超快光学 第章 放大.ppt

TheAmplificationofUltra shortLaserPulses FrancoisSalinCenterforIntenseLasersandApplications CELIA Universit BordeauxI Francesalin celia u bordeaux frGillesDarpentigny CELIA VincentBagnoud LLE AntoineCourjaud ClemensHonninger EricMottay AmplitudeSystemes LucVigroux AmplitudeTechnologies andsomeadditionalstufffromDanMittleman Rice Pulsecompressor t t Solidstateamplifier t Dispersivedelayline t Shortpulseoscillator Mostofthislecturecourtesyof Pulseenergyvs Repetitionrate Reprate pps Pulseenergy J Oscillator Cavity dumpedoscillator RegA Regen Regen multipass Regen multi multi pass 1Waveragepower A t Whatarethegoalsinultrashortpulseamplification Ipeak E Increasetheenergy E Decreasetheduration t Decreasetheareaofthefocus A Maximumintensityontarget Neededtostarttheexperiment Neededtogetusefulresults Pave Er Signalisproportionaltothenumberofphotonsonthedetectorperintegrationtime Maximumaveragepoweratthedetector Pulseenergy Reprate Pulseenergy Beamarea Pulselength IssuesinUltrafastAmplificationandTheirSolutions Pulselengthdiscrepancies Multi passamplifiersandregenerativeamplifiers Regens Damage Chirped PulseAmplification CPA Gainsaturation Frantz NodvickEquationGainnarrowing BirefringentfiltersThermaleffects coldandwavefrontcorrectionSatellitepulses Contrast andAmplifiedSpontaneousEmission Pockels cellsSystemscostlotsofmoney Earnmoremoney CavityDumping Beforeweconsideramplification recallthattheintracavitypulseenergyis 50timestheoutputpulseenergy Sowehavemorepulseenergy Howcanwegetatit Whatifweinsteadusedtwohighreflectors letthepulseenergybuildup andthenswitchoutthepulse ThisistheoppositeofQ switching itinvolvesswitchingfromminimumtomaximumloss andit scalled CavityDumping Cavitydumping thePockelscell APockelscellisadevicethatcanswitchapulse inand outofaresonator It susedinQ switchesandcavitydumpers Avoltage afewkV canturnacrystalintoahalf orquarter waveplate V IfV 0 thepulsepolarizationdoesn tchange IfV Vp thepulsepolarizationswitchestoitsorthogonalstate AbruptlyswitchingaPockelscellallowsustoextractapulsefromacavity Thisallowsustoachieve 100timesthepulseenergyat1 100therepetitionrate i e 100nJat1MHz Pockelscell voltagemaybetransverseorlongitudinal Polarizer AmplificationofLaserPulses inGeneral Verysimply apowerfullaserpulseatonecolorpumpsanamplifiermedium creatinganinversion whichamplifiesanotherpulse Nanosecond pulselaseramplifierspumpedbyothernslasersarecommonplace Laseroscillator Amplifiermedium Pump Energylevels Jpump lpump lL lL lpump Single passAmplificationMath AssumeasaturablegainmediumandJisthefluence energy area Assumeallthepumpenergyisstoredintheamplifier butsaturationeffectswilloccur Atlowintensity thegainislinear Athighintensity thegain saturates andhenceisconstant Intermediatecaseinterpolatesbetweenthetwo Jsto storedpumpfluence Jpump lpump lL Jsat saturationfluence materialdependent Single passAmplificationMath wherethesmallsignalgainperpassisgivenby ThisdifferentialequationcanbeintegratedtoyieldtheFrantz Nodvickequationfortheoutputofasaturatedamplifier Frantz Nodvickequation G 0 exp g 0 L exp J sto J sat Higherpumping Jsto meanshigherefficiencyandhighersaturationandsolowergain Soyoucanhavehighgainorhighextractionefficiency Butnotboth Gain Extractionefficiency Jout Jsto J sto J sat Jout Jin