光电检测技术英文

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英文原文

1.5ExperimentalSetup

Duetothemanyconceptsandvariationsinvolvedinperformingtheexperimentsinthisprojectandalsobecauseoftheirintroductorynature,Project1willverylikelybethemosttimeconsumingprojectinthiskit.Thisprojectmayrequireasmuchas9hourstocomplete.Werecommendthatyouperformtheexperimentsintwoormorelaboratorysessions.Forexample,powerandastigmaticdistancecharacteristicsmaybeexaminedinthefirstsessionandthelasttwoexperiments(frequencyandamplitudecharacteristics)maybeperformedinthesecondsession.

ANoteofCaution

Alloftheabovecommentsrefertosingle-modeoperationofthelaserwhichisaveryfragiledevicewithrespecttoreflectionsandoperatingpoint.Onemustensurethatbeforeperformingmeasurementsthelaserisindeedoperatingsingle-mode.Thiscanberealizedifasingle,broadfringepatternisobtainedorequivalentlyagoodsinusoidaloutputisobtainedfromtheMichelsoninterferometerasthepathimbalanceisscanned.Ifthisisnotthecase,thelaserisprobablyoperatingmultimodeanditscurrentshouldbeadjusted.Ifsingle-modeoperationcannotbeachievedbyadjustingthecurrent,thenreflectionsmaybedrivingthelasermultimode,inwhichcasethesetupshouldbeadjustedtominimizereflections.Ifstillnotoperatingsingle-mode,thelaserdiodemayhavebeendamagedandmayneedtobereplaced.

Warning

Thelasersprovidedinthisprojectkitemitinvisibleradiationthatcandamagethehumaneye.Itisessentialthatyouavoiddirecteyeexposuretothelaserbeam.Werecommendtheuseofprotectiveeyeweardesignedforuseatthelaserwavelengthof780nm.

ReadtheSafetysectionsintheLaserDiodeDriverOperatingManualandinthelaserdiodesectionofComponentHandlingandAssembly(AppendixA)beforeproceeding.

1.5.1SemiconductorDiodeLaserPowerCharacteristics

1.Assemblethelasermountassembly(LMA-I)andconnectthelasertoitspowersupply.Wewillfirstcollimatethelightbeam.Connectthelaserbeamtoavideomonitorandimagethelaserbeamonawhitesheetofpaperheldabouttwototen

(xzplaneinFigure1.1)isparallelwiththetablesurface.

2.Duetotheasymmetricdivergenceofthelight,thecross-sectionofthebeamleavingthelaserand,further,pastthesphericallensiselliptical.Thebeam,thus,hastwodistinctfocalpoints,oneintheplaneparallelandtheotherintheplaneperpendiculartothelaserdiodejunction.Thereisapointbetweenthetwofocalpointswherethebeamcross-sectioniscircular.Withtheinfraredimagerandawhitecard,roughlydeterminethepositionwherethebeamcross-sectioniscircular.

Figure1.9–Procedureforfindingastigmaticdistance.

3.Adjustthelaserdiodetolensdistancesuchthattherazorbladesarelocatedinthexyplanewherethebeamcross-sectioniscircular.

4.Movethelaserdiodeawayfromthelensuntilminimumbeamwaistisreachedattheplaneofrazorblades.Now,movethelaserdiodeabout200µmfurtherawayfromthelens.

5.Moverazorblade1inthexdirectionacrossthebeamthroughthebeamspreadθxandrecordthexpositionanddetectedintensityateachincrement(≤100µmincrements).TheexpectedoutputisshowninFigure1.9.Thederivativeofthiscurveyieldstheintensityprofileofthebeaminthexdirectionfromwhichthebeamdiameterisdetermined.

6.Repeatwithrazorblade2forθyintheydirection.

7.Movethelaserclosertothelensinincrements(≤50µm)throughatotalofatleastthan500µm.RepeatSteps5and6ateachzincrement,recordingthezposition.

