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121Chapter7ComponentsofFiber-121Chapter7ComponentsofFiber-Optic7.1SemiconductorOpticalTheneedforopticalamplifiersisclearfromtheattenuation-inducedweakeningofanopticalsignalasittravelsdowntheTherearetwomeanstoboostasignal:repeatersandopticalInspiteofseveralattractivefeatures,repeatersthemselvescannotbeusedtobuildWDMlinksandnetworks.Thus,theshiftfromTDMtoWDMlinksandfromlinkstofiber-opticnetworkscouldnothavebeendonewithoutoptical中德智能制造學(xué) OfSino-German 中德智能制造學(xué) OfSino-GermanIntelligentChapter7ComponentsofFiber-OpticNetworks342Chapter7ComponentsofFiber-342Chapter7ComponentsofFiber-OpticOptical-amplifiertypes:semiconductorandTwomajorclassesofopticalamplifiersareinusetoday:semiconductoropticalamplifiers(SOA)andfiberopticalamplifiers(FOA).Asemiconductoropticalamplifieris,inessence,anactivemediumofasemiconductorlaser.Inotherwords,anSOAisalaserdiodewithout,orwithverylow,opticalfeedback.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticThat’sexactlythemajorproblemthatconfrontedtheindustryseveralyearsagoand,inresponse,opticalamplifierswereTherearetwomajorclassesofopticalamplifiers:semiconductorandfiber.Themostpopularistheerbium-dopedfiberamplifier(EDFA),whichhasopenedthedoortothedeploymentofWDMfiber-opticcommunicationssystems.Butsemiconductorandothertypesofamplifiersarealsoinuseandevenmoretypesareintheprocessofresearchand中德智能制造學(xué) OfSino-German 563Chapter7ComponentsofFiber-563Chapter7ComponentsofFiber-OpticFiberamplifiers,specificallyerbium-dopedfiberamplifiers(EDFA),aretheworkhorsesintoday’sWDMnetworks.Erbium-dopedfiberamplifiersoperateonlyinthe1550-nmwindow,whilesemiconductoropticalamplifierscoverboththe1300-nmand1550-nmtransparentwindows.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticAfiberopticalamplifierisquitedifferentfromasemiconductoropticalamplifier.Essentially,itisapieceofspecialtyfibersplicedwithatransmissionfiberandconnectedtoapumplaser.Likeasemiconductoropticalamplifier,afiberamplifierworksontheprincipleofstimulatedemission.Energydeliveredbyapumplaserisusedtoexciteatomsattheupperenergylevel,wheretheyarestimulatedbythephotonsofaninformationsignaltofalltothelowerlevel.中德智能制造學(xué) OfSino-German 784Chapter7ComponentsofFiber-784Chapter7ComponentsofFiber-OpticJustimaginetheadvantage:ThesamepieceoffiberwouldserveasatransmissionandamplificationmediumEvenmoreremarkable,youwouldbeabletoplacethispieceataremotelocationfromyourpumpsource,thusfeedingandcontrollingamplificationfromacentralstation.Thesedevicesdefinitelypromisetoopennewvistasinopticalamplification.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticOthertypesofopticalThereare,ofcourse,othertypesofopticalamplifiersbesidesSOAsandEDFAs.Theseothersusenonlineareffectsforamplificationratherthanstimulatedemission.TwotypesofopticalfiberamplifiersthatareclosetoreachingpracticalimplementationusetheRamanandBrillouineffects.(ThesearediscussedinSection4.4.)Usingtheseeffectswouldmakeitpossibletobuilddistributed,notlump,amplificationofanopticalsignal.中德智能制造學(xué) OfSino-German 95Chapter7ComponentsofFiber-95Chapter7ComponentsofFiber-OpticSuchagainisneededtoincreasethespanbetweentwoadjacentamplifiers,butitresultsinlaunchinghigh-powerlightintotheopticalfiberimmediatelyaftertheamplifier.This,inturn,causesseverenonlineareffectsinanopticalfiber(seeSection4.4).PlacementofaRamanamplifierclosetotheendofeachspanbetweenEDFAsallowsthenetworkdesignertoreducethegainoftheEDFApositionedbeforetheRamanamplifierwhilekeepingthetotalgainoverthespanatthedesiredlevel.