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

♪ 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

ReceiverReceiver

Power Amplifier

Inline Amplifier

Transmitter A1

A2

ReceiverA3Pre

Amplifier

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

ASE Photons

Ground state

Excited state

Metastable stateTransition

Transition

Pump signal

Excited state

Metastable stateTransition

Pump signal Stimulated emission1550 nm

Signal photon 1550 nm

Ground state

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.

Energy Level Diagram of EDFAs

• Thanks to: Optical Communications Laboratory

Gain

Output Gain Photons plus Signal Photon

GainSpontaneous Emission Noise

(1.53 << 1.56 m)

4Incoming

Signal Photon

3

2

1

Excited State

Metastable State

Ground State

0.98

-m

pu

mp

1.48

-m

pu

mp

Fast Decay

0

000

00

Excited

State

Ground State

1.48

-μm

pum

p

Fast Decay

0.98

-μm

pum

p

Metastable State

λ1

λ2

λ3

λ4

Incoming signal photon

λ0

Output gain photons

plus signal photon

λ0λ0λ0λ0λ0

Spontaneous Emission Noise

(1.53<λ<1.56μm)

Gain

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

Thanks to: Applied Optoelectronic center

Input

1480 or 980 nm Pump Laser Erbium Doped Fiber

Output

IsolatorCoupler

• 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

Distributed 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

Source: Master 7_5

TransmitterOptical

ReceiverEDFA

Raman Pump Laser

Long Fibre Span

Distributed Raman Amplifier

Thanks to: Applied Optoelectronic center

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 SOA EDFA Raman

Amplification Band depends on pump power

depends on dopant (Er, Y, Th)

depends on pump power

Gain BW 60nm ~90nm(extended range)

20-50nm per pump

Flat gain 15-20nm

NOISE FIGURE 8 5 5

Noise ASE ASE Raman scatter, double Rayleigh

Pump wavelength Electrical pump 980/1.480nm for erbium

by 100nm shorter than amplified signal

range

Pump power <400mA ~10-300mW < 300mW

Saturation power depends on Bias current

Depends on dopant and gain

~power of pump

Direction Unidirectional Unidirectional Bidirectional

Simplicity Simpler more complex

(EDFA needed)

Simpler (no special fiber needed)

Cost Low Medium 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

Thank you