超快光學(xué)放大演示文稿

超快光學(xué)放大演示文稿當(dāng)前第1頁\共有49頁\編于星期五\15點Pulseenergyvs.RepetitionrateReprate(pps)Pulseenergy(J)10910610310010-310-910-610010-3OscillatorCavity-dumpedoscillatorRegARegenRegen+multipassRegen+multi-multi-pass1Waveragepower當(dāng)前第2頁\共有49頁\編于星期五\15點A

tWhatarethegoalsinultrashortpulseamplification?Ipeak=EIncreasetheenergy(E),Decreasetheduration(t),Decreasetheareaofthefocus(A).MaximumintensityontargetNeededtostarttheexperimentNeededtogetusefulresultsPave=ErSignalisproportionaltothenumberofphotonsonthedetectorperintegrationtime.MaximumaveragepoweratthedetectorPulseenergyRepratePulseenergyBeamareaPulselength當(dāng)前第3頁\共有49頁\編于星期五\15點IssuesinUltrafastAmplificationandTheirSolutionsPulselengthdiscrepancies:Multi-passamplifiers

andregenerativeamplifiers(“Regens”).Damage:Chirped-PulseAmplification(CPA)Gainsaturation:Frantz-NodvickEquationGainnarrowing:BirefringentfiltersThermaleffects:coldandwavefrontcorrectionSatellitepulses,Contrast,andAmplifiedSpontaneousEmission:Pockels’cellsSystemscostlotsofmoney:Earnmoremoney…Pockelscellpolarizergainpumpinput/output當(dāng)前第4頁\共有49頁\編于星期五\15點CavityDumpingBeforeweconsideramplification,recallthattheintracavitypulseenergyis~50timestheoutputpulseenergy.Sowehavemorepulseenergy.Howcanwegetatit?Whatifweinsteadusedtwohighreflectors,letthepulseenergybuildup,andthenswitchoutthepulse?ThisistheoppositeofQ-switching:itinvolvesswitchingfromminimumtomaximumloss,andit’scalled“CavityDumping.”E=ToutputEintracavityTransmissionofoutputcoupler:~2%R=100%R=98%EintracavityE當(dāng)前第5頁\共有49頁\編于星期五\15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當(dāng)前第6頁\共有49頁\編于星期五\15點AmplificationofLaserPulses,inGeneralVerysimply,apowerfullaserpulseatonecolorpumpsanamplifiermedium,creatinganinversion,whichamplifiesanotherpulse.Nanosecond-pulselaseramplifierspumpedbyothernslasersarecommonplace.LaseroscillatorAmplifiermediumPumpEnergylevelsJpump(lpump/lL)lLlpump當(dāng)前第7頁\共有49頁\編于星期五\15點Single-passAmplificationMathAssumeasaturablegainmediumandJisthefluence(energy/area).Assumeallthepumpenergyisstoredintheamplifier,butsaturationeffectswilloccur.Atlowintensity,thegainislinear:Athighintensity,thegain“saturates”andhenceisconstant:Intermediatecaseinterpolatesbetweenthetwo:AmplifiermediumpumpJinJpumpJoutJsatlLlpumpJstoJsto=storedpumpfluence=Jpump(lpump/lL)Jsat=saturationfluence(materialdependent)當(dāng)前第8頁\共有49頁\編于星期五\15點Single-passAmplificationMathwherethesmallsignalgainperpassisgivenby:ThisdifferentialequationcanbeintegratedtoyieldtheFrantz-Nodvickequationfortheoutputofasaturatedamplifier:當(dāng)前第9頁\共有49頁\編于星期五\15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當(dāng)前第10頁\共有49頁\編于星期五\15點Anotherproblemwithamplifyingultrashortlaserpulses…Anotherissueisthattheultrashortpulseissomuchshorterthanthe(nsorms)pumppulsethatsuppliestheenergyforamplification.Soshouldtheultrashortpulsearriveearlyorlate?Early:Late:Pumpenergyarrivestoolateandiswasted.timepumppumptimeEnergydecaysandiswasted.Inbothcases,pumppulseenergyiswasted,andamplificationispoor.當(dāng)前第11頁\共有49頁\編于星期五\15點Soweneedmanypasses.Allultrashort-pulseamplifiersaremulti-pass.Thisapproachachievesmuchgreaterefficiency.timepumpTheultrashortpulsereturnsmanytimestoeventuallyextractmostoftheenergy.當(dāng)前第12頁\共有49頁\編于星期五\15點TwomainamplificationmethodsMulti-passamplifierpumpinputoutputgainPockelscellpolarizergainpumpinput/outputRegenerativeamplifier當(dāng)前第13頁\共有49頁\編于星期五\15點Anothermulti-passamplifierAPockelscell(PC)andapairofpolarizersareusedtoinjectasinglepulseintotheamplifier.當(dāng)前第14頁\共有49頁\編于星期五\15點RegenerativeamplifiergeometriesThisisusedfor10-20-Hzrepetitionrates.IthasalargerspotsizeintheTi:sapphirerod.PockelscellFaradayrotatorthin-filmpolarizerPockelscellTheTi:Sapphirerodis~20-mmlonganddopedfor90%absorption.ThisdesignisoftenusedforkHz-repetition-rateamplifiers.當(dāng)前第15頁\共有49頁\編于星期五\15點Pulseintensitiesinsideanamplifiercanbecomesohighthatdamage(oratleastsmall-scaleself-focusing)occurs.Solution: Expandthebeamand

