optical amplifier
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Optical Optical AmplifiersAmplifiers
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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.
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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
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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
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ApplicationsApplications
Power AmpConfigurations
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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
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Generic optical amplifierGeneric optical amplifier
Continuous Wave(Constant)
Energy is transferred from the pump to signal
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Coherence
Incoherent light waves Coherent light waves
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Atomic Transitions
Stimulated absorption
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Stimulated emission
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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
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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.
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Solid State AmplifierSolid State Amplifier Gain VS Power Gain VS Power
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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
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~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
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Fig. 11-4: Erbium energy-level diagramFig. 11-4: Erbium energy-level diagram
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EDFA EDFA configurationsconfigurations
Co-Directional Pumping
Counter Directional
Dual Pumping
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Gain versus EDFA lengthGain versus EDFA length
There is an optimum length that gives the highest gain
Negative gain if too long
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Gain versus pump levelGain versus pump level
Gain decreases at large signal levels
Signal dependant gain
This increases with the pump power
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Amplified Spontaneous Emission (ASE) Amplified Spontaneous Emission (ASE) NoiseNoise
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EDFA Noise Figure EDFA Noise Figure = (Input SNR)/(Output SNR)= (Input SNR)/(Output SNR)
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SNR degradation due to SNR degradation due to amplificationamplification
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Fig. 11-12a: Gain-flattened Fig. 11-12a: Gain-flattened EDFA-AEDFA-A
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Fig. 11-12b: Gain-flattened EDFA-BFig. 11-12b: Gain-flattened EDFA-B
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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
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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.
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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
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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)
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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
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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
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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.
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EDFAsEDFAs
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SOASOA
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Raman AmplifiersRaman Amplifiers
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Latest technology in Optical Latest technology in Optical AmplifiersAmplifiers
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ReferencesReferences