FiberOpticNetworkDesignX.Wu,M.Y.Li,H.M.YanDept.ofOpt.Engr.,ZJU2011Class6

OpticalAmplificationEDFAStructureParametersSelectionGainflatteningExerciseAnopticalamplifierisadevicewhichamplifiestheopticalsignaldirectlywithouteverchangingittoelectricity.Thelightitselfisamplified.

Reasonstousetheopticalamplifiers:

ReliabilityFlexibilityWavelengthDivisionMultiplexing(WDM)LowCostVarietyofopticalamplifiertypesexists,including:SemiconductorOpticalAmplifiers(SOAs)ErbiumDopedFiberAmplifiers(EDFAs)TraditionalOpticalCommunicationSystemLosscompensation:Repeatersatevery20-50kmOptically

AmplifiedSystemsEDFA=ErbiumDopedFiberAmplifierVarietyofopticalamplifiertypesexist,including:SemiconductoropticalamplifiersOpticalfibreamplifiers(ErbiumDopedFibreAmplifiers)Distributedfibreamplifiers(RamanAmplifiers)OpticalfibreamplifiersarenowthemostcommontypeOneofthemostsuccessfulopticalprocessingfunctionsAlsousedasabuildingblockinDWDMsystemsSource:Master7_5OpticalAmplificationErbiumdopedfiberamplifiersAmplifierapplicationsIssues:GainflatteningandNoiseOverviewBasicEDFAmplifierDesignErbium-dopedfiberamplifier(EDFA)mostcommonCommerciallyavailablesincetheearly1990’sWorksbestintherange1530to1565nmGainupto30dB(1000photonsoutperphotonin!)

Opticallytransparent“Unlimited”RFbandwidthWavelengthtransparentInput1480or980nmPumpLaserErbiumDopedFiberOutputIsolatorCouplerPumpSourceWDMErbiumDopedFibreIsolatorIsolator

=FusionSpliceInputOutputApumpopticalsignalisaddedtoaninputsignalbyaWDMcouplerWithinalengthofdopedfiberpartofthepumpenergyistransferredtotheinputsignalbystimulatedemissionForoperation~1550nmthefiberdopantisErbiumPumpwavelengthiseither980nmor1480nm,pumppower~50mWGainsof30-40dBpossibleErbiumDopedFiberAmplifierFibreinput/outputErbiumdopedfibreloopPumplaserWDMFibrecouplerSource:Master7_5InteriorofanErbiumDopedFibreAmplfier(EDFA)PumpSourceWDMErbiumDopedFibreIsolatorIsolator

=FusionSpliceInputOutput980nmsignal1550nmdatasignalPowerlevel980nmsignal1550nmdatasignalPowerlevelPowerinterchangebetweenpumpanddatasignalsOperationofanEDFAPhysicsofanEDFAErbium:rareelementwithphosphorescentpropertiesPhotonsat1480or980nmactivate

electronsintoametastablestateElectronsfallingbackemitlightin

the1550nmrange

SpontaneousemissionOccursrandomly(timeconstant~1ms)StimulatedemissionByelectromagneticwaveEmittedwavelength&phaseare

identicaltoincidentoneErbiumProperties1480980820540670GroundstateMetastablestateErbiumDopedFibreAmplifiersFiberlength:typicallytenmetresorsoDoping:asmallcontrolledamountoftherareearthelementerbiumaddedtotheglassintheformofanion(Er3+).LDpower:between10and200mW

WavelengthsLightateither980or1,480nm.)