uselargeamplifiermedia.Okay,wedidthat.Butthat’sstillnotenough.Solution: Expandthepulseintime,too.Okay,sowhatnext?當(dāng)前第16頁\共有49頁\編于星期五\15點Chirped-PulseAmplificationChirped-pulse

amplificationin-

volvesstretchingthe

pulsebeforeamplifyingit,

andthencompressingitlater.Wecanstretchthepulsebyafactorof10,000,amplifyit,andthenrecompressit!G.Mourouandcoworkers1983CPAisTHEbigdevelopment.PulsecompressorttSolidstateamplifiertDispersivedelaylinetShortpulseoscillator當(dāng)前第17頁\共有49頁\編于星期五\15點Stretchingandcompressingultrashortpulsesdf2ffdgratinggratingOkay,thislooksjustlikea“zero-dispersionstretcher”usedinpulseshaping.Butwhend≠f,it’sadispersivestretcherandcanstretchfspulsesbyafactorof10,000!Withtheoppositesignofd-f,wecancompressthepulse.Pulsestretcher當(dāng)前第18頁\共有49頁\編于星期五\15點ApulsestretcherThisdevicestretchesan18-fspulseto600ps—afactorof30,000!Araytraceofthevariouswavelengthsinthestretcher:

Pulsestretchercharacteristics:Inputpulsewidth:18fsOutputpulseduration:600psBandwidthpassed:>105nmPulseenergyout:~0.5nJ當(dāng)前第19頁\共有49頁\編于星期五\15點AlexandriteTi:sapphireExcimers0,00010,0010,010,11101001101001000104105106Nd:GlassDyesDirectAmplificationFluence(J/cm2)PulseDuration(fs)CPAvs.DirectAmplificationCPAachievesthefluenceoflongpulsesbutatashorterpulselength!當(dāng)前第20頁\共有49頁\編于星期五\15點RegenerativeChirped-PulseAmplificationat~100kHzreprateswithacwpumpCoherent

RegAamplifierAfsoscillatorrequiresonly~5Wofgreenlaserpower.AnArgonlaserprovidesupto50W.Usetheresttopumpanamplifier.Today,weuseanintracavity-doubledNd:YLFpumplaser(~10W).Microjoulesat250kHzrepetitionrates!當(dāng)前第21頁\共有49頁\編于星期五\15點Regenerativechirped-pulseamplificationwithakHzpulsedpump

