超快光學放大演示文稿

超快光學放大演示文稿目前一頁\總數(shù)四十九頁\編于十九點Pulseenergyvs.RepetitionrateReprate(pps)Pulseenergy(J)10910610310010-310-910-610010-3OscillatorCavity-dumpedoscillatorRegARegenRegen+multipassRegen+multi-multi-pass1Waveragepower目前二頁\總數(shù)四十九頁\編于十九點A

tWhatarethegoalsinultrashortpulseamplification?Ipeak=EIncreasetheenergy(E),Decreasetheduration(t),Decreasetheareaofthefocus(A).MaximumintensityontargetNeededtostarttheexperimentNeededtogetusefulresultsPave=ErSignalisproportionaltothenumberofphotonsonthedetectorperintegrationtime.MaximumaveragepoweratthedetectorPulseenergyRepratePulseenergyBeamareaPulselength目前三頁\總數(shù)四十九頁\編于十九點IssuesinUltrafastAmplificationandTheirSolutionsPulselengthdiscrepancies:Multi-passamplifiers

andregenerativeamplifiers(“Regens”).Damage:Chirped-PulseAmplification(CPA)Gainsaturation:Frantz-NodvickEquationGainnarrowing:BirefringentfiltersThermaleffects:coldandwavefrontcorrectionSatellitepulses,Contrast,andAmplifiedSpontaneousEmission:Pockels’cellsSystemscostlotsofmoney:Earnmoremoney…Pockelscellpolarizergainpumpinput/output目前四頁\總數(shù)四十九頁\編于十九點CavityDumpingBeforeweconsideramplification,recallthattheintracavitypulseenergyis~50timestheoutputpulseenergy.Sowehavemorepulseenergy.Howcanwegetatit?Whatifweinsteadusedtwohighreflectors,letthepulseenergybuildup,andthenswitchoutthepulse?ThisistheoppositeofQ-switching:itinvolvesswitchingfromminimumtomaximumloss,andit’scalled“CavityDumping.”E=ToutputEintracavityTransmissionofoutputcoupler:~2%R=100%R=98%EintracavityE目前五頁\總數(shù)四十九頁\編于十九點Cavitydumping:thePockelscellAPockelscellisadevicethatcanswitchapulse(inand)outofaresonator.It’susedinQ-switchesandcavitydumpers.Avoltage(afewkV)canturnacrystalintoahalf-orquarter-waveplate.VIfV=0,thepulsepolarizationdoesn’tchange.IfV=Vp,thepulsepolarizationswitchestoitsorthogonalstate.AbruptlyswitchingaPockelscellallowsustoextractapulsefromacavity.Thisallowsustoachieve~100timesthepulseenergyat1/100therepetitionrate(i.e.,100nJat1MHz).Pockelscell(voltagemaybetransverseorlongitudinal)Polarizer目前六頁\總數(shù)四十九頁\編于十九點AmplificationofLaserPulses,inGeneralVerysimply,apowerfullaserpulseatonecolorpumpsanamplifiermedium,creatinganinversion,whichamplifiesanotherpulse.Nanosecond-pulselaseramplifierspumpedbyothernslasersarecommonplace.LaseroscillatorAmplifiermediumPumpEnergylevelsJpump(lpump/lL)lLlpump目前七頁\總數(shù)四十九頁\編于十九點Single-passAmplificationMathAssumeasaturablegainmediumandJisthefluence(energy/area).Assumeallthepumpenergyisstoredintheamplifier,butsaturationeffectswilloccur.Atlowintensity,thegainislinear:Athighintensity,thegain“saturates”andhenceisconstant:Intermediatecaseinterpolatesbetweenthetwo:AmplifiermediumpumpJinJpumpJoutJsatlLlpumpJstoJsto=storedpumpfluence=Jpump(lpump/lL)Jsat=saturationfluence(materialdependent)目前八頁\總數(shù)四十九頁\編于十九點Single-passAmplificationMathwherethesmallsignalgainperpassisgivenby:ThisdifferentialequationcanbeintegratedtoyieldtheFrantz-Nodvickequationfortheoutputofasaturatedamplifier:目前九頁\總數(shù)四十九頁\編于十九點Frantz-NodvickequationG0exp(g0L)exp(JstoJsat)Higherpumping(Jsto)meanshigherefficiencyandhighersaturationandsolowergain.Soyoucanhavehighgainorhighextractionefficiency.Butnotboth.11,21,41,61,822,22,42,600,20,40,60,81012345GainExtractionefficiency(Jout/Jsto)Jsto/JsatJout/Jin目前十頁\總數(shù)四十九頁\編于十九點Anotherproblemwithamplifyingultrashortlaserpulses…Anotherissueisthattheultrashortpulseissomuchshorterthanthe(nsorms)pumppulsethatsuppliestheenergyforamplification.Soshouldtheultrashortpulsearriveearlyorlate?Early:Late:Pumpenergyarrivestoolateandiswasted.timepumppumptimeEnergydecaysandiswasted.Inbothcases,pumppulseenergyiswasted,andamplificationispoor.目前十一頁\總數(shù)四十九頁\編于十九點Soweneedmanypasses.Allultrashort-pulseamplifiersaremulti-pass.Thisapproachachievesmuchgreaterefficiency.timepumpTheultrashortpulsereturnsmanytimestoeventuallyextractmostoftheenergy.目前十二頁\總數(shù)四十九頁\編于十九點TwomainamplificationmethodsMulti-passamplifierpumpinputoutputgainPockelscellpolarizergainpumpinput/outputRegenerativeamplifier目前十三頁\總數(shù)四十九頁\編于十九點Anothermulti-passamplifierAPockelscell(PC)andapairofpolarizersareusedtoinjectasinglepulseintotheamplifier.目前十四頁\總數(shù)四十九頁\編于十九點RegenerativeamplifiergeometriesThisisusedfor10-20-Hzrepetitionrates.IthasalargerspotsizeintheTi:sapphirerod.PockelscellFaradayrotatorthin-filmpolarizerPockelscellTheTi:Sapphirerodis~20-mmlonganddopedfor90%absorption.ThisdesignisoftenusedforkHz-repetition-rateamplifiers.目前十五頁\總數(shù)四十九頁\編于十九點Pulseintensitiesinsideanamplifiercanbecomesohighthatdamage(oratleastsmall-scaleself-focusing)occurs.Solution: Expandthebeamand