Anotherproblemwithamplifyingultrashortlaserpulses Anotherissueisthattheultrashortpulseissomuchshorterthanthe nsorms pumppulsethatsuppliestheenergyforamplification Soshouldtheultrashortpulsearriveearlyorlate Early Late Pumpenergyarrivestoolateandiswasted time pump pump time Energydecaysandiswasted Inbothcases pumppulseenergyiswasted andamplificationispoor Soweneedmanypasses Allultrashort pulseamplifiersaremulti pass Thisapproachachievesmuchgreaterefficiency Theultrashortpulsereturnsmanytimestoeventuallyextractmostoftheenergy Twomainamplificationmethods Multi passamplifier Regenerativeamplifier Anothermulti passamplifier APockelscell PC andapairofpolarizersareusedtoinjectasinglepulseintotheamplifier Regenerativeamplifiergeometries Thisisusedfor10 20 Hzrepetitionrates IthasalargerspotsizeintheTi sapphirerod TheTi Sapphirerodis 20 mmlonganddopedfor90 absorption ThisdesignisoftenusedforkHz repetition rateamplifiers Pulseintensitiesinsideanamplifiercanbecomesohighthatdamage oratleastsmall scaleself focusing occurs Solution Expandthebeamanduselargeamplifiermedia Okay wedidthat Butthat sstillnotenough Solution Expandthepulseintime too Okay sowhatnext Chirped PulseAmplification Chirped pulseamplificationin volvesstretchingthepulsebeforeamplifyingit andthencompressingitlater Wecanstretchthepulsebyafactorof10 000 amplifyit andthenrecompressit G Mourouandcoworkers1983 CPAisTHEbigdevelopment Pulsecompressor t t Solidstateamplifier t Dispersivedelayline t Shortpulseoscillator Stretchingandcompressingultrashortpulses Okay thislooksjustlikea zero dispersionstretcher usedinpulseshaping Butwhend f it sadispersivestretcherandcanstretchfspulsesbyafactorof10 000 Withtheoppositesignofd f wecancompressthepulse Pulsestretcher Apulsestretcher Thisdevicestretchesan18 fspulseto600ps afactorof30 000 Araytraceofthevariouswavelengthsinthestretcher Alexandrite Ti sapphire Excimers Nd Glass Dyes DirectAmplification Fluence J cm2 PulseDuration fs CPAvs DirectAmplification CPAachievesthefluenceoflongpulsesbutatashorterpulselength RegenerativeChirped PulseAmplificationat 100kHzreprateswithacwpump CoherentRegAamplifier Afsoscillatorrequiresonly 5Wofgreenlaserpower AnArgonlaserprovidesupto50W Usetheresttopumpanamplifier Today weuseanintracavity doubledNd YLFpumplaser 10W Microjoulesat250kHzrepetitionrates Regenerativechirped pulseamplificationwithakHzpulsedpump Wavelength 800nm Repetitionratesof1to50kHz HighEnergy 2mJat1kHzPicosecond 80ps 0 7mJat1kHzShortPulse 0 7mJat1kHz SpectraPhysicsregen the Spitfire Pumplaserforultrafastamplifiers Coherent Corona highpower Q switchedgreenlaserinacompactandmorereliablediode pumpedpackage 15mJ ns ata10kHzreprate 150Wavepower Extractedenergy Beamdiameter Pumppower100W AveragePower Reprate Averagepowerforhigh powerTi Sapphireregens Theseaveragepowersarehigh Andthispumppowerisalso Ifyouwantsub 100fspulses however theenergieswillbeless CPAisthebasisofthousandsofsystems It savailablecommerciallyinnumerousforms Itworks Buttherearesomeissues especiallyifyoutrytopushforreallyhighenergies Amplifiedspontaneousemission ASE Gainsaturation gainvs extractionefficiencyGainnarrowingThermalaberrationsContrastratioDamagethresholdvsextractionefficiency AmplifiedSpontaneousEmission ASE Fluorescencefromthegainmediumisamplifiedbefore andafter