Question#1:Whynotincreasedopinglevelanddecreaselength?Question#2:What’stheadvantageofpumping1480nm?Er+3EnergyLevelsPump:980or1480nmPumppower>5mWEmission:1.52-1.57mLonglivingupperstate(10ms)Gain30dBEDFAOperationA(relatively)high-poweredbeamoflightismixedwiththeinputsignalusingawavelengthselectivecoupler.Themixedlightisguidedintoasectionoffiberwitherbiumionsincludedinthecore.Thishigh-poweredlightbeamexcitestheerbiumionstotheirhigher-energystate.Whenthephotonsbelongingtothesignal(atadifferentwavelengthfromthepumplight)meettheexcitederbiumatoms,theerbiumatomsgiveupsomeoftheirenergytothesignalandreturntotheirlower-energystate.Asignificantpointisthattheerbiumgivesupitsenergyintheformofadditionalphotonswhichareexactlyinthesamephaseanddirectionasthesignalbeingamplified.Thereisusuallyanisolatorplacedattheoutputtopreventreflectionsreturningfromtheattachedfiber.Suchreflectionsdisruptamplifieroperationandintheextremecasecancausetheamplifiertobecomealaser!TechnicalCharacteristicsofEDFAEDFAshaveanumberof

attractivetechnical

characteristics:EfficientpumpingMinimalpolarisationsensitivityLowinsertionlossHighoutputpower(thisisnotgainbutrawamountofpossibleoutputpower)LownoiseVeryhighsensitivityLowdistortionandminimalinterchannelcrosstalkAmplifiedSpontaneousEmissionAmplifiedspontaneousemission(ASE)Randomspontaneousemission(SE)AmplificationalongfiberErbiumrandomlyemitsphotonsbetween1520and1570nmSpontaneousemission(SE)isnotpolarizedorcoherentLikeanyphoton,SEstimulatesemissionofotherphotonsWithnoinputsignal,eventuallyallopticalenergyisconsumedintoamplifiedspontaneousemissionInputsignal(s)consumemetastableelectronsmuchlessASEGainversusAmplifierlengthGainversuspumplevelNoiseSourcesEDFAEDFAhasrevolutionizedopticalcommunicationsAllopticalandfibercompatibleWidebandwidth,20~70nmHighgain,20~40dBHighoutputpower,>200mWBitrate,modulationformat,powerandwavelengthinsensitiveLowdistortionandlownoise(NF<5dB)PumpSource980nmlowASE,lownoiseamplifier1480nmhigherpowerpumplaserhighoutputpowernotasefficientdegreeofpopulationinversionislowerEDFABehaviouratGainSaturationTherearetwomaindifferencesbetweenthebehaviourofelectronicamplifiersandof

EDFAsingainsaturation:1)

As

inputpowerisincreasedontheEDFAthetotalgainoftheamplifierincreasesslowly.Anelectronicamplifieroperatesrelativelylinearlyuntilits

gainsaturatesandthenitjustproducesallitcan.Thismeansthatan

electronicamplifieroperatednearsaturationintroducessignificantdistortions

intothesignal(itjustclipsthepeaksoff).2)Anerbiumamplifieratsaturationsimplyapplieslessgaintoallofitsinput

regardlessoftheinstantaneoussignallevel.Thusitdoesnotdistortthesignal.

ThereislittleornocrosstalkbetweenWDMchannelseveninsaturation.

Totaloutputpower:Amplifiedsignal+Noise(AmplifiedSpontaneousEmissionASE)EDFAisinsaturationifalmostallErbiumionsareconsumedforamplificationTotaloutputpowerremainsalmostconstant,regardlessofinputpowerchanges

Pin(dBm)TotalPout-3dBMax-20-30-10GainSaturationinEDFAsGainCompressionTotaloutputpower:

Amplifiedsignal+ASEEDFAisinsaturationifalmostallErbiumionsareconsumedforamplificationTotaloutputpowerremainsalmostconstantLowestnoisefigure

Preferredoperatingpoint

PowerlevelsinlinkstabilizeautomaticallyPin(dBm)TotalPout-3dBMax-20-30-10GainAmplifierLengthAsthesignaltravelsalongthelengthoftheamplifieritbecomesstrongerdueto

amplification.Asthepumppowertravelsthroughtheamplifieritsleveldecreases

duetoabsorption.Thus,boththesignalpowerlevelandthepumppowerlevel

varyalongthelengthoftheamplifier.Atanypointwecanhaveonlyafinite

numberoferbiumionsandthereforewecanonlyachieveafinitegain(andafinite

maximumpower)perunitlengthoftheamplifier.

