Laser beam profiler - Wikipedia

Complete E-field beam profiling Laserbeamprofiler FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Alaserbeamprofiler. Alaserbeamprofilercaptures,displays,andrecordsthespatialintensityprofileofalaserbeamataparticularplanetransversetothebeampropagationpath.Sincetherearemanytypesoflasers—ultraviolet,visible,infrared,continuouswave,pulsed,high-power,low-power—thereisanassortmentofinstrumentationformeasuringlaserbeamprofiles.Nosinglelaserbeamprofilercanhandleeverypowerlevel,pulseduration,repetitionrate,wavelength,andbeamsize. Contents 1Overview 2Applications 3Measurements 3.1Beamwidth 3.2Beamquality 3.2.1Beamqualityparameter,M2 3.2.2CompleteE-fieldbeamprofiling 3.2.3Power-in-the-bucketorStrehldefinitionofbeamquality 3.3Beamdivergence 3.4Beamastigmatism 3.5Beamwanderorjitter 3.6Misrepresentationofbeamprofilermeasurementsforlasersystems 4Techniques 4.1Scanning-aperturetechniques 4.2CCDcameratechnique 4.2.1BaselinesubtractionforD4σwidthmeasurements 4.2.2Averagingtogetbettermeasurements 4.2.3Attenuationtechniques 4.2.3.1Neutraldensityfilters 4.2.3.2Diffractivebeamsampler 4.2.3.3Opticalwedges 4.2.4OptimalbeamsizeontheCCDdetector 4.2.5Pixelsizeandnumberofpixels 4.2.6Far-fieldbeamprofiler 5Specialapplications 5.1Irisreplacementwithmicroradianalignmentaccuracy 5.2Simultaneousmultipleportmonitoringoflasersystem 6Seealso 7References Overview[edit] Laserbeamprofilinginstrumentsmeasurethefollowingquantities: Beamwidth:Thereareoverfivedefinitionsofbeamwidth. Beamquality:Quantifiedbythebeamqualityparameter,M2. Beamdivergence:Thisisameasureofthespreadingofthebeamwithdistance. Beamprofile:Abeamprofileisthe2Dintensityplotofabeamatagivenlocationalongthebeampath.AGaussianorflat-topprofileisoftendesired.Thebeamprofileindicatesnuisancehigh-orderspatialmodesinalasercavityaswellashotspotsinthebeam. Beamastigmatism:Thebeamisastigmaticwhentheverticalandhorizontalpartsofthebeamfocusindifferentlocationsalongthebeampath. Beamwanderorjitter:Theamountthatthecentroidorpeakvalueofthebeamprofilemoveswithtime. Instrumentsandtechniquesweredevelopedtoobtainthebeamcharacteristicslistedabove.Theseinclude: Cameratechniques:Theseincludethedirectilluminationofacamerasensor.ThemaximumspotsizethatwillfitontoaCCDsensorisontheorderof10 mm.Alternatively,illuminatingaflatdiffusesurfacewiththelaserandimagingthelightontoaCCDwithalensallowsprofilingoflarger-diameterbeams.Viewinglasersoffdiffusesurfacesisexcellentforlargebeamwidthsbutrequiresadiffusesurfacethathasuniformreflectivity(<1%variation)overtheilluminatedsurface. Knife-edgetechnique:Aspinningbladeorslitcutsthelaserbeambeforedetectionbyapowermeter.Thepowermetermeasurestheintensityasafunctionoftime.Bytakingtheintegratedintensityprofilesinanumberofcuts,theoriginalbeamprofilecanbereconstructedusingalgorithmsdevelopedfortomography.Thisusuallydoesnotworkforpulsedlasers,anddoesnotprovideatrue2Dbeamprofile,butitdoeshaveexcellentresolution,insomecases<1 μm. Phase-fronttechnique:Thebeamispassedthrougha2DarrayoftinylensesinaShack–Hartmannwavefrontsensor.Eachlenswillredirectitsportionofthebeam,andfromthepositionofthedeflectedbeamlet,thephaseoftheoriginalbeamcanbereconstructed. Historicaltechniques:Theseincludetheuseofphotographicplatesandburnplates.Forexample,high-powercarbondioxidelaserswereprofiledbyobservingslowburnsintoacrylateblocks. Asof2002[update],commercialknife-edgemeasurementsystemscost$5,000–$12,000USDandCCDbeamprofilerscost$4,000–9,000USD.