光纖通信-Optical-Sources-and-Amplifiers課件

Chapter6OpticalSources

andAmplifiersChapter61Chapter6OpticalSources

andAmplifiers6.1Light-emittingDiodes6.2Light-emittingDiodeoperatingCharacteristic6.3LaserPrinciples6.4LaserDiodes6.5Laser-diodeoperatingCharacteristic6.7OpticalAmplifiers6.8FiberLasers6.9Vertical-CavitySurface-emittingLaserDiodesChapter6OpticalSources

2LightsourceLight-emittingdiodeLaserdiodeLightsourceLight-emittingdio3ModulationLaser

DirectModulationofLaserDiodeBias+DATAIssues--ComplexDynamicsYield

ExternalModulationofLaserDiodeLaserModulatorBiasBias+DATAIssues--AdditionalComponentModulationLaserDirectModul4ALight-emittingDiodesisapn-junctionsemiconductorthatemitslightwhenforwardbiased.

Circuit6.1Light-emittingDiodesALight-emittingDiodesisap5Intheupper-energyband,calledtheconductionband,electronsnotboundtoindividualatomsarefreetomove.Inthelowerband,thevalenceband,unboundholesarefreetomove.Holeshaveapositivecharge.6.1Light-emittingDiodesTwoallowedbandsofenergiesareseparatedbyaforbiddenregion(abandgap)whosewidthhasenergyWg.Intheupper-energyband,call66.1Light-emittingDiodesInaword,radiationfromanLEDiscausedbytherecombinationofholesandelectronsthatareinjectedintothejunctionbyaforwardbiasvoltage.PNpn-junctionflash366.1Light-emittingDiodesIna76.2Light-emittingDiode

operatingcharacteristicmA0501001507654321mWTheopticpowergeneratedbyanLEDislinearlyproportionaltotheforwarddrivingcurrent.6.2Light-emittingDiode

oper8DigitalmodulationcurrenttimeOutputpowerinputcurrenttimeOpticalpowerThediodeismodulatedbyacurrentsource,whichsimplyturnstheLEDONorOFF.DigitalmodulationcurrenttimeO9AnalogmodulationAnalogmodulationrequiresadcbiastokeepthetotalcurrentintheforwarddirectionatalltimes.OpticalpowertimetimecurrentAnalogmodulationAnalogmodula10Asweknow,theopticspectrumofthesourcedirectlyinfluencesmaterialandwaveguidedispersion.Pulsespreadingduetothesecausesincreaseslinearlywithsourcespectralwidth.LEDsoperatingintheregion0.8-0.9mgenerallyhaswidthof20-50nm,andLEDsemittinginthelonger-wavelengthregionhavewidthsof50-100nm.6.2Light-emittingDiode

operatingcharacteristic6.2Light-emittingDiode

oper11Couplingefficiencydependsheavilyontheradiationpatternofaemitter.

-90°090°BEAMANGLEBEAMINTENSITYθsurface-emittingLEDCouplingefficiencydependshe12Raysincidentonafiber,butoutsideitsacceptanceangle,willnotbecoupled.TheacceptanceangleforafiberhavingNA=0.24isonly14°,soalargeamountofthepowergeneratedbyasurfaceemitterwillberejected.-90°090°BEAMANGLEBEAMINTENSITYθsurface-emittingLEDRaysincidentonafiber,but13Edgeemittersconcentratetheirradiationsomewhatmorethansurfacedevices,providingimprovedcouplingefficiency.-90°-45°0°45°90°120°30°BEAMANGLEBEAMINTENSITYPARALLELPLANEPERPENDICULARPLANEθedge-emittingLEDFlash38Edgeemittersconcentratethei146.3LaserPrinciplesHereisalistofsomecharacteristicsthatalllaserspossessandthatareimportantintheirutilization:

Pumpingthreshold

Thepowerinputtoalasermustbeaboveacertainthresholdlevelbeforethedevicewillemit.

Outputspectrum

Thelaseroutputpowerisnotatasinglefrequencybutisspreadoverarangeoffrequencies.

