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

Unstimulated Population densities in 2 level atomEnergy levels 1 and 2 and their decay times. By means of pumping, the population density of level 2 is increased at the rate R2 while that of level 1 is decreased at the rate R1For large DN or No (also called inversion density)We want t2 long, but t21 not too small, t1 and R1 largeIdealy t21~tsp

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