[1]ThecostofCCDbeamprofilershascomedowninrecentyears,primarilydrivenbylowersiliconCCDsensorcosts,andasof2008[update]theycanbefoundforlessthan$1000USD. Applications[edit] Theapplicationsoflaserbeamprofilinginclude: Lasercutting:Alaserwithanellipticalbeamprofilehasawidercutalongonedirectionthanalongtheother.Thewidthofthebeaminfluencestheedgesofthecut.Anarrowerbeamwidthyieldshighfluenceandionizes,ratherthanmelts,themachinedpart.Ionizededgesarecleanerandhavelessknurlingthanmeltededges. Nonlinearoptics:Frequencyconversionefficiencyinnonlinearopticalmaterialsisproportionaltothesquare(sometimescubedormore)oftheinputlightintensity.Therefore,togetefficientfrequencyconversiontheinputbeamwaistmustbesmall,andlocatedwithinthenonlinearmaterial.Abeamprofilercanhelpmakeawaistoftherightsizeattherightlocation. Alignment:Beamprofilersalignbeamswithordersofmagnitudebetterangularaccuracythanirises. Lasermonitoring:Itisoftennecessarytomonitorthelaseroutputtoseewhetherthebeamprofilechangesafterlonghoursofoperation.Maintainingaparticularbeamshapeiscriticalforadaptiveoptics,nonlinearoptics,andlaser-to-fiberdelivery.Inaddition,laserstatuscanbemeasuredbyimagingtheemittersofapumpdiodelaserbarandcountingthenumberofemittersthathavefailedorbyplacingseveralbeamprofilersatvariouspointsalongalaseramplifierchain. Laserandlaseramplifierdevelopment:Thermalrelaxationinpulse-pumpedamplifierscausestemporalandspatialvariationsinthegaincrystal,effectivelydistortingthebeamprofileoftheamplifiedlight.Abeamprofilerplacedattheoutputoftheamplifieryieldsawealthofinformationabouttransientthermaleffectsinthecrystal.Byadjustingthepumpcurrenttotheamplifierandtuningtheinputpowerlevel,theoutputbeamprofilecanbeoptimizedinreal-time. Far-fieldmeasurement:Itisimportanttoknowthebeamprofileofalaserforlaserradarorfree-spaceopticalcommunicationsatlongdistances,theso-called"far-field".Thewidthofthebeaminitsfar-fielddeterminestheamountofenergycollectedbyacommunicationsreceiverandtheamountofenergyincidentontheladar'starget.Measuringthefar-fieldbeamprofiledirectlyisoftenimpossibleinalaboratorybecauseofthelongpathlengthrequired.Alens,ontheotherhand,transformsthebeamsothatthefar-fieldoccursnearitsfocus.Abeamprofilerplacednearthefocusofthelensmeasuresthefar-fieldbeamprofileinsignificantlylessbenchtopspace. Education:BeamprofilerscanbeusedforstudentlaboratoriestoverifydiffractiontheoriesandtesttheFraunhoferorFresneldiffractionintegralapproximations.OtherstudentlaboratoryideasincludeusingabeamprofilertomeasurePoisson'sspotofanopaquediskandtomapouttheAirydiskdiffractionpatternofacleardisk. Measurements[edit] Beamwidth[edit] Mainarticle:Beamdiameter Thebeamwidthisthesinglemostimportantcharacteristicofalaserbeamprofile.Atleastfivedefinitionsofbeamwidthareincommonuse:D4σ,10/90or20/80knife-edge,1/e2,FWHM,andD86.TheD4σbeamwidthistheISOstandarddefinitionandthemeasurementoftheM2beamqualityparameterrequiresthemeasurementoftheD4σwidths.[2][3][4]TheotherdefinitionsprovidecomplementaryinformationtotheD4σandareusedindifferentcircumstances.Thechoiceofdefinitioncanhavealargeeffectonthebeamwidthnumberobtained,anditisimportanttousethecorrectmethodforanygivenapplication.[5]TheD4σandknife-edgewidthsaresensitivetobackgroundnoiseonthedetector,whilethe1/e2andFWHMwidthsarenot.Thefractionoftotalbeampowerencompassedbythebeamwidthdependsonwhichdefinitionisused. Beamquality[edit] Beamqualityparameter,M2[edit] Mainarticle:Msquared TheM2parameterisameasureofbeamquality;alowM2valueindicatesgoodbeamqualityandabilitytobefocusedtoatightspot.ThevalueMisequaltotheratioofthebeam'sangleofdivergencetothatofaGaussianbeamwiththesameD4σwaistwidth.SincetheGaussianbeamdivergesmoreslowlythananyotherbeamshape,theM2parameterisalwaysgreaterthanorequaltoone.Otherdefinitionsofbeamqualityhavebeenusedinthepast,buttheoneusingsecondmomentwidthsismostcommonlyaccepted.