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RoleofsemiconductoropticalamplifiersinadvancednetworkingBecauseoftheirversatility,SOAsmaysoonappearinawidevarietyofapplications.TONYKELLYandIVANANDONOVIC,KamelianLtd.WDMopticalnetworkinghasevolvedrapidlyfrombeingsimplyamoreefficientwayofincreasingthroughputtoaplatformforimplementingotherphotonicfunctionssuchasadd/dropandswitchingdirectlyintheopticaldomain.Itisalsobecomingevidentthatthemassdeploymentoftheseadvancedphotonictechniquesreliesondevelopingroutestotheintegrationofpassivewaveguidingtechnologieswithactivesemiconductordevices.Aplatformtechnologyisrequiredthatwilladdresstheneedtomanufactureadvancedcomponentsinascalablewayandmeetthevolumeandcostdemandsofsystemsintegrators.Oneapproachintegratescustomized(mode-matched)semiconductoropticalamplifiers(SOAs)onasiliconmicrobenchwithpassivecomponentstorealizehybridmodules.TheadvancedfunctionalitiesenabledbySOAsrangefromlinearopticalamplifierstoon/offswitchgatesthroughtoall-opticalwavelengthconvertersandregenerators.SemiconductoropticalamplifiersAnSOAisalaserdiodeoperatingbelowthreshold.ThemajordifferencebetweenSOAsandtypicallaserdiodesisthattheSOA'sfacetshavemultilayerantireflectioncoatingstopreventinternalfeedback.Thesedevicesarereferredtoastraveling-waveamplifiers(TWAs),sincesignalstraversethroughthecavityonlyonceandemergeintensifiedattheoutput(seeFigure1).Asisthecasewithsemiconductorlasers,onecanselectthegainpeakwithinalargewavelengthrange(0.6-1.6microns)thatcoversthewavelengthscommonlyusedintelecommunicationsnetworks.Thisversatilecomponentcanbeusedinseveralmodes,includinginlinelinearopticalamplification,high-extinctionopticalon/offgateswithinherentgain,andall-opticalwavelengthconvertersandregenerators.OpticalgaininSOAsisprovidedbynetstimulatedemissionthroughthepopulationinversionoftheactiveregioncreatedbytheapplicationofcurrentacrossthep-njunction.Thisstimulatedemissionisalwaysaccompaniedbyspontaneousemission,wherephotonsofrandomphaseandpolarizationareaddedtothesignal.TheSOAsmallsignalgainisgovernedbythematerialgain,internalloss,devicelength,andconfinementfactor.Inpracticehowever,themaximumachievablegainislimitedbytheself-saturationduetospontaneousemission.Thespontaneousemissionisamplifiedasittravelsalongthecavity,resultinginaloweredcarrierdensityandthereforereducedgain.InternalSOAgainscanbeashighas40dB,butthecouplinglossperfacetis1-4dB,yieldingtypicalgainsof25to35dB.The3-dBgainbandwidthisusuallyabout45nm,butstructuresbasedonmultiquantumwells(MQWs)canexhibitbandwidthsofupto80nm.Saturationoutputpowersgreaterthan10dBmcanbeachieved.ThedominantnoisegeneratedbytheSOAisspontaneousemission.Theadditionofspontaneousnoiseaddstwobeat-noisetermsatthesquarelawdetector.Signal-spontaneousbeatnoiseisgeneratedduetomixingofthesignalandtheamplifiedspontaneousemission(ASE);spontaneous-spontaneousnoiseisgeneratedduetomixingoftheASEcomponents.Thenoiseperformanceofanopticalamplifierischaracterizedbythenoisefigure,definedastheamountofdegradationinthesignal-to-noiseratiocausedbytheamplificationprocess.Inpractice,anarrowopticalfilterisusedbeforedetection,whichreducesthelevelofspontaneous-spontaneousbeat-noise.Inmostpracticalscenarios,forlargegains,signalASEbeat-noisedominatesoverothersourcesofnoise.TheSOAfundamentallowerlimitforthenoisefigureisnotsubstantiallydifferentfromanyphase-insensitivelinearamplifiersuchasanerbium-dopedfiberamplifier(EDFA).ThemajordifferencewhencomparedtotheEDFAisthecouplinglosses;hence,theeffectivenoisefigurerisestobetween6and10dB,comparedto4dBfortheEDFA.ThefactthatSOAscanbeintegratedwithpassiveplanarlightwavecomponentssuchasarrayedwaveguidegratings,splitters,andcombiners,however,allowsthelossespriortotheSOAtobereducedcomparedtofiber-basedamplification.Thatcanresultinloweroverallsubsystemnoisefigures.Figure3:Semiconductoropticalamplifierscanbeusedtoconstructall-opticalwavelengthconvertersbasedoncross-gainmodulation.Thestrongersignalonλ1modulatesthegainoftheamplifier,andthismodulationisimposedonλ2.Inanyopticalcommunicationsystem,thestateofpolarizationatanyinlinecomponentisunknown,simplybecausethemostwidelyinstalledtypeofopticalfiberdoesnotpreservethestateofpolarization.Thus,apartfromafewspecializedapplications,theSOAhastobepolarization-insensitive.Thereareseveralwaysinwhichthatcanbeachieved-eitherthroughsymmetricalwaveguide-design,tensilestraintobalancethedifferingconfinementfactors,orthroughpolarization-insensitiveMQWdesigns.Verylowpolarizationsensitivity(0.2dB)canbeachieved.ForeffectiveuseofSOAsinopticalnetworking,thedeviceinputandoutputhastobecoupledefficientlyintoasinglemodeopticalfiber.Thatcanbeachievedusinglensedfiberorbybulkmicro-opticstofocustheopticalfieldintoafiber.Sinceinanormaldevicethewaveguidemodeisverytightlyconfined,andthereforehighlydivergent,positionalalignmentwithsubmicrontolerancesisrequiredtoachievelowcouplinglossataconsiderableexpenseoverhead.ThepositionaltoleranceforpackagingcanbeincreasedbyexpandingtheoutputmodeoftheSOA,mostreadilyachievedthroughlateralorverticaltaperingoftheactiveregion.Thegoalistoexpandthemodesothatthefarfieldisassy