8.Usingthecollecteddata,determinethebeamintensityprofilesinthexandydirectionsasafunctionofthelenspositionz.Thisisdonebydifferentiatingeachdatasetwithrespecttoposition.Then,calculatethebeamdiameterandplotasafunctionofz.Thedifferenceinzfortheminimuminθxandθyistheastigmaticdistanceofthelaserdiode.Useofcomputersoftware,especiallyindifferentiatingthedata,ishighlyrecommended.

Ifthelaserjunctionisnotparalleltothetablesurface,thenforeachmeasurementabove,youwillobtainanadmixtureofthetwobeamdivergencesandthemeasurementwillbecomeimprecise.Ifthelaserisorientedat45°tothesurfaceofthetable,theastigmaticdistancewillbezero.

Differentlaserstructureswillhavedifferentangularbeamdivergencesand,thus,differentastigmaticdistances.Ifyouhaveaccesstoseveraldifferentlasertypes(gainguided,indexguided),itmaybeinstructivetocharacterizetheirastigmaticdistances.

1.5.3FrequencyCharacteristicsofDiodeLasers

Inordertostudyfrequencycharacteristicsofadiodelaser,wewillemployaMichelsoninterferometertoconvertfrequencyvariationsintointensityvariations.Anexperimentalsetupforexaminingfrequencyand,also,amplitudecharacteristicsofalasersourceisillustratedinFigure1.10.

1.Inthisexperiment,itisverypossiblethatlightmaybecoupledbackintothelaser,thereby,destabilizingit.Anopticalisolator,therefore,willberequiredtominimizefeedbackintothelaser.Asimpleisolatorwillbeconstructedusingapolarizingbeamsplittercubeandaquarterwaveplate.Weorientthequarterwaveplatesuchthatthelinearlypolarizedlightfromthepolarizerisincidentat45°totheprincipalaxesofthequarterwaveplatesothatlightemergingfromthequarterwaveplateiscircularlypolarized.Reflectionschangeleft-circularpolarizedlightintoright-circularorviceversasothatreflectedlightreturningthroughthequarterwaveplatewillbelinearlypolarizedand90°rotatedwithrespecttothepolarizertransmissionaxis.Thepolarizer,then,greatlyattenuatesthereturnbeam.

Inassemblingtheisolator,makesurethatthelaserjunction(xzplaneinFigure1.1)isparalleltothesurfaceofthetable(thenotchonthelaserdiodecasepointsupward)andthebeamiscollimatedbythelens.Thelaserbeamshouldbeparalleltothesurfaceoftheopticaltable.Setthepolarizerandquarterwave(λ/4)plateinplace.Placeamirroraftertheλ/4plateandrotatetheλ/4platesothatmaximumrejectedsignalisobtainedfromtherejectionportofthepolarizingbeamsplittercubeasshowninFigure1.11.Whenthissignalismaximized,thefeedbacktothelasershouldbeataminimum.

2.ConstructtheMichelsoninterferometerasshowninFigure1.12.Placethebeamsteeringassembly(BSA-II)ontheopticaltableandusethereflectedbeamfromthemirrortoadjustthequarterwaveplateorientation.Setthecubemount(CM)ontheopticalbreadboard,placeadoublesidedpieceofadhesivetapeonthemount,andputthenonpolarizingbeamsplittercube(05BC16NP.6)ontheadhesivetape.Next,placetheotherbeamsteeringassembly(BSA-I)andthedetectormount(M818BB)inlocationandadjustthemirrorssothatthebeamsreflectedfromthetwomirrorsoverlapatthedetector.

Whenlongpathlengthmeasurementsaremade,theinterferometersignalwilldecreaseordisappearifthelasercoherencelengthislessthanthetwowayinterferometerpathimbalance.Ifthisisthecase,shortentheinterferometeruntilthesignalreappears.Ifthisdoesnotwork,thencheckthelaserforsingle-modeoperationbylookingforthefringepatternonacardorbyscanningthepiezoelectrictransducerblock(PZB)inBSA-IIandmonitoringthedetectoroutputwhichshouldbesinusoidalwithPZBscandistance.Ifthelaserdoesnotappeartobeoperatingsingle-mode,realigntheisolatorand/orchangethelaseroperatingpointbyvaryingthebiascurrent.Additionally,toensuresingle-modeoperation,thelasershouldbeDCbiasedabovethresholdbeforeapplyingACmodulation.Overdrivingthelasercanalsoforceitintomultimodeoperation.