[6] Beamqualityisimportantinmanyapplications.Infiber-opticcommunicationsbeamswithanM2closeto1arerequiredforcouplingtosingle-modeopticalfiber.LasermachineshopscarealotabouttheM2parameteroftheirlasersbecausethebeamswillfocustoanareathatisM4timeslargerthanthatofaGaussianbeamwiththesamewavelengthandD4σwaistwidthbeforefocusing;inotherwords,thefluencescalesas1/M4.TheruleofthumbisthatM2increasesasthelaserpowerincreases.Itisdifficulttoobtainexcellentbeamqualityandhighaveragepower(100WtokWs)duetothermallensinginthelasergainmedium. TheM2parameterisdeterminedexperimentallyasfollows:[2] MeasuretheD4σwidthsat5axialpositionsnearthebeamwaist(thelocationwherethebeamisnarrowest). MeasuretheD4σwidthsat5axialpositionsatleastoneRayleighlengthawayfromthewaist. Fitthe10measureddatapointsto σ 2 ( z ) = σ 0 2 + M 4 ( λ π σ 0 ) 2 ( z − z 0 ) 2 {\displaystyle\sigma^{2}(z)=\sigma_{0}^{2}+M^{4}\left({\frac{\lambda}{\pi\sigma_{0}}}\right)^{2}(z-z_{0})^{2}} ,[7]where σ 2 ( z ) {\displaystyle\sigma^{2}(z)} isthesecondmomentofthedistributioninthexorydirection(seesectiononD4σbeamwidth),and z 0 {\displaystylez_{0}} isthelocationofthebeamwaistwithsecondmomentwidthof 2 σ 0 {\displaystyle2\sigma_{0}} .Fittingthe10datapointsyieldsM2, z 0 {\displaystylez_{0}} ,and σ 0 {\displaystyle\sigma_{0}} .Siegmanshowedthatallbeamprofiles—Gaussian,flattop,TEMXY,oranyshape—mustfollowtheequationaboveprovidedthatthebeamradiususestheD4σdefinitionofthebeamwidth.Usingthe10/90knife-edge,theD86,ortheFWHMwidthsdoesnotwork. CompleteE-fieldbeamprofiling[edit] Beamprofilersmeasuretheintensity,|E-field|2,ofthelaserbeamprofilebutdonotyieldanyinformationaboutthephaseoftheE-field.TocompletelycharacterizetheE-fieldatagivenplane,boththephaseandamplitudeprofilesmustbeknown.TherealandimaginarypartsoftheelectricfieldcanbecharacterizedusingtwoCCDbeamprofilersthatsamplethebeamattwoseparatepropagationplanes,withtheapplicationofaphaserecoveryalgorithmtothecaptureddata.ThebenefitofcompletelycharacterizingtheE-fieldinoneplaneisthattheE-fieldprofilecanbecomputedforanyotherplanewithdiffractiontheory. Power-in-the-bucketorStrehldefinitionofbeamquality[edit] TheM2parameterisnotthewholestoryinspecifyingbeamquality.AlowM2onlyimpliesthatthesecondmomentofthebeamprofileexpandsslowly.Nevertheless,twobeamswiththesameM2maynothavethesamefractionofdeliveredpowerinagivenarea.Power-in-the-bucketandStrehlratioaretwoattemptstodefinebeamqualityasafunctionofhowmuchpowerisdeliveredtoagivenarea.Unfortunately,thereisnostandardbucketsize(D86width,Gaussianbeamwidth,Airydisknulls,etc.)orbucketshape(circular,rectangular,etc.)andthereisnostandardbeamtocomparefortheStrehlratio.Therefore,thesedefinitionsmustalwaysbespecifiedbeforeanumberisgivenanditpresentsmuchdifficultywhentryingtocomparelasers.ThereisalsonosimpleconversionbetweenM2,power-in-the-bucket,andStrehlratio.TheStrehlratio,forexample,hasbeendefinedastheratioofthepeakfocalintensitiesintheaberratedandidealpointspreadfunctions.Inothercases,ithasbeendefinedastheratiobetweenthepeakintensityofanimagedividedbythepeakintensityofadiffraction-limitedimagewiththesametotalflux.[8][9]Sincetherearemanywayspower-in-the-bucketandStrehlratiohavebeendefinedintheliterature,therecommendationistostickwiththeISO-standardM2definitionforthebeamqualityparameterandbeawarethataStrehlratioof0.8,forexample,doesnotmeananythingunlesstheStrehlratioisaccompaniedbyadefinition. Beamdivergence[edit] Mainarticle:Beamdivergence Thebeamdivergenceofalaserbeamisameasureforhowfastthebeamexpandsfarfromthebeamwaist.Itisusuallydefinedasthederivativeofthebeamradiuswithrespecttotheaxialpositioninthefarfield,i.e.,inadistancefromthebeamwaistwhichismuchlargerthantheRayleighlength.Thisdefinitionyieldsadivergencehalf-angle.