Radiationpattern

Therangeofanglesoverwhichalaseremitslightdependsonthesizeoftheemittingareaandonthemodesofoscillationwithinthelaser.6.3LaserPrinciplesHereisa15

thesemiconductorlaserdiodethegaslaserthebulkNd:YAGthefiberlasercommonkindsoflaserAlaserisahigh-frequencygenerator,oroscillator.Foroscillationstooccur,asystemneedsamplification,feedback,monkindsoflaserAlaser16Lightamplificationbystimulatedemissionofradiation

laserLightamplificationbylaser17stimulatedemissionenergyEnergyissuppliedfromoutsideandatomentersexcitedstate.E1groundstateE2excitedstatestimulatedemissionenergyEnerg18stimulatedemissionE2E1hArrivingphotonPhotonarrivesandinteractswithexcitedatom.stimulatedemissionE2E1hArriv19stimulatedemissionE2E1hArrivingphotonAtomemitsadditionalphotonandreturnstothegroundstate.hhstimulatedemissionE2E1hArriv20stimulatedemissionE2E1hArrivingphotonWhenanewphotonisemittedithasidenticalwavelength,phaseanddirectioncharacteristicsastheexcitingphoton.hhstimulatedemissionE2E1hArriv21stimulatedemissionPopulationinversionThenumberofatomsintheupperlevelexceedsthoseinthelowerlevel.stimulatedemissionPopulation22PopulationinversionThenumberofphotonswillincreaseastheypropagate.Morephotonwillencounterupperlevelatoms(causinggenerationofadditional)thanwillmeetlowerlevelatoms(whichwouldabsorbthem).Amediumwithpopulationinversionhasgainandbehavesasanamplifier.PopulationinversionThenumber23energylasingM1M2mirrorPartialmirrorlaserenergylasingM1M2mirrorPartial24mirrorPartialmirroroscillationmirrorPartialmirroroscillatio25LaseroutputLaseroutput266.4LaserDiodesMETALLIZATIONn-AlGaAs,Wg=1.8eVCONFINEMENTn-AlGaAs,Wg=1.55eVACTIVELAYERn-AlGaAs,Wg=1.55eVCONFINEMENTGaAsSUBSTRATEP-GaAs,CONTACTSiO2,INSULATIONMETALLIZATIONSTRIPECONTACT0.1-0.3μm-1μm-1μm-1μmThestructureofan

AlGaAslaserdiode6.4LaserDiodesMETALLIZATIONn27powerConfinementLayerConfinementLayerActiveLayerRefractiveIndex6.4LaserDiodespowerConfinementLayerConfinem28Manylaserdiodesareedgeemitters.Underforwardbias,chargesareinjectedintotheactivelayer,causingthespontaneousemissionofphotons.Someoftheinjectedchargesarestimulatedtoemitbyotherphotons.Ifthecurrentdensityissufficientlyhigh,thenalargenumberofinjectedchargesareavailableforstimulatedrecombination.Theopticgainwillbelarge.Thethresholdcurrentisreachedwhenthegainislargeenoughtooffsetthediodelosses.Atthispoint,laseroscillationstart.6.4LaserDiodesManylaserdiodesareedgeemi29GAINOFTHEAMPLIFYINGMEDIUM819820821WAVELENGTH(nm)OutputpowerofalaserdiodeDiodesradiatinga

spectrum

containingnumerous

longitudinalmodes.GAINOFTHEAMPLIFYINGMEDIUM8306.5Laser-diodesoperatingcharacteristicoperatingcharacteristicoutputpowerlinewidthtemperaturesensitiveoperatingcharacteristic6.5Laser-diodesoperatingcha31(1)OutputopticpowerOPTICALPOWER(mW)CURRENT(mA)050ITH10015054321Outputopticpowerisplottedagainstforwardinputcurrent.(1)OutputopticpowerOPTICA32Digitalmodulationof