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticARamanamplifierrecentlyappearedonthemarketinbothdistributedanddiscreteforms.ItsmostpopularconfigurationtodayisahybridEDFA/Ramanamplifier,adeviceinwhichtheRamanamplifiercompensatesfortheEDFA’slackofgaininthewavelengthrangefrom1570to1630nm.AnotherimportantapplicationofadistributedRamanamplifieristopreventthedeleteriouseffectcausedbythelargegainofanEDFAamplifier.中德智能制造學(xué) OfSino-German 6Chapter7ComponentsofFiber-6Chapter7ComponentsofFiber-OpticAbooster,alsocalledapost-amplifier,isapoweramplifierthatmagnifiesatransmittersignalbeforesendingitdownafiber.(SeeFigure12.13(a).)Aboosterraisesthepowerofanopticalsignaltothehighestlevel,whichmaximizesthetransmissiondistance.Themainrequirementofthisamplifieristoproducemaximumoutputpower,notmaximumgain,becausetheinputsignalhereisrelativelylarge;itcomesalmostimmediatelyfromatransmitter.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticFunctionaltypesofopticalOpticalamplifiersarecategorizedintermsofthefunctiontheyperform.Thethreebasictypesareboosters,in-lineamplifiers,andpreamplifiers.(SeeFigure7.13.)Figure7.13Functionaltypesofoptical(a)(b)in-line(c)中德智能制造學(xué) OfSino-German 7Chapter7ComponentsofFiber-7Chapter7ComponentsofFiber-OpticHence,themainrequirementofthistypeofamplifierisstabilityovertheentireWDMbandwidth.Sincemanyin-lineamplifiersmaybecascaded,similarityingaincharacteristicsisalsoofconcernwhenworkingwiththisKeepingnoiseattheminimallevelandperforminggoodopticalinteractionwithatransmissionfiberareotherrequirementsofthistypeofopticalamplifier.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticAnadditionalbenefitofusingaboosteristhatitrelievesthetransmitterofthenecessityofproducingmaximumopticalpower,enablingtransmitterdesignerstoconcentrateonimprovingabroadrangeoftransmittercharacteristicsratherthanalwaysworryingaboutpowerrequirements.Anin-lineamplifieroperateswithasignalinthemiddleofafiber-opticlink,asFigure12.13(b)shows.Itsprimaryfunctionistocompensateforpowerlossescausedbyfiberattenuation,connections,andsignaldistributionin中德智能制造學(xué) OfSino-German 8Chapter7ComponentsofFiber-8Chapter7ComponentsofFiber-OpticThenumberofboostersandpreamplifiersrequiredforaspecificnetworkisdeterminedbythenumberoftransmittersandreceiversituses.Butthenumberofin-lineamplifiersneededdependsbothonthelengthofthefiber-opticlinkandthenetwork’sForlong-haullinks,suchastransoceanicandtranscontinental,in-lineamplifiersareusuallyinstalledevery80to100km.Theseamplifierscompensateforlossescausedbyfiberattenuationandsplices.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticApreamplifiermagnifiesasignalimmediatelybeforeitreachesthereceiver.(SeeFigure12.13(c).)ThistypeofopticalamplifieroperateswithaweakHence,goodsensitivity,highgain,andlownoisearemajorrequirementshere.Noisebecomesanextremelyimportantfeatureofapreamplifierbecauseareceiver’sperformanceislimitednotbyitsownnoisebutbythenoiseofapreamplifier.Usingapreamplifierlessensthestringentdemandthatotherwisewouldbemadeonareceiver’ssensitivityandeventuallyallowsanetworktooperateatahigherbitrate.中德智能制造學(xué) OfSino-German 9Chapter7ComponentsofFiber-9Chapter7ComponentsofFiber-OpticFigure12.14Useofin-line(a)Compensationoffiberattenuationand(b)compensationofsignal中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticHowever,in-lineamplifiersarealsoneededforshort-distancenetworkstocompensateforlossescausedbysignaldistributioninalocalareanetwork,asshownschematicallyinFigure12.14.Whenwetalkaboutboosters,in-lineamplifiers,orpreamplifiers,weusuallyimplytheuseoferbium-dopedfiberamplifiers.