; 14(3), pp . 64---66. 1978 . W . A , G amblin g, A . H . Hartog and C. M . Ragda le. 'Optic al fibre tra nsmission lines', Radio Electron. Eng. J . tes«. 5 1(7/ 8), pp. 313- 32 5, 19 8 1. D. K Payne and W. A. Ga mbling. ' Zero material d i ~petsion in optical fi bres'. Electron. t.eu.; 11 (8 ), pp. 176- 178, 1975. F. P. Kapron a nd D. B. Ked, ' Pulse transmissio n through a d ielectric opt ical wa veguide'. Appl. Opt.. 10(7), pp . 151 9--1523, 197 1. M. DiDomcnio Jr, ' Malerial dispc rsjun in o ptical fiber wnveguidcs', Aw l- Opl., ll. pp. 652--654, 1972. F. G. Strernle r,Inrroduct fon /Q Communication SJ'slems, 2nd EOn., AddisonWesley, 19 82. D. Botez and G , J. Herkskcwiu, 'Components for ~'p l ic al co mmu nication systems: a review', PrM . 1t:f:.'E. 6&,6), pp. 689- 730, 19 sa . A . G ha tak a nd K. T hyaga raja n, ' G raded inde ll optical wa veguides : a review', in E. Wolf (Ed.). Progress in Optics, pp. 1- 109, North -Holland Publishing, 1980. D . Gloge and E. A . Marea tili'- ' M ult imod(' theory of graded-core fibers' , Bell S.vst. Tech. J.. S2, pp. 156 3- 157&. 1973. J. E. Midwinter, Oplicol Hbe rs fo r Transmission. J ohn Wile) , 1979. R. Ols ha nsky a nd D . B. Kcck . ' Pulse broadening in grad ed-inde x optical fib ers' , Appr. Opt.; 15(12). pp. 48 3-491, 1976. D. Gloge, 'D ispersion in wea kly guiding fibers", Appl. Opt., 10(1 1), pp. 2442-2445. 1971. W . A. Gambling, H . Matsumura and C. M. Ragd ale, ' MoJ e dispersion. material d isp ersion and profile dispersion in graded index single-mode fibers", ft.-H}. M icrowa ves, Optics and A coustics (GB) , 3 (6), pp. 239-246, 1979. J. W . Fleming, ' Material dis persion in lightguid e glasses', Electron, Lell., 14{ 11), pp. 326-328, 1978. J, I, Yamada, M. Saruwalari, K. A m lani, H. Tsucbiya, A. Ka wan a, K. S uaiy ama and T . Ki mura, ' High speed o ptical pulse tran smissio n at 1.29 tJ.rn wav elength Uiinalow·loS! sinl le-mode fibe rs', IEEE J. QUQntum Etectron.; Q E-

onucot

e

14, pp, 79 1-800. 197&. • "

,

I !,

110 41 42

!I ,

\

I

43 44

45 4. 47

48 4" 50 51 52 153 54 55

56 57

58

59 60 61

OPTICAL FIBER CO M M UNICATIONS: PRINCIPLES A ND PRACTICE

F. P. Kapron, ' Ma xim um info rmation capacity of fibre-o ptic waveguides", Electron. L en.; 13(4 ). pp. l.i6-97, 1977. D. N. Payne and A. H. Hartog, 'Detcrminauon o f t he wa velength of zero ma terial dispersio n in optica l fitores by p ulse-delay measurements, I:.'leclrlJl/. L eft.• 1l(2 1)., pp. 627- 6 28. 19 77. L G . Cohe n, C. Lin and W. G . French , ' Tailo ring TCro chromatic dispersion into the 1.5-1 .6 pm tow-to ss spectral regio n of single-mode fibres', Electron. Lett.. 15(1 2), pp. 334- 335, 1979. A. W. Snyde r and R. 1\. Sammut. 'Dispersion in graded single-mode fi bres', Electron, L ett; 15(10 ). pp. 269- 270. 1979. W. A. Gambling. H. Matsumura li nd C. M. Ragdale, ' z ero mode dispersjun in s ingle mode fibre : . Electron, Leu.; 14 ( 19), pp. 618-6 20, 1978. R. E. Epwort h, ' The pheno meno n of modal noise in analogue and digjtal optical fi bre systems', in PrucwdillXS of tne 4lh European Conference 011 Ointeat Communicauon, Italy, pp. 492 - 50 1. 1978. A. R. G od win, A. W. Da vit... P. A. K jrkb y. R. E. Ep.....onb and R. G . Plumb. ' N arrow stripe and semicond uctor la ser for improved performance of opt ical comm unicat ion systems' , Proc . 5th Euro pean Conf Opt. Comm ., Thc Netherland s. paper 4-3. 19 7':1. K. Sato and K. Asamni. 'A nalogue baseband T V tran smission experiments using semiconducto r laser diod es', Electron. Lett.. 1~ ( 2 4 ) . pp. 794- 795, 1979. B. C ulshaw, 'Minimisation o f modal noise in optical- fi bre connectors', Electron, L ett., 1~ (l 7) , pp . 529 - 53 1, 1979. G. D e M arc his, S. Piazzo lla and B. Dain u, ' Modal noise in optical fi bres ', in Proceed ings oj ' he 61h Eu ropean Confer ence on Optical Commumcction, UK , pr. 76--79, 1980. M. Monerie. D. Moutonnet a nd L. Jeunhomme, 'Polarisatio n studies in long lengt h single mode fi bres'. in Proceedings of the 61h European Cortference on Opncat CommunicOl ;fl n. UK, pp. lO7- I I I, 1980. l. P. Kaminow, ' Pola rlzanon in fi bers', L aser Focus, 16(6). pp. gQ---i! 4, 1980. 1. P. K amino w, ' Polariz ation in optica l fibers', I EEE J . Qua nl w11 Electron.. Q Emn, pp. 15- 22, 198 1. S. C. Ras hleigh and R. Ulrich , ' Polarization mod e dispersion in single-mode fibe rs', Opt. L eu.; 3. PI', 60-62. 1978. \T. R am aswamy, R. D. Stan dley, D _Sze and W. G . f rench, ' Pola risation effects in ..hort length. single mode fibres', Bell SySI. Tech . J., 57, pp. 635---f>5 1, 1978. A. Papp a nd H . Harms, -pola r iaat io n optics of inde x- gradient opncal waveguide fi bers'. Appl. Opt.• 14. pp. 2406-24 11, 19 75. A. J. Ba rlow. D. N. Payne . M. P. va rnham and R . D . Birch, ' Po larisation c haracteristics o f fi bres fo r coherent detect ion systems', l E E Co lloq. on Co herence in Opt. F ibre S)'f.t., Londo n, 25th Ma y 19 1(2, D . N. Payne, A. J. Barlow and J. J. Ramskov H ansen, ' D evelopment o f low and high birefringence optica l fibres'. I EEE J. Quant um Elettro n.; QE- 18(4), pp. 477-487, 1982. R . Ulrich, 'Polarisat ion sta bilisation on singl e-mode fi bre' , Appt, Phys. Leu., 35 , pp. 840-842, 1979. M. J. Adams, D. N . Payne and C. M. Ragdale, ' Birefringence in o pt ical fibres with elliptica l cros s-section ', Electron . Len., 1~ ( IO), pp , 298- 299, 197 9. T . K at suyama , H. Matsu mu ra and T. Sug anuma. ' Low lou single-polarisation fibre s', E lectron. Lett" 17(1 3), pp. 4 73-474 , 1981. '

4 Optical Fibers, Cables and Connections

4.1

INTRODUCTION

Optical fiber waveguides and their transmission characteristics have been considered in some detail in Chapters 2 and 3. However, we have yet to discuss the practical considerations and problems associated with the production, application and installation of optical fibers within a line transmission system. These factors arc of paramount importance if optical fiber communication systems arc to be considered as viable replacements for conventional metallic line communication systems. Optical fiber communication is of little use if the many advantages of optical fiber transmission lines outlined in the previous chapters may not be applied in practice in the telecommunications network without severe degradation of their performance. It is therefore essential that: (a) Optical fibers may be produced with good stable transmission characteristics in long lengths at a minimum cost and with maximum reproducibility. (b) A range of optical fiber types with regard to size, refractive indices and index profiles, operating wavelengths, materials, etc., be available in order to fulfill many different system applications. (c) The fibers may be converted into practical cables which can be handled in a similar manner to conventional electrical transmission cables without problems associated with the degradation of their characteristics or damage. (d) The fibers and fiber cables may be terminated and connected together {jointed) without excessive practical difficulties and in ways which limit the effect of this process on the fiber transmission characteristics to keep them within acceptable operating levels. It is important that these jointing techniques may be applied with ease in the field locations where cable connection takes place. In this chapter we therefore pull together alSociated with optical fiber communications. preparina optical fibers (both liquid and vapor able for telecommunications applications are

the various practical elements Hence the various methods for phase) with characteristics suit" outlined in Sections 4.2 to 4.4.

112

OPTICAL FIBER COM MUN ICATIONS : PRINCIPLES AND PRACTICE

Th is is followed in Section -t.5 with consideration of commercially a vaila ble fibers describing in general terms both the type s and their cha racteri..tics. The requ irement.. for optical fi ber cabling in relat ion to fi ber prote ction are then disc ussed in Section 4.6 prio r 10 con siderat ion of cable design in Section 4,7. In Section 4.8 we deal with the losses inc urred when optical fibers a re connec ted together. This disc ussion provides a basis for consideration of the techniq ues employed for jointing optical fibers. Permanent fiber joints (or splices) a rc then dealt with in Section 4.9 prior to discussion of the various types of demountable fi ber connector in Sections 4.10 to 4.12.

4.2

PREPARATION OF OPTICAL FIBERS

From t he considera tions of o ptical waveguidmg of C ha pter 2 it is clea r that a variation of refractive index inside the optical fiber (i.e. between t he core and the cladding) is a funda mental necessity in the fabrication of fi bers for light transmission . Hence at least two different materials which a re tran sparent to light over the operating wavelength range (0.8- 1.6 1Ufl) arc required. In practice these materials rnust exhibit rela tively low optical attenuation and they must therefore have low intrinsic absorption a nd sca ttering losses. A number of organic and inorganic insulating substances meet these conditions in the visible a nd ncar infrared regions of the spectr um. Ho wever. in order to avoid scattering losses in excess of the fund amental intrinsic losses. scattering centers such as bubbles. strains and grain boundaries must be eradicated. This tends to limit the choice of suitable materials for the fabrication of optical fi bers to either glasses (or glass-like ma terials) a nd monocrystalline structures (certain plastics). It is also useful, a nd in the case of graded index fibers essential, that the refracti ve index of the material may be varied by suitable doping with another com patible material. Hence these two materials ..hould have mutual solubility over a relatively wide range of concentrations. This is. only achieved in glasses or glass-like materials. and the refore monocrysta lline materials are unsuitable for the fabrication of graded index fi bers. but may be used for step index fi bers. H owever, it is apparent tha t glasses exhibit the best overall material characteristics for use in the fa brication of low loss optical fibers. They a rc therefore used almost exclusively in the preparation of fibers for telecomm unications applications. Plast ic cl ad IReI'. 1] and all plastic fibe rs fi nd some use in short-haul, low bandwidth applications. In this section the discussion will therefore be confi ned to the prepa ration of gla ss fi bers. This is a two stage process in which initially the pure glass is produced a nd con verted into a form (rod or preform) suitable for making the fiber. A dra wing or pulling technique is then employed to acquire the end product. The methods of preparing the extremely pure optical glas ses aeneraJly fall into two major categories which are :

113

OPTICAL FIB ERS. CABLES A ND CONN ECTI ONS

(a ) conventional glass refi ning techniques in which the glas!> is processed in the molten Mate (melt ing methods producing a mulncompon ent glass structure: (b) vapor phase depositio n methods producing s ilica-rich glasses which have melting temperatures that a re too high to allow the conventional melt process. These processes. with th eir respective dra wing techniq ues. are described in the fo llo wing section s.

4.3

LIQUID PHASE (M ELT IN G) TECHNIQUES

T he first stage in thi s process is the preparation of ultra pure material powders which are usu ally oxides or carbonates of the required con stituents. These include oxides such as SiO• • GeO" 8 2 0 1 and A 2 0 J • and carbo nates such as

w.< i old

1 - - 1-- -

1..... ..."

+---t--\i
, .. ...,

G'lItI1l.ldng lu rn. e- fot t!'le prod uction •

~

(" 1'\"'_- lIn,.,-

hlgil c•.riW gl.. ,n

l ~.

4).

114

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

-----------

~h)ltcll ~hm

(n",Ne-----

Fig. 4.2

High purity glass melting using a radiofrequency rooucuon tumace [Refs. 6-8].

Na, CO" K 2 CO" CaCD) and BaCD.] which will decompose into oxides during the glass melting. Very high initial purity is essential and purification accounts for a large proportion of the material cost: nevertheless these compounds are commercially available with total transition metal contents below 20 parts in 109 and below 1 part in 109 for some specific impurities IRef. 21The purification may therefore involve combined techniques of fine filtration and coprecipitation, followed by solvent extraction before recrystallization and final drying in a vacuum to remove any residual OH ions (Ref. 31. The next stage is to melt these high purity, powdered, low melting point glass materials to form a homogeneous, bubble-free multicomponent glass. A refractive index variation may be achieved by either a change in the composition of the various constituents or by ion exchange when the materials are in

'.

'~,.'-'

OPTICAL FIBERS, CABLES AND CONN ECTIONS

115

the molten phase. The melting of these multicomponent glass systems occurs at relat ively lo w tempera tures between 900 and 1300 °C and may tak e place in a silica crucible as shown in Fig. 4. 1 [Ref. 4]. However. conta mination can arise du ring melting from several sources including the furnace environment and the crucible. Both fused silica and platinum crucibles have been used with some success although an increa se in impurity content was observed when the melt was held in a platinum crucible at high temperatures over long periods

IRef. 51. Silica crucibles can give dissolution into the melt which may introduce inhomogeneities into the glass especially at high melting temperat ures. A technique for avoiding this involves melting [he glass directly into a radiofreq uency (RF approxima tely 5 MHz) induction furnace while cooling the silica by ga s or water flow a s shown in Fig. 4.2 IRefs. 6-81. The materials are p reheated to around 1000 ClC where they exhibit sufficient io nic conductivity to enable cou pling between t he melt and the RF fi eld. The melt is also protected from any impu rities in the crucible by a thin layer of solidifi ed pure glass which forms due to the temperature difference between the melt and the cooled silica crucible. In both technique s the glass is homog enized and dr ied by bu bbling pure gases through the melt. whil st protecting agai nst any airborne du st particles either originating in the melt furnace or present a s atmospheric conta mination. Arter the melt has been suita bly processed. it is cooled a nd formed into long rods (cane) o f multicomponent glass.

4 .3.1

Fiber Drawing

The trad itional technique for producing fine optical fi ber waveg uides is to make a preform using the rod in tube pro cess. A rod of core glass is inserted into a lube o f cladding glass and the preform is drawn in a vertical muffle furnace as illustrated in Fig.4.3 IRef.91. This technique is useful for the production o f step index fi bers with large core and cladding d iameters where the achievement o f low attenua tion is not critical as there is a danger o f including bubbles and particulate mailer at the core-cladding interface. Another technique which i!i a lso suitable for the prod uction o f large core dia meter step index fibers. and red uces the core-cladding interface pr oblems, is called the stratified melt precess. This process, develop ed by Pil kington Labor atories [Ref. 10J, involves pour ing a layer of cladding glass over the core glass in a platinum crucible as sho wn in Fig. 4.4 [Ref. I ll . A bait glass rod is dipped into the molten combination and slowly withd rawn giving a composite ce re-clad preform which may be then drawn into a fibe r. Subsequent development in the d rawing of o ptical fi bers (especially graded lndell) produced by liquid pha se techniq ues has concentrated on the double crucible method. In this method th e core and cladding glass in the fonn of IIp&rate rods Is fed i;nto two concentric platinum crucibles as illustrated in •

116

OPTICAL FIBER COMMUNICATIONS: PRINCIPLE S AND PRACTICE

----- ~,-

[• I

'
I I, I Hi""

~- -_.

Flg.4.3

Opti cal fiber from a oretc-m lRef. 9].

Fig. 4.5 IRef. 41. Tbe assembly is usually located in a muffle furnace capable of hea ting the c rucible co ntents to a temperature of between 800 a nd 1200 °C. Th e crucibles have nozzles in their bases from which the clad fiber is drawn directly from the melt as sho wn in Fig. 4.5. Index gradi ng may be achieved through the diffusion of mobile ions across the core-cladd ing interface within the molten glass. It is possible to achieve a reasonable refractive: index profile via this diffusion process, although due to lack of precise control it is not possible to obtain the optimum nea r paraboli c profil e which yields the

I'~'"

t,

p"""rm

r ,/ ~ PJ .I",um

fig . 4.4

•

'''''''\>Ie

The sU"lItifi ttd me1t proce ss (gl.. ~~ on glass technique' for prOClt.IClnll

rod. or preform. [Ref. 11 I.

;rU' ~.d

117

OPTICA L FIB ERS, CA BLES AN D CON NECTIONS

-

Cc ~ · ~I" ... ( ·', d,!i n~

.d ....

_

FiG.4.5

_

r~' tin~b.I IL

The dou ble crucible m et h
mi nimu m pulse dispersion (see Section 3.9.2). Hence graded inde x fi bers produced by this technique arc substantially less dispersive tha n step index fi bers. b ut do not ha ve the bandwidth-length prod ucts of optimum profi le fi bers. Pulse dispersion of 1- 6 ns km J [Refs, 12. 13J is quite typical.depending on the material system used. Some of the material systems used in the fabrication of multicornpo nent glass step index and graded index fibers are given in T able 4.1. Using very high purity melting techniques and the double crucible drawing method, step index and graded index fibers with atten uations as low as 3.4 dB krrr ' [Ref. 14) and 1. 1 dB km' " [Ref 2J respectively have been produced. However. such low losses c annot be consistently obtained using liquid phase techniques and typical lo sses for multicompon ent glass fi bers prepared continuo usly by these methods are between 5 and 10 d B km 1 .

Therefore, liquid phuc techniques have the inherent disadvantage of obtaining

118

OPTICAL FIBER COMMUNICATIONS; PRINCIPLES AND PRACTICE Table 4.1

Material systems used in the fabrication of multicomponent glass fibers by the double crucible technique

Step Index

Core glass

Cladding glass

Na,,-B,O,-Si0 2 Na,-LiO-CaO-SiO, Na,-CaO-GeO, TI,O-Na,O-B,O,-GeO,-BaO-CaO-Si0 2

Na20-B20~-Si02

Na,O-BaO-GeO,-B,O~-SiO,

P,O.-Ga,O,-GeO,

Na,O-Li,O-CaO-SiO, Na20-CaO-Si0 2

NalO-B10~-Si02

Na,O-B,O,-Si0 2 P,O.-Ga,O,-SiO,

Gradadindex Base glass

Diffusion mechanism

R,O-Ge0 2-CaO-SiO, R,O-B,O,-SiO,

Nat '" K+

Na,O-B20~-SiO,

NalO-B10~-Si02

T!" .. Na" Na20 diffusion CaO, BaO diffusion

and maintaining extremely pure glass which limits their ability to produce low loss fibers. The advantage of these techniques is in the possibility of continuous production (both melting and drawing) of optical fibers.

4.4

VAPOR PHASE DEPOSITION TECHNIQUES

Vapor phase deposition techniques are used to produce silica-rich glasses of the highest transparency and with the optimal optical properties. The starting materials are volatile compounds such as SiCI4 , GeCI 4 , SiF 4 , Bel), O 2 , BBr) and POcl, which may be distilled to reduce the concentration of most transition metal impurities to below one part in 109 giving negligible absorption losses from these elements. Refractive index modification is achieved through the formation of dopants from the nonsilica starting materials. These vapor phase dopants include Ti0 2 , Ge0 2 , P205 , A1 20 3 , 8 20 3 and F, the effects of which on the refractive index of silica are shown in Fig. 4.6 [Ref. 21. Gaseous mixtures of the silica-containing compound, the doping material and oxygen are combined in a vapor phase oxidation reaction where the deposition of oxides occurs. The deposition is usually onto a substrate or within a hollow tube and is built up as a stack of successive layers. Hence the dopant concentration may be varied gradually to produce a graded index profile or maintained to give a step index profile. In the case of the substrate this directly results in a solid rod or preform whereas the hollow tube must be collapsed to give a solid preform from which the fiber may be drawn.

OPTICAL FIBERS, CABLES A ND CONNECTIONS

"9

~",o, ~,o )

(bulk )

b, D, (fiber)

, ,440

Fig.4.6

1

t 10 1~ 110 Dop.nt ,o""'nlnltion (mnal

4

l4

16

The va riation in t he re fr;lcl ive ind ex of since us ing va r;ous dopan ts, Reproduced with perm iss ion from t he p ub lisners. sccje tv of Gl aSli Te chnolo gy_ Ph ys. Chem. Glasses, 21. p 5, 19 80 ,

There are a number of variations of vapor pha se deposition which have been successfully utilized to produce low loss fibers. The major techniques are illustrated in Fig. 4.7, which also indicates the plane (horizont al or vertical) in which the deposition lake s place as well as the fo rmation of the preform. These va por pha se depositio n techniques fall into two broad ca tegories: fl ame hyd rolysis and chemical vapor d e pos itio n (C V D ) m ethod s. The individual tec hniques are con sidered in the fo llowing sections.

'erial.---- - --;==i._--.

Sl o:rh "l ...

r-r-_

_

-'1

1

1

VOF'" .. 1Ol I\ql<>lioo. (\'Alll

(),>lsId. ._ pt>ul: <».il»1"'" ~ IO\'fQ'

IL-

• 1

1

.."",r"d _ .. ' ''1'<'' d
~4<\"~tod


<1<,.,...""" lKVD ,

mcvr»

1

U Coll' l' " '0 l' ,.d Qr n\

~\\ II~: PI,. 4.7

=

~

r;1.",,;n~

h, " ><JU'Q<

t'ihft dm ' ;IIJ

I

Sc hltm. tIc W",l t r. tio n of t he .... ~ Of' ph ss., d epos llion teck1nique s used in m e prIIPII1I d on CI' low ION Ol'tic. 1 fibera.

..

120 4.4.1

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Outside Vapor Phase Oxidation (OVPOI Process

This process which uses flame hydrolysis stems from work on 'soot" processes originally developed by Hyde IReI'. 171 which were used to produce the first fiber with losses of less than 20 dB km-' IReI'. 181. The best known technique of this type is often referred to as the outside vapor phase oxidation process. In this process the required glass composition is deposited laterally from a 'soot" generated by hydrolyzing the halide vapors in an oxygen-hydrogen flame. Oxygen is passed through the appropriate silicon compound (i.e. SiC~) which is vaporized, removing any impurities. Dopants such as GeCI 4 or TiCI. are added and the mixture is blown through the oxygen-hydrogen flame giving the following reactions:

SiCl4 + 2H l O (vapor) (vapor)

heat

sio,

•

+ 4HCI (solid) (gas)

(4.1)

and SiC~ (vapor]

GeCl 4 (vapor)

+ 0,

heat

sio, +

•

(gas)

+ 0, (gas)

(solid) heat

•

GeO!

2CI! (gas)

(4.2)

+

(4.3)

(solid)

2CI! (gas)

0'

TiCI. + 0, (vapor) (gas)

heat

•

no, (solid)

+ 2CI l (gas)

(4.4)

The silica is generated as a fine soot which is deposited on a cool rotating mandrel as illustrated in Fig. 4.8(a) [Ref. 19 J. The flame of the burner is traversed back and forth over the length of the mandrel until a sufficient number of layers of silica (approximately 200) are deposited on it. When this process is completed the mandrel is removed and the porous mass of silica soot is sintered (to form a glass body) as illustrated in Fig. 4.8(b). The preform may contain both core and cladding glasses by properly varying the dopant concentrations during the deposition process. Several kilometers (around 10 km of 120 11m core diameter fiber have been produced fRef. 2]) can be drawn from the preform by collapsing and closing the central hole as shown in Fig. 4.8(c). Fine control of the index gradient for graded index fibers may be achieved using this process as the gas flows can be adjusted at the completion of each traverse of the burner. Hence fibers with bandwidth-length products as high as 3 GHz km have been achieved [Ref. 201 through accurate index grading with this process. The purity of the glass fiber depends on the purity of the feedina: materials

121

OPTICAL FI8ERS, CA BLES AND CONNECTION S

, "

, ',

ro

l [-

] I fig. 4 .8

'" '" Sche matic d iagra m of th e OVP O precess far m e pre pa ration of optica l fibe rs : (a) soot deposition, Ib) prefor m einte rmq: (c) nb er dra w ing IRef. 191.

and a lso upon the amount of OH impurity fro m th e exposure of the silica to water vapor in the flame following the reactio ns given in Eqs. (4. 1) to (4 .4). T ypicall y the O H content is between 50 and 200 parts per million a nd this contributes to the fiber atte nuation. It is possible to reduce the OH im purity content by employing gaseous chlorine as a d rying agent during sinrering. This has given losses a s low as I d B km- ' a nd 1.8 d B km- ' at wavelength s of 1.2 and 1.55 um respectively [Ref. 2 1 J in fibers prepared using the OVPO process. O ther problems stem from the u se of the m and rel which ean create some diffi culties in the fo rm ation of th e fiber preform. C racks may fo rm due to stress concentratio n on the surface of the inside wall when the mandrel is rem oved . Also the refract ive index profil e has a central depressio n d ue to th e collapsed hole when the fiber is drawn. Therefore a lthough the OV PO process is a useful fiber prepara tion technique, it has several drawbacks. Furthermore it is a batch process which limit'! it.. use for the volume production of optical fibers.

4 .4.2

Vapor Axial Deposition (VA D)

This pr ocess was developed by lzawa et at. IRef. 22J in t he se arch for a continuous (rather th an batch) technique for the production of low loss o ptical fibers . The VAD technique uses an en d-on deposition onto a rot ating fused silica ta rget as illustrated in Fig. 4.9 IRef. 23 1. The va porized constu ucnts are injected from burners and react to form silica soot by name hyd rolysis. This is deposited on the end of the sta rtin g ta rget in the axial direction fo rming a solid pcrcue alau preform in the shape of a boule. The preform which is growing in the axial direction i. Pulled upwards It a rate which corresponds to the growth

..

122

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

_ _ CarbOll I"ur",,,<

Pow", prcfur:T1

=_-0,

SO("l,

/~o:=' • t

sm, BBr,

s-o,

( ;0(1,

0, + II,

Fig.4.9

POCI, 0, + II,

The VAD process [Ref. 231.

rate. It is initially dehydrated by heating with SOCl 2 using the reaction: H2 a

+ SOCl 2

(vapor)

(vapor)

heal

•

2HCl (gas)

+ S02

(4.5)

(gas)

and is then sintered into a solid preform in a graphite resistance furnace at an elevated temperature of around 1500 "C. Therefore, in principle this process may be adapted to draw fiber continuously, although at present it tends to be operated as a batch process. A spatial refractive index profile may be achieved using the deposition properties of Si02--GeOl particles within the oxygen-hydrogen flame. The concentration of these constituents deposited on the porous preform is controlled by the substrate temperature distribution which can be altered by

',"

123

OPTICAL FIBERS, CA BLES AND CON NECTIONS

changing the gas flow conditions. Fibers produced by the VAD process still suffer from some O H imp urity content due to the flam e hydrolysis and hence very lo ll.' loss fibers have not been achieved using this method. Ne vertheless, fibers with atte nuation in the range 0.7-2.0 d B km- l a t a wavelengt h of L1 8 1J.rn have bee n reported IRef. 24 1.

4.4.3

Modified Chemical Vapor Deposition (MCVD)

Chemical va por d epo sitio n techniques are oommonly used at very low deposition rates in the semicond uctor ind ustry to produce protective SiO! films on silicon semiconductor devices. Usually an easily oxidized reagent such as SiH, diluted by inert gas es an d mixed with oxygen is brought into contact with a heated silico n surface where it forms a glassy tran sparent silica fil m. Th is heterogeneous reaction (i.e. req uires a surface to take place) was pioneered for the fabrication of o ptical fibers using the inside surface of a fused quartz tu be [Ref 25 J. However, these processes gave low deposition rates and were prone to O H contamination due to the use of hydride reactants. Th is led to the de velopment of the modified che mical vapor deposition (MC VD) process by Bell Telephone La boratories t Ref. 261and Southampton Un iversity, UK [Ref. 2 71. which overcomes these problem s a nd has found widespread application throughout the world. T he MC VD process is also an in side vapor phase oxidation (I VPO ) technique taking place inside a silica tube as sho wn in Fig. 4 ,10. However, the vapor phase reactants (halide and o xygen) pas." through a hot zo ne so that a substantial part o f the reaction is ho mogeneous {i.e. involves only one phase; in

1)"p o,iteJ core .11(! d ." (l ; ,, ~

'.j

-

(hI

- - - - - - - - - - - ~ ~ ~".,

,,' ptll.4.10

Scl1emat ie di'gr. m sh ow ing the M CVD method for the P!'e p. r't ion of ccrc et f l ~ !1I : (I I dtp Oli tiOtl; (ol coI1I P" to prod..lce a pt'eform; (el fl ber dr, winij.

..

124

OPTICAL FIBER COM MU NICATIONS: PRINCI PLES AN D PRACTICE

th is case the vapor phase). G lass pan icles formed d uring this reaction travel with the gas now and are deposited o n th e wa lls of the silica tube. The tube ma y form the cladding material but usually it is merely a supporting structu re which is heated on the outside by an o xygen- hyd ro gen name to tem peratures between 1400 °C and 1600 vC . Thus a hot zone is created which enco urages high temperature o xid ation react io ns such as tho se given in Eqs. (4 .2) and (·U ) or (4.4) (not Eq. (4.1)). These react ion s reduce the OH impurity co ncentration to levels below those found in fibers prepared by hyd ride oxidation or fl ame hydrolysis. The hot zone is moved ba ck and forth along the tube allowing the pan icles to be deposited o n a layer by layer ba sis giving a sm tered transparent silica film o n the walls o f lhe tube. The film may be up to 10 ....m in thickness and un iformity is maintained by ro tating the tu be. A graded refractive index profile c an be created by changing the composition of the la yers as the glass is deposited . Usually when suffi cient thickness has been fo rm ed by successive traverses of the burner for the cladding, vaporized chlorides of germa nium (GeCI4 ) or phosphoru s (POCI.1 ) are added to the gas fl ow. The core glass is then formed by the deposition of successive lay ers of germanosilicatc or pho sphosilicate glass. T he cladding layer is important a s it acts as a barrier which sup presses OR ab sorption losses due to the diffu sion of O H ions from the silic a tube into the core glass as it is deposited. After the deposition is completed the temperature is increa sed to between 1700 and 1900 °C. The tu be is then collapsed to give a solid preform which may then be drawn into fiber at temperatu res of 2()()(}..- 2200 ° C a s illustra ted in Fig. 4. 10. This technique is the most widely used at present as it allows the fabrication of fi ber with the lo west lo sses . Apart from the red uced on impurity con tamination the MC YD process ha s t he advantage th at deposition occ urs within an enclosed reacto r wh ich en sures a very clean environ ment. Hence gaseo us and p articulate impurities may be avoided du ring both the layer deposition and the preform collapse phases. The process also allo ws the use of a variety of materi als and g lass compositions. It h as prod uced G eO] doped silica single mode fibe r with minimum losses of o nly 0.2 d O krrr' at a wavelength o f 1.551J.1l1 (Ref. 281. More generally the GcO l - 0 10) -Si0 2 system (° 1 0 ) is added to reduce the viscosity and assis t fining) has shown minimum losses of 0.34 d B krr r ' with multimode fi ber at a wavelength of 1.55 um IRef. 291. Also graded index germa nium phosph ostltcatc fibers have exhibited losses near the intrinsic level for their composition of 2.8, 0.45 and 0.35 dB km-' at wavelengths of 0.82, 1.3 and 1.5 lim respectively

[Ref. 301. The MCVD process has also demonstrated the capability of producing tibers with very high bandwid ths. although sti ll well below the theoretical values whic h may be achieved. Multimode graded ind ex fibers with measured bandwidth-length products of4 .3 GHz km and 4.7 G HL.km at wavelengths of 1.25 and 1.29 fU11 h ave been reported [Ref J 11. LarI e-tcaJc bitch production ., -....

.-..

OPTICAL FIBERS, CABLES AND CONNECTIONS

125

( 30,000 km} of 50 urn core graded index fiber has maintained bandwidthlength products of 8 25 Mll z km and 735 MH z km at wavelengths o f 0.825 a nd 1.3 urn respectively IRef. 30}. The median att enuation obtained with this fiber was 3.4 d B km -' at 0.825 11m and 1.20 dB km I at t J 11m. Hence, although it is not a con tinuo us pro cess, the M C VD technique has proved suitable for the mass production of high performance o ptical fi ber . 4.4.4

Plasma-activated Chemical Vapor Deposition (PCVD)

A variation on the MCVD technique is the use of various types of plasma to supply energy for the vapor phase o xidatio n of halides. This method, fi rst developed by Kuppers and Koenings [Ref 32 J. involves plasma-induced chemical vapor deposition inside a silica tube as shown in Fig, 4, 11. The essential difference between this technique and the MCVD process is the stimulation of o xide fo rmation by means of a nonisothermal plasma maintained a t low pressure in a microwave ca vity (2.45 G Hz) which surrounds the tube. Volatile reactants arc introd uced into th e tube where they react heterogeneously within the microwave ca vity a nd no partic ulate matter is formed in the va por phase. The reaction zone is moved backwards a nd fo rwards along the tube by control of the microwave cavity and a circularly symmetric layer growth is formed. Rotation of the tube is unnecessary and the deposition is virtually !()()q(, efficient. Film dep osition can occur at temperatures as low as 500 °e, but a high chlorine content may ca use expansiviry a nd crac king of the film. Hence the tube is heated to around 1000 °C du ring deposition using a stationary furn ace. The high deposition efficiency allows the composnion of the layers to be accurately varied by con trol of the va por phase reactants. Also when the pla sma zon e is moved r apkll~' back wards and forward s a long the tube very thin layer deposition may be achieved giv ing t he formation of up to 2000 individual layers. This enables very good graded index profil es to be realized which are a close appro ximation to the o ptimum ncar pa rabolic profile. Thus low pulse dispersion of less th an 0.8 ns km"" , for fibers with attenuations of between 3 an d 4 dB km- ", at a wavelength of 0.85 11m have been reported

[Ref. 21. \l"""t

,""""~'"" ,,,,, " ------.-...-1;]

~ """" Ih,·, "",I rl",,,,.

.'

TtI4• 'llP.r.l u, ulllized in me PCVD process. •

•

to,· '"

126

OPTICAL FIBER COMMUNICATIONS : PRINCIPLES AND PRACTICE

A furt her PC VD technique uses an inducti-..e1y coupled radiofrcq oency argon plasma which operates at 3 frequency of 3.4 M Hz IRef. 331. The deposition ta kes place at I atmosph ere pressure and is predominant ly a hom ogeneous vapor phase reaction which, via the high temperature discharge. causes the fu sion of the deposited material into glass. This technique has proved to have a reaction rate five times faster than the conventional MCVD process. However. fiber attenuation is somewhat higher with losses of 6 dB km-' at a wavelength of 1.06 urn. Variations on this theme operating at frequencies of 3- 6 MHz and 27 MHz have produced Gc Ol - P1 0 ~ -S i 01 fibers with minimum losses of 4-5 dB km' at a wavelength of 0.85 urn (Rer. 341.

4 .4.5

Summary of Vapor Pha... Depo.ition Technique.

The salient features of the major vapor phase deposition techniques are summarized in Table 4.2 IRef. 351. T.bl.4.2

S ummar" 01 vapo r phase depositio n l echn iq ves used ;n the pre paration of lo w lOSS o ptical n be-s

Reaction type Fla me t1 ~ d ro l y si s Hig h temperature o . ida tio n l ow le mpe ra ture o..ida ti o n

OV ~O .

VAD

MCVD

pevo

Deposit iOf1 a l d ire ct ion

OUlsk1e la ve r de pcelncn Inside la ye r deposilion

OVPO MCVD. pew

A ..ial la yer d epo sjllOrl

VAO

Refract ive Index profi le for mati o n laye r a pproxima tion Simulta neous toema uon

ow o. MCVD. PCVD VAO

Process

Ba tch Conl inuous

4.8

oveo. MCVD. PCVD VAO

OPTICAL FIBERS

In order to plan the use of optical fibers in a variety of line communication applications it is necessary to consider the various optical fibers currently available. The following is a summary of the major optical fiber types with an

indication of their aeneral characteristics. The performance ch.r.cteristic8 or the v.rloul flbor typel dllculled var)' consider.bl>- depen upon the

OPTlCAL FIBERS. CABLES AND CONNECTIONS

127

materials used in the fabrication process and the preparation techniq ue involved. The values qu oted are largely based upon manu fac turers' and suppliers' data {Refs. 4Q---44 J for commercially a vailable fi bers. presented in a gene ral fonn rather than for specific fi bers . Hence the fibers may appear to have somewhat poorer perfo rman ce cha racteristics than those stared for the eq uivalent fiber types prod uced by the best possible techniq ues and in the best possible cond itions which were indicated in C hapter 3. However, it must be remembered tha t the high performance va lues qu oted in C hapter 3 were generally for fibers prod uced and tested in the laboratory. T here the pu rsuit of enhanced performance was the predominant criterion, whereas the fibers COIIsidcrcd in this section are those already man ufactured in bulk for the com mercial market. T his section. therefore, refl ects the time delay bet ween the achieveme nt o f fi ber performan ce in th e laborato ry (and possibly achieved in a work ing environment by orga nizations which have a fiber prod uction capability and are also in a position of servicing the telecommunications networks), and the general co mmercial availability o f such fi bers. Nevertheless. it is certain that as the momentum generated in this field increa ses, fibers with much imp roved performance characteristics will become mo re generally available. especially those in the longer wavelength regio n ( 1.1 - 1.6 um]. It must be noted that the performance values given thro ugho ut this section are for the sho rter wa velength region (0.8-0.9 urn) un less otherwise stated , as comme rcially availa ble fib ers are more frequently specifi ed at wavelengths in thi s region . However. although these fibers are not predominantly designed for use in the longer wavelength region it is generally th e ca se th at the silica glass fibers o perate more effi ciently over this wavelength range. Finally, the bandwidths quoted are specified over a I km length o f fiber (i.c. 8 up[ x L ). These a re generally o btained from man ufacturer s' data which does not always indicate whet her the electrical or th e optical bandwidth has been meas ured. It is likely that these arc in fact optical bandwidths which a re signifi cantly greater than their electrica l equivalents (see Section 7.4.3 ).

4.5.1

Muftimode Step In de. Flber1l

Multimode step index fibers ma y be fabricated fro m either multicomponer u glass compounds or doped silica. T hese fibers can have reasonably large core diameters and large numerical apert ures to facilit ate efficient coupling to inc oherent light sources such as light emitting diodes (LEDs). The performance characteristics of this fiber type may vary considerably depending on the materials used and the method o r preparation: the doped silica fi bers exhibit the best perfo rma nce. M utt lcomponent gjass a nd do ped silica fi bers are often rcrerred to as multicomponent glass/glass (glass-d ad glass) a nd silica/silica (. IIIc:a-c11d .ilica) respectively, altho ugh the glass-clad glass terminology is lomeUmll pMd aomewhat vaguely to denote both types. A ty pical structure far • mp index nbtr ~ tbown in Fil. 4.12.

128

OPTICAL FIBER COM MUNICATIONS: PR INCIPLES A ND PRACTICE

---,'---,

---

r;::---' " <3

~,·1. . S ~ ,. 1 _ ~ 1

F"eg.4.12

Ty piGaI structure fOl a g lass m ult im o de step imle R t iber.

Structure Core diameter:

S().....4()() JUTl.

125- 500 urn.

Cladding diameter: BuITer j acket diameter: Numerical aperture:

250- 1000 urn. 0.16-0,5.

Perf ormance characteristics Attenuation : 4-50 d B krn-' limited by absorption or scattering. The wide V3Tl3t10n in attenua tion is d ue to the large diffe renc es both within and between the two overall preparation method s

"l<...

t,<>~

(dB k", "

100

sc

'"

." ace

' till

000

,~,

~ • • • I.,, ",~ I~ "' j

'" ,\U'"",,;,,"

(J ~ ~'"--' )

,r~~,--,;t;;~~~:;;::~~~~;~ t>OU

h l(1

SIJO

<>00

IUOO

I l OCI

\\-. , """ ~lh , " "' )

FIg.4.13

'" AnenUllllo n spectr, lor mvltirnode ste p index fiber,: III multleom ponenl gl... titll' ; lbl cIo~ .1Ile. t iber. P:' Olrod uc:' d wit'" p, rml,,1on of e , tIlog

~,

Ltd.

129

OPTICAL FIBERS, CABLES AND CONNECTIONS

(melting and deposition). To illustrate this point Fig.4.13 shows the attenuation spectra from suppliers' data IRef. 431 for a multicomponent glass fiber (glass-clad glass) and a doped silica fiber (silica-clad silica). It may be observed that the multicomponent glass fiber has an attenuation of around 40 dB krrr ' at a wavelength of 0.85)lm whereas the doped silica fiber has an attenuation of less than 5 dB krrr ' at a similar wavelength. Bandwidth: 6-25 MHz krn. Applications: These fibers are best suited for short-haul, limited bandwidth and relatively low cost applications. 4.5.2

Multimode Graded Index Fibers

These multimode fibers which have a graded index profile may also be fabricated using multicomponent glasses or doped silica. However, they tend to be manufactured from materials with higher purity than the majority of multimode step index fibers in order to reduce fiber losses. The performance characteristics of multimode graded index fibers are therefore generally better than those for multimode step index fibers due to the index grading and lower attenuation. Multimode graded index fibers tend to have small core diameters than multimode step index Fibers although the overall diameter including the buffer jacket is usually about the same. This gives the fiber greater rigidity to resist bending. A typical structure is illustrated in Fig. 4.14.

Structure Core diameter:

30-60 urn, a standard of 50 11m has been established

Cladding diameter:

for telecommunications applications. 100-150 urn, a standard of 12511m has been established for telecommunications applications.

Buffer jacket diameter: Numerical aperture:

250-1000 urn. 0.2-0.3.

Buffer j,ek,,' PrimoI)' coating

(:0''-

",

'" ,,~,

.......14

'1';VPICII

,,

Itructure for a glass multi mode graded index fiber .

"; ' 1.48 "j" 1.46

130

OPTICAL FIBER COMM UNICATIONS: PRINCIPLES AND PRACTIC E

Performance characteristics Attenua tion : Bandwidth : Applications :

2- 10 dB km J . gene rally a scattering limit. 150 MH z km to 2 G Hz krn. These fibers a re best suited for medium -haul, medium to high bandwidth application!' using incoherent and coherent multimode sources {i.c . LED!' and inject ion lasers respect ively).

It is useful to note (hat the re are a number of partially graded index Fibers commercially a vailable. T hese fibers generally exhibit slightly better performance cbarecterisrics than co rre spond ing muhimode step index fi bers bu t are so mewhat inferior to the fully graded index fibers described above. 4 .6.3

Single Mode FiberB

Sing le mode fi ber s can have eit her a step ind ex o r gra ded index profile. Howev er. the benefits of using a gr aded index profile arc by no means as significant as in the case of multimode fi bers (see Section 2,5). The refore at present commercially available single mode fibers are almo st exclusively step ind ex. They are high quality fi bers for wideband, long-haul tran smission and a re gener ally fa bricated fro m do ped silica (silica-cl ad silica ) in order to reduce a ttenuation. Although single mode fi ber" have small core dia meters to allow single mode propaga tion . the cladding d ia meter must be at least ten times the core diameter to avo id lo sses from the eva nescent field. Hence with a bu ffer jacket to provide protec tion and strength. single mode fibers h ave simila r o verall diameters to mult imode fi bers. A typ ica l example of a single mode step index fiber is shown in Fig . 4. 15.

Structure Core d iameter: Cladd ing diameter : Buffer jack et diameter : Nu merical a perture :

3- 10 am. 50- 125 Jim. 250-1000 urn. 0 .08--0.15. usually a ro und 0 . 10.

", <.J

fl• . 4 .1 8

Typlc,l IrrUC1u ' l for I Imea lingle moae s le p i nde.. fibirr.

N, - I."r.o

". -I."'"

131

OPTICA L FIBERS, CABLES AND CO NNECTIONS

Performance characteristics A ttenua tion : 2-5 dB km- ' with a scattering limit of a ro und I d B km' a t a

Bandwidth :

Applica tions :

4.5.4

waveleng th o f 0 .85 urn . Significantly lower lo sses a re possible in the lo nger wavelength region. Greater than 500 MHz km, In th eo ry the bandwidth is limited by waveguide and materi al dispersion to approximately 40 G Hz km at a wavelength of 0.8 5 urn . These fiben are idea lly suited for hi gh bandwidth very lo ngha ul applicat ion.. using single mode injection laser sources.

Plaltic-elad Fibers

Plasti c-clad fibers are m ultirnode and have either a step index or a graded index profile. They have a plast ic cladding (often a silicone rub ber) and a glass core whic h is frequentl y silica (i.e. pla stic clad silica- Pe S fibers). The pes fiber s exhibit lower radiation-induced losses than silica clad silica fibers an d, therefo re. have an im proved performance in certain environments. Plastic-clad fibers arc gene rally slightly c heaper than the corresponding gla ss fibers, but usually have more limited performance characteristics . A ty pical structure for a ste p index plastic-clad fibe r (which is more common) is shown in Fig. 4. 16.

S tructure 100-500 .... m.

Step index

C o re d ia meter :

50- 100 u m.

G raded index C ladding diameter: Step Index Graded index Buffer j ac ket diameter : Step index Graded inde x Step index N umerical ape rture: G raded index

Performance characteristics Attenuation; Step index G raded index

300-800 urn. 125- 150 um 500-1 000 g m .

250-1 000 urn . 0 .2-1).5 . 0 .2-1).J. 5-50 dB/km. 4-1 5 dB/km.

CI.~ 'li n ~ -f­ (p l "tlo)

Cu" _

r-r-r-": "

....." .

'•.0:-

~, . , ~,

~ , . ' j"

132

OPTICA L FIBER COMMU NICATIONS : PRINCIP LES AND PRACTICE

Bandwidth : Applications:

4.5.5

Step ind ex 5-25 M Hz km. G raded index 200-400 MHz km. These fi bers are genera lly used o n lower band wid th. shorterhaul links where fiber costs need to be limited. They also have the advan tage o f easier termination over glas s-clad multimod e fibers.

All-plastic A be...

All-plastic fibers are exclusively of the multimode step index type with large core a nd cladding diameters. lienee there is a reduced requ irement for a buffer jac ket for fi ber protection a nd st rengthening. These fibers are chea p to produce And are ea sier to handle than the corresponding glass v ariet)'. However, their performance (especially for optical transmission in th e infrared) is severely restricted, giving th em very limited use in communication applications. A ll-plastic fibers generally have large nu merical apertures which allow easier coupling of light into the fiber from a multimode source. A typical structure is illustr ated in Fig. 4.1 7.

S tructure Core d iameter : C ladding diameter : Numerical aperture:

200-600 urn. 450- I000 urn.

0.5-0.6. ------

Performance characteristics Att enuation : 3So-l000dB km-' at a wavelength o f 0_65IJrn. Bandwidth : This is not usually specified as tra nsmission is generally limited to tens of meters . Applicatio ns : Th ese fi bers ca n only be used for very short-haul (i.e. 'inbouse') low cost links. However, fi ber coupling and termination a rc relatively easy and do not require so phisticated techmques.

( 0 ' '' -

8 ' - -"""- - - - - --- -

flI .4.t7

~ ""." o; ,·

Typical structu r. for ." I II-pintle fiber.

",

.,

..

.. """"

n o- 1.50 »v- 1.40

L OPTICAL FIBERS, CABLES AND CONNECTIONS

4.6

OPTICAL FIBER CABLES

It was indicated in Section 4.1 that if optical fibers arc to be alternatives to electrical transmission lines it is imperative that they can be safely installed and maintained in all the environments (e.g. underground ducts) in which metallic conductors arc normally placed. Therefore when optical fibers arc to be installed in a working environment their mechanical properties are of prime importance. In this respect the unprotected optical fiber has several disadvantages with regard to its strength and durability. Bare glass fibers are brittle and have small cross-sectional areas which makes them very susceptible to damage when employing normal transmission line handling procedures. It is therefore necessary to cover the fibers to improve their tensile strength and to protect them against external influences. This is usually achieved by surrounding the fiber by a series of protective layers which arc referred to as coating and cabling. The initial coating of plastic with high clastic modulus is applied directly to the Jiber cladding as illustrated in Section 4.5. lt is then necessary to incorporate the coated and buffered fiber into an optical cable to increase its resistance to mechanical strain and stress as well as adverse environmental conditions. The functions of the optical cable may be summarized into four main areas. These are: (a) Fiber protection. The major function of the optical cable is to protect against fiber damage and breakage both during installation and throughout the life of the fiber. (b) Stability of the fiber transmission characteristics. The cabled fiber must have good stable transmission characteristics which are comparable with the uncabled fiber. Increases in optical attenuation due to cabling are quite usual and must be minimized within the cable design. (c) Cable strength. Optical cables must have similar mechanical properties to electrical transmission cables in order that they may be handled in the same manner. These mechanical properties include tension, torsion. compression, bending, squeezing and vibration. Hence the cable strength may be improved by incorporating a suitable strength member and by giving the cable a properly designed thick outer sheath. (d) Identification and jointing of the fibers within the cable. This is especially important for cables including a large number of optical fibers. If the fibers are arranged in a suitable geometry it may be possible to use multiple jointing techniques rather than jointing each fiber individually.