uselargeamplifiermedia.Okay,wedidthat.Butthat’sstillnotenough.Solution: Expandthepulseintime,too.Okay,sowhatnext?目前十六頁\總數(shù)四十九頁\編于十九點Chirped-PulseAmplificationChirped-pulse

amplificationin-

volvesstretchingthe

pulsebeforeamplifyingit,

andthencompressingitlater.Wecanstretchthepulsebyafactorof10,000,amplifyit,andthenrecompressit!G.Mourouandcoworkers1983CPAisTHEbigdevelopment.PulsecompressorttSolidstateamplifiertDispersivedelaylinetShortpulseoscillator目前十七頁\總數(shù)四十九頁\編于十九點Stretchingandcompressingultrashortpulsesdf2ffdgratinggratingOkay,thislooksjustlikea“zero-dispersionstretcher”usedinpulseshaping.Butwhend≠f,it’sadispersivestretcherandcanstretchfspulsesbyafactorof10,000!Withtheoppositesignofd-f,wecancompressthepulse.Pulsestretcher目前十八頁\總數(shù)四十九頁\編于十九點ApulsestretcherThisdevicestretchesan18-fspulseto600ps—afactorof30,000!Araytraceofthevariouswavelengthsinthestretcher:

Pulsestretchercharacteristics:Inputpulsewidth:18fsOutputpulseduration:600psBandwidthpassed:>105nmPulseenergyout:~0.5nJ目前十九頁\總數(shù)四十九頁\編于十九點AlexandriteTi:sapphireExcimers0,00010,0010,010,11101001101001000104105106Nd:GlassDyesDirectAmplificationFluence(J/cm2)PulseDuration(fs)CPAvs.DirectAmplificationCPAachievesthefluenceoflongpulsesbutatashorterpulselength!目前二十頁\總數(shù)四十九頁\編于十九點RegenerativeChirped-PulseAmplificationat~100kHzreprateswithacwpumpCoherent

