Optical Signal Amplification

Optical Amplifiers Two types ¤ Opto-electronic conversion ¤ All Optical

ALL OPTICAL AMPLIFIERS ☼ Boosting of an optical signal without conversion of an optical signal into an electrical signal. Why we go for such an amplifiers? ۩Cheap ۩Few Repeaters ۩Noise Reduction ۩No electronic restriction on Bandwidth

CHARACTERISTICS OF AN OPTICAL AMPLIFIER

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Gain Gain Efficiency Gain Bandwidth Gain Saturation Noise

TYPES OF OPTICAL AMPLIFIERS ¤ Semiconductor Optical Amplifiers ¤ Fiber Amplifiers ¤ Erbium Doped Fiber Amplifier ¤ Raman Fiber Amplifier

MODES OF APPLICATION ¤ ¤ ¤ ¤

Power Amplifier In Line Amplifier Preamplifier Functions of the Amplifier Transmitter

A1

Power Amplifier A2

A3

Inline Amplifier

Pre Amplifier

Receiver Receiver Receiver

SEMICONDUCTOR OPTICAL AMPLIFIERS(SOA) Laser diodes with or without end mirrors which have fiber attached to both the ends. Two types  Fabry perot SOA  Travelling wave SOA ♫ They work for the optical windows both 1310 and 1550nm. ♫ Transmit bidirectionally.

SOA - OPERATION PRINCIPLE Excited state

Tra

nsi

tio

n

an Tr

Pump signal

Metastable state

ASE Photons

n

io sit

Ground state Excited state

Tra

nsi

tio

n

Pump signal Signal photon 1550 nm Ground state

Metastable state Stimulated emission 1550 nm

Merits ♫ Cheap solution ♫ Can be easily integrated with other devices like MUX/DEMUX ( AWG’s ) ♫ Good for use in Metro WDM Limitations ♪ High coupling loss ♪ Polarization dependent ♪ High Noise figure ♪ Matching with the fiber is also a problem

Erbium Doped Fiber Amplifier Why Erbium? • Erbium ions (Er3+) have quantum levels that allows them to be stimulated to emit in the 1540nm band. • Erbium's quantum levels also allow it to be excited by a signal at either 980nm or 1480nm.

Excited Excited State State FasFa t Dst D ecaec yay

1.48-μm pump 1.48-µ m pump

0.98-μm 0.98-µ mpump pump

Energy Level Diagram of EDFAs

Ground

Ground State State

Metastable Metastable State State λ2 λ 2

λ4 λ 4 λ3 λ 3

λ1

λ 1 Spontaneous Emission Spontaneous Emission Noise Noise (1.53 < λ < 1.56 µ m) (1.53<λ<1.56μm)

λ λ0 0 λ λ0 0 λ λ0 0 λ0 λ 0 λ λ0 0 Output Gain Photons Output gain photons plus Signal Photon plus signal photon

λ0 λ 0 Incoming Incoming signal Signal photon Photon

Gain Gain

Gain

• Thanks to: Optical Communications Laboratory

Operation Details • Erbium atoms emit photons at same wavelength and in same phase and direction as incoming photons  Cascading photons effectively amplify incoming signal  Signal amplified in direction of travel only  Similar to laser action

• Isolator put at output to prevent reflections from returning to amplifier and disrupting operation Input 1480 or 980 nm Pump Laser

Coupler

Isolator Output

Erbium Doped Fiber

Thanks to: Applied Optoelectronic center

• Advantages ~ High gain per mW of pump power ~ Low crosstalk ~ Happen to operate in most transparent region of the spectrum for glass fiber ~ Extremely long excited state lifetime (of the order of 10 ms)

Limitations ~ Can only work at wavelengths where Er+3 fluoresces ~ Requires specially doped fiber as gain medium ~ Three-level system, so gain medium is opaque at signal wavelengths until pumped ~ Requires long path length of gain medium (tens of meters in glass) ~ Gain very wavelength-dependent and must be flattened

Raman Fiber Amplifier Raman Amplification Ŕ Stimulated Raman scattering occurs when light waves interact with vibrations of atoms in a crystalline lattice ( optical fiber ). The atom absorbs the light and re-emits new photons with an energy which is lower than the original energy ( with a wavelength which is about 100nm longer than the original WL at 1550nm ). Ŕ Raman amplification is possible for the S-band and even for the 2nd optical window ( pump WL about 13 THz higher frequency ). Ŕ Raman amplification excellent for use in new ultra long haul DWDM systems: ‫۝‬ ‫۝‬ ‫۝‬

High channel count ( more than 80 ) High modulation speed ( 40 Gbit/s ) Longer distances between regeneration

Types Ŕ 2 types of Raman amplifiers: Ŕ

Discrete Raman amplifiers: Signal is not amplified in the transmission fiber, but in a special fiber within a box with other components, like EDFA ! Ŕ Distributed Raman amplifiers: The transmission medium ( fiber ) is used to achieve gain.

Ŕ Distributed Raman amplifiers benefits: Ŕ Reduces the overall Noise Figure ( NF ) → longer links without regeneration & higher modulation rates become possible. Ŕ Flat gain can be achieved with the use of more than one pump laser with different wavelengths ( Also possible with Discrete Raman amplifiers ).

Discrete Amplifier

DistributedDistributed Raman Amplifier Raman Amplification (I)

 Raman pumping takes place backwards over the fiber.  Gain is a maximum close to the receiver and decreases in the transmitter direction

Long Fibre Span Transmitter

EDFA

Optical Receiver

Raman Pump Laser

Thanks to: Applied Optoelectronic center Source: Master 7_5

Advantages ¤ Variable wavelength amplification possible ¤ Compatible with installed SM fiber ¤ Can be used to "extend" EDFAs ¤ Can result in a lower average power over a span, good for lower crosstalk ¤ Very broadband operation may be possible

Limitations ☼High pump power requirements, high pump power lasers have only recently arrived ☼Sophisticated gain control needed ☼Noise is also an issue

Comparison: Property Amplification

Band

Gain BW

SOA

EDFA

Raman

depends on pump power

depends on dopant (Er, Y, Th)

depends on pump power

60nm

~90nm(extended range)

20-50nm per pump

Flat gain NOISE FIGURE

15-20nm 8

5

5

ASE

ASE

Raman scatter, double Rayleigh

Electrical pump

980/1.480nm for erbium

by 100nm shorter than amplified signal range

<400mA

~10-300mW

< 300mW

depends on Bias current

Depends on dopant and gain

~power of pump

Direction

Unidirectional

Unidirectional

Bidirectional

Simplicity

Simpler

Noise Pump wavelength

Pump power Saturation power

Cost

Low

more complex (EDFA needed) Medium

Simpler (no special fiber needed) high

References: • • •

• •

DWDM Pocket Guide, Ines Brunn, Acterna Eningen GmbH,Postfach 12 62, 72795 Eningen u. A., Germany. Semiconductor Optical Amplifiers, Michael J. Connelly, Kluwer Academic Publishers, New York. Erbium-Doped Fiber Amplifiers Fundamentals and Technology, P.C. Becker, N.A. Olsson and J.R. Simpson, Elsevier Academic Press, San Diego. Raman Amplification in Fiber Optical Communications System, Clifford Headley and Govind P. Agrawal, Elsevier Academic Press, Amsterdam. Electro-Optics Handbook, Ronald W. Waynant, Marwood N. Ediger, Second Edition, McGraw Hill, Inc., New York

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