英文原文1.5ExperimentalSetupDuetothemanyconceptsandvariationsinvolvedinperformingtheexperimentsinthisprojectandalsobecauseoftheirintroductorynature,Project1willverylikelybethemosttimeconsumingprojectinthiskit.Thisprojectmayrequireasmuchas9hourstocomplete.Werecommendthatyouperformtheexperimentsintwoormorelaboratorysessions.Forexample,powerandastigmaticdistancecharacteristicsmaybeexaminedinthefirstsessionandthelasttwoexperiments(frequencyandamplitudecharacteristics)maybeperformedinthesecondsession.ANoteofCautionAlloftheabovecommentsrefertosingle-modeoperationofthelaserwhichisaveryfragiledevicewithrespecttoreflectionsandoperatingpoint.Onemustensurethatbeforeperformingmeasurementsthelaserisindeedoperatingsingle-mode.Thiscanberealizedifasingle,broadfringepatternisobtainedorequivalentlyagoodsinusoidaloutputisobtainedfromtheMichelsoninterferometerasthepathimbalanceisscanned.Ifthisisnotthecase,thelaserisprobablyoperatingmultimodeanditscurrentshouldbeadjusted.Ifsingle-modeoperationcannotbeachievedbyadjustingthecurrent,thenreflectionsmaybedrivingthelasermultimode,inwhichcasethesetupshouldbeadjustedtominimizereflections.Ifstillnotoperatingsingle-mode,thelaserdiodemayhavebeendamagedandmayneedtobereplaced.WarningThelasersprovidedinthisprojectkitemitinvisibleradiationthatcandamagethehumaneye.Itisessentialthatyouavoiddirecteyeexposuretothelaserbeam.Werecommendtheuseofprotectiveeyeweardesignedforuseatthelaserwavelengthof780nm.ReadtheSafetysectionsintheLaserDiodeDriverOperatingManualandinthelaserdiodesectionofComponentHandlingandAssembly(AppendixA)beforeproceeding.1.5.1SemiconductorDiodeLaserPowerCharacteristics1.Assemblethelasermountassembly(LMA-I)andconnectthelasertoitspowersupply.Wewillfirstcollimatethelightbeam.Connectthelaserbeamtoavideomonitorandimagethelaserbeamonawhitesheetofpaperheldabouttwototencentimetersfromthelaserassembly.Slowlyincreasethedrivecurrenttothelaserandobservethespotonthewhitecard.Thethresholddrivecurrentratingofthelaserissuppliedwitheachlaser.Increasethecurrenttoabout10-20mAoverthethresholdvalue.Withtheinfraredimagerorinfraredsensorcard,observethespotonthecardandadjustthecollimatorlenspositioninthelaserassemblyLMA-Itoobtainabrightspotonthecard.Movethecardtoabout30to60centimetersfromthelensandadjustthelenspositionrelativetothelasertoobtainaspotwheresizedoesnotvarystronglywiththepositionofthewhitecard.Whenthespotsizeremainsroughlyconstantasthecardismovedcloserorfurtherfromthelaser,theoutputcanbeconsideredcollimated.Alternatively,thelaserbeammaybecollimatedbyfocusingitatadistanceasfarawayaspossible.Protectfellowco-workersfromaccidentalexposuretothelaserbeam.2.Placean818-SLdetectoronapostmount(assemblyM818)andadjustitspositionsothatitsactiveareaisinthecenterofthebeam.Thereshouldbeadequateopticalpowerfallingonthedetectortogetastrongsignal.Connectthephotodetectortothepowermeter(815).Reducethebackgroundlighting(roomlights)sothatthesignalbeingdetectedisonlyfromthelaser.Reducethedrivecurrenttoafewmilliamperesbelowthresholdand,again,checktoseethatroomlightisnotthedominantsignalatthedetectorbyblockingthelaserlight.3.Increasethecurrentandrecordtheoutputofthedetectorasafunctionoflaserdrivecurrent.YoushouldobtainacurvesimilartoFigure1.2.Ifdesired,thediodetemperaturemayalsobevariedtoobservetheeffectsoftemperatureonthresholdcurrent.Whenexamininglaserdiodetemperaturecharacteris...