RegAamplifierAfsoscillatorrequiresonly~5Wofgreenlaserpower.AnArgonlaserprovidesupto50W.Usetheresttopumpanamplifier.Today,weuseanintracavity-doubledNd:YLFpumplaser(~10W).Microjoulesat250kHzrepetitionrates!目前二十一頁\總數(shù)四十九頁\編于十九點Regenerativechirped-pulseamplificationwithakHzpulsedpump

Wavelength:800nm

(Repetitionratesof1to50kHz)HighEnergy:<130fs,>2mJat1kHzPicosecond:~80ps,>0.7mJat1kHzShortPulse:<50fs,>0.7mJat1kHzSpectraPhysicsregen:the“Spitfire”目前二十二頁\總數(shù)四十九頁\編于十九點PumplaserforultrafastamplifiersCoherent“Corona”highpower,Q-switchedgreenlaserinacompactandmorereliablediode-pumpedpackage15mJ(ns)ata10kHzreprate(150Wavepower!)目前二十三頁\總數(shù)四十九頁\編于十九點1kHz10kHz100kHz20mJ1.8mJ0.2mJ20W18W20W3mm1mm250μmExtractedenergyBeamdiameterPumppower100WAveragePowerReprateAveragepowerforhigh-powerTi:SapphireregensTheseaveragepowersarehigh.Andthispumppowerisalso.Ifyouwantsub-100fspulses,however,theenergieswillbeless.目前二十四頁\總數(shù)四十九頁\編于十九點CPAisthebasisofthousandsofsystems.It’savailablecommerciallyinnumerousforms.Itworks!Buttherearesomeissues,especiallyifyoutrytopushforreallyhighenergies:Amplifiedspontaneousemission(ASE)Gainsaturation:gainvs.extractionefficiencyGainnarrowingThermalaberrationsContrastratioDamagethresholdvsextractionefficiency目前二十五頁\總數(shù)四十九頁\編于十九點AmplifiedSpontaneousEmission(ASE)Fluorescencefromthegainmediumisamplifiedbefore(andafter)theultrashortpulsearrives.Thisyieldsa10-30nsbackgroundwithlowpeakpowerbutlargeenergy.Dependsonthenoisepresentintheamplifieratt=0ASEsharesthegainandtheexcitedpopulationwiththepulse.Amplificationreducesthecontrastbyafactorofupto10.目前二十六頁\總數(shù)四十九頁\編于十九點GainNarrowing(andASE)Oneachpassthroughanamplifier,thepulsespectrumgetsmultipliedbythegainspectrum,whichnarrowstheoutputspectrum—andlengthensthepulse!Asaresult,thepulselengthens,anditcanbedifficulttodistinguishtheultrashortpulsefromthelongerAmplifiedSpontaneousEmission(ASE)目前二十七頁\總數(shù)四十九頁\編于十九點GainnarrowingexampleTi:sapphiregaincrosssection10-fssech2pulsein00.20.40.60.8100.511.522.536507007508008509009501000NormalizedspectralintensityCrosssection(*10^-19cm^2)Wavelength(nm)65-nmFWHM32-nmFWHMFactorof2lossinbandwidthfor107gainMostTerawattsystemshave>1010smallsignalgainlonger

pulseout目前二十八頁\總數(shù)四十九頁\編于十九點BeatinggainnarrowingBirefringentplatePolarizerPolarizerEEEwith