3.TheMichelsoninterferometerhasthepropertythatdependingonthepositionofthemirrors,lightmaystronglycouplebacktowardthelaserinputport.Inordertofurtherreducethefeed-back,slightlytiltthemirrorsasillustratedinFigure1.13.Ifstillunabletoobtainsingle-modeoperation,replacethelaserdiode.

4.Placeawhitecardinfrontofthedetectorandobservethefringepatternwiththeinfraredimager.Slightlyadjustthemirrorstoobtainthebestfringepattern.Trytoobtainonebroadfringe.

5.Positionthedetectoratthecenterofthefringepatternsothatitinterceptsnomorethanaportionofthecenteredpeak.

6.Byapplyingavoltagetothepiezoelectrictransducerblockattachedtothemirror(partPZB)inonearmoftheinterferometer(i.e.BSA-II),maximizetheoutputintensity.Theoutputshouldbestableoveratimeperiodofaminuteorso.Ifitisnot,verifythatallcomponentsarerigidlymounted.Iftheyare,thenroomaircurrentsmaybedestabilizingthesetup.Inthiscase,placeabox(cardboardwilldo)overthesetuptopreventaircurrentsfromdisturbingtheinterferometersetup.

7.Placetheinterferometerinquadrature(pointofmaximumsensitivitybetweenmaximumandminimumoutputsoftheinterferometer)byvaryingthevoltageonthePZB.

8.Theoutputsignaloftheinterferometerduetofrequencyshiftingofthelaserisgivenby∆I∝∆φ=2π/c∆L∆νwhere∆Listhedifferenceinpathlengthbetweenthetwoarmsoftheinterferometerand∆νisthefrequencysweepofthelaserthatisinducedbyapplyingacurrentmodulation.RememberthatinaMichelsoninterferometer∆Listwicethephysicaldifferenceinlengthbetweenthearmssincelighttraversesthislengthdifferenceinbothdirections.∆Lvaluesof3-20cmrepresentconvenientlengthdifferenceswiththelarger∆Lyieldinghigheroutputsignals.

Beforeweapplyacurrentmodulationtothelaser,notethattheinterferometeroutputsignal,∆I,shouldbemadelargerthanthedetectororlasernoiselevelsbyproperchoiceof∆Landcurrentmodulationamplitudedi.AlsorecallfromSection1.3thatwhenthediodecurrentismodulatedsoisthelaserintensityaswellasitsfrequency.Wecanmeasurethelaserintensitymodulationbyblockingonearmoftheinterferometer.Thiseliminatesinterferenceandenablesmeasurementoftheintensitymodulationdepth.We,then,subtractthisvaluefromthetotalinterferometeroutputtodeterminethetruedI/diduetofrequencymodulation.Applyalowfrequency,smallcurrentmodulationtothelaserdiode.Notethatwhentheproperrangeisbeingobserved

and

fortheamplitudechangeonly.Recalling

,,

or

whereKisadetectorresponseconstantdeterminedbyvarying∆L.

9.Withtheinterferometeranddetectionsystemproperlyadjusted,varythedrivefrequencyofthelaserandobtainthefrequencyresponseofthelaser(Figure1.4or1.10a).Youwillneedtorecordtwosetsofdata:(i)themodulationdepthoftheinterferometeroutputasafunctionoffrequency,and(ii)thelaserintensitymodulationdepth.ThedifferenceofthetwosetsofcollecteddatawillprovideanestimateoftheactualdI/diduetofrequencymodulation.Alsonotethatifthecurrentmodulationissufficientlysmallandthepathmismatchsufficientlylarge,thelaserintensitymodulationmaybenegligible.YoumayneedtoactivelykeeptheinterferometerinquadraturebyadjustingthePZBvoltage.

