Pulsed Lasers Q-Switching and Mode Locking脉冲激光器的调Q锁模

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1、PulsedLasers:Q-SwitchingandModeLockingPulsedlasersarevaluablewhenpeak(orinstantaneous)powerratherthanaveragepowerismostimportant.Examples:NonlinearphotochemicalprocesseswhereRATEµIn(←integer>1)PEAKPOWER469IfE=pulseenergy(J),thenPeakPower≈AveragePower=E/pulse·ExampleAKrFlaser(l=248nm)

2、producespulseshavingenergiesof250mJandtemporalwidthsof20nsataPRFof80Hz.469PeakPower==12.5MWAveragePower=J·80s–1=20W!Pulsingalasermayalsobeanecessityifthethresholdpumpingpower(specificpowerloading:W-cm–3)cannotbelongsustained.469Example:KrFAgainsSE=2.6Å2gth=3·10–3cm–1DNth=1.2·1013cm–

3、3tsp=(A21)–1=5·10–9sFormationefficiencyforupperlaserlevel=15%  ThresholdPumpPower  469ApproachestoPulsingLasers1.Pulsingtheexcitationitself;requireshighpowerelectronics;typicallyhighpeakpowerbutlowPRF(andlowaveragepower).2.Q-switching3.Mode-locking—OperatelaserCWbut,withoptics,canin

4、ducelasertoproducea“train”ofshortpulses;activeorpassivemode-locking.469Q-SwitchingTheideaissimple:SpoiltheQofanopticalcavity,allowingDNtorisewellaboveDNth.Then,restoreQsuddenly…→“GiantPulse”!Example:SpinningMirror469AlthoughQ-switchingdoesnotproducetheshortestpulsesavailable(mode-loc

5、kingdoesthat),ithasseveraladvantages:1.It’sinexpensive!2.Easytoimplement3.Efficientinextractingenergystoredinupperlaserlevel.4691.Pumpatlowratewithshutterclosedtopreventlasing.2.DNrisesto>DNth3.Openshutter,laseroscillationbuildsuprapidly.4.2→1stimulatedtransitionsdepleteDNquicklyande

6、fficiently.4695.DNcangobelowDNthbeforepulseterminates.6.Closeshutter.469AnalysisofQ-SwitchingImmediatelyaftertheshutteropens,theintensityIgrows“fromthenoise”asI(z)=I0egzÞ=gIand469Lettingj≡totalnumberofphotonscirculatingintheopticalcavity,thenwheretp=photonlifetime=469Definet=(dimensi

7、onlesstime)andsoBut≡gth(1)469Note,however,thatforeach2→1stimulatedtransitionthatoccurs,DNchangesby2.469ni=DN(t=0)nf=DN(t→∞)469PulsedLasers:Q-Switching&ModeLockingI.MethodstoProduceShortLaserPulsesA.Pulsedlaserstypicallyhavehigherpeakpower(thoughperhapsloweraveragepower)thancontinuous

8、waveCWlasers