In order to consider the cabling requirements for fibers with regard

(a) (b), it is necessary to discuss the fiber strength and durability as well as pOlliblc sources,of degradation of the fiber transmission characteristics

and an)' which ,

art .

I

likely to occur due to cabling. ,

(0

134

4.6.1

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Fiber Strength and Durability

Optical fibers for telecommunications usage are almost exclusively fabricated from silica or a compound of glass (multicomponent glass). These materials are brittle and exhibit almost perfect elasticity until their breaking point is reached. The bulk material strength of flawless glass is quite high and may be estimated for individual materials using the relationship 1Ref. 44]:

s ~ ,

(y,E)! 4/

(4.6)

•

where 8 1 is the theoretical cohesive strength, Yp is the surface energy of the material, E is the Young's modulus for the material (stress/strain), and fa is the atomic spacing or bond distance. However, the bulk material strength may be drastically reduced by the presence of surface flaws within the material. In order to treat surface flaws in glass analytically, the Griffith theory I Ref. 491 is normally used. This theory assumes that the surface flaws are narrow cracks with small radii of curvature at their tips as illustrated in Fig. 4.18. It postulates that the stress is concentrated at the tip of the crack which leads to crack growth and eventually catastrophic failure. Figure 4.18 shows the concentration of stress lines at the crack tip which indicates that deeper cracks have higher stress at their tips. The Griffith theory gives a stress intensity factor K 1 as: (4.7)

where S is the macroscopic stress on the fiber, Y is a constant dictated by the shape of the crack (e.g. Y = 1tt for an elliptical crack as illustrated in Fig. 4.18) and C is the depth of the crack (this is the semi-major axis length for an elliptical crack). Further, the Griffith theory gives an expression for the critical stress intensity factor K rc where fracture occurs as: (4.8)

fib<:r

""0" kn,iQIl

--i-

U "" of ,onstall[ ,t",",

FlII.4.18

An elliptical surfaca crack in a tensioned optical fiber,

,

,,'

135

OPTICAL FI BERS , CABLES AN D CONNECTIONS

Combining Eqs. (4.7) and (4.8) gives the Griffith equation for fracture stress or a crack Sr as : (4.9)

It is interesting to note that Sf is proportional to C-+. Therefore Sf decreases by a factor of 2 for a fourfold increase in the crack. depth C.

bamfM 4.1 The S i--O bond has a theore tica l co hesive st .ength of 2.6" 10 6 \lsi which ccnesponos to 11 bond di st ance of 0. 16 nm. A sil ic a opt ical f iber has an ellipt ical crac k of dep t h 10 nm at a p oint alo ng its leng t h. Estimate: (a) the frac t ure stress in p si f or the f iber II it is dependen t upon th is crack. (0) the perc ent ag e st rain at t he break.

The Younqs modulu s f o r si lica is i1 p p ro ~ ; m il t e l y 9 X 10 ' 0 N m- 2 and 1 ps i ... 689 4. 76 N m 2

Sotu tton, lei Us in g Eq. 14 .61 . t he t heo ret ical cohesive st rengt h for t he 5 i- O bo nd is:

5,=(Y 'E)' 4', Hence

= 2 29 J The fracture stress f or me si lica fiber may be ob lainoo from

s, ~

ec. 14

91 w here:

(2Fy'TC, )'

For an e llipt ica l crac k:

2 X 9 \ 10 10 X 2 .2 9 ) ' ( ll x1 0 8 .". 3.62 x 10 9 N m · l

= 5.25

x 10 5

It may be noted

psi

t hat the f ractu re stress is reduced from th e theor 91l c al va lu e fo r ' Is wilis silica of 2 ,6 x 10 6 psi by a f ac to r of e pp roximat efv 5. (b ) Yool'g', modulu s ;, defined as:

E _stress _"" /

anein

•

136

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Therefore stress strain =

s,

---cc-- ~ E

E

3,62 x 10 9

~

--c-:c:>c

= 0.04

Hence the strain at the break is 4%, which corresponds to the change in length over the original length for the fiber.

In example 4.1 we considered only a single crack when predicting the fiber fracture. However, when a fiber surface is exposed to the environment and is handled, many flaws may develop. The fracture stress of a length of fiber is then dependent upon the dominant crack (i,c. the deepest) which will give a fiber fracture at the lowest strain. Hence the fiber surface must be protected from abrasion in' order to ensure high fiber strength. A primary protective plastic coating is usually applied to the fiber at the end of the initial production process so that mechanically induced flaws may be minimized. Flaws also occur due to chemical and structural causes. These flaws are generally smaller than the mechanically induced flaws and may be minimized within the fiber fabrication process. There is another effect which reduces the fiber fracture stress below that predicted by the Griffith equation. It is due to the slow growth of flaws under the action of stress and water and is known as stress corrosion. Stress corrosion occurs because the molecular bonds at the tip of the crack are attacked by water when they arc under stress. This causes the flaw to grow until breakage eventually occurs. Hence stress corrosion must be taken into account when designing and testing optical fiber cables. It is usual for optical fiber cables to have some form of water-protective barrier as is the case for most electrical cable designs. In order to predict the life of practical optical fibers under particular stresses it is necessary to usc a technique which takes into account the many flaws a fiber may possess, rather than just the single surface flaw considered in example 4.1. This is approached using statistical methods due to the nature of the problem which involves many flaws of varying depths over different lengths of fiber. Calculations of strengths of optical fibers are usually conducted using Weibull statistics [Ref 501 which describe the strength behavior of a system that is dependent on the weakest link within the system. In the case of optical fibers this reflects fiber breakage due to the dominant or deepest crack. The empirical relationship established by Weibull and applied to optical fibers indicates that the probability of failure F at a stress S is given by: (4.10)

"y

137

OPTICA L FIBERS, CABLES AN D CONN ECTION S

where m is the Weibull distribution parameter, So is a scale parameter, L is the fiber length and La is a constant with dimensio ns of length . T he expression given in Eq. (4. 10) may be plotted for a fiber under test by breaking a large number of 10-20 m fiber lengths and measuring the strain at the b reak . Th e various strains are plotted against the cu mulative probability of their occurrence to give the Weibull plot as illustrated in Fig. 4.19 IRef. 5 11. It ma y be observed from Fig. 4. 19 that most of the fiber tested brea ks at strain due to the prevalence of many shallow surface fl aws. However, some of the fi ber tested contains deeper flaws (possibly d ue to external damage) giving the failure a t lower stra in depicted by the tail o f the plot. T his red uced strength region is of greatest interest when determining the fiber's lifetime under stress. F inally the additional problem of stress co rrosion must be added to the informa tion on the fiber under stress gained from the Weibull plot. The stress co rrosion is usually predicted using an empirical relationship fo r the crack in terms of the a pplied stress intensity facto r KJ , where (Ref. .5 1]: velocity

"c

.." -- AK"I

(4.1 1)

T he constant n is called the stress corrosion susceptibility (typically in the range 15- 50 for glass), and A is also a constant for the fiber material. Equation (4. 11) allows estimation of the time to failure of a fiber under stress corrosio n co nditions. Therefore from a combinatio n of fi ber testing. (Weibull plot) a nd stress corrosio n information estimates o f the ma ximum allowable fiber strain can be made available to the cable designer. T hese estimates may be co nfirmed by straining the fiber up to a specified level (p roof testing) such as 1% st ra in. F iber which survives this test can be a ccepted. However, proof testing presents further problems. as it may cause fiber damage. Also it is necessary to derate the maxi mum allowa ble fiber strain from the proof test value to increas e confidence in fiber survival under stress cond itio ns. It is suggested lRef. 5 11 that a reasonable derating for use by th e ca ble d esigner for fiber which has survived a I % strain proof lest is around 0.3% in order that the fiber has a reasona ble chance of surviving with a continual strain for 20 years,

~

'11,4.1' A I chemat!e hm M. H.

~'ualltatiOl'l

RH~ .

..

of a We ibull

•. Son. ll

~OL

~ . ,,~

Reproduced wit ... ce-mjssion

TM Radio . "d EI.crfOfl. E" g., 11, p. 327. 1981 .

138

4.6.2

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Stability of the Fiber Transmission Characteristics

Optical fiber cables must be designed so that the transmission characteristics of the fiber are maintained after the cabling process and cable installation. Therefore increases in the optical attenuation and reduction in the bandwidth of the cabled fiber must be avoided. A problem which occurs in the cabling of optical fiber is the meandering of the fiber core axis on a microscopic scale within the cable form. This phenomenon, known as microbending, results from small lateral forces exerted on the fiber during the cabling process. Such random bending of the fiber axis causes coupling of power between modes (see Section 2.3.7) which results in losses due to radiation in both multimode and single mode fibers. Thus excessstve micro bending can easily create additional fiber losses to an unacceptable level. To avoid deterioration in the optical fiber transmission characteristics resulting from mode coupling induced by microbending, it is important that the fiber is free from irregular external pressure within the cable. Carefully controlled coating and cabling of the fiber is therefore essential in order to minimize the cabled fiber attenuation. Furthermore the fiber cabling must be capable of maintaining this situation under all the strain and environmental conditions envisaged within its lifetime.

4.7

CABLE DESIGN

The design of optical fiber cables must take account of the constraints discussed in Section 4.6. In practice these constraints may be overcome in various ways which are, to some extent, dependent upon the cable's application. Nevertheless, generally cable design may be separated into a number of major considerations. These can be summarized into the categories of fiber buffering, cable structural and strength members, and cable sheath and water barrier. 4.7.1

Fiber Buffering

It was indicated in Section 4.6. that the fiber is given a primary coating during

production in order to prevent abrasion of the glass surface and subsequent flaws in the material. The primary coated fiber is then given a secondary or buffer coating (jacket) to provide protection against external mechanical and environmental influences. This buffer jacket is designed to protect the fiber from microbcnding losses and may take several different forms. These generally fall into one of three distinct types which are illustrated in Fig. 4.20 [Ref. 52]. A tight buffer jacket is shown in Fig. 4.20(a) which usually consists of a hard plastic (e.g. Nylon, Hytrel, Tefzel) and is in direct contact with the primary coated fiber. This thick buffer coating (0.25-1 mm in diameter) provides stiffening for the fiber against outside micro bending influences, but it must be applied in such a manner as not to cause microbending losses itself. '. i'e" -

OPTICAL FIBERS, CABLES AND CONNECTIONS

Ti ~:lt

!luff"

j,,,k('L

Loose [,ull., j,chl

139

Fill'd luo,"

hufrcr jackot

'" Techniques for buffering of optical fibers f,)

fig.4.20

lei

fRef. 52J: (a) tight buffer jacket; (bl loose buffer jacket; lei filled loose buffer jacket.

An alternative approach which is shown in Fig. 4.20(b) is the use of a loose buffer jacket. This produces an oversized cavity in which the fiber is placed and which mechanically isolates the fiber from external forces. Loose buffering is generally achieved by using a hard, smooth, flexible material in the form of an extruded tube, or sometimes a folded tape with a diameter between I and 2 mm. Finally Fig.4.20(c) shows a variation of the loose buffering in which the oversized cavity is filled with a moisture-resistant compound. This technique, which combines the advantages of the two previous methods, also provides a water barrier in the immediate vicinity of the fiber. The filling material must be soft, self-healing and stable over a wide range of temperatures and usually consists of specially blended petroleum or silicone-based compounds.

4.7.2

Cable Structural and Strength Members

One or more structural member is usually included in the optical fiber cable to serve as a core foundation around which the buffered fibers may be wrapped, or into which they may be slotted as illustrated in Fig. 4.21 [Refs. 51 and 52]. The structural member may also be a strength member if it consists of suitable material (i.e. solid or stranded steel wire or Kevlar (DuPont Ltd.) yarns). This situation is shown in Fig. 4.21(a) where the central steel wire acts as both a structural and strength member. In this case the steel wire is the primary loadbearing element. Figure 4.21(b) shows an extruded plastic structural member around a central steel strength member. The primary function of the structural member in this case is not load-bearing, but to provide suitable accommodation for the bulTered fibers within the cable. Structural members may be nonmetallic with plastics, fiberglass and Kevlar often being used. However, for strength members the preferred features include a hiah Young's modulus, high strain capability, flexibility and low weight per ,unit lenath. Therefore although similar materials are frequently utilized for both Itrenath and Itructural members, the requirement for additional tensile Itrln.th- of the lu.nlth member must be considered within the cable design.

140

OPTI CAL FIBER COM M UNICATIONS: PRINCIPLES A ND PRACTICE

S,«;I '''m, :!. 5 nUD, \.

2() k~

k,. - '

llodd tJII: b ye:r 3. 2 DUD T~I .,.j

r,.....

; " "'. 1! , _ h,-'

'......e"'."'_

H -Hut b&mu <>' 1 /

!.. tlull

.... ," ."." ...

' ~ k& W"'-·

f oly" h,I ," , >bo.th 4l Wj k ", - ', II mm

I.)

Fig . 4 .21

Structural an d s tre ng th m e mbe rs in optical fibe r clIb!@s : (8) ce nt ra l steel wire structural lind s tren gth membe r fRef. 511: (b) No rthe rn Teleco m unit co re cable w ith ce ntral steel streng th membe r and e xtruded pintle: str uctural member [Ref, 521.

FIe....ibility in strength members formed o f materials with high Young's moduli ma y be improved by using a stranded or bunched asse mbly of smaller units a s in the case of steel wire. Sim ilar techniques are also em ployed with other materials us ed fo r strength mem bers which include plastic m onofilamerits (i.e. specially processed polyester). textile fiber (Nylon, Terylene, Da cron and the widely used K evler} and carbon and gla ss fibers . These ma teri al s provide a variety of tensile strength s for d ifferent cable ap plicat ions. H owever. it is worth noting th ai Kevlar, an aromatic polyester. ha s a very high Young's modulus (up to 13 x 10 10 N m" } wh ich gives it a stre ngth to weight ratio ad vantage four limes tha t of steel. It is usual when utilizin g a stranded strengt h mem ber to cover it with a coating of extruded plastic , o r helically applied tape. This is 10 p rovide the strength member with a s mooth (cushioned) surface which is especi ally im portant for the prevention of microbending lo sses when the member is in contact with the buffered optical fibers.

4 .1.3

Cable Sheath and Water Barrier

T he cable is normally cov ered with a substantial outer pla stic sheath in order to reduce abra sion and to provide the cable with extra protection a gainst external mechanical effects such as crushing. The cable sheath is said to contain the cable core and may vary in complexity fro m a single extruded plastic jacket to

,

\ _ .-

OPTICAL FIBERS, CABLES AND CONNECTIONS

141

a multilayer structure comprising two or more jackets with intermediate armoring. However, the plastic sheath material (e.g. polyethylene, polyurethane) tends to give very limited protection against the penetration of water into the cable. Hence an additional water barrier is usually incorporated. This may take the form of an axially laid aluminum foil/polyethylene laminated film immediately inside the sheath as used by British Telecom [Ref. 53] and illustrated in Fig.4.21(a). Alternatively the ingress of water may be prevented by filling the spaces in the cable with moisture-resistant compounds. Specially formulated silicone rubber or petroleum-based compounds are often used which do not cause difficulties in identification and handling of individual optical fibers within the cable form. These filling compounds are also easily removed from the cable and provide protection from corrosion for any metallic strength members within the fiber. Also the filling compounds must not cause degradation of the other materials within the cable and must remain stable under pressure and temperature variation.

4.7.4

Examples of Fiber Cables

Many different cable designs have been proposed and a large number have been adopted by different organizations throughout the world. At present there are no definite standards for optical fiber cables incorporating either a particular number of fibers or for specific applications. However, as discussed previously there is a general consensus on the overall design requirements and on the various materials that can be used for cable construction IRef. 521. In this section we therefore consider some further examples of optical fiber cable construction in order to give the reader a feel for the developments in this important field. Figure 4.22 [Ref 40] shows two examples of cable construction for single fibers. In Fig. 4.22(a) a tight buffer jacket of Hytrel is used surrounded by a layer of Kevlar for strengthening. In this construction the optical fiber itself acts as a central strength member.

Oplic:u fil>cr-==;{: Buffcrlube_

Fi ber

Outet j.. ket, Hytrol

\Inn" jack«,

Inner jacket--j

cl.ddiLl ~

Outer j acket

Hytrel

'0> PIg.4.21

Slngll flblr clble8 [Ref. 40J: Jlcklt dnlgri.

(e}

tight buffer jacket design; (b) loose buffer

142

OPTICAL FIBER COMMUNICATIONS : PRINCIPLES AND PRACTICE

The cable co nstructio n illustra ted in Fig. 4.22(b) uses a loose t ube buffer a round the central optical fi ber. This is s urrounded by a Kevlar strength mem ber which is protected by an inner shea th or j ack et before the o uter sheath layer . Thc strength mem bers of single optical fi ber cables are not usually incorporated at the center o f the cable (u nless the fi ber is acting a s a strength member) but are placed in the surround ing cable form as illustrated in Fig. 4 .2 2(b ). Cable designs for multifi ber cables may also take this general form with the strength member surro unding the fibers at the center of the cable. Examples of th is construction are illustrated in Fig. 4 .23 [Ref. 521. F igure 4.2 3(a) shows se ven fibers at the cab le center surro unded by a helically laid Kevla r strength mem ber . Figure 4 .23(b) shows a ribbon cable configuration with a strength mem ber of polyp ropylene yarn s in the surrounding cable form. It may also be noted thai this d esign utilizes a rmo ring o f stainless steel wires placed in the o uter shea th. Two more ca ble designs which allow the incorporatio n o f a larger num ber of fiber-s are sho wn in Fig. 4 .24 {R ef. 52 1. The configuration illustrated in Fig. 4 .24 (a) is a stranded d esign where the buffered fiber s are arranged in on e or more layers. Alt ernatively, Fig. 4 .24(b) shows 8 multi-unit design wh ere each u nit contains seven buffered fi bers. In this case the design al lows 4 9 fibers to be included within the cable. F inally a c able d esign which has proved successful in install ations in the United States is shown in Fig. 4.25 [Ref. 54]. T he cable has a central copper wire for strengthening and also to provide possible electrical conouttc n surro und ed by a pla stic structural memb er. Up to 12 optic al fi bers are placed in a flat rib bon between plastic tape s and in corpo rated into a helical groove in the extr uded pla st ic str uctural mem ber. A nother diamet rically o pposite groove is designed fo r the placement o f up to seven plastic insula ted metallic pairs o r a lterna tively the incorpo ratio n o f o ther ribbon o r o ptical fibers. T he princ ipal stre ngth mem ber i... a loose aluminu m tu be litted over the c able core which also acts a s a water barrier. T his is surrounded by an inn er pol yeth ylene jacket or

"'.\-- l'~l' ptoy rl",,' ~ .ll" H IWI '. inner , h, " h

_ I'nly u r" h. " , ino " j :>
, -

_

III+ + - GI,,, cor< (rLhhO,,, l

T Fl "I'"

_ PoI)·"" lh, n.

·-

""".. j. , ko,

HDPE prc......•.. l ru<\od

w I•• "",.u.

'0' Ftg.4.23

Multffiber ca bles w ;thout centla. &Intngtll arK! Str\J ctural m emb ar [R, f. 1521: la! ITT ISV111n fibe r e ~ l8 rf'l al lfflmg l l1 member ea bl,;(bl AT &. Tribootl cabl,.

••

.,

143

OPTICAL FIBERS. CABLES AN D CON NECTIONS

,.

r

• ,

·---

-

l.1ontfOl ......1>of

.-

..... _

Crntral momb«

1'£

~~::=Ol'lO;lII

v-- (.'"... fi""~

... _..-

Ii\,.,.-

--,<_Bu'I.,.ja.:l("

...... "'"..... '

., Fig.4.24

'"

b a mp les of mullifiber cable deslg 'l [Ref. 521: fa) S illCOl ' 18 fiber d uct ca ble ; fbI s secc r 49 tibe r ...n it c a ble.

sheat h followed by armoring con sisting of corrugated steel tape with longitudinal overlap. A second polyethylene jacket acts as an cuter cable sheath givinj;t the cable an overall diameter of around 2.5 em. The use of the alum inum tube also allows the ca ble to be operated under pressurized condiHom which gives the addit ional advantages of: (a) an alarm in the event of sheath perforation ; (b) s heath fault location; (c) the exclusion or reduction of water ingress et a sheat h fault. Trials of various optical fiber cable designs have taken place t hro ughout the world since J977 with little indication of failure due to the possible degradation

_

u~

'0 7 -.1010<..


-

........

-

_

.

,J""" ",,,",,,,,", ,....... '"....., ....... ,

_ lh ld'
A a.ntnl cabl~ Cotpalltlon mu ltifiber cable IRel. 641. •

144

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

mechanisms. It is therefore suggested [Ref. 51 j that there is a possibility that current commercial optical fiber cables manufactured for telecommunications purposes have been 'over-engineered' and are thus working very successfully. Hence it is likely that the future development of optical fiber cables will concentrate on simpler designs which will bring both production and cost benefits as optical fiber systems are utilized more fully in telecommunication networks.

I ,,

4.8

OPTICAL FIBER CONNECTION

Optical fiber links, in common with any line communication system, have a requirement for both jointing and termination of the transmission medium. The number of intermediate fiber connections or joints is dependent upon the link length (between repeaters), the continuous length of fiber cable that may be produced by the preparation methods outlined in Sections 4.2--4.4, and the length of fiber cable that may be practically or conveniently installed as a continuous section on the link. Current practice allows single lengths of fiber cable of around 1 km to be installed. However, it is anticipated [Ref. 56] that this will be increased to several kilometers, especially for submarine systems where continuous cable laying presents fewer problems. Repeater spacing on optical fiber telecommunication links is a continuously increasing parameter with currently installed digital systems operating over spacings of up to 30 km together with the prospect of repeater spacings of many tens and even over 100 kilometers for the long wavelength single mode systems of the near future. (For example, 100 km operation without repeaters was achieved by British Telecom in the laboratory (uncabled fiber) at the beginning of 1982 with a 140 M bit s 1 single mode system operating at a wavelength of 1.55 11m. In this case the fiber produced by a MCVD process was jointed (spliced) at 6 km intervals.) It is therefore apparent that fiber-fiber connections with low loss and minimum distortion (e.g. modal noise with multimode fibers) is of increasing importance within optical fiber communications in order to sustain the repeater spacings required for developing systems. However, in this context optical fiber jointing has to a certain extent lagged behind the technologies associated with the other components of optical fiber communication systems (fiber, sources, detectors, etc.). Nevertheless in recent years there has been an increasing interest in this topic and significant advances have been made. Therefore in this and the following sections we review the theoretical and practical aspects of fiber-fiber connections with regard to both multimode and single mode systems. Fiber termination to sources and detectors-is not considered as the important aspects of these topics are discussed in the chapters covering sources and detectors (Chapters 6, 7 and 8). Nevertheless the discussion on fiber jointing is relevant to both source and detector coupling, as many manufacturers supply these electro-optical devices already terminated to a fiber optic pigtail in order to facilitate direct

I_;;;;;;;:::~fi~'b~e;r=fi;'b~e_r_connection to an optical tiber link,

J '.":~!~~~:

145

OPTICA L FIBERS, CABLES AND CONN ECTION S

Before we consider fi ber-fiber connection in fu rther det ail it is necessary to indicate the two maj or ca tegories o f fiber joint c urrentl y both in use a nd development. These a re : (a) Fiber splices : these are semipermanent o r permanen t j oin ts which find major use in most o ptical fiber telecommuni cation systems (analogous to electrical soldered joints), (b) D emo untable fiber connectors o r simply connectors : these are removable joints which allow eas y fa st manual coupling and unco upling of fiber s (a nalogous to electrical plugs and sockets).

A majo r consideration with all types of fiber-fiber connection is t he o ptical loss e ncountered at the interface. Even when the two jointed fiber ends are smooth and perpendicular to the fiber axes, and the two fi ber axes are perfectly aligned, a small proportion o f the light may be reflected back into the transm itting fiber causing att enuatio n at the joint. Thi s phenomenon, known as Fresnel reflection, is as sociated with the step changes in refract ive index at the j oi nted interface (i.e. glass- air -glass). The magnitude of this partial reflection of the light transmitted through the interface may be estimat ed using the classical Fresnel formul a fo r light o f normal incidence and is given by [Ref.

l7):

r~

(n ,-n ) ' n, n

(4. 12)

+

"I

where r is the fraction of the ligh t reflected at a single interface, is the refractive ind ex o f the fiber core and n is the refractive ind ex o f the medium between the two jointed fi bers (i.c. for a ir n = 1). H owever in orde r to determine the amount of light reflected at a fiber joint, F resnel refl ection at both fiber interfaces must be taken into account . The loss in decibels due to Fresnel reflection at a single interface is given by :

Loss-,... = - 10 loglo ( I - r)

(4.13)

H ence using the relation ships given in Bqs. (4.12) and (4 .1 3) it is possible to determ ine the optical attenuatio n due to Fresnel reflection at a fiber-fiber j oint. Jt is apparent that F resnel refl ection may give a significant loss at a fiber joint even when all other aspect s of the connection are ideal. However, the effect of Fresnel reflection at a fiber-fiber connection can be reduced to a very low level through the use of an index matching fluid in t he gap between the jointed fibers . When the index matching fl uid has the same refractive index as the fiber core, losses due to Fresnel reflection are in theo ry er adi cated. Unfortunately F res nel refl ection is only one possible so urce of optical loss at • Aber joinL A potentia lly gre ater source of loss at a tiber-fiber connection is " Ililolid by m!I&llpmenl o f the two jointed fibers. In o rder to appreciate the ' ~ _ ....e tue:en' DC vuioulJ connection techniques it is useful

,

n-..

l.lll:

dmll,

146

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Example 4.2 An optical fiber has a core refractive index of 1 5. Two lengths of the fiber with

smooth and perpendicular {to the core axes] end faces are butted together. Assuming the fiber axes are perfectly aligned, calculate the optical loss in decibels at the joint {due to Fresnel reflection) when there is a small air gap between the fiber end

tecaa. Solution: The magnitude of the Fresnel reflection at the fiber-air interface is given by Eq. (4,12) where:

,~

("' -" )' n, + n

1.5- 10 )'

( 1.5+1.0

)' (~ 2.5 0,04

The value obtained for r corresponds to a reflection of 4% of the transmitted light at

the single interface. Further, the optical loss in decibels at the single interface may be obtained using Eq. {4.13) where: Los5fre.

=

-10 10910 {1 - r)

=

-10 IOg10 0.96

=0.18dB A similar c~lculation may be performed for the other interface (air-fiber). However from considerations of symmetry it is clear that the optical loss at the second interface is also 0.18 dB. Hence the total loss due to Fresnel reflection at the fiber joint is approximately 0.36 dB.

4.8.1

Fiber Alignment and Joint Loss

Any deviations in the geometrical and optical parameters of the two optical fibers which are jointed will affect the optical attenuation (insertion loss) through the connection. It is not possible within any particular connection technique to allow for all these variations. Hence there are inherent connection problems when jointing fibers with, for instance: (a) (b) (c) (d)

different core and/or cladding diameters; different numerical apertures and/or relative refractive index differences; different refractive index profiles; fiber faults (core ellipticity, core concentricity, etc.).

The best results are therefore achieved with compatible (same) fibers which are manufactured to the lowest tolerance. In this case there is still the problem of the quality of the fiber alignment provided by the jointing mechanism. Examples of possible misalignment between coupled compatible optical fibers are illustrated in Fig. 4.26 [Ref. 581. It is apparent that misalignment may _'.,f

... j

-

"'.•

. . . ,. . .

OPTICAL FIBER S. CABLES AN D CONNECTIONS

147

.., rlg.4.28

The tr se e poS$ible t'IPe ~ of misalignme nt w hich may c cc ur when jo'ming compatible optical fibe rs IRef. 58 J; (a) longitudinal m isalignme nt; (bI la te ral misalig nment; Ie! ang ula r misalig nme nt .

oc cur in three d imensions, the separation between the fibe rs (longitudinal misalignment), the offset perpendicular to the fiber core 3llCS (lateral/radial/axial misalign ment) and the angle between the core axes (angular misalignment). Optical losses resulting from these three types of misalignment depend upon the fiber type, core diameter and the distribution of the optical power between th e propagating modes. Example s of the measured optical losses d ue to the various types of misalignment are shown in Fig. 4.27. Figu re 4.27(a) [Ref 581 sho ws the attenuation cha rac teristic for both longitudinal and lateral misalignmen t of a 50 um core diameter graded index fiber. It may be observed that the lateral misalignment gives s ignilk a ntly greater losses per unit displacement tha n the lo ngitudinal misalignment. For instance in this case a latera l displacement of 10 urn giv·es a bout I dB insertion loss whereas a similar longitudinal displacement gi v'es an insertion loss of around 0 .1 dB. Figure 4.27(b) lRef. 59 1 shows the attenuation characteristic for the an gular misalignment of two multimode step index fibers with n umerical a pertures of 0.22 and 0.3. An inser tion loss of a rou nd 1 d B is o btained with ang ular misalignment of 4 0 and 5 0 for the 0.22 NA and 0.3 NA fibers respectively. II may also be observed in Fig. 4.27(b) that the effect of an index mat ching flu id in the fiber gap cau ses increased losses with angular misalignment. Therefore it is clear that relatively small levels of lateral and/or angular misalignment can cause significan t attenuation at a fibe r joint. This is especially the case fo r small core d iameter (less than 150 urn) fibers which are currently employed for most telecommunication purposes. Theoretical and experiment al studies of fiber misalignment in optical fiber connections [Refs. 60- 721 allow approximate determination of the losses encountered with the various misalignments of different fi ber types . We conelder here some of the expressio ns used to calculate losses d ue to lateral and Inlular misalignment of optical fiber joints. Longitudinal misalignment is not dl.cuned in detail 8!1 it tends to be the least important effect and ma y be larlof)' avoU!td Ie Abet cceeeeooe. Also there is some disalreemenl over the Mlp1tudI ol-tbt-lOlili chat to lonaftudlnaJ mluJi,nment when it is calculated . "

148

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

In,mion

La«,,]

1o," (
2.0

o

·5

10

15

20

25

311 35 4(1

45

;;0

lli'l'h""",,,,' i~,n)

,,,'

Inde, ","clL," g"l'

HI

~A'IJ.2'

I"""ioll 10>1

uno

Air l'l'P

lil

~'A

= (J,22

2.0

3

Fig.4.27

4

5

b

7

g

9

10

Insertion loss characteristics for jointed optical fibers with various types of

misalignment: (a) insertion loss due to lateral and longitudinal misalignment for a 50 IJ.m core diameter graded index fiber. reproduced with permission from P. Mossman, The Radio and Electron. Eng., 51. p. 333, 1981: (bl insertion loss due to angular misalignment for joints in two multlmode step index

fibers with numerical apertures of 0.22 and 0.3. Reproduced with permission from C. P. Sandbank Iedl, Opticaf Fiber Communication Systems, John Wiley & Sons, 1980.

theoretically between Miyazaki et al. [Ref. 61J and Tsuchiya et al. lRef. 62]. Both groups of workers claim good agreement with experimental results which is perhaps understandable when considering the number of variables involved in the measurement. However, it is worth noting that the lower losses predicted by Tsuchiya et at. agree more closely with a third group of researchers [Ref. 63]. Also all groups predict higher losses for fibers with larger numerical •

OPTICAL FIBERS, CABLES AND CONNECTIONS

149

apertures which is consistent with intuitive considerations (i.e. the larger the numerical aperture, the greater the spread of the output light and the higher the optical loss at a longitudinally misaligned joint). Theoretical expressions for the determination of lateral and angular misalignment losses are by no means definitive although in all cases they claim reasonable agreement with experimental results. However, experimental results from different sources tend to vary (especially for angular misalignment losses) due to difficulties of measurement. It is therefore not implied that the expressions given in the text are necessarily the most accurate, as at present the choice appears somewhat arbitrary. Lateral misalignment reduces the overlap region between the two fiber cores. Assuming uniform excitation of all the optical modes in a multimode step index fiber the overlapped area between both fiber cores approximately gives the lateral coupling efficiency TlI'I' Hence the lateral coupling efficiency for two similar step index fibers may be written as [Ref. 62J:

_

Tllat-

16(n,/n)' I { 2cos (I + (n l/n))4 rt

_,(y) [1 -(Y)']'} - (Y) 2a

a

2a

(4.14)

where n l is the core refractive index, n is the refractive index of the medium between the fibers, y is the lateral olTset of the fiber core axes, and a is the fiber core radius. The lateral misalignment loss in decibels may be determined using: Loss lat = -10 loglo

T\lat

dB

(4.15)

The predicted losses obtained using the formulae given in Eqs. (4.14) and (4.15) are generally slightly higher than the measured values due to the assumption that all modes are equally excited. This assumption is only correct for certain cases of optical fiber transmission. Also certain authors [Refs. 61 and 7l] assume index matching and hence no Fresnel reflection which makes the first term in Eq. (4.14) equal to unity (as nl/n = 1). This may be valid if the two fiber ends are assumed to be in close contact (i.e. no air gap in between) and gives lower predicted losses. Nevertheless, bearing in mind these possible inconsistencies, useful estimates for the attenuation due to lateral misalignment of multimode step index fibers may be obtained. Lateral misalignment loss in multimode graded index fibers assuming a uniform distribution of optical power throughout all guided modes was calculated by Gloge [Ref. 65]. He estimated that the lateral misalignment loss was dependent on the refractive index gradient (l for small lateral offset and may be obtained from:

Ll=~

e) (::~)

whlre the Iltltll cOUpllnl efficiency

forO~y~O.2a

was given by:

(4.16)

150

OPTICAL FIBER COMMU NICATIO NS : PRINCIP LES AND PRACTICE

(4. 17)

H ence Eq. (4. 17) may be utilized to obtain the lateral misalignment loss in decibels . With a parabolic refract ive index profile where a = 2, Eq. (4.16) grves : (4. 18) A further estimate includ ing the leaky modes, gave a re vised expression for the lateral misalignment loss given in Eq. (4.17) of O.7S(y l a). This an alysis was also extended to step index fibers (where a = co) and gave lateral misalign ment losses of O.64(y/a) and O.5(y/ a) fo r the cases of guided mo des only an d both guided plus lea ky modes respectively.

Example

4 .3

A step irnJell f ib er h as a core ref ract iv e inde ll of 1.5 and a 0C\f 8 dia m et er 0 1 SO 10m .

The fi b e r

IS

jolnle d w it h a lilt!!' ''1 m i5alig nment b etwe e n t he c ore ..xe s of 5 I' m.

Esti m at e t he insertio n 10S5 at t he join t d ue to t ne I,n e ral m isalignm enT assumi ng a un ilo rm d islfibut ion of powe r b etw een all g u;ded modes w hen ' (a) t here is a sm a ll air g ap at ttle join t; (b l th e join t is con si dered in de~ ma t ctled ,

S olutio n ' (a) Th e co upl in g efficienc y for a m u lt im od e st ep i ndex f iber w ith uniform ill u m inat io n of all pro pli gat in g mod es is gi ven by ECl. 14 ,14 1 as:

2 16 (1. 51

1

tI + l, 5 j4

l':

~

= 0.293 12(1.47 1) - 0.210 99 1; 1

- 0.804 The i nsertio n loss due to l at era l m isalignmen t is g iven b y ECl ' (4 . 151 w tle'e Lo sSj., = - 10 lo g , o '1')1 ., = - 10 10910 0 ,804

= 0 ,9 5 dB Hence assu mi ng a sm al l a ir gap at t he joint the insert io n lo ss is ap pro xi m ate ly 1 dB whe n th e lat era l offset is 10"'£ of t hO!! fi ber d iameter. (bl W hel'l til e jo int is co nsid ered inde x m at ched u.e. 1'10 ai r gap l t he c ou pti ng efficien cy m ay b e aga in o bt ained f ro m Eq. 4,14 where :

151

OPTICAL FIBER S. CA8 LES AN D CO NNECTIONS =

0 3 16 1211.47 1) _ O.2!O.99IJ }

=

0.8 72

Therefore the insertio n 10 $$ is :

Loss,••

= - 10

IOg lO 0 .8 72

= 0 .5 9

dB

With index matching the in sertion loss at the joint in exa mple 4.3 is red uced to approximately 0.36 d B. It may be noted that the difference between the losses obta ined in parts (a) a nd (b) corresponds to the o ptical lo ss due to F resnel ref lection at the similar fiber-air-fiber interface determined in e xa mple 4.2. The result may be checked using the formulae derived by G loge for a muhimode step index Fiber where the lateral misalignment loss assuming uniform illumination of all guided modes is obtained using:

t; = 0.64

( ~) = 0.64 ( : 5 )

=0. 128

Hence the lateral coupling efficiency is given by Eq. (4.17) as:

111"l

=

I - 0.128

=

0.872

A gain using Eq. (4. 15), the insertion loss d ue to the lateral misalignment a ssuming index matching IS:

LosslM = - 10 log lo 0.872 = 0.59 d B H ence using the expression d erived by Gloge we o beain the same value o r approximately 0.6 d B fer the insertion loss .....i th the inhere nt assum ption that there is no change in refractive index at the joint int erface. Altho ugh this est imate or insertion loss may be shown to agree with certain experimental res ults IR ef. 6 Il a value of around I d B insertion loss for a )0% lateral d isplacement with regard to the core diameter (as esti mated in example 4.3(a) is more usually found to be the case with m uhimode step index fi bers (Refs. 59. 72 and 731. Further it is generally accepted that the lateral offset must be kept below 5% of the fiber core diameter in o rder to reduce insert ion loss at a j oint to below 0.5 d B IRef. 721.

A grl ded Index fiber h ~, a cerebcnc refractive index profile (a = 2) and a core dllmeter of 50 11m. ESl imil te th e lnsertlon loss due to a 3 urn lateral misalignment at I flblr Joint when there I, index matching and ~ ssu m i n ll :

(.1 fbI

Ih erl !I unifor m illumination 01 a ~ guided modes only: th ere rl uniform lilum lnal ion ot all guided and leaky modes.

J oIIJrkJn: I_I Anuf¥llnV uniform II!urnll'\ltion of gu id~ mod" ooly. the m isalignment lOll mlf be OOt_lned \IIin; Eq. 14.181, where

.•

,

. -

152

-

-

OPTICA L FIBER COM M UNICATIONS: PRINCIP LES AND PRACTICE

i , = 0.85 H UI coupling

~ffitie"(:v

e)

3 = 0 8S ( 2 5 ) = 0. 102

i, g iven b V Eq, (4.1 7) a s :

11,,! "':" 1 - L , = 1 - 0 .10 2

=

0 89 8

Hence t ne inse rti <:>n loss due to th e l at era l m isa lign ment is g;ve " by Eq. 14 .1 51. w he re : l~ ..

= - 10 10910 0 .898 = 0 47 d B

lb l When assu ming t he u nifo rm illumi na tio n 01 both g uided lind lea ky modes Gloge"s formu la becomes :

it

= 0 75 (: ) = O.7S( :5 ) =0,090

Therefo re the coupling efficiency is

111_1 = 1 - 0. 0 9 0 _= 0,91 0 and t he in sertion loss d ue 10 lat eral m isa lign m ent is: los~ . t

"" - 10 lo g 10 0 9 10

=

0 .4 1 dB

It ma y be noted by observing Fig. 4 .27(a) w hich sho ws the measured lateral misal ignment loss fo r a 50 J.U1l diameter graded index tiber that the losses pre-dicted above are very pessimistic (the loss for 3 tJ.I11 offset shown in Fig. 4 .27(.1) is less than 0 .2 d B). A model whic h is found to predict insertion loss due to lateral misalignment in graded index fibe rs with greater accuracy was proposed by Miller and Mettler {Ref. 661. In this model they assumed the po wer distribution at the fiber ou tpu t to be of a G aussian form. U nfortunately the analysis is too detailed for this text as it involves integratio n using numerical techniques. We therefore limit estimates of insert ion losses d ue to lateral misalignment in multimod e graded index fibe rs to the use o f Gloge's formula. Angul a r misalignment losses at joints in multimode step index fibers may be predicted with reasonable accuracy using an expression fo r the angular coupling efficiency "an ~ given by (Ref. 621: (4.19) where 9 is the ang ular displacement in radian s and .6. is the relative refractive index difference for the fibe r. The insertion loss due to angular misalignment may be obtained fro m the a ngu la r coupling efficiency in the same manner as the lateral misalignment lo ss following :

Loss.... - - 10 loalO 1'1u,

.

.

(4.20)

153

OPTICAL FIBERS, CABLES AND CONNECTIONS

The formulae given in Eqs. (4.19) and (4.20) predict that the smaller the values of d the larger the insertion loss due to angular misalignment. This appears intuitively correct as small values of d imply small numerical aperture fibers which will be more affected by angular misalignment. It is confirmed by the measurements shown in Fig. 4.27(b) and demonstrated in example 4.5.

Example 4.5 Two multi mode step index fibers have numerical apertures of 0.2 arid 04 respectively, arid both have the same core refractive index which is 148. Estimate the insertion loss at a joint in each fiber caused by a 5° angular misalignment of the fiber core axes. It may be assumed that the medium between lhe fibers is air. Solution: The angular cOlJpling efficiency is given by Eq. {4. 191 as

The numerical aperture is related to the relative refractive index difference following Eq. (2.10) where:

Hence

TJ ol1g ~

16(n,lnI

2

(1 + In! /n))

[ 4

n ] 1 --llNA

For the 0.2 NA fiber.

16(148)2 TJ a ng

~

=

(1+1.481

4

1 - 5ll/180 ] [

II

0.2

0.797

The insertion loss due to the angular misalignment may be obtained from Eq. {4.20), where: Loss. ng = -10 109'0 TJono

., -10 log10 0.797

0.98 dB For the 0.4 NA fiber:

TJang

~ 0.926

[1

. :ll/180 ] 110.4

::= 0.862

The tneerncn loss due to the angular misalignment is therefore: LOSS.~g

= -10 10910 0.862

_ 0.64 dB

154

OPTICAL FI BER COMM UNICATIONS: PRINCIPLES A ND PRACTICE

Hence it may be noted from exa mple 4. 5 th at the insertio n loss due to a ngula r misalignment is reduced by u sing fi bers with large nu merica l a pertures. This is t he o pposite trend to the increasing insertion lo ss with numerical a perture for fiber longitudinal misalignm ent at a joint. Misalignment losses at co nnection s in single mode fi bers have been theoretically co nsidered b y Marcu se IRef. 681 and Gam bling 1'1 a L [Refs. 69 and 701. The theoretical anal ysis which was instigated by Marcusc is based upon the G aussian or near G au ssian shape of the modes prop agating in single mode fi bers regardless of the fiber type (i.e. step ind ex or graded index). f urther d evelopment of this theory by G ambling 1'1 at. I Ref. 70J gave sim plified fo rmulae for both th e lateral a nd an gula r misalignment losses at joi nts in single mode fi bers. In the abse nce o f a ngula r misal ignment G am bling el ttl. calculated tha t the loss T j due to lateral. offset y was given by:

Y ) ' dB 1', = 2.17 ( 000

(4.21)

where roo is the spot size of the fundamenta l mode. T he spot size is usually defi ned as t he width to Il l' inte nsity of the LP OI mode, or in terms of the spot size of an incident G a ussian beam which gives maxim um launc hing effi ciency IRef. 141. However, the spot size fo r the LPOI mode (correspo nds to H E mode) ma y be o btained from the e mpirical formula IRefs. 68 a nd 69 1: ' 5 + 2.88V- 6 ) 1.6 2 V,, (0".6"-5_+--"'-----;;:;-_ _ ~

~ = o -

21

(4.22)

where ">0 is the spot size in 11m. a is the fiber core radius an d V is the normalized frequency for the fi ber. Alternatively the insertion loss T. caused by an angular mi salignment lin rad ians) at a j oint in a single mode fi ber may be given b y:

a

T. = 2.17 (

eroonI

V

aNA

) ' dB

(4.23)

where 11 1 is the fi ber core refract ive index and NA is the nume rical aperture of the fibe r. It must be noted that the formulae given in Eq s. (4.21) and (4. 23) assum e that the spot sizes of the modes in the two coupled fibers are the same. G am bling es al. IRef. 70J also deri....ed a somewhat complicated fo rm ula which gave a good approximatio n fo r the combined losses d ue to both lateral and ang ular misal ignment at a fibe r joint. However they ind icate that fo r small total lo sses (less than 0 .75 d B) a reasonable approximation is o btained by sim ply c o mbining Bqs. (4.2 1) and (4.23).

OPTICAL FIBERS, CABLES AND CONNECTIONS

155

Exsmpls 4.6

A single mode fiber has the following parameters: normalized frequency (VI = 2.40 core refractive index (n 1 ) = 1.46 core diameter (2a1 = 8).l.m numerical aperture (NA) = 0.1 Estimate the total insertion loss of a fiber joint with a lateral misalignment of 1 um and an angular misalignment of 1°. Solution: Initially it is necessary to determine the spot size in the fiber. This may be obtained from Eq. (4,221 where:

rou

10.65+ 1,62V

15

+2,88V 6)

=

a

~

10.65 + 1,62(2.41- 1 5 + 2.88(2.41- 5 1 4----:cc-----

2'

The loss due to the lateral offset is given by Eq. 14.21) as:

71 = 2 . 1 7 ( - , - ) 2 =2.17(_'_)' (l)o 3.12 = 0.22 dB

The loss due to angular misalignment may be obtained from Eq. l4.23) where:

=

2.17

(In/180)X3.12Xl.46X 2.4) 4 x 0.1

=

0,49 dB

Hence the total insertion loss is TT

~

7, + T» =0.22 + 0.49

0.71 dB

In this example the loss due to angular misalignment is significantly larger than that due to lateral misalignment. However. aside from the actual magnitudes of the respective misalignments, the insertion losses incurred are also strongly dependent upon the normalized frequency of the fiber. This is especially the case with angular misalignment at a single mode fiber joint where insertion losses of less than 0.3 dB may be obtained when the angular

168

OPTICAL FI BER COMM UNICATIONS: PRIN CIPLES ANO PRACTICE

misal ignment is 1 D with fibers o f ap propriate V value. Nevertheless for low loss single mode fiber jo ints it is im po rta nt that a ng ula r a lignment is better than 1°. We h ave considered in some detail the optical attenu ation a t fiber-fiber connection s. H owever we have not yet d iscu ssed the pos ~ib le distortion of the transmitt ed signal at a fiber joint. Although work in this area is in its infancy, in creased interest h as been generated with the use of highl y coherent sources (inj ection lasers ) and very low d ispe rsion fibers. It is apparent that fiber connections strongly affec t the signal transmission causing modal noise (see Sectio n 3. 1 1) and nonlinear distortion (Ref. 76 ] when a coherent light source is u tilized with a multimode fi ber. A lso it h as been reported IRef. 77) th at the transmission I~ of a connection in a coherent multimode system is e xtremely wavelength-depend ent exhibit ing a possible 10% change in the transmitted optical wa velength for a very small change (0 .001 nm ) in the laser emission wavelength. Nevertheless it has been found that these problem s may be red uced by the use of single mode o ptical fiber (R ef. 761 . F urthermore the above modal effe cts become negligible wh en an incoherent source (light em itting diode) is used with multimode fi ber. H owever, in this ins tance there is often some mode conversion at the fiber joint which can make the connection effectively a ct as a mode mixer o r filter [R ef. 781. Indication s are tnat this phenomeno n which has been in vestigated (R ef. 791 with regard to fiber splices, is more pronounced with fusion splices than with mecha nical splices, both of whi ch are described in Section 4.9.

4, 9

FIBER SPLICES

A permanen t joint formed between tw o individual optical fibers in the field or factory is kno wn a s a fi ber splice. Fiber splicing is frequently used to establish lo ng-haul optical fiber link s where smaller fiber lengths need to be joined, and there is no requirement for repeated connection and disco nnect ion. Splices may be divided into two broad categories depending upon the splicing technique utilized. These are fusion splici ng or welding and mechanical splicing. F usion splicing is accomplished by applying localized heating (e.g. by a n ame or an electric a rc) at the interface between two butted , p realigned fiber ends c ausing them to soften and fuse, Mechanical splicing, in which the fibers a re held in alignment by so me mechanical means, may be achieved by various methods including the use o f tubes around the fiber ends (tu be splices) or v-grcoves into which the butted fibers are placed (groove splices ). All these te chniques seek to optimize the splice performance (i.e. red uce the insertion loss at the joint) through both fiber end preparation and alignment of the two j ointed fibers. T yp ical a verage sp lice insertion losses for multimode fibers are in the range 0.1-0.2 dB IRef. 81) w hich is generally a better performance than that e xhibited by demountable connections (see Sections 4.10-4.12). It may be

OPTICA L FIBERS. CABLES AN D CON NECTIONS ~ " " ~ . «I#

157

i"i'i. .. ,

,..... ~ prop... tioo

'V

Fig .4..28

Op l'cal fi ber end prepSla llOfl : the principle of scribe e nd b re ak c utt in g [Re f. 821.

noted that the insertion losses of fiber splices are generally much less than the possible F resnel reflection lo ss at a butted fiber-fiber joi nt. This is because there is no large step change in refractive index with the fusion splice as it forms a continuous fi ber connection, and some method of index matching (e.g. a fluid] tends to be utilized with mechanical splices. However, fiber splicing (especially fusion splicing) is at present a somewhat difficu lt process to perform in a field environment and suffers from pract ical problems in the development of field -usable tools. A requirement with fibers intended fo r splicing is that they have smooth and squa re end faces. In general this end preparation ma y be achieved using a su itable tool which cleaves the fi ber a s illustrated in Fig. 4.28 [Ref. 82). This process is often referred to as scribe and break or score a nd break as it involves the scoring of the fiber surface under tension with a cutting 1001 (e.g. sapphire. d iamond, tungsten carbide blade). The surface sco ring creates failure as the fiber is tensioned and a clean, reasonably square fi ber end can be produced. Figure 4.28 illustrates this process with the fi ber tension ed around a curved mandrel. However. stra ight pull, scribe a nd break tools are also utilized , which arguably give better results [Ref. 83 1.