Wavelength:800nm

(Repetitionratesof1to50kHz)HighEnergy:<130fs,>2mJat1kHzPicosecond:~80ps,>0.7mJat1kHzShortPulse:<50fs,>0.7mJat1kHzSpectraPhysicsregen:the“Spitfire”當(dāng)前第22頁\共有49頁\編于星期五\15點PumplaserforultrafastamplifiersCoherent“Corona”highpower,Q-switchedgreenlaserinacompactandmorereliablediode-pumpedpackage15mJ(ns)ata10kHzreprate(150Wavepower!)當(dāng)前第23頁\共有49頁\編于星期五\15點1kHz10kHz100kHz20mJ1.8mJ0.2mJ20W18W20W3mm1mm250μmExtractedenergyBeamdiameterPumppower100WAveragePowerReprateAveragepowerforhigh-powerTi:SapphireregensTheseaveragepowersarehigh.Andthispumppowerisalso.Ifyouwantsub-100fspulses,however,theenergieswillbeless.當(dāng)前第24頁\共有49頁\編于星期五\15點CPAisthebasisofthousandsofsystems.It’savailablecommerciallyinnumerousforms.Itworks!Buttherearesomeissues,especiallyifyoutrytopushforreallyhighenergies:Amplifiedspontaneousemission(ASE)Gainsaturation:gainvs.extractionefficiencyGainnarrowingThermalaberrationsContrastratioDamagethresholdvsextractionefficiency當(dāng)前第25頁\共有49頁\編于星期五\15點AmplifiedSpontaneousEmission(ASE)Fluorescencefromthegainmediumisamplifiedbefore(andafter)theultrashortpulsearrives.Thisyieldsa10-30nsbackgroundwithlowpeakpowerbutlargeenergy.Dependsonthenoisepresentintheamplifieratt=0ASEsharesthegainandtheexcitedpopulationwiththepulse.Amplificationreducesthecontrastbyafactorofupto10.當(dāng)前第26頁\共有49頁\編于星期五\15點GainNarrowing(andASE)Oneachpassthroughanamplifier,thepulsespectrumgetsmultipliedbythegainspectrum,whichnarrowstheoutputspectrum—andlengthensthepulse!Asaresult,thepulselengthens,anditcanbedifficulttodistinguishtheultrashortpulsefromthelongerAmplifiedSpontaneousEmission(ASE)當(dāng)前第27頁\共有49頁\編于星期五\15點GainnarrowingexampleTi:sapphiregaincrosssection10-fssech2pulsein00.20.40.60.8100.511.522.536507007508008509009501000NormalizedspectralintensityCrosssection(*10^-19cm^2)Wavelength(nm)65-nmFWHM32-nmFWHMFactorof2lossinbandwidthfor107gainMostTerawattsystemshave>1010smallsignalgainlonger

pulseout當(dāng)前第28頁\共有49頁\編于星期五\15點BeatinggainnarrowingBirefringentplatePolarizerPolarizerEEEwith