alaserdiodeTIMEiTIMEIdcCURRENTOPTICALPOWERisidcDigitalmodulationof

alaser33AnalogmodulationofalaserdiodeTIMEIdcCURRENTOPTICALPOWERisidcITHTIMEisAnalogmodulationofalaserd346.5Laser-diodesoperatingcharacteristicoperatingcharacteristicoutputpowerlinewidthtemperaturesensitiveoperatingcharacteristic6.5Laser-diodesoperatingcha35(2)temperaturesensitiveCURRENT(mA)OutputPower(mW)20°40°60°0LaserdiodesaremuchmoretemperaturesensitivethanareLEDsAsthetemperatureincreases,thediode’sgaindecreases,andsomorecurrentisrequiredbeforeoscillationcanbegin-----thethresholdcurrentbecomesgreater.(2)temperaturesensitiveCUR36(2)temperaturesensitiveCURRENT(mA)OutputPower(mW)20°40°60°0Ataconstantcurrent,theoutputpowerofalaserdiodewilldiminishifthetemperaturerisesTherearetwotechniquesforovercomingthisproblem:thermoelectricallycoolingthediode,andchangingthebiascurrenttocompensateforchangedthreshold.(2)temperaturesensitiveCUR37(2)temperaturesensitiveThelaseremissionwavelengthalsodependsuponthetemperature.Thiseffectarisesfromthedependenceofthematerial’srefractiveindexontemperature.(2)temperaturesensitiveThel386.5Laser-diodesoperatingcharacteristicoperatingcharacteristicoutputpowerlinewidthtemperaturesensitiveoperatingcharacteristic6.5Laser-diodesoperatingcha39(3)linewidth-2.5-1.5-2.5WAVELENGTH(nm)2.01.51.00.50.0INTENSITYLaserdiodestypicallypossesslinewidthsof1-5nm,considerablysmallerthanthooutputspectraofLEDs.Whenthedrivecurrentisjustabitabovethethreshold,laserdiodesproducemultimodespectra(3)linewidth-2.5-1.5-040(3)linewidth-2.5-1.5-2.5WAVELENGTH(nm)2.01.51.00.50.0INTENSITYWAVELENGTH(nm)2.01.51.00.50.0INTENSITY-2.5-1.5-2.5Asthecurrentincreases,thetotallinewidthdecreases,andthenumberoflongitudinalmodesdiminishes.Atasufficientlyhighcurrent,thespectrumwillcontainjustonemode.Itiscalled

single-longitudinal-modelaser.(3)linewidth-2.5-1.5-0416.6Narrow-spectral-widthandTunablelaserdiodes6.6.1Distributed-feedbacklaserDiode(DFB)TheDFBlaserdiodeisasingle-longitudinal-modelaserdiode.

PnMETALIZEDLAYERGRATINGACTIVELAYERCLEAVEDFACETOutput6.6Narrow-spectral-widthand426.6.1Distributed-feedbacklaserDiode(DFB)OperatingwavelengthisdeterminedfromBragg’slaw6.6.1Distributed-feedbacklas43DFBlasershaveanumberofuniquepropertiesarisingfromthegratingstructure.Inadditiontotheirnarrowlinewidths(typically0.1-0.2nm),whichmakethemattractiveforlonghigh-bandwidthtransmissionpaths,theyarelesstemperaturedependentthanaremostconventionallaserdiodes.6.6.1Distributed-feedbacklaserDiode(DFB)DFBlasershaveanumberofun44pngain

phase

BraggIGIPIB6.6.2TunableLaserDiodesThegaincurrentIGdeterminestheamplificationintheactiveregionandthelevelofoutputlaserpower.pngainphase45pngain

phase

BraggIGIPIB6.6.2TunableLaserDiodesThephasecurrentIPcontrolsthefeedbackfromtheBraggreflectionregion.pngainphase46pngain