4 .9.1

Fusion Splice.

T he fusion splicing of single fiber s involves the heating o f the two prepared fiber ends to their fusing point with the application of suffic ient axial pressure between the two optical fibers. It is therefore essential that the stripped (of cabling and buffer coating) tiber ends are adequately positioned and aligned in order to ac hieve good continu ity of the transmission medium at the junction point . Hence the fibers are usually positioned and clamped with the aid of an Inspection microscope. F lame heating sources such a s microplasma torches (argon and hydrogen) IDd olhydric nucroburnetl (oxYlen, hydrogen and alcohol vapor) have been utillzld wilh tome I\KlCtIi (Ret'. 841. However, the mon widely used heating

158

OPTICAL FIBER COMM UNICATIONS: PRINCIPLES AND PRACTICE

source is an electric arc. T his technique offers advantages of consisten t. easily controlled heat with adaptability for use under field co nditions. A schematic dia gr am of the basic arc fusion meth od is given in Fig. 4.29(a) IRefs. 8 J and 85 1illustrating how the two fibers are welded together. Figure 4.29(b) (Rer. 73 1 sho ws a developmen t of the basic arc fusion process which involves the ro unding of the fiber ends with a low energy discharge before pressing the fi bers together and fusing with a stro nger a rc. This tec hnique. known as prefusion, removes the requirement for fiber end prepa ration which has distinct

I. )

o

", Fig. 4 .29

Electric arc: fu Si on splicing : 1. 1 an examp le of fusi on lo Fcing IDl)trl tu. [R.f• .

81 . 00 851: lb) sc:t1emat lc il luetratio n of t ~ 1 p·efu.lon n" ethod for ICIMltl ly spl 'cirog op tic, t fl be rl lRef. 731.

..--

159

OPTIC AL FIBERS, CABLES AND CONNEc n ONS

Hb« ""'"

!

•

\

(

•

I

I

!

,

j

)

,

I I

1.1

, ( Fig . 4 .30

•

'" ,

I I

'"

Self- alignm ent phenom enon w hich tak es otace d u ri n ~ fusion splicing . tal betc re fusion: Ib } du rin ~ fusion; (c) after fusion IRefs. B5. 67 and B8J.

ad vanta ge in the field environme nt. It has been utilized with multimode fibers giving average splice losses of 0.09 d B I Ref. 86J. F usion splicing of single mode fibers with typical core diam eters between 3 an d 10 u m presents problems o f more critica l fiber alignment (i.e. lateral offsets o f less than I urn are required for low lo ss joints). However, splice insertion losses below 0 .3 dB may be achieved due to a self-alignment phenomenon which partially co mpensate" for an y lateral offset. Self-alignment. illustrated in Fig. 4.30 IRefs. 85. 87 and 881. is caused by surface tension effects between the two fiber ends during fusing. A recently reported [Ref. 89J fi eld trial of single mode fiber fusion splicing over a 31.6 km li nk gave mean splice insertion lo sses o f 0.18 and 0 .12 dB at wavelength s of 1.3 and 1.5 5 urn respectively. A po ssible d raw back with fus ion splicing is thai the heat necessary to fuse the fibers may weaken the fiber in the vicinity of th e splice. It has been found that even with careful handling. the tensile strength ofthe fused fiber may be as low as 3Q9(, of that of t he uncoated fi ber before fusion IRef. 9 11. The fiber fractu re generally occurs in the heat-affected zone adjacent to the fused joint. The red uced tensile strength is attributed [Refs. 9 1 and 92 1 to the combined effects of surface d a mage caused by handling. s urface de fect growth during healing and induced residential stresses due to changes in chemical composition . It is th erefore necessary th at the co mpleted splice is packaged so as to red uce tensile loadin g upon the fiber in the vicinity of the splice.

4.9.2

Mech.nlcel Splice.

A number of mechanical techniques for splicin g ind ividual optical fibers have been d eveloped. A common method invo lves the use of an accurately produced rliid aHiRmenl lube into which the prepared fi ber ends are permanently bonded. TN. ll1ul tubc splice is illustrated in F ia. 4.3 1(a} [Ref 94) and may udlhl .a ...... or otr&m!c: =apiUary with an inner diameter j ust larae

160

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE Square ero» "'diun "pilla,)'

"

Opti",1 fiiler

(.)

Fig. 4.31

(Il)

Techniques for tube splicing .ot optical fibers: lal snug tube splice [Ref. 941: (b) loose tube splice utilizing square cross section capillary [Ref. 96J.

enough to accept the optical fibers. Transparent adhesive, (e.g. epoxy resin) is injected through a transverse bore in the capillary to give mechanical sealing and index matching of the splice. Average insertion losses as low as 0.1 dB have been obtained IRef. 95] with multimode graded index and single mode fibers using ceramic capillaries. However, in general, snug tube splices exhibit problems with capillary tolerance requirements. A mechanical splicing technique which avoids the critical tolerance requirements of the snug tube splice is shown in Fig. 4.31(b) IRef. 961. This loose tube splice uses an oversized square section metal tube which easily accepts the prepared fiber ends. Transparent adhesive is first inserted into the tube followed by the fibers. The splice is self-aligning when the fibers are curved in the same plane, forcing the fiber ends simultaneously into the same corner of the tube, as indicated in Fig. 4.31(b). Mean splice insertion losses of 0.073 dB have been achieved [Refs. 88 and 97] using multimode graded index fibers with the loose tube approach. An alternative method of obtaining a tight fitting splice is by use of the collapsed sleeve splicing technique which is illustrated in Fig. 4.32 [Ref. 981. Cullap",d ,h"

,

~c::=={"'~ .

, Flg.4.32

'"

(oj

Th. eeuee.. d 11•• vt.pllcln" technique {Ref, B8],

Optical flber

'B'

OPTICAL FIBERS. CABLES A ND CONN ECTIONS

Ii

I

I

T his method utilizes a Pyre's glass sleeve which ha s a lower melt ing point than the fibers to be jointed. When the sleeve is heated to its softening point it collapses due to surface tension, eventua lly forming a solid rod . F igure 4_32(a) shows a partially collapsed Pyrex sleeve formed by local heating of the sleeve. With t he collapsed sleeve splicing technique th e glass sleeve is collap sed arou nd one of the prepared liber ends to form a tight lilting socket as shown in Fig. 4.J U b). The second fiber is then inserted into the socket and the who le a ssembly is bonded with epoxy resin a s illust rated in Fig.4.J2(c). Hence an index matched splice is created . T his technique is useful in the splicing of two fibers with different diameters. In this case the sleeve is collapsed over the larger diameter fi ber before the insertion of the second. smaller diameter fiber. The collapsing is the n continued to form a socket of an appropriate size for a clo se fit to the smaller fiber. Collapsed sleeve splices are generally protected by enclosure in a metal ferrule. They hav e exh ibited insertion losses in the range O.2~. J d B IRef. 591 when using multimode graded index fibers with avera ge tosses of 0.5 d B in the field. Other co mmon mechanical splicing techniques involve the use of grooves to secure the fibers co be join ted . A simple method utilizes a V-groove into which th e two prepared Fiber ends are pressed. The v -groove splice which is illustrated in Fig. 4.33(a) [Ref 99 1gives alignment o f the prepared fib er end s thro ugh insertion in the groove. The splice is made permanent by securing the fiber s in the v -grcove with ep oxy resin. Jigs for producing v -groove splices

,I

,

Fik" ~ .. n «l lOp" ':ll-~

V1ll'""" ,-d ..........

l T

,.,

162

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

__

, ,,

SpriLl~

-----------------c.,

/

-

------

/

(yliIlJri,,1

;J""

,., Fig.4.34

The Scrinqroove" splice [Ref. 101J: la) expanded view of the splice; Ibl schematic cross section of the splice.

have proved quite successful, giving joint insertion losses of around 0.1 dB [Ref. 82]. V-groove splices formed by sandwiching the butted fiber ends between a Vgroove glass substrate and a flat glass plate as shown in Fig. 4.33(b) have also proved very successful in the laboratory. Splice insertion losses of less than 0.01 dB when coupling single mode fibers have been reported [Ref. 100] using this technique. However, reservations are expressed regarding the field implementation of these splices with respect to manufactured fiber geometry, and housing of the splice in order to avoid additional losses due to local fiber bending. A slightly more complex groove splice known as the Springroovew splice utilizes a bracket containing two cylindrical pins which serve as an alignment guide for the two prepared fiber ends. The cylindrical pin diameter is chosen to allow the fibers to protrude above the cylinders as shown in Fig. 4.34(a) IRef. 101]. An elastic element (a spring) is used to press the fibers into a groove and maintain the fiber end alignment as illustrated in Fig. 4.34(b). The complete assembly is secured using a drop of epoxy resin. Mean splice insertion losses of 0.05 dB [Ref. 88) have been obtained using multimode graded index fibers with the Springroove'" splice. A similar mechanical splicing technique is illustrated in Fig. 4.35 [Ref. 941. In this case the spring is replaced Hoot ,11I';llbhk

,hw_~

--

-

-, Fig. 4.36

The precision pin splice [Ref. 94J.

~.

-

-« --

.~.

- - - - - ~ -----=:::Cl'lindrical pin'

•, I

163

OPTICAL FIBER S, CABLES AND CO NNECTIONS

by a third cylindrical pin and the whole as sembly is held in place with a heat shrinka ble sleeve. This precision pin splice has given mean insertion losses of around 0.2 dB IRef. 881with multimode fibers.

4.9.3

Multiple Splice.

M ultiple simultaneous splicing ha s mainly been attempted using mechanical splicing methods. Groove splicing techniques have been utilized for the sim ultaneous splicing of an a rray of fibers within a nat ribbo n cable. F igure 4.36 IRef. 102] shows a groove splice for a five fiber ribbon cable. II utilizes a grooved metal substra te with the groove spacing equal 10 the spacing of the fi bers in the array. The plastic coating is removed from the fi ber ends and they are prepared using a suitable scribe and break tool. Then the two ribbon ends are placed into the grooves, the Fibers are pressed together and held in position with a rubber sheet and 8. cover plate. Finally epoxy resin is added to provide index matching as well as securing a permanent splice. A 12 fiber version of this splice using an injection moulded plastic substrate gave an average splice loss of 0.2 dB IRef. 97J. A ribbon splice using etched silicon chips is shown in Fig. 4.37 [Ref. 103]. Sim ilar chips may be utilized to form a 12 x 12 array of 12 ribbon cables each containing 12 fibers IRef. 821. The whole assembly is clamped together to form a single multiple splice. Multiple fiber splicing of a circular cross section cable has also bern achieved. A splice or this type is shown in Fig. 4.38 1Ref. 1041 and consists of matched sets of precision moulded circular collars (o ne shown) which contain a series of groo....es around the circumference. The fi bers are inserted into the precision grooves and bonded with adhesive. They are then cut. polished and covered with index matching material at the jointing ends. before the two collars are brought together and fastened with semicylindrical shells and pins.

, ,.K ul>i"t ~u" l

Multlpl. fiber IPllclng of f be, ribbon ClIble using a Qroon

I.... 1021./

alignm6'f'11 techniQlle

j!

I

164

Fig . 4 .37

OPTICAL FIBER COMMUNICATIONS: PRINCIP LES A ND PRAcnCE

Mul t ip le fiber sp licing of Slacked rib bon cables using precision mu tn-v-q roove

silicon chips [Ret 10 3 1.

• Fig.4.38

Cro ss sect ion of the stet -core ca ble for mult i ple fiber mech anical So 'icing [Ref.

l 04J.

When the two grooved collars a re produced from a single piece of alumina, average splice insertion losses of 0.3 dB [R ef 88J have been obtained with multimode step index fibers.

I. ,,.

4.10

FIBER CONNECTORS

Demou nta ble fiber connectors are more difficult to achieve than optical tibet splices. This is because they must maintain similar tolerance requirements to •

OPTICAL FIBERS. CABLES AND CONNECTIONS

165

splices in order to couple light between fibers efficiently, but they must accomplish it in a removable fashion. Also the connector design must allow for repeated connection and disconnection without problems of fiber alignment which may lead to degradation in the performance of the transmission line at the joint. Hence to operate satisfactorily the demountable connector must provide reproducible accurate alignment of the optical fibers. In order to maintain an optimum performance the connector must also protect the fiber ends from damage which may occur due to handling (connection and disconnection), must be insensitive to environmental factors (e.g. moisture and dust) and must cope with tensile load on the cable. Additionally, the connector should ideally be a low cost component which can be fitted with relative ease. Hence optical fiber connectors may be considered in three major areas, which are: (a) the fiber termination which protects and locates the fiber ends; (b) the fiber end alignment to provide optimum optical coupling; (c) the outer shell which maintains the connection and the fiber alignment, protects the fiber ends from the environment and provides adequate strength at the joint. The use of an index matching material in the connector between the two jointed fibers can assist the connector design in two ways. It increases the light transmission through the connection whilst keeping dust and dirt from between the fibers. However, this design aspect is not always practical with demountable connectors, especially where fluids are concerned. Apart from problems of sealing and replacement when the joint is disconnected and reconnected, liquids in this instance may have a detrimental affect, attracting dust and dirt to the connection. There are a large number of demountable single fiber connectors, both commercially available and under development, which have insertion losses in the range 0.2-3 dB. Fiber connectors may be separated into two broad categories: butt jointed connectors and expanded beam connectors. Butt jointed ccnnectors rely upon alignment of the two prepared fiber ends in close proximity (butted) to each other so that the fiber core axes coincide. Expanded beam connectors utilize interposed optics at the joint (i.c. lenses or tapers) in order to expand the beam from the transmitting fiber end before reducing it again to a size compatible with the receiving fiber end.

4.11

BUTT JOINTED CONNECTORS

Butt jointed connectors are the most widely used connector type and a subItantial number have been reported. In this section we review some of the more common butt Jointed~ connector designs which have been developed primarily

.,,'.

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

166

for usc with multimode fibers. Nevertheless in certain cases as indicated in the text, similar designs have been used successfully with single mode fibers. 4.11.1

Ferrule Connector

The basic ferrule connector (sometimes referred to as a concentric sleeve connector), which is perhaps the simplest optical fiber connector design. is illustrated in Fig.4.39(a) [Ref. 58]. The two fibers to be connected arc permanently bonded (with epoxy resin) in metal plugs known as ferrules which have an accurately drilled central hole in their end faces where the stripped (of buffer coating) fiber is located. Within the connector the two ferrules are placed in an alignment sleeve which, using accurately machined components, allows the fiber ends to be butt jointed. The ferrules are held in place via a retaining mechanism which in the example shown in Fig. 4.39(a) is a spring. It is essential with this type of connector that the fiber end faces are smooth and square (i.e. perpendicular to the fiber axis). This may be achieved with varying success by either: (a) cleaving the fiber before insertion into the ferrule; (b) inserting and bonding before cleaving the fiber close to the ferrule end face;

Forru!,

R'laining

~-:::;::::r=;==,,--I-~='

Ali~llnl<"'

,Icc"

'PrL"~

('on""wr ~lCll

{,)

Staink" ""el [erruk

I

AJhC'i\'e~

,

L

I

.I,," , """

Il

,

Iii

k

T

I I

IVatchj,wd

Fig. 4.39

'"'

PI"", ,'o,t"'f

or fih,',

Ferrule connectors: lal structure of a baSIC ferrule connector [Ref. 58); lb) structure of a watch jewel connector ferrule [Ref. 59].

,.7

OPTICAL FIBERS. CAB LES AND CONNECTIONS

(c) using either (a) or (b) and polishing the fiber end face until it is fl ush with the end of the ferrule. Polishing the fiber end face after insert ion and bonding provides the best results but it lends 10 be time-consumin g and incon venient especially in the field. T he fiber alignment accuracy of the ba sic ferrule connector is largely dependent upon the ferrule ho le into which the fiber is inserted. Hence some ferrule co nnectors incorporate a watch jewel in the ferrule end face (je welled ferrule connector) as iIIuslrated in Fig. 4.39(b) [Ref. 59 1. In this ca se the fiber is ce ntered with respect 10 the ferrule th rough the watch jewel hole. The use of the watch jewel allows the close diameter and tolerance requ irements of the ferr ule end face hole to be obtained more easily than simply th rou gh drilling of th e ferrule end face alone. Nevertheless. typical concentricity errors between the fiber core and the outside diameter of the jewelled ferrule are in the ran ge 2--6 urn giving insertion tosses in the range 1-2 dB with multimode step index fibers.

4 .11.2

Blconical Connector

A ferrule type connecto r which is widely used as pan of jumper ca ble in a variety of applications in t he Bell system is the biconical plug connector (Refs. 81 and 105). The plugs are either transfer moulded d irectly onto the fiber o r cast around the fiber using a $oiliea-Ioaded epox y resin ensuring concentricity to within 5 urn. After plug a ttach ment, the fiber end faces are polished before the plugs are inserted and aligned in the biconical moulded center sleeve as shown in Fig. 4.40 [Ref. 811. Mean insertion losses as low as 0.21 d B have been reported [Ref. 1051 when using this connector with SO urn core di ameter graded inde x fibers. In the original design transparen t silicone resin pad s were placed o ver the fiber e nd face s to provide index matching. Ho wever, currently the polished fiber end faces are bu tted directly, the gap a nd

P....
moul"'=d '! ok...<

_f

PVC Col" "

.... ...-0

I ~ .. tll

Croll Hetlon of Ill. blcon lCi I connector [Refs. B1 I nd 105J.

/

168

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

parallelism of the end faces being controlled to a degree that gives insertion losses better than the level normally exhibited by Fresnel reflection. 4.11.3

Ceramic Capillary Connector

An approach giving accurate ferrule alignment is used in the ceramic capillary connector shown in Fig. 4.41 [Refs. 107 and 1081. The special feature of this concentric sleeve connector is the mounting of the optical fiber in a ceramic capillary which is set in the tip of the ferrule. This gives accurate positioning of the fiber core and allows the fiber end face to take a good polish. Average insertion losses 01'0.4 dB have been reported [Ref. 107] when using multimode fibers. This connector has also been used for the coupling of single mode fibers giving average insertion losses of 0.5 dB [Ref. 108J when using a 10 11m core diameter step index fiber operating at a wavelength of 1.3 11m.

AJlti-rot>t;on pin

Axi,] comp"",;on ,p'h,g COUpli"! nn! C"i~i"~

pipe

Slittod ,Icc", ho,,,ing Slit!ed

.Ji~nment

,keYC Ad,p101

Fig. 4.41

The ceramic capillary connector showing the ferrule plug and the adaptor into which two plugs are located [Refs. 107 and 108).

4.11.4

Double Eccentric Connector

The double eccentric connector does not rely on a concentric fixed sleeve approach but is adjustable, allowing close alignment of the fiber axes. The mechanism, which is shown in Fig. 4.42 [Refs. 58 and 62], consists of two eccentric cylinders within the outer plug. It may be observed from Fig. 4.42 that the optical fiber is mounted eccentrically within the inner cylinder. Therefore when the two connector halves are mated it is always possible through rotation of the mechanism to make the fiber core axes coincide. This operation is performed on both plugs using either an inspection microscope or a peak optical adjustment. The mechanisms are then locked to give permanent alignment. This connector type has exhibited mean insertion losses of 0.48 dB with multimode graded index fibers: use of index matching fluid within the connector has reduced these losses to 0,2 dB. The double eccentric connector-

16.

OPTICA L FI BERS, CABLES AND CONN ECTIONS

-

T",o <.<:«rtt" c ,..
Fig . 4 .42

St ructure of the do ub le ecce ntric co nnecto r plug [Aels. 58 a nd 62 J.

design has also been utilized with single mode fib ers where its adjustable nature has proved advantageo us for alignment of the sma ll core diameter

flbers. 4 .11 .5

Triple BaA Connector

T here are a number of connectors which utilize kinematic design principles. A noteworthy example develo ped by British Teleco m is the triple ball connector ill ustrated in Fig. 4.43 IRef. 10 91. Three accurately ground tungsten sphe re'S

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/

r

/

.1 1SS4

<'

~ 'lg.4.431

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./

/'.

;.

,-/ ;; '

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-, .-

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,

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~"

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/

f•

I

.1

)'

I Fil>or

'" The Iriple ball conneClo r [R, f. 10 91 : tal coonector cross secuco showing the !oClIio n of tta fiber In I groove be lween three contacting balls ; lbl plan view of thl ca nn.ctor lIhowln; the two In/erlockil'lg " III of th r" ba lls and t oo pesttlotl of tM MI« and..

170

OPTICAL FIBE R COM MU NICATIONS: PRINCI PLE S AND PRACTIC E

gripping the fiber are ho used in a bush as shown in Fig. 4 .43(a ). Two sets of the spheres a re nested together with a relative rotation of 60° ( Fig. 4.43(b» bringi ng the two butt jointed fibers into align ment. The reported [Ref. 109] a verage insertion loss using multimode step index fibers is 0.49 d B without index matching and O.lg dB with matching. A recent improvement to this connector is described in Section 4.12. 4 .11.6

Singl. Mod. Rbet COM.ctOt

Although t he cera mic ca pillarv and the double eccentric connector have been utilized with single mode fibe rs they were no t initially designed for this purpose. A connector which was designed specifically fo r use with single mode fib ers is illustrated in Fig. 4 ,44 IRef. 1101. It consists of a pair of fi ber plugs a nd a sleeve with ball bearing a rr ays o n its inside surface. These ball bearings prov ide plug alignm ent ac curacy as well as smooth det achment. Each fiber plug contains two eccentric tubes which by means of rotation allow the single mode fiber to be accurately centered within the plu g. Average insertion losses of 0.46 dB I Ref. IIOJ were encountered with the co nnector when using 5.7 urn core diameter ( 150 urn cladding. diameter) single mode fi ber. A development [Ref II Dl of this co nnector design using a simpler cy lind rica l plug structure (not requiring eccentric tu bes) with a n accurately machined hole to locale the fi ber exhibited simila r average insertion losses (0.47 d B). Ho wever, th e simpler plug design makes co nnecto r assembly possible without special equipment. making it far more co nvenient for practical use.

_

_ _ _ (..~""" r....·' 1 lb., pb.

filii . 4 .44

S ing~e modeopl ical fibe rco nne etor stru ct u re [Re i. 1 10 ).

4.1'.7

Multiple Connectors

In comparison with th e large number of single fiber co nnecto rs few multiple fiber connectors have been developed to date. Nevertheless two multiple connector designs suitable for jointing ribbon fiber cable are illustrated in Fig. 4.45 [Refs. 11 2 a nd 1131. The connector shown in Fig. 4.45(a) employs plastic moulded multiple termmadon s which a re jointed in an alignment sleeve consisting of grooved silicon chips. This technique is very similar to the multiple groove splicing methods for r ibbon c able de scribed in Section 4.9.3. 1:'l ln,"

' ,(f<.

171

OPTICA L FIBERS, CABLES AN D CON NECTIONS

(. 1

....

~ r-, ~~-~.~

~>->~,

""'" ~"' Ul .od 'liod

("0 « .

,.,

. Ag.4.45

(" ~~ ....... .

Mult iple fiber co nnectors: leI connector wit h g rooved i1 lignment sleeve a nd mou lded fiber ribbon e nd le rminalio ns IRef. 11 21: (bl liber ribbon connector using V-grooved ~ I i con c hip [Ref. 1131.

multimode graded index fib ers. insertion losses in the range 0.2-0.32 dB were obtained [Ref. 11 2] with this device. F igure 4.45(b) also shows a multiple connector design which utilizes v-grooved silicon chip s. H owever, in this case ribbon fi bers are mounted and bonded in the V-grooves in order to form a plug together with precision metal ,uidini rods and coil springs. The butt jointed fi ber co nnections are I.ccomplished by butt jointing the two pairs o f guiding rod s in slitted sleeves Joel.ted in the adaptor also shown in Fig. 4,45(b). This co nnector exhibited IVlrl,. inu rtiotl Jonet of 0.8 dB which were reduced to 0.4 dB by the use of

I.... Il\IlOhlnl fluld•.

172

4 .12

OPTICAL FI BER COM MUNICATIONS: PRINCIPLES AND PRACTICE

EXPANDED BEAM CONNECTORS

An alternative to connection via direct butt joints between optical fibers is offe red by the principle of the expa nded beam . Fiber connection utilizing this principle is illustrated in Fig. 4.4 6 which shows a con nector consisting of two lenses for collimating and refocusing the light from one fiber into the other. The use of this interposed optics makes the achievement of lateral alignment much less critical than with a butt j ointed fiber connector. Also the longil udinal separation between the two mated halves of the connector ceases to be c ritical. However, this is achieved at the expense of more stringent angular alignment. Nevertheless, e xpanded beam connectors a re useful for multifiber connection and edge connection for printed circ uit boards where lateral and longitudinal alignmen t are frequently difficult to achieve. Two examples of lens coupled expanded beam connecto rs are illustrated in Fig, 4.47 IRefs.1l 4 and 115J. The con nector shown in Fig. 4.47(a) utilizes spherical microlenses for beam expansion and reduction. It exhibited average ' losses of I dB which were reduced to 0.7 dB with the application of an antireflection coating on the lenses and the use of 50 11m core diameter graded index fiber. Figure 4.47(b) shows an improvement on the triple ball connector (Section 4.11.5) which utilizes a glas s bead lens formed on the end of each fiber. This is achieved u ~ i n g an electric a rc discharge. and the beads assist the centeri ng of the fi ber ends between the clusters of spheres. The bead lens reduces the need for index matc hing within the connector a nd gives average insertion losses of 0.8 dB with multimode fibers. It has also been used with si ngle mode fibers [Refs. 116 and 1171 exhibiting losses in the range ]-2 dB due to the greater alignment difficulties with the smaller core diameter fiber. Tapers have also been utilized to provide expanded beam fiber connection. A cladded fi ber taper is shown in Fig. 4.48 [Ref 118 1. It is fused to the end face o f each of the fibers to be connected which increases tile o ptical beam width at the connection by se veral limes. The two tapers a re butt jointed in order to complete the expanded bea m connector. As with the lens connccnors, the increased beam dia meter reduces the effect of d ust a nd dirt on connector loss. However, the difficulties involved in producing accura tely dimensioned tapers currently preelude wide use of this connector type.

Fig.4.4I

Scl>ematlc Illustration of . n elq)ll'lCied beam conl'lK!or .howll'lV ttl. r:J ope.... tion.

ortl'lel~

173

OPTICAL FIBERS, CABLES AND CONNECTIONS

,

Micl'O""'"

, ,'r,r,', ,', " " , " ,',

,-----------7~

'-'_._--------, '

. , '""--::')~~=~+---tJ' i ,'

1

" "

:

Optkd fib';r IL.

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,

,

j

'

I

L.,

,,-

" ,::

,

,

. .:

:

<,

J' Opk,' rd",

," , ,

I,

,

,.~ ~.J

'"

Ibll emi,,·

],;Il,,' 1',,,,11<;><;,(,,,

Ell ball du,k,

'" Fig. 4.47

Lens coupled expanded beam connectors: lal schematic diagram of a connector with two mtcroranses making a 1 : 1 image of the emitting fiber upon the receiving one [Ref. 114]; (bl triple ball. fiber bead connector [Ref. 115J.

Fu,ion

Flg.4.48

,pi,,,

Fil>e'

Expanded beam connector using tapers [Ref. 118].

PROBLEMS 4.'

Describe in general terms liquid phase techniques for the preparation of , multloomponent Ilalies for optical fibers. Discuss with the aid of a suitable dlqram Ont meltlnl method for the preparation of multicomponent glass.

174

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

4.2

Indicate the major advantages of vapor phase deposition in the preparation of glasses for optical fibers. Briefly describe the various vapor phase techniques currently in use.

4.3

(a) Compare and contrast, using suitable diagrams, the outside vapor phase oxidation (OYPO) process and the modified chemical vapor deposition (M.CYD) technique for the preparation of low loss optical fibers. (b) Briefly describe the salient features of vapor axial deposition (YAD) and the plasma-activated chemical vapor deposition (PCYD) when applied to the preparation of optical fibers.

4.4

Discuss the drawing of optical fibers from prepared glass es with regard to: (a) multicomponent glass fibers; (b) silica-rich fibers.

4.5

List the various optical fiber types currently on the market indicating their important features. Hence briefly describe the general areas of application for each type.

4.6

Briefly describe the major reasons for the cabling of optical fibers which are to be placed in a field environment. Thus state the functions of the optical fiber cable.

4.7

Explain how the Griffith theory is developed in order to predict the fracture stress of an optical fiber with an elliptical crack. Silica has a Young's modulus of 9 x 1010 N m- 2 and a surface energy of 2.29 J. Estimate the fracture stress in psi for a silica optical fiber with a dominant elliptical crack of depth 0.5 urn. Also determine the strain at the break for the fiber (I psi = 6894.76 N m").

4.8

Another length of the optical fiber described in problem 4.7 is found to break at I % strain. The failure is due to a single dominant elliptical crack. Estimate the depth of this crack.

4.9

Describe the effects of stress corrosion on optical fiber strength and durability. It is found that a 20 m length of fused silica optical fiber may be extended to 24 m at liquid nitrogen temperatures (i.c. little stress corrosion) before failure occurs. Estimate the fr~c.ture. stress in ~si for the fiber u~der these conditions. Young's modulus for silica IS 9 x 10 1 Nm-2 and I PSI= 6894.76 Nm- 2.

4.10

Discuss optical fiber cable design with regard to: (a) fiber buffering; (b) cable strength and structural members; (c) cable sheath and water barrier. Further, compare and contrast possible cable designs for multifiber cables.

4.11

State the two major categories of fiber-fiber joint, indicating the differences between them. Briefly discuss the problem of Fresnel reflection at all types of optical fiber joint, and indicate how it may be avoided. A silica multi mode step index fiber has a core refractive index of 1.46. Determine the optical loss in decibels due to Fresnel reflection at a fiber joint with: (a) a small alr aap; (b) an index matchina epoxy which has a refractive Index of 1.40,

OPTICAL FIBERS, CABLES AND CONNECTIONS

175

It may be assumed that the fiber axes and end faces are perfectly aligned at the joint.

4.12

The Fresnel reflection at a butt joint with an air gap in a multimode step index fiber is 0.46 dB. Determine the refractive index of the fiber core.

4.13

Describe the three types of fiber misalignment which may contribute to insertion loss at an optical fiber joint. A step index fiber with a 200 urn core diameter is butt jointed. The joint which is index matched has a lateral offset of 10 urn but no longitudinal or angular misalignment. Using two methods, estimate the insertion loss at the joint assuming the uniform illumination of all guided modes.

4.14

A graded index fiber has a characteristic refractive index profile (a) of 1.85, and a core diameter of 60 urn. Estimate the insertion loss due to a 5 urn lateral offset at an index matched fiber joint assuming the uniform illumination of all guided modes.

4.15

A graded index fiber with a parabolic refractive index profile (a = 2) has a core diameter of 40 jim. Determine the difference in the estimated insertion losses at an index matched fiber joint with a lateral offset of 1 jim (no longitudinal or angular misalignment). When performing the calculation assume (a) the uniform illumination of only the guided modes and (b) the uniform illumination of both guided and leaky modes.

4.16

A graded index fiber with a 50 urn core diameter has a characteristic refractive index profile «(1) of 2.25. The fiber is jointed with index matching and the connection exhibits an optical loss of 0.62 dB. This is found to be solely due to a lateral offset of the fiber ends. Estimate the magnitude of the lateral offset assuming the uniform illumination of all guided modes in the fiber core.

4.17

A step index fiber has a core refractive index of 1.47, a relative refractive index difference of 2% and a core diameter of 80 urn. The fiber is jointed with a lateral offset of 21l-m, an angular misalignment of the core axes of 3° and a small air gap (no longitudinal misalignment). Estimate the total insertion loss at the joint which may be assumed to comprise the sum of the misalignment losses.

4.18

Describe what is meant by the fusion splicing of optical fibers. Discuss the advantages and drawbacks of this jointing technique. A multimode step index fiber with a core refractive index of 1.52 is fusion spliced. The splice exhibits an insertion loss of 0.8 dB. This insertion loss is found to be entirely due to the angular misalignment of the fiber core axes which is 7°. Determine the numerical aperture of the fiber.

••, .

Describe, with the aid of suitable diagrams, three common techniques used for the mechanical splicing of optical fibers. A mechanical splice in a multimode step index fiber has a lateral offset of 16% of the fiber core radius. The fiber core has a refractive index of 1.49, and an Index matchlnl fiuld with a refractive index of 1.45 is inserted in the splice bttWItft thllNtt~ted fiber end•• AlluminS no longitudinal or angular mis.U.MItiIIl, ....... the InHrdon 10.. or the .pUce. -

.. ;., .',.L, ,;:- ;.,

176

4 .20

OPTICAL FIBER COM MUNICATIONS: PRINCIPLES AN D PRACTICE

Discu ss the principles of o peration of the two majo r ca tego ries of demo untable o ptical fibe r connector. Desc ribe in detail a common techniq ue for ac hieving a butt j ointed fi ber eonneoor. A butt jointed fiber connecto r used on a m ultimode step mdex fiber with a core refractive indet of 1.4 2 and a relative refract ive indt'll difference of 1% has an angular misajignmem uf\}o . There is no longilUd inaJ Of' lateral misalignment but there is a small a ir gap between the fi bers in the co nnector. Estimate thc inser tion lo ss of the connector .

4 .21

Bridl y d escribe the t )·pe~ o f d emou ntable connec ter thaI may be used .....ith single mode fibers. Furthe r, ind icate the problems in volved with t he co nnectio n of single mode reers. A single mode fi ber c on nector is used with a 6 u rn co re dia meter silica (refractive index 1.46) step index. fi ber which h a.. a no rmalized freq uency of 2.2 a nd a numerica l aperture o f 0 .09. The connecto r has a lateral offset of 0 .7 gm a nd a n ang ula r misalign ment of 0 .8 ° . Estima te th e total ins ert ion loss of the connector assu ming that the joint is index matched and tha t there is no longitudina l mi salig nment .

4 .22

A 10 urn core diameter single mode fiber has a normalized freq uency of2.0. A fusion splice at a point alon g its length exhibit s an insertion loss of 0.15 d ll . A ssuming only later al misalign ment contributes to the splice insertion loss, estim ate the m agnitude of th e late ral misalignment .

4 .23

A 5 g m core d ia meter single mode step index fi ber has a nor malized freq uency

of 1.7. a cor-e refractive inde>; o f lAS a nd a numerical a perture of 0 . 14-. The loss in deci bels due to llngular misalignmen t a t a fus ion splice with a lateral offset of 0.4 pm is twice t hat d ue to the lateral off~t. Estimale the m agnit ude in degrees of the a ngula r misa lignment .

4 .24

Given the following parameters for a single mode step index fiber with a fusion splice estimate ( a) t he fiber con: d ia meter; and (b) the numerical ap ertu re for the tiber. Fiber norm alized freq uency = I .t} Fiber cere rd ract ive inde x - L. 46 Splice lateral offset = 0 .5 IJlTI Splice latera l offset loss = 0 .0 5 d B Splice a ngular misa lign ment = 0 .3 0 Splice a ngular misa lignment loss = 0 .04 d B

Answers to Numerical Problema 4 .7 4 .8 4.' 4 .11 4 .12 4 .13 4 .14 4. ••

7.4 3 x 10' pst, 0.6% 0.2 urn 2.6 1 x 1if psi (a) 0.31 dB ; (b) 3.8 x 10 4 dB 1.59 0 .29 d B 0 .67 dB (a) 0 . 19 dB ; ( b) 0 .17 d B; ditTerence 0.02 d B

4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23 4..4

4.0 urn 0. 71 dB 0. 35 0 .4 7 d B J.S J d B 0.54 d B 1.2 IlJ11 0.6$ 0 (a) 7.0 pm ; (b) 0.10

OPTICA L FIBERS, CAB LES AN D CONN ECTIONS

177

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26 21

28 29 30

31

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34 35 38

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40 41

42 43 44

Optical Flbrt Conflmlltlcallo"s. DevlcfS, e lm/Its and $YSlfMl, pp, 18~24 9, Jolm Wiley, 1980,

OPTICAL FIBERS, CABLES AND CONNECTIONS

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63 54

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56 57

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pp. 1439-1441. 1973. II

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.I!I.~

.,.

1I"·}'17•

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70

71 72 73 74

75 76 77

78 79 80

81

82 83 84

85 86

87 8&

89

90 91

92

OPTICAL FIBERS, CABLES AND CONNECTIONS

sa 94 95

96 97 98 99

100 101 102 103

104 105

106 107

108 109 110 111

112 113

"..

181

J. Cook and P. K. Runge, 'Optical tiber connectors', in S. E. Miller (Ed.), Optical Fiber Telecommunications, PP. 483-497, Academic Press, 1979. T. G. Giallorcnzi, 'Optical communications research and technology', Proc, IEEE, 66(7), pp. 744-780, 1978. K. Nawata, Y. Iwahara and N. Suzuki, 'Ceramic capillary splices for optical fibres', Electron. Leu; 15(15), pp. 470-472, 1979. C. M. Miller, 'Loose tube splice for optical fibres', Bell Syst. Tech. J., 54(7), pp. 1215-1225, 1975. D. Gloge, A. H. Cherin, C. M. Miller and P. W. Smith, 'Fiber splicing', in S. E. Miller (Ed.), Optical Fiber Telecommunications, pp. 455-482, Academic Press, 1979. D. G. Dalgoutte, 'Collapsed sleeve splices for field jointing of optical fibre cable', Proceedings of Srd European Conference on Optical Communication (Munich), pp. 106-108, 1977. P. Hensel, J. C. North and J. H. Stewart, 'Connecting optical fibers, Electron. Power, 23(2), pp. 133-135, 1977. A. R. Tynes and R. M. Derosier, 'Low-loss splices for single-mode fibres,' Electron. LeU., 13(22), pp. 673-674, 1977. Y. Toda, O. Watanabe, M. Ogai and S. Seika, 'Low-loss fusion splice of single mode fibre', Internal. Commun. Conf (IEEE), paper 27.7, 1981. E. L. Chinnock, D. Gloge, D. L. Bisbee and P. W. Smith, 'Preparation of optical fiber ends for low-loss tape splices', Bell Syst. Tech. J., 54, pp. 471--477, 1975. C. M. Miller, 'Fiber optic array splicing with etched silicon chips', Bell Syst. Tech. J., 57(1), pp. 75-90, 1978. G. Le Noaoe, 'Optical fibre cable and splicing technique', 2nd European Conference on Optical Fiber Communication (Paris), pp. 247-252, 1976. W. C. Young, P. Kaiser, N. K. Cheung, L. Curtis, R. E. Wagner and D. M. Folkes, 'A transfer molded biconie connector with insertion losses below 0.3 dB without index match', Proceedings of 6th European Conference on Optical Communication, pp. 310-313, 1980. G. Le Noane, 'Low-loss optical-fibre connection systems', Electron. Lett., 15(1), pp. 12-13, 1979. N. Suzuki and K. N awata, 'Demountable connectors for optical fibre transmission equipment', Rev. Elect. Commun. Labs (NT7). 27(11-12), pp. 999-1009, 1979. N. Suzuki, Y. Iwahara. M. Saruwatari and K. Nawata, 'Ceramic capillary connector for 1.3 11m single-mode fibres', Electron. Lett., 15(25), pp. 809-810, 1979, P. Hensel, 'Triple-ball connector for optical fibres', Electron. Len., 13(24), pp. 734-735, 1977. N. Shimuzu and H. Tsuchiya, 'Single-mode fibre connection', Electron. Lett., 14(19), pp. 611-613,1978. N. Shimizu, H. Tsuchiya and T. Izawa, 'Low-Joss single mode fibre connectors', Electron. Len.; '5(1), pp. 28-29, 1978. P. W. Smith, D, L. Bisbee, D. Gloge and E, L. Chinnock. 'A moulded-plastic technique for connecting and splicing opticalfibre tapes and cables', Bell Syst. Tech. J., 54(6) pp. 971-984, 1975. Y, Fujii, J. Minowa and N. Suzuki, 'Demountable multiple connector with precise V-aTOoved silicon', Electron. Lett., 15(14), pp. 424-425, 1979. A. Nicil, 'Prlcticallow·loss lens connector for optical fibres', Electron. Lett.,

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D. B. Payne and C. A. Miller, 'Triple-ball connector using fibre-bead location" Electron. LeU., 16(1), pp. 11-12, 1980. 116 CA. Miller and D.S. Payne, 'Monomode fibre connector using fibre bead location', Proceedings of tnh European Conference on Optical Communication {UK), pp. 306-309, 1980. 117 D. B. Payne and D. J. McCartney, 'Splicing and connectors for single-mode fibres', tntemat, Commun. Con]. (IEEE), paper 27.6, 1981. 118 M. A. Bedgood, J. Leach and M. Matthews, 'Demountable connectors for optical fiber systems', Elect. Commun., 51(2), pp. 85-91, 1976. 119 S. Nagasawa and H. Murata, 'Optical fibre connectors using a fused and drawn multi-glass-rod arrangement', Electron. Len; 17(7), pp- 268-270, 1981. 120 G. Khoe, H. G. Kock, D. Kuppers, J. H. F. M. Poulissen and H. M. De Vrieze, 'Progress in monomode optical-fiber interconnection devices', J. Lightwave Technol., LT-2 (3), pp. 217-227, 1984.

5 Optical Fiber Measurements

5.1

INTRODUCTION

In this chapter we aTC primarily concerned with measurements on optical fibers which characterize the fiber. These may be split into three main areas: (a) transmission characteristics: (b) geometrical and optical characteristics; (c) mechanical characteristics. Data in these three areas are usually provided by the optical fiber manufacturer with regard to specific fibers. Hence fiber measurements aTC generally performed in the laboratory and techniques have been developed accordingly. This information is essential for the optical communication system designer in order that suitable choices of fibers, materials and devices may be made with regard to the system application. However, although the system designer and system user do not usually need to take fundamental measurements of the fiber characteristics there is a requirement for field measurements in order to evaluate overall system performance, and for functions such as fault location. Therefore we also include some discussion of field measurements which take into account the effects of cabled fiber, splice and connector losses, etc. There are a number of major techniques used for the laboratory measurement of the various fiber characteristics. The transmission characteristics of greatest interest are those of optical attenuation and dispersion (bandwidth), whereas the important geometrical and optical characteristics include size (core and cladding diameters), numerical aperture and refractive index prolile. Measurements of the mechanical characteristics such as tensile strength and durability were outlined in Section 4.6.1 and arc therefore pursued no further in this chapter. When attention is focused on the measurement of the transmission properties of multimode fibers, problems emerge regarding the large number of modes propagating in the fiber. The various modes show individual differences with reiard to attenuation and dispersion within the fiber. Moreover, mode coupllnl occun,lvinl transfer of energy from one mode to another (see hctkm 2;3.7). Tht modi coupUn. which Is associated wtth perturbations in

184

I

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

the tiber composition or geometry, and external factors such as microbcnds or splices, is for instance responsible for the increased attenuation (due to radiation) of the higher order modes. These multimode propagation effects mean that both the fiber loss and bandwidth are not uniquely defined parameters but depend upon the fiber excitation conditions and environmental factors such as cabling, bending, etc. Also these transmission parameters may vary along the fiber length (i.e. they are not necessarily linear functions) due to the multimode propagation effects. making extrapolation of measured data to different fiber lengths less than meaningful. It is therefore important that transmission measurements on multimode fibers are performed in order to minimize these uncertainties. In the laboratory, measurements are usually taken on continuous lengths of uncabled tiber in order to reduce the influence of external factors on the readings (this applies to both multimode and single mode fibers). However, this docs mean that the system designer must be aware of the possible deterioration in the fiber transmission characteristics within the installed system. The multimode propagation effects associated with fiber perturbations may be accounted for by allowing or encouraging the mode distribution to reach a steady-state (equilibrium) distribution. This distribution occurs automatically after propagation has taken place over a certain tiber length (coupling length) depending upon the strength of the mode coupling within the particular fiber. At equilibrium the mode distribution propagates unchanged and hence the fiber attenuation and dispersion assume well-defined values. These values of the transmission characteristics are considered especially appropriate for the interpretation of measurements to long-haul links and do not depend on particular launch conditions. The equilibrium mode distribution may be achieved by launching the optical signal through a long (dummy) fiber to the fiber under test. This technique has been used to good effect [Ref. 1] but may require several kilometers of dummy fiber and is therefore not suitable for dispersion measurements. Alternatively there are a number of methods of simulating the equilibrium mode distribution with a much shorter length of fiber. Mode equilibrium-may be achieved using an optical source with a mode output which corresponds to the steady-state mode distribution of the fiber under test. This technique may be realized experimentally using an optical arrangement which allows the numerical aperture of the launched beam to be varied (using diaphragms) as well as the spot size of the source (using pinholes). In this case the input light beam is given an angular width which is equal to the equilibrium distribution numerical aperture of the fiber and the source spot size on the fiber input face is matched to the optical power distribution in a cross section of the fiber at equilibrium. Other techniques involve the application of strong mechanical perturbations on a short section of the fiber in order to quickly induce mode coupling and hence equilibrium mode distribution within I m. These devices which simulate. mode equilibrium over a short length of fiber are known as mode scramblers-or

185

OPTICAL FIBER MEASUREMENTS

mode mixers. A simple method [Ref. 2] is to sandwich the fiber between two sheets of abrasive paper (i.e. sandpaper) placed on wooden blocks in order to provide a suitable pressure. Two slightly more sophisticated techniques are illustrated in Fig. 5.1 [Refs. 3 and 41Figure 5.1(a) shows mechanical perturbations induced by enclosing the tiber with metal wires and applying pressure by use of a surrounding heat shrinkable tube. A method which allows adjustment and therefore an improved probability of repeatable results is shown in Fig. 5.1(b). This technique involves inserting the fiber between a row of equally spaced pins, subjecting it to sinusoidal bends. Hence the variables are the number of pins giving the number of periods, the pin diameter d and the pin spacing s. In order to test that a particular mode scrambler gives an equilibrium mode distribution within the test fiber, it is necessary to check the insensitivity of the far field radiation pattern (this is related to the mode distribution, see Section 2.3.6) from the tiber with regard to changes in the launch conditions. It is also useful to compare the far field patterns from the mode scrambler and a separate long length at the test fiber for coincidence [Ref II. However, it must be noted that at present mode scramblers tend to give only an approximate equilibrium mode distribution and their effects vary with different fiber types. Hence measurements involving the use of different mode scrambling methods can be subject to discrepancies. Nevertheless, the majority of laboratory measurement techniques to ascertain the transmission characteristics of multimode optical fibers use some form of equilibrium mode simulation in order to give values representative of long transmission lines. We commence the discussion of optical tiber measurements in Section 5.2 by dealing with the major techniques employed in the measurement of fiber attenuation. These techniques include measurement of both total fiber attenuation and the attenuation resulting from individual mechanisms within the fiber (e.g. material absorption, scattering). In Section 5.3 fiber dispersion measurements in bot-h- the time and frequency domains are discussed. Various techniques for the measurement of the fiber refractive index profile are then con-

________ ._____-Fikr 1,5,,,,, _ _ Wire' '00 ~m

~--I{""t ~lTlllk"blc llIbe 1'1

PI•••. ,

(hi

Mode Icremtilerl; (8) heel shrinking technique IRef. 31; Ib) bending technique

[AI'. 41, '

186

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

sidered In Section 5.4. In Section 5.5 we discuss two simple methods for measuring the fiber numerical aperture. Measurement of the fiber outer diameter is then dealt with in Section 5.6. Finally, field measurements which may be performed on optical fiber links, together with examples of measurement instruments, are discussed in Section 5.7. Particular attention is paid in this concluding section to optical time domain reflectometry (OTDR).

5.2

I, 1

FIBER ATTENUATION MEASUREMENTS

Fiber attenuation measurements techniques have been developed in order to determine the total fiber attenuation of the relative contributions to this total from both absorption losses and scattering losses. The overall fiber attenuation is of greatest interest to the system designer, but the relative magnitude of the different loss mechanisms is important in the development and fabrication of low loss fibers. Measurement techniques to obtain the total fiber attenuation give either the spectral loss characteristic (see Fig. 3.3) or the loss at a single wavelength (spot measurement).

5.2.1

Total Fiber Attenuation

A commonly used technique for determining the total fiber attenuation per unit length is the cut back or differential method. Figure 5.2 shows a schematic

r.l L, Vkwing Order or mg lilt«

",

Chopp"r

I wnn lil'il!

