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Chapter8.3:LaserPresentedby:LeiWang8.3:LaserLaser:LightAmplificationbyStimulatedEmissionofRadiationSomeadvantages:HighlydirectionalMonochromaticCoherent28.3.1spontaneousemissionandstimulatedemissionSpontaneousemissionStimulatedabsorptionThisradiationwillbemonochromaticsinceeachphotonwillhaveanenergyofpreciselyandwillbecoherent,becauseallthephotonsreleasedwillbeinphase.StimulatedEmission38.3.1spontaneousemissionandstimulatedemission

A21:spontaneousemissionprobability;W21

：stimulatedemissionprobability;W12：stimulatedabsorptionprobabilityA21,B21andB12arecalledEinsteincoefficients58.3.2opticalresonantcavity

(Fabry-Perot)Thewaytoenhancethestimulatedemissionistohaveaverylargephotonfieldenergydensity(12):anopticalresonantcavityinwhichthephotondensitycanbuilduptoalargevaluethroughmultipleinternalreflectionsatcertainfrequencies(12).68.3.2opticalresonantcavity(Fabry-Perot)Howtobuildup?Theremustbethemediuminthecavitytoprovidethegain.78.4SemiconductorLasers8.4.1PopulationInversionataJunctionWhatisthedegeneratesemiconductor?Whatwillbethebanddiagramwhenap-njunctionisformedbetweenap-typedegeneratesemiconductorandann-typedegeneratesemiconductor?98.4.1PopulationInversionataJunctionWhatisthedegeneratesemiconductor?

Forheavily-dopedsemiconductors,impuritiesbecomesocloselypackedwithinthelatticethatinteractionsbetweenthemcannotbeignored.Forexample,inann-typesemiconductor,donorstatesformaband,whichmayoverlapthebottomoftheconductionband.TheFermilevellieswithintheconductionband.Similarly,foraheavilydopedp-typesemiconductor,theFermilevelwillliewiththevalanceband.

degeneratesemiconductors.108.4.1PopulationInversionataJunctionEcEvEvEcEvEFEcEFEFIntrinsicn-typep-typeEcEvEvEcEvEFEcEFEFIntrinsicHeavilydopedn-typeHeavilydopedp-type11Populationinversionisbestdescribedbytheuseofconceptofquasi-Fermilevels.？TheseparationofFnandFpatanypointisameasureofthedeparturefromequilibrium.Thedepartureisconsiderableintheinversionregion,itisgreaterthanthebandgap.138.4.1PopulationInversionataJunction

Fortwo-levelsystem,populationinversioncanbedescribedasn2>n1.

Forap-njunctioninsemiconductor,populationinversionexistswhen148.4.1PopulationInversionataJunctionWhenFnandFpliewithintheirrespectivebands,stimulatedemissioncandominateoverarangeoftransitions,fromhν=Fn-Fptohν=Eg.Thedominanttransitionsforlaseractionsaredeterminedlargelybytheresonantcavityandthestrongrecombinationradiationoccurringnearh=Eg.158.4.1PopulationInversionataJunction(2)DFBCavity(DistributedFeedbackCavity)orDBR(DistributedBraggMirror)DFBLaser178.4.1PopulationInversionataJunctionDBRLasers188.4.1PopulationInversionataJunction(3)VC(VerticalCavity)orVCSELs(垂直腔面發(fā)射激光器）198.4.2OutputCharacteristicsofasemiconductorlaserP-ICharacteristicsThresholdcurrentexists.~severalmA,evenμA.Thresholdcurrent218.4.2OutputCharacteristicsofasemiconductorlaserModesandSpectraMultimodeLasersSinglemodelasers

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