光纤通信课件：Chapter6-Optical-Source-and-Amplifiers

Chapter6OpticalSourceandAmplifiers6.1Light-EmittingDiodespnLEDiV+-pnEnergyLevelDiagramforV=0(zerobias)WgConductionBandValenceBand-------++++++HoleEnergyElectronEnergyPNEnergyLevelDiagramforV>0(forwardbias)WgConductionBandValenceBand-----------------------++++++++++++HoleEnergyhf=WgElectronEnergyWg=bandgapenergyItsvaluedependsonthesemiconductorused.Thereleasedenergytakestheformofaphoton.(6.1)WgisinjoulesandlisinmetersSupposeWgisgivenineV.Computetheoutputwavelength.(6.2)withWgineVandlinmm.Material (m) Wg(eV)GaAs 0.9 1.4AlGaAs 0.8-0.9 1.4-1.55InGaAs 1.0-1.3 0.95-1.24InGaAsP 0.9-1.7 0.73-1.35GaInP 0.64-0.68 1.82-1.94CommonCharacteristicsofLED6.2LEDOperatingCharacteristics05OutputPower(mW)InputCurrent(mA)LinearRegionNonlinearRegion100(6.3)(6.4)whereWgisineVandPisinWattswhereWgisinjoulesandPisinWattsdecreasesathighcurrents,creatingthenonlinearsaturationregion.+-LostPhotonsLEDModulatinganLED(Digital)OpticalPowerOpticalPowerOutput111100InputCurrent(Signal)ttSignalLEDiModulatinganLED(Analog)iThetotaldiodecurrentis i=Idc+IspsintwisthemodulationfrequencyisIdcBiasSignalLEDOpticalPowerOpticalPowerInputCurrent(Signal)tModulatinganLED(Analog)tPdcIdciPspIsp0JunctionandparasiticcapacitancethecapacitivereactanceThereactancegetssmallerasthemodulationfrequencywgetslarger,isPotrttOutputPowerLargeincreasesthepulserisetimetr.Largebroadenstheoutputpulse.InputCurrentThesechargesproducethesephotons.2)

Non-zerocarrierlifetime()

(6.8)TypicalLEDbandwidthsareontheorderof1MHzto100MHz.TheelectricalbandwidthoftheLEDistheniLEDitr10%90%PttThe10%to90%risetimeisshown.StepinputcurrentOutputpowerTherelationshipbetweenelectricalbandwidthandrisetimeis(approximately),(6.9)Wewilluseitfordeterminingthefrequencyresponseoflightsources,lightdetectors,andthefiber.TypicalvaluesoftrforLEDsareafewnsto250ns.RadiationPatternsofLEDsnpIntensitycos60º90º1.00.5BeamIntensity0At=0º,cos=1At=60º,cos=0.5Totalhalf-powerbeamwidthis120ºExample:LetthefiberNA=0.24FiberAcceptanceConeSourceRadiationCone||nppnJunctionLEDPackagingGlassCover(orLens)MetalCapHeaderLEDElectricalLeadsLEDMountedonaHeaderSource-to-FiberCouplingofaGlass-CoveredLEDLED FiberLED FiberA)WithoutaLens B)WithaLensClearCoverMetalCapHeaderLEDElectricalLeadsMicrolensedLEDMicrolens6.3LaserPrinciplesHeNeLaser

=0.633m(red)Fiber2NA=sinFiberScatteredLightOutputLightStructureMirrorMirrorLHeNeGasTubePowerSupplyOutputWindowzEnergyLevelDiagramHe Neon170,000160,000150,000Energy(J)WuWlWpNeatomsarenowintheexcitedstateTheNeatomsdropdowntothelevelAtom-PhotonInteractionsforNeon=filledstate=formerlyfilledstateLowerLevelHigherLevelPhotonAbsorption=filledstate=formerlyfilledstateLowerlevelHigherlevelPhotonSpontaneousemission

orstimulatedemission.

PossibilitiesLowerLevelHigherLevelphotonpopulationinversionLowerLevelHigherLeveloriginalphotonnewphotonWenowhavethefollowing:1500MHzffffGainCavityResonancesOutputSpectrumShownarethethreelongitudinalmodesofthelaser.TheoutputhasaGaussianbeamintensitypattern.0.1351rI/I0w=10mm0.56.4LaserDiode

10m~0.1-0.3m300mJunctionRegionnpMetalStripeContactLaserDiodeMetalVFeedback

Example:AlGaAs,n=3.6AmplificationWg---------------------------------+++++++++++++++++++++HoleEnergyElectronEnergyOriginalPhotonFreeElectronsFreeHolespStimulatedPhotonnIncomingPhotonForwardBiasedpnJunctionTuningThebandgapenergydeterminestheoutputwavelength.Previously,wefound(6.2)whereisinmandWgisineV.theenergytransitionoccursbetweenbandsofenergy(notdiscretelinesasinagaslaser).-------------------

--------------+++++++++++++++++++++WgLinewidth(3.26)(3.25)Assume:0=0.82m,L=300m,n=3.6,=2nm(laserlinewidth)ThecavityalsoaffectstheoutputspectrumExample:ThecavityresonantwavelengthspacingisLnLnccooc2222lll==DWeconcludethattherearesixlongitudinalmodes.Thenumberoflongitudinalmodesisapproximately819 820 821Gain0.311nm(nm)(nm)CavityResonancescForthelaserdiode,wehave:(nm)cOutputSpectrum6.5LaserDiodeOperatingCharacteristicsP(mW)50I(mA)ITH=thresholdcurrentExample:ITH=75mADiodeVoltages1.2-2Volts