" 0'"

'iHLr,"

k

-..

LOll,

Len.

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L..Jopti"

mode

!trippe'

\1od,' "r,m~

, 1/

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-,

~

~lQnochTOmalor

Rd,',,"c('

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Fiber'

/

){",'0.0'"

/di, Pl.y

r-', /

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."'plif,er

1 I

10000x m,!cb,d

photod,tcotm

Fig.6.2

,I -f "' CI.ddlnl; InOode stripper

A typical experimental arrangement for the measurement 01 Ipectral lOll In optical fibers using the CUI back technique.

./

OPTI CAL FIBER MEASURE.M ENT S

187

diagram of the typical experimen tal set-up for measu rement of tbe spectra l le ss to obta in (he overall attenuatio n spectru m for the fi ber. It co nsists o f a ' white' light SO UTCt:. usually a tungsten halogen or xenon arc lamp. The focused light is mec hanically cbopped at a low frequency of a few hundred hert z. This enables the lock-in amplifier at the receiver to perform ph ase-sensitive de tection. The chopped light is then fed through a monochromator which utilizes a prism or diffrac tion grating a rrangement \ 0 select the required wavelength at which the attenuation is to be mea sured. Hence the light is filtered before being focused onto the fi ber by means of a microscope objective lens. A beam splitter may be incorporated before the fi ber to pro vide light for viewing optics and a re fe rence signa l used to compensate for o utput power fluctuations. As ind icated in Section 5.1. when the measurement is performed on rnultimode fibers it is very dependent o n the optical launch conditions. T herefore unless the la unch optics are arranged to give the steady -sta te mode distribution at the fibe r input. o r a d um my fi ber is used, then a mode scram bling device is attached to the fiber within the first meter. T he fibe r is also usually put through a cladding mode stripper, which may co nsist of an S-shaped groove cut in the T efl on and filled with glycerin. Thi s device removes light launched into t he fiber cladding thro ugh r adiation into the index matched (or-slightly higher refractive index) glycerin. A mode stripper can also be included at t he fi ber outp ut end to remo ve any optical power which is scattered from the co re into the cladding do wn the fiber len gth. T his tends to be pronounced when the Iiber cladd ing consists o f a low refra ct ive inde.. silicone resin. The optical power at the receiving end of the fiber is detect ed using a p---i-n or ava lanche phorodiod e. In order to o btain reprod ucible re!'ults the photode tector surface is us ually index matched to the fiber outpu t end face using epoxy resin or an index matching cell I Ref. 51. F inally the electrical outpu t from the photodetector is fed to a lo ck-in amplifier ; the o utput of which is reco rded. T he cut back method invo lves taking a set of optical output power measurements over the req uired spectrum using a lo ng length of fiber (usually at least a kilometer). This fi ber is genera lly uncabled ha ...-ing only a primary protecti ve co ating. Increased losse s d ue to cabling (see Section 4.6.2) do not tend to cha nge the shape of the attenua tion spectrum as they a re entirely radiative, which for multimode fi bers are almost wavelength independent. The fiber is the n cut back to a point a few meters (e.g. 3 m ) from the input end and, maintaining the same launch conditions, another set of power output measurements are taken. The follow ing relationship for the optical atten uation per unit length adB for the fiber may be obtained from Eq. (3.3):

(5.1)

L l and L 3 art the orfa1nI1lnd cut bult fiber lengths respectively, and POI and

•••

188

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

are the corresponding output optical powers at a specific wavelength from the original and cut back tiber lengths. Hence when L] and L 2 arc measured in kilometers, ces has units of dB km I. Furthermore Eq. (5.l) may be written 10 the form:

P02

(5.2)

where VI and V2 correspond to output voltage readings from the original fiber length and the cut back fiber length respectively. The electrical voltages VI and V 2 may be directly substituted for the optical powers POI and Pm of Eq. (5.1) as they arc directly proportional to these optical powers (see Section 7.4.3). The accuracy of the results obtained for u JB using this method is largely dependent on constant optical launch conditions and the achievement of the equilibrium mode distribution within the fiber. It is indicated [Refs. 6 and 71 that for constant launch conditions IT.JB may be determined with a precision of around ±O.l dB over 1 km lengths of fiber. Alternatively the total uncertainty L 2 ) dB krrr ' in the measured attenuation is quoted [Ref 81 as ±0.2/(L where L 1 and L 2 are in kilometers. Hence the approximate uncertainty for a I km fiber length is +0.2 dB km-". J

-

Example 5.1 A 2 km length of multi mode fiber is attached to apparatus for spectral loss measure men!. The measured output voltage from the photoreceiver lIsing the full 2 krn fiber length is 2.1 Vat a wavelength of O.85I!m, When the fiber is thHn CuI back to IpaVA a 3 m length the output voltage increases to 10.5 V. Determine the attenuation per kilometer for the fiber at a wavelength of 085j,un and estimate the eccuracv of the result, Solution: The attenuation per kilometer may be obtained from Eq {5.21 where:

10

cae

10

V.

~ '-~--log'0-'=

L,

L,

V, =

10,5

109 10 -

1.997

-

2,1

3.5 dB km

1

The uncertainty in the meesurec Bttenuation may be estimat ed using:

±O,2 Uncertainty

±O,2

~-

:::: ±0.1 dB

L,-L 2

1 997

The dynamic range of the measurements that may be taken depends upon the exact configuration of the apparatus utilized, the optical wavelength and the fiber core diameter. However, a typical dynamic range is in the region 3~0 dB when using a white light source at a wavelength of 0.8S ~m and multimode fiber with a core diameter around SO um. This may be lncreeaed to'

189

OPTICA L FIBER MEASUREM ENTS

aro und 60 d B by use of a laser source operating at the same wa velengt h. It must be noted tha t a la ser source is only suita ble for making a single wa velength (spot) measurement as it does not emit across a broad band of spectral wavelengths. Spot measurement s may be performed on experimental set-up similar to that shown in Fig. 5.2. However interference filters are frequently used instead of the monochrom ator in order to obtain a measurement at a particular optical wavelength . T hese provide greater d ynamic range ( to- 15 dB improvement) th an the monochromator but are of limited use for spectral measurements d ue to the reduced number orwavelengrhs that a rc genera lly availa ble for measurement. A typical optica l confi guration for spot attenuation measurements is sho wn in Fig. 5.3. Th e interference filters are located on a wheel to allow mea surem ent at a selection of dilTcren t wavelengths. In the experimental a rrangement shown in F ig. 5.3 the source spot size is defined by a pinhole and the beam angular width is varied by using different diaphragms. However, the electro nic equipment utilized with this set-up is similar to that used for the spectral loss measurements illustrated in Fig. 5.2. Therefore dete rm ination of the optical loss per unit length for the fiber at a particular wavelength is performed in exactly the same mann er, using the cut' back method . Spot attenualion measurements are sometimes utilized after fiber c abling in order to o btain info rmatio n on any degradation in the fiber attenuation resulting from the cabling process. . Altho ugh widely used, the c ut back measure-ment method has the major , d rawback of being a destructive techniq ue. Therefore, although suita ble for

an

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./

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... ...

Art

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m " IlI~ "'tI

.rnn;-mlnt tor mallJog spot Isingle whelel'lgthl a ll8ouat ion ullng IMlrlI"'nl;1 flit." ancl e rnDloylng t he cut N ell tlchnique.

190

OPTICAL FIBER COM M UNICATIONS: PRINCIPLES A ND PRACTICE

la boratory measu rement it is far from ideal for atten ua tio n measurements in the field . Severa l nondestructive techniques exist wh ich allow the fiber lo sses to be ca lc ulated th rough a single read ing of the optical o utput power at the far end of the fiber after determination of the near end po wer level. The sim plest is t he insertion loss techni que which utilizes the same experim ental configuration as the cut b ack met hod. H owever, the fi ber to be tested is spliced, o r connected by means o f a demo untable connecto r. to a fiber with a known optical output at th e wavelength of interest. w hen all the o ptical power is co m pletel y coupled between the two fibers, or when the insertion loss of the splice or connector are kn ow n, then the measurement of the optical output power from the second fiber gives the loss resulting from the insertion of thi s second fiber into the system. Hence the insertion loss due to the second fiber provides measure ment of its attenuation per unit length. Un fortunately the acc uracy of this mea su rement method is dependent on the coupling between the two fiber s and is therefore somewhat uncertain. The most popular nondestructive attenuatio n mea surement techn iq ue for both labora tory and field use only requires access to one end of the fiber. It is the backscatter meas urement method which uses opti cal time domain refkctometry and also provides measurement of splice and co nnector losses as well as fault location. Optical time domain rellectometry fi nds major usc in field mea surements a nd therefore is discussed in detail in Sectio n 5.7. 1.

5.2.2

Fiber Absorption Loss Measurement

It was indicated in the p revio us section that there is a requirement fo r the optical fibe r manufacturer to be able to separate the total fiber atten uat ion into the contributio ns from the major loss mechanism s. Material absorption lo ss measurements a llow the level o f impurity content within the fiber material to be checked in the manufacturing process. The measurem ents a re ba sed on cal orimetric method s which determine the temperat ure rise in th e fiber o r bulk m aterial resulting from the ab sorbed optical energy within the struc ture. The apparatus shown in Fig. 5.4(a) [Ref. 121 wh ich is use d to mea sure the absorption loss in o ptica l fibers wa s modified from an earlier version which measured the a bsorption lo sses in bu lk glasses (Ref. 131. This temperature measurement techniq ue, illustrated d ia grammatically in F ig. 5.4(b), has been widely adopted for absorption lo ss meas urements. The two fiber sam ples shown in Fig. S.4(b) are mounted in capillary tubes sur ro unded by a low refractive index liquid (e.g. meth anol) for good electri cal conta ct, within the same enclo su re of the apparatus sho wn in F ig. S.4(a). A thermocouple is wo un d aro und the fiber c on tainin g capillary tubes using one of them as a reference junctio n (d ummy fibe r). light is launched from a laser source ( Nd :YAG or krypton io n depending o n the wavelength of interest) th rough the m ain fiber (not the dummy), and the temperature rise due to abso rption is measured by the thermocou ple a nd indicated o n I nancvcltmeter. Electrical '

OPTICAL FIBER MEASUREMENTS

191

rc ".nomltme'el' M."i,'c

ro I',fe",,,c< "mpl,'

,"dO'lLJ"

-,

rh
I

L.""chi"~

Sili,'. /',uh"

Fib",

,-011

I

, )'lode ,tripp..

/

Ethyl

I

;,ic"hol

objcc",',

// llypoocrmic ,,,he

,",

, 111crmocollplo jU"Cli<.><,

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,

1101 jundion

I

\ I

Silica ",pill",;e,

Fig.5.4

!

ThermocoUPle

Calorimetric measurement of fiber absorptionlosses: (a) schematic diagram of a version of the apparatus Iner. 121: [b} the temperature measurement technique using a thermocouple.

calibration may be achieved by replacing the optical fibers with thin resistance wires and by passing known electrical power through one. Independent measurements can then be made using the calorimetric technique and with electrical measurement instruments. The calorimetric measurements provide the heating and cooling curve for the fiber sample used. A typicalexample of this curve is illustrated in Fig. 5,5(a). The attenuation of the fiber due to absorption a. b , may be determined from this heating and cooling characteristic. A time constant te can be obtained from a plot of(T", - T t ) on a logarithmic scale against the time t, an example of which shown in Fig. 5.5(c) was obtained from the heating characteristic displayed in Fig, 5,5(b) [Ref. 131. Too corresponds to the maximum temperature rise of the fiber under test and T1 is the temperature rise at a time t. It may be observed from Fig, 5,5(a) that Too corresponds to a Itead)' ,tate tempUlture for the flber when the heat loss to the surrounding

192

OPTICAL FIBER COM M UNICATIONS: PRIN CIP LES AND PRACTICE

-,

-- - --.;;.-~ (

.,

, 10)

, , ,-

"!-----*----~,,--.,, : 00 , 1<) '0' :

:

"" I

I-

I

•

Fig. 5.5

I (' )

(a ) A typical h@al i"9andcooli ng curvefor a glass fiber sam ple. (bi A. hell ing curve

a nd Ic) too corrlls pond lll9 plo t of (T - Tt l ltga irnJt time for I tllT'p t, gI ... rod (bulk material measurement ). Reproduced with p41f1"'liulon from 1(. I. White Ind. J. E. Midwi'ller.ODto-elecrronlcs.I .p. 323, 19 73.

OPTICAL FIBER MEASUREMENTS

193

balances the heat generated in the fiber resulting from absorption at a particular optical power level. The time constant Ie may be obtained from the slope of the straight line plotted in Fig. 5.5(c) as:

(5.3) where II and t2 indicate two points in time and to is a constant for the calorimeter which is inversely proportional to the rate of heat loss from the device. From detailed theory it may be shown [Ref 131 that the fiber attenuation due to absorption is given by: (5.4) where C is proportional to the thermal capacity per unit length of the silica capillary and the low refractive index liquid surrounding the fiber, and Popl is the optical power propagating in the fiber under test. The thermal capacity per unit length may be calculated, or determined by the electrical calibration utilizing the thin resistance wire. Usually the time constant for the calorimeter Ie is obtained using a high absorption fiber which gives large temperature differences and greater accuracy. Once t, is determined, the absorption losses of low loss test fibers may be calculated from their maximum temperature rise Tei:;' using Eq. (5.4). The temperatures are measured directly in terms of the thermocouple output (microvolts), and the optical input to the test fiber is obtained by use of thermocouple or optical power meter.

Example 5.2

Measurements are made using a calorimeter and thermocouple experimental arrangement as shown in Fig. 54 in order to determine the absorption loss of an optical fiber sample. Initially a high absorption fiber is utilized to obtain a plot of (Tc>o - Ttl on a logarithmic scale against t. It is found from the plot that the readings of (Tro- T~ after 10 and 100 seconds are 0.525 and 0,021 uv respectively. The test fiber is then inserted ln.tha calorimeter and gives a maximum temperature rise of 4.3 x 10-0 °C with a constant measured optical power of 98 mW at a wavelength of 0.75 lim. The thermal capacity per kilometer of the silica capillary and fluid is calculated to be 1.64 x 10' J °C-'. Determine the absorption loss in dB km-". at a wavelength of 0.75 urn. for the fiber under test. Solution: Initially, the time constant for the calorimeter is determined from the meaeurements taken on the high absorption fiber using Eq. (5.31 where:

194

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE 100-10

90 In (O.5251-ln 10.021) 28.0 s

=

Then the absorption loss of the test fiber may be obtained uslnq Eq. (54) whPrH:

CT""

1.64 x 10' x 4.3 x 10-4 98x 10 )x28.0

=

2.5 dB km-'

Hence direct measurement of the contribution of absorption losses to the total fiber attenuation may be achieved. However, fiber absorption losses are often obtained indirectly from measurement of the fiber scattering losses (see the next section) by subtraction from the total fiber attenuation, measured by rit~{J, on, of the techniques discussed in Section 5.2.1.

5.2.3

Fiber Scattering Loss Measurement

The usual method of measuring the contribution of the losses due to scattering within the total fiber attenuation is to collect the light scattered from a short length of fiber and compare it with the total optical power propagating within the fiber. Light scattered from the fiber may be detected in a scattering cell as illustrated in the experimental arrangement shown in Fig. 5.6. This may consist of a cube of six square solar cells (Tynes cell IRef. 141) or an integrating sphere Soa"" ;,," cub, li",,1 with ,Ix ,olar ",II, Chiliel ill~

("h'~'I'"

mou,

Cl.dd,n~

",il'pe,

"'Lrpcr

/ Los" sourc,'

InO"'-

•• \

1/

\ \ \

/

••

"

I ""

P,,-

. •

0'

nef"",I'" ,igndl

InJex ",,,ohi,

_ _ I"'"grating

-..,.-,,,

'1'1""

Sign.] detce'o< P"

Fig. &.6

An experimental 50t-UP for measurement of fiber scattarlng loss Illustrating both the solar call cuba and Integrating sphara scattering celli.

195

OPTICAL FIBER MEASUREMENTS

and detector IRef. 151. The solar cell cube which contains index matching fluid surrounding the fiber gives measurement of the scattered light, but careful balancing of the detectors is required in order to achieve a uniform response. This problem is overcome in the integrating sphere which again usually contains index matching fluid but responds uniformly to different distributions of scattered light. However, the integrating sphere does exhibit high losses from internal reflections. Other variations of the scattering cell include the internally reflecting cell [Ref. 161 and the sandwiching of the fiber between two solar cells IRef. 171. A laser source (i.e. He-Ne, Nd: Y AG, krypton ion) is utilized to provide sufficient optical power at a single wavelength together with a suitable instrument to measure the response from the detector. In order to avoid inaccuracies in the measurement resulting from scattered light which may be trapped in the fiber, cladding mode strippers (see Section 5.2.1) are placed before and after the scattering cell. These devices remove the light propagating in the cladding so that the measurements are taken only using the light guided by the fiber core. Also to avoid reflections contributing to the optical signal within the cell, the output fiber end is index matched using either a fluid or suitable surface. The loss due to scattering Usc following Eq. (3.3) is given by:

u,"

=

10 loglo {(km)

(P",) p," P,>pt -

dB km

,

(5.5)

where {(km) is the length of the fiber in km contained within the scattering cell, Po pt is the optical power propagating within the fiber at the cell and Psc is the optical power scattered from the short length of fiber l within the cell. As P o Pt ;t> P,o, then the logarithm in Eq. (5.5) may be expanded to give:

Usc

=

4.343 {(km)

(P,c ) dB km' - '-

Po pt

(5.6)

Since the measurements of length are generally in centimeters and the optical power is normally registered in volts, Eq. (5.6) can be written as: 5

.~ '0

4.343 X 10 {(em)

(.

V'o ) -r --dBkm VUp l

(5.7)

where V", and VOPI are the voltage readings corresponding to the scattered optical power and the total optical power within the fiber at the cell. The relative experimental accuracy {i.e. repeatability) for scatter loss measurements are quoted as ±O.2 dB [Ref. 6J using the solar cell cube and around 5% [Ref. 81 with the integrating sphere. However, it must be noted that the absolute accuracy or the measurements is somewhat poorer, being dependent on the cIllbrldon Or the ICiatterina cell and the mode distribution within the fiber.

196

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Example 5.3

,I I

A He-Ne laser operating at a wavelength of 063 J.lm was used w it h a solar cell cube to measure the scanering loss in an optical fiber sample. With a constant optical output power the reading from the solar cell cube was 8 14 nV. The optiGal power measurement at the cube without scattering was 153.38 ).IV. The length of till! filwr in the cube was 2.92 em. Determine the loss due to scattering in dB krn 'for the fiber at a wavelength of 0.63 urn. Solution: The scattering loss in the fiber at a wavelength of 0.63 J.lm may be obtained directly using Eq. 15.71 where:

I (em I 4.343 x 10"

2.92 =

&.3 ,

,,I!

6.14XlO-') (

153.38x 10 "

6.0 dB km-"

FIBER DISPERSION MEASUREMENTS

Dispersion measurements give an indication of the distortion to optical signals as they propagate down optical fibers. The delay distortion which, for example, leads to the broadening of transmitted light pulses, limits the information-carrying capacity of the fiber. Hence as shown in Section 3.7 the measurement of dispersion allows the bandwidth of the fiber to be determined. Therefore besides attenuation, dispersion is the most important transmission characteristic of an optical fiber. As discussed in Section 3.7 there are three major mechanisms which produce dispersion in optical fibers (material dispersion, waveguide dispersion and intermodal dispersion). The importance of these different mechanisms to the total fiber dispersion is dictated by the fiber type. For instance, in multimode fibers (especially step index), intermodal dispersion tends to be the dominant mechanism, whereas in single mode fibers intermodal dispersion is nonexistent as only a single mode is allowed to propagate. In the single mode case the dominant dispersion mechanism is intramodal (i.e. material dispersion). The dominance of intermodal dispersion in multimode fibers makes it essential that dispersion measurements on these fibers are only performed when the equilibrium mode distribution has been established within the fiber, otherwise inconsistent results will be obtained. Therefore devices such as mode scramblers must be utilized in order to simulate the steady-state mode distribution. Dispersion effects may be characterized by taking measurements of the impulse response of the fiber in the time domain, or by measuring the

197

OPTICAL FIBER MEA SUREMENTS

baseband frequency response in lhe frequency domain. If it is assumed that the fiber response is linear with regard (0 power IRef. 191. a mathemati cal description in the time domain for the optical output power Po(t) from the fiber may be o btained by convoluting the power impulse respon se h(t) with the o ptical input po wer Pj(t) a s: (5.8) where the asterisk • denotes conv olution. The conv olution of h(t) with P,(t) sbo wn in Eq. (5.8) may be evaluated using the convolution integral IRef. 201 where:

P,«(l ~

.r:

P,«(- x )h(xl dx

( 5.9)

In the frequency domain the power transfer function l1 (w) is the Fo urier transform of h(t) and therefore by taking the Fourier tran sforms of all the fu nctions in Eq. (5.8) we obtain. '1'0(0)) = H (ro)Pj(m)

(5.10)

where w is the base band angular frequency. The frequency domain representation given in Eq. (5.10) is the lea st mathematically complex, and by performing the Fou rier tran sformation (or the inverse Fourier transformatio n) it is possible to switch between the time and frequency dom ains (or vice versa) by ma thema tical means. Hence. independe nt measurement of either h(l) o r H (ro) allows determinatio n of the o vera ll d ispersive properties of the o ptical fiber. Thus fiber dispersion measu rements can be made in eit her the time o r Irequcncy domains.

5 .3.1

Time Domain Me.surement

The most common method fo r time dom ain mea surement of pulse dispersion in optical fibers is illustrated in Fig. 5.7 [R ef, 2 11. Short o ptical pulses .....t. ...1..

Ilea...
-1"'k

pM t<>d_

,/

/ V., •• ol,

Pulsed

dto"" rsll,n •

L,n.

"

>U-

Fober

dnYc,

]

.....I.n'lw ph<>l<>di
.... L7

0

bptrlm.nla l .,..~ .m.nt tor melti"o fib er dlsD. rsiol'l mM 'Ur&m&nts in t oo lima dOflllln {Rtf. 2 11. . .'

198

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

(100-400 ps) are launched into the fiber from a suitable source (e.g. AIGaAs injection laser) usmg fast driving electronics. The pulses travel down the length of fiber under test (around I km) and are broadened due to the various dispersion mechanisms. However, it is possible to take measurements of an isolated dispersion mechanism by, for example, using a laser with a narrow spectral width when testing a multimode fiber. In this case the intramodal dispersion is negligible and the measurement thus reflects only intermodal dispersion. The pulses are received by a high speed photodetector (i.e. avalanche photodiode) and are displayed on a fast sampling oscilloscope. A beam splitter is utilized for triggering the oscilloscope and for input pulse measurement. Alternatively, after the initial measurement of output pulse width, the long fiber length may be cut back to a short length (1-2 m) and the measurement repeated in order to obtain the effective input pulse width. The fiber dispersion is obtained from the two pulse width measurements which are taken at any convenient fraction of their amplitude. However, unlike the considerations of dispersion in Sections 3.7-3.10 where rms pulse widths are used, dispersion measurements are normally made on pulses using the half maximum amplitude or 3 dB points. If Pi(t) and Po(t) of Eq. (5.8) are assumed to have a Gaussian shape then Bq. (5.8) may be written in the form 1Ref. 201: (S.ll) where 'ti(3 dB) and t o (3 dB) are the 3 dB pulse widths at the fiber input and output respectively and 't(3 dB) is the width of the fiber impulse response again measured at half the maximum amplitude. Hence the pulse dispersion in the fiber (commonly referred to as the pulse broadening when considering the 3 dB pulse width) in ns km-' is given by: 't(3 dB) =

(t;(3 dB) - t! (3 dB»'

ns km

I

(5.12)

L

where 't(3 dB), 't;(3 dB) and 'to (3 dB) arc measured in ns and L is the fiber length in km. It must be noted that if a long length of fiber is cut back to a short length in order to take the input pulse width measurement then L corresponds to the difference between the two fiber lengths in km. When the launched optical pulses and the fiber impulse response are Gaussian then the 3 dB optical bandwidth for the fiber Bo pl may be calculated using [Ref. 22]:

B o pt

X

't(3 dB) = 0.44 GHz ns

(5.1 J) = 0.44 MHz ps

Hence estimates of the optical bandwidth for the fiber may be obtained from the measurements of pulse broadening without resorting to rigorous mathematical analysis.

199

OPTICA L FIBER M EASU REM ENTS

Pulse d isp ersiOf"t mea su rem en ts are t ake n Ower a 1.2 1ml leng th o l l.ar1ia l1v gr
la) Ihe JdB p ulse broaden in g for Ine fi ber in ns km · l • [b] the f ibe r b il l'ldwi tl l ll--l englh p roduc t

Solulfon : (iI) Th e 3 d B p ulse broad en ing mil y be ob t ained u ~in g

«a dB I =

(12 .6~ _ 0.321 t

1.2

Eq.:S. 12 ) w her e :

11 58 .7 6 _ 0 09 ) l

=-- c-::- 1.2

_ 1 0 .5 ns . m- 1 (b) The o pt ical bal'ldwidt h for It>e liber is g iv en by

0. 4 4 BOfJ' ."

era dB)

rc, !S. 131 as:

0 44 -- G Hz ~ m

10 5 = 4 1.9 MHz km

T he v alue coreioeo for B opt corresponds 10 the b ,md w kft h-l.m g th product f o r the fi ber bec ec se t he pulse broad ening in part (a) w as c alculated o v er a 1 1<m fih.::! r leng th . Also i l ma y be 1'1 0100 th at in t his case t he n a"ow input pu lse w id th m a1
re.o.

The dispersio n measurement tech nique discussed above'ls reasonably simple and gives a direct measurement of the pulse broadening: However. it bas some dra wbacks when performing measurements o n lo w dispersion fi bers. At present the time resolutio n of commercially a vailable detectors is limited to a few tenths of a nanosecond. Hence inaccuracies occur in th e measurement when the pulse broadening a long the fiber is less than 0.5 ns, For low dispersion fibers this may dictate the testing of a considerable length of fiber. A more convenient method of measuring t he tempora l dispersion of an optical pulse within a fiber whic h does not require a long fi ber" length is the shuttle pulse technique. T he experi mental set-up reported by Cohen [Ref 23J is shown in Fig. 5.8. Both ends of a short fiber length are terminated with partially transparent mirrors and a pulse launched from a GaAs injection laser travels through o ne mirror into the fiber and then shuttles back and forth between the fiber ends. This technique has an added adva ntage in that it allows the length dependence of the impulse response to be st udied by sampling t he pWH tJ\er each 2N - 1 (where N = I, 2. J. etc.I transits. The pulse a t the '., OQtput end it diJplayed on a sampling oscilloscope through the partially transpanat mirror. Htnoethe puJs. broadcnina may be meuured by comparing the ,

200

OPTICAL FI BER COMM UNICATIONS : PRI NCI PLES AND PR ACTICE

."

Fig.5.8

;o n

S che m at ic diagra m s how ing the a ppa ra t us used in the s hutt le pulse technique fo r ti me do main dispe rsion measure m e nts in o ptica l fi be rs IRef . 2 31.

widlh s of the observed o utput puls es. To ensure symm etrica l reflectio n at the mirrors it is importa nt that they a re perpendicular to the fi ber a xis. T he mirrors a re therefore mounted in cylind rical holde rs which ha ve g rooves precisely milled to a ccurately mach ined faces. A n index matchin g fluid is also utilized between the fiber end faces and the mirrors in order to ach ieve o ptimum optical t ransm ission. Nevertheless t he techniq ue does have drawba cks including o ptica l loss at each end reflection, T herefore accurate measurements over multiple refl ections cannot always be en sured.

5 .3.2

Frequency Domain Measurement

F req ue ncy do main measurement is the preferred method fo r acqumng the band wid th of optica l fiber s. This is because the ba seband frequ ency response lI(w ) of the fiber may be o btai ned directly from these measurements using Eq. (5.10) without the need for any as su mptions of Gaussian sh ape, or altern atively. the mat hematically complex deco nvolution of Eq . (5.8) which is necessary with measurements in the time domain . Thus the optical ba ndwidth of a fiber is best obtained fro m frequency domain me asurements. O ne of two frequency domain measurement technique s is generally used . Th e fi rst utilizes a simila r pu lsed so urce to th at employed for the time dom ain measurements shown in Fig. (5 .7). However, the sampling oscilloscope is rep laced by a spectrum analyzer which takes the Fourier transform of the pu lse in the time d omain and hence d isplays its constituent frequency components. T he experimental arrani ement is .illusteeted in Fl,. 5.9.

201

OPTICA L FI BER M EAS UREMENTS

", .aun.o-~ .

,

~-

'<

=

/

'I

o . . ,"," "';' ~'

r.,',.od J .....

I

t>

"

r hOIW lOJ<

J

Fli><.

'-

lli:J S p" O t ""~

Fig.5.9

. ,,,,Ly, ..

Ex pl!lime"Tal set-u p fo r makirlQ tibe r dis pe rsion meas ure ments in the do ma in using a pulsed lase r source .

fre qu l! n.c ~

Comparison of the spectrum a t the fiber output '1',,(00) with [he spect rum at the fiber input 'P,(ro) provides the baseband frequency response for the fi ber under tes t following Eq. (5.10) where:

(5. 14) The second technique involves launching a sin usoida lly modulated optical signal at different selected fr equencies using a sweep oscillator . Therefore the signal energy is concentrated in a very narrow frequency band in the baseban d region, unlike the pulse measurement method where the signal energy is sp read over the entire baseband region. A po ssible experime ntal arrangement for this swept frequency measurement method is shown in F ig. 5.10 [Ref. 24J. The optical sou rce can be an LED o r an injection laser. both of which may be

-

.-. _1lIo""

Op' ir" ~~I."'"

""-"

~.

... 1, ;.

"

,~

"./

f 11<01.:':"«><'"''

1 ~IIJi''''"~

~"''1 '' ' '' 'Y

. m ~ l i f i ..

"'"'

Sp,,,,,~," ' ''111~"r

N<,_ _ ." ~..

'.10

ol ~

...,

.d,c,i"

,

./

.~ " I. "d . phot odiod .

Iol' ''' '''l' a nd l'oor_UIIOI

Block 1CM'l'IIllc ,how'"il lin ellj)8"mentar 3lTanv" ment fat' the fo~l fIeqUincy m . ..urlrnenl mlt l'lod to provide fiber dll ~ion measu rements in me frtqllt"ICY ltOm' ln (Rtf. 2.].

,

202

OPTICAL FIBE R CO MMU NICATIONS: PRI NCI PLES A ND PRACTICE

d irectly mod ulated (see Section 7.5) from the sweep oscillato r. A spectrum a na lyzer may be used in order to obtain a contin uou s display of the swe pt frequency signal. Again, Eq. (5. 14) is utilized to ob tain the baseband frequency response. However, th e spectru m a nalyzer pro vides no info rmation on the phase of the received sign al. T herefore a vector voltmeter or ideally a network analyzer can be employed to give both the frequ ency and phase information

(Ref. 71. 5.4

FIBER REFRACTIVE INDEX PROFILE MEASUREMENTS

T he refractive index profile of t he fi ber core pla ys a n important role in c ha racterizing the properties o f o ptical fibers. It a llows d etermination of the fiber' s numerical aperture and the nu mber o f mode s propagating within the fibe r core, whilst largely de fining any intermodal an d/ or profi le dispersion ca used by the fiber. Hence a detail ed knowl edge of the refractive index pro file enables the impulse respon se of the fiber to be predicted. Also as the impulse response and consequently the information-carrying capacity of the fiber is strongly depend ent on the refractive index profil e, it is essentia l t hat th e fiber manufa cturer is a ble to produce particular profiles with great accuracy, e specially in the C.1.SC of gra ded index fibers (i.e. o ptimu m profile). There is therefore a requirement for accu rate measurement o f the refractive index profile. These measurements. may be performed using a number o f different techniques each o f which exhibit certain advantages and drawback s. In this sectio n we will discu ss. some of th e more popular meth od s which may be relati vely easily interpreted theoretically, without attempting to review all the possible techniques which have been developed.

5.4.1

Interferometric Method.

Interference microscopes (e.g. Mach-Zehnder, Michelson) ha ve been widely used to determine the refractive index profiles of o ptical fi bers. The technique usua lly involves th e prepa ration of a thin slice of fiber (slab method) which has both ends acc urately polished to obta in sq uare (to th e tiber a xes) and optically fl.a t surfaces. T he slab is often im mersed in an index matching fl uid, and the assembly is examined with a n interference microscope. T wo major methods are then emplo yed; using either a trans mitted light interferometer (MachZehnder [Ref. 25J) or a rel1ected light interferometer (Michelson [Ref. 261). In both ca ses light from the micro scope travels nor mal to the p repared fiber slice faces (parallel to the fibe r axis), and differences in refr act!....e index result in d ifferent o ptical path lengths . Thi s situation is illustrated in the case of the Mach -Zehnder interferometer in Fig . S. l l(a). When the phase of the incident light is compared with the phase of the emerging lipt, a field of par allel

'

..

'

... "'"

203

OP TI CA L FIBER M EA SUREMENT S

, ..

. '-" ~op

" ~'------1c--"",

f ',,11

lk•., "'11lt" f'~~·'---~'v i"-·-----,~

rt:o.~:;' ,,' Fig. 6 .11

(a) T he principle of t he M ach-Zehnd er rnter tercme ter [ Ref 251. (bl The int erference fring e patte rn obt ai ned w it h an i nte rf erence mi cros cope fr om a grad ed in dex fi ber. Reprod uced w ith permis sion fr om L, G. Cohen, P. Kaise r, J. B. Mecch esnev. P, B O'Con ner and H, M. Presby , Appl. Phys. Lett.. 26, p. 472,19 7 5 ,

interference fri nges are ob served . A photograph of the fri nge pattern may then be taken. an example of which is shown in Fig. 5.1 I(b) IRef. 28 1. The fringe disp lacements for the points within th e libel' core a rc then measured using as reference the pa rallel fringes outside the libel' core (in the fi ber cladding). The refractive index d ifference betwee n a po int in the fi ber core (e.g. the core axi s) and lhe cladding c an tc obtained from the fringe shift q. which co rres ponds to a number of fringe displacem ents. T his d iffe rence ill. refractive index 5/1 is given by (Ref. 6 1: q ).

6,, = -

(5. 15)

x

w here x is the thickness o f the fi ber slab and A is t he incident o ptical wa velength. The slab method gives an accurate measurement of the refractive ind ex. profile, although computation o f the individual po ints is somewhat tediou s un less an automated technique is used [R ef. 281. Figure 5.12 [Ref. 281 shows the refractive index profile obtained from the fringe patt ern ind icated in

Fig. S.II(b). A limitation of this method is the time required to prepare the fi ber slab. However, another interfero metric technique ha s been deve loped IRef. 30] which requires no sample prep aration . In this method the light beam is incident to the fiber perpend icular to its axis : this is known as transverse shearing ' Interferometry. Again fringes are observed from which the fi ber refract ive Index prolUe may be obtained. ,

,

,.

204

OPTICAL FIBER COMM UNICATIONS: PR INCIPLES A ND PRACTICE

• •

"

•

•

,. ,,

,

•

I •

I·

.,

,\ , , :

\,

5.4.2

I

•,

0

••

,

\

I''''

R. ~....

1 ' - _ . _. _ . _ . -

-

~

_

..-

,

'J Fig.5.12

I

•

, ,

I

~4

't •

,,

" . l4 l 'fft'I 1JI_.Il"d ~

,

•

I

,

't

,

•

lU \

I

" • :!-o

•

The fi be r refra ctive inde ~ p rofi le compe ted fro m tbe lorertereece patt e rn snown in Fig . 5 .1 1Ibl. Reproduced with per miss io n trom L G . Cohen. P. Ka iser, J . 8 M acChesne y, P. B. O'Conner and H. M. Presby. A pfll. Phys. Lett.• 26. p. 4 72, 1975.

Near Field Scanning Method

The near field scanning method utilizes the close resemblance that exists between t he ncar field intensity distribution and the refractive index profile, for a tiber with all the guided modes equally illuminated. It provides a reasonably straightforward and rapid method far acquiring the refractive index profile. When a diffuse Lambertian source {e.g, t ungsten filament lamp or LED) is used to excite all the guided modes then t he near fiel d optical power density at a radius r from the core axis P[)(r ) may be expressed as a fraction of the core axis near field optical power den sity Po(O) following IRef. 3 11 :

PD(r) [ n1 (r) - ni -:".",, = C(u)

PolO)

,

nHOl - nj

]

(5.l6)

205

OPTICAL FIBER MEASUREMENTS

where nl(O) and nl(r) are the refractive indices at the core axis and at a distance r from the core axis respectively, n: is cladding refractive index and C(r, z) is a correction factor. The correction factor which is incorporated to compensate for any leaky modes present in the short test fiber may be determined analytically. A set of normalized correction curves is, for example, given in Ref. 33. An experimental configuration is shown in Fig. 5.13. The output from a Lambertian source is focused onto the end of the fiber using a microscope objective lens. A magnified image of the fiber output end is displayed in the plane of a small active area photodetector (e.g. silicon p-i-n photodiode]. The photodetector which scans the field transversely receives amplification from the phase sensitive combination of the optical chopper and lock-in amplifier. Hence the profile may be plotted directly on an X-V recorder. However, the profile must be corrected with regard to C(r, z) as illustrated in Fig. 5.14(a) which is very time consuming. Both the scanning and data acquisition can be automated with the inclusion of a minicomputer IRef. 31]. The test fiber is generally less than I m in length to eliminate any differential mode attenuation and mode coupling. A typical refractive index profile for a practical step index fiber measured by the near field scanning method is shown in Fig. 5.14(b). It may be observed that the profile dips in the center at the fiber core axis. This results from the collapse of the fiber preform before the fiber is drawn during the manufacturing process (see Sections 4.3 and 4.4). Measurements of the refractive index profile may also be obtained from the far field pattern produced by laser light scattered by the fiber under test. This 'technique, generally known as the scattered pattern method, requires complex analysis of the forward or backward patterns in order to determine the refractive index profile [Ref. 31]. Therefore, it is pursued no further in this text.

Chopp
x400bj

..c v

Pr
-

/'

•

Ira"e,·,aol, detector

-

Flb.r •

,,0Obj

,

o

....... ,

C\

Look·ln amplifier

-

"'00'00<

, Limbo,ti", IOU"•

EKptrlmentel let-up for the near field scanning measurement of the refractive Ind'll profll, IR.f. 321.

206

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Normalj"d

",---------,

inlcn,ily Refrncliw

i",lo,

'"

C""',lo
11.8

i"t"n~ty

near fi
'"

I,

Di,tant'c

o.a

I 0.2

I

0.4

0.6

0.8

1.0

'" Fig.5.14

(al The refractive index profile of a step index fiber measured using the near field scanning method, showing the near field intensity and the corrected near field intensity. Reproduced with permission from F. E. M. Siaden, D. N. Payne

and M. J. Adams, Appl. Phys. Lett., 28, p. 225. 1976. (b) The refractive index profile of a practical step index fiber measured by the near field scanning

I

method [Ref. 31].

I

,

i

5.4.3

I

The refractive index at any point in the cross section of an optical fiber is directly related to the reflected power from the fiber surface in air at that point following the Fresnel reflection formula of Eq. (4.12). Hence the fraction of light reflected at the air-fiber interface is given by:

I

End Reflection Method

r = reflected optical power = ( n l - 1 )' incident optical power nj + 1

(5.17)

where n, is the refractive index at the point on the fiber surface (usually within the fiber core). For small changes in the value of the refractive index: (,--,n,_-_1:.,.)

&=4-:-

(n] + I)J

on[

(5.18)

Therefore combining Bqs. (5.18) and (5.17) we have:

[ni~l]on[

(5.19)

Equation (5.19) gives the relative change in the Fresnel reflection coefficient

OPTICAL FIBER MEASUREMENTS

207

r which corresponds to the change of refractive index at the point of measure-

ment. However, when the measurement is performed in air the small changes in refractive index on l that must be measured give only very small changes in r, as demonstrated in example 5.5.

Example 5.5 Refractive index measurements are to be made using the and refieCliofl method on a step index fiber in air. The fiber core refractive index is nominally 1.5 EstirnatH the percentage change in the Fresnel reflection coefficient that must be measured in order to allow a change in refractive index of 0.001 to be resolved, Solurion: The relative change in the Fresnel reflection codfieir"t is giVHnby Eq, 15.19). where:

on,

", Therefore when

n,

rs

5 and the requirement for

on,

is 0.001:

0: [1:5 ]0,001 0,0032 =

0.32%,

Hence the change in the Fresnel reflAc\ion coefficient which must be measured is

It is clear from example 5.5 that for a fiber in air very accurate measurement of r is needed to facilitate a moderate resolution of on l • However, this problem may be overcome by immersing the fiber in a suitable index matching oil, as illustrated in example 5.6.

Example 5.6 The step index fiber of example 5,5 is immersed in oil with a refrac;tivH index of 1,45, Estimate the percentage change in the Fresnel reflection eoelli"iH"t which must be measured in order to obtain the resolution in required in exarnplH 55 10.0011. Solution: In this case mL!st refer back to the Fresnel .etrccnoo romHJla of Eel. 14.121 where:

on,

we

"'- " ~ ("' 1.45 ~ ( n + n ,il, + 1.45 , ,

Hence (n, - 1,45)

6r=4

(n,

. &1,

+ 1,45)J

208

OPTICAL FIBER COM M UNICATIONS: PRINCIPLES AND PR ACTIC E

' ed

",

-

=

4 ] 0 _00 1 = 0 0 2 72 __ [ 2 2 5 2. 103

The re fo re , tile chdn g .. in th.' Fres ne l re fle c tio n coe ffici,mt ""I,id , m u!o l be measu red fo r tile fibe r in tha o il il'o 2 .72'1(•.

The result from example 5.0 illustrates the increased sensitivity in t he rneas . uremcnt with index matching ove r that calculated in exa mple 5.5 (0.32%) when no index match ing wa s used. Also the sensitivity may be increased furth er with improved index matching giving very accu rate profile mea surements . Two experimental arrangements for performing end refl ection measurement s are illustr ated in Fig. 5. 15 [Refs. 34 and 35 1. Both techniques utilize a focused laser beam incident on the fiber end face in o rder to provi de the nece ssary spatial resolutio n. Figur e 5. 15(a) shows end reflection measurements witho ut index ma tching of the fiber input end face. The laser beam is initially di rected thro ugh a polarize r and a )../ 4 plate in o rder to prevent feed back of the reflected optical power fro m both the fiber end face and the intermediate optics. ca using mod ulat ion of the la ser o utput t hro ugh interference. The circ ularly polarized light beam from the A/4 plate is th en spatially fil tered and expanded to pr ovide a suitable spot size. A beam splitter i~ used to prov ide both a reference fro m the input light beam which is monitored with a solar cell, and two beams fro m the fiber end face reflectio n. The reflect...' d bcams are used fo r measurement via a ,ri-n photodiode, lock. -in am plifier combina tio n, and lor visual chec k of the alignment on the fiber end face using a screen. Focusing o n the fi ber end face is ach ieved with a microscope o bjective lens. and t he fi ber end i~ sca nned slowly a cross the focal spot using precisio n tra nslatio n stages. The reflected optical power is monitored as a function of the fi ber linear positio n on an X-Y recorder and the refractive index pr o l1le may be obtained directly using Eq. (5. 19). Possible refl ections from the other fiber end face are av oided by immersing it in an index matching liquid. The experimental arra ngement shown in Fig. 5. 15(b) provides increased sensitivity by immersing the fibe r in an index matching o il as demonstrated in example 5.6. In this case the laser beam , which is again incide nt on a polarizer, an d )./ 4 plate is deflected vertica lly using a mirror. An oil immersion objective is utilized to focus the beam o nto the immersed fi ber end. Thi s apparat us has sho wn sensitivity co mparable w'ith the near field method. However. there is a need for careful alignment of the apparatus in order to avoid stray reflections.

209

OPTI CAL FIBER ME A SUREM ENTS

f ok,

Lo
"

"

l'itol o
Oil imm",lon obj " l;"

indo rel="nofollow"> ", . 'dli"~ 1 "l ~~ 1

fig . 5 .15

Experime ntal a. rilllge me llls lor eed reflectio n me ~ su P'l! .....e nt of fiber refra ctjve iflde~ p rofile : (i1) w ithOUl ifldell ..... atching of fibel irput e nd lace IRef. 3 41: (b ) w ith index m atch iflg 01 fiber inpu t e nd lace IRef. 3 5 1.

Also in both techniques it is essential tha t the fiber end face sho uld be perfectly n at (clea ved but not polished), b ecame the reflected powe r is severely alTected by surface irregularities.

&.6

FIBER NUMERICAL APERTURE MEASUREMENTS

T he numerical aperture is an important o ptical fiber parameter a s it affects characteristics such as the light -gathering efficiency and th e normalized fr eq uency of the fiber ( V). This in turn dictates the nu mber o f modes propagating within the fiber (also definina the single mode region) which has consequent IfTKU on botb the fibt r diaper.ion (i.e. intermodaJ) and. possibly. the fiber 0"

210

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

attenuation (i.e. differential attenuation of modes). The numerical aperture (NA) is defined for a step index fiber by Eq. (2.8) as: (5.20)

NA = sin 8" = (n1-ni)J-

where e, is the maximum acceptance angle, n l is the core refractive index and n2 is the cladding refractive index. It is assumed in Eq. (5.20) that the light is incident on the fiber end face from air with a refractive index (no) of unity. Although Eq. (5.20) may be employed with graded index fibers. the numerical aperture thus defined represents only the local NA of the fiber on its core axis (the numerical aperture for light incident at the fiber core axis). The graded profile creates a multitude of local N As as the refractive index changes radially from the core axis. For the general case of a graded index fiber these local numerical apertures NA(r) at different radial distances r from the core axis may be defined by: NA(r) = sin 8,(r) = (nr(r) - ni){

(5.21)

Therefore. calculations of numerical aperture from refractive index data are likely to be less accurate for graded index fibers than for step index fibers unless the complete refractive index profile is considered. However, if refractive index data is available on either fiber type from the measurements described in Section 5,4, the numerical aperture may be determined by calculation. Alternatively, a simple, commonly used technique for the determination of the fiber numerical aperture involves measurement of the far field radiation pattern from the fiber. This measurement may be performed by directly measuring the far field angle from the fiber using a rotating stage. or by calculating the far field angle using trigonometry. An example of an experimental arrangement with a rotating stage is shown in Fig. 5.16. The tiber end faces are prepared in order to ensure square smooth terminations. The fiber output end is then positioned on the rotating stage with its end face parallel to the plane of the photodetector input, and so that its output is perpendicular to

H,;" 'I",'hm'

,

Fil'" 1\Oldel'

•

r-; Fiber

-,

I(ef",,>c' ';3",1 [rom opti,,1

<, la"k-ill

""'l'lir",

, , 'V

.

0"

<;hol'l~'T

,

"\

\

iI

L,,~,

,,"'"

i'lJatoJ,-t",o'

Ro,atlllg ,tag"

Fig.5.16

Fiber numerical aperture measurement using a sClmnlng photcdetectcr lind II rotating stage.

211

OPTIC AL FIBER M EASUREM ENTS

tile 3J1: is of rotation. li ght is la unched into the fiber li t a ll possible angles (overfi lling, the fiber) using an optica l system similar 10 that used in the spot a ttenua tion measuremen ts (Fig. 5.3). T he phorodetector, which may be either a small area device or an apert ured la rge arca device, is placed 10-20 em from thc fiber and positioned in order to obtain a maximum signal with no rotation (0°). He nce when the rotat ing stage is tu rned the limits of the fa r field pattern ma y be recorded. The o utput power is monitored and plotted as a fun ct io n of angle; the maximum acceptance angle being o btained when the power drops a predetermined amount (e.g. 10%). Thus the numerica l aperture of the fi ber c an be o bta ined fro m Eq. (5.20). This far field scann ing measurement may also be performed with the phorodetector located o n a rotationa l stage and the fi ber positio ned at the center o f rotat ion [Ref. 7 ). A complementary techniqu e utilizes a plane wave input to the fiber , which is then rotated around t he in put beam axis whilst its outpu t is directly monitored. A less precise measurement of the numerical aperture can be obtained from the fa r field pattern by trigono metric means. The experimental apparatus is shown in F ig. 5.17, where the end prepared fiber is located on an optical base plate or slab . A gain light is launched into the fiber under test over the full range of its numerical aperture, and the fa r field pattern from the fiber is display ed on a scree n which is po sitioned a kno wn distance 0 from the fiber o utput end face. The test fiber is then aligned so that the o ptical intensity o n (he screen is ma ximized . Fina lly, the pen em size on the screen A is measured using a calibra ted vernier caliper. T he numerical aperture can be obtained from simple ir igonometr ical relationships where : A

(5.22)

Elllltmple 5.7 A t r;g(lI'o m etrical m easu re me nt is perfo rTnm l in order to d etermine I he o um .,rir:al ep en u re 0 1 a st ep i nde ~ f iber The screen is positio ned 10.0 em f.u m th e fiber e nd f ace. Wll en illu m ir"lll l ed f ro m a WId e angled visible &(Ill ' roC t he m eiOsured o utput pattern size is 6 .2 em. catcora te th e epproxlrrt at e Il u m prical an erture 01 t he fi ber, Solution: The n um eric al apert u re m ay be d Plf!rmin cd directl y lI sing Eq (5 271

wh ere : 6.2

= =

- c = = O.30

138 .44 + 4 00 )~

It m U8t be noted that the acc uracy o f this measurem ent techniq ue is dtptndent upon the visu el a ssessmen t of the fa r field pattern from the fiber. The above meaaurement techniqu es are generally emplo yed with multimode .'