phase

BraggIGIPIB6.6.2TunableLaserDiodesThecurrentIBcontrolstheBraggwavelengthbychangingthetemperatureintheBraggregion.pngainphase476.7OpticalAmplifiersOpticalamplifierswillnotsolvetheproblemofreconstructingsignalwaveshapes,buttheywillallowextensionofpower-limitedlinks.Inotherwords,bandwidth-limitedsystemwillnotbehelped,butpower-limitedoneswill.6.7OpticalAmplifiersOptical486.7OpticalAmplifiersSemiconductorOpticalAmplifier(SOA)Erbium-DopedFiberAmplifier(EDFA)Erbium-DopedWaveguideAmplifier(EDWA)FiberRamanAmplifier(FRA)6.7OpticalAmplifiersSemicond496.7.1SemiconductorOpticalAmplifiers(SOA)MirrorInputOutputMirrorCurrentARCoatInputOutputARCoatFabry-PerotamplifierTraveling-waveamplifier6.7.1SemiconductorOpticalAm506.7.1SemiconductorOpticalAmplifiers(SOA)R1R2ISOASOAcanbeconstructedbyusingstimulatedemission,similartolaser.Achievingenoughgainanddoingsowithoutaddingtoomuchnoisehasbeenaproblem.ThegainofSOAispolarizationdependent.6.7.1SemiconductorOpticalAm51SOAProductSOAProduct52Inputsignal1530nm-1570nmAmplifiedoutputsignalPowerlaser(Pump)980nmor1480nmFibercontainingerbiumdopant6.7.2Erbium-DopedFiberOpticalAmplifier(EDFA)HighgainWavelengthofamplificationLargebandwidthLownoiseInputsignalAmplifiedoutputs53Energystatesandtransitionsenergy1550nmemissionW2980nm1480nmW31550nmW1Erbium-dopedglassfiberEnergystatesandtransitionse546.7.2Erbium-DopedFiberOpticalAmplifier(EDFA)INPUTSIGNAL1550nmWDMWDM980nm980nmPUMP-LASERDIODESINPUTSIGNAL1550nmISOLATORISOLATORERBIUM-DOPEDFIBERLOOPThepumpinglightisabsorbedbytheerbiumatoms,raisingthemtoexcitedstatesandcausingpopulationinversion.6.7.2Erbium-DopedFiberOptic556.7.2Erbium-DopedFiberOpticalAmplifier(EDFA)INPUTSIGNAL1550nmWDMWDM980nm980nmPUMP-LASERDIODESINPUTSIGNAL1550nmISOLATORISOLATORERBIUM-DOPEDFIBERLOOPTheexcitederbiumatomsarethenstimulatedtoemitbythelongerwavelength1550nmphotons,amplifyingthesignal.6.7.2Erbium-DopedFiberOptic566.7.2Erbium-DopedFiberOpticalAmplifier(EDFA)INPUTSIGNAL1550nmWDMWDM980nm980nmPUMP-LASERDIODESINPUTSIGNAL1550nmISOLATORISOLATORERBIUM-DOPEDFIBERLOOPThesignalbeamandthepumpingbeamfromthelefttraveltogetherdownthefiber.Thesignalbeamcontinuallyincreasesinstrengthwhiledepletingthepumppower.6.7.2Erbium-DopedFiberOptic576.7.2Erbium-DopedFiberOpticalAmplifier(EDFA)INPUTSIGNAL1550nmWDMWDM980nm980nmPUMP-LASERDIODESINPUTSIGNAL1550nmISOLATORISOLATORERBIUM-DOPEDFIBERLOOPTheisolatorsarerequiredtoattenuatereflectedwaves(feedback),whichwouldbeamplifiedandcouldcauselaser-typeoscillation.6.7.2Erbium-DopedFiberOptic58EDFAoperatingcharacteristicoperatingcharacteristicoperatingbandwidthgainsaturationErbium-dopedfiberlengthoperatingcharacteristicEDFAoperatingcharacteristico59(1)operatingbandwidth15391569Operatingbandwidthofmorethan30nmareachievable,soanumberofwavelength-division-multiplexingchannelscanbeamplifiedsimutaneously.(1)operatingbandwidth153915660Dual-bandamplifierL-bandEBFADual-bandamplifierL-bandEBFA61EDFAoperatingcharacteristicoperatingcharacteristicoperatingbandwidthgainsaturationErbium-dopedfiberlengthoperatingcharacteristicEDFAoperatingcharacteristico62(2)Erbium-dopedfiberlengthTheErbium-dopedfiberlengthsaretypicallyafewtensofmeters.Theoptimumlengthdependsontheamountofpumppoweravailable.(2)Erbium-dopedfiberlengthT63(2)Erbium-dopedfiberlengthThepumppowerdecreasesasittravelsdownthroughthefiber,andeventuallyitbecomessoweakthatthegainreducedtozero,andthepumpedfiberbecomesabsorbingratherthanamplifying.(2)Erbium-dopedfiberlengthT64EDFAoperatingcharacteristicoperatingcharacteristicoperatingbandwidthgainsaturationErbium-dopedfiberlengthoperatingcharacteristicEDFAoperatingcharacteristico65(3)gainsaturation3dBPout,satgainPin(dBm)saturationSaturationisthedecreaseingainthatoccurswhentheamplifiedpowerreacheshighlevels.(3)gainsaturation3dBPout,sat666.7.3Erbium-DopedWaveguideOpticalAmplifierWDMEDWLDINPUTSIGNALOUTPUTSIGNALThewaveguideisdopedwitherbiumatoms.Integrationissimpler,moreeconomical,reducessize,reducesinsertionlosses.6.7.3Erbium-DopedWaveguideO676.7.4RamanAmplifierTheEDFAprovidessignificantamplificationintheC-band.AmplifiersusingstimulatedRamanscatteringhavebeendevelopedforapplicationsinotherbands.Development

