Semiconductor Optical Amplifiers And Their Applications .

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Semiconductor Optical Amplifiersand their ApplicationsMichael ConnellyDept. Electronic and Computer EngineeringUniversity of LimerickLimerick, IrelandTel: 353 61 202173, fax: 353 61 338176e-mail: michael.connelly@ul.ieKey words. Semiconductor optical amplifier,optical communication systems.Output signaland noiseCurrentOutput facetActive regionand waveguideInputsignalInput facetFig. 1: Schematic diagram of an dabsorptionE21. Introduction.There has been rapid growth in the deploymentand capacity of optical fibre communication networksover the past twenty-five years. This growth has beenmade possible by the development of new optoelectronictechnologies that can be utilised to exploit the enormousbandwidth of optical fibre. Today, systems are operationalwhich operate at bit rates in excess of 100 Gb/s. Opticaltechnology is the dominant carrier of global information.It is also central to the realisation of future networks thatwill have the capabilities demanded by society. Thesecapabilities include virtually unlimited bandwidth to carrycommunication services of almost any kind, and fulltransparency that allows terminal upgrades in capacityand flexible routing of channels. Many of the advances inoptical networks have been made possible by the opticalamplifier.Optical amplifiers can be divided into two classes:optical fibre amplifiers (OFA) and semiconductor opticalamplifiers (SOAs). The former has tended to dominateconventional system applications such as in-lineamplification used to compensate for fibre losses.However, due to advances in optical semiconductorfabrication techniques and device design, the SOA isshowing great promise for use in evolving opticalcommunication networks. It can be utilised as a generalgain element but also has many functional applicationsincluding an optical switching and wavelengthconversion. These functions, where there is no conversionof optical signals into the electrical domain, are requiredin transparent optical networksThis paper reviews SOA technology and theapplications of SOAs in emerging optical communicationnetworks.PhotonInducing photonEnergy gapStimulated photonHoleE1Electron (carrier)Fig. 2: Spontaneous and stimulated processes in a twolevel system.FeatureMaximum internal gain (dB)Insertion loss (dB)Polarisation sensitive?Pump source3 dB gain bandwidth (nm)Nonlinear effectsSaturation output power (dBm)Noise figure (dB)Integrated circuit compatible?Functional device possibility?OFA30 - 500.1 - 2NoOptical30Negligible10 - 153-5NoNoSOA306 - 10Weak ( 2 dB)Electrical30 - 50Yes5 - 207 - 12 dBmYesYesTable 1: Comparison between OFAs and SOAs.The gain of an SOA is influenced by the inputsignal power and internal noise generated by theamplification process. As the input signal power increasesthe gain decreases as shown in Fig. 3. This gain saturationcan cause significant signal distortion. It can also limit thegain achievable when SOAs are used as multichannelamplifiers in wavelength division (WDM) multiplexedsystems.A schematic diagram of an SOA is shown in Fig.1. The device is driven by an electrical current. The activeregion in the device imparts gain, via stimulated emission,to an input signal (Fig. 2). The output signal isaccompanied by noise. This additive noise, amplifiedspontaneous emission (ASE), is produced by theamplification process. A comparison between OFAs andSOAs is given in Table 1.SOAs are polarisation sensitive. This is due to anumber of factors including the waveguide structure andthe gain material. Polarisation sensitivity can be improvedby the use of square-cross section waveguides andstrained quantum-well material.Gain (dB)2. SOA basics.Output signal power (dBm)Fig. 3: Typical SOA gain versus output signal power.SOAs are normally used to amplify modulatedlight signals. If the signal power is high then gainsaturation will occur. This would not be a serious problemif the amplifier gain dynamics were a slow process.