212

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

--I)~:++-T

,I

I

Fig.5.17

Apparatus for trigonometric fiber numerical aperture measurement.

fibers only, as the far field patterns from single mode fibers are affected by diffraction phenomena. These are caused by the small core diameters of single mode fibers which tend to invalidate simple geometric optics measurements. However, more detailed analysis of the far field pattern allows determination of the normalized frequency and core radius for single mode fibers, from which the numerical aperture may be calculated using Eq. (2.69) [Ref 361Far field pattern measurements with regard to multimodc fibers are dependent on the length of the fiber tested. When the measurements are performed on short fiber lengths (around 1 m) the numerical aperture thus obtained corresponds to that defined by Eqs. (5.20) or (5.21). However, when a long fiber length is utilized which gives mode coupling and the selective attenuation of the higher order modes, the measurement yields a lower value for the numerical aperture. It must also be noted that the far field measurement techniques give an average (over the local NAs) value for the numerical aperture of graded index fibers. Hence, alternative methods must be employed if accurate determination of the fiber's NA is required [Ref. 371.

5.6 5.6.1

FIBER DIAMETER MEASUREMENTS Outer Diameter

It is essential during the fiber manufacturing process (at the fiber drawing

stage) that the fiber outer diameter (cladding diameter) is maintained constant to within I%. Any diameter variations may cause excessive radiation losses and make accurate tiber-tiber connection difficult. Hence on-line diameter measurement systems are required which provide accuracy better than 0.3% at a measurement rate greater than 100 Hz (i.e. a typical fiber drawing velocity is 1 m S-I). Use is therefore made of noncontacting optical methods such as fiber image projection and scattering pattern analysis. The most common on-line measurement technique uses fiber image projection (shadow method) arid is illustrated in Fig. 5.18 [Ref. 38 J. In this method a laser beam is swept at a constant velocity transversely across the fiber and a

213

OPTICAL FIBER MEASUREMENTS c

'" "

[mil

'"

..11,

,

Ia
C,

ri,

hi,.,

,

~' Photo
.\1,

>c "I'''''''':1
Scope
Pub,'

f-

3'"

r-e-

S"'
,i""it

eFig.5.18

0"

~

Ai"'l'

'''''' Time ill""Hl J,
Prill',",

The shadow method lor the on-line measurement of the fiber outer diameter [Ref, 381

measurement is made of the time interval during which the fiber intercepts the beam and casts a shadow on a photodetector. In the apparatus shown in Fig. 5.18 the beam from a laser operating at a wavelength of 0.6328 11m is collimated using two lenses (G J and G 2 ) . It is then reflected ofT two mirrors (M 1 and M 2 ) . the second of which (M 2 ) is driven by a galvanometer which makes it rotate through a small angle at a constant angular velocity before returning to its original starting position. Therefore, the laser beam which is focused in the plane of the fiber by a lens (G,) is swept across the fiber by the oscillating mirror, and is incident on the photodetector unless it is blocked by the fiber. The velocity ds/dt of the fiber shadow thus created at the phctodetector is directly proportional to the mirror velocity d$/dl following: ds

d$

dt

dt

-~l­

(5.23)

where 1 is the distance between the mirror and the photodetector. Furthermore the shadow is registered by the photodetector as an electrical pulse of width W, which is related to the fiber outer diameter do as:

do

=

d' W.dl

(5.24)

Thus the fiber outer diameter may be quickly determined and recorded on the printer. The measurement speed is largely dictated by the inertia of the mirror rotation and its accuracy by the risetime of the shadow pulse.

214

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Example 5.8

The shadow method is used for the on-line measurement of the outer diameter of an optical fiber. The apparatus employs a rotating mirror with an angular velocity of 1 4 rad 5- which is located 10 em from the photorlPler;tor. At a particular instant in

time a shadow pulse of width 300 liS is registered by the photodHector Determine the outer diameter of the optical fiber in urn at this instant in lima

Solution: The shadow velocity may be obtained from Eq. 15.23) where: ds

dojl

-~/-

dt

0.1 x4 __ OAms- 1

dr

__ 0.4 IJ.m I!S-1 Hence the fiber outer diameter do in urn is given by Eq (524):

d, do = We -

,"

= 300 IlS X O.41J.rTl 1-\5->

120 11m

Other on-line measurement methods, enabling faster diameter measurements, involve the analysis of forward or backward far field patterns which are produced when a plane wave is incident transversely on the fiber. These techniques generally require measurement of the maxima in the center portion of the scattered pattern from which the diameter can be calculated after detailed mathematical analysis [Refs. 39--421. They tend to give good accuracy (e.g. +0.25 11m l Ref 421) even though the theory assumes a perfectly circular fiber cross section. Also for step index fibers the analysis allows determination of the core diameter, and core and cladding refractive indices. Measurements of the fiber outer diameter after manufacture (off-line) may be performed using a micrometer or dial gage. These devices can give accuracies in the order of +0.5 11m. Alternatively, off-line diameter measurements can be made with a microscope incorporating a suitable calibrated micrometer eyepiece.

5.6.2

Core Diameter

The core diameter for step index fibers is defined by the step change in the refractive index profile at the core-cladding interface. Therefore the techniques employed for determining the refractive index profile (interferometric. near field, end reflection, etc.) may be utilized to measure the core diameter. Graded index fibers present a more difficult problem as, in general, there is a continuous transition between the core and the cladding. In this case it is necessary to define the core as an area with a refractive index above a certain

OPTICAL FIBER MEASUREMENTS

215

predetermined value if refractive index profile measurements are used to obtain the core diameter. Core diameter measurement is also possible from the near field pattern of a suitably illuminated (all guided modes excited) fiber. The measurements may be taken using a microscope equipped with a micrometer eyepiece similar to that employed for off-line outer diameter measurements. However, the corecladding interface for graded index fibers is again difficult to identify due to fading of the light distribution towards the cladding, rather than the sharp boundary which is exhibited in the step index case. A second possible definition of the fiber core diameter is that it may be considered as the dimension of an area in which a certain fraction (e.g. 90 or 95%) of the total transmitted optical power is propagated. Hence measurements may be performed to establish the optical power distribution within the fiber either using near field scanning techniques [Ref. 7], or by measuring the power transmitted through a calibrated variable diameter iris positioned upon an enlarged image of the fiber. Unfortunately these transmission measurements are dependent on the fiber length and the optical launch conditions which tend to make the measurement an effective rather than true diameter reading. Other possible techniques include selectively etching the core material [Ref. 71, and the analysis of the far field scattering patterns (for step index fibers) as indicated when the measurement of fiber outer diameter was discussed.

5.7

FIELD MEASUREMENTS

The measurements discussed in the previous sections are primarily suited to the laboratory environment where quite sophisticated instrumentation may be used. However, there is a requirement for the measurement of the transmission characteristics of optical fibers when they are located in the field within an optical communication system. It is essential that optical fiber attenuation and dispersion measurements, connector and splice loss measurements and fault location be performed on optical fiber links in the field. Although information on fiber attenuation and dispersion is generally provided by the manufacturer, this is not directly applicable to cabled, installed fibers which are connected in series within an optical fiber system. Effects such as microbending (see Section 4,6,2) with the resultant mode coupling (see Section 2.3.7) affect both the fiber attenuation and dispersion. It is also found that the simple summation of the transmission parameters with regard to individual connected lengths of fiber cable does not accurately predict the overall characteristics of the link [Ref 43J, Hence test equipment has been developed which allows these transmission measurements to be performed in the field. In leneral. field test equipment differs from laboratory instrumentation in a numbtr of IlpICtl 11~'1t is required to meet the exacting demands of field , , (;,,,

216

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

measurement. Therefore the design criteria for field measurement equipment include: (a) Sturdy and compact encasement which must be portable. (b) The ready availability of electrical power must be ensured by the incorporation of batteries or by connection to a generator. Hence the equipment should maintain accuracy under conditions of varying supply voltage and/or frequency. (c) In the event of battery operation, the equipment must have a low power consumption. (d) The equipment must give reliable and accurate measurements under extreme environmental conditions of temperature, humidity and mechanical load. (e) Complicated and involved fiber connection arrangements should be avoided. The equipment must be connected to the fiber in a simple manner without the need for fine or critical adjustment. (f) The equipment cannot usually make use of external triggering or regulating circuits between the transmitter and receiver due to their wide spacing on the majority of optical links. Even if the above design criteria arc met, it is likely that a certain amount of inaccuracy will have to be accepted with field test equipment. For example, it may not be possible to include adjustable launching conditions (i.e. variation in spot size and numerical aperture) in order to create the optimum. Also, because of the large dynamic range required to provide measurements over long tiber lengths, lossy devices such as mode scramblers may be omitted. Therefore measurement accuracy may be impaired through inadequate simulation of the equilibrium mode distribution. A number of portable, battery-operated, optical power meters are commercially available. These devices often measure absolute optical power in dBm and dBIl (i.e. 0 dBm is equivalent to 1 mW and 0 dBIl is equivalent to 1 IlW; sec example 5.9) over a specified spectral range (e.g. 0.4-1.15 11m). In a number of cases the spectral range may be altered by the incorporation of different demountable sensor heads (photodetectors} However, it must be noted that although these devices often take measurements over a certain spectral range this simply implies that they may be adjusted to be compatible with the center emission frequency of particular optical sources so as to obtain the most accurate reading of optical power. Therefore these devices do not generally give spectral attenuation measurements unless the source optical output frequency is controlled or filtered to achieve single wavelength operation. A typical example is the United Detector Technology S 550 fiber optics power meter shown in Fig. 5.19. This device may be used for measurement of the absolute optical attenuation on a fiber link by employing the cut back technique. Other optical system parameters which may also be obtained usina this type of power meter arc the measurement of individual splice and son-

217

OPTICAL FIBER MEASUREMENTS

Fig.5.19

The United Detector Technology S 550 fiber optics power meter, (Courtesy of United Detector Technology.)

nectar losses, the determination of the absolute optical output power emitted from the source (see Sections 6.5.3 and 7.4.1) and the measurement of the responsivity or the absolute photocurrent of the pbotodetector in response to particular levels of input optical power (see Section 8.6).

Example 5.9 An optical power meter records optical signal power in either dBm or tlBJ.l {al Convert the optical signal powers of 5 mW and 20 J.lW tu dBm. (b) Convert optical signal powers of 0.3 mW and 80 nW to dB).l.

Solution: The optical signal power can be expressed in decibels using: dB=10109,o

(:~)

where Po is the received optical siqnul power and P r ls a reference power level. (a) For a 1 mW reference power level: dBm

=

10 log,o (

Po

)

Im W

Hence an optical signal power of 5 mW is equivalent to Optlcat signal power

=

10 IOg10 5

=

6.99 dBM

and an optical signal power of 20 IlW is equivalent to:

Optical signal power = 10 10glo 0.02 = -16,99 dBm

218

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE (b) For a 1 IlW reference power level:

Therefore an optical signal power of 0.3 mW is equivalent to:

Optical signal power = 10109,0 (

Pc ) "W

= 10 log,o 30

= 14.77 dBJ.l

and an optical signal power of 800 nW is equivalent to: Optical signal power = 10 109,0 0.8 = -0.97 dBIi

,,

There are a number of portable measurement test sets specifically designed for fiber attenuation measurements which require access to both ends of the optical link. These devices tend to use the cut back measurement technique unless correction is made for any difference in connector losses between the link and a short length of similar reference cable. A block schematic of an optical attenuation meter consisting of a transmitter and receiver unit is shown in Fig. 5.20 [Ref. 43]. Reproducible readings may be obtained by keeping the launched optical power from the light source absolutely constant. A constant optical output power is achieved with the equipment illustrated in Fig. 5.20 using an injection laser and a regulating circuit which is driven from a reference output of the source derived from a photodiode. Hence any variations in the laser output power are rectified by automatic adjustment of the modulating voltage, and therefore current, from the pulse generator. A large area photodiode is utilized in the receiver to eliminate any effects from differing fiber end faces. It is generally found that when a measurement is made on multimode fiber a short cut back reference length of a few meters tS insufficient to obtain an equilibrium mode distribution. Hence unless a mode scrambling device together with a mode stripper are used, it is likely that a reference length of around 500 m or more will be required if reasonably accurate measurements are to be made. When measurements are made without a steady-state mode distribution in the reference fiber a significantly higher loss value is obtained which may be as much as I dB km" above the steady-state attenuation IRefs. 22 and 441. Several field test sets are available for making dispersion measurements on optical fiber links. These devices generally consist of transmitter and receiver units which take measurements in the time domain. Short light pulses (:::!::200 ns) are generated from an injection laser and are broadened by

1-.', ""\,_

219

OPTICAL FIBER MEASUREMENT S ~ ._-

Jl l-

Fib« ..., "" , m
--

Inj-o..'lioD

W......,.,.

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D ~ ' r~ '

t-

I

,

... ~

1""("""

1'1 rel="nofollow"><'_

>4<

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Ampl.r« ,

Rt.bf.,(

V.';.I,,,, , .iot . on)~i 'i<'

'\0 . ~~.-

«0'. ..... ......

II

~

._ ,.- , ..-

,

I>

I

1% rc

,

.s,

nI

,

-Tmm-mi licr_._._- ,

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UI

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~I,,' I, ~'

I>

,- ,n I-'I- a I ':" :1 ':": ,I "

L .. .

Flg.5.20

An oplical atl e n ual io n me te. IRef. 43].

tran smission do wn the optical link before being received by a fast response phot odetector (i.e. ava lanche photod iode) and displayed on a sampling osc illoscope. This is similar to the dispersion measurements in the time domain discussed in Section 5.3. If it is assumed that the pulses ha ve a near Gaussian shape. Eq. (5.12) may be utilized to determi ne th e pulse broadening on the link, and hence the 3 d B optical bandwidth may be o btained.

5.7.1

Optical T ime Doma in ReflK10metry t OTDR)

A measurement techn ique which is far more sophisticated and which finds wide a pplication in both the labor atory a nd the field is the use or optical time do main reflectometry (OT DR). This technique is often ca lled th e backscatter measurement method. It provides meas urement or the attenuation on an optical link down its entire length giving information on the length dependence of the link loss. In this sense it is superior to th e optical attenuat io n measurement methods discussed previously (Section 5.2) which only tend to provide an averaged loss over the whole length measured in dB krn-' , When the attenuation on the link varies with length. the averaged loss informa tion is inadequate. OTDR also allows splice and conn ecto r lo sses to be evaluated
.er

•

I,

220

OPTICAL FIBER COMMUNICATIONS : PRINCIPLES AND PRACTICE

aperture due to Rayleigh scattering (see Section 3.4. 0 . Hence the backscattering meth od which was first described by Ba m oski and Jensen IRef. 451 has the advantages of being no ndestru ctive (i.c. does req uire the c utting back of the fiber) and of requiring access to one end of the optical link onl y. The back sca ttered o pt ical power as a function of time PRa(l) may be o btained from the following relationship I Ref. 46 1: (5.25) where Pi is the optical power lau nched into the fi ber, S is the fraction of captured optical power. 'fR is the Rayleigh sca ttering coefficient (backscatter 101>5 per unit length), W " is the input o ptical pulse width• •,~ is the g roup velocity in the fiber a nd '( is the attenuation coefficie nt per unit length for the fi be r. T he fraction of captured optical power S is given by the ratio of the solid acceptance angle for the fi ber to the tot al solid a ngle as:

ttNA1

NA '

S " ':::"'-c,.. _ 4n:nr 4nr

(5 .26)

It must be noted that the relation ship given in Eq. (5.26) applies co step index fibers and th e pa rameter S for a graded inde x fiber is generally a factor of 2/ 3 lower th an for a step index fi ber with the same numerical aperture tRef. 471. Hence using Eqs. (5.25) a nd (5.26) it is possible 10 determine the backscattered optical power from a point along the lin k length ill relation to the

forward o ptical pow er at tha t poin t.

ElI&mple 5 .10

A n op t ic al f iber lin k con sist s of st ep tnde« fib er w h ich h as 0 ll u m erir.al ~l p'H l u re 01 0 .2 a nd a ce re . c n a c tive i,..,de ~ of 1.5. The Rayle ig h l;caUeting c(w fficil:lfll fo r the fib er is 0 . 1 km- ' . W hen lig ht p urses 01 501'>5 r1 ural ioll a , ~ 11Iuncnc d inl n t he l i lJcf~ c a lcu lat e the rat io ;'1 dl'cibels o f th e hack eca tteeed o pt ical po..... cr 10 the fo~w a rd

o ptica l po w e r a t tne fibe r inpu t. The verccrv a t light in a vacuum is 2 99 8 x

tos m s-'. S olution: The b acksc at te red op t ica l pow er

PI\~ ( t )

is (l iven by Eq (5 ,2 51 wh ere:

PJII, ltl = ; PoSYfl W" Vg exp 1- Y"!< fl At t h e fi be r inp u l t , - O. bence ' he p ower ra tIo is:

-,

,

•,

Sub st itul in g for S from Eq. 15. 2 6 1 g ~\/iU :

P". (OI

PI

'i

'•

~" •

.

",,2. 2

[NAI Y~WO vg] A~

221

OPTICAL FIBER M EASUREM ENTS The U.oup ve locily i" lhe fibe. " D is def i" om by EQ 12 .32) as'

c vq = -

c '="' -

»,

. N,

Therefore

= ~ [ 10 .0 21' 0 .7 " 10 ..... )( 50 " 10-' " 2 .998 " , 0' ]

2

411 .5P

= 1.5 5 5 x 10-'

In d e cibe ls

.- - 4 8 ,' dB

Hence in example S.IO the backscattcred optical power at th e fiber input is -1.8.1 d B down on the forw ard optical power. The backscanered optical power should not be confu sed with any F resnel reflection at the fiber inpu t end face resulting from a refractive index misma tch. This co uld be considerably greater th an the backsca ttered light from the Iiber, p resenting measurement problems with OT DR if it is allowed to fall on to the receiving pho todete ctor of the equipment described below. A block sche matic of the backscatter measurement method is shown in Fig. 5.2 1 [Ref. 49 1. A light pulse is launched into the fi ber in the fo rward d irection I ,1..,

C""pl<1

hlW ,-"

)

/

i'110 t 0<1 ,' Ie' ,' L,"

,\ PD

I

fit.

".1

"", ".or

1.0'

( :Ilon

;"\qlI'>""

>mpb' ..,

R"Co r, k r

Optica l lim a d o m. ln refleeto me try o r the ba cks ca tt 9'l' me aSl,l remanl me thod.

222

i

OPT ICAL FiSER COM MU NICATIONS: PRINCIPLE S AN D PRACTIC E

from a n injection laser using either a d irectional coupler o r a system of cxter nal lenses with a beam s plitter (usually o nly in the laborato ry). The back scattered light is detected using a n avalanc he photodiode receiver which drives a n integrator in o rder 10 improve the received signa l to noise ratio by giving an arithmetic average o ver a nu mber o f measurem ents tak en at o ne po int within the fiber. This is necessary as the r eceived o ptica l signal pow er from a particular point along the fiber length is at a very low level comp ared with the forw ard power at th at point by some 4 5-60 d B (see e ..ample S. IO). and is also swamped wit h noise. The signal from th e int egrator is fed through a lo garithmic amplifier and averaged measurements for successive points within the fiber arc plotted o n a c ha rt recorder. This provides locatio n-dependent a tt enuation values wh ich give a n o verall picture of the o ptical loss down the lin k. A possible backscatter plot is illustrated in Fig. 5.22 IRef. 50 1 which sho ws the initial p ulse ca used by reflection and backsc at ter from the input co uple r followed by a long tail caused by th e distributed Rayleigh scatt ering from the inp ut pulse a s it travels down the link. Also shown in the plot is a pulse corresponding to the d iscrete reflection from a fi ber joint, as well as a discontinuity due to excessive loss at a fiber imperfection or fa ult. T he end of the fiber link is indicated by a p ulse corresponding to the Fresnel refl ection inc urred at th e ou tput end face of the fi ber. Such a plot yield s the attenuation per unit length for the fiber by simply compu ting the slo pe o f t he curve over the length required . Also the locatio n and insertio n losses of jo ints and/or faults ca n be obtained fro m the power drop at t heir respective pcsinons on t he link. F inal ly the overall lin k len gth ca n be determined from the time difference between reflections fro m the fi ber input a nd o utput end faces . Hence optical time do main reflecto metry is a very powerful technique for field measurement on optic al fiber links. A n umber of opti cal time domain refl ectometers are commercially avail ab le fo r operation in the shorter wavelength region below 1.0 urn. These devices emit a series of short ( lD- loo ns), intense optical pulses ( 100-500 mw ) from whic h the backscattered light t.. received, an alyzed and displayed on an oscillo scope, o r plotted o n a c hart recorder. A typical example which will o perate o ver a dy na mic ran ge o f 40 d B two-way o ptical lo ss (often quoted as 2 x 20 dB since the single wa y o ptical lo ss is 20 d B) with locatio n and attenuation accuracies of +4 m and ± 10% respectively is shown in F ig. 5.23. In ad dition this device is capabl e of detecting reflecting breaks (i.e. fro m the 4% F resnel refl ection) over a single way dynamic range of up to 38 d B. A major drawback of this technique, especially wh en using commercial optical time dom ain reflectometers, is the limited dynamic range of the mea surement system. As indicated above t his is currently aro und 40 dB (2 x 20 d B) for high performance d evices. Hence. depe nding upon the fi ber and coupling lo sses, the length of optical lin k which can be fully tested is restricted to a t very best around IS km. However, a method of optical time domain refleetometry by photon countina IRef. 5 11 has shown some promile

OPTICAL FIBER MEASUREM ENT S

223

••• •

~• •

D

,a

D

N

N

• <•

..

OPTICA L FIBER COM MUNICATIONS : PRINCIPLES A ND PRACTICE

224

,.,

•

." .. _. . :~;.,t'l·

•

.. .....

~~ :::

_

..-' ...., :::

~ ~ ~: : :: , : :: : : -~

.mn:: ......... •

--.

•

··•• ••, -..••. •

•... .....-:: ::: :

' .. ,,,.,, ~ :: _. ~

-

•

_

..

-..-

-

. ... ...... ..• ....... .'" ..: -

"

-

'

- '-

-

-"

Fig.5.23

-

The STC OFR3 oprtcat tlm e domain rene ctorn ara r. (Courtesy of STC Components.t

especially when used as a diagnostic tool for fault locat ion. In this method the avala nche phorodiode is opera ted in a Geiger tu be brea kdown mode IRef. 52 1 by biassing the device above its normal operating voltage where it ca n detect a single photon. Experimental rRet: 511 measurements using this technique ha ve demonstrated its a bility to cope with single wa y losses of up to 40 dB (i.e.

!

a two-way dy namic range o f mode fibers.

80 d B) when detecting reflectin g brea ks in m ulti-

PROBLEMS 5.1

Describe what is meant by 'equilibrium mode distribution' and ' cladding mode stripping' with regard to tran smission measurements in optical fibers. Briefl y outline method s by which these conditions may he achieved when opt ical tiber measurements are perfo rmed.

5 .2

Discuss w ith t he aid of a suitable diagram the cut back technique used for the measurement of the tota l auen uarion in an o p(iea l fiber. Indicate the differences in the apparatu s utilized for spectra l lou and spot attenuation measurement. A spot measurement of fiber attenuation is performed on a 1.5 1m lenlth of optical fi ber at a wavelon.th of 1. 1 jU11. The mouurod optical oulp'l1 powtr

22.

OPTI CAL FIBER M EASURE M ENTS r. _ T,

,

, ~ V)

1.1 1l

6. IIS

-

,, ,,, , _ _ _ _L ,

--- - - - --:;.1. 51l

_

,'----
,..

fig . 5 .24

' ''n

•

"'l

I

hl

'"

Fibe r a bsorplio n for meas ure ments tor pro blem 5.3 ; lal plol 01 ( r~ - Tr I againsl lime l or a high a~rplion fiber; (b ) t he healing a nd COOling eU1V1t for the te st 'iber .

fr om the 1.5 km length of fiber is 50. '

~W .

W hen th e fiber is cu r b ack to a 2 m length, the measured optical outpu t power is 385. 4 ILW. Determine the attenuation per kilo meter for the fiber at a wavelength of 1.1 um, an d estimate the accuracy of the result.

' .3

'_4

•••

1.1

Briefly outline the principle behind the calorimetric methods used for the measurement of absorp tion loss in optical fibers. A high ab sorption opticnl fl ber was used to obt ain the plot of ( T ,", - T ,)(on a logarithmic scale) against lime shown in Fig. 5.24(a). The measurements were achieved using a c alorimeter 8I"Id thermocouple experimental arrangement. Subsequently a d ifferent test filler was passed three times through the same ca lorimeter befo re further mea suremen lS were taken. Mea surements o n the t est fiber prod uced lite heating a nd cooling curve shown in Fig. 5.24(b) when a constant 76 mW of optical p ower, at a wavelength o f 1.06 p m, was passed through it. The constant C for the experimental arrangement was calculated 10 be 2.32 x I O~ J -c-' . C alculate the absorption loss in decibels per kilometer . at a wavele ngth of 1.06 ~ . for th e fiber under test. Discuss the measurement of fiber scattering lo ss by de scribing the: use of two co mmon scattering ce llfi. A Nd : YAG laser operating at a wavelength o f 1.064 J.l rn is used with a n integra ting sphere to measure the scauenng loss in an optical fiber sam ple. The optical power propagating within the fiber at the sphere is 98.45 pw and 5 .3 1 n W of optical power is scau ered within the sphere. Th e length of fiber in the sp here is 5.99 em . Determine the optical loss due to scattering for the fiber at a wavelength of 1.064 um in decibels per kilometer. Fib er scattering lo ss mea surement s arc taken at a wavelength of O.75 lJ m using a solar cell cube. The readin g of the input optical power to the cube is 7.78 V with a gain settin g o f lOs . T he corresponding re ading from the scattering cell which incorporates a 4. 12 em length o f fiber is 1.56 V with a gain setting of 1 0~ . Previo us measurements of the total fiber attenuation at II wavelength of 0. 75 urn gave a value of 3.2 1 dB km- l . Calculate the absorptio n los s for the fiber at a wavelength of 0 .75 urn in decibels per kilometer. DilCUU with the aid of suitable d il &fams the measurement o f di~persion in optical tl.bet1- Corllidct both time and frequency domain measurement

., "",,",,-

-,

-'

22.

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Pulse dispersion measurements arc taken on a graded index fiber in the time domain. The 3 dB width of the optical output pulses from a 950 ill fiber length is 827 ps. When the fiber is cut back to a 2 ill length the 3 dB width of the optical output pulses becomes 234 ps. Determine the optical bandwidth for a kilometer length of the fiber assuming Gaussian pulse shapes.

5.7

Pulse dispersion measurements in the time domain arc taken on a rnultirnodc and a single mode step index fiber. The results recorded are: Input pulse width

(3 dB)

Output pulse

Fiber

width (3 dB)

length (km)

(,) Multimode fiber (b) Single mode fiber

400 ps

31.20 ns

200 ps

425 ps

I. 13 2.35

Calculate the optical bandwidth over one kilometer for each fiber assuming Gaussian pulse shapes.

5.8

Describe the end reflection method for the detailed measurement of the refractive index profile of an opticalliber. Indicate how the resolution of this technique can be improved. The end reflection technique is to be used in order to measure the refractive index profile of a graded index fiber in air. The fiber has a core axis refractive index of 1.46 and a relative index difference of 1%. It is envisaged that 20 point measurements will be made between the core axis and the cladding (inclusive). Estimate the percentage change in the Fresnel reflection coefficient which must be measured in order to facilitate these readings, assuming no index matching at the fiber input end face.

5.9

The fraction of light reflected at an air-fiber interface r is given by:

,~(",~l)' nl

+I

where nl is the fiber core refractive index at the point of reflection. Show that the fractional change in the core refractive index onl!nl may be expressed in terms of the fractional change in the reflection coefficient or/r following: 5nl_ nl

(~)

&-

1-r

r

Hence, show that for a step index fiber with n I of 1.5, a 5% change in r corresponds to only a I % change in n I'

5.10

A step index fiber has a nominal core refractive index of 1.48. The fiber input end face is immersed in oil with a refractive index of 1.51 prior to taking refractive index measurements using the end reflection method. Determine the anticipated resolution in the core refractive index measurement for a 2% change in the Fresnel reflection coefficient.

B.11

Compare

and contrast two simple techniques used for the m088uremont of the numerical aperture of optical fiberl.

227

OPTICA L FIBER M EASUREMENT S

Nu merical aperture mea suremen ts a re performed on a n o ptical Fiber, The angula r limit o f the far liekl pa ttern is found to be 26, 10 when t he finer is rotated from a cen ter ze ro poim. The far field panem is then di1>pla,.ed o n a screen where its size is measured as 16.7 em. Determine the numerical apert ure for the fiber and tht' d ist ance of the fiber O ~lpu l end face from the screen.

5.12

Describe, with the aid of a su itable diagram, the shado w metbcd used for thc on-line measurement of the o uter diam eter of a n optical fiber . The sh aJ o w met hod is used for th e measuremen t o f the o uter d iameter of an o ptical fiber. A fiber outer d iameter o f 347 p rn generates a shadow pulse o f SSO us ....-hen the rot ating mirro r has an a ngular velocity of 3 red s -I . Calc ulate the distance between the rot atin g mirror and the optical fi ber,

5.13

Oulline the major dcsign criteria of an optical fiber po wer meter for use in the rteld . Suggest any p roble ms a ssociated with field measurement s using such a de vice. Convert. the following optical power meter readings to numerical values of power: 2S dBm, - 5.2 dB m, 3.8 dBI!-

5.14

Describe what is meant by optical time dom ain reflectomct ry. Discuss how the technique may be used to lake tield measurements o n optical Il bers. Indicate t he advantag es of this technique o ver other measurement methods to determine attenuati on in optical fi bers. A backscatter plot for an o ptical fiber link provid ed by O T D R is sho wn in F ig. 5.25. Determine : (a} t he at tenuation o f the o ptical link for the regions indicated A , B a nd C in decibels per kilometer. (b) the insertion lou of the joint at the point X .

.-

Rei " ;""

,ul<..... t
• 1.0 - - - - - - - - - - - - - - -

" 10,0

'" .... U.

• 'I'ht

•

N'~lCtttt r

,

,

•

pfeil for til. oolicil link 01 protIl.rn 11.14.

,

II 228

5.15

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Discuss the sensitivity of QTDR in relation to commercial rcflcctometers. Comment on an approach which may lead to an improvement in the sensitivity of this measurement technique. The Rayleigh scattering coefficient for a silica single mode step index fiber at a wavelength of 0.80 urn is 0.46 km- 1 . The fiber has a refractive index of 1.46 and a numerical aperture of 0.14. When a light pulse of 60 ns duration at a wavelength of 0.80)..lm is launched into the fiber, calculate the level in decibels of the backscattcrcd light compared with the Fresnel reflection from a clean break in the tiber. It may be assumed that the fiber is surrounded by air.

Ans""ers to Numerical Problems 5.2

5.3 5.4 5.5

5.S 5.7 5.8 5.9

5.92 us km"!, ±0.13 dB 1.77 dB km- I 3.91 dB km- I 1.10 dB km"! 525.9 MHz km (a) 15.9 MHz k m: (b) 7.3 GHz km 0.26% 1.0'1'0

5.10

5.11 5.12 5.13 5.14 5.15

4.5 x 10- 4 0.44, 17.0 cm 21.0 em 316.2 mW. 302 IlW, 2.40 IlW (a) 2.0 dB km"", 3.0 dB km", 2.5 dB km I ; (b) 1.0 dB -37.3 dB

REFERENCES 1

2

3

I

I

4

5 6

7 8

9 10

11

M. Tateda, T. Horiguchl, M. Tokuda and N. Uchida, 'Optical loss measurement in graded index tiber using a dummy fiber', Appl. Opt.. 18(19). pp. 3272-3275, 1979. M. Eve, A. M. Hill, D. 1. Malyon, 1. E. Midwinter, B. P. Nelson, 1. R. Stern and J. V. Wright, 'Launching independent measurements of multimode fibres', 2nd European Conference on Optical Fiber Communication (Paris). pp. 143-146, 1976. ' M. Ikeda, Y. Murakami and K. Kitayuma, 'Mode scrambler for optical fibres'. Appl. Opi., 16(4), pp. 1045-1049, 1977. S. Seikai, M. Tokuda, K. Yoshida and N. Uchida, 'Measurement of baseband frequency response of multi mode fibre by using a new type of mode scrambler', Electron. Leu.. 13(5), pp. 146-147. 1977. 1. P. Dakin. W. A. Gambling and D. N. Payne, 'Launching into glass-fibre waveguide', Opt. Commun., 4(5), pp. 354-357, 1972. L. C. Cohen, P. Kaiser, P. D. Lazay and H. M. Presby. 'Fiber characterization', in S. E. Miller and A. G. Chynoweth (Eds.), Optical Fiber Telecommunications, pp. 343-400, Academic Press, 1979. B. Costa and B. Sordo, 'Fibre characterization', Optical Fibre Communication, Technical Staff of CSELT, pp. 145-308. McGraw-Hill, 1981. G. J. Cannell, R. Worthington and K. C. Byron, 'Measurement techniques" in C. P. Sandbank (Ed.), Optical Fibre Communications Systems, pp. 106-55.10hn Wiley, 1980. D. Charlton and P. R. Reitz, 'Making fiber measurements" Laser Focus, pp. 52-64, Sept. 1979. R. Boui1Jie and L. Jeunhommc, 'Measurement techniques for physical characteristics of optical fibers', ICC International Conference on Communication (Boston, USA), IEEE Pt 3, pp. 1--4, 1979. J. E. Midwinter, Optical Fibers for Transmission, John Wiley, 1979.

OPTI CAL FIBER MEASUR EM ENT S

12

13

14

15 16

17 18 19 20 21 22

23 24

25 28

27

28 29 30

31 32

33

229

K.. I. Whi te, 'A calo rimetric met hod fo r thc mcasur emem o f low o ptical absorption losses in o ptical communica tion fibres', Opt. Quant/un Electron., 8. pp. 73-75, 1976. K. I. White and J. E. Midwinter. 'A n im proved technique for the measurement of low o ptical absorption losses in bulk glass', Opto-eiectrontcs, " pp. 323- 334, 1973. A . R. Ty nes, 'Integrating cube scat tering detector" •.(ppl. Opt... 9( 12), pp. 210 6--2710, 1970 . F. W. Osterme yer and W. A. Benson, ' Integrating sp here for mea surin g scattering loss in optical fiber wa ~'egllides', Appl. Opt., U (8), pp. 1900- 1905, 1974. S. de Vito and 8. So rde, ' Misure di anenuazlone e diffusio ne in fibre otliche multi-modo' , LXX V Riuniuo ne AEI, Rome, 15-21 Sept, 19 74. J. P. Dakin. 'A simplified photo meter for r api d measu rement o f total scauermg auc nuation of fibre op tical waveguides' , Opt. Commun., 12( 1), PI'. 83---88, 1974. L G. Cohen, P. Kais.er a nd C . Lin, ' Ex perimental techniq ues for evefueuon of ti ber transmission lo ss a nd dispersion', r roc. l EE! ', 68( 10), pp. 1203-1208. 1980. S. D . Personick, ' Base band lineari ty a nd equalization in fiber optic d igital communication systems' , Bell SYSI . Tech. J., 52(7). pp. 1175-11 94, 1973. B. P. Lathi, Random Signals and Communication Theory, International T extboo k Company, 1968. D. Gloge, E. L. Chinnock an d T, P. Lee, 'Self pulsing O aA ~ laser for fiber dispersion measurement', IEEE J. Quantum E lectron., QE·8. pp. 844-846, 1972. F . Krah n, W. Meiningh aus and D . Rittich, ' Measu ring and les t eq uipment for optical ca ble', Phillips Teieco mmun. Ree.; 37(4), pp. 241 -249, 19 79 . L. G. C ohen, 'Shuttle pulse measurements of pulse spre ading in an optical fibre' , Appl. Opl.. 14(6), pp. 1351-1 356, 1975. I. K okayashi, M . Ko yama a nd K . A oyama, ' Meas ure ment of optical fibre t ransfer functions. by swept frequency technique and discussion of fibre ch aracteristics', Electron . Commu". Jpn, 6O-C(4), pp. 126-133, 19 77. W. E . Martin, ' Refractive index profile measurement s of diffused optical wavegu ides', App l. Opt.; 13(9), pp. 2112- 21 16, 1974. H . M. Presby , W. Ma mmel and R . M. D ero sier, ' Refractive ind ex profiling of gr aded inde'\ optical fibers ', Rf'V. Sci. Instr., 4 7(3), pp. 341\- 352, 1976. 8 . Costa and G. De Marchis, ' Test methods (o ptical fibres)', Telecomm. J. {Engl. Ed.) Switzerkm d; 48(11 ), pp. 666--673, 1981. L. G. Co hen. P. Kaiser, J. 8. MacChesney, P. B. O 'Co nner and H. M, Presby, ' T ransmission properties of a low-loss near-parabojic-inde a fiber' , Appl. Phys. Lett .. 26(8), pp. 472--4 74, 1975. C . Lin, ' Measurement techniques in fiber optics', IFOC tm, Fiber Opt. Commun. ( USA), 2(3), pp . 18- 20, 52-53. 198 1. M . E . Marhic, P. S. Ho and M. Epstein, ' N ondestructive refractive index profile measurement of clad optical fibers' , Appl. Phy t , Lell., 26(10), pp. 574- 575. 19 75. ' P. L. Chu, 'Measurements in optical fibres', Proc. IEEE Australia , 40(4), pp. 102- 114, 1979. F . E. M. Sladen, D . N . Payne and M. J. Ad ams, 'Determin a tion of op tical fibre refractive index profile by ncar fi eld scann ing technique', Appl. Phys. Lett., 28(5), pp, 255-258, 1976. , M, J. Adams, D. N. Pa yne and F. M. E, Sladen, ' Correction recto rs for deter" mination of optical fibre refr active-index profiles by nea r-Field sca nning technfquu', £1«tf'Ol1. Lm., 11( 11 ), PI', 281 -283, 1976.

..

230 34 35 36 37 38 39 40

41

42 43 44

45 46

47 48 49

50

51

52 53

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

W. Eickhoff and E. Weidel, 'Measuring method for the refractive index profile of optical glass fibres', Opt. Quantum Blectron.; 7, pp. 109-113,1975. B. Costa and B. Sordo, 'Measurements of the refractive index profile in optical fibres, comparison between different techniques', 2nd European Conference on Optical Fiber Communication (Paris), pp. 81-86, 1976. W. A. Gambling, D. N. Payne and H. Matsumura, 'Propagation studies 'on single mode phosphosilicate fibres', 2nd European Conference on Optical Fiber Communication (Paris), pp. 95-100, 1976. F. T. Stone, 'Rapid optical fibre delta measurement by refractive index tuning', Appl. Opt.; 16(10), pp. 2738-2742, 1977. L. G. Cohen and P. Glynn, 'Dynamic measurement of optical fibre diameter', Rev. Sci. Instrum., 44(12), pp. 1745-1752, 1973. H. M. Presby, 'Refractive index and diameter measurements of unclad optical fibres', J. Opt. Soc. Am., 64(3), pp. 280-284, 1974. P. L. Chu, 'Determination of diameters and refractive indices of step-index optical fibres', Electron. Len., 12(7), pp. 150-157, 1976. H. M. Presby and D. Marcuse, 'Refractive index and diameter determinations of step index optical fibers and preforms', Appl. Opt., B( 12), pp. 2882-2885, 1974. D. Smithgall, L. S. wakins and R. E. Frazee, 'High-speed ncncontact fibrediameter measurement using forward light scattering', Appl. opt; 16(9), pp. 2395-2402, 1977. F. Krahn, W. Meininghaus and D. Rittich, 'Field and test measurement equipment for optical cables', Acta Electronica, 23(3), pp. 269-275, 1979. R. Olshansky, M. G. Blankenship and D. B. Keck, 'Length-dependent attenuation measurements in graded-index fibres', Proceedings of 2nd European Conference on Optical Communication (Paris), pp. 111-1l3, 1976. M. K. Barnoski and S. M. Jensen, 'Fiber waveguides: a novel technique for investigating attenuation characteristics', Appl. Opt., 15(9), pp. 2112-2115, 1976. S. D. Pensonick, 'Photon probe, an optical fibre time-domain reflectometer', Bell Syst. Tech. J., 56(3), pp. 355-366, 1977. E. G. Newman, 'Optical time domain reflectometer: comment', Appl. Opt., 17(11), p. 1675, 1978. E. A. Lacy, Fiber Optics, Prentice-Hall, 1982. M. K. Bamoski and S. D. Personick, 'Measurements in fiber optics', Proc. IEEE, 66(4), pp. 429-440, 1978. J. D. Archer, Manual ofFibre Optics Communication, STC Components Group, UK, I'J~L P. Healey, 'Optical time domain reflectometry by photon counting', 6th European Conference on' Optical Communication (UK), pp. 156-159, 1980. P. P. Webb et al., 'Single photon detection with avalanche photodiodes', Bull. Am. Phys. Soc. If, IS, p. 813, 1970. P. Healey, 'OTDR by photon counting', lEE Colloquium on Test Equipmentfor Optical Fibre Commun. Syst. (London), paper 4/1, May 1981.

6 Optical Sources 1: The Laser

6.1

INTRODUCTION

T he o ptic al source is often considered to be the a ctive component in an optical fiber communication system. Its fund amental function is to con vert electrical energy in the form of a current into optic al energy (light) in an efficient manner which allows the light output to be effectively launched or coupled into the

optical fiber. Three main types of optical light source are available. These are: (a) wideband 'continuous spectra' sources (incandescent lamps); (b) monoc hromatic incoherent sources (light emitting diodes' LEOs); (e) monochromatic coheren t sources (la sers).

To aid consideration of the sources currently in major use the historical aspect must be men tioned. In the earl y stages of o ptical liber communication the most powerful n arrowband coherent light sources were necessary d ue to severe attenu atio n and dispersio n in the fibers. Th erefore initially gas lasers (helium-neon) were utilized. However, the develop ment of the semico nductor injection laser and the LE D, together with the substantial improvement in the properties of optical fibers, has given prominence to these two specific sourc es. To a large extent these two sources fulfil the major requirem ents for an o ptical fiber emitter which arc outlined below.

(a ) A size and configuration compatible wn b launch ing light into an o ptical fiber. Ideally the light output sho uld be highly directio nal. (b) Mu st accurately track the electrical i.nput signal to minimize distortion and noise. Ideally the source should be linear. (c) ShouJd emit light at wavelengths where the fiber has low losses and low dispersion and where the d etectors are efficient . (d) Preferably capable of simple sign al modulation (i.e. direct- see Section 7.5) over a wide bandwidth extending from a udio frequencies to beyond the aigahertz range. (e) Must couple sufficient optical power to overcome attenuatio n in the fi ber plu. additional connector losses and leave adequate power to drive the detector. 231 .'

,

232

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

(f) Should have a very narrow spectral bandwidth (linewidth) in order to minimize dispersion in the fiber. (g) Must be capable of maintaining a stable optical output which is largely unaffected by changes in ambient conditions (e.g. temperature). (h) It is essential that the source is comparatively cheap and highly reliable in order to compete with conventional transmission techniques.

In order to form some comparison between these two types of light source the historical aspect must be enlarged upon. The first generation optical communication sources were designed to operate between 0.8 and 0.9 urn (ideally around 0.85 1IDl) because initially the properties of the semiconductor materials used lent themselves to emission at this wavelength. Also as suggested in (c) this wavelength avoided the loss incurred in many fibers near 0.9 urn due to the OH ion (see Section 3.3.2). These early systems utilized multimode step index fibers which required the superior performance of semiconductor lasers for links of reasonable bandwidth (tens of megahertz) and distances (several kilometers). The LED (being a lower power source generally exhibiting little spatial or temporal coherence) was not suitable for long distance wideband transmission, although it found use in more moderate applications. However, the role of the LED as a source for optical fiber communications was enhanced following the development of multimode graded index fiber. The substantial reduction in intermedal dispersion provided by this fiber type over multimode step index fiber allowed incoherent LEDs emitting in the 0.80.9 urn wavelength band to be utilized for applications requiring wider bandwidths. This position was further consolidated with the development of second generation optical fiber sources operating at wavelengt¥ between 1.1 and 1.6 urn where both material losses and dispersion are greatly reduced. In this wavelength region wideband graded index fiber systems utilizing LED sources may be 0E~r.!1:t_t?~Lo.Y~TJQngciistances, without the need for intermediate repeaters. Furthermore, LEDs offer the advantages of relatively simple construction and operation with the ~rent effects of these factors on cost .end _extended, trouble-free life. In parallel with these later developments in multimode optical propagation came advances in single mode fiber construction. This has stimulated. the development of single mode laser sources to take advantage of the extremely low dispersion offered by single mode fibers. These systems are ideally suited to extra wideband, very long-haul applications and are currently under intensive investigation for long-distance telecommunications. On the other hand, light is usually emitted from the LED in many spatial modes which cannot be readily focused and coupled into single mode fiber. Hence to date the LED has been utilized almost exclusively as a multimode source which will only give adequate coupling efficiency into multimode fiber. However, in this role the LED has become a primary multimode source which is extensively

'~

/

OPTICAL SOURCES 1;, THE LASER

233

used for increasingly wider bandwidth, longer-haul applications. Therefore at present the LED is chosen for many applications using multimode fibers and the injection laser tends to find more use as a single mode device in single mode fiber systems. Although other laser types (e.g. Nd:YAG laser, see Section 6.11) as well as the injection laser may eventually fmd limited use in optical fiber communications, this chapter and the following one will deal primarily with major structures and configurations of semiconductor sources (injection laser and LED) taking into account recent developments and possible future advances. We begin by describing in Section 6.2 the basic principles of laser operation which may be applied to all laser types. Immediately following in Section 6.3 is a discussion of optical emission from semiconductors in which we concentrate on the fundamental operating principles, the structure and the materials for the semiconductor laser. Aspects of practical semiconductor injection lasers are then considered in Section 6.4 prior to a more specific discussion of the structure and operation of multimode devices in Section 6.5. Following in Section 6.6 is a brief discussion of the single mode injection laser which provides a basis for the description of the major single mode structures presented in Section 6.7. As the preceding sections have primarily dealt with injection lasers operating in the shorter wavelength region (0.8--0.9 11m), a brief account of longer wavelength (1.1-1.6 11m) devices is given in Section 6.8. In Section 6.9 we describe the operating characteristics which are common to all injection laser types before a short discussion of injection laser to optical fiber coupling together with device packaging is presented in Section 6.10. Finally, in Section 6.11 nonsemiconductor lasers are briefly considered, the discussion concentrating on the neodymium-doped yttrium-aluminum-garnet (Nd :YAG) device.

6.2

BASIC CONCEPTS

To gain an understanding of the light-generating mechanisms within the major optical sources used in optical fiber communications it is necessary to consider both the fundamental atomic concepts and the device structure. In this context the requirements for the laser source are far more stringent than those for the LED. Unlike the LED, strictly speaking, the laser is a device which amplifies light. Hence the derivation of the term LASER as an acronym for Light Amplification by Stimulated Emission of Radiation. Lasers, however, are seldom used as amplifiers since there are practical difficulties in relation to the achievement of high gain whilst avoiding oscillation from the required energy feedback. Thus the practical realization of the laser is as an optical oscillator. The operation of the device may be described by the formation of an electromalnetic standing wave within a cavity (or optical resonator) which pro'vide. an output of monochromatic highly coherent radiation. By contrast

:1 :

234

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

the LED provides optical emission without an inherent gain mechanism. This results in incoherent light output. In this section we elaborate on the basic principles which govern the operation of both these optical sources. It is clear, however, that the operation of the laser must be discussed in some detail in order to provide an appreciation of the way it functions as an optical source. Hence we concentrate first on the general principles of laser action. 6.2.1

Absorption and Emission of Radiation

The interaction of light with matter takes place in discrete packets of energy or quanta, called photons. Furthermore the quantum theory suggests that atoms exist only in certain discrete energy states such that absorption and emission of light causes them to make a transition from one discrete energy state to another. The frequency of the absorbed or emitted radiationfis related to the difference in energy E between the higher energy state £2 and the lower energy state E 1 by the expression: (6.1)

where h = 6.626 X 10-34 J s is Planck's constant. These discrete energy states for the atom may be considered to correspond to electrons occurring in particular energy levels relative to the nucleus. Hence different energy states for the atom correspond to different electron configurations, and a single electron transition between two energy levels within the atom will provide a change in energy suitable for the absorption or emission of a photon. It must be noted, however, that modern quantum theory [Ref. I) gives a probabilistic description which specifies the energy levels in which electrons are most likely to be found. Nevertheless, the concept of stable atomic energy states and electron transitions between energy levels is still valid. Figure 6.1(a) illustrates a two energy state or level atomic system where an atom is initially in the lower energy state E 1 • When a photon with energy (E 2 - £1) is incident on the atom it may be excited into the higher energy state E 2 through absorption of the photon. This process is sometimes referred to as stimulated absorption. Alternatively when the atom is initially in the higher energy state E 2 it can make a transition to the lower energy state £1 providing the emission of a photon at a frequency corresponding to Eq. (6.1). This emission process can occur in two ways: (a) by spontaneous emission in which the atom returns to the lower energy state in an entirely random manner; (b) by stimulated emission when a photon having an energy equal to the energy difference between the two states (E 2 - E I) interacts with the atom in the upper energy state causing it to return to the lower state with the creation of a second photon.

