光纤通信第四章课件

OpticalSourcesCHAPTER4LD:Laser-Diodes

OpticalOutputCoherent,Monochromatic,DirectionalLED:Light-EmittingDiodes

OpticalOutputIncoherent,BroadSpectral,LargebeamLDcoupledintoSMForMMFLEDcoupledintoMMFOverviewofthisChapterSemiconductormaterialfundamentalsCharacteristicsofLED&LDTemperatureresponsesofopticalsourcesPure-crystalenergy-banddiagramTopicsFromSemiconductorPhysicsCHAPTER4.1EnergyBandsn-typematerialTopicsFromSemiconductorPhysicsCHAPTER4.1p-typematerialTopicsFromSemiconductorPhysicsCHAPTER4.1TopicsFromSemiconductorPhysicsCHAPTER4.1IntrinsicandExtrinsicMaterialIntrinsicMaterial:purityExtrinsicMaterial:smallquantitiesimpuritiesElectricalconductivityproportionalcarrierconcentrationMajoritycarriers:Electronsinn-typeorholesinp-typeMinorityCarriers:Electronsinp-typeorholesinn-typeElectrondiffusionacrossapnjunctionTopicsFromSemiconductorPhysicsCHAPTER4.1Thep-nJunctionReversebiasconditionTopicsFromSemiconductorPhysicsCHAPTER4.1ForwardbiasconditionTopicsFromSemiconductorPhysicsCHAPTER4.1DirectBandGapTopicsFromSemiconductorPhysicsCHAPTER4.1IndirectBandGapTopicsFromSemiconductorPhysicsCHAPTER4.1TopicsFromSemiconductorPhysicsCHAPTER4.1SemiconductorDeviceFabricationCrystalStructure:SingleAtoms(Si,Ge)GroupofAtoms(NaCl,GaAs)LatticeSpacingsareafewangstroms(Å)Epitaxiallayersformedbyvapor,liquidormolecularphaseLight-EmittingDiodes(LEDs)CHAPTER4.2LEDsaslightsourcefor100~200Mb/swithMMF

SimpledrivecircuitryNothermaloropticalstabilizationcircuitBefabricatedlessexpensivelyAnLEDmusthave:AhighradianceoutputAfastemissionresponsetimeAhighquantumefficiencyDouble-heterostructureconfigurationSurface-emittingLEDLight-EmittingDiodes(LEDs)CHAPTER4.2Diameter50µm,Thickness2.5µm120°Edge-emittingLEDLight-EmittingDiodes(LEDs)CHAPTER4.2LightSourceMaterial

Musthaveadirectbandgap.

Material:

Ⅲelements:Al,Ga,InⅤelements:P,As,Sb

800-900nm:Ga1-xAlxAs(ratioxdeterminesbandgap)

1.0-1.7um:In1-xGaxAsyP1-y

Forsimplicity,GaAlAsandInGaAsParegenerallyused.Light-EmittingDiodes(LEDs)CHAPTER4.2BandgapenergyLight-EmittingDiodes(LEDs)CHAPTER4.2EmissionwavelengthLight-EmittingDiodes(LEDs)CHAPTER4.2Light-EmittingDiodes(LEDs)CHAPTER4.2ThepeakemissionwavelengthFor

Ga1-xAlxAs:ForIn1-xGaxAsyP1-y

:when0<x<0.47,y≈2.20xLEDspectralpatternsLight-EmittingDiodes(LEDs)CHAPTER4.2In1-xGaxAsyP1-yLight-EmittingDiodes(LEDs)CHAPTER4.2QuantumEfficiencyExcesscarrierdensitydecay:Light-EmittingDiodes(LEDs)CHAPTER4.2Rrisradiativerecombinationrate;Rnrisnonradiativerecombinationrate.ConstantcurrentflowintoLEDCarrierrecombinationrate:InternalQuantumEfficiency:Light-EmittingDiodes(LEDs)CHAPTER4.2LEDpowerOpticalpowergeneratedinternallytotheLED:Light-EmittingDiodes(LEDs)CHAPTER4.2OpticalpoweremittedfromtheLED:ExternalQuantumEfficiency:Light-emissionconeLight-EmittingDiodes(LEDs)CHAPTER4.2Light-EmittingDiodes(LEDs)CHAPTER4.2ModulationCapabilityIntensityoftheopticaloutput3dBelectricalpoint:I2(ω)=I2(0)/2Light-EmittingDiodes(LEDs)CHAPTER4.23dBopticalpoint:

I(ω)=I(0)/23-dBbandwidthpointsLight-EmittingDiodes(LEDs)CHAPTER4.2LasertransitionprocessesLaser

Diodes(LDs)CHAPTER4.3Spatial&temporalcoherenceHighlymonochromaticdirectionalDensityofexcitedelectronsissmall,stimulatedemissionisnegligiblePumpingtechniquesPopulationinversionFabry-PerotresonatorcavityLaser

DiodeModesandThresholdConditionsCHAPTER4.3.1Responsetimes<1nsOpticalbandwidth<2nmOutputpower≈milliwaltsFabry-Perotresonatorapairofflat,PartiallyreflectingmirrorsDFBlaserLaser

DiodeModesandThresholdConditionsCHAPTER4.3.1Distributed-feedbacklaserThelaserisobtainedfromBragggratingsLaser

DiodeModesandThresholdConditionsCHAPTER4.3.1Theopticalfieldintensity:g:thegaincoefficientГ:TheopticalconfinementfactorMirrorFresnelreflectioncoefficientsR:Laser

DiodeModesandThresholdConditionsCHAPTER4.3.1Theopticalfieldintensity:R1,R2:themirrorreflectivityConditionsofasteady-stateoscillation:Opticaloutputvs.drivecurrentLaser

DiodeModesandThresholdConditionsCHAPTER4.3.1ExternalQuantumEfficiencyCHAPTER4.3.3Typical:15-20%Highquality:30-40%ExternaldifferentialQuantumEfficiencyResonantFrequenciesCHAPTER4.3.4ResonantFrequencies

Whenanintegernumbermofhalf-lengthspanstheregion,thecavityresonates(astanding-wavepatternexists).Fabry-PerotlaserspectrumResonantFrequenciesCHAPTER4.3.4ResonantFrequenciesCHAPTER4.3.4ResonantFrequencies

Considertwosuccessivemodesofvm-1andvmResonantFrequenciesCHAPTER4.3.4Basicrequirements:Confinetransverseopticalandcarrierbetweenlayers;Currentflowberestrictedtoanarrowlengthstripe;Limitthenumberoflateralmodestoasinglefilament;Stabilizethelateralgain;Ensurearelativelowthresholdcurrent.LDStructureandRadiationPatternCHAPTER4.3.5LDStructureandRadiationPatternCHAPTER4.3.5Gain-inducedguideHighopticalpowers;Stronginstabilities;Highlyastigmatic.positive-indexguideWell-collimatebeam;Bell-shapedcurvenegative-indexguideWell-collimatebeam;NarrowsidelobesSingleModeFiberCHAPTER4.3.6DistributeFeedbackLasers(DFB)SingleModeFiberCHAPTER4.3.6SingleModeFiberCHAPTER4.3.6DistributeBragg-ReflectorLasers(DBR)ModulationofLaserDiodesCHAPTER4.3.7Directmodulation:Imposeinformationonalightstreambydirectlyvaryingthelaserdrivecurrent.Nonlineareffects,suitableforlow-speedsystemExternalmodulation:Imposeinformationonalightstreambyexternalmodulator.suitableforhigh-speedsystem.ModulationofLaserDiodesCHAPTER4.3.7ThedelaytimeTherelaxationfrequencyTheexampleofrelaxationoscillationpeakModulationofLaserDiodesCHAPTER4.3.