Tutorial On Optical Fiber-02

Fibre Optic System Design IDC

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To be discussed » Determining the fundamental link design parameters – aquire information » Design loss calculations – power budget » Costing

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Design considerations ●

Transmission technology » Chosen around the data / IT requirements of the organisation

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Transmission parameters » Data rates » Bandwidth » Capacity » Transmission distances

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Future growth of transmission capacity » An extremely important issue » If there is free capacity available someone will find a reason to use it !!! » The cost of extra fibres is insignificant in the total cost of the installation » Therefore install many additional spare fibres

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Cable costs versus distance *This does not include cable installation costs

100 90 80 70

Cable Cost as % 60 of Total Cost* 50 40 30 20 10

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10

100

Distance (Km)

1000

10000

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System reliability » Quality of equipment? Refer to – Consultants – Other users – Trade magazines

» Do not over design the system » Trade off between cost and reliability » Route cables through quiet areas » Mark all cables clearly at termination points » Cont

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» Keep up to date documentation » Route duplication? » Consider other telecommunications technologies ●

Choice of wavelength » One of the first design considerations » Dictated by the application (FDDI uses 1300nm) » Use only one over the entire site » Generally use the shortest possible wavelength IDC

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Cable selection and installation route » Multimode or monomode? » Network topology? (Ring, Star or Bus)

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Repeaters or amplifiers ? » Needed for extra long cable runs » Require power supplies, enclosures, maintenance etc » Best to avoid where possible » Amplifiers (analog) contribute noise » Repeaters (digital) are preferable

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Transmitters and receivers » Where possible use – Those that adhere to international standards – From one manufacturer only

» Use LEDs where possible – Cheaper – Less affected by the environment – Less sensitive to vibration and stress

» Selection considerations – Maximum data rate – Wavelength of operation IDC

» Cont

– Max transmission distance – Losses in each link – Max dispersion over each link – Max rise time for each component – Max rise time for the system – Led or laser – NA and diameter of the fibres – Matching TX & RX modules to the fibre – Data encoding to be used – Commercial or industrial application – Reliability requirements – Link availability requirements IDC

System Design Parameters ●

Transmitter power – Measured at end of 2m attached fibre. .

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Minimum transmit power – Quoted by manufacturer for TX operating life. – Use either peak or average power for all measurements in system (not both )

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Receiver sensitivity – Minimum input signal level for a BER of 10-9 at a specified data rate (eg. 100 Mbps)

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System gain – Difference between TX output power and RX sensitivity

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System losses

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– Natural fibre attenuation – Splicing – Connectors – Coupling – Dispersion – Ageing – Temperature – Physical stress – Damaged fibres

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Safety margin – To account for aging, environmental losses, design errors and future in line splices – Add the manufacturers specified “receiver power penalty” ●

Accounts for jitter (phase variations in a digital signal), bandwidth limitations, dispersion, clock recovery problems etc

– Recommend 5 to 10 dB ●

Dynamic range

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– Limit to the power into the receiver (to prevent damage, distortion) – Difference between maximum receiver input power and receiver sensitivity – Ensure sufficient link attenuation.

Coupling losses ●

Transmitter coupling losses » LED has large surface area compared to fibre core. – LED loses 15 dB with 50 µm core – LED loses 35 dB with 8.5 µm core

» Laser has small surface area compared to fibre core – Laser loses very little ● IDC

Receiver coupling losses » Negligible because photodiode is always larger than fibre core

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Link loss budget – Safety margin minus the system gain – Maximum signal loss allowable for cable, splice and connector losses

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Fade margin – Link loss budget minus known losses (unused system gain)

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Link Loss Budget and Safety Margin

-15 Min TX power (62.5/125/250) -20

Connector and splice losses Link loss budget

Optical Power -25 (dBm)

Effective dispersion losses

Cable attenuation

-30 Safety margin

Receiver Sensitivity

-35

1

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2

3

4

5

Maximum Cable Distance

6

7 Distance (Km)

Example Power Budget Calculation IDC

Bandwidth calculations ●

Calculated using time responses of fibre and TX & RX components. » Slowest response time allowed from system: where the system output pulse has risen to 90% of the input pulse value in 70% of the time

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Pulse duration And time response T = Pulse Duration R = Data Rate

T

(a) T=

1 R

T=

1 2R

NRZ T

(b) RZ T 90%

(c) IDC

Filtered NRZ 0.7T

Tr = 0.7T = rise time = Max response Time of Link

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Total system response time must be faster than 0.7 of the input signal period (pulse duration). » Maximum allowable bandwidth is inverse of the calculated fastest system response time. T s = 0.7 * T where Ts = system time response And T = 1/R for NRZ T = 1/2R for RZ IDC

where R = data rate

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System time response is sum of – Fibre time response – Transmitter time response – Receiver time response

» Fibre time response affected by chromatic and modal dispersion » TX and RX rise times in specification sheets

Ts = ( Tt2 + Tr2 + Tf2)1/2 where Tt = response of transmitter, Tr = response of receiver IDC

Tf = response of fibre

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Tf

=

Fibre response time (Tf) (Tfm2 + Tfc2 )½

Where Tfm = time response from modal dispersion (ns)

Tfc = time response from chromatic dispersion (ns) Tfm = Dm x L where Dm = modal dispersion (ns/km) L = Length (km) Dm =

350 Bandwidth of fibre(quoted) OR Dm = 1 00 IDC Modal Bandwidth of fibre(quoted)

Tfc = time response from chromatic dispersion (ps) Tfc = Dc x ∆λ x L where Dc = chromatic dispersion (ps/nm-km)

∆λ = spectral spread (nm) L = Length (km)

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Example

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BANDWIDTH DESIGN CALCULATIONS VIDEO