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EE 230: Optical Fiber Communication Lecture 7From the movieWarriors of the NetOptical Amplifiers-the Basics
Amplifier Types and ApplicationsFiber Optics Communication Technology-Mynbaev & ScheinerAmplifiers are used to overcome fiber loss They are used in 4 basic applications:
In-line amplifiers for periodic power boosting
Power Amplifier to increase the power to greater levels than possible from the source
Pre-amplifier to increase the received power sensitivity
Distribution loss compensation in local area or cable networks
Characteristics of all amplifiersThey operate by creating a population inversion, where there are more individuals in a high energy state than in a lower oneThe incoming pulses of signal on the fiber induce stimulated emissionThey saturate above a certain signal powerThey add noise to the signal
Comparison of Real and Ideal Amplifier
Inhomogeneous Gain BroadeningLasers-SiegmanInhomogeneous broadening
The individual atomic responses within and inhomogeneously broadened transition all add up to yield the measured lineshapeA Gaussian inhomogeneously broadened atomic lineshape such as produced by doppler broadening in atoms
Interaction of Atoms with Light
Rate Equations and Populations
Population densities with a strong resonant signal
Ideal Amplifier SystemPump process with large crosssectionThird excited state with very short lifetime,no fluorescenceSecond excited state with very longlifetime and high cross section for stimulated emissionEnergy gap between first andsecond excited states matches telecommunication frequenciesFirst excited state with very shortlifetime
Amplified Spontaneous Emission
Noise Figure MeasurementFiber Optics Communication Technology-Mynbaev & Scheiner
Noise Figure
3 main types and 3 Big IdeasThe main types of optical amplifiers are:
Semiconductor amplifiers (lasers that arent lasing)Doped fiber amplifiersRaman and Brillouin Amplifiers
The three big ideas
Gain and gain bandwidthGain saturationNoise and noise figure
Laser Amplifiers
Semiconductor Optical AmplifiersFiber Optics Communication Technology-Mynbaev & Scheiner
Types of SOAFabry-Perot AmplifierHigh gain but non-uniform gain spectrumTraveling wave amplifierBroadband but very low facet reflectivities are neededGain as a function of frequencyRipples are caused by the cavity modesThe overall gain curve is due to the width of the atomic transition in the semi-conductorFundamentals fo Multiaccess Optical Fiber Networks Dennis J. G. Mestgagh
Amplifier BandwidthsFiber Optics Communication Technology-Mynbaev & ScheinerComparison of the bandwidths of Fabry Perot and Traveling wave amplifiers
Traveling Wave SOAFiber Optics Communication Technology-Mynbaev & ScheinerTo make a traveling wave Semiconductor Optical Amplifier the Fabry-Perot cavity resonances must be supressed. To accomplish this the reflectivity must be reduced.
Three approaches are commonly used:
Anti-reflection coating
Tilted Active Region
Use of transparent window regions
Saturation PowerFiber Optics Communication Technology-Mynbaev & ScheinerGain saturation and saturation powerSemiconductor Optical amplifiers saturate silmilarly to a 2 level atom
The typical saturation output power for
SOAs is around 5-10 mW
Crosstalk in Semiconductor AmplifiersRate equation for pump current
If suddenly goes to zero, as in 1-0 sequence,
Time constant is(ns)
If suddenly turns on,which is smaller
Parameters on previous slideN=carrier density (cm-3)I=pump current (amp=coul/s)q=charge on electron (coul)L,w,d=cavity dimensions (cm3)=recombination lifetime (s)=confinement factor (unitless)=photon density (cm-3)a=gain coefficient (cm-1)
Crosstalk in semiconductor amplifiersIf time constant for spontaneous decay of excited state is shorter than the bit duration, the population of the excited state will vary sharply with the optical power in the fiber, and gain will depend on the fraction of 1s and 0s in the data stream. If time constant is long, then the population in the excited state will be constant, dependent upon the pump power but not the signal power.
Reduction of Polarization DependenceFiber Optics Communication Technology-Mynbaev & ScheinerThree main approaches
Connect the amplifiers in seriesResidual facet reflectivitycan cause undesired coupling between amplifiers resulting in poor noise and dynamic performance
Connect them in parallelGood solution but complex
Double pass with polarizaion rotationAutomatic 6 db loss due to coupler
Undesired effects in an SOAFiber Optics Communication Technology-Mynbaev & ScheinerCross saturation can cause undesired coupling between channelsThis can be used for wave length conversion and controlling light with light
If used for multiple channels in a switched network gain must be adjusted as channels are added and dropped
Four wave mixing is also quite pronounced in SOAs
Causes undesired coupling of light between channelsCan however also be used to advantage in wavelength converters.
High coupling loss
Polarization sensitive gain
Short Pulse Amplification in SOAs
Semiconductor amplifier advantagesAre the right size to be integrated with waveguide photonic devices (short path length requirement)Can easily be integrated as preamplifiers at the receiver endUse same technology as diode lasersGain relatively independent of wavelengthAre pumped with current, not another laser
Semiconductor amplifier disadvantagesPolarization dependenceSelf-phase modulation leading to chirpCross-phase modulation Four-wave mixing and crosstalkExtremely short (ns) excited state lifetimes