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticInjectioncurrentdeliverstheexternalenergynecessarytopumpelectronsattheconductionband.Theinputsignalstimulatesthetransitionofelectronsdowntothevalencebandandtheemissionofphotonswiththesameenergy—thatis,thesamewavelength—thattheinputsignalThus,theoutputisanamplifiedoptical中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticPrincipleofOperationAnSOAusestheprincipleofstimulatedemissiontoamplifyanopticalinformationsignal.HowanSOAisconnectedtoafiberlinkisshownschematicallyinFigure12.15.Anopticalinputsignalcarryingoriginaldataentersthesemiconductor’sactiveregionthroughcouplingoptics.CouplingisrequiredbecausetheMFDofasinglemodefiberbeamistypically9.3μm,whilethesizeoftheactiveregionislessandcanevenbeontheorderoftenthsofmicrometers.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticGainOfanThetwobasicSOAtypesaretheFabry-Perotamiplifierandthetraveling-waveamplifierTheFPAhasthesameconfigurationasaFabry-PerotLightenteringtheactiveregionisreflectedseveraltimesfromcleavedfacetsand,havingbeenamplified,leavesthecavity.ThisexplanationisdepictedinFigure12.16(a),wheredifferentpathsofareflectedbeamareshownforillustrativepurposesonly,nottorepeatourdiscussionofthestructureandprincipleofoperationofaFabry-Perotsemiconductorlaser中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticFigure12.15Semiconductoropticalamplifier中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticChapter7ComponentsofFiber-OpticGainofanIfwedenotethepower-reflectioncoefficientsofcleavedfacetsasR,thelengthofanactiveregionasL,thevelocityoflightwithintheactivemediumwithrefractiveindexnasv=c/n,andthesingle-passpoweramplificationfactorasGs,thenthegain(G)ofanFPAcanbeexpressedasw + whereωandω0arethecurrentandcenterangularfrequencies,中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticFigure12.16Fabry-Perotandtraveling-(a)Fabry-Perot(b)traveling-wave(c)gainasafunctionof中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticResonantfrequenciesfromFormula12.5aregiven wherev=c/nisthespeedoflightwithinaresonatorYoucaneasilycalculatethespacingbetweenadjacentresonantfrequencies.Withtheseexplanationsinmind,youcanappreciatethedependenceofFPAgainonfrequency,whichisshowninFigure12.16(c).中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticAsingle-passageamplificationfactor,Gs(ω),isassumedtohaveaGaussian-shapedependenceonfrequency,asshowninFigure12.16(c).AnFPAexhibitspeaksofgain,calledgainripple,atresonantfrequencies(wavelengths).ThesearefrequenciesthataresonatormadefromfacetsseparatedbydistanceLcansupport.Theseresonantwavelengths(λN)lN=2L/ whereNisanintegerandListhelengthoftheactiveregionthatisequaltoresonatorlength.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticUsingaFabry-Perotresonator,whichprovidesopticalfeedback,cansignificantlyincreasethegainofanSOA.Thehigherthereflectance(R),thehigherthegainattheresonantfrequencies.Butbearinmindthatincreasingthereflectancebeyondacertainpointcanputusintoanoscillationmode,thatis,turnouramplifierintoalaser.Indeed,thecloserRGsisto1,then,asfollowsfromFormula12.4,thehigherwillbetheGFPA(ω).Intheextreme,whenRG=1,GFPA(ω)goestoinfinity,thatis,tothepointwhereanFPamplifierstartstogeneratelight.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticIfweconsiderreflectionoccurringatthenaturalsemiconductor-airinterface,whichisequaltoabout0.32,we’llseepeaksofFPAgainattheresonantfrequencies.Thesmallerthereflectance,thelesspronouncedthegainpeaks,asshowninFigure12.16(c)foxR=0.03.Ultimately,wearriveatR=0,whichisaTWAsemiconductoramplifier.ThisissimplyaGaussiancurve,Gs(ω),aswehave中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticExample中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticExample中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticAsingle-passgain(GS)canbeexpressedthroughtheparametersofanSOAasfollows: whereΓistheconfinementfactorthataccountsfortheguidingofradiatedphotonsbythewaveguidestructureofanactiveregion;g(1/m)isthegaincoefficientofanactiveregionperunitofα(1/m)isthelosscoefficientofacavityperunitofInasense,Formula12.9definesthegainofaTWADesignerhasfourmeanstoincreasethegainofanamplifierwithoutreflectivefacets:increaseΓ,g,andL,anddecreaseα.