(Sometimes,fullanglesareusedintheliterature;thesearetwiceaslarge.)Foradiffraction-limitedGaussianbeam,thebeamdivergenceisλ/(πw0),whereλisthewavelength(inthemedium)andw0thebeamradius(radiuswith1/e2intensity)atthebeamwaist.Alargebeamdivergenceforagivenbeamradiuscorrespondstopoorbeamquality.Alowbeamdivergencecanbeimportantforapplicationssuchaspointingorfree-spaceopticalcommunications.Beamswithverysmalldivergence,i.e.,withapproximatelyconstantbeamradiusoversignificantpropagationdistances,arecalledcollimatedbeams.Forthemeasurementofbeamdivergence,oneusuallymeasuresthebeamradiusatdifferentpositions,usinge.g.abeamprofiler.Itisalsopossibletoderivethebeamdivergencefromthecomplexamplitudeprofileofthebeaminasingleplane:spatialFouriertransformsdeliverthedistributionoftransversespatialfrequencies,whicharedirectlyrelatedtopropagationangles.SeeUSLaserCorpsapplicationnote[10]foratutorialonhowtomeasurethelaserbeamdivergencewithalensandCCDcamera. Beamastigmatism[edit] Seealso:Astigmatism Astigmatisminalaserbeamoccurswhenthehorizontalandverticalcrosssectionsofthebeamfocusatdifferentlocationsalongthebeampath.Astigmatismcanbecorrectedwithapairofcylindricallenses.Themetricforastigmatismisthepowerofcylindricallensneededtobringthefocusesofthehorizontalandverticalcrosssectionstogether.Astigmatismiscausedby: ThermallensinginNd:YAGslabamplifiers.Aslabthatissandwichedbetweentwometalheatsinkswillhaveatemperaturegradientbetweentheheatsinks.Thethermalgradientcausesanindexofrefractiongradientthatisverysimilartoacylindricallens.Thecylindricallensingcausedbytheamplifierwillmakethebeamastigmatic. Unmatchedcylindricallensesorerrorinplacementoftheseoptics. Propagationthroughanonlinearuniaxialcrystal(commoninnonlinearopticcrystals).Thex-andy-polarizedE-fieldsexperiencedifferentrefractiveindices. Notpropagatingthroughthecenterofasphericallensormirror. AstigmatismcaneasilybecharacterizedbyaCCDbeamprofilerbyobservingwherethexandybeamwaistsoccurastheprofileristranslatedalongthebeampath. Beamwanderorjitter[edit] Everylaserbeamwandersandjitters—albeitasmallamount.Thetypicalkinematictip-tiltmountdriftsbyaround100μradperdayinalaboratoryenvironment(vibrationisolationviaopticaltable,constanttemperatureandpressure,andnosunlightthatcausespartstoheat).Alaserbeamincidentuponthismirrorwillbetranslatedby100matarangeof1000 km.ThiscouldmakethedifferencebetweenhittingornothittingacommunicationssatellitefromEarth.Hence,thereisalotofinterestincharacterizingthebeamwander(slowtimescale)orjitter(fasttimescale)ofalaserbeam.ThebeamwanderandjittercanbemeasuredbytrackingthecentroidorpeakofthebeamonaCCDbeamprofiler.TheCCDframerateistypically30framespersecondandthereforecancapturebeamjitterthatisslowerthan30 Hz—itcan'tseefastvibrationsduetoone'svoice,60Hzfanmotorhum,orothersourcesoffastvibrations.Fortunately,thisisusuallynotagreatconcernformostlaboratorylasersystemsandtheframeratesofCCDsarefastenoughtocapturethebeamwanderoverthebandwidththatcontainsthegreatestnoisepower.Atypicalbeamwandermeasurementinvolvestrackingthecentroidofthebeamoverseveralminutes.Thermsdeviationofthecentroiddatagivesaclearpictureofthelaserbeampointingstability.Theintegrationtimeofthebeamjittermeasurementshouldalwaysaccompanythecomputedrmsvalue.Eventhoughthepixelresolutionofacameramaybeseveralmicrometres,sub-pixelcentroidresolution(possiblytensofnanometerresolution)isattainedwhenthesignal-to-noiseratioisgoodandthebeamfillsmostoftheCCDactivearea.