However in SOAs the gain dynamics are determined bythe carrier recombination lifetime (few hundredpicoseconds). This means that the amplifier gain willreact relatively quickly to changes in the input signalpower. This dynamic gain can cause signal distortion,which becomes more severe as the modulated signalbandwidth increases. These effects are even moreimportant in multichannel systems where the dynamicgain leads to interchannel crosstalk. This is in contrast tooptical fibre amplifiers, which have recombinationlifetimes of the order of milliseconds leading to negligiblesignal distortion.SOAs also exhibit nonlinear behaviour. Thesenonlinearities can cause problems such as frequencychirping and generation of intermodulation products.However, nonlinearities can also be of use in using SOAsas functional devices such as wavelength converters.High gainHigh saturationoutput powerLow noise figureLow polarisationsensitivityLow insertion lossOptical filterOptical isolatorsModulatedlaserFibreNot criticalNot criticalYesYesYesYesNot criticalNot necessaryYesYesNot criticalNot criticalYesYesNot criticalIncrease medium-haul optical transmission link distanceIncrease long-haul optical transmission link power budgetCompensate for splitting and tap losses in optical distributionnetworksSimultaneous amplification of WDM signalsModulatedlaserBoosteramplifiersPassive splitterOptical d demodulationReceiverRelative signalpower (dB)TransmitterPreampYesNot criticalTable 2: Optical amplifier requirements.The principal applications of SOAs in opticalcommunication systems can be classified into three areas:(a) Postamplifier or booster amplifier to increasetransmitter laser power, (b) in-line amplifier tocompensate for fibre and other transmission losses inmedium and long-haul links and (c) preamplifier toimprove receiver sensitivity (Fig. 4). The incorporation ofoptical amplifiers into optical communication links canimprove system performance and reduce costs. The mainrequirements of optical amplifiers for such applicationsare listed in Table 2.In-line amplifiersIn-line ampYesYesTable 3: Applications of optical booster amplifiers.3. Basic network e from transmitterFig. 4: Application of SOAs as booster amplifier, in-lineamplifiers and preamplifier in an optical transmissionlink.3.a. – Booster amplifier.The function of a booster amplifier is to increasea high power input signal prior to transmission. Theprinciple applications of booster amplifiers are listed inTable 3. Boosting laser power in an optical transmitterenables the construction of medium-haul links withincreased transmission distance. Such links simply consistof an optical fibre between the transmitter and receiver.As this involves no active components in the transmissionlink, reliability and performance are improved.In long-haul links the use of a booster amplifiercan increase the link power budget and reduce the numberof in-line amplifiers or regenerators required. Boosteramplifiers are also useful in distribution networks (Fig. 5),where there are large splitting losses or a large number oftaps. Booster amplifiers are also needed when it isrequired to simultaneously amplify a number of inputsignals at different wavelengths, as is the case in WDMtransmission.Fig. 5: Booster amplifier application in opticaldistribution networks.3.b. – Preamplifier.The function of an optical preamplifier is toincrease the power level of an optical data signal beforetodetection and demodulation. The increase in power levelcan increase receiver sensitivity. This allows longerunrepeatered links to be constructed. A schematicdiagram of a preamplified optical receiver is shown inFig. 6. The receiver consists of an optical preamplifier, anarrowband optical filter and photodiode followed bypost-detection circuitry and a decision circuit.Input lightsignalPath loss (ηL)Post-detectioncircuitry anddecision erDataoutp-i-nphotodiodeFigure 6: Preamplified optical receiver.3.c. – In-line amplifier.In loss limited optical communication systems, inline optical amplifiers can be used to compensate for fibreloss thereby overcoming the need for optical regeneration.The main advantages of in-line SOAs are: Transparencyto data rate and modulation format (unsaturated operationand at high bit rates), bidirectionality, WDM capability,simple mode of operation, low power consumption and