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticGainofaAtraveling-waveamplifierisessentiallyanactivemediumwithoutreflectivefacetssothataninputsignalisamplifiedbyasinglepassagethroughtheactiveregion,asshowninFigure12.16(b).Thegainofatraveling-waveamplifierisgivenbyFormula12.4,wherethereflectanceshouldbezero.Hence, Thus,thegainofaTWAisthegainofanFPAwithR=0,whichimmediatelyfollowsfromthedefinitionofaTWA.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticExample中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticExample中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticFigure12.17Makingatraveling-wavesemiconductoropticalamplifier:(a)Usingantireflection(AR)(b)tiltingactive(c)usingtransparent中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Optic中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-Optic中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticGain:ThedifferencebetweenFPAand中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-Optic中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticGain中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticBandwidthofanThebandwidthofanamplifier,bydefinition,isthefrequencyrangeatwhichthegaindropstwice(3dB)itsmaximumvalue.Todeterminethe3-dBbandwidthofanFPA,wehavetolookatFormula12.4again.Whiledetuning(ω-ω0)increases,gain(G)decreases.Thevalueof(ω-ω0)atwhichGisreducedbyafactorof2istheFPAbandwidth.Mathematically,thisisequivalenttorequiringthedenominatorofGinFormula12.4todouble.Thus,weobtain:中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticFigure12.18Gain中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticExample中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Optic which andthisformulaistruefortheseconditions:3-22<RGS<3+2thatis,0.17<RGS<Thisformuladeterminesthebandwidthofan中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticFromFormula12.8weeasily SubstitutingFormula12.17intoFormula12.16,we 1- FromFormula12.18,youcanclearlyseethetrade-offbetweenbandwidthandgaininanFPA.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticCanweincreaseanFPA’sbandwidth?Yes,butattheexpenseofthegainbecausetheonlyconstituenttoalterinFormula12.12istheRGSWecandemonstratethisgain-bandwidthtrade-offinexplicitformbystudyingthefollowingderivation.Since1-RGSinanFPAisverysmall,wecan /2}中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticButthecriteriongiveninFormula12.12statesjusttheopposite:AnamplifierisconsideredaTWAifRGs<0.17.Hence,Formula12.15cannotbeappliedtocalculateTWAHowever,qualitatively,aTWA’sbandwidthpropertycanbedescribedbasedonFormula12.18,modifiedtoapplytoaTWA'scondition: 1- 中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticLet’sconsiderthe3-dBbandwidthofaItwouldseemthatwecouldestimatethisbandwidthfromFormula12.15byplugginginR=0,butitdoesn’twork.Indeed,fromFormula12.15wecanfindthisveryevident dictatedbythepropertyofasinefunction.Asimplemanipulationyields 中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticFigure12.19Band-widthsofFabry-Perotandtraveling-waveamplifiers.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticFromFormula12.21,youcanseethesamebandwidth-gaintrade-offaswasthecaseforanFPA.Moreover,foraTWAwewanttohavethereflectances(R)muchsmallerthantheyareforanFPA;hence,theproductofthegainandbandwidthforaTWAislargerthanitisforanAsamatteroffact,aTWAwithasingle-passgainofabout30dBandabandwidthnear40nmiscommerciallyavailable.