6.7.4RamanAmplifierTheEDFA686.7.4RamanAmplifierfiber(a)Nopumppower(dB)1550nmpower(dB)1550nm1450nmfiber(b)Withpump1550nm1450nm1550nmSRScausesanewsignal(astokeswave)tobegeneratedinthesamedirectionasthepumpwavedown-shiftedinfrequencyby13.2THzprovidedthatthepumpsignalisofsufficientstrength.6.7.4RamanAmplifierfiber(a)696.7.4RamanAmplifierpower(dB)1550nm1450nmfiber1450nm1550nmOptimalamplificationoccurswhenthedifferenceinwavelengthisaround13.2THz.Thesignaltobeamplifiedmustbelowerinfrequency(longerinwavelength)thanthepump.6.7.4RamanAmplifierpower(dB)706.7.4RamanAmplifier6.7.4RamanAmplifier716.7.4RamanAmplifier6.7.4RamanAmplifier72WDMPUMPLASERINPUTSIGNALOUTPUTSIGNALOpticalfiberISOLATORISOLATORRamanamplifier6.7.4RamanAmplifierWDMPUMPLASERINPUTSIGNALOUTPU73zEDFANonlinearEffectsNoiseHighRAHighpumppower6.7.4RamanAmplifierpulseamplitudezEDFANonlinearEffectsNoiseHi74BroadbandRamanamplifierBroadbandRamanamplifier75BroadbandRamanamplifierBroadbandRamanamplifier76UltraflatamplifierUltraflatamplifier776.7.5NoiseFigureThenoisefigureFisameasureofthenoisecharacteristicsofanamplifier.Fgivesanindicationofthedegradationinasignalowingtoamplification.Amplificationincreasesthesignalpowertoausablelevel,butdoesdegradetheinformation.Itoftenexpressedindecibels:6.7.5NoiseFigureThenoisefi786.7.5NoiseFigureSemiconductorOpticalAmplifier(SOA):8dBErbium-DopedFiberAmplifier(EDFA):6dBErbium-DopedWaveguideAmplifier(EDWA):5dBFiberRamanAmplifier(FRA):4.5dB6.7.5NoiseFigureSemiconducto796.7.5NoiseFigureOpticalfiberpowersignalpowerASEnoiseOpticalSNRNumberofamplifier6.7.5NoiseFigureOpticalfibe806.7.6OpticalAmplifierApplicationsTXAAARXFIBERFIBERLAUNCHAMPINLINEAMPPREAMPPOWERLEVEL6.7.6OpticalAmplifierApplic816.8FiberLaserLaserdiodesandlight-emittingdiodesdon’tcouplethelighttheygenerateefficientlyintofibers.Thisproblemarisesbecauseofthedifferentgeometriesofsemiconductorsourcesandopticalfibers.Inaddition,theradiationpatternofthesourcedoesnotmatchtheacceptancepatternofthefiber,andtheemissionpatternofalaserdiodedoesnotmatchthesingle-modepatternofasingle-modefiber.6.8FiberLaserLaserdiodesan82Fiberamplifierscansolvethisproblem.AcommononeisFabry-Perotresonator,whichconsistsofapump,anamplifyingsection,andfeedbackintheform.

LaserDiodeActiveFiberλpλLM1M2MirrorM1transmitsthepumpwavelengthλpandreflectsthelaserWavelengthλL,whilemirrorM2istransmittingpartiallyatwavelengthλL.6.8FiberLaserFiberamplifierscansolvethi83GRATINGGRATINGWDM980nmPUMPLASERERBIUM-DOPEDFIBERLOOPOutputSignal1550nmErbium-dopedfiberlaser,thegratingsactaspartialmirrorsatthelaser-outputwavelength.6.8FiberLaserGRATINGGRATINGWDM980nmPUMPLAS846.9Vertical-CavitySurface-emittingLaserDiodesThisstructurehasseveraluniquecharacteristics:

Oneisthatthebeampatterniscircular,thesameshapeasthefiber.Thismatchimprovesthecouplingefficiency.