[11] Beamwanderiscausedby: Slowthermalizationofthelaser.Lasermanufacturersusuallyhaveawarm-upspecificationtoallowthelasertodrifttoanequilibriumafterstartup. Tip-tiltandopticalmountdriftcausedbythermalgradients,pressure,andlooseningofsprings. Non-rigidlymountedoptics Vibrationduetofans,peoplewalking/sneezing/breathing,waterpumps,andmovementofvehiclesoutsidethelaboratory. Misrepresentationofbeamprofilermeasurementsforlasersystems[edit] Itistomostlasermanufacturers'advantagetopresentspecificationsinawaythatshowstheirproductinthebestlight,evenifthisinvolvesmisleadingthecustomer.Laserperformancespecificationscanbeclarifiedbyaskingquestionssuchas: Isthespecificationtypicalorworst-caseperformance? Whatbeamwidthdefinitionwasused? IstheM2parameterforbothverticalandhorizontalcrosssections,orjustforthebettercrosssection? WasM2measuredusingtheISO-standardtechniqueorsomeotherway—e.g.powerinthebucket. Overhowlongwasthedatatakentocomeupwiththespecifiedrmsbeamjitter.(RMSbeamjittergetsworseasthemeasurementintervalincreases.)Whatwasthelaserenvironment(opticaltable,etc.)? Whatisthewarm-uptimeneededtoachievethespecifiedM2,beamwidth,divergence,astigmatism,andjitter? Techniques[edit] Beamprofilersgenerallyfallintotwoclasses:thefirstusesasimplephotodetectorbehindanaperturewhichisscannedoverthebeam.Thesecondclassusesacameratoimagethebeam.[12] Scanning-aperturetechniques[edit] Themostcommonscanningaperturetechniquesaretheknife-edgetechniqueandthescanning-slitprofiler.Theformerchopsthebeamwithaknifeandmeasuresthetransmittedpowerasthebladecutsthroughthebeam.Themeasuredintensityversusknifepositionyieldsacurvethatistheintegratedbeamintensityinonedirection.Bymeasuringtheintensitycurveinseveraldirections,theoriginalbeamprofilecanbereconstructedusingalgorithmsdevelopedforx-raytomography.Themeasuringinstrumentisbasedonhighprecisionmultipleknifeedgeseachdeployedonarotatingdrumandhavingadifferentanglewithrespecttobeamorientation.Scannedbeamisthenreconstructedusingtomographicalgorithmsandprovides2Dor3Dhighresolutionenergydistributionplots.Becauseofthespecialscanningtechniquethesystemautomaticallyzoomsinontothecurrentbeamsizeenablinghighresolutionmeasurementsofsubmicronbeamsaswellasrelativelargebeamsof10ormoremillimeters.ToobtainmeasurementofvariouswavelengthdifferentdetectorsareusedtoallowlaserbeammeasurementsfromdeepUVtofarIR.Unlikeothercamerabasedsystemsthistechnologyalsoprovidesaccuratepowermeasurementinrealtime Scanning-slitprofilersuseanarrowslitinsteadofasingleknifeedge.Inthiscase,theintensityisintegratedovertheslitwidth.Theresultingmeasurementisequivalenttotheoriginalcrosssectionconvolvedwiththeprofileoftheslit. Thisfusionbetweenknife-edgetechnologyandtomographicalgorithmscreatesanewfieldofbeamprofiling-CKET(ComputerizedKnife-EdgeTomography).Thiscreatescapabilityofaccuratemeasurementfromamicrontoover10millimeterswithadaptableresolutionoverawidespectrumrange,practicallyifasingle-surfacedetectorexistsforacertainwavelengthregion,thenusingthistechnologyanimage-likeprofilecouldbederived.[13] Thesetechniquescanmeasureverysmallspotsizesdownto1μm,andcanbeusedtodirectlymeasurehighpowerbeams.Theydonotoffercontinuousreadout,althoughrepetitionratesashighastwentyhertzcanbeachieved.Also,theprofilesgiveintegratedintensitiesinthexandydirectionsandnottheactual2Dspatialprofile(integratingintensitiescanbehardtointerpretforcomplicatedbeamprofiles).Theydonotgenerallyworkforpulsedlasersources,becauseoftheextracomplexityofsynchronizingthemotionoftheapertureandthelaserpulses.[14] CCDcameratechnique[edit] TheCCDcameratechniqueissimple:attenuateandshinealaserontoaCCDandmeasurethebeamprofiledirectly.Itisforthisreasonthatthecameratechniqueisthemostpopularmethodforlaserbeamprofiling.ThemostpopularcamerasusedaresiliconCCDsthathavesensordiametersthatrangeupto25 mm(1 inch)andpixelsizesdowntoafewmicrometres.Thesecamerasarealsosensitivetoabroadrangeofwavelengths,fromdeepUV,200 nm,tonearinfrared,1100 nm;thisrangeofwavelengthsencompassabroadrangeoflasergainmedia.TheadvantagesoftheCCDcameratechniqueare: Itcapturesthe2Dbeamprofileinreal-time Highdynamicrange.Evenawebcam'sCCDchiphasadynamicrangeofaround28.