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1、Thegoal measuringtheintensityandphasevs time orfrequency Theways TheSpectrometerandMichelsonInterferometerScanningAutocorrelationSingle shotautocorrelationTheAutocorrelationandSpectrumThird orderAutocorrelationInterferometricAutocorrelation MeasuringUltrashortLaserPulsesI Autocorrelation Inordertome
2、asureaneventintime youneedashorterone Tostudythisevent youneedastrobelightpulsethat sshorter Butthen tomeasurethestrobelightpulse youneedadetectorwhoseresponsetimeisevenshorter Andsoon So now howdoyoumeasuretheshortestevent PhotographtakenbyHaroldEdgerton MIT TheDilemma Ultrashortlaserpulsesarethesh
3、ortesttechnologicaleventsevercreatedbyhumans It sroutinetogeneratepulsesshorterthan10 13secondsinduration andresearchershavegeneratedpulsesonlyafewfs 10 15s long Suchpulseshavemanyapplicationsinphysics chemistry biology andengineering Youcanmeasureanyevent aslongasyou vegotapulsethat sshorter Sohowd
4、oyoumeasurethepulseitself Youmustusethepulsetomeasureitself Butthatisn tgoodenough It sonlyasshortasthepulse It snotshorter Techniquesbasedonusingthepulsetomeasureitselfhavenotsufficed Todeterminethetemporalresolutionofanexperimentusingit Todeterminewhetheritcanbemadeevenshorter Tobetterunderstandth
5、elasersthatemitthemandtoverifymodelsofultrashortpulsegeneration Tobetterstudymedia thebetterweknowthelightinandlightout thebetterweknowthemediumwestudywiththem Tousepulsesofspecificintensityandphasevs timetocontrolchemicalreactions Coherentcontrol Tounderstandpulse shapingeffortsfortelecommunication
6、s etc Becauseit sthere Whymeasureanultrashortlaserpulse Asamoleculedissociates itsemissionchangescolor i e thephasechanges revealingmuchaboutthemoleculardynamics notavail ablefromthemerespectrum oreventheintensityvs time Linear Linearornonlinearmedium Measuringtheintensityandphaseofthepulsesintoando
7、utofamediumtellsusasmuchaspossibleaboutthelinearandnonlineareffectsinthemedium StudyingMediabyMeasuringtheIntensityandPhaseofLightPulses Withalinearmedium welearnthemedium sabsorptioncoefficientandrefractiveindexvs Withanonlinear opticalmedium wecanlearnaboutself phasemodulation forexample forwhicht
8、hetheoryismuchmorecomplex Indeed theoreticalmodelscanbetested Time fs Intensity Phase Nonlinear Eaton etal JQE35 451 1999 Alaserpulsehasthetime domainelectricfield E I t 1 2 exp i w 0 t i t Intensity Phase t Re Equivalently vs frequency exp i j w w 0 Spectral Phase neglectingthenegative frequencycom
9、ponent E w Re S w w0 1 2 Wemustmeasureanultrashortlaserpulse sintensityandphasevs timeorfrequency Spectrum Knowledgeoftheintensityandphaseorthespectrumandspectralphaseissufficienttodeterminethepulse t d dt Theinstantaneousfrequency Example Linearchirp We dliketobeabletomeasure notonlylinearlychirped
10、pulses butalsopulseswitharbitrarilycomplexphasesandfrequenciesvs time Thephasedeterminesthepulse sfrequency i e color vs time Thespectrometermeasuresthespectrum ofcourse Wavelengthvariesacrossthecamera andthespectrumcanbemeasuredforasinglepulse PulseMeasurementintheFrequencyDomain TheSpectrometer Co
11、llimatingMirror Czerny Turner arrangement EntranceSlit CameraorLinearDetectorArray FocusingMirror Grating Imagingspectrometers allowmanyspectratobemeasuredsimultaneously Broad bandpulse PulseMeasurementintheTimeDomain Detectors Examples Photo diodes Photo multipliers Detectorsaredevicesthatemitelect
12、ronsinresponsetophotons Detectorshaveveryslowriseandfalltimes 1nanosecond Asfaraswe reconcerned detectorshaveinfinitelyslowresponses Theymeasurethetimeintegralofthepulseintensityfrom to Thedetectoroutputvoltageisproportionaltothepulseenergy Bythemselves detectorstelluslittleaboutapulse Anothersymbol
13、foradetector PulseMeasurementintheTimeDomain Varyingthepulsedelay Sincedetectorsareessentiallyinfinitelyslow howdowemaketime domainmeasurementsonorusingultrashortlaserpulses We lldelayapulseintime Andhowwillwedothat Bysimplymovingamirror Sincelighttravels300 mperps 300 mofmirrordisplacementyieldsade
14、layof2ps Thisisveryconvenient MovingamirrorbackwardbyadistanceLyieldsadelayof Donotforgetthefactorof2 Lightmusttraveltheextradistancetothemirror andback Translationstage Inputpulse E t E t t Mirror Outputpulse Wecanalsovarythedelayusingamirrorpairorcornercube Mirrorpairsinvolvetworeflectionsanddispl
15、acethereturnbeaminspace Butout of planetiltyieldsanonparallelreturnbeam Cornercubesinvolvethreereflectionsandalsodisplacethereturnbeaminspace Evenbetter theyalwaysyieldaparallelreturnbeam Hollowcornercubes avoidpropagationthroughglass EdmundScientific Measuringtheinterferogramisequivalenttomeasuring
16、thespectrum PulseMeasurementintheTimeDomain TheMichelsonInterferometer Pulseenergy boring Fieldautocorrelation maybeinteresting but TheFTofthefieldautocorrelationisjustthespectrum Beam splitter Inputpulse Delay Slowdetector Mirror Mirror E t E t t Okay sohowdowemeasureapulse V Wong I A Walmsley Opt Lett 19 287 289 1994 I A Walmsley V Wong J Opt Soc AmB 13 2453 2463 1996 Result Usingonlytime independent linearfilters completecharacterizationofapulseisNOTpossiblewithaslowdetector Translation Ifyou