ActualIdealITH100OpticalPowerOpticalPower111100InputCurrentorSignalttDigitalModulationisIdcITHiisIdcBias

Thedcbias,setjustat(ornear)threshold,makesthedeviceturnonquickerforbinaryones.DigitalModulationSignal

OpticalPowerOpticalPowerInputCurrent(Signal)tAnalogModulationtPdcIdciPspIspITHThedcbiasissettooperateinthemiddleofthelinearregionofthediode’scharacteristiccurve.AnalogModulationisIdcBias

Signal

iTemperatureDependenceP(mW)50i(mA)30°C80°C70100Example:WavelengthShift=0.1nm/oCT1=27°CGainCavityResonanceOutputCavityResonanceOutputT2=30°CBandwidth f3-dB=0.35/trExample:Lettr=0.1nsLightfromthelaserdiodecanbemodulatedatratesover40GHz(40Gb/s).LaserDiodeBandwidthHalf-PowerBeamwidths

||=10Parallel=35Perpendicular

RadiationPattern||7m0.2mLaserDiodePackageDiodeTerminalThreadedContactLDWindowCapGroundLaserDiodeWithIntegralFiberPigtailDiodeTerminalThreadedContactLDGroovedBlockCapGroundFiberPigtailGroovedBlockLDGroundPhotodetectorDiodeTerminalInsulatedStandoffSiliconSubstrateFiberAlaserdiodewithanintegralpowermonitorandfiberpigtailLDLDpn6.6.1DistributedFeedbackLaserDiodeDifferentMaterialsGratingCleavedFacetActiveLayerMetallization6.6Narrow-Spectral-WidthandTunableLaserDiodesLaserGainCavityResonancesGratingResonancesLaserOutput0DISTRIBUTED-FEEDBACKLDThegratingperiodthensatisfiesandtheresonantwavelengths,asmeasuredinfreespace,aregivenby:(6.11)Example:ConsideranInGaAsPDFBLD 0=1.55m,n=3.5,letm=1(firstorder)Determinethegratingperiod.Letm=2(secondorder)DistributedBraggReflectorLaserDiodepnIPIBIGGAINPHASEBRAGGCLEAVEDFACET6.6.2TunableLaserDiodesTypicaltuningis1percentofthecenterwavelength.

Ifthecenterwavelengthis1500nm,thetuningrangewouldbe15nm.IfWDMchannelswerespacedby0.1nm,thesystemcapacitywouldbe

N=15/0.1=150channelsSystemwithrepeatersFiberFiberFiberTransmitterRepeaterRepeaterReceiver6.7OpticalAmplifiers6.7.1SemiconductorOpticalAmplifierActivelayerARcoatingOutputfiberInputfibernPVARcoatingFiberFiberTransmitterEDFAReceiverG6.7.2Erbium-DopedFiberOpticalAmplifier98014801550l(nm)20nmAmplificationPump PumpNon-radiativetransitionsErbiumAbsorptionSpectrumErbium-Doped-FiberEnergy-LevelDiagramGroundstateEnergy1.48mmor0.98mmFasttransitions4I13/21.55mm4I15/2HighenergylevelUpperlaserlevelBasicConfigurationofEDFALDWMInputsignal1.55mm 1.55mmOutputsignalErbiumdopedfiberloop1.48mm1.55mm1.48mmPracticalConfigurationofEDFAIsolatorWMWMLDLD1.48mm1.48mmEr-dopedfiber1.481.48IsolatorInput1.55mmOutput1.55mm1.551.55WDMLDINPUTSIGNALEDWOUTPUTSIGNAL(a)6.7.3Erbium-DopedWaveguideOpticalAmplifierINTEGRATEDWDMANDEDWLD

INPUTSIGNALEDWOUTPUTSIGNALEDWA(b)WDMEDWAArrayWithIntegratedMultiplexerandSplitterWDMWDMEDWEDWINPUTSIGNAL1INPUTSIGNAL2PUMP(c)OUTPUTSIGNAL1OUTPUTSIGNAL2PSEDWAARRAYRamanScatteringW3W1GROUNDSTATEVIRTUALSTATEhf13hf32W2OPTICALPHONONhf13istheinputphotonenergyhf32istheoutputphotonenergy(W2–W1)isthephononenergyThephononfrequencyis:(W2–W1)/h=13.2THz6.7.4RamanAmplifierExample:Theinputwavelength(lin)is1450nm.WhatisthewavelengthoftheRamanscatteredphoton?Whatisthewavelengthshift?Solution:Fromtheenergyleveldiagramweconcludethathf13-hf32=hfphononhfin-hfout=hfphononfout=fin-fphononSettinglinto1450nmresultsinanoutputwavelengthof

lout=1549nmThewavelengthshiftis99nm.RamanScatteringW3W1GROUNDSTATEVIRTUALSTATEhf13hf32W2OPTICALPHONONStimulatedRamanScatteringW3W2W1GROUNDSTATEVIRTUALSTATEOPTICALPHONONhf13hf32hf32PUMPSIGNAL

RamanamplifiersareavailableintheS-,C-,andL-bands.Recall:BandWavelengthS1460-1530nmC1530-1565nmL1565-1625nmStimulatedRamanAmplifierWDMOPTICALFIBERINPUTSIGNALISOLATORPUMPLASEROUTPUTSIGNALISOLATORExample:apumpwavelength:1450nm,theamplificationwouldbeatacenterwavelengthof1549nm.Computetheamplificationbandwidthinnminthiscase.Solu