Fiberlength=F(signalpower,pumppower,Erconcentration,gainrequired)Inanamplifierdesignedformultiwavelengthoperationthereisanotherconsideration-theflatnessofthegaincurveovertherangeofamplified

wavelengths.Withacarefuldesignandoptimisationoftheamplifier'slengthwecanproduceanearlyflatamplifiergaincurve.OpticalGain(G)G=SOutput/SInput

SOutput:outputsignal(withoutnoisefromamplifier)

SInput: inputsignal

InputsignaldependentOperatingpoint(saturation)of

EDFAstronglydependson

powerandwavelengthof

incomingsignalWavelength(nm)Gain(dB)154015601580101520204030-5dBm-20dBm-10dBmPInput:-30dBmSeveralattractivefeaturesfornetworkuse:RelativelysimpleconstructionReliable,duetothenumberofpassivecomponentsAllowseasyconnectiontoexternalfibersBroadbandoperation>20nmBitratetransparentIdeallysuitedtolongspansystemsIntegralpartofDWDMsystemsUnderseaapplicationsforOFAsarenowcommonOFAsintheNetworkIn-lineAmplifierPowerAmplifierPreamplifierFibreLinkTransmitterTransmitterTransmitterOpticalAmplifiersOpticalAmplifierFibreLinkOpticalAmplifierFibreLinkOpticalReceiverOpticalReceiverOpticalReceiverOpticalAmplifierApplicationsAmplifierApplicationsPreamplifiersverylowalownoisecharacteristicisessentialonlyamoderategainfigureTypicallyapreamplifierwillnothavefeedbackcontrolasitcanberunwellbelowsaturation.PoweramplifiersMostDFBlasersoutput:2mW,Afiber:aggregatepowerlevelsofupto100to200mWbeforenonlineareffectsstarttooccur.TypicalEDFApoweramplifiershaveanoutputofaround100mWLineamplifiersReplacesarepeaterwithinalong

communicationlinehighgainattheinputandhighpoweroutputaverylownoisefigure.Thisisreallyapreamplifiercascaded

withapoweramplifier.Sophisticatedlineamplifierstodaytendtobemade

justthisway-asamulti-sectionamplifierseparatedbyanisolator.EDFACategoriesIn-lineamplifiersInstalledevery30to70kmalongalinkGoodnoisefigure,mediumoutputpowerPowerboostersUpto+17dBmpower,amplifiestransmitteroutputAlsousedincableTVsystemsbeforeastarcouplerPre-amplifiersLownoiseamplifierinfrontofreceiverRemotelypumpedElectronicfreeextendinglinksupto200kmandmore

(oftenfoundinsubmarineapplications)RXPumpTXPumpBestusedforsinglechannelsystemsinthe1550nmregion,Systemsaredesignedforuseasboosters,in-lineamplifiersorpreamplifiers.BandwidthisnotwideenoughforDWDM,specialEDFAneededExample:ConventionalEDFASource:Master7_5Gainflatnessisnowwithin1dBfrom1530-1560nmITU-TDWDMCbandis1530to1567nmGainFlattenedEDFAforDWDMTypeGainMaximumOutputpowerNoisefigurePowerAmplifierHighgainHighoutputpowerNotveryimportantIn-lineMediumgainMediumoutputpowerGoodnoisefigurePreamplifierLowgainLowoutputpowerLowvalue<5dBessentialSelectingAmplifiersPumpingDirectionsAdditionalpumpingoptionsMultistageEDFAsSomenewEDFAdesignsconcatenatetwooreventhreeamplifierstages.An

amplifier“stage”isconsideredtoconsistofanyunbrokensectionoferbiumdoped

fibre.Multistageamplifiersarebuiltforanumberofreasons:1.Toincreasethepoweroutputwhileretaininglownoise2.Toflattenthetotalamplifiergainresponse3.Toreduceamplifiedstimulatedemissionnoise