OPTICAL SOU RCES 1: THE LASE R

(."J7

,!>,~

I~it


IV

.... ....'"

235

_

•

• I

~

I

I

I C,,-

"-

-

_

-

I-

-

I

I

~

S ~m " J .kJ

,

<m...."'"

I I

I, '

fig . 6 .1

•

Energy sta le- d iagra m s ho '#\' ing: ·(01) s bsorpuon: lb ) sponta neous e rmss'on: 11:1st imulate-d emission, The blac1 d ill indic ates the s la te of Ihe ato m before and aft e r a tran s ition ta kes plac e.

Th ese two emission processe s are illustrated in Figs. 6.1(b) and (c) respectively. The random nature of the spontaneous emission process where light is emitted by electronic transitions from a large number of atoms gives incoherent radiation. A similar emission process in semiconductors provides the basic mechanism for light generation within the LED (see Section 6.3.2). It is the stimulated emission process, however, which gives the laser its special properties as an optical source. Firstly the photon produced by stimulated emission is generally" of an identical energy to the one which caused it and hence the light associated with them is of the same frequency. Secondly the light associated with the stimulating and stimulated photon is in phase and has the same polarization. Therefore, in contrast to spontaneous emission, coherent radiation is obtained. Furthermore this means that when an

• It. pfloton with enct&Y "'will not necnn ril!( always stimulate Ulotber pholOll with «I«'gJ PbokmJ may be .timulated over a small rani!! of CDCf"IieI UOllocl l(providin, an anisIIOClwlUW bu • fWtt lNqUll'lC')' or "". veIe:\rlb Ipread (lin1!Vt'idlb).

v:

•

•

236

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

atom is stimulated to emit light energy by an incident wave, the liberated energy can add to the wave in a constructive manner, providing amplification.

6.2.2

The Einstein Relations

Prior to discussion of laser action in semiconductors it is useful to consider optical amplification in the two level atomic system shown in Fig. 6.1. In 1917 Einstein [Ref 2] demonstrated that the rates of the three transition processes of absorption, spontaneous emission and stimulated emission were related mathematically. He achieved this by considering the atomic system to be in thermal equilibrium such that the rate of the upward transitions must equal the rate of the downward transitions. The population of the two energy levels of such a system are described by Boltzmann statistics which give:

(6.2) ~

g,

-

g,

oxp (Iif/KT)

where N[ and N z represent the density of atoms in energy levels E 1 and E z respectively with gl and gz being the corresponding degeneracies" of the levels, K is Boltzmann's constant and T is the absolute temperature. As the density of atoms in the lower or ground energy state E[ is N 1 , the rate of upward transition or absorption is proportional to both N j and the spectral density PI of the radiation energy at the transition frequency f Hence the upward transition rate R j 2 (indicating an electron transition from level I to level 2) may be written as: R 12 = N 1 PiB12

(6.3)

where the constant of proportionality BIZ is known as the Einstein coefficient of absorption. By contrast atoms in the higher or excited energy state can undergo electron transitions from level 2 to level 1 either spontaneously or through stimulation by the radiation field. For spontaneous emission the average time an electron exists in the excited state before a transition occurs is known as the spontaneous lifetime If the density of atoms within the system with energy £z is N 2 • then the spontaneous emission rate is given by the product of N z and

'21 .

... In many cases the atom has several sublevels of equal energy within an energy level which is then said to be degenerate. The degeneracy parameters 81 and 82 indicate the number of sublevels within the enerlY level. E l and E l respectively. If the system is not delenerale tben 81 and 81 may b, Nt to unity lRef. 1l,

OPTICAL SOURCES 1; THE LASER

237

Ih ll • This may be written as N 2.A 21 where A 21 • the Einstein coefficient of spontaneous emission . is equal to the reciprocal of the spontaneo us lifetime. The rate of stimulated d ownward transition of a n electron from level 2 10 level I may be obtained in a similar manner 10 tbe rate of stimulated upwa rd transition . Hence the rate of stimulated em ission is given by N 2Pf B 11• where B ll is the Einstein coefficient of stimulated emission. The total transition rate from level 2 to level 1, R 21 • is the sum of the spontaneous and stimulated con tributions . Hence: R 21 = N 2A u + N 1 Pj B1 l

(6.4)

For a system in th erm al equilibrium. the upward and downward transition rates must be equal and therefore R It = R 21 , or (6.5) It follows that ;

and

Pr ~ =

o4 2.1B 21

-c:-';-:,-":.,.---:-

(6.6)

( B 12N, / B2INl ) -1

Substituting Eq. (6.2) into Eq. (6.6) gives

All /B l l Pr ~ ~-=--=--=-",-..:.c.==,..-,­ ((xl B12/g2B21 ) exp (Iif/KD 1- 1

(6.7)

However, since the atomic system under consideration is in thermal equilibrium it produces a radiation density which is identical to black body radiation. Planck showed that th e rad ia tion spectral density for a black body radiating within a frequency range f to f + qr is given by IRef. 31:

8Mt' (

~ ~ c'

I

)

exp (hl /KT) - I

(6 .8)

Comparing Eq. (6.8) with EQ. (6. 7) we obtain the Einstein relations : (6.9)

and (6. 10) .'

I',

I,

I

I

238

OPTICA L FIBER COM MU NICATIONS: PRINCI PLES A ND PRACTiCE

It ma y be o bserved from Eq. (6.9) that when the degeneracies of the two levels are eq ual (gl = K:) then the probabilities of absorption and stim ulated emission are equal. Furthermore, th e rat io of the stimulated emission rate to the spontaneo us emission rate is given by:

B 21 P.r

stimulated emission rate

J ~

spontaneous emission rate

An

exp Vif"lKn - I

(6.11)

Exampl. 8 .1

Calc ula te th e ra tio of t he stimulated e miss io n ra te 10 the spo nta ne o us emi ssio n ra te fo r a n incandescent lamp oper atin g 818 te mpe ratu re of 1000 K. It ma y be assume d tha t th e ave ra ge opera ting wa ve len gth is 0. 5 urn. S olution : Thll average operating frequency is given by :

2 ,9 98 x 10 8

c ( = -

~6 .0 "

=

).

0.6 " 10 ..-6

10

.

HI

Us ing Eq . (6 . 1 1) thll ra tio re : stim l,llaled em ission ra te

1

seootaeeous em ission ra te

e xp ( 6 .6 26 " 10 -3<1 K 6 x 10

14

)

1.3 8 1 x 10 - 23 )( 1000

= exp 1- 2 8 .8 ) =

3 .1 " 10- 13

The resu lt obtained in example 6.1 indicates th at for systems in thermal equilibrium sponta neous emission is by far the dominant mechanism. Furthermore it illustrates th at the radiation emitted from ordinary o ptical sources in the visible spect rum occurs in a random manner, proving these sources are incoherent. It is apparent th at in order to produce a coherent optical source and amplification of a light beam the r ate of stimulated emission must be increased far above the level indicated by example 6.1. From consideration of Eq. (6.5) it ma y be noted that for stimulated emission to dominate over absorption a nd spontaneous emission in a two level system both the radia tion density and the population density of the upper energy level N z must be increased in relation to the population density of the lower energy level N 1 •

6.2.3

Population Inver.lon

Under the conditions of thermal equilibrium given by the Boltzmann distribution (Eq. (6.2» the lower energy level E , of the two level atomic system contains more atoms than the upper energy level E z• This situation which it

OPTICAL SOURCES 1: THE LASER

239

normal for structures at room temperature is illustrated in Fig. 6.2(a). However, to achieve optical amplification it is necessary to create a nonequilibrium distribution of atoms such that the population of the upper energy level is greater than that of the lower energy level (i.e. N 2 > N]). This condition which is known as population inversion is illustrated in Fig. 6.2(b). In order to achieve population inversion it is necessary to excite atoms into the upper energy level E 2 and hence obtain a nonequilibrium distribution. This process is achieved using an external energy source and is referred to as 'pumping'. A common method used for pumping involves the application of intense radiation {e.g, from an opticaillash tube or high frequency radio field). In the former case atoms are excited into the higher energy state through stimulated absorption. However, the two level system discussed above does not lend itself to suitable population inversion. Referring to Eq. (6.9), when the two levels are equaJly degenerate (or not degenerate) then 8]2 = 8 2 ] , Thus the probabilities of absorption and stimulated emission are equal, providing at best equal populations in the two levels. Population inversion, however, may be obtained in systems with three or four energy levels. The energy level diagrams for two such systems which correspond to two nonsemiconductor lasers are illustrated in Fig. 6.3. To aid attainment of population inversion both systems display a centraJ metastable state in which the atoms spend an unusually long time. It is from this metastable level that the stimulated emission or lasing takes place. The three level system (Fig. 6.3(a)) consists of a ground level Eo, a metastable level E] and a third level above the metastable level E 2 • Initially the atomic distribution will follow the Boltzmann law. However, with suitable pumping the electrons in

,

Energy IE)

I I \ Ic",

",

\"pl \

\'''1'( t../ieF) \

F/i:I)

,

", \

, ,

,, , ,

,

-,

-,

",

,,

: ,'

"l 1"' _ _ N,

Den,i'}' of

Dc,,,i',' of

""o>«·\')

,iomd.\') (I»

Flg.8.2

Populations in a two energy level system: lal Boltzmann distribution for a ,y'tem in thermal equlllbrlum: (bl a nonequltlbrlurn ctsntooncn showing popula110n Inversion. '

240

OPTICAL FIBER COM MU NICATIONS: PRINCIPLES AND PRACTICE

•

c.

.....

,, --

d.. ""

• --- -- - -

"'''''''''= • -- - -

P . ",~ "'lI:

o S1 t• FIg .6.3

--

,,-

~

,.>

.

It: . } ol ~"

,~l~

,~ .

1141"d 4."a~

••

'"

Energy level di agrams sho w ing population inversion and lasing for t wo no nsem iconductor lasers : la) t hree level system-ruby (cryst al) laser : (b) four level svstern-c-He-Ne Igasl laser.

some of t he atoms may be excited from the ground state into th e higher level E 2 _ Sin ce £ 2 is a normal level the electrons will ra pidly decay by nonradiative processes to either E I o r directly to E o- Hence empty states will always be provided in £ 2_ The metastable level E 1 exhibits a much longer lifetime than £ 2 wh ich allows a large number of a to ms to accumulate at E I • O ver a period the density of atoms in the meta stable state N I increa ses above those in the ground st ate No and a population inversion is obt ained bet ween these two levels. Stimulated emission and hence la sing can then occur creating radiative electron transitions between levels E l and Eo. A drawback with the three level sy stem such as the ruby laser is that it generally req uires very high p ump powers because the terminal state o f th e laser transition is the gro und state. Hence more than half the grou nd state atoms must be pumped into the meta sta ble sta te to achieve population inversion. By co ntrast a four level system such as the He-Ne laser illustrated in F ig. 6.3(b) is characterized by much lower pumping requirements. In this case the p umping excites the atoms fro m the ground state into energy level E J and they d ecay rapidly to the metastable level £1' However. since the populations of E; and £1 remain essentially unchanged a small increase in the number of atoms in energ y level £ 1 creates population inversion , and lasing takes plac e between this level and level E I '

8.2.4

Optical Feedback end La.er Oscillation

Light amplification in the laser OCC\lrJ when a photon conidin. with an atom. in the excited C:llerar ,talc UUs.e1 the ltimulued c:millioo of a tccond photon and

OPTICAL SOURCES 1: THE LASER

l.

24'

then both these photons release two more. Continuation of this process effectively creates avalanche multiplication, and when the electromagnetic waves associated with these photons are in phase, amplified coherent emission is obtained. To achieve this laser action it is necessary to contain photons within the laser medium and maintain the conditions for coherence. This is accomplished by placing or forming mirrors (plane or curved) at either end of the amplifying medium as illustrated in Fig. 6.4. The optical cavity formed is more analogous to an oscillator than an amplifier as it provides positive feedback of the photons by reflection at the mirrors at either end of the cavity. Hence the optical signal is fed back many times whilst receiving amplification as it passes through the medium. The structure therefore acts as a FabryPerot resonator. Although the amplification of the signal from a single pass through the medium is quite small. after multiple passes the net gain can be large. Furthermore, if one mirror is made partially transmitting, useful radiation may escape from the cavity. A stable output is obtained at saturation when the optical gain is exactly matched by the losses incurred in the amplifying medium. The major losses result from factors such as absorption and scattering in the amplifying medium. absorption, scattering and diffraction at the mirrors and non-useful transmission through the mirrors. Oscillations occur in the laser cavity over a small range of frequencies where the cavity gain is sufficient to overcome the above losses. Hence the device is not a perfectly monochromatic source but emits over a narrow spectral band. The central frequency of this spectral band is determined by the mean energy level difference of the stimulated emission transition. Other oscillation frequencies within the spectral band result from frequency variations due to the thermal motion of atoms within the amplifying medium (known as Doppler broadening") and by atomic cotnsionst. Hence the amplification within the

Opt;"1 • wl!,u!

,-------------------

\I;'",r

Flg.6.4

\lirror

The basic laser structure incorporating plane mirrors.

• Doppler broadening is referred to as an inhomogeneous broadening mechanism since Individual groups of atoms in the collection have different apparent resonance Frequencies. t Atomic collisions provide homogeneous broadening as every atom ill the collection has the .Ime relonant frequency and spectral spread.

242

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

laser medium results in a broadened laser transition or gain curve over a finite spectral width as illustrated in Fig. 6.5. The spectral emission from the device therefore lies within the frequency range dictated by this gain curve. Since the structure forms a resonant cavity, when sufficient population inversion exists in the amplifying medium the radiation builds up and becomes established as standing waves between the mirrors. These standing waves exist only at frequencies for which the distance between the mirrors is an integral number of half wavelengths. Thus when the optical spacing between the mirrors is L the resonance condition along the axis of the cavity is given by [R'f. 41,

Aq

L~­

(6.12)

2n

I

I,

where A is the emission wavelength, n is the refractive index of the amplifying medium and q is an integer. Alternatively discrete emission frequenciesfare defined by

I I

(6.13)

where c is the velocity of light. The different frequencies of oscillation within the laser cavity are determined by the various integer values of q and each constitutes a resonance or mode. Since Eqs. (6.12) and (6.13) apply for the case when L is along the longitudinal axis of the structure (Fig. 6.4) the frequencies given by Eq. (6.13) are known as the longitudinal or axial modes. Furthermore from Eq. (6.13) it may be observed that these modes are separated by a frequency interval of where: c

oJ~­

(6.14)

2nL

){"I,li,,' ,""I,li(i,,'iol,

(;"ilO

,'eIVe'

"rnv.'lop,'

Fig.6.5

The relative amplification in the laser amplifying medium 8howlng the broadened laser transition line or gain curve.

243

OPTICAL SOURCES 1: THE LASER

Example 6.2

A ruby laser contains a crystal length 4 cm with a refractive index of 1.78. The peak emission wavelength from the device is 0.55 urn. Determine the number of longitudinal modes and their frequency separation. Solution: The number of longitudinal modes supported within the structure may be obtained from Eq. 16.12) where: 2nL 2 x 1.78 x 0,04 q~--= =2.6xl0 5

A

0.55xl0

6

Using Eq. (6.14) the frequency separation of the modes is: 2.998 X 10 8

6f

2 x 1,78 x 0,04

=2.1 GHz

Although the result of example 6.2 indicates that a large number of modes may be generated within the laser cavity, the spectral output from the device is defined by the gain curve. Hence the laser emission will only include the longitudinal modes contained within the spectral width of the gain curve. This situation is illustrated in Fig. 6.6 where several modes are shown to be present in the laser output. Such a device is said to be multimode. Laser oscillation may also occur in a direction which is transverse to the axis of the cavity. This gives rise to resonant modes which are transverse to the

,

1""",i'

,

,

,

-1-1--

,I lI. ,I , ,

,, I \ \

\

pt,_ 1.1

,,

,0> (I!I) The modes in the Issar cavity, (bl The longitudinal modes in the laser output.

I 244

OPTICAL FIBER COM M UNICATIONS : PRINCIPLES AN D PRACTICE

L,,,,', "I,l i..1 ,. ,'i1~

,,,;,,,,,

n " ...

Rg.6.7

rUt,.

TE~ "

The low er c rce r tra nsve rse mod es a f a lase r.

direction of propagation. These transverse elect romag netic modes a re designated in a similar manner to transverse modes in waveguides (Section 2.3.2) by TEM[m where the integers I and m indicate the number of tr ansverse modes (see Fig. 6.7). Unlike the longitudinal modes which contribute o nly a single spot of light to the laser output, tr ansverse modes may give rise to a penern of spots at the output. This may be observed from the low order transverse mode patterns shown in Fig. 6.7 on which the direction of the electric field is also indicated. In the case of the TEM ou mode all pans of the propagating . wavefront a re in phase. This is not so, however, with higher o rder modes (TEM Hl , T EM u ' etc.) where phase reversals produce the various mode patterns. Th us the greatest degree of coherence together with the highest level of spectral purity may be obtained from a laser which operates in only the TEM co mode. H igher order transverse modes only occ ur when the width of the cavity is sufficient for them to oscillate. Consequent ly they ma y be eliminated by suitable narrowing of the laser cavity.

6 .2.5

TIlt••hoId Condition for Leser Oscillation

It has been indicated that steady state conditions for laser oscillation are achieved when the gain in the amplifying medium exactly balances the tot al losses." Hence although population inversion between the energy levels providing the laser transition is necessary for oscillation to be esta blished, it is not alone sufficient for lasing to occur. In addition a minimu m o r threshold gain within the amplifying med ium must be attained such th at laser oscillations are initia ted and sustained. This threshold gain may be determined by considering the c hange in energy of a light beam as it passes through the amplifying medium. F or simplicity all the losses except those due to transmission through the mirrors may be included in a single loss coefficient per unit length crrr ' . A gain we assume the amplifying med ium occupies a length L completely filling the region between the two mirrors which ha ve ref lectivi ties 1'1 a nd 1'1' On each

a

• This a pplies to C W luer w~.K:h give!! a co:UinuoUI output" rather than pulsed devices for which diihtly difJcn:l1 cooditior.s nist. for OKillation to ~m:r.e nce the fractional i lin InCI lou mun be mlttc!lcd.

245

OPTICAL SOURCES 1: THE LASER

round trip the beam passes through the medium twice. Hence the fractional loss incurred by the light beam is: Fractional loss = r l "a e-1iiL

(6.15)

Furthermore it is found that the increase in beam intensity resulting from stimulated emission is exponential [Ref. 41. Therefore if the gain coefficient per unit length produced by stimulated emission is it cm-! , the fractional round trip gain is given by Fractional gain = e2jL

(6.16)

Hence

and (6.17) The threshold gain per unit length may be obtained by rearranging the above expression to give: _ _ 1 1 g'h=u+-In-2£ r l r,

(6.18)

/ The second term on the right hand side of Eq. (6.18) represents the transmission loss through the mirrors." For laser action to be easily achieved it is clear that a high threshold gain per unit length is required in order to balance the losses from the cavity. However it must be noted that the parameters displayed in Eq. (6.18) are totally dependent on the laser type.

6.3 6.3.1

OPTICAL EMISSION FROM SEMICONDUCTORS The

o-n

Junction

To allow consideration of semiconductor optical sources it is necessary to review some of the properties of semiconductor materials, especially with regard to the p-n junction. A perfect semiconductor crystal containing no impurities or lattice defects is said to be intrinsic. The energy band structure IRef. II of an intrinsic semiconductor is illustrated in Fig. 6.8(a) which shows the valence and conduction bands separated by a forbidden energy gap or bandgap E~, the width of which varies for different semiconductor materials. • Thll term II sometimes .expreased in the form mirrored endl, II equal to Y(rlrJ.

in: In i/r,

where r. the reflectivity of the

246

OPTI CAL FIB ER COMMU NICATi ONS: PRINCIPLES AND PR ACTICE

c,, "~U< ,

..'" "'...J

•• • ••• • • • ..

..

m

r

' - - - - - - - --------.J • I I.·. , ,,,. .

", Fig. 6.8

~

,, ---,, _.-

,, , ,, ,

11,,1,..

• , ",

(81 Th e e nergy band st ruc w re o f an intrins ic semicond uctor at a te mperat ure a bo ve absolut e zer o s ho w in g a n e q ua l n umbe r o f etect rc e s a nd ho les in t he cond uctio n ba nd a nd The va ler'l c e ba nd res pective ly. Ib) The Fe rm i- Dira c p robab ility d ist ribution co rres po nding to [a].

Fig ure 6.8(a) shows the situation in the semiconductor at a temperat ure above a bsolute zero where thermal excitation raises some electrons from the valence band into the conduction band leaving empty hole states in the valence ba nd. These thermally excited electrons in the conduction band and the holes left in the valence band allow conduction throu gh the material. a nd are called earners. For a semiconductor in the rmal equilibrium the energy level occupation is desc ribed by the Fermi-D irac distribution function (rather than the Boltzmann). Consequently the probability P(E) that an electr on gains suffi cient therm al energy at a n absolute temperature T t hat it will be found occupying a part icular energy level E. is given by the F ermi-Dirac distribution (Ref. I I: (6.19)

where K is Boltzmann's constant and E F is known as the Fer mi energy or Fermi level. T he Fermi level is only a mathematical parame ter but it gives an indication of the distribution of carriers within the mat erial. This is shown in Fig. 6.8(b) for the intrinsic semiconductor where the Fermi level is at the center of the bandgap, indicating that there is a small probability of electrons occupying e nergy levels at the bottom of the conduction band and a corresponding number of holes occupying energy levels at the top of the valence band. To create an extrinsic semiconductor the material is doped with impurity atoms which either create more free electrons (donor impurity) or holes (a cceptor impurity). These two situations are shown in Fia_ 6.9 where the donor impurities form energy levels j ust below the conduction band whilst acceptor impurities .form eneray levels j ust above the VBJ CDCC band.

247

OPTICAL SOURCES 1: THE LASER

["",,,, r - - - -- - - - -, (...... 100<...... .....

• •• •••• ••• • • • •• ------ - -----!.,-

--£,.-- " 'CO Pl O' " Il I'" , i l y l, ,'
----- --

(I (I

_,,,_I

,

c

_

.,

G

"

Fig.6.9

El"lergv band d iagrams: (a) " t yp e

t'" ~ m;cond uctor ,

(bl p type sem icond uc tor,

When donor impurities are ad ded , thermally excited electrons from the donor levels are raised into the con duction band to create an excess o f negative charge ca rriers and the semiconductor is said to be n type, with the majority carriers being electrons. The Fermi level corresponding to this carrier distribut io n is raised to a position above the center of the bandgap as illustrated in Fig, 6.9(a). When accepto r impurities a re added as sho wn in Fig. 6.9(b) thermally excited electrons are raised fro m the va lence band to the acceptor imp urity levels leaving an excess of positive charge carriers in the valence band and creating a p type semiconductor where the majority carriers are holes. In \ this case the Fermi level is lowered belo w the centre of me bandga p. The (HI junction diode is formed by creating adjoining p and n type sem ico nductor layers in a single crysta l as shown in Fig. 6.1O(a). A thin depletion region or layer is fo rmed at the junction thro ugh carrier recombination which effectively leaves it free of mobile charge carriers (both electrons and holes). This establishes a potential barrier between the p and n type regions which restricts the interdifTusion of majority carriers fro m their respective regions a s illustrated in Fig. 6.IO(b). In the absence o f an externally a pplied voltage no current flows as th e potential barrier prevents the net flow of carriers from on e region to another. W hen the j unction is in this equilibrium state the Fermi level for the p and n type semiconductor is the same as sho wn in Fig. 6.1O(b). The width of the deplet ion region a nd thus the magnitud e of the potential barrier is dependent upo n the carrier concentrations (doping) in the p and n type regions, and any extern al applied voltage. When an external positive voltage is applied to the p type region with respect to the n type, both the depletion region width and the resulting potential barrier are reduced and the diode is said to be forward biassed. Electrons from the n type region and holes from the p type region can flow more read ily across the j unction into the opposite type reaiol1. These minority carriers are effectively injected across the j unction by the appUcatioa or the external voltaae end form a current tlow th rough the

248

OPTICAL FIBER COM MUNICATIONS: PRINCIPLES A ND PR ACTICE ' ''ple tion 10\<,,,

.:•

p .t J I't 0

,.,

0

0 0

e 0

•

0

c

e

u ~ Yi><

o

0 0

, e ,) e o

e

" • •0 0

; ,0

0

. I

0

9

e

0

•

0

a

I

e I

0

•• ••

• • :• •

•

•

• Ek<.1Jom ~

•

o

II<.."

il.< <
. ~

'- - - - --- ,;-- , - ----,-- - - •

•

I I _ _ _ _ _ _ 1__.__ _ _ _

,0>

-- -

--

G €l

',;; ,> c C O Cl I ~)()O

I L

_

J

O () I

o

N"....

@

(i)

1 · '

,

e I•••••• -1-

_

_

I ,

I

a

I

Fig . 6 .10

""'on,..1

I@

J

"----+-'_ ,----__ 0

fa) Th e im p urities .,nd c harge c arr iers at a p-rt junctio n . fb) Th e e nergy band

di a g ram cor respo nd ing 10 (a ).

dev ice as they continuously ditfuse a way from the interface. Ho wever, this situa tio n in suitable semicond uc to r materials allows carrier recombina tion with the emission of light .

6 .3.2

Spontaneous Emis s ion

The increased concentration of minority carriers in the opposite type region in th e forward biassed p--n d iode leads to the recombination of carriers acro ss the bandga p. This process is shown in F ig. 6. 11 for II d irect bandgap (see Section

••• • ••••

__1.

- ---------1- - ± -----,•

.

"

<j O o o r) (l r) O
c " ;,c o

0

(";

C

<'io ' - ............. ~ O C O ') C 0':". 0

fig . 1 .11

II ......

The p-n junction wit" torwl rd biN g'vlng Ij)OnllnKl\.oI .mlilion

at photoN..

249

OPTICAL SOURCES 1 : THE LASER

6.3.3) semiconduct or materia: where the normally empty electron states in the conduction band o f the p type material a nd the normally empty hole states in the valence band o f the n type material are popu lat ed by injected carriers w hich recom bine a cross the bandgap. The energy released by thi s electronhole recom bin ation is approximately equal to th e bandgap energy E8 • E xcess carrier population is therefore dec rea sed by recombin ation which may be r ad iative or nonradiative. In nonradiative recombina tion the energy released is dissipated in the form of lattice vib rat ion s and th us heal. However, in ba nd to band radiative recombination the energy is released with the creation o f a photon (sec Fig. 6.1 1) with a freq uency following Eq. (6.20) where the energy is approximately equal to the bandgap energy Ea and therefore : E~ ~

(6.20)

hi

This spontaneous emission of light fro m within the diode structure is known as etectroluminescence." The light is emitted at the site of carrier recom bination which is primarily close to the j unction. although recombination may take place throughout the diode struct ure as carriers diffuse away from the junction reg ion (see Fig. 6. 12). Ho wever, the amount of radiative recombination and the emis sion area within the structure is dependent upon the semiconductor m at erials u sed and the fabrication of the device.

- - P\lu''''''' , ,,•• ,

, •

P- t~pe--+ ~ .

- "- - - - -- - - - - -- - - - - - - - ~ - - - - - - - - - -J--- P-~ juo"l.""

• •

.

•

~

• e fig.. e .12

An illust rat ion of cemer recom bina tlcn giving spo m enecus emisskm of Iigh l in a p-n junction diode .

• Tbe term dectroIum iDtlccn« i.

fJl an . lectric fkId.

-

usee whnl the op-JcaI a mssion m1u1ts from the apphca lion

2 50

6 .3.3

OPTICAL FIBER COMM UNICATIONS: PRINCIPLES AND PRA CTICE

Carrier Recombination

6 .3.3.1 Direct and Indirect Bandgap Semiconductors In order to encourage electrolumlnescence it is necessary to select an appropriate semiconductor material. The most useful materials for this purpose are direct bandgap semiconductors in which electrons and holes on either side of the forbidden energy gap have the same value of crystal momentum and thus direct recombination is possible. This process is illustrated in Fig. 6.13(a) with an energy-momentum diagram for a direct bandgap semiconductor. It may be observed that the energy maximum of the valence band occurs at the same (or very nearly the same) value of electron crystal momentum- as the energy minimum of the conduction band. Hence when electron- hole recombination occurs t he momentum of the electron remains virtually constant and the energy released, which co rresponds to the bandgap energy E~, may be emitted as light. This direct transition of an electron across the energy gap provides an effi cient mechanism for photon emission and the average time the minority carrier remains in a free state before recombination (the minority carrier lifetime) is short (Io-R _ l o- IO s). Some commonly used direct bandgap semiconductor materials are shown in Ta ble 6,1 [Ref. 3],

[I",-..O~ "'~m

·-

n..~·, .....

, , ,,

~ PI>o , ,,,,

, , ,, ,

\1,,,,..

,, ~ u , ,,

'"

"""" ,, tu n' M

,,·. " Wd Of k

wa" .,.o,lor k

Fig,6.13

Energy - mom en tu m di agrams show i ng the types of transit ion: te} direct b andgap sem icondu ctor ; (b) i ndi rect bandgap semi conduct or .

• T he cry_ Mal momentum p is related 10 the weveveetor k for .n electron in • cryetl1 by p = 2rtltk , where h is F..nek's COftUartl [Ref. I]. Hence the .bKUII of F1&-6.13 Is oltta . now n u the e:tctron .....vevector n ther thlll roomcntum.

~

- "

251

OPTICAL SOURCES 1: THE LASER TIIb~

' .1

Some d ire ct and ind irecl bandga p s e miconducto rs with calc uialed re co mbln e ncn coetl,e ie nt5

S e miconducto r ma terial

Energy ba ndg ap ( , V}

Recombinalion coefficient 8 , (cm ' s· ' j

G,""

Dire ct : 1.4 3 Dire ct 073 Direct: 0 .35 Direct: 0 ,18

7.2 1 )( 10 - 10 2 .39 X 10- 10

Indire ct. 1.12 Indirect : 0 .67 Indire ct: 2.26

1,7 9)(1 0-' · 5,2 5 )( 10'\· 5.37 x 10- 1•

GaSb

''"'"

InSb

s; Ge

G,P

8.5 )< , 0. 11 4 .58 x 10-"

In indirect bandgap semiconducto rs, however, the maximum a nd minim um energies occur at differen t va lues o f crystal momentum (Fig. 6.13(b)). For electro n- hole recombination to take place it is essential that the electron loses momentum such that it has a value of momentum corresponding to the maximum energy of the valence ba nd . The conserv..ation of momentum requires the emission o r abso rptio n of a third particle, a phonon. This three particle recombination process is fa r less probable tha n the two part icle process exhibited by direct bandgap semicond ucto rs. Hence. the recombinenon in indirect ba ndgap sem icond uctors is relatively slow (Io-z- 1Q-4 s). T his is reflected by a much longer mino rity carrier lifetime togeth er with a gre at er \ probability of nonradlad ve tran sition s. The competing nonradiative recombination processes which involve latt ice defects and impurities (e.g. precipitates of commonly u sed dopants) become more likely a s they allow carrier recombin ation in a relativ ely short time in most materials. Thus the indirect bandg ap emitters such as silico n and germanium sho wn in Table 6 .1 give insignifican t levels of electrolummescence. T his d isparity is further illustrated in T able 6. 1 by the values of the recombination coefficient B , given for both the direct and indirect bandgap recombination semiconductors shown. The recombination coefficient is obtained from the measured absorption coefficient of the semiconductor, and for low injected minority carrier density relative to the majority carriers it is related approximately to the radi ative mino rity carrier lifetime' 't, by (R ef. 41 : t, ~ IS,(N

+ P))-'

(6.21)

where Nand P are the respective majority carrier concentrations in the n and p type regions. The signifi cant difference between the recom bination coefficients for the direct and indirect bandgap semiconductors shown, underlines the importance of the use of direct bandgap materials for electroluminescent

e The radllttv. miDorit)' carrier lifeti me il Oefined as the average ume a minorily eacriCJ call ulIt In • free IUle befOl'1l radiative ~ mbillatioo takn p1&cc.

Z5Z

OPTICAL FIBER COMMUNICATIONS; PRINCIPLES AND PRACTIC£

sources. Direc t bandga p semiconducto r devices in general have a much higher internal quantum effi ciency. This is the ratio of the numbe r of radiative reco mbinations (photons prod uced within the structure) to the nu mber of injected carriers which is often expressed as a percentage.

Com pare t h8 apc rodmate radi ative minority carrier tltetrm es in Qaliium arsenide ar'ld silicon w hen t he m inority carriers are electro ns inj ected into the p t ype region w hi ch has a hole concentration of 10 ' $ cm- 3 . The injecte d electron densit y is sm all compared wit h t he majo rity car rier del'1sit y. SQl u tion ; EQuation (6. 2 1l giv@s tn ll rad iative m inority cerrier lifetime t , as

t, ~ lB,W + Pll-l In t he p type regio" t"a "ole concentrat ion oererrntoes (he radiative carrier lifellme 115 P > N . Hence,

Th\ol s for gallium arsenid e:

I, ~ [7 2 1 = 1.39

10- 10

'It 'It

'It

10 . 81 - 1

10-9

= 1.39 ns f or sil icon:

I , ~ (1 .79 X 10.. 15 )( 10'8 J-' = 5.58

x 10, 4

= 0 .56 m s Thus th e direct b8ndgap gallium arsen ide has a radiative carrier lifetime factol 01 IlfOund 2 .5 )( 10 · 1! less than rne lndiret:t bandgap silicon.

6 .3.3.2 Other Radiative Recombination Processes In th e previous sections only full bandgap transitions have been considered to give r adiative recombinatio n. Ho w ever energy levels may be introduced into the bandga p by impurities or lattice defects within the material struct ure which may gr eatly inc rease the electron-hole recombination (effectively red uce the carrier lifetime). The recombina tio n process through such impurity or defect centers may be either radiative or nonradiative. Major radiative reco mbination processes at 300 K oth er than band to band transitions are shown in Fig. 6.14. These are band to impurity cent er o r impurity center to band, don or level to acceptor level and reco mbination in volving Isoelectrc nic impurities. Hence an indirect bandgap semicond ucto r may be made into a more useful electroluminescent material by the addition of impurity centers which will effectively convert it into a direct bandgap material. An example of this is the introduction 0( nitrogen as an impurity into gallium pho sphide. In this ca.c the . -. :'.- ~-<'

253

OPTICAL SOURCES 1: THE LA SER

.

('o<><)uc,,,"', - - - - -, 1-1,'" ..",

~

I

I

- - - - - "--- - -

I

...

,~ ,

~- --""-,, -- . . ,

k ...I

""«I"'" 'nol""'l.

...1_

~"

Flg.6.t4

Major radiativ e recom btnatlon p roce ss e s at 300 K: (el ccnuucnon to valence ba nd (ba nd to ba nd] tr an sitio n: Ibl co nd uction ban d 10 ac ce pto r imp uri ty, an d do nor imp urity to valen ce ba nd tran sition ; (c l dO!1O r impurity 10 acceptor im purity tra ns ition; (d) re co mbinat ion from an isoelect ronic impur ity to Ihe va le nce ba oc.

nitro gen form s an isoelectromc impurity as it ha s the sa me num ber o f valence \ (o uter shell) electrons as pho sphorus but with a different covalent rad ius and hig.her electronegativity [Ref. 11. The nit rogen impurity center th us captures an electron and acts as an isoelectronic trap which ha s a la rge spread of momentum. This trap then att racts the oppositely charged carrier (a hole) an d a d irect transition takes p lace between the impurity center and the valence band. Hence gallium phosph ide may become an efficient light emitter when n itrogen is incorporated. However, such conversio n of ind irect to direct bandgap transitions is only rtad ily achi eved in mat erials where the direct and indi rect bandgaps ha ve a small energy d ifference. This is the case with gallium phosphide but not ....'ith silicon o r germanium.

8 .3.4

Stimulated Eml••lon and Lasing

The general concept of stimulated emission via population inversion was indicated in Section 6.2.3. Carrier population inver sion is achieved in an intri nsic (undoped) semiconductor by the injection of electrons into the conduction band of the material. T his is illustrated in Fig. 6. 15 where the electron energy an d the corresponding fill ed states are shown. Figure 6.15(a) shows the .Itu l tion at absolute zero when the 'conduction band contains no el ectron s. EloctroDl injected into the material fill the low er energy states in th e conduction band up to the injection energy o r the quasi Fermi level for electrons. Ilft;t char.o ceutrality j, conserved within the material an equal density of •

254

OPTICAL FIBE.R COM MUNICATIONS : PRINCIPLES AND PRA CTICE r,lkd .1,." 1",,, ,r.1....

O.m~u< t; u"

h . "d

'" f lg.6.15

,..

The filled electron STales fo r an intrins ic direct ba ndgap se m ico nd ucto r a t ab solute zero [Het. 51: (8) in eq uilibrium ; (b! w it h hig h ca rrie r inject io n.

holes is cre ated in the top of the valence band by the absence of elect rons as shown in Fig. 6. 15(b) (Ref. 51. Incid ent photons with energy E~ but less than the separatio n energy of the q uasi Fermi levels E q = E Fe - E r • cannot be absorbed beca use the necessary cond uction band states are occupied. However. these photons ca n induce a downward transitio n of an electron from the filled conductio n band slates into the empty valence band sta tes thu s stim ulating the emission of another photon. The basic condition for stimulated emission is therefore dependent on the quasi Fermi level separation energy as. well as. the bandgap energy and may be defined as: (6.22)

I

i

However. it must be noted tha t we have described an idea l situ ation whereas. at no rmal o perating tempera tures the distribution of electrons and holes is less well defined but the condition for stimulated emission is largely maintained. Popu lation inversion may be o bta ined at a p--n junction by heavy doping (degenerative doping) of both the p and n type material. Heavy p type doping with acceptor impurities causes a lowering of the Fermi level or boundary between the filled and empty states into the valence band. Similarly degenerative n type doping causes the Fermi level to enter the co nduction band of the material. Energy band diagrams of a degenerate p-n junction a re shown in Fig. 6.16. The position of the Fermi level and. the electron occ upation (shading) with no applied bias are shown in F ig. 6. I6(a). Since in this case the junction is in thermal equilibrium, the Fermi energy has the same value thro ughout the material Fia;ure 6.16(b) sho ws the p-n junction when I forward bi.a nearly equal to tile bandilP voltaiC it appUcd and hMee there It

OPTICAL SOURCES 1 : lH E LASER

255

,

•

Fill
II '"''''

<\0,." "' 0

,•

'"

-----------

• ~£"

,,' Fig. 6 .16

\

The dege ne rate o-n [uncttcn: (a ) with no a pplied bias: Ibl w ith strong for w ard b ias suc h tha r nle seoareuco of tile Qua si Fe lmi Ie v els is hign a f tha n m e electio n- hole fe comblna tiOn e nergy h I ill the nanow a ctive reg io ll. He lice s timulated e miss ion is obtained in th is region.

direct conduction. At high injec tion carrier density" in such a junction there exists an active region near the depletio n layer th at contains simultaneously degener ate populations of electrons and holes (sometimes termed doubly degenerate). For this region the conditio n for stimulated emission of Eq. (6.22) is satisfied for electromagnetic radiation of frequency E1/h
• rbl. may be Ilr,ely ecnetdered to be dectron l injected into the P-Il region becau se of their ....t..

mobility, •

,

256

OPTICAL FIBE R COM M UNICATIONS: PRINCIPLES AND PRACTICE

states. Furthermore the tra nsitio ns ma y termina te o n acceptor states which because of their high concentration also extend as a band into th e energy gap. In this wa y the lasing transitions rna)' occur at energies less than the bandgap energy E• . When transitions of this type dominate, the lasing peak energy is less than the bandgap energy. B ence the effective lasing wavelength ca n be varied within th e electroluminescent semico nd ucto r used to fabrica te the j unction la ser through variatio n o f the impurity concentration. For example, the lasing wavelength of gallium ars enide may be varied between 0.85 and 0.9 5 11m althoug h the best performance is usually achieved in the 0,88 to 0,91 11m band (see problem 6.5). However, a further requirement of the junction diode is necessary to establish lasing. This involves the provision of optical feedback to give laser oscillation. It may be achieved by the formatio n of a n optical cavity (f abryPerot cavity. see Section 6.2.4) within the structure by polishing t he end faces of t he j unction diode to ac t as mirro rs. Each end of the j unctio n is polished or cleaved and the sides are roughened to prevent an y unwanted light emission and hence wasted population inversion. In common with all other laser types a requireme nt for the initiation and maintenance of lase r oscillation is th at the optical gai n matches the optical losses within the cavity (see Section 6.2.5). For the p-n j unctio n o r semiconduct or la ser this occurs at a particular phot on energy within the spectrum of spontaneous emission (usually near the peak wavelength of spontaneous emission), Thus when extremely high currents are passed through the device (l.e. injection levels of around 1018 carriers cm-' ), spontaneous emission with a wide spectrum (Ii.newidth) becomes lasing (when a current th reshold is passed) and the linewidth subsequently na rrows. An idealized optical o utput power against current characteristic (aIM) called the light o utput against curren t characteristic) for a semiconductor laser is shown in Fig. 6. 17. The current thre shold is indicated and it may be observed that the device gives little light output in the region below th e threshold current

~ 'H"" ,.I",I

,·In;. lIOn

Sp""tal\' ou.


'"~'''''

""lI"'"

c......., Fie.e.'7

-.. . .

~- ,

The

id l l l l ~ l'It

output 19a 1nt! eurre.,t ee rl ctlri l t ic: tor I n Injl ction II " ',

257

OPTI CAL SOU RCES 1; THE LASER

which correspond s to spontaneous emission only within the structure. However, after the Lhreshold cu rrent is reached, the light output increases substantially for small increases in current t hrough the device. This corresponds to the region of stimulated emission when the device is acti ng a s an am plifier of light. For strongly confined structures the threshold current density for stimulated em ission J lh is to a fair app ro ximation [R ef. 41 related to the threshold gain coefficient gtll for the laser cavity through:

i ...

=

flitl .

(6.23 )

-

where the gain factor P is a constant appropriate to specific devices. Detailed di scussion of the more exact relations hip is given in R ef. 4 . Substituting for gl~ fro m Eq . (6. 18) and rearranging we o btain :

J lh =

~ [Ci + "':"'10 _1_] P 2L r 1r ,

(6.24)

Since fo r the semiconductor laser the mirrors are formed by a dielectric plane and often uncoated, the mirror reflectivities r 1 and rx may be calculated using the Fresnel reflection rela tionship of Eq. (4.12).

bemple 6 .4 A GaAs il1jtH;tiol1 la!ief has a n o pliul ce vttv ot le ngth 2 50 J,lm and w;dtt1 100 lJ.m. At no rm al o peratin g tem perature the gaif1 lec tor ~ is 21 x 10 - 3 A c m-J and the loss coefficie"t a per c m is 10. DeTermine the tt"(lshold c urren l dens ily end he nce Il'1 e Ih ....s hold c un ee t fo r the devlCfl. II ",ay be assumed mat 11'1 08 cle.aoved m irrors ,lie uncoa ted and thai the cu mm t is res triCted TO tt>e optical cavity. The refractive i nde~ 01 GaAs mey be la ke" as 3.6. Solution : The reflectivity for normal Incidence of 8 pTaf1e wa ve o n Ga As-lI ir lf1t.rface m ey be c eteroee fro m Eq 14.121 w here :

'1='2=' = •

("-' )' 0+ ,

( 3.6 - 1 )2 ;;:;0.32 3 6 -+ 1

The t hreshold cu rrent d8ns lty m ay be obtained fro m Eq . \6.2 4 1 wf'le ~ ;

, , ] J 1h ", -1 [ ii..--ln-

B.

L

r

258

OPTICAL FIBER COM M UNICATIONS: PRINCIPLES AND PRACTICE

~ 2 1 " 110 3

[ 10 +

== 2.65 " 10 3 A. cm

1 In...2.-J 4 2 50)( 10 0 .32 2

The t hre shold c urrent ' In is give n by :

' tn = J ' n x area of the optical cavity '" 2 .6 5 x 10 3 x 250 x 100 x 10- 8

==

663 mA

Therefore the thres ho ld c urre nt l or t his device is 663 mA. if the c urrent flow is

ll!I$tnc ted 10 the optical CSvil.,...

As the stimulated emission mino rity carrier lifet ime is much shorter (typically 10-- 11 s) tha n that due to sponta neous emissio n, furth er increases in input cu rrent above the threshold will result alm ost entirely in stimulate d emission, giving a high intern al quantum efficiency (50- 100%). Also, whereas incoherent spontaneous emission has a linewidth of tens of nan ometers, stimula ted coherent em ission ha s a linewidth of a nanom eter or less.

6.3.5

Het.rojunctlons

The previo us sections have considered the pbotoemis s ive properties of a single p----n j unction fabricated from a single crystal semiconductor materia l. T his is known as a homojunction. However the radiative properties of a junction diode may be improved by the use of heterojunct ions. A heterojun ction is an interfa ce between two adjoining single crystal semiconductor s with different bandgap energies. Devices which are fabricated with heterojunctions are said to have he terostru cture. Heterojunctions are c lassified into either an tsorype (n-n o r p-p) or an anisotype (p--n). The isotype heterojunction pro vides a potential ba rrier within the structu re which is useful for the confinement of minority carriers to a small active region (carrier confinement). It effectively reduces th e carrier diffus ion length and thus the volume within the structure where radiative recombination ma y take place. This technique is widely used for the fabrication of injection laser s a nd high radiance LEOs. Isotype heterojunctions a re also extensively used in LEO s to provide a transparent lay er close to the active region which substantially reduces the absorption of light emitted from the structure. A lternatively anisorype heterojunctions with sufficiently large bandgap differen ces improve the injection efficiency of either electrons o r botes. Both types of bererojuncuon provide a dielectric step due to the different refractive indices at either side of the junction. Thi s ma y be used to provide radiation confinement to the active region (i.e. the walls of an optical waveauide). The efficienc;y of the containment depends usee the mqnitude of tbt tltlP wblch

259

o pnCAL SOURCES 1; TI-l E LASER

n

CO'

') 100 -

0

-r -'-. U, ., . ,' • • .- . • • ._-. _. •

•

'"

0

l rode -

,I lle• • od "',,,.,. I

,

•, ,•

•

•

I

!

~

.-

e

•

[ o 0 o c o c

o

';'

c

"

C O

c

Q

0

<)

00

,

"

'"

II

G

--,, ,

(.)

,...... " • •••1.

The doubll heteroj" oC1;on injlM;tiOlllaser: tel the laver s tructo.lre. shown with I n epplii d forward 1)4'1; (bl energy ba od diag ram indicati ng a frP beterolcnc110n on thl left end. p-n hltl roJunctlon on the right; Ie) the correspo ndi ng I'IIr1otlvl Indu dl. grlm Ind I llctrlc fliid diau;Dution. •

260

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

is d ictated b y the difference in bandgap energies and the wavelength of the r adialion. It is useful to consider the application o f heterojunctions in the fabrication of a particular device. They were first used to provide potential barriers in injection laser s. Wh en a dou ble heterojunction (DR) structure wa s implemented, the resulting carrier and optical confinement red uced the threshold curren ts necessa ry for lasing. by a factor of a round 100. Thus stim ulated emission was o bt ained with relatively smaU threshold curren ts (30-200 mAl. The layer structure and an energy band diagram for a DH injection lase r are illustrated in Fig. 6.1 S. A beterojunction is shown either side of the active layer for laser oscillation. The forward bias is supplied by connecting a positive electrode o f a supply to the p side of the structure and a negative electrode to the n side. When a voltage which corresponds to the bandgap energy of the active layer is applied, a large number of electrons (or holes) are injected into the active layer a nd la ser oscillatio n commences. These ca rriers are confined 10 the active layer b y the energy barriers provided by the heterojunctio ns which are placed within the diffusio n length of the injec ted carrier s. It may also be observed from Fig. 6.18(c) that a refractive index step (us ually a difference of 5-1 0%) a t the heteroj unctions provides radiat ion containment to the active layer. In effect the active layer forms the center of a dielectric waveguide which strongly confines the electroluminescence within this region as illustr ated in Fig. 6. I 8(c). The refractive index step shown is the same for each heterojunctton which is desirable in order to prevent losses due to lack of waveguiding which can occ ur if the structure is not symmetrical . Careful fabrication of the heteroj unctions is also important in order to reduce defects at the interfaces such as misfit dislocations or inclusions which cause nonradiauve recombi nation a nd thus red uce the intern al q uantum effi ciency. Lattice matching is therefore an important criterion for the materials used to form the interface. Ideally heterojunctions should have a very small lattice parameter mis ma tch of no greater tha n 0.1%. However, it is often not possible to obtain such good lattice parameter matching with the semico nd uc tor materials required to give emission at the desired wavelength a nd therefore much higher lattice parameter mismatch is often toler ated (.... 0.6%).

6.3.6

Semiconductor Material.

