Fig4-1Pure-crystalenergy-banddiagram图4-1纯晶体的能带图

LightSourcesforOpticalCommunicationsEE8114XavierFernandoRCLLabRequirementsSmallphysicaldimensionstosuitthefiberNarrowbeamwidth

tosuitfiberNA

Narrowspectralwidth(orlinewidth)toreducechromaticdispersionFastresponsetime(highbandwidth)tosupporthighbitrateHighoutputpowerintothefiberforlongreachwithoutrepeatersConsiderations…AbilitytodirectlymodulatebyvaryingdrivingcurrentLinearity(outputlightpowerproportionaltodrivingcurrent)importantforanalogsystemsStability

LEDbetterthanLASERDrivingcircuitissuesimpedancematchingReliability(lifetime)andcostSolidState(Semiconductor)LightSourcesLightEmittingDiode(LED)SimpleforwardbiasedPNjunctionLASERSpecializedLEDwithstimulatedemissiontoprovide:lowlinewidth,lowbeamwidth,highpowerandcoherencySemiconductorPhysicsLEDsandlaserdiodesconsistofapnjunctionconstructedofdirect-bandgapIII-Vmaterials.Whenthepnjunctionisforwardbiased,electronsandholesareinjectedintothepandnregions,respectively.Theinjectedminoritycarriersrecombineeitherradiatively(aphotonofenergyE=h

isemitted)ornonradiatively(therecombinationenergyisdissipatedasheat).The

pnjunctionisknownastheactiveorrecombinationregion.Energy-BandsPureGroup.IV(intrinsicsemiconductor)materialhasequalnumberofholesandelectrons.Thermalexcitationofanelectronfromthevalencebandtotheconductionbandenableittofreelymove.n-typematerialDonorlevelinann-type(GroupV)semiconductor.Theionizationofdonorimpuritiescreatesanincreasedelectronconcentrationdistribution.p-typematerialAcceptorlevelinanp-type(GroupIII)semiconductor.TheionizationofacceptorimpuritiescreatesanincreasedholeconcentrationdistributionIntrinsic&ExtrinsicMaterialsExtrinsicmaterial:donororacceptortypesemiconductors.Majoritycarriers:electronsinn-typeorholesinp-type.Minoritycarriers:holesinn-typeorelectronsinp-type.Theoperationofsemiconductordevicesisessentiallybasedontheinjectionandextractionofminoritycarriers.Intrinsicmaterial:Aperfectmaterialwithnoimpurities.IndirectBandGapSemiconductorsPhysicalDesignofanLEDDoubleheterostructureisusedtoimprovelightoutput(2ptypeand2ntypematerials)Eachregionshallalsohavetherightrefractiveindextoguidethelight(opticalproperty)Lightisfirstconfinedintheactiveregion(highref.index)duetowaveguideoperationThenitexistsviathefront(surfaceemittingLED)ortheside(edgeemittingLED)Double-HeterostructureconfigurationLight-EmittingDiodesLEDfeatures:MadeofGaAlAs(850nm)orInGaAsP(S-Lbands)Broadspectraloutput(50to150nm)Opticaloutputpowerslessthan-13dBm(50μW)CanbemodulatedonlyupafewhundredMb/sLessexpensivethanlaserdiodesEdge-emitterorsurfaceemitterstructuresEdge-EmittingLEDTheactiveregionisembeddedintoawaveguidestructuresothatthelightisdirectedanedgeLargeractiveregionMoredirectionalradiation(similartoLASER)Wavelength,EgandtheRatiobetweenSemiconductorsRelationshipbetweenthecrystallatticespacing,Eg,emissionλatroomtemp.TheshadedareaisforthequaternaryalloyIn1–xGaxAsyP1–y

BandgapEnergyThesourceemissionwavelengthdependsonthebandgapenergyofthedevicematerial.16BandgapEnergyForIn1–xGaxAsyP1–ycompositionsthatarelattice-matchedtoInP,thebandgapineVvariesas17Bandgapwavelengthsfrom920to1650nmarecoveredbythismaterialsystem.SurfaceandEdgeEmittingLED18GenerallyanLEDisabroadbandlightsourceRateequationsandQuantumEfficiencyofLEDsWhenthereisnoexternalcarrierinjection,theexcessdensitydecaysexponentiallyduetoelectron-holerecombination.nistheexcesscarrierdensity,BulkrecombinationrateR:tn(t)WithanexternalsuppliedcurrentdensityofJtherateequationfortheelectron-holerecombinationis:Inequilibriumcondition:

dn/dt=0tn(t)Bulkrecombinationrate(R)=Radiativerecombinationrate(Rr)+Nonradiativerecombinationrate(Rnr)Forexponentialdecayofexcesscarriers:Radiativerecombinationlifetime τr=n/Rr