VCSELshaveshortcavitylengths,whichtendtodecreaseresponsetimes.ThisresultisthatVCSELscanbemodulatedatveryhighspeeds.6.9Vertical-CavitySurface-em856.9Vertical-CavitySurface-emittingLaserDiodesMonolithictwo-dimensionallaser-diodearrays6.9Vertical-CavitySurface-em86LightsourceLight-emittingdiodeLaserdiodeLEDsarenormallychosenformultimodeSIlinks.GRINfiberandanLEDcancombinetoproduceasystemtransmittingmoderatelyhighdataratesoverfairlylongdistances.Becauseofhigherinitialcostsandincreasedcircuitcomplexity,laserdiodesareusedonlywhennecessary.Thelargestrate-lengthproductsareachievedwhenasingle-modelaserdiodeismatchedwithasingle-modefiberandoperatedinthelow-loss,longer-wavelengthregionsuchastheCorLbands(1530

to1625nm)LightsourceLight-emittingdio87謝謝騎封篙尊慈榷灶琴村店矣墾桂乖新壓胚奠倘擅寞僥蝕麗鑒晰溶廷籮侶郎蟲林森-消化系統(tǒng)疾病的癥狀體征與檢查林森-消化系統(tǒng)疾病的癥狀體征與檢查謝謝騎封篙尊慈榷灶琴村店矣墾桂乖新壓胚奠倘擅寞僥蝕麗鑒晰溶廷88Chapter6OpticalSources

andAmplifiersChapter689Chapter6OpticalSources

andAmplifiers6.1Light-emittingDiodes6.2Light-emittingDiodeoperatingCharacteristic6.3LaserPrinciples6.4LaserDiodes6.5Laser-diodeoperatingCharacteristic6.7OpticalAmplifiers6.8FiberLasers6.9Vertical-CavitySurface-emittingLaserDiodesChapter6OpticalSources

90LightsourceLight-emittingdiodeLaserdiodeLightsourceLight-emittingdio91ModulationLaser

DirectModulationofLaserDiodeBias+DATAIssues--ComplexDynamicsYield

ExternalModulationofLaserDiodeLaserModulatorBiasBias+DATAIssues--AdditionalComponentModulationLaserDirectModul92ALight-emittingDiodesisapn-junctionsemiconductorthatemitslightwhenforwardbiased.

Circuit6.1Light-emittingDiodesALight-emittingDiodesisap93Intheupper-energyband,calledtheconductionband,electronsnotboundtoindividualatomsarefreetomove.Inthelowerband,thevalenceband,unboundholesarefreetomove.Holeshaveapositivecharge.6.1Light-emittingDiodesTwoallowedbandsofenergiesareseparatedbyaforbiddenregion(abandgap)whosewidthhasenergyWg.Intheupper-energyband,call946.1Light-emittingDiodesInaword,radiationfromanLEDiscausedbytherecombinationofholesandelectronsthatareinjectedintothejunctionbyaforwardbiasvoltage.PNpn-junctionflash366.1Light-emittingDiodesIna956.2Light-emittingDiode

operatingcharacteristicmA0501001507654321mWTheopticpowergeneratedbyanLEDislinearlyproportionaltotheforwarddrivingcurrent.6.2Light-emittingDiode

oper96DigitalmodulationcurrenttimeOutputpowerinputcurrenttimeOpticalpowerThediodeismodulatedbyacurrentsource,whichsimplyturnstheLEDONorOFF.DigitalmodulationcurrenttimeO97AnalogmodulationAnalogmodulationrequiresadcbiastokeepthetotalcurrentintheforwarddirectionatalltimes.OpticalpowertimetimecurrentAnalogmodulationAnalogmodula98Asweknow,theopticspectrumofthesourcedirectlyinfluencesmaterialandwaveguidedispersion.Pulsespreadingduetothesecausesincreaseslinearlywithsourcespectralwidth.LEDsoperatingintheregion0.8-0.9mgenerallyhaswidthof20-50nm,andLEDsemittinginthelonger-wavelengthregionhavewidthsof50-100nm.6.2Light-emittingDiode

operatingcharacteristic6.2Light-emittingDiode

oper99Couplingefficiencydependsheavilyontheradiationpatternofaemitter.

