Optical Optical AmplifiersAmplifiers

Necessity of Optical Necessity of Optical amplifiers?amplifiers?To Transmit a signals over long

distances (>100km), to compensate attenuation losses.

Initially this was accomplished with an optoelectronic module consisting of optical RX, regenerator, equalizer, & an optical TX to send the data.

Although functional this arrangement is limited by optical to electrical & electrical to optical conversions.

IntroductionIntroductionAn optical amplifieroptical amplifier is a device

which amplifies the optical signal directly without ever changing it to electricity. The light itself is amplified.

Reasons to use the optical amplifiers:

ReliabilityFlexibilityWavelength Division

Multiplexing (WDM)Low Cost

Basic ConceptsBasic ConceptsMost optical amplifiers use stimulated

emissionAn optical amplifier is basically a laser

without feedbackOptical gain is realized when the amplifier is

pumped optically (or electrically) to achieve population inversion

Gain depends on wavelength, internal light intensity and amplifier medium

Three types: semiconductor optical amplifiers, Raman Amplifiers and fiber doped amplifiers

ApplicationsApplications

Power AmpConfigurations

Selecting AmplifiersSelecting Amplifiers

Type GainMaximum

Output power Noise figure

Power Amplifier

High gainHigh output

powerNot very important

In-line Medium gain Medium

output powerGood noise

figure

Preamplifier

Low gainLow output

powerLow value < 5 dB essential

Generic optical amplifierGeneric optical amplifier

Continuous Wave(Constant)

Energy is transferred from the pump to signal

Coherence

Incoherent light waves Coherent light waves

Atomic Transitions

Stimulated absorption

Stimulated emission

Condition for Amplification by

Stimulated Emission

Population Inversion:More Electrons in higher energy level

Pumping:Process to achieve population inversion

usually through external energy source

In general if N2 > N1 then MEDIA IS SAID TO BE ACTIVE

Semiconductor Optical Semiconductor Optical AmplifiersAmplifiers

Similar to Laser diodes but the emission is triggered by input optical signal

Work in any wavelength (+)Have high integration, compact and low

power consumption (+)Gain fluctuation with signal bit rate (-)Cross talk between different wavelengths

(-)Two types: Fabry-Perot or Traveling Wave

Amp.

Solid State AmplifierSolid State Amplifier Gain VS Power Gain VS Power

Distributed Fiber Distributed Fiber AmplifiersAmplifiersThe active medium is created by lightly

doping silica fiber core by rare earth element Ex: Erbium (Er)

Long fiber length (10-30 m)Low coupling loss (+)Transparent to signal format and bit rateNo cross talkBroad output spectrum (1530 – 1560

nm)

Works only in specific Wavelengths

~1550 nm

980 nm

RadiationlessDecay

~1550 nm

Pump

Signal

Output

Optical Pumping to Higher Energy levels Rapid Relaxation to "metastable" State

Stimulated Emission and Amplification

N1

N2

N3

N1

N2

N3

N1

N2

N3

Amplification Process of EDFAAmplification Process of EDFA

Fig. 11-4: Erbium energy-level diagramFig. 11-4: Erbium energy-level diagram

EDFA EDFA configurationsconfigurations

Co-Directional Pumping

Counter Directional

Dual Pumping

Gain versus EDFA lengthGain versus EDFA length

There is an optimum length that gives the highest gain

Negative gain if too long

Gain versus pump levelGain versus pump level

Gain decreases at large signal levels

Signal dependant gain

This increases with the pump power

Amplified Spontaneous Emission (ASE) Amplified Spontaneous Emission (ASE) NoiseNoise

EDFA Noise Figure EDFA Noise Figure = (Input SNR)/(Output SNR)= (Input SNR)/(Output SNR)

SNR degradation due to SNR degradation due to amplificationamplification

Fig. 11-12a: Gain-flattened Fig. 11-12a: Gain-flattened EDFA-AEDFA-A

Fig. 11-12b: Gain-flattened EDFA-BFig. 11-12b: Gain-flattened EDFA-B

Raman Fiber Amplifiers (RFAs) rely on an intrinsic non-linearity in silica fiber

Variable wavelength amplification:

Depends on pump wavelength

For example pumping at 1500 nm produces gain at about 1560-1570 nm

RFAs can be used as a standalone amplifier or as a distributed amplifier in conjunction with an EDFA

Source: Master 7_5

Raman AmplifiersRaman Amplifiers

Stimulated Raman Stimulated Raman ScatteringScattering

Stimulated Raman Scattering (SRS)Stimulated Raman Scattering (SRS) causes a new signal (a Stokes wave) to be generated in the same direction as the pump wave down-shifted in frequency by 13.2 THz (due to molecular vibrations) provided that the pump signal is of sufficient strength.

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

TransmitterOptical

ReceiverEDFA

Raman Pump Laser

Long Fiber Span

With only an EDFA at the transmit end the optical power level decreases over the fiber length

With an EDFA and Raman the minimum optical power level occurs toward the middle, not the end, of the fiber.

Source: Master 7_5

Distance

Op

tica

l Po

wer

EDFA+

Raman

EDFA only

Animation

Distributed Raman Amplification (II)Distributed Raman Amplification (II)

Raman amplification can provides very broadband amplification

Multiple high-power "pump" lasers are used to produce very high gain over a range of wavelengths.

93 nm bandwidth has been demonstrated with just two pumps sources

400 nm bandwidth possible?

Source: Master 7_5

Broadband Amplification using Broadband Amplification using Raman AmplifiersRaman Amplifiers

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

Disadvantages

High pump power requirements, high pump power lasers have only recently arrived

Sophisticated gain control needed

Noise is also an issue Source: Master 7_5

Advantages and Disadvantages of Advantages and Disadvantages of Raman AmplificationRaman Amplification

ConclusionConclusion

Optical amplifiers perform a critical function in modern optical networks, enabling the transmission of manyterabits of data over long distances of up to thousands of kilometers.

EDFAsEDFAs

SOASOA

Raman AmplifiersRaman Amplifiers

Latest technology in Optical Latest technology in Optical AmplifiersAmplifiers

ReferencesReferences