[15] Softwaretypicallydisplayscriticalbeammetrics,suchasD4σwidth,inreal-time SensitiveCCDdetectorscancapturethebeamprofilesofweaklasers Resolutiondowntoabout4μm,dependingonthepixelsize.Inaspecialcasearesolutionof±1.1 μmwasdemonstrated.[15] CCDcameraswithtriggerinputscanbeusedtocapturebeamprofilesoflow-duty-cyclepulsedlasers CCD'shavebroadwavelengthsensitivitiesfrom200to1100 nm ThedisadvantagesoftheCCDcameratechniqueare: Attenuationisrequiredforhigh-powerlasers CCDsensorsizeislimitedtoabout1inch. CCDsarepronetobloomingwhenusedneartheedgeoftheirsensitivity(e.g.closeto1100 nm)[16][17] BaselinesubtractionforD4σwidthmeasurements[edit] TheD4σwidthissensitivetothebeamenergyornoiseinthetailofthepulsebecausethepixelsthatarefarfromthebeamcentroidcontributetotheD4σwidthasthedistancesquared.ToreducetheerrorintheD4σwidthestimate,thebaselinepixelvaluesaresubtractedfromthemeasuredsignal.ThebaselinevaluesforthepixelsaremeasuredbyrecordingthevaluesoftheCCDpixelswithnoincidentlight.Thefinitevalueisduetodarkcurrent,readoutnoise,andothernoisesources.Forshot-noise-limitednoisesources,baselinesubtractionimprovestheD4σwidthestimateas N {\displaystyle{\sqrt{N}}} ,where N {\displaystyleN} isthenumberofpixelsinthewings.Withoutbaselinesubtraction,theD4σwidthisoverestimated. Averagingtogetbettermeasurements[edit] AveragingconsecutiveCCDimagesyieldsacleanerprofileandremovesbothCCDimagernoiseandlaserbeamintensityfluctuations.Thesignal-to-noise-ratio(SNR)ofapixelforabeamprofileisdefinedasthemeanvalueofthepixeldividedbyitsroot-mean-square(rms)value.TheSNRimprovesassquarerootofthenumberofcapturedframesforshotnoiseprocesses–darkcurrentnoise,readoutnoise,andPoissoniandetectionnoise.So,forexample,increasingthenumberofaveragesbyafactorof100smoothsoutthebeamprofilebyafactorof10. Attenuationtechniques[edit] SinceCCDsensorsarehighlysensitive,attenuationisalmostalwaysneededforproperbeamprofiling.Forexample,40 dB(ND4or10−4)ofattenuationistypicalforamilliwattHeNelaser.Properattenuationhasthefollowingproperties: ItdoesnotresultinmultiplereflectionsleavingaghostimageontheCCDsensor Itdoesnotresultininterferencefringesduetoreflectionsbetweenparallelsurfacesordiffractionbydefects Itdoesnotdistortthewavefrontandwillbeanopticalelementwithsufficientopticalflatness(lessthanonetenthofawavelength)andhomogeneity Itcanhandletherequiredopticalpower ForlaserbeamprofilingwithCCDsensors,typicallytwotypesofattenuatorsareused:neutraldensityfilters,andwedgesorthickopticalflats. Neutraldensityfilters[edit] Mainarticle:Neutraldensityfilter Neutraldensity(ND)filterscomeintwotypes:absorptiveandreflective. Absorptivefiltersareusuallymadeoftintedglass.Theyareusefulforlower-powerapplicationsthatinvolveuptoabout100 mWaveragepower.Abovethosepowerlevels,thermallensingmayoccur,causingbeamsizechangeordeformation,becauseofthelowthermalconductivityofthesubstrate(usuallyaglass).Higherpowermayresultinmeltingorcracking.Absorptivefilterattenuationvaluesareusuallyvalidforthevisiblespectrum(500–800 nm)andarenotvalidoutsideofthatspectralregion.Somefilterscanbeorderedandcalibratedfornear-infraredwavelengths,uptothelongwavelengthabsorptionedgeofthesubstrate(around2.2 μmforglasses).Typically,onecanexpectabout5-10%variationoftheattenuationacrossa2-inch(51 mm)NDfilter,unlessspecifiedotherwisetothemanufacturer.TheattenuationvaluesofNDfiltersarespecifiedlogarithmically.AND3filtertransmits10−3oftheincidentbeampower.PlacingthelargestattenuatorlastbeforetheCCDsensorwillresultinthebestrejectionofghostimagesduetomultiplereflections. Reflectivefiltersaremadewithathinmetalliccoatingandhenceoperateoveralargerbandwidth.AnND3metallicfilterwillbegoodover200–2000 