-90°090°BEAMANGLEBEAMINTENSITYθsurface-emittingLEDCouplingefficiencydependshe100Raysincidentonafiber,butoutsideitsacceptanceangle,willnotbecoupled.TheacceptanceangleforafiberhavingNA=0.24isonly14°,soalargeamountofthepowergeneratedbyasurfaceemitterwillberejected.-90°090°BEAMANGLEBEAMINTENSITYθsurface-emittingLEDRaysincidentonafiber,but101Edgeemittersconcentratetheirradiationsomewhatmorethansurfacedevices,providingimprovedcouplingefficiency.-90°-45°0°45°90°120°30°BEAMANGLEBEAMINTENSITYPARALLELPLANEPERPENDICULARPLANEθedge-emittingLEDFlash38Edgeemittersconcentratethei1026.3LaserPrinciplesHereisalistofsomecharacteristicsthatalllaserspossessandthatareimportantintheirutilization:

Pumpingthreshold

Thepowerinputtoalasermustbeaboveacertainthresholdlevelbeforethedevicewillemit.

Outputspectrum

Thelaseroutputpowerisnotatasinglefrequencybutisspreadoverarangeoffrequencies.

Radiationpattern

Therangeofanglesoverwhichalaseremitslightdependsonthesizeoftheemittingareaandonthemodesofoscillationwithinthelaser.6.3LaserPrinciplesHereisa103

thesemiconductorlaserdiodethegaslaserthebulkNd:YAGthefiberlasercommonkindsoflaserAlaserisahigh-frequencygenerator,oroscillator.Foroscillationstooccur,asystemneedsamplification,feedback,monkindsoflaserAlaser104Lightamplificationbystimulatedemissionofradiation

laserLightamplificationbylaser105stimulatedemissionenergyEnergyissuppliedfromoutsideandatomentersexcitedstate.E1groundstateE2excitedstatestimulatedemissionenergyEnerg106stimulatedemissionE2E1hArrivingphotonPhotonarrivesandinteractswithexcitedatom.stimulatedemissionE2E1hArriv107stimulatedemissionE2E1hArrivingphotonAtomemitsadditionalphotonandreturnstothegroundstate.hhstimulatedemissionE2E1hArriv108stimulatedemissionE2E1hArrivingphotonWhenanewphotonisemittedithasidenticalwavelength,phaseanddirectioncharacteristicsastheexcitingphoton.hhstimulatedemissionE2E1hArriv109stimulatedemissionPopulationinversionThenumberofatomsintheupperlevelexceedsthoseinthelowerlevel.stimulatedemissionPopulation110PopulationinversionThenumberofphotonswillincreaseastheypropagate.Morephotonwillencounterupperlevelatoms(causinggenerationofadditional)thanwillmeetlowerlevelatoms(whichwouldabsorbthem).Amediumwithpopulationinversionhasgainandbehavesasanamplifier.PopulationinversionThenumber111energylasingM1M2mirrorPartialmirrorlaserenergylasingM1M2mirrorPartial112mirrorPartialmirroroscillationmirrorPartialmirroroscillatio113LaseroutputLaseroutput1146.4LaserDiodesMETALLIZATIONn-AlGaAs,Wg=1.8eVCONFINEMENTn-AlGaAs,Wg=1.55eVACTIVELAYERn-AlGaAs,Wg=1.55eVCONFINEMENTGaAsSUBSTRATEP-GaAs,CONTACTSiO2,INSULATIONMETALLIZATIONSTRIPECONTACT0.1-0.3μm-1μm-1μm-1μmThestructureofan

AlGaAslaserdiode6.4LaserDiodesMETALLIZATIONn115powerConfinementLayerConfinementLayerActiveLayerRefractiveIndex6.4LaserDiodespowerConfinementLayerConfinem116Manylaserdiodesareedgeemitters.Underforwardbias,chargesareinjectedintotheactivelayer,causingthespontaneousemissionofphotons.Someoftheinjectedchargesarestimulatedtoemitbyotherphotons.Ifthecurrentdensityissufficientlyhigh,thenalargenumberofinjectedchargesareavailableforstimulatedrecombination.Theopticgainwillbelarge.Thethresholdcurrentisreachedwhenthegainislargeenoughtooffsetthediodelosses.Atthispoint,laseroscillationstart.6.4LaserDiodesManylaserdiodesareedgeemi117G