compactness. The latter two advantages are important forremotely located optical components.3.d. – Transmission experimentAn example of a WDM transmission experiment isshown in Fig. 7 [2]. The transmitter consists of eightlasers combined by an 8:1 coupler. The wavelengths arein the range 1558-1570 nm with a channel spacing of 200GHz. The channels are externally modulated at 20 Gbit/s.Three booster SOAs are used to compensate for thecoupler and modulator losses. The transmission link iscomprised of four amplified 40 km single-mode fibrelinks including dispersion compensating fibre. The spanloss is 13 dB. The 12 to 14 dB of gain available from eachamplifier is adequate to compensate for the link loss. Thereceiver consists of two SOA preamplifiers betweenwhich the signal is demultiplexed to 10 Gbit/s by anLiNiO3 modulator. The demultiplexed data is thendetected by a photodiode. All the detected channels hadBERs 3 x 10-13.Transmitter20 Gb/smodulatorλ1λ840 km SMF DCFSOA2 kmx4SOASublinkCombinerOBPFAttenuator20 10demuxClockrecoveryReceiverFigure 7: 8-channel WDM transmission experiment.DCF: Dispersion compensating fibre ([2]).4. SOA nonlinearities.SOAs can also be used to perform functions thatare useful in optically transparent networks. These alloptical functions can help to overcome the ‘electronicbottleneck’. This is a major limiting factor in thedeployment of high-speed optical communicationnetworks. Many of these functional applications are basedon SOA nonlinearities. The development of photonicintegrated circuits (PICs) has made feasible thedeployment of complex SOA functional subsystems.Nonlinearities in SOAs are principally caused by carrierdensity changes induced by the amplifier input signals.The four main types of nonlinearity are: Cross gainmodulation (XGM), cross phase modulation (XPM), selfphase modulation (SPM) and four-wave mixing (FWM).4.a. – Cross gain modulation.The material gain spectrum of an SOA ishomogenously broadened. This means that carrier densitychanges in the amplifier will affect all of the input signals,so it is possible for a strong signal at one wavelength toaffect the gain of a weak signal at another wavelength.This non-linear mechanism is called XGM. The mostbasic XGM scenario is shown in Fig. 8, where a weakCW probe light and a strong pump light, with a smallsignal harmonic modulation at angular frequency ω, areinjected into an SOA. XGM in the amplifier will imposethe pump modulation on the probe. This means that theamplifier is acting as a wavelength converter.PumptimetimeSOACW probeModulated probeFilterFig. 8: Simple wavelength converter using SOA XGM.The most useful figure of merit of the converter isthe conversion efficiency, defined as the ratio between themodulation index of the output probe to the modulationindex of the input pump. Typical efficiency bandwidthsare of the order of 10 GHz.4.b. – Cross phase modulation.The refractive index of an SOA active region is notconstant but is dependent on the carrier density and so thematerial gain. This implies that the phase and gain of anoptical wave propagating through the amplifier arecoupled via gain saturation. This strength of this couplingis related to the linewidth enhancement factor αl. Plots ofαl versus photon energy are shown in Fig. 9.If more than one signal is injected into an SOA,there will be cross-phase modulation (XPM) between thesignals. XPM can be used to create wavelength convertersand other functional devices. However, because XPMonly causes phase changes, the SOA must be placed in aninterferometric configuration to convert phase changes inthe signals to intensity changes using constructive ordestructive interference.4.c. – Four-wave mixing.Four-wave mixing (FWM) is a coherent nonlinearprocess that can occur in an SOA between two opticalfields, a strong pump at angular frequency ω0 and aweaker signal (or probe) at ω0 - Ω, having the samepolarisation. The injected fields cause the amplifier gainto be modulated at the beat frequency Ω. This gainmodulation in turn gives rise to a new field at ω0 Ω, asshown in Fig. 10. FWM generated in SOAs can be used inmany applications including wavelength converters,dispersion compensators and optical demultiplexers.12αl 8400.802. 0.84 0.86Photon energy0.88Figure 9: Calculated linewidth enhancement factor versuswavelength for undoped InGaAsP. The parameter iscarrier density (x1024 m-3) ([3]).

PumpSignal (ω0 - Ω)SignalConjugate signalSOACW pump (ω0)γCW (λ2)1-γλ11-γSOA 1Converted output (λ2)γSOA 2Input signal (λ1)Asymmetric MZI wavelength converterω0 - Ω ω0 ω0 ΩOutput spectrumInput signal (λ1)γSOA 1CW (λ2) γSOA 2Figure 10: SOA FWM.γγConverted output (λ2)5. Functional applications.5.a. – Wavelength conversionAll-optical wavelength converters will play animportant role in broadband optical networks. Their mostimportant function will be to avoid wavelength blockingin optical cross-connects in WDM networks. Wavelengthconverters increase the flexibility and capacity of anetwork using a fixed set of wavelengths. Wavelengthconversion can be used to centralise networkmanagement. In packet switching networks, tuneablewavelength converters can be used to resolve packetcontention and reduce optical buffering requirements. Wehave already seen how XGM in an SOA can be used forwavelength conversion. SOA XPM can also be used forwavelength conversion if SOAs are placed in a MachZehnder configuration as shown in Fig. 11. Thesewavelength converters have a superior power efficiencycompared to those devices based on XGM.In the asymmetric MZI wavelength converter theCW input at λ2 is split asymmetrically to each arm of theMZI by a coupler. The intensity modulated signal at λ1saturates each SOA asymmetrically inducing differentphase shifts in the input CW signal. The output couplerrecombines the split CW signals where they can interfereconstructively or destructively. The actual state ofinterference depends on the relative phase differencebetween the interferometer arms, which relies both on theSOA bias currents and on the input optical powers.SOA FWM can be used to construct wavelengthconverters. The basic scheme is shown in Fig. 10, whereCW pump and modulated probe input signals are injectedinto an SOA, generating a new modulated signal.However the conversion efficiency is relatively low formoderate values of frequency detuning between the pumpand probe signals. For efficient FWM to occur in an SOA,the polarisation states of the pump and probe must beidentical. An example of a more efficient FWMwavelength converter is shown in Fig. 12. In this schemethe input pump is polarised at 45o relative to thepolarisation axes of polarisation beam splitter PBS1. Thismeans that half of the pump power is delivered to eachSOA along with a co-polarised component of the signal.These mix in each SOA to produce a conjugate signalwith the same polarisation. The orthogonal polarisedconjugate signals from the SOAs are recombined at theoutput in polarisation beamsplitter PBS2. If the SOAshave the same gain and conversion efficiencies then thescheme will be polarisation independent.Symmetric MZI wavelength converterFig. 11: Mach-Zehnder interferometer (MZI) SOAwavelength converters.PTE STESignalSOA BPTM STMSOA APumpPBS1PBS2TMSignal (S)θSignalPump (P)O45TEPumpConvertedλ2λ12λ1-λ2λPolarisation diversityFig. 12: Polarisation diversity wavelength converter.5.b. – Optical gates.Future high-speed WDM and TDM opticalcommunication networks require high-speed opticalswitches (or gates) that can either be optically orelectrically controlled. Such optical switches can beconstructed using SOAs. The simplest method to controlan SOA gate is by turning the device current on or off.The great advantage of SOA gates is that they can beintegrated to form gate arrays. In the 2 x 2 switch moduleshown in Fig. 13, an incoming data packet can be routedto any output port by switching on the appropriate SOA.The switching time of a current switched SOA isof the order of 100 ps. Much faster switching times can beachieved using SOAs placed in non-linear loop mirrors(Fig. 14). Switching is achieved by placing an SOA offsetfrom the centre of an optical fibre loop mirror andinjecting data into the loop via a 50:50 coupler. The ronously at the SOA. A switching pulse is timed toarrive after one data pulse but just before its replica. Theswitching pulse power is adjusted to impart a phasechange of π radians onto the replica, so the data pulse isswitched out when the two counter-propagatingcomponents interfere on their return to the coupler. Thisdevice is also known as a TOAD (terahertz opticalasymmetric demultiplexer) because it can also be used todemultiplex high-speed TDM pulse streams.