theultrashortpulsearrives Thisyieldsa10 30nsbackgroundwithlowpeakpowerbutlargeenergy Dependsonthenoisepresentintheamplifieratt 0ASEsharesthegainandtheexcitedpopulationwiththepulse Amplificationreducesthecontrastbyafactorofupto10 GainNarrowing andASE Oneachpassthroughanamplifier thepulsespectrumgetsmultipliedbythegainspectrum whichnarrowstheoutputspectrum andlengthensthepulse Asaresult thepulselengthens anditcanbedifficulttodistinguishtheultrashortpulsefromthelongerAmplifiedSpontaneousEmission ASE Gainnarrowingexample Ti sapphiregaincrosssection 10 fssech2pulsein Normalizedspectralintensity Crosssection 10 19cm 2 Wavelength nm 65 nmFWHM 32 nmFWHM Factorof2lossinbandwidthfor107gainMostTerawattsystemshave 1010smallsignalgain longerpulseout Beatinggainnarrowing Introducesomelossatthegainpeaktooffsetthehighgainthere Gainandloss Spectrum beforeandafter Gain NarrowingConclusion Gainnarrowingcanbebeaten Wecanuseuptohalfofthegainbandwidthfora4 levelsystem Sub 20fsinTi sapphireSub 200fsinNd glassSub 100fsinYb XX Intensity arb units Wavelength nm Verybroadspectracanbecreatedthisway A100 nmbandwidthat800nmcansupporta10 fspulse Heatdepositioncauseslensingandsmall scaleself focusing Thesethermalaberrationsincreasethebeamsizeandreducetheavailableintensity Ipeak E A T Wewantasmallfocusedspotsize butthermalaberrationsincreasethebeamsize nottomentionscrewingitup too Nowtheaveragepowermatters Therepetitionrateiscrucial andwe dlikeittobehigh buthighaveragepowermeansmorethermalaberrations ThermalEffectsinAmplifiers Lowtemperatureminimizeslensing CalculationsforkHzsystemsCryogeniccoolingresultsinalmostnofocalpower Insapphire conductivityincreasesanddn dTdecreasesasTdecreases Murnane Kapteyn andcoworkers StaticWave frontCorrection 2 5timesimprovementinpeakintensityhasbeenachieved CUOS DynamicCorrectionofSpatialDistortion 50mmdiameter37actuators CUOS Contrastratio Whydoesittakeover2yearsbetweenthefirstannouncementofanewlasersourceandthefirstsuccessfulexperimentusingit Becausethepulsehasleadingandfollowingsatellitepulsesthatwreakhavocinanyexperiment Ifapulseof1018W cm2peakpowerhasa little satellitepulseonemillionthasstrong that sstill1TW cm2 Thiscandosomeseriousdamage Ionizationoccursat1011W cm2soat1021W cm2weneeda1010contrastratio Majorsourcesofpoorcontrast Nanosecondscale pre pulsesfromoscillatorpre pulsesfromamplifierASEfromamplifier Picosecondscale reflectionsintheamplifierspectralphaseoramplitudedistortions 0 1 2 3 4 5 6 7 8 9 10 Front Back time Spectralphaseaberrations Pre pulses ASE 0 ps 10ns ns FWHM Amplifiedpulsesoftenhavepoorcontrast Log Energy Pre pulsesdothemostdamage messingupamediumbeforehand Typical3rd orderautocorrelation Amplifiedpulseshavepre andpost pulses APockelscell PulsePicker APockelscellcanpickapulsefromatrainandsuppresssatellites Todoso wemustswitchthevoltagefrom0tokVandbackto0 typicallyinafewns V Time Voltage fewns Switchinghighvoltagetwiceinafewnsisquitedifficult requiringavalanchetransistors microwavetriodes orotherhigh speedelectronics amplifier oscillator stretcher compressor Pockelscells 10 2 10 3 10ns Pockelscellssuppresspre andpost pulses Unfortunately Pockelscellsaren tperfect Theyleak 1 Contrastimprovementrecipes APockelscellimprovesthecontrastbyafew100to1000 Weneedatleast3Pockelscellsworkinginthebestconditions ona