Makeanynecessaryfunctiongeneratoramplitudeadjustmentstokeepthecurrentmodulationdepthofthelaserconstantasyouvarythefrequency.Thisisbecausethefunctiongenerator/drivercombinationmaynothaveaflatfrequencyresponse.Theeffectofthisisthatthecurrentmodulationdepthdiisnotconstantandvarieswithfrequency.Sotoavoidunnecessarycalculations,monitorthecurrentmodulationdepthbyconnectingtheLASERMONITORconnectoronthelaserdiodedriversystemtoanoscilloscopeandkeepthemodulationdepthconstantbyadjustingtheamplitudeoftheappliedsinusoidalwaveasafunctionoffrequency.Recordthefrequencyforyourlaseratwhichthethermalcontributiontodν/dibeginstobecomenegligibleanddν/didropsoff(seeSection1.3).

10.Keepingtheaboveequationsinmind,wewill,now,measuretheFMchirpcharacteristicsofthelaser.Ataconstantcurrentmodulationfrequency(chooseamodulationfrequencywheredν/divariesrapidly,i.e.wheretheslopeofyourgraphfromStep9,whichshouldbesimilartoFigure1.10a,ismaximum),varythecurrentmodulationdepthdifordifferentlaserbiaslevelsandderiveacurvesuchastheoneinFigure1.10b.Theoutputdνshouldnotvarysignificantlyexceptaroundthresholdandathighcurrents.

Caution

Donotexceedthespecifieddrivecurrents/outputpowerratingsofthediodeoritmaybedamaged.

11.Thephasenoisecharacteristicbehavior(Section1.4)asafunctionofinterferometerpathlengthimbalance∆Lmaybedeterminedbyinducingphasenoisethroughapplicationoflasercurrentmodulation.Makesurethattheinterferometerisinquadrature.

Setthelaserdiodecurrentabovethreshold,applyasmallcurrentmodulation,andfixthemodulationfrequencyatadesiredvalue.Convenientfrequenciesmayinclude50Hz,2kHz,and50kHz(seeReference1.5).Monitorthedetectoroutputwithaspectrumanalyzeroranoscilloscopeandrecordthepeak-to-peakoutputintensityatinterferometerquadrature.YoumayaccomplishthisbymanuallysweepingthePZBvoltagetocauseaminimumofπ/2phaseshift,recordingthemaximumpeak-to-peakintensityasafunctionofpathlengthimbalance.Itisimportanttoensurethatinstrumentnoiseisbelowthesignallevelsexpectedanditisassumedthatsingle-modeoperationofthelaserismaintained.CurvessimilartoFigure1.10cshouldbeobtained.

1.5.4AmplitudeCharacteristicsofDiodeLasers

ThemeasurementsoftheintensitycharacteristicsaretakenbyplacingthedetectorbeforetheinterferometerasinFigure1.10orbyblockingonemirrorintheinterferometer.Again,thelasermustbeoperatedsingle-modedwithminimumfeedbackorthenoiselevelandfunctionalitywilldrasticallychange.Therelativeintensitynoise(RIN)isdefinedas20log(dI/I)wheredIistheRMSintensityfluctuationssothatfordI~10-4,theRINis-80dB.Normally,thesemeasurementsaremadewithaspectrumanalyzeranda1Hzbandwidth.

WhenmakingRINmeasurements,electronicandphotodetectorshotnoisemustbebelowtheRINlevels.(OPTIONAL)Youmaydeterminetheshotnoiseusinganincoherentsource(e.g.lamp)withanintensitylevelsimilartothatofthelaser.Theresultantfrequencyspectrumofnoisewiththelightsourceexcitedgivesameasureoftheshotnoiselevelwhichshouldbeadjustedtobeatleast10dBgreaterthanelectronicnoiselevels.ThemeasuredshotnoiseshouldbecheckedwithEquation0.47.

1.Varythelaserdrivecurrentfrombelowthresholdt