nm.Theattenuationwillrapidlyincreaseoutsidethisspectralregionbecauseofabsorptionintheglasssubstrate.Thesefiltersreflectratherthanabsorbtheincidentpower,andhencecanhandlehigherinputaveragepowers.However,theyarelesswellsuitedtothehighpeakpowersofpulsedlasers.Thesefiltersworkfinetoabout5Waveragepower(overabout1 cm2illuminationarea)beforeheatingcausesthemtocrack.Sincethesefiltersreflectlight,onemustbecarefulwhenstackingmultipleNDfilters,sincemultiplereflectionsamongthefilterswillcauseaghostimagetointerferewiththeoriginalbeamprofile.OnewaytomitigatethisproblemisbytiltingtheNDfilterstack.AssumingthattheabsorptionofthemetallicNDfilterisnegligible,theorderoftheNDfilterstackdoesn'tmatter,asitdoesfortheabsorptivefilters. Diffractivebeamsampler[edit] Diffractivebeamsamplersareusedtomonitorhighpowerlaserswhereopticallossesandwavefrontdistortionsofthetransmittedbeamneedtobekepttoaminimum. Inmostapplications,mostoftheincidentlightmustcontinueforward,"unaffected,"inthe"zeroorderdiffractedorder"whileasmallamountofthebeamisdiffractedintoahigherdiffractiveorder,providinga"sample"ofthebeam. Bydirectingthesampledlightinthehigherorder(s)ontoadetector,itispossibletomonitor,inrealtime,notonlythepowerlevelsofalaserbeam,butalsoitsprofile,andotherlasercharacteristics. Opticalwedges[edit] Opticalwedgesandreflectionsfromuncoatedopticalglasssurfacesareusedtoattenuatehighpowerlaserbeams.About4%isreflectedfromtheair/glassinterfaceandseveralwedgescanbeusedtogreatlyattenuatethebeamtolevelsthatcanbeattenuatedwithNDfilters.TheangleofthewedgeistypicallyselectedsothatthesecondreflectionfromthesurfacedoesnothittheactiveareaoftheCCD,andthatnointerferencefringesarevisible.ThefarthertheCCDisfromthewedge,thesmallertheanglerequired.Wedgeshavethedisadvantageofbothtranslatingandbendingthebeamdirection—pathswillnolongerlieonconvenientrectangularcoordinates.Ratherthanusingawedge,anoptical-qualitythickglassplatetiltedtothebeamcanalsowork—actually,thisisthesameasawedgewitha0°angle.Thethickglasswilltranslatethebeambutitwillnotchangetheangleoftheoutputbeam.Theglassmustbethickenoughsothatthebeamdoesnotoverlapwithitselftoproduceinterferencefringes,andifpossiblethatthesecondaryreflectiondoesnotilluminatetheactiveareaoftheCCD.TheFresnelreflectionofabeamfromaglassplateisdifferentforthes-andp-polarizations(sisparalleltothesurfaceoftheglass,andpisperpendiculartos)andchangesasafunctionofangleofincidence–keepthisinmindifyouexpectthatthetwopolarizationshavedifferentbeamprofiles.Topreventdistortionofthebeamprofile,theglassshouldbeofopticalquality—surfaceflatnessofλ/10(λ=633 nm)andscratch-digof40-20orbetter.Ahalf-waveplatefollowedbyapolarizingbeamsplitterformavariableattenuatorandthiscombinationisoftenusedinopticalsystems.Thevariableattenuatormadeinthisfashionisnotrecommendedforattenuationforbeamprofilingapplicationsbecause:(1)thebeamprofileinthetwoorthogonalpolarizationsmaybedifferent,(2)thepolarizationbeamcubemayhavealowopticaldamagethresholdvalue,and(3)thebeamcanbedistortedincubepolarizersatveryhighattenuation.Inexpensivecubepolarizersareformedbycementingtworightangleprismstogether.Thegluedoesnotstandupwelltohighpowers—theintensityshouldbekeptunder500 