filterwithoutfilterSpectrumWavelength(nm)Gain&modulation650850900Wavelength(nanometers)70075080011.522.595020%GainmodulationBeforeAfterIntroducesomelossatthegainpeaktooffsetthehighgainthere.GainandlossSpectrum:beforeandafter當(dāng)前第29頁\共有49頁\編于星期五\15點Gain-NarrowingConclusionGainnarrowingcanbebeaten.Wecanuseuptohalfofthegainbandwidthfora4-levelsystem.Sub-20fsinTi:sapphireSub-200fsinNd:glassSub-100fsinYb:XX當(dāng)前第30頁\共有49頁\編于星期五\15點00.20.40.60.81750800850Intensity(arb.units)Wavelength(nm)Verybroadspectracanbecreatedthisway.A100-nmbandwidthat800nmcansupporta10-fspulse.當(dāng)前第31頁\共有49頁\編于星期五\15點Heatdepositioncauseslensingandsmall-scaleself-focusing.Thesethermalaberrationsincreasethebeamsizeandreducetheavailableintensity.Ipeak=EATWewantasmallfocusedspotsize,butthermalaberrationsincreasethebeamsize,nottomentionscrewingitup,too.Nowtheaveragepowermatters.Therepetitionrateiscrucial,andwe’dlikeittobehigh,buthighaveragepowermeansmorethermalaberrations…ThermalEffectsinAmplifiers當(dāng)前第32頁\共有49頁\編于星期五\15點Lowtemperatureminimizeslensing.CalculationsforkHzsystemsCryogeniccoolingresultsinalmostnofocalpowerInsapphire,conductivityincreasesanddn/dTdecreasesasTdecreases.Murnane,Kapteyn,andcoworkers當(dāng)前第33頁\共有49頁\編于星期五\15點StaticWave-frontCorrection2.5timesimprovementinpeakintensityhasbeenachievedCUOS當(dāng)前第34頁\共有49頁\編于星期五\15點DynamicCorrectionofSpatialDistortion50mmdiameter37actuatorsCUOS當(dāng)前第35頁\共有49頁\編于星期五\15點ContrastratioWhydoesittakeover2yearsbetweenthefirstannouncementofanewlasersourceandthefirstsuccessfulexperimentusingit?Becausethepulsehasleadingandfollowingsatellitepulsesthatwreakhavocinanyexperiment.Ifapulseof1018W/cm2peakpowerhasa“l(fā)ittle”satellitepulseonemillionthasstrong,that’sstill1TW/cm2!Thiscandosomeseriousdamage!Ionizationoccursat1011W/cm2soat1021W/cm2weneeda1010contrastratio!當(dāng)前第36頁\共有49頁\編于星期五\15點MajorsourcesofpoorcontrastNanosecondscale: pre-pulsesfromoscillator pre-pulsesfromamplifier ASEfromamplifierPicosecondscale: reflectionsintheamplifier spectralphaseoramplitudedistortions當(dāng)前第37頁\共有49頁\編于星期五\15點0-1-2-3-4-5-6-7-8-9-10FrontBacktimeSpectralphaseaberrationsPre-pulsesASE0ps10nsnsFWHMAmplifiedpulsesoftenhavepoorcontrast.Log(Energy)Pre-pulsesdothemostdamage,messingupamediumbeforehand.當(dāng)前第38頁\共有49頁\編于星期五\15點Typical3rd-orderautocorrelationAmplifiedpulseshavepre-andpost-pulses.當(dāng)前第39頁\共有49頁\編于星期五\15點APockelscell“PulsePicker”APockelscellcanpickapulsefromatrainandsuppresssatellites.Todoso,wemustswitchthevoltagefrom0tokVandbackto0,typicallyinafewns.VTimeVoltagefewnsSwitchinghighvoltagetwiceinafewnsisquitedifficult,requiringavalanchetransistors,microwavetriodes,orotherhigh-speedelectronics.當(dāng)前第40頁\共有49頁\編于星期五\15點amplifieroscillatorstretchercompressorPockelscells10-2-10-310nsPockelscellssuppresspre-andpost-pulses.Unfortunately,Pockelscellsaren’tperfect.Theyleak~1%.當(dāng)前第41頁\共有49頁\編于星期五\15點ContrastimprovementrecipesAPockelscellimprovesthecontrastbyafew100to1000.Weneedatleast3Pockelscellsworkinginthebestconditions:onaxis(donottiltPockels’cells)broadbandhigh-contrastpolarizers(notdielectric)fastrisetime(<<2ns10-90%)collimatedbeams

TemperaturedriftisalsoaprobleminPockelscells.Also:Goodpumpsynchronizationgivesafactor3-10當(dāng)前第42頁\共有49頁\編于星期五\15點Multiple-stagemulti-