Nonradiativerecombinationlifetime τnr=n/Rnr

Forhighquantumefficiency,Rr>>Rnr

τr

<<τnr

QuantumEfficiencyInternalquantumefficiencyistheratiobetweentheradiativerecombinationrateandthesumofradiativeandnonradiativerecombinationratesWhere,thecurrentinjectedintotheLEDisI,andqisthechargeofanelectron.InternalQuantumEfficiency&OpticalPowerOpticalpowergeneratedinternallyintheactiveregionintheLEDisequaltothenumberofphotons/seconds(I/q)timesenergyperphotons(hv)timestheinternalquantumefficiency[4-9]ExternalEfficiencyNotallthelightinternallygeneratedexitstheLEDTheactuallightoutputdependson:therefractiveindexoftheactiveregion,therefractiveindexofthesurroundingmaterialincidentangleoflighttotheinterfaceFresnelReflectionIftherefractiveindexnofthemediumseparatingthesourceandthefiberendisdifferentfromthecoreindexn1,then,forperpendicularfiberendfaces,thepowercoupledintothefiberreducesbythefactorRistheFresnelreflectionorthereflectivityatthefiber-coreendface;Tisthetransmissivity(R+T=1)Thereflectioncoefficientr=(n1-n)/(n1+n)relatestheamplitudesoftheincidentandreflectedwave.FresnelReflectionExample25IngeneralAtthesurfaceofanytwomaterialwithn1andn2refindices,therewillbeFresnelLoss FresnelLoss=-10Log(T)[4-12][4-13][4-14]Lightemissionconen1n2ModulationofanLEDTheresponsetimeofanopticalsourcedetermineshowfastanelectricalinputdrivesignalcanvarythelightoutputlevelIfthedrivecurrentismodulatedatafrequencyωandP0isthepoweremittedatzeromodulationfrequency,theopticaloutputpowerofthedevicewillvaryas3-dBbandwidthsOpticalPower

I(f);ElectricalPower

I2(f)ElectricalLoss=2xOpticalLossModulationofLEDThefrequencyresponseofanLEDdependson:1-Dopinglevelintheactiveregion2-Injectedcarrierlifetimeintherecombinationregion,.3-ParasiticcapacitanceoftheLEDIfthedrivecurrentofanLEDismodulatedatafrequencyofω,theoutputopticalpowerofthedevicewillvaryas:Electricalcurrentisdirectlyproportionaltotheopticalpower,thuswecandefineelectricalbandwidthandopticalbandwidth,separately.[4-15][4-16]ElectricalandOpticalBandwidthsDrawbacksofLEDLargelinewidth(30-40nm)Largebeamwidth(Lowcouplingtothefiber)LowoutputpowerLowE/OconversionefficiencyAdvantagesRobustLinearHalfPowerBeamWidth(θ1/2)TheangleatwhichthepowerishalfofitspeakvalueL=1ForLambertiansourceSource-to-FiberPowerLaunchingAssumeasurface-emittingLEDofradiusrslessthanthefiber-coreradiusa.ThetotalopticalpowerPsemittedfromthesourceofareaAsintoahemisphere(2πsr)isgivenby33IntermsofPstheopticalpowercoupledintoastep-indexfiberfromtheLEDisSource-to-FiberPowerCouplingComparisonoftheopticalpowerscoupledintotwostep-indexfibers34LensesforCouplingImprovementIfthesourceemittingareaissmallerthanthecorearea,aminiaturelenscanimprovethepower-couplingefficiency.35EfficientlensingmethodRequiresmoreprecisealignmentFiber-to-FiberJointsDifferentmodaldistributionsoftheopticalbeamemergingfromafiberresultindifferentdegreesofcouplingloss.36Asteady-statemodalequilibriumhasbeenestablishedintheemittingfiber.Allmodesintheemittingfiberareequallyexcited.Achievingasteady-stateinthereceivingfiberresultsinanadditionalloss.MechanicalMisalignmentForareceivingfibertoacceptalltheopticalpoweremittedbythefirstfiber,theremustbeperfectmechanicalalignmentbetweenthetwofibers,andtheirgeometricandwaveguidecharacteristicsmustmatchprecisely.Mechanicalalignmentisamajorprobleminjoiningfibers.37AxialDisplacementAxialorlateraldisplacementresultswhentheaxesofthetwofibersareseparatedbyadistanced.Thismisalignmentisthemostcommonandhasthegreatestpowerloss.Forthestep-indexfiber,thecouplingefficiencyissimplytheratioofthecommon-coreareatothecoreend-facearea:38AngularMisalignmentWhentwofiberendsareseparatedlongitudinallybyagaps,notallthehigher-modeopticalpoweremittedintheringofwidthxwillbeinterceptedbythereceivingfiber.Thelossforanoffsetjointbetweentwoidenticalstep-indexfibersis39OpticalFiberConnectorsPrincipalrequirementsofagoodconnec