The semiconductor materials used for optical sources mu st bro adly fulfill several criteria. These a re: (a) p-n j unction formation. The ma terials must lend themselves to the format ion of p-n j unctio ns with suitable characteristics for carrier injection. (b) Efficient electrolumlnescence. The devices fabricated must have a hiah probability of radiative transition! and therefore a high intern al quantum. etnciency. Hence the material, utilized muat be either direct blnd.ap

,

261

OPTICAL SOURCES 1: THE LASER

semiconductors or indirect bandgap semiconductor s with appropriate impurity centers. (c) Useful emission wavelength. The materials m ust emit light at a suitable wavelength to be utilized with current o ptical fi bers and detecto rs (0.8- 1. 7 pm). Ideally th ey should allow bandgap variation with appropriate doping and fabrication in order that emission at a desired specitic wavelength may be achieved. Initial investigation of electrolu minescent ma terials for LEOs in the early 19605 centered around the direct bandgap III-V alloy semiconductor s including the binary compounds gallium arsenide (G uAs) and gallium phosphide (G a P) and the ternary gallium arsenide phosphide (G a As" PI _".). Gallium arsenide gives efficient ejectrclu rrdnescence over an appropriate wavelength band (0.88-0.9 1 urn) and for thc first generation optical fi ber communication systems was the first material to be fab ricated into homojun ction semiconductor lasers operating a t low temperature [Ref. 8 i. It was quickly realized that improved devices co uld be fabricated with heterojunction structures which thro ugh ca rrier and radiation co nfinement would give enhanced light output for drastically reduced device currents. These heterostructure devices were first fabricated using liquid ph ase epitaxy (LPE) to produce GaAs!AI" Ga 1_J As single hetercjunctton lasers. This process inv olves the precipitation of material from a cooling solution onto an underlying substra te. When the substrate consists of a single crystal and the lattice constant or parameter of the precipitating material is the same or very similar to th at of the substrate (i.e. the unit cells within the two crystalline structures are of a similar dimension), the precipitating material form s an epitaxiallayer on the substrate surface. Subseq uer.Uy the sa me technique was used to produce do uble heterojunctions consisting of At.. Ga 1_X As/GaAsl Alx G al--> As epitaxial layers, which gave continuous (CW) operation at room temperature (Refs. 9 and 10]. Some or the common material systems now utilized fo r double heterojunction device fabrication together with their useful wavelength ranges are shown in Table 6.2. Table 6 .2

So me ccm mc e mateflal svste ms use-d in the fabricatio n of electrc hrminescent sources lor OOl ical fibe r cc enm unlcatio o s

Maler ial s yste ms active laye r/confi ning la ye rs

Useful w avele,.,gt h

range Iuml

Su bs trate

0 8 --0.9 0.9 065-0.9 0.8 5- 1.1

GaA$ GaAs

0 ,9- 1.1

1.0-1.7 0.9 2-1.7

G.'"

GaAs GaAs GaS b

I,.

262

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

The GaAs/AIGaAs DH system is currently by far the best developed and is used for fabricating both lasers and LEDs for the shorter wavelength region. The bandgap in this material may be 'tailored' to span the entire 0.8--0.9 11m wavelength band by changing the AlGa composition. Also there is very little lattice mismatch (0.017%) between the AlGaAs epitaxial layer and the GaAs substrate which gives good internal quantum efficiency. In the longer wavelength region (1.1-1.6I1m) a number of III~V alloys have been investigated which are compatible with GaAs, InP and GaSb substrates. These include ternary alloys such as GaAs1_xSbx and InxGa1_xAs grown on GaAs. However, although the ternary alloys allow bandgap tailoring they have a fixed lattice parameter. Therefore, quaternary alloys which allow both bandgap tailoring and control of the lattice parameter (i.e. a range of lattice parameters is available for each bandgap) appear to be of more use for the longer wavelength region. The most advanced are Inl_xGaxAsYPI_Y lattice matched to InP and GaJ_yAI"Asl_xSbx lattice matched to GaSb. Both these material systems allow emission over the entire 1.0--1.7 urn wavelength band. At present the InGaAsP/InP material system is the most favorable for both long wavelength light sources and detectors. This is due to the ease of fabrica, tion with lattice matching on InP which is also a suitable material for the active region with a bandgap energy of 1.35 eV at 300 K. Hence InP/lnGaAsP (active/confining) devices may be fabricated. Conversely GaSb is a low bandgap material (0.78 eV at 300 K) and the quaternary alloy must be used for the active region in the GaAlAsSb/GaSb system. Thus compositional control must be maintained for three layers in this system in order to minimize lattice mismatch in the active region whereas it is only necessary for one layer in the InP/InGaAsP system.

6.4

THE SEMICONDUCTOR INJECTION LASER

The electroluminescent properties of the forward biassed p-n junction diode have been considered in the previous sections. Stimulated emission by the recombination of the injected carriers is encouraged in the semiconductor injection laser (often simply called the injection laser) by the provision of an optical cavity in the crystal structure in order to provide the feedback of photons. This gives the injection laser several major advantages over other semiconductor sources (e.g. LEDs) that may be used for optical communications. These are: (a) High radiance due to the amplifying effect of stimulated emission. Injection lasers will generally supply milliwatts of optical output power. (b) Narrow linewidth of the order of 1 nm (10 A) or less which Is useful in minimizina the 'effects of material dispersion.

2.3

OPTICAL SOURCES 1: THE LASER

(c) Modulation capabilities which at present extend up into the gigahertz range a nd will undoubtedly be improved upon. (d) Relative temporal coherence which is con side red essential to allow heterodyn e (coherent) detection in high capacity systems, but at present is primarily o f use in single mode systems. (e) Good spatial coherence which allows the o utput to be focused by a lens into a spot .....hich has a greater intensity than the dispersed unfocused emission. This permits efficient coupling o f the o ptical o ut put power into the fiber even fo r fibers with low n umerical apertu re. The spatial field matching to the optical fiber which may be obtained with the laser source is not po ssible with an incoherent emitter and con sequently coupling efficiencies are much. reduced. Th ese advantages, together with the compatibility of the injection laser with optical fi bers (e.g. size) led to the early developments of the device in the 19605. Early injectio n lasers had the form of a Fabry-Perot cavity often fabricated in gallium arsenide which was the major III-V compound semiconductor with electroluminescent properties at the appropriate wavelength for first generation systems. The basic structure o f this homoj unctioo device is shown in Fig. 6. 19, where the cleaved ends of the crystal act as partial mirrors in order to encourage stimulated emission in the cavity when electrons are injected into the p type reg ion . However, as men tioned previously these devices had a high threshold current den sity (greater than 10" A cm-") due to th eir lack of carrier containment and proved inefficient light sources. The high current densities required dictated that these devices when operated at 300 K were largely utilized in a pulsed mode in order to minimize the j unction temperature and thus avert d amage. Improved carrier containment and thus lower threshold current densities (around 1()3 A cm- 2 ) were achieved using heterojunction structu res (see Section 6.3.S). The do uble heterojunction injection laser fabri cated from lattice matched Ill-V alloys provided both carrier and optical confinement on both

..(h¥ft!-

/

Fo....,. ,."",


--------- -L~')

,H:.;."

-- 7 -- , .,'~ rxl_/ "I. 1 .1'

/

Sel'ltlml 1!e dl'fjf,lTI 0"

elvlly.

- -'-

/-

.,~. ~

0.".,,,, ..,.., taot

...

....... ~""...,

G,A, I'Iomojunc1ion injection laser w'lh a Fabry-Perot

264

,

I e

OPTICAL FIBER COM M UNICATIONS: PRINCIPLES AND PRACTICE

side s of the p-n j unction giving the injection laser a greatly enhanced performance. This en abled these devices with the appropriate heat sinking to be o perated in a continuous wave (CW) mode at 300 K with obvious ad vantages for o ptical communications (e.g. analog transmission ). Ho wever, in o rder to provide reliable CW operation o f the 0 H injection laser it was necessary to provide further carrier and op tical confinement which led to the introduction of stripe geometry D H laser configurations. Prior to discussion of this structure. ho wever, it is useful to consider the efficiency of the semiconductor injection laser as an optical source.

6 .4.1

Efficiency

There are a number o f ways in which the operatio nal efficienc y o f the semicond uctor laser may be defined . A useful definition is th at of the differential external quantum efficiency 1'1 0 which is the ratio of the increase in photon output rate for a given increase in the number of injected electrons. If P, is the optical p ower emitted from the device, I is the current, e is the charge on an electron. and hi is the photon energy, then:

dP.lhf 1'1 0 =

dP. dIle "" d/(E

}

(6.25)

J

I

where: E. is the bandgap energy expressed in electronvolt s. It may be noted that 1'1 0 gives a measure of the rate of change of the optica l output power with curren t and hence defines the slope o f the output characteristic (Fig. 6.17) in the lasing region. for a particular device. Hence 110 is sometimes referred to as the slo pe quantum efficiency . F or a CW semicond uctor laser it u sually has values in the range 4Q.-.60%. Alternatively the internal q uantum efficiency of the semiconductor laser TI" wh ich wa s d efined in Sec tion 6.3.3.1 as : n umbcr of photons produced in the laser ca vity 1\ =

number of injected electrons

( 6.26)

may be quite high with values usually in the range 5O-1 OO%.lt is related to the d ifferential external quantum efficiency by the express io n IRef. 41:

11.D = 11.i

[I + C2liLl:n(1 /r rz)) ] I

(6.27)

a

where is the loss coefficient of the laser cavity , L is the length of the laser cavity an d r l , r2 are the cleaved mirror reflectivities. An other parameter is the total efficienc y (external q uant um efficiency) l1T wh ich is efficiency defined as : lotaJ numbcr of outpUI phalons

t lr = total number of injected c1OCtron5

I

. •''''',..'jl.f.l~

(6.2 8)

OPTICAL SOURCES 1: THE LASER

265

Pel Jif r, .-0-,-'- ~ lIe IE,

(6.29)

As the power emitted P, changes linearly when the injection current I is greater than the threshold c urrent Ilb ' then :

Fo r high injection current (e.g. 1= 5/... ) then '1T ~ '1 D ' wherea s for lower c urrents (I "- lllh ) the total efficienc y is lower and around 15-25%. The extern al power efficiency o f the device (or device efficiency) 'lop in converting elect rical input to opt ical o utput is given by :

11

r,

= -

P

OJ'

r,

x 100 = -

IV

x 100%

( 6.3 1)

where P = I V is the d.c . electrical input power. Using Eq. (6.29) for the total efficiency we find :

( ~~)

'lop = 'IT

x 100%

(6.32)

The tote r e fficie ncy of an inje ctio n ja se r w ilh 2 GolAs. a Cl ive ' eg'on is 18%. The VOlta ge app lied to the dllvice is 2 5 V and the band ga p e-nergy fo r GeAs is 1,4 3 eV. Ca lculat e the e l le mal powe r efficie r"lcy of the device. Solvrion : Us ing Eq. (6.3 21. the e xte rr.a r powe r eff.e iency is given by:

'l.p = 0. 18

('.43) -

2 .5

l(

l 00 ~

10%

This res ull indi&a lE-s the possibilily o f a ch ie ving high overall power e fl ic iencies fro m semtcond uctc r lase rs whic h are much la rger tha n for o th.., reser types.

6.4.2

Stripe Geometry

Th e 0 H laser structure pro vides optical confinement in the vertical direction through the refractive index step at the heterojun ction interfaces, but lasing takes place across the whole width of the device. Th is situation is illustrated in Fig. 6.20 which shows the broad area D R laser where the sides of the cavity are simpJy form ed by roughening the edges o f the dev ice in o rder to reduce unwanted eminion in th ese directions EU1d limit the number of horizontal trantVerte moael. Howner. the broad emission area creates several problems tnc1udin, difficult heat ~lna. w in,from. multiple filaments in the relatively

•

266

Fig.6.20

OPTICAL FIBER COMMUNICATIONS: PRI NCI PLES AND PRACTICE

A broad area GilAs/AIGaAs OH injection laser.

wide active area and un suitable light output geometry for efficient coupling to the cylindrical fibers. To overcome these problems whilst also reduci ng the req uired threshold cu rrent, laser structures in which the active region does no t extend to the edges of the de vice were de veloped. A co mmon technique Involved the introduction of stripe geometry to th e structure to provide o ptical containment in the ho rizontal plane. The structure of a DH stripe contact laser is s hown in Fig. 6.21 where the major current now th rough the device and hence t he active region is within th e stripe. Generally the stripe is form ed by the creatio n of high re sistance areas on either side by techniques such as proton bombardment IRef. 91 or oxide isolation (Ref. 101. The stripe there fore acts as a guiding mechani sm which overcomes the major problems of the broad a rea device, However , ahhough the active area width is reduced the light out put is still not

F"tg. 8.21

;

2.7

OPTICAL SOURCES 1: THE LA SER

particularly well collimated due to isotropic ermssron from a small active region and diffraction within the structure. The optical output and far field emission pattern ate also illustrated in Fig. 6.21. The output beam divergence is typically 45 ° perpendicular Co the plane of the junction and 9° parallel to it. Nevertheless this is a substantial Improvement on the broad area laser. The stripe contact device also gives. with the correct balance of guiding, single transverse (in a direction parallel to the ju nction plane) mode opera tion whereas the broad area device tends to allow mukimode operation in this horizontal plane. Numerous stripe geometry laser structures have been investigated with stripe widths ran ging from 2 to 6S tun. and the DH stripe geometry structure is universally utilized for optical fiber communications.

6.5

MULTIMODE INJECTION LASERS

6. 5.1

laM r Modea

The typical outpu t spectrum for a broad area injection laser is shown in Fig. 6.22(8). It does not consist of a single wavelength o utput but a series of wavelength peaks corresponding to different longitudinal (in the plane of the junction. along the optical cavity) modes within the structure. As indicated in Section 6.2.4 me spacing o f these modes is dependent o n the o ptical cavity length a s each one corresponds to an integ ral number of lengths. They are generally separated by a few tenth s of a nanometer, and the laser is said to be a multimode device. However, Fig. 6.22(8) also indicates some broadening of the longitudinal mode peaks due to subpeaks caused by higher order ho rizontal

(, )

/ / :'\ .

,

....

I~

H

R.I.,... ;nl.'n",y

,.,

- ---

..... •

' .m_ ~ •.

'.12

Outpul ' lactr, for multl mode injection la sers: (sl broad area device with mutrlIr.D,vI ..I mod..: (b) .tripi glomllry device with single t rensvarse mode.

268

OPTICAL FIBER COM MU NICATIONS: PRINCIPLES AND PRACTICE

transvers e modes." These h igher order lateral modes may e xist in the broad area device due to the unrestricted width o f Lhe active regio n. The correct stripe geometry inhibits the occurrence of the higher o rder lateral modes by limiting the width of the optical ca vity leaving onJy a single lateral mode w hich gives the output spectrum shown in F ig. 6.22(b) where o nly the longitudin al modes may be observed. This represents the typical output spectrum for a good multimode inj ection laser.

6 .5.2

Structurea

F abrica tion of m uUimode injection lasers with a sing le later al mode is achieved by the use of stripe geometry. The constriction of the current llow to the stripe is realized in the structure eith er by implanting the regio ns outside the stripe with protons (proton isolated stripe) to make them highly resistive. or by oxide or p-n junction isolation. The structure for an aluminu m gallium arsenide oxide isolated stripe 011 laser is shown in F ig. 6.21. 11 has an active region of gallium arsenide b ounded on both sides by aluminum gallium arsenide regions. This technique h as been widely applied especially for laser st ructures used in the shorte r wavelength region. The current is confi ned by etching a narrow stripe in a silico n dioxide film. The other two basic tec hniq ues are illustrated in Figs. 6.23(a) and (b) which sho w the proton isolated st ripe and the p-n junction isolated stripe structures respectively. In Fig.6.24(a) the resistive region formed b y the proto n bombardment gives better curre nt confinem ent tha n the simple oxide stripe and bas

r

I ;:;.d w_

'4' >-

......ll""'. p-...... .." .

"

..." 11;. ....

tKi.....

s Flg.6.23

L

'"

Schem ati c representarfon of struct ures for stripe geom etry injaction lasers: {a] proton isolat ed strip ll GaAs/A IGaAs laser; (bl p -n jun ction Isolated diffu sed pla nar stripe) GaAs/AIG aAs laser.

• Tran,verse modes in the plane of the jll1lCtiotr. an oIlm «lIt
269

OPTICAL SOU RCES 1 : THE LASER Rd.lt ...

LIlli"

<''' '1 ''''

(m 'ili )

;,,,, "'; ty ~

( mil' l

,

•

l-..",~ oo

~,I(l

, 00

. ()rl

,.,

( ~ ' n-Jl l lo ~ ~ '

Fig.6.24

'"

la l Ttle ligh t o u tp,,1 ag a inst c u rre nl cneeacteosnc fo r a n injectio n laser with non linellrities or a kink. in t he Slim ula led e mission region lb) A typica l nea r field intensity d istributio n (patte rn; in th e plan e of t he junction lor a n inje ctlc n la s e r,

superior thermal properties due to the ab sence of the silicon dioxide layer. p-e j unction isolation invokes a selective diffusio n through the n type surface region in order to reach the p type layers as iUustrated in Fig. 6.24(b), None of th ese structures confines all the radiation and current to the stripe region and spreading occurs on both sides of the stripe.

1.5.3

Optical Output Power

The optical output power ag ainst current ch aracteristic for the ideal semiconductor laser was ill ustrated in Fig. 6.17. However, with many practical laser diodes this characteristic is n ot linear in the stimulated emission region but exhibits kinks. T hese kinks may be clas sified into two broad categories. The first type of kink results from changes in the dominant lateral mode of the laser as the current is changed. The output ch aracteri stic for laser A in F ig. 6.24(a) ifluetrates this ty pe o f kink where la sing from the device changes from the fundamental lateral mode to a higher order lateral mode (second order) in a current region corresponding to a ch ange in slope. The second type o f kink involve s a ' spike' as o bserved for laser B of F ig. 6.24(a). These spikes have been shown to be associated with filamentary beh avior with in the active region of the device [Ref. 41. The liIa~nls result from defects within the crystal structure. Both these mechanisms a rr~ t the near and far fie ld inte nsity distributions (patternl) obtained from the laser. A typical near field intensity distributio n corrolpondina to a sinjle optical output power level in the plane of the junction il _hown in Fia. 6.24(b}. AI this distribution is in the lateral dire ction, it is

270

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

determined by the nature of the lateral waveguide. The single intensity maximum shown indicates that the fundamental lateral mode is dominant. To maintain such a near field pattern the stripe geometry of the device is important. In general relatively narrow stripe devices (";;;10 11m) formed by a planar process allow the fundamental lateral mode to dominate. This is especially the case at low power levels where near-field patterns similar to Fig. 6.24(b) may be obtained.

6.5.4

Recent Developments

Recent developments in multimode laser fabrication utilizing aluminum gallium arsenide have been involved with three important areas:

(a) to reduce the required threshold current at room temperature and thus the power consumption of the devices; (b) to obtain a stable, narrow, near field intensity distribution for efficient coupling of the emitted light to the optical fibers and to remove or reduce any kinks in the light output against current characteristic; (c) to increase the reliability of the devices. Various stripe widths have been utilized in an attempt to optimize these factors. The 20 urn wide oxide stripe laser may be fabricated so that its light output against current characteristic is relatively free of kinks [Ref. 11] (see Fig. 6.25), but it requires a fairly high threshold current of the order of 180 mA. If attempts are made to lower the threshold current by shortening the cavity then the first order longitudinal mode has a lower output power. Also the light output against current characteristic, although free from kinks, tends to be nonlinear in the lasing region and is therefore of little use for analog modulation. The narrow stripe laser which utilizes an oxide-defined stripe of less than 5 urn appears largely to overcome these difficulties. The light output against

io

',m 'lripe

Light

outpul (mWl

.511

\

Fig.6.25

I

.

........

.. :"

10!'m ,tripo

100 ISO 200 Curront (mAl

The light output against current cherecterteece for 120 11m wiele Inell 3 11m wide oxide stripe injection laser [Refe. 11 and 121.

OPTICAL SOURCES 1: THE LASER

271

current characteristic for a 3 urn stripe laser [Ref 12J is also shown in Fig. 6.25 and it may be observed tha t the th reshold curren t is of the o rder of 70 mA. This de vice also has a very line ar light output against c urrent characteristic. In addition it provides an em ission of man)' longitudinal modes which tends to reduce modal noise (see Section 3.11) in the fiber, F urther reduction in threshold current to 27 mA has been ach ieved with a 12 1J.fJl wide ox ide-defined stripe AI..G s1_.. As la ser {Ref. 13J by red ucing the cavity length to 100 urn and limiting the current spread by a thin p type gallium arsenide cap, Also a d ielectric stack was added 10 the back mirror facet. T his provided substantially imp roved reflectivity from this fa cet ensuring that virtually all the optical power wa s emitted from the other facet thus maintainin g a good external efficiency with a reduced current de nsity through the device. These devices have nearly do ubled the useful output efficiencies to value s as high as 14%, and similar laser structures have been operated in CW mode to light output levels abo ve 30 mW. Optical output power levels as high as 130 m W have been achieved with wide (65 J.UTl) oxide-defined stripe lasers iR ef. 141. H owever, at these light levels reliability b ecome s a problem as the degradati on mechanism s are accelerated (see Section 6.9 .6).

6.6

SINGLE MODE INJECTION LASERS

These de vices are becoming increasingly important in optical fiber communications. The develop ment o f high radiance LEOs for u se with low loss. low dispersion graded index fibers h as diminished the req uirement for multimode laser sources. Early optical fiber system design utilized LED sources only for short-haul, low -band width ap plications with step index fibe rs, More recently advances made in both LED fabrication (see Section 7.3) and graded indelt fibe rs have led to the use of L ED sources in medium-haul . mediumbandwidth system s. H owever. for long-h aul. high-b andwidth applications it is necessary to use single mode fiber where the only c urrent suita ble source is the single mode laser. H ence there h as been much activity in the area o f single mode laser fabrication in recen t years along with the developments in low attenuation single mode fiber . This bas proved to be o f special interest in the longer wavelength region, around 1.3 and 1.55 urn, where low fi ber attenu ation may be achieved. H owever. this does not mean that the sh orter wavelength region has been neglected with re,ard to single mode lasers. The expertise a nd knowledge of fabricating with the GaAs/AIGaAs system has led to the development of a nu mber of . tru~urel which aUow single mode C W o peration. The well-proven a nd lucces.ful AlOaA. sinale mode laser stnrctures will therefore be discussed before consideration of the application of similar structu res to the longer

.,lvtIoaath " &len. •

.1

212

6 .6.1

OPTICAL FIB ER COMMUNICATIO NS: PRINCIPLES AND PRACTICE

Single Mode Operation

For sin gle mode operation, the optical output from a laser must contain only a single longitudin al a nd single transverse mode. Hence the spectral width of the emission from the single mod e device is far smaller than the broadened transition Iinewidth discussed in Section 6.2.4. It was indicated that an inhomogeneously broadened laser can support a number of longitudinal and tran sverse modes simultaneously giving a mult imode output. Single transverse mode operation. however. may be obtained by reducing the aperture of the resonant C8\11Y such that only the TEM oo mode is supported. To obtain single mode operation it is then necessary to eliminate all but one of the longitudinal modes. O ne method of achieving single longitudinal mode operation is to reduce the length L of the cavity until the frequency separation of the adjacent modes given by Eq. (6.14) as &f= c/ 2nL is larger than the la ser transition line width or gain curve. Then only the single mode which falls within the transition linewidth can oscillate within the la ser cavity. However, it is clear that rigid control of the cavity parameters is es sential to provide the mode stabilization necess ary to achieve and m aintain this single mode o peratio n.

6.6.2

Mode Stabilization

The structures required to give mode stability have been discussed with regard t o the m ultimode injection laser (see Sectio n 6.5.2) and similar techniques are req uired to produce a laser emitting a single longitudinal and transverse mode. The correct DH structure restricts the vertical width oflhe waveguiding regi on to le ss than 0.4 j.UT\ allowin g only the fundamental transverse mode to be supported and removing any interference of the higher order tra nsverse modes o n the emitted longitudinal modes. The lateral modes (in the plane of the j unction) are confined by the restrictions o n the current now provided by the stripe geometry. In general o nly the lo wer order modes are excited which a ppear as satellites to each of the longitudinal modes. H owever, as mentioned previo u sl y (Section 6.5.3), stripe contact devices often have instabilities and strong nonlinearities (e.g. kin ks) in th eir light output against current ch aracteristic s. Tight current confinem ent as well a s good waveguiding are therefore essential in order to achieve only the required longitudinal modes which form between the mirror facets in the plane of the j un ction. Finally, as ind icated in the previous section, single mode operatio n m ay be obtained through control of the optical ca vity length such that o nly a single longitudina l m ode falls within the gain bandwidth o f the device. Fig ure 6.26 shows a typical output spectrum for a single mode device. However, injection lasers with short cavity lengths (around 50 pm) are difficult to handle and have DOt been particularly successful. A number of

273

OPTICAL SOURCES 1: THE LASER

'--~""iI.S S~---oFig. 6 .2 6

Typical single longitudinal mode output spectrum from a single ro oce injecti on laser.

other str uctures art available which give electrical and optical containment end allow single mode operation.

8.7

SINGLE MODE STRUCTURES

6.7 .1

Buried Heterostructure IBH ) Laser

The BB laser is obtained by etching. a narro w mesa stripe (as small as 1 11m in width) in O H semiconductor material a nd effect ively burying it in high resistivity, lattice matched n type mate rial ....-ith a n appropriate bandgap and low refractive index. T his process involves a rather complicated double liq uid phase epitaxial (L PE) growth to give the structure illustrated in Fig. 6.27. These devices may heve very small active regions to allow single mode operation. The small active region also gives low threshold currents of 10 mA or less [Refs. IS and 16] and good linearity with out kinks, Wide modul ation bandwidths of 2 GHz have also been obtained IRef. 16] but the maximum reliable optical output power is restricted. In practice the CW o ptical po.....er mu st be kept below 1 m WIfam where lifetimes in excess of 2()(X) hours at 70 0 C have been reported [Ref. 161. T he other major drawback is the beam divergence in the junction plane (40-50°) from the small active region.

~

1.17

Scne malic

~1''''''UlUO"

.,ructUl't l.., r,

of the

Str\
of a G.As/AIGaAs buried neterc -

274

"" 1~.7.2

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Transverse Junction Stripa (TJS) Laser

This structure is one of the most promising for single mode operation. It has had substantial development since its conception in the mid-1970s and is now fabricated on a semi-insulating substrate which has largely removed the earlier problem of excessive temperature-dependent leakage current. A recent form of the device is shown in Fig. 6.28 and consists of a DH multilayer in n type semiconductor material. The lateral waveguide is achieved by two consecutive zinc diffusions in order that the structure is confined by p" -n and p-n gallium arsenide homojuncttons. Carrier injection is obtained laterally across these homojunctions in the central layer and the DH structure confines the carriers to the central active region. The device has good characteristics for single longitudinal and transverse mode operation with typical threshold currents of the order of 20 rnA giving CW optical output power of around 3 mW IRef.

18L Extrapolated device lifetimes of lO6 hours at room temperature have been reported [Ref. 19] for the AIGaAs structure which appear to be aided by the moderate junction temperature rise (approximately 10 "C) due to the low values of drive current. However, the structure has a strong threshold current dependence on temperature (see Section 6.9.1) which makes control of the optical emission more difficult at elevated temperatures.

Z" diffused

,{ I'{

Fig.6.28

6.7.3

+--

i-GoA,
The structure of a GaAs/AIGaAs transverse junction stripe laser.

Channelled Substrate Lasers

There are several single mode structures which rely on the growth by liquid phase epitaxy over channelled substrates. These include the channelled substrate planar (CSP), the plano-convex waveguide (P'Cw) and the constricted double heterojunction (CDH) which are illustrated in Fig. 6.29. The CSP laser structure is fabricated by growing a DH layer on a substrate into which a shallow channel has already been etched. This is shown in Fig. 6.29(a) where the n type AlGaAs fills the channel giving a flat active layer. Mode selection is thought to be the result of higher order transverse modes undergoing a large propagation loss induced by light absorption in the GaAs substrate on either side of the channel. Single mode operation is dependent On the achievement of

I 27.

OPTICAL SOURCES 1: THE LASER

,,, Active "lion

I

r-

•

M,tl!

p-<:.A.

(

a thin flat layer over the channelled substrate and on deep zinc diffusions (wider than the channel) in order to create a uniform current across the channel. Typically these devices have a threshold current around 70 rnA and a median lifetime of 780 hours at 70 "C (Refs. 18 and 22J. The pe w laser structure utilizes a lens-shaped lateral wa veguide grown by LPE over the channel as shown in Fig. 6.29(b). Lateral confinement of both current and radiation is provided by the variation in thickness over the lens shape which also tend s to focus the light giving a narrow active region of 2-3 pm. A stripe contact is used to restrict the current now to the active regio n. Single mode operation with c urrent thresholds around 40 rnA giving C W optical outp ut to 10 mW and linear light output a gainst current characteristics have been obtained (Ref. 231. The CDH laser structure is grown with a 'double-dovetail' channel con figuration as illustrated in Fig. 6. 29(c). T he resulting constricted active region is defined by a stripe contact approximately 10 urn wide and provides the lingle mode operation of the device. Tight current confinement is not required with this structure as the optical cavity is on the least resistive path. This offers a distinct advantage over the CSP and pew la ser struct ures. The d evices operate with thmbold currents in the range 4t)-10 mA giving CW single . ~tudizW mode output to twice current thrcshokt ( 10- 15 mV( optical output

,.....> (Rift. 18 aad 2<1).

278

OPTICAL FIBER COMMUNICATIONS : PRINCIPLES AND PRACTICE

6.7.4

Distributed Feedb.ck (DFB) La.er.

Th ese devices consist of a complex structure which determines the wavelength of the longitudinal mode em ission rather than the ma terial composition as in the more conventional cleaved mirror structures. An optical gr ating is incorporated into the heterostructure waveguide to provide periodic vari ations in refr active index along the direction of wave propagation so that feedback of optical energy is obtained through Bragg scattering (see Section 1 l. 8.3). The corrugated grating may be applied over the whole active length of the device where it gives what is known as distributed feedback a nd eliminates the need for end mirrors. OriginaUy the corrugations were applied directly to the active layer (Ref. 251 but the performance of s uch devices deteriorated rapidly at temperatures above 80 K. It is believed this resulted from excessive non radiauve recombination from defect s in contact with the active layer introduced by the processes (e.g. ion milling) employed in the format ion of the corrugations. An improved mesa stripe geometry D FB laser structure fabricated with the GaAs/ AlGaAs system which employs a separate confinement heterostructure is sho wn in Fig. 6.30 [Ref. 26]. In th is device the corrugations are separated from the active layer and formed in an Alo. o7 G8cl.~J As layer on the p side of the junction. The n type Alo.j G llo.7 As and p type Alo,11GBo.uAs layers conftne the o ptical field to the p type GaAs active layer. However. sufficient optical power leaks across the th in p type (~. I urn} Alc. nG80,u As buffered layer into the corrugated region such that distributed feedback is obtained. Furthermore the

" .c".,. .. ..b
_ .,,_ p.A I... , [ ; . ~ " ,>"

1'-01.1,. " (;. , .,, A,

Fig.

a 30

The structure of 8 m~ stripe geomelry A1GaAa c:h trib\ltecl feedback (OF!) teeer IRef. 2 6 1.

"

OPTI CAL SOURC ES 1: THE LASER

277

P type Alo.n G80.u As layer also acts as a barrier for injected electrons confi ning them to the active layer. thus avoiding exces sive nonradiative recombine. tion in the corru gation region. The emission frequency from th e structure is determined by the corrugation period (see Section 11.8.3). Hence the DFB structure can provid e superior longitudinal mode d iscrimination o ver con ventional F abry-Perot struct ures where single longitudin al mode operation is dependent on the gain spectru m of the optical cavity. When suitably fabricated and operati ng with a single longitudinal and transverse mode such DFB lasers can give narrow emissio n linewidths of less than 1 nm in comparison with 1-2 nm for convention al DH injection lasers. F abrication, however. on the channelled substrate requires great accuracy to ensure longitudina l mode selection and tran sverse mode cont rol becau se the cleaved edges of the d evice are not p ari of the optical cavity. A further ad vantage of the DFB structure over the Fa bry- Perot cavity design is that it exhibits a reduced wavelength sensitivity to changes in temperature and injection current . T he emission from a F abry- Perot injection laser follows the temperature dependence o f the energy gap whereas the lasing from the D F B structure fo llows the smaller temperature dependence of the refractive index. The typical wavelength shift with tem perature for a D FB laser is 0-0.05 nm K-I while the ordinary D H stru cture gives a typical shift of 0.2-0.5 nm K -I [Ref. 271. Therefore. d espite the constructional complexity the DFB laser offers interesting possibilities for applicatio n in optical fiber communication s. Th is is especially the case in relation to integrated optical techniques (Section 11.7) which are starting to be used in the fabrication of components and circuits for optical fi ber systems.

8.7.5

Large Optical Cavity (LOC) Lasers

Ideally, lon g dist ance wideband app lications require light sources with high power single mode C W optical outputs in the range 15-30 mW. To obtain sucb high power operation with current technology it is necessary to increase the lasing spot size (active region) both transversely and laterally whilst maintaining the single mode selectivity. T wo method s o f achieving this increase in the transverse direction are by use of very thin active layers of 0.05-0.06 J.L m instead of the more typical value around 0.2 J.L m [Refs. 18 a nd 311. or with the large optical cavity. The LOC laser utilizes an additional guide layer with a refractive index intermediate between that of the active layer and of the n type AlOaAs layer as illustrated in F ig. 6.3 1 [Ref. 181. It is within thi s l ulde layer th at the optical mode is mainly propagated whilst o btai ning optical lain from the active layer. The LOC technique bu been applied to BH. pew and COH devices. with th' 8H and COH Itructurel Jivina increased spot size in both the tranSVef!le

278

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

Output 18

puw,', (,nW)

16

Met.l Active

rc~iol\

----00

Fig. 6.31

20 40 60 Currellt \m,\)

{a) Schematic representation of the structure of a high power, single mode buried heterostructure large optical cavity laser, !b) The light output against current characteristic for the BH LOC laser. Reproduced with permission from D, Botez. Proc. SPIE (USA), 224, p. 102, 1980.

and lateral planes [Ref. 32]. The structure shown in Fig. 6.3I(a) is for a BH LOC laser. However, a problem with the LOC technique is that the guide layer allows carrier leakage as well as the spread of the optical mode. This leakage is reduced in the structure of Fig. 6.31(a) by the reverse biassed ~n junction which is formed during the second LPE growth. The resulting current confinement gives low threshold currents around 20 rnA as may be seen from a typical characteristic shown in Fig. 6.31(b). It may also be noted that the device gives CW optical output power approaching 20 mW. Bandwidths in the range of 2 GHz have also been reported [Ref. 16] for these devices which exhibit a virtually flat frequency response.

6.8

LONGER WAVELENGTH INJECTION LASERS

Semiconductor materials currently under investigation for the longer wavelength (1.1-1.6 urn) region were outlined in Section 6.3.6. The work has centered on the InGaAsP/InP and AIGaAsSb/GaSb systems, the former prepared by both liquid phase and vapor phase epitaxial techniques; the latter generally prepared by LPE. Especially promising characteristics have been obtained from devices utilizing InGaAsP prepared by both techniques [Ref. 141Figure 6.32(a) shows the structure of a 12 urn stripe contact InGaAsP/InP CW injection laser grown by VPE and emitting at 1.25 gm [Ref. 361. The

_,:,L~':,_

-

"

.

,

279

OPTICAL SOURCES 1: THE LASER

s

•

,

~ oo

100

d< <~ .... "ll "'~' 1, 1

Fig . 8.32

l bl

l sl Structure of an !roGaAsP/l nP laser for em ission at 1.25 urn, lb) lhe light output ag.8:' r'\.51current charect enstic for the device ~n Ie). Reproduced .....ith perrmssr c n Imm G, H. Olsen. C. J Newoe and M . Eltlll'1 burg. A ppl. Phys. Lett.• 34. p . 262. 1979.

device has a current threshold of 85 rnA which may be seen from Fig. 6.32(b) and the light output against current characteristic is linear up to 4 mW where a kink occurs. It exhibits single longitudinal mode optical output up to this power level which indicates that the radiation is essentially confined to the contact stripe. However, improved single mode operation may be obtained using the modeconfining techniques discussed in the previous section. BH and TJS laser structures have been found to give consistent single mode operation with the InGaAsP system. Threshold currents as sow as 22 rnA have been reported (Ref. 37 J for BII devices with a 1 um wide active region and single mode operation to 8 m W has been observed in wider structures oftb is type (Ref. 38). The structure of a 811 lnGaAsP/ln P laser which operates at 1.5 urn is illustrated in Fig. 6.33(a) (Ref. 391. It comprises a semi-insulating Fe-doped lnP substrate onto which art grown successively an n type lnP confining layer. an InGaAsP active layer. a p type InP confming layer and ap ty pe InGaAlP cap layer. The buried heterostructure so formed provides an optical cavity of length 2S0 um and width 2.S urn. [0 addition, the epitaxial layers are mesa etched down to the n type InP layer (right hand side of Fig. 6.3 3a), leaving a 5 urn length for the n type electrode. The p type electrode is formed on the p type InGaAsP cap layer. Hence, the two electrodes are provided on the same side of the laser chip. Althouih the previously reported devices operating in the 1.5-1 .7 urn wlveJenJth band have exhibited fairly high threshold currents (200-- 300 mAl [Ref. 40]. thIJ device was reported to have a threshold current as low as 38 rnA for CW operation at 25 ac. This i. demonstrated by the light output apiDIt OWTtat ,ebltae:tlriltic for the device lhown in Fia. 6.33(b). It may allO

280

OPTICAL FIBER COMM UNICATIONS: PRINCIPLES AND PRACTICE

.,~

,

p- I. r

1,, =J 8 m....

Ifll..;."'.r

wo

(' '' ' i" ol o.y ,, )

'" f lg.6.33

( il )

(al S c he ma tic rl! Drl!sen ta tio n of the structure of a BH InGa AsP/l nP laser on a sem i-ins ula tin g s u bstra te. (b! The lig ht outp ut ag ains t cvrrent c ha racte ris tic for t he BH lase r unde r CW op e ratio n a t 2 5 °C. Re prod uce d with pe rmission fro m T. Ma tsuok a, K. 1 a l
19 8 1.

be noted that this c haracteri stic is kink free to an optical output power of

to mw. H owever. single mod e operation was obtained only at 1.4 times threshold or about 2 mW optical output power. Another InGaAsP/lnP 011 structu re emitting near the 1.5 J.1m wavelength is the separated multiclad layer (SML) stripe la ser [Ref. 4 11. This device has a novel waveguide and internal current confinement structure which is illustrated in Fig. 6.34. The structure is fou r layered with the stripe region separating the multiclad layer. A cou pled waveguide perpendicular to the junction plane is formed by the active layer and the n type InG aAsP layer. There is a difference in the propagation constant between the mode within the stripe and the modes

( , \

"<

~~

M, t rel="nofollow">Hiu ti""

1>-lnG. A-l nP n-ln G " AU' ,,-lnC. .. .,P p-Jnl' ~, nG.A ,F "r Kl g l"~ .,1"" I, )·. , )

~

,

n · l,,!,

~

8.34

Structure of 811 IIlG8A.1P/lnP tep8Ulled multiclltd I• .,.r .trlpe I...r IRef. 411.

,

.

281

OPTICAL SOURCES 1: THE LASER

of the coupled waveguide outside the confines of the stripe which contains the optical field and stabilizes the trans...erst mode. The current is confined to the stripe by the reverse biassed p-n heterojunction. For a del-ice with a stripe width of 6 pm and cavity length 250 urn the CW threshold current was found to be 90 rnA at 2S °C. Single mode optical output (longit udinal and transverse) was obtained at 1.5 times threshold current or around 5 roW optical output power. A maj or problem with the InG aAsP/lnP sy stem is its high temperature dependence on threshold c urrent in comparison with the GaAs/AIGaAs shorter wavelength system (see Section 6.9.1). This temperature sensitivi ty dictates th e use of proper heat sinking SO that the device temperature does not rise much above room temperature. It is often necess ary to use thermoelectric cooling in o rder to maintain a specified working point However, as demand for longer wavelength injection lasers rises due to the tremendous interest in long-haul. high-bandw idth systems. it is likely that device performance will improve. A reduction in temperature dependence may be brought about through improved electrical contacts and device mounting techniques. Also lower threshold currents, enhanced modal stability and improved dynamic response are to be expected as the technology for the fabricat ion of these devices matures. Hence the requirements of the external techniques for temperature stabilization may be reduced in the future.

6.9

INJECTION LASER CHARACTERISTICS

When considering the use of the injection laser for o ptical fiber co mmunications it is necessary to be aware of certain of its characteristics which may affect its efficient operation. The following section s outline the major oper ating characteristics of the device (the ones which have not been dealt with in detail previously) which apply to aU the various materials and structures previously discussed although there is substan tial variation in behavior between them.

6 .9.1

Thr••hold Current Temp.... ture Dependence

Figure 6.33 shows the typical variation in threshold current with temperature for an injection laser. The threshold current tends to increase with temperature, the temperature dependence of the threshold current density J1h being approximately exponential [Ref. 4] for most common structures. It is given by:

J th o: exp -

T

(6.33)

T,

where T I, the device absolute temperature and To is the threshold temperature cotfftcilm wbich 1I a characteristic temperature describing the qullity of the •

2BZ

OPTICAL FIBER COMM UNICATIONS : PRI NCIPLES AND PRACTICE

..,,, •

../-

,

-I-'---f......·....,.. . "-'."''' '6

,

100

:

' ..1=0 ' 0

FI{,. e.35

:!QG

:

t"l0 cu ",,~t

300

'.. (
(...,\1

I .. ( ll(l "t )

A typical lig ht out put against c urre nt c ha ract eris tic tor a n injectio n lase, s howing t he va ria tio n in threshold c urrent wit h te mp e ratu re.

material. but which is also affected by the structure of the device. For AlGaAs devices, To is usually in the range 120-190 K, whereas for InGaA sP devices it is between 40 and 75 K [Ref. 42]. This emphasizes the stronger temperature dependence of InGaAsP structures which is illustrated in example 6.6. The increase in threshold current with temperature for AIGaAs devices can be

accounted for with reasonable accuracy by consideration of the increasing energy spread of electrons and holes injected into the conduction and valence bands. Par InGaAsP lasers it appears that the high temperature sensitivity results from an add itional large and rapidly rising non radiative component of the recom bination current in the active layer (Ref. 4 ). bMnpl.8.6 Com~re

tl-e ratio of t he thres"lOk:I current densiti es et 20 ·C 800 80 · C fOf" a A IG8As inj ecti oro l asel' w ith To =' 160 K 8nd th e similar rltio for 8n lnG IIAsP device wittl To = 55 K. S olution: From Eq . (8 _3 31 the thr eshold current density :

T J ...

0:: e~p­

To

For th e AIGlI As device :

293

J,~

(20 · C) 0:: exp - - = 6.24 160

353 J'h laO · C) <X: exp = 9.08 160 Hent:e the ratio of the current den siti es:

-=-- - _J .... 180 · CI

J... 120 · CI

,...,

)

..

9 .08 6.24

= 1.48

283

OPTICAL SOURCES 1: THE LASER For me Il'lG sAsP eevce:

293 J' h 120 "el a:: eKp -

55

= 205 .88

353

J'h (80

"el IX:. exp -

= 61 2 .89

'5 Hence t hll ratio of the current densiti es:

J' h (80 eCI

6 12.89

..:::.- - = _ 2 .98 J th (20 eCI 205.88

Thus in example 6.6 the threshold current density for the AIG aAs device increases by a factor of I .S over the temperature range. whereas the threshold current density for the [nGaAsP device increases by a factor of J . Hence the stronger dependence of threshold current on temperature for InOaAsP structures is shown in this comparison of two average devices. It may also be noted that it is important to obtain high values of To for the devices in order to minimize temperature dependence. However, carrier leakage due to low potential barriers within the laser structure causes To to be reduced. Hence To for the best AlGaAs TJS devices is only 95 K . and for LOe structures it is around 100 K rather than the higher values quoted for the standard OH structures. Conversely the threshold current variation for AIGaAs COH structures appears much less than the standard DH devices with To of around 250 K reported (Ref. l SI. In all cases adequate heat sinking along with consideration of the working environment are essential in order that the devices operate reliably over the anticipated current range. 6.9.2

Dynamic Reaponae

The dynamic behavior of the injection laser is critical especially when it is used

in high-bit rate (wideband) optical fiber communication systems. The application of a current step to the device results in a switch on delay often followed by high frequency (of the order of 10OHz) damped oscillations known as relaxation oscillations (RO). These transient phenomena occur whilst the electron and photon populations within the structure come into equilibrium and are illustrated in Fig. 6.36. The switch-on delay td may last for 0.5 ns and the RO (or perhaps twice that period. At data rates above 100 Mbits s-' this behavior can produce a serious deterioration in the pulse shape. Thus reducing 'do and dampina the relaxation osciDations is highly desirable. Thl .witcb-on delay it caused by the initial build-up of photon density rnultiDal'rom Itimull1ld em1Il'on. It iJ related to the minority carrier lifetime .'

284

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

1--', --I Fig.6.36

The possible dynamic behavior of an injection laser showing the switch-on delay and relaxation oscillations.

"

!

and the current through the device [Ref. 7]. The current term, and hence the switch-on delay, may be reduced by biassing the laser near threshold (prebiassing), However, damping of the ROs is less straightforward. They are a basic laser phenomenon which vary with device structure and operating conditions; however, RO damping has been observed, and is believed to be due to several mechanisms including lateral carrier diffusion [Refs. 43 and 44], feeding of the spontaneous emission into the lasing mode [Ref. 45) and gain nonlinearities [Ref. 46]. Narrow stripe geometry DH lasers and all the mode stabilized devices (i.e. BH, TJS, esp, CDH and LOC lasers) give RO damping, but it tends to coincide with a relatively slow increase in output power. This is thought to be the result of lateral carrier diffusion due to lack of lateral carrier confinement. However, it appears that RO damping and fast response may be obtained in BH structures with stripe widths less than the carrier diffusion length (i.e. less than 3 urn) [Ref. 47].

6.9.3.

Self Pulsations

Injection lasers also exhibit another type of output fluctuation (apart from ROs) when operating in the CW mode. These self pulsations which are observed in aged and poor-quality devices are a major problem as they can occur after several hundred hours operation. Both transient as well as continuous self-sustained pulsations occur in the frequency range 0.2-4 GHz. The pulsation frequency is related to both the spontaneous recombination lifetime of the carriers (approximately 2 ns) and the photon lifetime in the cavity (approximately 10 ps). Mechanisms within the device structure believed to cause these self pulsations include quantum noise effects (see Section 9.2.3), defects and filament formation and temperature-induced changes. More recent work [Refs. 48 and 49] traces the pulsations to the absorbing dark line defects (see Section 6.9.6), and to regions of carrier depletion. Also there appears to be correlation between the occurrence of self pulsations and deep level traps [Ref. 51] as well as the presence of visible defects [Ref. 52]. It is likely that the preseece of

285

OPTICAL SOURCES 1: THE LASER

absorbing regions within the active layer o f the device will be effected by an}' slight increase: in ph oton density which will tend to reduct their absorption capability (l.e. excess electron-hole pairs created in the absorption region bringing it near to transparency). The subsequent decrease in loss in the optical cavity will enhance the net gain o f the de vice and increase the photon density giving a peak in the output. H owever. stimulated emission fro m the device will red uce the amount of population inversion in the amplifying mat erial and allow the laser to fall below threshold. thus switching the device off. The absorption of the absorbing regions will therefore again be enhanced. a nd the am ount of population inversion will be increa sed by the contin ued injection current. Jlence the threshold will be reached again and the whole cycle repeated. External means may be adopted to suppress these self-sustained pul sations in the la ser response. These include optical injection IRef. 531. feedback. by an external mirror {Ref. 54} and ch ange s in d rive circuit reacta nce [Ref 551. However, it is Likely that these pulsations will ca use difficulties until a better understanding of the nonlinear absorption and nonlinear gain mechanisms within inj ection la sers is obtained .

6.9.4

Noi ••

An other Important characteristic of injection laser operation involves the noise behavior o f the device. This is especially the case when considering a nalog tr ansmiss ion. T he sources of noise are : _ (a) quantum noise (see Section 9.2.3); (b) instabilities in operation such as kink s in the light output against current characteristic and self pu lsation; (c) reflection of light back into the devi ce; (d) partition noise in mulrimode devices.