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-Optic中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticWe’veseenthatforamplificationofasinglechannel(wavelength),SOAsarefarandawaytheleadingsystemsinuse.Butsupposewewanttoamplifyseveralchannels(wavelengths)simultaneously,aswedoinWDMnetworks.Thefirstthingwehavetoworryaboutinsuchacaseiscrosstalk,whichisanydistortionofachannelcausedbythepresenceofanotherchannel.(ThetermtracesitsorigintotelephoneThepenetrationofonevoicechannelintoanotherlinewasthetypeofcrosstalktelecommunicationsengineersfirst中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-Optic中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticFigure12.20Crosstalkinasemiconductoroptical(a)Signalamplificationwithoutcrosstalk;(b)interchannel中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticFigure12.21Reducingpolarizationdependenceinan(a)Polarization-dependentgain;(b)connectionoftwoSOAsin(c)connectionoftwoSOAsinparallel;(d)doublepassthroughan中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticThegainofsemiconductoropticalamplifiersdependsonthestateofpolarizationoftheinputsignal.Inotherwords,theamplificationoftransverse-electric(TE)andtransverse-magnetic(TM)modesisdifferentasshowninFigure12.21(a).ThephysicalreasonsforthedependenceofSOAgainonpolarizationincludetherectangularshapeandthecrystalstructureoftheactiveregion.Thesetwocharacteristicsmakethegain(g)and(r)coefficientsdependentonpolarization.Thedifferenceingainbetweentwoorthogonalpolarizationscanreach5to7dB.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-Optic中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Optic中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticFigure12.22Measurementofnoise(a)measurementof(b)measurementof(SNR)out中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Optic中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticAmplifiedspontaneousemissionThenoisegeneratedbyanactivemediumofanopticalamplifieriscausedprimarilybyamplifiedspontaneousemission(ASE).Thephysicsunderlyingthisphenomenonisasfollows:Thevastmajorityofexcitedcarriersareforcedbystimulatedemissiontofalltoalowerlevel,althoughsomeofthesecarriersdosospontaneously.Whentheydecay,thesecarriersradiatephotonsThespontaneouslyemittedphotonsareinthesamefrequencyrangeastheinformationsignal,buttheyarerandominphasesanddirections.中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticExample中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticYoumayrecallthatemissionistermedspontaneousifitoccurswithoutexternalstimulation.Spontaneousemissiondependsontherelativepopulationsoftheupper(excited)andlowerenergylevels.(Insemiconductors,aswelearned,theupperlevelistheconductionbandandthelowerlevelisthevalenceband.)Aspontaneous-emissionfactor,orpopulation-inversionfactor(nsp),canbedefinedasnsp=N2/(N2-N1 whereN2andN1arepopulationsoftheexcitedandlowerlevels,中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-OpticThespontaneouslyemittedphotonsthatfollowinthedirectionoftheinformationsignalareamplifiedbyailactiveThesespontaneouslyemittedandamplifiedphotonsconstituteamplifiedspontaneousemission(ASE).Sincetheyarerandominphase,theydonotcontributetotheinformationsignalbutgeneratenoisewithinthesignal’s中德智能制造學(xué) OfSino-German Chapter7ComponentsofFiber-Chapter7ComponentsofFiber-OpticThepointtorememberhereisthatspontaneousemissioninanopticalamplifieroccurswithinthesamebandofwavelengths(frequencies)inwhichsignalamplificationtakesplace.Thisiswhyspontaneousemissionisthemainmechanismaddingnoi