filterwithoutfilterSpectrumWavelength(nm)Gain&modulation650850900Wavelength(nanometers)70075080011.522.595020%GainmodulationBeforeAfterIntroducesomelossatthegainpeaktooffsetthehighgainthere.GainandlossSpectrum:beforeandafter目前二十九頁\總數(shù)四十九頁\編于十九點Gain-NarrowingConclusionGainnarrowingcanbebeaten.Wecanuseuptohalfofthegainbandwidthfora4-levelsystem.Sub-20fsinTi:sapphireSub-200fsinNd:glassSub-100fsinYb:XX目前三十頁\總數(shù)四十九頁\編于十九點00.20.40.60.81750800850Intensity(arb.units)Wavelength(nm)Verybroadspectracanbecreatedthisway.A100-nmbandwidthat800nmcansupporta10-fspulse.目前三十一頁\總數(shù)四十九頁\編于十九點Heatdepositioncauseslensingandsmall-scaleself-focusing.Thesethermalaberrationsincreasethebeamsizeandreducetheavailableintensity.Ipeak=EATWewantasmallfocusedspotsize,butthermalaberrationsincreasethebeamsize,nottomentionscrewingitup,too.Nowtheaveragepowermatters.Therepetitionrateiscrucial,andwe’dlikeittobehigh,buthighaveragepowermeansmorethermalaberrations…ThermalEffectsinAmplifiers目前三十二頁\總數(shù)四十九頁\編于十九點Lowtemperatureminimizeslensing.CalculationsforkHzsystemsCryogeniccoolingresultsinalmostnofocalpowerInsapphire,conductivityincreasesanddn/dTdecreasesasTdecreases.Murnane,Kapteyn,andcoworkers目前三十三頁\總數(shù)四十九頁\編于十九點StaticWave-frontCorrection2.5timesimprovementinpeakintensityhasbeenachievedCUOS目前三十四頁\總數(shù)四十九頁\編于十九點DynamicCorrectionofSpatialDistortion50mmdiameter37actuatorsCUOS目前三十五頁\總數(shù)四十九頁\編于十九點ContrastratioWhydoesittakeover2yearsbetweenthefirstannouncementofanewlasersourceandthefirstsuccessfulexperimentusingit?Becausethepulsehasleadingandfollowingsatellitepulsesthatwreakhavocinanyexperiment.Ifapulseof1018W/cm2peakpowerhasa“l(fā)ittle”satellitepulseonemillionthasstrong,that’sstill1TW/cm2!Thiscandosomeseriousdamage!Ionizationoccursat1011W/cm2soat1021W/cm2weneeda1010contrastratio!目前三十六頁\總數(shù)四十九頁\編于十九點MajorsourcesofpoorcontrastNanosecondscale: pre-pulsesfromoscillator pre-pulsesfromamplifier ASEfromamplifierPicosecondscale: reflectionsintheamplifier spectralphaseoramplitudedistortions目前三十七頁\總數(shù)四十九頁\編于十九點0-1-2-3-4-5-6-7-8-9-10FrontBacktimeSpectralphaseaberrationsPre-pulsesASE0ps10nsnsFWHMAmplifiedpulsesoftenhavepoorcontrast.Log(Energy)Pre-pulsesdothemostdamage,messingupamediumbeforehand.目前三十八頁\總數(shù)四十九頁\編于十九點Typical3rd-orderautocorrelationAmplifiedpulseshavepre-andpost-pulses.目前三十九頁\總數(shù)四十九頁\編于十九點APockelscell“PulsePicker”APockelscellcanpickapulsefromatrainandsuppresssatellites.Todoso,wemustswitchthevoltagefrom0tokVandbackto0,typicallyinafewns.VTimeVoltagefewnsSwitchinghighvoltagetwiceinafewnsisquitedifficult,requiringavalanchetransistors,microwavetriodes,orotherhigh-speedelectronics.目前四十頁\總數(shù)四十九頁\編于十九點amplifieroscillatorstretchercompressorPockelscells10-2-10-310nsPockelscellssuppresspre-andpost-pulses.Unfortunately,Pockelscellsaren’tperfect.Theyleak~1%.目前四十一頁\總數(shù)四十九頁\編于十九點ContrastimprovementrecipesAPockelscellimprovesthecontrastbyafew100to1000.Weneedatleast3Pockelscellsworkinginthebestconditions:onaxis(donottiltPockels’cells)broadbandhigh-contrastpolarizers(notdielectric)fastrisetime(<<2ns10-90%)collimatedbeams

TemperaturedriftisalsoaprobleminPockelscells.Also:Goodpumpsynchronizationgivesafactor3-10目前四十二頁\總數(shù)四十九頁\編于十九點Multiple-stagemu