It is possible to reduce. if not remove. (b), (c) and (d) by using mode sta bilized devices and optical isolators. Quantu m noise, however, is an intrinsic property of all laser types. It results from the discrete and random spontaneous or stimulated transitions which c ause intensity fluctuation s in the optic al emission. F or injection lasers operating a t freq uencies less th an 100 MHz qua ntum noise levels are usually lo w (signal to noi se ratios less than - 80 dB) unless the device is biassed within 1096 of threshold . Over this region the noise spectrum is flat. H owever, for wideband systems wh en the las er is operating above threshold quantum noise b ecomes more pronounced . This is esp ecially the case with m ultimode devices (signal to noise ra tios of around - 60 d B). The hiJber noise level results from a peak in the noise spectrum due to a relax ation reecnance which typically occurs between 200 MHz and I GHz lR ef. 7). Sln,le mode lisen hi ve shown IRef. 551 greater noise immunity by as m uch I. 30 dB when the current i' raised above threshold. T his is a further

--

288

OPTICAL FIBER COMMUNICATIONS ; PRINCIPlES AND PRACTIC E

It " :

Fig.6.31

The e ttect of pa rt ilion no i58 in 8 multimode il'ljeC1;on lase'. It h. displa yed a s a va riation in the d is t ribut io n of the various 10flgitudil'la l modes e rmrted f rom the device.

advantage of the use of single mode lasers as sources for high data rate systems. Partition noise is a phenomenon which occurs in multimode lasers when the modes are not well stabilized. Temperature changes can cause a variation in the distribution of the various longitudinal modes as illustrated in Fig. 6.37. This leads to increase d dispersion on the link and hence, if not allowed for, may cause errors on a digital cha nnel. 8.9.5

Mode Hopping

The single longitudinal mode output spectr um of a single mode laser is illustrated in Fig. 6.38(a). Mode hopping to a longer wavelength as the current is increased above threshold is demonstrated by compari son with the output spectrum shown in Fi.g. 6.38(b). This behavior occurs in all single mode injection lasers and is a consequence of increases in temperature of the device junclion . The transition (hopping) from one mode to another is not a continuous function of the drive current but occurs suddenly over only 1-2 rnA. Mode hopping alters the light output against current characteristics of the laser, and is responsible for the kink s observed in the characteristics or man y single mode devices.

~ .lo'ih·

------1__ ["",,',.:

(n output ,,,,.. ,,

(" t. ", ;".

,0> _.l>._-,?,...I.J...~~~~ "",..
Fig. 8 .38

U U Mode ho pping in a sil'lgle mode inje C1 ioo lase.: lal $;n; le lo ng ilud ina t mod e optical o utput . lb l mode ho p 10 a lo nge r pes t eminlon w.v. l, r glh it an inentased Dlnie,' output DOwer.

OPTICA L SOU RCES 1: THE LASER

287

Between hops the mode tends to shift slightly with temperature in the range 0.05-0.08 nm K-l. St abilization agai nst mode hopping and mode shift may be o btained with adequate heat sm king o r thermoelectric cooling. However, at constant heat sink tempera ture, shifts due to thermal increases can only be fuUy controlled by the use of feed back from external or internal grating structures (see Section 10.2.3).

6 .9.6

Reliability

De vice reliability ha s been a major problem with injection la sers and although it has been extensively studied, not a U aspects o f the failure mechanisms are fully understood. Nevertheless. much progress has been made since the early d ays when device lifetime s were very sho rt (a few hours). The degradation behavior may be: separated into two maj or processes kn own as 'catastrophic' and ' grad ual' degradation. C atastrophic degradation is the result of mechanical damage of the mirror fa cets and leads to the partial or com plete laser failure. It is ca used by the average optical flux density within the structure at the facet and therefore may be limited by using the device in a pu lsed mode. H owever, its occurre nce may severely restrict the operation (to low optical power levels) and lifetime of CW devices. G rad ual d egradation results from internal d amage caused by the energy released by nonradiative carrier recombination. It is gener ally accepted (Refs. 4 and 511 that this energy enhances point defect (e.g. vacancies and inte rstitials) displ acement leading 10 the accum ulation of defect s in the active region. Hence: if nonradiative electron-hole recombination occurs, for instance at the damaged surface of a laser whc:rc: it has been roughened, this accelerates the diffusion of the point defect into the active region of the device. The emis sion characteristics of the active region therefore gradually deteriorate through the accumulation of point defects until the device is no long er useful. Mobile impurities formed by the precipitation process such as oxygen. copper or in terstiti al beryllium or zjnc atoms may also be displaced into the active region. These atoms tend to cluster around e xisting d islocations encouraging h igh local absorptio n o f photons. This causes 'dark lines' in the output spectrum which are a major problem with gradual deg radation. Over recent years techniques have evolved to reduce, if not eliminate, the introduction of defects into the injection laser active region. These include the use of substrates with low d islocation den sities (i.e. less than J()l crrr" ), passivating the mirror facet s to avoid surface-related effects and mounting with soft solde rs to avoid extern al strain . Together with im provements in crystal growth, device fabrication and material selection this has led to CW Injection lasers with reported me an lifetimes in excess o f I Ct hours. or more than 100 yean. These projections heve been reported (Ref. 581fo r a variety of O&AJIAlOIA. lucr ltTUetUr Cl. In the longer wavelength region where techDIq\MI ItI DOt u Wen. Idvlncecl reponed [Ref's.41 and .59) extrapolated •

288

OPTICAL FIBER COM MU NICATIONS: PRINCIPLES AND PRACTICE

lifetimes for CW InG aAsP/lnP structures are around 10' hours. These predictions bode well for the future where it is clear that injection lasers will no longer be restricted in their application on the grounds of reliability.

6.10

INJECTION LASER COUPLING AND PACKAGING

A lthough injection lasers are relatively directional the divergence of the beam must be considered when coupling the device to an optical fiber. A lens assembly is therefore required to direct the beam within the numerical aperture of the fiber if reasonable coupling efficiencies are to be obtained. A method of achieving this is by use of a fiber lens as illustrated in Fig. 6.39 (Refs. 57 and 591. Th e fiber lens is mounted in a v -groove directly in (ra nt of the laser chip a nd at right angJes to the fiber pigtail and gives a coupling efficiency of a bout 30%. Th e whole assembly is incorporated in a hermetically sealed composite package (for increased reliability) with a monitor detector for feedb ack control (see Section 10.2.3) along with possibly some of the control and drive circuits. Optical power is ta ken out of the package by means of the optical fiber pigtail which itself passes through the hermetic seal. The monitor detector is mounted behind the laser chip and monitors the optic al power emerging from the rear face of the device. Alternatively a specially fabricated beam splitter may be used in order to obtain optical power from the mean output to the monitor detector. The whole assembly has the dimensions of a large scale integra ted circuit and can therefore be placed on a

Fi b" pi~ ..il to l'
Ag.8.39

Schematic diagram o f a co mposite package for an InjectIo n le nl g:"'; ng incru sed COl.lpl ln g effICiency [Ref. 5 71.

_c _~ -j~Z::S,;·J'''!'''i Ui C-• f . - .-- -'. - - ..• • __ ~.

--. - __ •~ _ •MoaiiJ: .. '

,_ - _ c.~

..

~. H r

wl tt't I f bl r

289

OPTICAL SOURCES 1: THE LASER lotio""..., _ .

,

! •

~

Fit""

) •, (

Wj«to<>oo 1- .

fig . 8 .40

Coupling the light ou tput from an irojectiOf1 la ser to the optical filler usi'lg a eylindtical micmleros [Ref, 6 11.

printed circuit board along with other components. This type o f packagi ng also uses low inductance drive leads a nd the compact nature of the drive circuit reduces stray capac itance which eases the problems o f driving the laser with large current pulses at high bit rates. Mo re recently similar composite packages have been utilized with cylindrical rnic rolenses for coupling the laser emission to the optical fiber [Ref. 6 1]. This coupling is illustrated in Fig. 6.40, and experimental versions of this configu ration have already succeeded in launching more than 1 mW of optical power at 1.3 urn wavelength into single mode fiber using an 8 1.lm diameter microlens. A lso hemispherical lenses grown on the fiber end may be uti lized to d irect the laser emission into the fibe r (Ref. 42 1. and the resulting action gives a coupling efficiency of around 20%. These various techniques are still under investigation and as yet no standard method has emerged.

8.11

NON$EMICONDUCTOR LASERS

Although at present injection la sers are the major la sing source for optical fiber communications it is possible that in the future c ertain non sem iconductor sources will find application in high -bandwidth, long -haul systems. Solid state lasers doped with neodymium which emit in the 1.0 5-1 .3 um range appear to be att ractive sources even though their application to optical fi ber com municat io ns is at a n early stage. The most advanced of there promising solid state sources is the neody mium-doped yttrium-aluminum-garnet (Nd :Y AG) laser. T his structure has several important properties which may enhance its use as an optical fiber communication source : ..

__.

(a) Single mode operation near 1,064 and 1.32 urn, making it a suitable source for single mode systems. (b) A narrow linewidth (,,0.Ql nm) which is useful for reducing dispersion on optical links. (e) A potentially long lifetime, a lthough comparatively litt le d ata an: available. (eI) The pOllibility that the dimensions of the laser may be reduced to match

_

of the Iln&Ie mO
,

-,

'I,

29.

OPTICAL FIBER COMM UNICAT IONS; PRINCIPLES AND PRACTICE

,I

'r

~

1 nll,.J.....~n l ...... plin~

I

t

(\ ~- l<1

U>< PI" """

~

1D.1« , ;. 1

Fig. 6 .41

Schematic d iagram of an end pum ped Nd : YAG laser.

However, the Nd:YAG laser also has several drawbacks which are common to all neodymium doped solid state devices:

I

(a) The device must be optically pumped. However. long lifetime AIGa As LEOs may be utilized which improve the overall lifetime of the laser. (b) A long fluorescence lifetime of the order of I ()'-'" seconds which only allo ws direct modulation (see Section 1.5) of the device at very low ban dwidths. Thus an extern al o ptical modulator is nece ssary if the laser is to be usefully utilized in optical fiber communications. (c) The device cannot take advantage of the well-developed tec hnology a ssociated with semiconductors and integrated circ uits. (d) The above requ irements {i,e. p umping a nd modulation) lend to give a cost d isadvantage in comparison with semiconducto r lasers . An illustration of a typical end pumped Nd : YAG la ser is shown in Fig. 6.41. The Nd : YAG laser is a four level system (see Section 6.2.3) with a number of pumping bands and fluorescent transitions. The strongest pumping bands are at wavelengths of O.7S a nd 0.81 urn giving major useful lasing transitions at 1.064 and 1.32 urn . More complex neodym ium-based compounds (rather than doped) are also under investigation fo r so lid state o ptical sources. These include neodymium pentaphosphate (N dP4014) and lithium neodymium tetraphosphate (LiN d P4011) which may give high power single mode emission with less optic al pumping. Oth er types of lasa system (e.g. gas) do not appear useful for optical fiber com munication systems owing to problem s of size. fragility a nd high operating vo ltages. However, these devices are often used for laboratory evaluations of o ptical fiber s and co mponents where their high optical o utput power is an asset .

,I

PROBLEMS

i

6 .1

I

Briefl y outline the general requirements for

II.

source in optical fiber com-

munications. Di SCU$$ the areas in which the injection laser fulfils these requimnenu. tnd comment on ar.y drawbacb of using thit device u an optical fiber com·

munication source.

OPTICAL SOURCES 1 : THE LASER

291

6.2

Briefly describe the two processes by ",..hich light can be emitted from an atom, Discuss Ihe requirement for popctaucn inversion in order lbat stimulated emission may dominate over spontaneous emission. lllusu ate your answer with an energy level diagram of a common nnnsemiconductor laser.

8.3

Discuss the mechanism of optical feedback to provide oscill ation and hence amplification within the laser. Indicate how this provides a distinctive spectral output from the d eyice. The longitudinal modes of a g&l.lium arsenide injection laser emitting at a wal'elenglh of 0.81 um are separated in frequency by 218 0 Hz. Determine the Imgt:h of tbe optical cavity and the nu mber of longitudinal modes emitted. The refracnve indell of ga:lium arsenide i. 3.6.

8.4

When GaSb is used in the fabrication of an electroluminescent source, estimate the necessary hole concentration in the p type region in order that the radiative minority carrier lifetime is I ns.

8 ,5

The energy bandgap for lightly doped gallium arsenide at room temperature is 1.43 eV. When the material is beavily doped (degenerative) it is found that tbe lasing transitions involve 'bandtail' Slates which effectively reduce the bandgap transition by 8%. Determine the difference in the emission wavelength of the liaht between the lightly doped a nd thi~ heavily doped case.

8,8

With the aid of suitable diagrams, discuss the principles of operation of the injection laser. Outline the semiconductor materials used for enisslcn over the wavelength range 0.8- L7 JIDl and give reasons for their choice.

1.7

Describe the techniques used to give both elecericel and optical confinement in

multimode injection Iesers. 8.8

A DH il\iectioo laser has an optical cavity of length ~ o urn and width 15 urn, At normal operating temperature the loss coefficient is 10 em"! and the current threshold is ~ O rn A. When the mirror reflectivity at each end of the optical cavity is 0.3, estimate the gain factor ~ for the device, It may be assumed that the cu rrent is confined to tm: optical cavity.

1.9

The coat ed mirror reflectivity at eithef end of the 3~ 0 urn long optical cavity of an injection IlLSCT is 0,5 and 0 . 6 ~ , At normal operating temperature the threshold current density for the device is 2 x 10 3 A cm- 2 and the gain factor p is 22 x 10-] em A- I, Estimate the 1068 coefficient in the optieal cavity,

eo,o

A gallium arsenide injection laser with a cavity of length 500 IJm has a loss coefficie nt of 20 cm- t . T he measured differential external quantum efficiency of the device is 4 5%. Calculate the internal quantum effICiency of the laser. The refractive index of gallium arsenide is 3.6.

""

Octcribe, with the lid of hlitabl.e diagrams, the major structures utilized in the rabrl~ of l!Jlala mode Injection 1&Ien. Q ivt: reasons fOf' the cumm mkTest 1ft the.. d..!""

292

,

OPTICAL FIBER COMMUNICATIONS; PRINCIPLES AND PRACTICE

6.12

Compare the ideallig,bt output against current characteristics for the injection laser with o ne from a more typical device. Di~u" the majoc points on the characteristics and indicate why the two differ.

6.13

The th reshold current density for a stripe geo:neuy AlGaAs laser is 3000 A em" at a temperature of 15 " C. Estimate the required th reshold current at a temperature of 60 "C when the threshold temperatu re coefficient To for the device is 180 K, and the contact stripe is 20 x 100 um .

6.14

Briefl)' describe what is meant by the following terms when they are used in relation to injection lasers: (a) relaxation oscillations ; (b) self pulsations; (c) mode h opping; (d) partition noise.

6 .16

Discuss deyadalion mec hlUlisms in injection la sers. Commen t on these with regard to the C W lifetime of the devices.

Anewen to Numerical Problems 0,3 6,4

150 pm, 124 1 4.2 x 1018 cm- J

6.' 0.8

0.07 101m

3.76 x 10- 2 em A-I

0.•

28 crrr'

6.10 6.13

84.5%

71.0 rnA

REFERENCES 1 2 3 4 5

6 7 8

9

1

C. K ittd, In troduction to S olid S tate Physics, John Wiley, 19 71. E. S. Yang. Fundamentals 01 S t mkondut:tor 1Jnlces, McGraw-Hill, 1978. Y. P. Varshni, ' Band to band radiative recombination in groups IV, VI and III- V semiconductot r, Ph)'s. Stat. S olidi (Germany). 19( 2), pp. 459-5 14, 1967. H . Kre~ a nd J. K. Butler, S tnrlCONhu::tor Losers Gild Heteroj Undltm L EDs, Academic Press, 1977. H. Kressel, 'Bececluminescent sources for- fiber systems', in M. K. Barno.sti (Ed.), Fu ndamentals alOplical Fiber Communication', p p. 109-141 , Acad emic P ress, 197 6. S. M. $:e, Physics 01 semiconductor devices (2nd Edn.), John Wiley, 1981. H. C. C asey and M. B. Parish. Heterostructure L asers: Part A and B , Acad emic Press, 1978. R. N . H all, G. E. Fenner, J. D. Kingsley, T. J. Soltys and R. O. C arlso n, 'Co herent light emission from Ga AI junctions', Ph" . Rtt>. Uti., 19, p. 366, 1962. I . C. Dyment. L A. D'Asaro, J. C. Norbt, B. I. Miller Itld J. E. Ripper, ' Prot.on· bombardment formation of nripe-acometry hetet'Oltnl.eture laser. for 300 K CW operatio n', Proc. IEEE, 60, pp. 726-728, 1972.

-"

,/' .~

<0-= -_

opnCAl SOURCES 1: THE LASER

10

11 12

13

14 15

16

17

18 19

20 21 22 23

24 25

28

27

21

21

293

H. Kresse! a nd M . E!tenburJ, ' Low-threshold double, hetercj uncuon A1G a Asi GaAs laser dioces : theory and experiment', J. Appl. Phys., 47(8), pp- 353335 37, 1976. P . R. Selway, A . R. Goodwin and P. A. K irby, 'Semiconductor laser light sources for optical fiber ccmmunceuces', in C . P. Sand bank (Ed.), Optical Fiber Communication Systems, pp. 156-1 83, John Wiley. 1980. A.R. Goodwin, A.W. D avis. P.A. K irby, R.E. Epworth and R. G . Plumb. 'N arro'..... stripe semiconductor laser for improved performance o f optical communica tion systems', Proceed ings of the Jth European Co'fjerence on Opticol Communications (Amsterdam), paper 4- 3, 1979. M. Bttenburg and H . F. Lockwood, 'Low-threshold -current CW injection lasers, Fiber Integ, Opt., 2(1), pp . 47-60, 1979. C. 1. Neuse, ' Advan ces in heterojunction lasers fo r fiber optics application s', Opl. E ng., " (1), PP. 20-24, 19 79. K. Saito, N. Shige, T. K ajimur a, T. T SlOkada., M. Maeda and R . Ito, ' Buriedheterostructure lasers as light sources in fiber optic communications', Technical Digest of 1977 Internationa l Conff'rena on Integrated Opllcs and Optical Fiber CommunIcation (Tokyo, Japan), pp. 65-68, J977 . K. Saito a nd R. Ito, 'Buried -heterostruct ure AIG aAs lasers', l £ t E J. Qualltum Electron" I QE-16(2), pp . 205- 215, 1980. N. Bar-chal m, J. Katz, 1. Uray and A. Yariv, ' Buried heterostructure A IGaAs laser s o n ~i-irn.ulatir.g substrates', Electro". Leu.; 17(3), pp. 10 8- 109. 198 1. D. Berea, 'Single mode AIGaA 9 laser diodes', Proc. S oc. Pitoto-Opt. lnstrum, Eng. (US A).. 224 , pp. 102-112, 1980. S. N ita, H_N llIIIiu ki, S. Takam iya and W. SllsalU, 'Single-mode junction-up TJS laser s with estimated lifetime 106 hours" I EEE J. Quantum Electron.. QE15(1 1), pp. 1208- 1210, 1979. H. K umbe, T. T anaka and H . Narlsaki, 'High TE M single-mode CW operation with junction laser', App l. Phys. L ett.; 33( 1), pp. 38-39, 1978. D. C . O ·Shea. W. Russell Callen and W. T. Rhodes, l ntrodm:tion to L asers and Their Applicalioru , Adclison-Wesley, 197 8. K. A iki, M. Nakam ura, T. K urod a, J. Umede, n.uc . N. Chinone and M. Maeda. ' Transverse mode sta bilized Al",Ga l _Jr A9, injection lasers with channet-subsrrareplan ar structures', IE E E J. Quantum Electron.; Q E·I 4(2), pp. 89-97. 1978. T. Furuse, J. Sak uma, Y. Ide, K. Nishida and F. Saito, ' Tr an sver se mode stabilized A IOa As DH la ser having b uilt in plano-convex waveguide', Proceedings of the 5th European Co nference on Optical Co mmunications ( Amsterd am), paper 2.2, 19 79. D . Botea, 'Single-mode C W operation of "dot.:ble -dovetarI" constricted DH (Al Ga)A, diode lasers' , Appl. Phys. L ett ., .B (10), pp. 872-874, 1978 . H . C . C a sey J r, S. Somek.h and M. I!egerns, "Room-temperature o peration of low-threshold separate confinement heterostruct ure injection laser with distrib uted feedback', Appl. Phys. Leu., 27(3), pp . 142- 144, 1975. M. Nak amura, K. Aidi, 1. Umeda and A. Y ariv, ' CW oper ation of nisrrib utedfeedback, G aAs-GIlAIAs diO\1e lasers at tem peratu res up to 300 K ', Appl. Phys. Lett., 27(7), pp. 403-405 , 19 75 . A. Vanv and M . Nakamora, ' Periodic structures for integrated optics" I EJ::E J . Quatltum E lectron.. QE-I 3(4), pp. 233-2S3, 1977. A, Yany, Introduction to Opllcal El«tronics ( 2nd Edn), Holt, Rineh an and Wlnaton, 1976. C. A. Burrul. H. Ct.1a Caley and T. U , 'O ptical sou rces', in S. E. Miller (Ed .), OptfCGIFtb,r r"lfGmm.,,,tcat/ons, ~p. 499-SS6, Academic Press, 1979.

294

30

31 32

33 34 36 38

37 38

39 40 41

42 43

44 45 46

47 48 49

50

OPTICAL FIBER COM MUNICATIONS: PRINCIPLES AND PRACTICE

L. R. Tomasetta, H. D . Law. K. Nukano and J. S. lIarri$. ' OaAIAsSb/Ga5b lanice ma tched laser operating a t 1.25-1.40 pm', IE E E l ntem at, Semuxmductor LQJer Colif. (San Fmnctsco. CAl. paper 11- 13, 1918. D. Botez, 'Near and far field a nalytical approJrimations fo r the fundammtal mode in symmetric wl1ieguide DH lasers', RCA. RtJ'e...., 19(4), pp. 577--603, 1918. N . C hinone, K.. Saito, R. It o, K. Alki and N . Shige, ' H ighl)' efficient (O a Al)As b uried heterostructure: Jasen with buried optic al guide', Appl. Phy s. Len; l S(7). pp. 51 3--51 6, 1979. H. F. W olf, 'Optica l sources', in H. F. W olf (Ed.), Handbook of F~r Optics, pp . 153--201. Granad a, 1979. R. C. Goodfellow and R. Da1iies., 'Opticai source devices', in M. J. Howes and D . V. M organ (Eds.). Optical Fibre Communications. pp. 27- 106, John Wiley. 1980. S. Akiba. K. Sakai. Y. Matsushima and T . Yamamo to, ' Room tempera ture CW operation of InG aAsP/inP heterostruct ure lasers emitting at 1.56 u rn', Electron. Lett.. U (19). pp. 606-607. 19 79. G . H . Olsen, C. J. Neuse and M. Ettenburg, ' Low-threshold 1.25/lm vapourgrown InGaAsP CW lasers'. Appt. Phy s. L ett., 34, pp. 262- 264, 1979. M . Nakamura, 'Semiconductor injection lasers for long wavelength optical communications'. Techn ical Dlgest- 1980 lmeg., and Gulded·Wove Opt., Opt. Soc. of America. Paper MO-l , 1980 . H. Kan s and K . Sugiyam a, ' O peration characteristics of bu ried stripe GaInAsP/ InP DH lasers by melt back method'. J . Appl. Phy s., SO, pp. 7934-7938, 1979. T. M atsuoka , K. Takahei, Y. Noguchi and H. Nagai, ' 1.5 urn region InPI GalnAsP buried heterost ruc ture lasers on semi-insulating substrates'. Electron. Leu.• n ( l). pp. 11-1 4. 198 1. G . H _ Olsen, ' InGaAsP laser diodes', Proe. Soc. Photo·.QpI. I"strum. Eng.• 224. pp. 113- 12 1. 19 80_ H . l ma i, H . Ishikawa, T. Tan ahashi and M. Tak u\agawa., ' InG aAsP/ ln P separated multicl ad layer stripe geometry lasers emitting at 1.5 .... m', Elecrron. Leu.. 17(1). pp. 17-1 9, 1981. O. Bctez and G . J. Herkskowltz, ' Components for optical com munication s systems : a review ', Proc. IEEE. 68(6), pp. 689-730 , 1980 . T . Jkegami, 'Spectrum brawenini and tailing effect in direct -mod ulated injection lasers'. Proceedings of 1st EUrQpta!I Conj'erf!/'lCf! on Optical Fiber Com muntca11011 ( London., IJK). p. II I. 1975. K. Furuya, Y. Suema tsu and T. Hoc&. ' Reduction of resonance like peak in direct modulation d ue to carrier diffusion in injection laser'. Appl. Opl., 17(1 2). pp. 1949-1952, 19 78. P. M . Boers, M . T . Vlaardingerbroek and M. Danielson, 'D ynamic behaviour o f semicond uctor lasers', Electron. Lett., 11(10), pp. 206-208, 1975. D . J. Channin, 'Effect of gain saturation on injection laser switching', J. App l. P hys., 50(6), pp. 3858-3860, 1979. N. Chinane, K. Aiki, M. Nakamura and R. Ito. ' Effect s of lat eral mode and carrier density profile on dyna mic behaviour of semiconductor lasers', IEEE J. Quan tum Eiectron ., Q E-14 , pp. 625- 631, 1977. G. Arnold and K. Peter mann, 'Self pulsing phenomena in (G a Al)A s doubleheterostructure injection lase rs'. Opt. Quantum E lectron., 10. pp. 31 1- 322, 1978. R. W. Dixon and W. B. Jo yce. 'A possible model fot su stained o scillations (pulsation s) in (AlG a)As double-heterostructure ' a sers·. IEE£ J. QuolWm Electron.. QE-U, pp. 470-474. 1979. G. A. Ackc:t and K . Koelman, ' Recent developmentl in HmIconductor iIlje.ct;on Ium', A cta Electro". {FrQIIl:,}, 12(4), pp. 295-300, 1979. -:

", . .~ '' '~' d~ .,. ......,,:';' ~t. ~••,_ I~ .

. -',,... • =-, '-

295

OPTICAL SOURCES 1: THE LASER

51

52 53

54

66 56

57

68 69 60

61 62 63 64

R.1. Hartman and R. W. Dixon, 'Reliability of DH GaAs lasers at elevated temperatures', Appl. Phys. Leu.. 26, pp. 239-242, 1975. J. A. Copeland, 'Semiconductor-laser self pulsing due to deep level traps', Electron. Lett., 14(25), pp. 809-810, 1978. R. Lang and K. Koboyashi, 'Suppression of the relaxation oscillation in the modulated output of semiconductor lasers', IEEE J. Quantum Electron., QR12(3), pp. 194-199, 1976. N. Chinone, K. Aiki and R. Ito, 'Stabilization of semiconductor laser outputs by a mirror close to the laser facet', Appl. Phys. Lett., 33, pp- 990-992, 1978. Y. Suematsu and T. Hong, 'Suppression of relaxation oscillations in light output of injection lasers by electrical resonance circuit', IEEE J. Quantum Electron., QE-13(9), pp. 756-762, 1977. R. E. Nahory, M. A. Pollock and J. C. De Winter, 'Temperature dependence of InGaAsP double-heterostructure laser characteristics', Electron. Lett., 15(21), pp. 695-696, 1979. I. Garrett and J. E. Midwinter, 'Optical communication systems', in M. J. Howes and D. V. Morgan (Eds.), Optical Fibre Communications, pp. 251-299, John Wiley, 1980. H. Koge1nik, 'Devices for optical communications', Solid State Devices Research Corif, (ESSDERC) and 4th Symposium on Solid Device Technology (Munich, W. Germany), 53, pp. 1-19, 1980. T. Yamamoto, K. Sakai and S. Akiba, 'IOOOO-h continuous CW operation of Inl_XGaxAsyPl_y/InP DH lasers at room temperature', IEEE J. Quantum Electron., QE-15(8), pp. 684-687, 1979. N. Chinone and H. Makashima, 'Semiconductor lasers with thin active layer', Appl. Opt., 17(2), pp. 311-315, 1978. P. A. Kirby, 'Semiconductor laser sources for optical communication', Radio Electron. Eng., J. [ERE, 51(7/8), pp. 363-376, 1981. J. Stone and C. A. Burrus, 'Self contained LED pumped single crystal Nd:YAG fiber laser', Fiber Integ. Opt., 2, p. 19, 1979. J. K. Butler (Ed.), Semiconductor Injection Lasers, IEEE Press, 1980. K. Shirahata, W. Suseki and H. Namizaki, 'Recent developments in fiber optic devices', IEEE Trans. Microwave Theory and Techniques, MTT-30(2), pp. 121-130,1982.

--------

ri

•

Optical Sources 2: The Light Emitting Diode

7.1

INTRODUCTION

Spontaneous emis sion of radiation in the visible and infrared regions of the spectrum from a forward biassed p-n junction was discussed in Section 6.3.2. The normally empty conduct ion band of the semiconductor is populated by electrons injected into it by the forward current through the junction, and light is generated when these electrons recombine with holes in the valence band to emit a photon. Th is is the mechanism by which light is emitted from an LED, but stimulated emission is not encouraged as it is in t he injection laser by the addition of an optical cavity and mirror facets to provide feedback of photons. The LED can therefore ope rate at lower current densities than the injection laser. bu t the emitted photons have ra ndom phases and the device is an incoherent optical sou rce. Also the energy of the emitted photons is only roughly equal to the bandgap energy of the semicond uctor material, which gives a much wider spectrallmewidth (pos sibly by a fact or of 100) than the injection laser. The linewidth for an LED is typically 1-2 KT, where K is Boltzmann's constant a nd T is the absolute temperature. This gives linewidths of 30-40 om at room temperature. Thus the LED supports many optical modes within its structure and is generally a multimode source which may only be usefully utilized with multimode step index or graded index fiber. At present LEDs ha ve several furth er dra wbacks in compariso n with injection laser s. These include : (a) generally lower optical power coupled into a fiber (microwau s); (b) relatively small modulation bandwidth (often less than SO MHz); (e) harmonic distortion. However, although these problems may initially appe ar to make the LED a far less attractive optical source t han the injection laser, the device has a number of distinct advantages which nave given it a prominent place in optical fiber communications : (8) Simpler fabrication. T here are no mirror facet. and in some struetw'e. no Itriped geometry.

- -

,.... -'..

-

OPTICAL SOURCES 2 : TH E UGHT EMlTIING DIODE

297

(b) C OSL The simpler construction o f the LED leads to much reduced cost which is always likely to be maintained. (c) Reliability. The LE D doe s not exhibit catastrophic degrad ation and ha s proved far less sensitive to gradual d egradation th an the injection laser. It is also immune to self pulsation and modal noise problems. (d) Less temperat ure dependence. T he light output against current characteristic is less affected by temperature than the co rresponding characteristic for the injectio n la ser. Furthermore the LED is not a threshold device and therefore raising the temperature cannot increase the threshold current above the operating point and hence halt operation. (e) Simpler dri ve circuitry. Thi s is due to the generally lower drive currents and redu ced temperature depen da nce which makes temperature compensa tion circuits unnecessary . (f) Linearity. Ideally the LED has a linear light output against current characteristic (sec Section 7.4.1) unlike the injection laser. This can prove advantageo us where analog modulation is concerned.

1.

These advantag es coupled with the development o f high radiance medium bandwid th d evices have made the LE D a widely used optical source for cornmunication s applications. Structures fa bricated using the GaAs/AIGaAs material system are well advanced for the shorter wavelength region. There is also much interest in LEDs for the longer wavelength region especially around Ld um where m aterial dispersion in silica-based fibers goes through zero and where the wide . linewidth of the LED imposes far less limitati on on link length than intermodal dispersion within the fibe r. Furth ermore the reduced attenuation allows longerhaul LED systems. As with injection lasers InGaAsP/lnP is the material structure currently fa vored in th is region for the high rad iance devices. These longer wavelength systems utilizing graded index fiber s are likely to lead to the development of wider bandwidth devices as data rates of hundreds of Mbit s-' arc alrea dy feasible. It is therefore likely that in the near future injection la sers will o nly find majo r use as single mode devices within single mode fi ber systems for the very long-haul, ultra -wide band applica tions whilst LEOs will become the primary so urce for all other system applications. Having dealt with. the basic operating principles for the LED in Section 6.3.2, we contin ue in Section 7.2 with a discussion of LED efficiency in rete tion to the launching of light into o ptical fibers. Moreover, at the end of this section we include a brief account of the oper ation of an efficient LED which employs a double heterostructure. This lead s into a discussion in Section 7.3 of the major practical LED structures where again we have regard of their light couplina efficiency. The various o perating characteristics a nd limitations on LED pedonnance are deiCribed in Section 7.4. Finally, in Section 7.S we tDelude a brief dilc:ullion OD the possible modulation tech niqU'c.s... for semieoa-

4._

optical

IOIltCeL

I 298

7.2

OPTICAL FIBER COMMUNICATIONS : PRINCIPLES AND PRACTICE

LED EFFICIENCY

The absence of optica l a mplification through stimulated emission in the LED tends ( 0 limit the internal quantu m efficiency (ratio of photons generated to injected electrons) of the: device. R eliance on spontaneous emission allows nonradiative recombination to take place within th e structure due to crystalline imperfections and impurities giving at best an internal quantum efficiency of 50 % for simple homojunction devices. However. as with injection lasers d ou ble heterojunction (D H) structu res have been implemented whic h recombination lifetime measurements suggest (Ref. II give interna l quantu m efficiencies o f 6()....80%. Ahhough the possible intern al quantum efficiency can be relatively high the radiation geometry for an LED which em its thro ugh a planar surface is essentially Lambertian in th at the surface radiance (t he po wer radiated from a unit area into a unit solid angle; given in W sr" m' " } is constant in all d irections. T he Lambertian intensity distribution is illustr ated in Fig. 7. ] where the maximum intensity 10 is perpend icular to the planar surface but is reduced on th e side s in proportion to the cosine of the viewing angle 9 as th e a pparent area varies with this angle. T his reduces the external power efficiency to a few per cent as mo st of the light generated within the device is trapped by total internal reflection (see Section 2.2. L) when it is radia ted at greater than the critical angle for the crystal-air interface. As with the injection laser (see Section 6.4. 1) the external p ower efficiency T\q, is defi ned as the ratio of the optical power emitted externa lly Pr to the electrical power provided to the device P or:

r,

Tk , =:.: -P

X

100%

(7.1 )

A lso the optical power emitted P, into a medium of low refractive inde x n from the face of a planar L ED fabricated fro m a material of refractive inde x nx is given approximately by IRef. 21 :

r. Fn'

P,

~ "'-'4"' 0'-'-;'-

,

\'1"-"---- --1',

fig. 7.1

;'" " -;, '

,

The Lamblllrtifln in1e nsity d'fltribUl io n typ;ct l 01. pl. n.r LEO.

(7.2)

OPTICAL SOURCES 2 : TH E LIGHT EM ITTING DIODE

299

where P;nt is the po...... er generated intern ally and F is the transm ission fact or of the semicond uctor-external interface. Hence it is possible to estimate the percentage of optical power e mitted.

A plSOlI r LED is fabricated from llal1ium a rserlide wt1 ich ha s a reflactive inde x o t 3.6 . la l Calc ulate \:t1e o ptica l powe r emitted into air as a percen tage of Il'1 e intema l optical POWer" tor the device w he n me transmission factOr It the c rystal-air imert8~ is 0 .68. lbl When the optica l power 9,mel'8led ifT te m ally is 5 0% of the elec tlical power SU!:lplted, detelmine \:t1e e xterna l power efficiency. Solution: le I The o ptical power emit1 ed is given by Ell. (7. 2 1. in w hich 11'18 rej-active Inde x n for air is 1. P im O.68 x 1

4(3 .6 l2

=

0. 0 13 Pin'

He nc e th8 pow e r emitted 15 only 1.3 % of the opt ical power gllnllrated internally. lb] The external powe r effi cien cy is given by Eq. 17. 11, wh ere .... ~

P~

=- x

Pr"

toc e o.ota c--.«

P

10 0

P

Also ttMt optical power gerUtrated Illlema lly Pint = 0 .51'. Hen~

0 .013 P..., ~

x 100 = 0 .65%

=

2Pio'l

A further loss is encountered when coupling the light output into a fiber. Consideration s of this coupling efficiency are very complex; ho wever. it is possible to use an a pproximate simplified a pproach (Ref. 31. If it is assumed (or ste p index. fibers that all the light incident on the exposed end of the core within the accept ance angle 6a is coupled, then for a fiber in air using Eq . (2.8), (7.3)

Also incident light at angles greater th an 6a will not be coupled, For a Lambertien source, the radiant intensity at an angle 6,I(6) is given by (set: Fig. 7.1): 1(6) = 1Q cos 6

(7.4)

where 10 is the rad1,et inten&ity Ilona the line 6 = O. Con siderins a source which 11 .maIler tllan.; udln c10te ProJlimity to. the fiber oore.'and anumin&

300

OPTICAL FIBER COMMUNICATIONS : PRINCIPL£S AND PRACTICE

cylindrical symmetry. t he coupling efficiency 1'\t is given by :

'10

~

J---,-0e. J

1(0) sin 8

ee

_

%/1 /

(0) sin 0

(7.5)

ae

v

Hen ce substituting from Eq. (7.4)

J

1'\0 =

lla 10 cos

---,- -- - .' 71/2

J

•

Io cos Osin OdO

J e.. Jo~

=2 ·

'10 =

0 sin 0dB

10 sin 20 dO

_

10 sin 20 d8

[-10 cos 29/2Jt

1-10

cos 2e/2l~

= sin 1 9•

(7.6)

Fu rthermo re from Eq. (7.3),

n, = sin 2

e.

= (NA)!

( 7.7)

Eq uation (7.7) for the coupling efficiency allows estimates for the percentage of optical power coupled into the step index fibe r relative to the amount of optical power emitted from the LED. b . . . . . 7.2 Th e li1:lht o u lput from the GilAs LEO o f e xemp le 7. 1 i5 co upled Inlo a s te p Ifldel( f iber wllh II oumert cere certure at 0.2. a co re refractive indell of 1.4 an d a d ia mete r la rg e r tha n the dia me ter of the devic e. Estim ate: (a) The co upling effic iency Into th e fiber when the LED Is in close p rOl( imity to th e fib er CO 'II . lb ) The optical loss in decibe ls. relative to the power e mitt ed from the LED. whefl COUpliflg the ligh t ooQ u t into the fbe r. te} The loss relat iv e to me In tern a lly Ilen erlll ted opt ical power in th e d evic e w l'1en cou p ling the light o utPUt into t he fibe r wh ,!II\ the re is a &m a ll a i, ga p be twellf'l the LED lind t he fiber core. S olution: IIlJ From Eq. 17.71. the couplirtg efficlQr'lCY ls QIVilo by:

'It - (NAj2 _ (0 .2j2 _ 0 .041 ';

'

..'

301

opnCAL SOURCES 2 : THE UGHT EMITTING DIODE

Th us abou t 4 % of the extemellv e-mitted o ptical powe r is coupled i"to the fi be r. (b ) Le t the optic a l power cou pled inlO tne fh e' be Pc ' The n the o ptica l loss in d ecib e ls re la tive 10 p . when co upling t he light o ut put iJ'1 10 lhe fibe ' is:

P, loss = -1 0 10g lO

-

p.

= - 10 log, O'lc

Hence, Loss = - 10 log10 0 .04 = 14 0 dB Ic) When th e LED Is emitting int o ai r, from exam ple 7.1 .

r , = O O' 3 Pi" , A.sSll ming a val Y s m a ll a ir g a p (i .e. cylind rical symm etry u naft ecl ed l ; th en fro m (a l me power co up led i"l o the fi ber is:

Pc = O.04P. = 0.04

;0;

O.013Pinl

= 5.2 )( 10"" P im

Henc e in this c a se Of'1 ly about 0.05% of th e inte ma l o ptica l pow er .s ccc p' eo into the

"'-.

The loss in de c ibe ls rela tive to Pinl is ;

P,

Loss = - 10 10910 = - 10 109 10 5 .2 x 10 P inl

~

= 32.e d B

Therefore, beating In mimI th e assum ptio ns ma d e. t he re is a 3 .7 d B ad va ntag e in th is example in e ns uring the fib er is in direct co nta ct with the LEO so urce .

If significa nt optical power is to be coupled from a n incoherent LED into a low NA fiber the device mu st exhibit very high radiance. This is especially the case when considering graded index fiber s where the Lambertian coupling efficiency ....-ith the same N A (same refractive index difference) and a:::::: 2 (see Sectio n 2.5) is about half that into step index fibers (Ref. 4 1. To obtain the necessary high radi ance, direct bandgap semicond uctors (see Section 6.3.J. L) must be used fabricated with D H structures which may be driven at high current densities. The principle of operation of such a device will now be considered prior to discussion of ... arious LED structures.

7.2.1

The Doubl. Heterojunctlon LED

The principle of operation of the DH LED is illustrated in Fig. 7.2. The device shewn consists of • P type GaAs layer sandwic hed between a p type AIGaAs and aa fI type A10 " , II )'er. When a forward bias is applied ta s indicated in Fla. 7.2(a» e1eeuOlli from the fI type layer are injected through·the p-n junction lDto &hI P type O~ ~y.r where they become minority The..

camerl.

302

OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND PRACTICE

minority carriers diffuse away from the junction IRef. 5] recombining with majority carriers (holes) as they do so. Photons are therefore produced with energy corresponding to the bandgap energy of the p type GaAs layer. The injected electrons are inhibited from diffusing into the p type AlGaAs layer because of the potential barrier presented by the p-p heterojunction (see Fig.7.2(b)). Hence elecrroluminescence only occurs in the GaAs junction layer, providing both good internal quantum efficiency and high radiance emission. Furthermore light is emitted from the device without reabsorption because the bandgap energy in the AlGaAs layer is large in comparison with that in GaAs. The DH structure is therefore used to provide the most efficient

~-:--

0-__

I

o---{

CC'___

,---

c"---

, ,

C

I

c ___(>--- ~,

",

, , ,

, , , , ,

''---

o.,...

...

, , , , , , , ,

y

"b ,':.:" (

(

, "', ~~ ,

:--,

lklcIQjunction,o=-l-- ---- - -- - -- -- -- -- -----:

:

,-------t-.---,

:

I'

,

Op"cal

"'-",,,,(

I

I'

,

I

"

. ...- ••

• • ••

_

\.-

•

Inj'"io"

,loW"",

.

•

'" c

, u ,

"

c'

Hole,

"

Flg.7.2

c

n

"

o C

" ", " U

U

The double heterojunction LED: (a) the layer structure, shown with an applied forward bias; (b) the corresponding energy bend diagram.

OPTICAL SOURCES 2 : THE LIGHT EMITTING DIODE

.0.

incoherent sources for ap plication within optical fiber com munications. Nevertheless these devices generally exhibit the previou sly discussed constraints in relation to coupling effic iency to optical fibers. This and other LED structures a re con sidered in greater detail in the following sectio n.

7.3

LED STRUCTURES

There a re fou r maj or types o f LED structure although only two ha ve found extensive use in optical fiber communications. These are the etched well surface emitter. often -simply called the surface emitter. or Burrus (after the o riginator) LED, and the edge emitter. The other two stru ctures, the planar and dome LEO s, find more application as cheap plastic encapsulated visible d evices for use in such areas as intruder alarms, T V channel c ha nges and industrial counting. However, infrared versions of these d evices have been used in optical communications mainly with fiber bundles and it is therefore useful to consider them briefly before progressing on to the high radiance LED structures.

7 .3.1

Planar LED

The planar LED is the simplest of the structures th at are available and is fabrica ted by either liquid o r vapor phase epitaxial processes o ver the whole surface of a GaAs substrate. This involves a p type d iffu sion into the " type substrate in order to create the junction illustrated in Fig. 7.3. Forward current now through the junction gives Lambertian spo ntaneo us emission and the device emits light from all surfaces. However, only a limited amount of light escapes the structure due to total internal reflection a s discussed in Section 7.2, and therefore the radiance is low.

t • •

304

7.3.2

OPTICAL FIBER COMM UNICATIONS; PRINCIPLES AND PRACTICE

Dam _ LED

The structure: of a typical dome LEO is shown in Fig. 7.4. A hemisphere of n type GaAs is formed around a diffused p type region. The diameter of the dome is c hosen to maximize the amount of internal emission rea ching the surface within the critical angle of the GaAs-air interface. Hence this device has a higher external power efficiency than the planar LED. However. the geometry of the structure is such that the dome must be far larger th an the active recombi nation area, which gives a greater effective emission area and thus reduces the radiance.

Fig.7 .4

Thll str uctu re of a do me LEO.

7.3.3

Surface Emitter (Bu rrus Type) LED

A method for obtaining high radiance is to restrict the emission to a small active region within the device. The technique pioneered by Burrus and Da wso n (Ref. 61 with homoetructure devices was to use an etched well in a GaAs substrate in order to prevent heavy absorption of the emitted r adiation, and physically to accommodate the fiber. Th ese structures have a low thermal impedance in the active region el jowing high c urrent densities and giving high radiance emission into the o ptical fiber. Furthermore considerable advantage may be obtained by employing DII structures giving increased efficiency from electrical and optical confinement as well as less absorption of the emitted radiation. The structure of a high radiance DH surface emitter for the 0.8-O.9Ilm wavelength band is shown in Fig. 7.5 lRef. 7]. The internal absorption in this device is very low due to the larger bandgap confining layers, and the reflection coeffici ent at the back crystal face is high giving good forward radiance. The emission from the active layer is essentially isotropic although the external emission distribution may be considered Lambertian with a beam width of 120 0 due to refraction from a high to a low refractive index at the OaAs-tiber interface. The power coupled Pc into a step index tiber may be estimated from

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the relationship (Ref. 81 : (7.8)

where r is the Fresnel reflection coefficient at the fiber surface. A is the smaller of the fiber core c ross section o r the emission area of the source and Ro is the r adian ce o f the source. However, t he power coupled into the fiber is also dependent on many other fac tors including the distance and alignment between the emission area a nd the fi ber. the LED emission pattern and the medi um between the emitting area and the fiber . F or insta nce the addition of epoxy resin in the etched well tends to red uce the refractive index mismatch and increase the external power efficiency of the device. Hence, D H surface emitters often give more coupled optical power than predicted by Eq . (7.8). Nevertheless Eq. (7.8) may be used to gain an estimate of the power co upled altho ugh accurate results may o nly be obtained through measurem ent

A DH surface emit ter w hich h~s an emiSsi<m area diamet er of 50 J.l.rn Is butt jQiN ed to en eo p.m core step indeJl fi Der w ith a numerical apert ure oj 0. 15. The device has 1 a r.diance of 30 W 5 r- cm- 2 at II oonatent operating dnve current . Estimate the Optical pow er coupled Into th e fiber if 11 is essumec that thl! Fresnel reflection coeffIcittl t I t me IndeJl m etcn ed fi ber surface is 0 .0 1. S o/urlon: UI;ng EQ. 17.8 I, th e optical power coupled into t he fiber P r is give" by:

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II

OPTI CAL FIBEA COM MUNICATIONS: PRINCIPLES AND PRACTICE

306 Th us,

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However, for graded index fiber optimum direct coupling requires that the source diameter be about one half the fiber core diameter. In both cases lens coupling may give increased levels of optical power coupled into the fiber but

at the cost of additiona l complexity. Another facto r which complicates the LED fi ber coupling art the t ransmission characteristics of the leaky modes or large angle skew rays (see Section 2.3.6). Much of the optical power from an incoherent source is initially coupled into these large angle rays, which fall within the acceptance angle of the fiber but have much higher energy than meridional rays. Energy from these rays goes into the cladding and may be lost. Hence much of the light co upled into a multimode fiber from an LED is lost within a few hundred meters. It must therefore be noted that the effective optical power coupled into a short length of fiber significantly exceeds that cou pled into a longer length,

,

7 .3.4

Len. Coupling

It is apparent that much of the light emitted from LEOs is no t cou pled moo the generally narrow acceptance angle o f the fi ber, Even with the etched well surface emitter where the low N A fiber is b utted d irectly into the em itting aperture of the device. coupling efficiencies are poor (of the order o f 1- 296). However, it has been fou nd that greater coupling efficiency may be obtained if lenses are used to collimate the emission from the L ED . There are several lens coupling configurations which include sph erically polished stru ct ures not unlike the dome LED. spherical-ended fiber coupling. tr uncated spherical microlenses and integral lens structures. A G a As!AIGaAs spherical-ended fiber coupled LED is illustrated in Fig. 7,6 (Ref. 91. It consists of a planar surface emitting structure with the spherical-ended fiber attached to the cap by epoxy resin. An emitting diameter of 3.5 urn was fabri cated into th e device and the light was co upled into fibers with core diameters of 7.5 and 110 pm. The geometry o f the situation is such that it is essential that the active diameter of the device be substantially less (factor of 2) than the fiber core diameter if increased co upling efficiency is to be obtained . In this case good performance was obtained with coupling efficiencies around 6%, This is in agreement with theoretical IRef. 101 and o ther experimental {Ref. II I results which suggest an increased coupling efficie ncy of 2- .5 times th rough the sp herical fiber lens.

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Another common lens coupling technique employs a truncated spherical mlcrolens. This configuration is shown in Fig. 7.7 for an etched well InG aAsP/ InP DH surface emitter [Ref. 12J operating at a wavelength of 1.3 urn. Again a requirement for efficient coupling is that the emission region diameter is much sm aller than the core diameter o f the fiber . In this case the best results were obtained with a 14 Jl111 active diameter and an 85 Jl111 core diameter step index fi ber with a numerical aperture of 0.1 6. The coupling efficiency was increased by a factor of 13, again supported by theory (Ref. 10) which SUU ests possible inc reases of up to 30 times. However, the overall power conversion efficiency ~ which is defined as the ratio o f the optical power cou pled into the fiber Pc to the electrical power applied at the terminals of the device P and is therefore given by : (7.9)

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