Laser Beam Profile Measurement - Newport

Devices which measure and analyze the spatial distribution of a laser beam are called beam profilers and come in many different types, each with its own ... 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Resources TechnicalNotes LaserBeamProfileMeasurement LaserBeamProfileMeasurement Giventhedifferenttypesoflaserbeamspatialdistributionsandtheirevolutionastheypropagateinfreespaceandthroughopticalelements,measurementofthebeamprofileiscriticaltoquantifytheirradianceatthepointoftheapplication.Monitoringthebeamprofilecanalsoidentifylaserbeamanomalies.Thisfacilitatesoptimalalignmentofalasercavityorbeamdeliveryopticsresultinginsignificantimprovementsinlaserprocessing(seeFigure1,forexamples).Deviceswhichmeasureandanalyzethespatialdistributionofalaserbeamarecalledbeamprofilersandcomeinmanydifferenttypes,eachwithitsownadvantagesandchallenges.Thebasictypesincludearray-basedorcamera-basedprofilers,mechanicalscanningapertures,knife-edgescanners,andotherdevices.Sincecamera-basedandscanningapertureprofilersarethemostcommontechniques,theyarethesubjectsofthissection.Camera-basedsystemsaregenerallyphotodiode-basedCCD(charge-coupleddevices)orCMOS(complementarymetal-oxide-semiconductor)devices.However,thereareapplicationsusingarrayswiththermalsensorsfordetectionoflongerwavelengths.Scanningaperturesystemscombineamovingslitandsingle-elementphotodetectortoreproduceabeamprofilebyspatiallysamplingit. Thelaserandapplicationdictatethecriteriaforselectingthebestbeamprofiler Criteriaincludelaserwavelength,beamsize,power,pulsedorCW,andhowtheprofilerwillbeused.ThewavelengthdictatesthetypeofsensortobeusedwithSiprovidingsensitivityintheVISandNIR,GeandInGaAscoveringtheNIR,andpyroelectricscoveringtheUVandIR.Aprofilermusthavealargeenoughaperturetoaccommodatethesizeofthebeamwhilesmallerbeamswillbelimitedbytheresolutionoftheprofiler,i.e.,pixelsizeforacamera-basedprofilerandslitsizeforascanningprofiler.Sinceeveryprofilerhasasensor,theyaresusceptibletosaturationandopticaldamage,whichlimitstheallowableincidentlaserpower.Thepowercanbeattenuatedorreducedpriortoreachingthesensortoaccommodatehigherpowerswhilescanningslitprofilersmaynotrequireattenuationsincetheyonlysampleasmallportionofthebeam’spower.Generally,scanningapertureprofilersareonlyeffectiveforpulsedsystemsabove1-2kHzduetotheirmechanicalnaturewhilecameraprofilescanbeusedforeitherpulsedorCWsources.Cameraprofilersprovideatruetwo-dimensionalimageofthebeamandwillshowfinestructureandhotspots,whichascanningslitmayintegrateout.Slit-basedprofilerswilltypicallyprovideamoreaccuratebeamsizemeasurementandcanmeasureverysmallbeamsdirectly. Figure1.Measuredlaserbeamsusingacamera-basedbeamprofiler.ToprowshowsanindustrialNd:YAGlaserexhibitingaGaussianbeam(left),whichgivesrisetoamulti-peakeddistributionfollowinganincreaseinpowerof1.7times.BottomrowshowsaCO2laserprofilebefore(left)andaftercavityalignment(right). Camera-BasedProfilers Figure2showsacamera-basedbeamprofilersystemconsistingofacamera,profilersoftware,andabeamattenuationaccessory.Anarraydetectorcontainsmanysensorsarrangedinadensegridpatternand,sinceallthesensorscanrespondtoincidentlightsimultaneously,thesedetectorscantransformanopticalimageintoanelectronicsignaltobemeasuredandanalyzed.Camera-basedprofilersusearraydetectorsandthemajordifferencesbetweenthemlieinthetypeofsensorused,thesizesoftheindividualsensorelementsorpixels,andthesignalhandlingandreadoutmethods.Thesensorsaretypicallysemiconductorphotodiodeswhosespectralsensitivitydependsonthetypeofsemiconductormaterialemployed.However,thermaldetectorssuchaspyroelectricsensorsandbolometers(athermometricdevicewhichchangesitselectricalresistanceasafunctionofabsorbedradiation)arealsousedifsensitivityintheIRisdesired.Thecameraconsistsofanarrayofphotosensorsorpixels.Thespatialresolutionofthecameradependsonthepixelpitch,i.e.,pixel-to-pixeldistance,andthisvariesdependingonthetypeofsensor.ForSiphotodiodesensors,thepitchcanbebelow5µmwhileGeandInGaAssensorshavepitchesinthe10’sµmrange.Pyroelectricarrayshavepitchesthatcanreachslightlylowerthan100µm.Theelectronicsignalsfromthepixelsmustthenbereadout.CCDstransferthechargegeneratedoneachpixeltotheadjacentpixelandsoonuntilitreachestheedgeofthearray.Thesignalsarethereforehandledinaserialmannerbytheassociatedcircuitry.Bycontrast,CMOSdetectorsreadeachpixelindividuallyandthereforethesignalsarehandledinparallel.ThisenablesamuchfasterresponseforCMOSdevicesand,becausetheyarebasedonwidelyusedmanufacturingtechnology,theyarelessexpensivethanCCDs.However,CCDsgenerallypossessbetterlinearityandnoisecharacteristics. Figure2.Camera-basedbeamprofilersystemconsistingofaCCDcamera,profilersoftware,andbeamattenuationoptics. Whiletheindividualphotodiodesthatmakeupmostcamerashaveverylargedynamicranges(seePhotodiodeSensorPhysics),thisisreducedconsiderablyforarraydetectorsmainlyduetothesignalhandlingandreadout.Asaresult,detectorsaturationoccursatmuchlowerincidentirradiances,andattenuationopticsareoftenplacedinthebeampathpriortothedetector.Beamprofilingsoftware,coupledtothecamera,typicallyprovidesbothimageprocessinganddataanalysiscapabilities.Imageprocessingcanincludeproperbaselining,whichhelpseliminatebackgroundnoise,electronicgainadjustmenttomaximizethedynamicrangeofthedetector,andaveraging,whichcanreducenoise.Dataanalysisshouldprovideinformationonbeamsize,position,ellipticity(ordeviationfromacircularlysymmetricbeam),andreal-timestatisticsofthesevalueswhilealsoallowingforfittingofthebeamprofiletoaGaussianfunction.Thebenefitofacamera-basedsystemisthatthebeamcanbeimageddirectlyandinrealtimetorevealnotonlythesizeandpositionofthebeambutthedistributionofthepower/energyinthebeam. ScanningSlitProfilers Figure3showsascanningslitbeamprofilerinwhichanarrowslitmovesthroughthebeamandthelightpassingthroughtheslitgeneratescurrentinasinglephotodetector.Thus,astheslitscansthroughthebeam,thedetectorsignalislinearlyproportionaltothespatialbeamirradianceprofileintegratedalongtheslit.Adigitalencoderprovidesaccurateslitposition.Thephoto-inducedsignalisdigitizedandanalyzedtoobtainthebeamprofileinthedirectionoftranslationoftheslit.Typically,twoorthogonalslitsaresequentiallymovedthroughthebeamprovidingprofilesintheseperpendiculardirections.Fromtheseprofiles,importantspatialinformationsuchasbeamwidth,beamposition,beamquality,andothercharacteristicsaredetermined. Ascanningprofilerhasseveraladvantagesoveracameraprofiler.Sinceonlyasinglephotodetectorisemployed,sensorswithverydifferentspectralsensitivitiescanbeused,enablingprofilingfromUVtoIR.Asingledetectoralsohasalargerdynamicrangecomparedtoanarray.Sincetheslitonlyallowsafractionofthebeamtoimpingeonthedetector,muchhigherpowerscanbemeasuredwithoutrequiringadditionalattenuation.Becausetheslitplaneisthemeasurementplane,itisalsopossibletomeasurethetightlyfocusedbeamswithsmallspotsizesthatcannotbedirectlymeasuredwithacamera.However,thescanningslitprofileronlyprovidesmeasurementsalongtwoorthogonaldirectionsofabeamandthereforelacksthetwo-dimensionalinformationavailablefromacamera.Additionally,becausethesignalfromtheslitisintegratedinonespatialdimension,thedeviceismostappropriateforGaussian-likelaserbeams.Duetothespeedassociatedwiththemechanicalmovementoftheslit,theprofilercanonlymeasureCWandrelativelyhighfrequencypulsedbeams.Beamswithlowpulserepetitionrates,<1kHz,shouldbemeasuredwithacamera. Figure3.Scanheadforscanningslitbeamprofiler(left)andenlargedviewofscanningslitthatshowsrecordingofbeamprofile(right). RelatedTopics OpticalSensors ThermopileSensorPhysics PyroelectricSensorPhysics PhotodiodeSensorPhysics BeamCharacterization LEDCharacterization LaserBeamSpatialProfiles Foradditionalinsightsintophotonicstopicslikethis,downloadourfreeMKSInstrumentsHandbook:Principles&ApplicationsinPhotonicsTechnologies RequestaHandbook SignIn EmailAddress: Required Password:  Required Needanewaccount? Register PasswordReset Enteryouremailaddressbelowtoresetyouraccountpassword. EmailAddress: Required Cancel PasswordReset Close EmailVerificationRequired Close CartMerge Success Close RemoveProduct Removethisproductfromyourcomparisonlist? Cancel CheckOrderStatus Provideanordernumberandpostalcodetocheckthestatusofanorderordownloadaninvoiceforanorderthathasshipped. Logintoviewyourcompleteorderhistory. 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