Necessity of Optical 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.
Introduction An optical amplifier is a device whichamplifies the optical signal directlywithout ever changing it to electricity. Thelight itself is amplified. Reasons to use the optical amplifiers: Reliability Flexibility Wavelength Division Multiplexing(WDM) Low Cost
Basic 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 inversionGain depends on wavelength, internal light intensity and amplifier mediumThree types: semiconductor optical amplifiers, Raman Amplifiers and fiber doped amplifiers
Selecting Amplifiers Maximum Type Gain Noise figure Output power Power High output Not very High gainAmplifier power important Medium Good noise In-line Medium gain output power figurePreamplifi Low output Low value < 5 Low gain er power dB essential
Generic optical amplifier Continuous Wave (Constant) Energy is transferred from the pump to signal
Condition for Amplification by Stimulated EmissionPopulation Inversion: More Electrons in higher energy level Pumping: Process to achieve population inversionusually through external energy sourceIn general if N2 > N1 then MEDIA IS SAID TOBE ACTIVE
Semiconductor Optical AmplifiersSimilar to Laser diodes but the emission is triggered by input optical signalWork 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.
Distributed Fiber AmplifiersThe 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
Amplification Process of EDFA N3 N3 Radiationless Decay980 nm N2 N2Pump N1 N1 Optical Pumping to Higher Energy levels Rapid Relaxation to "metastable" State N3 ~1550 nm ~1550 nm N2 Signal N1 Output Stimulated Emission and Amplification
Raman Amplifiers 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
Stimulated Raman ScatteringStimulated Raman Scattering (SRS) causes anew signal (a Stokes wave) to be generatedin the same direction as the pump wavedown-shifted in frequency by 13.2 THz (dueto molecular vibrations) provided that thepump 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 Long Fiber Span OpticalTransmitter EDFA Receiver Raman Pump Laser
Distributed Raman Amplification (II) 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. EDFA + Raman Optical Power EDFA only Distance Source: Master 7_5 Animation
Broadband Amplification using Raman Amplifiers 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
Advantages and Disadvantages of Raman Amplification 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 Source: Master 7_5 Noise is also an issue
ConclusionOptical amplifiers perform a criticalfunction in modern optical networks,enabling the transmission of manyterabits of data over long distances of up tothousands of kilometers.