PolymerwavguidesInputfibresOutputfibresSOAFig. 13: 2 x 2 hybrid SOA switch module.tuneable filter as shown in Fig. 17. The filter can be tunedby changing its current. The selected wavelength channelis reflected by the filter, amplified a second time by theMQW section and extracted to a drop port using acirculator. The remaining channels pass through the filtersection to which it is a simple matter to add a newwavelength channel.Data A (λ1)Data B (λ2)SOATruth tableCompensator PolariserAXOR output0 1B0 0 11 1 0(a)Switchingpulses (λ1)Data pulse 1Data pulse 2Data A (λ1)Coupler1SOA waveguidesCW light in (λ3)2Data B (λ2)Truth tableReflective endA0 10 0 1B1 1 1OR output (λ3)(b)Input datapulses (λ2)Data and switchingpulses outData pump A (λ1)Data pump B (λ2)Probe (λs)Fig. 14: Optical switch using a TOAD.5. c. – Optical logic.Optical logic can be useful for all-optical signalprocessing applications in high-speed optical networks.Three SOA configurations that can be used to realiseoptical logic gates are shown in Fig. 15.5. d. – Multiplexers.Optical time division demultiplexers (OTDDMs)and add/drop multiplexers (ADMs) are key componentsrequired by optical time division multiplexed (OTDM)network nodes. In an ADM one channel is dropped froman incoming TDM data stream leaving the other channelsundisturbed. A new channel can be added by insertingdata pulses into the vacant time slot.MZI switches incorporating SOAs can also beused as ADMs. Many configurations are possible, one ofwhich is shown in Fig. 16. In this configuration the inputdata signal at 40 Gb/s is split into two drive signals. Oneof the drive signals is delayed by a half a bit period. Theinterferometer is configured such that when an undelayedsignal pulse is present in the upper arm of theinterferometer an input 10 GHz pulse is directed to thedrop port. At the same time the 3 x 10 GHz pulse streamis directed to the through port. When the delayed signalpulse is present in the lower arm of the interferometer thedata is directed away from the drop port. The amplitudesof the drop and through pulses are modified by the SOAgain saturation induced by the input data pulses so pulseamplification and reshaping also occurs, i.e. the devicefunctions as a 2R regenerator. If it is combined withoptical clock recovery for retiming it will function as a 3Rregenerator. Data can be added to the vacant time slot inthe output data simply by sending the add channel datapulses to the add port.The ability to add and drop wavelength channels inWDM networks is useful for wavelength routing. Thefunction of a wavelength ADM is to separate a particularwavelength channel without interference from adjacentchannels. This can be achieved by a wave

optical networks have been made possible by the optical amplifier. Optical amplifiers can be divided into two classes: optical fibre amplifiers (OFA) and semiconductor optical amplifiers (SOAs). The former has tended to dominate conventional system applications such as in-line amplification used to compensate for fibre losses.

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