Two-StageEDFALineAmplifierwithSharedPump.Pumppowerwould

typicallybesplitinaratiodifferentfrom50:50.CommercialDesignsInputEDFOutputIsolatorTelemetry&RemoteControlPumpLasersOutputMonitorEDFInputMonitorIsolatorSecurity/SafetyFeaturesInputpowermonitorTurningontheinputsignalcancausehighoutputpowerspikesthatcandamagetheamplifierorfollowingsystemsControlelectronicsturnthepumplaser(s)downiftheinputsignalstaysbelowagiventhresholdformorethanabout2to20μs

BackreflectionmonitorOpenconnectorattheoutputcanbealasersafetyhazardStraightconnectorstypicallyreflect4%ofthelightbackBackreflectionmonitorshutstheamplifierdownifbackreflectedlightexceedscertainlimitsOutputSpectraASEspectrumwhennoinputsignalispresentAmplifiedsignalspectrum(inputsignalsaturatestheopticalamplifier)1575nm-40dBm1525nm+10dBmErbiumcanprovideabout40-50nmofbandwidth,from1520to1570nmGainspectrumdependsontheglassused,eg.silicaorzblanglassGainspectrumisnotflat,significantgainvariationsWavelength(nm)

152015301540155015603020100Gain(dB)EDFAgainspectrumEDFAGainSpectrumGainCharacteristicsofEDFAGain(amplifier)-istheratioindecibelsofinputpowertooutputpower.Gain

at1560nmissome3dBhigherthangainat1540nm(thisistwiceasmuch).In

mostapplications(ifthereisonlyasinglechannelorifthereareonlyafew

amplifiersinthecircuit)thisisnottoomuchofalimitation.WDM

systemsusemanywavelengthswithintheamplifiedband.Ifwehaveaverylong

WDMlinkwithmanyamplifiers

thedifferenceinresponseinvariouschannelsaddsup.FlatteningoftheGainCurveTechniquesOperatingthedeviceat77°K->amuchflattergain,notallthatpractical.Introducingotherdopantmaterials(suchasaluminiumorytterbium)alongwith

theerbiumintothefibrecore.Amplifierlengthisanotherfactorinfluencingtheflatnessofthegaincurve.Controllingthepumppower(throughafeedbackloop)isroutinetoreduceamplifiedspontaneousemission.AddinganextraWDMchannellocallyattheamplifier(“gainclamping”).Manipulatingtheshapeofthefiberwaveguidewithintheamplifier.Atthesystemslevelthereareotherthingsthatcanbedonetocompensate:Using“blazed”fiberBragggratingsasfilterstoreducethepeaksinthe

responsecurve.TotransmitdifferentWDMchannelsatdifferentpowerlevelstocompensatefor

lateramplifiergaincharacteristics.GainFlatteningConceptUsedtoreducevariationinamplifiergainwithwavelength,usedinDWDMsystemsThegainequalisationisrealisedbyinsertingtaperedlongperiodgratingswithintheerbiumdopedfibre.DesignedtohaveapproximatelytheoppositespectralresponsetothatofanEDFAInlineDicongainflatteningfilterInlineDicongainflatteningfilterspectralresponseGainFlatteningFilters(Equalizers)SpectralHoleBurning(SHB)GaindepressionaroundsaturatingsignalStrongsignalsreduceaverageionpopulationHolewidth3to10nmHoledepth0.1to0.4dB1530nmregionmoresensitive

toSHBthan1550nmregion

ImplicationsUsuallynotanissueintransmission

systems(singlelorDWDM)Canaffectaccuracyofsome

lightwavemeasurements1545155015601540Wavelength(nm)7nm0.36dBPolarizationHoleBurning(PHB)PolarizationDependentGain(PDG)Gainofsmallsignalpolarizedorthogonaltosaturatingsignal0.05to0.3dBgreaterthanthelargesignalgainEffectindependentofthestateofpolarizationofthelargesignalPDGrecoverytimeconstantrelativelyslow

ASEpoweraccumulationASEpowerisminimallypolarizedASEperpendiculartosignalexperienceshighergainPHBeffectscanbereducedeffectivelybyquicklyscramblingthestateofpolarization(SOP)ofthein