mW/mm2.Single-elementpolarizersarerecommendedforhighpowers. OptimalbeamsizeontheCCDdetector[edit] TherearetwocompetingrequirementsthatdeterminetheoptimalbeamsizeontheCCDdetector.Onerequirementisthattheentireenergy—orasmuchofitaspossible—ofthelaserbeamisincidentontheCCDsensor.Thiswouldimplythatweshouldfocusalltheenergyinthecenteroftheactiveregioninassmallaspotaspossibleusingonlyafewofthecentralpixelstoensurethatthetailsofthebeamarecapturedbytheouterpixels.Thisisoneextreme.Thesecondrequirementisthatweneedtoadequatelysamplethebeamprofileshape.Asaruleofthumb,wewantatleast10pixelsacrosstheareathatencompassesmost,say80%,oftheenergyinthebeam.Therefore,thereisnohardandfastruletoselecttheoptimalbeamsize.AslongastheCCDsensorcapturesover90%ofthebeamenergyandhasatleast10pixelsacrossit,thebeamwidthmeasurementswillhavesomeaccuracy. Pixelsizeandnumberofpixels[edit] ThelargertheCCDsensor,thelargerthesizeofbeamthatcanbeprofiled.Sometimesthiscomesatthecostoflargerpixelsizes.Smallpixelssizesaredesiredforobservingfocusedbeams.ACCDwithmanymegapixelsisnotalwaysbetterthanasmallerarraysincereadouttimesonthecomputercanbeuncomfortablylong.Readingoutthearrayinreal-timeisessentialforanytweakingoroptimizationofthelaserprofile. Far-fieldbeamprofiler[edit] Afar-fieldbeamprofilerisnothingmorethanprofilingthebeamatthefocusofalens.ThisplaneissometimescalledtheFourierplaneandistheprofilethatonewouldseeifthebeampropagatedveryfaraway.ThebeamattheFourierplaneistheFouriertransformoftheinputfield.Caremustbetakeninsettingupafar-fieldmeasurement.Thefocusedspotsizemustbelargeenoughtospanacrossseveralpixels.Thespotsizeisapproximatelyfλ/D,wherefisthefocallengthofthelens,λisthewavelengthofthelight,andDisthediameterofthecollimatedbeamincidentuponthelens.Forexample,ahelium-neonlaser(633 nm)with1 mmbeamdiameterwouldfocustoa317μmspotwitha500 mmlens.Alaserbeamprofilerwitha5.6μmpixelsizewouldadequatelysamplethespotat56locations. Specialapplications[edit] TheprohibitivecostsofCCDlaserbeamprofilersinthepasthavegivenwaytolow-costbeamprofilers.Low-costbeamprofilershaveopenedupanumberofnewapplications:replacingirisesforsuper-accuratealignmentandsimultaneousmultipleportmonitoringoflasersystems. Irisreplacementwithmicroradianalignmentaccuracy[edit] Inthepast,alignmentoflaserbeamswasdonewithirises.Twoirisesuniquelydefinedabeampath;thefartheraparttheirisesandthesmallertheirisholes,thebetterthepathwasdefined.Thesmallestaperturethataniriscandefineisabout0.8 mm.Incomparison,thecentroidofalaserbeamcanbedeterminedtosub-micrometreaccuracywithalaserbeamprofiler.Thelaserbeamprofiler'seffectiveaperturesizeisthreeordersofmagnitudesmallerthanthatofaniris.Consequently,theabilitytodefineanopticalpathis1000timesbetterwhenusingbeamprofilersoveririses.Applicationsthatneedmicroradianalignmentaccuraciesincludeearth-to-spacecommunications,earth-to-spaceladar,masteroscillatortopoweroscillatoralignment,andmulti-passamplifiers. Simultaneousmultipleportmonitoringoflasersystem[edit] Experimentallasersystemsbenefitfromtheuseofmultiplelaserbeamprofilerstocharacterizethepumpbeam,theoutputbeam,andthebeamshapeatintermediatelocationsinthelasersystem,forexample,afteraKerr-lensmodelocker.Changesinthepumplaserbeamprofileindicatethehealthofthepumplaser,whichlasermodesareexcitedinthegaincrystal,andalsodeterminewhetherthelaseriswarmedupbylocatingthecentroidofthebeamrelativetothebreadboard.Theoutputbeamprofileisoftenastrongfunctionofpumppowerduetothermo-opticaleffectsinthegainmedium. Seealso[edit] Laserbeamquality References[edit] ^R.Bolton,"Giveyourlaserbeamacheckup,"PhotonicsSpectra,June2002.Table1. ^abISO11146-1:2005(E),"Lasersandlaser-relatedequipment—Testmethodsforlaserbeamwidths,divergenceanglesandbeampropagationratios—Part1:Stigmaticandsimpleastigmaticbeams." ^ISO11146-2:2005(E),"Lasersandlaser-relatedequipment—Testmethodsforlaserbeamwidths,divergenceanglesandbeampropagationratios—Part2:Generalastigmaticbeams." ^ISO11146-1:2005(E),"Lasersandlaser-relatedequipment—Testmethodsforlaserbeamwidths,divergenceanglesandbeampropagationratios—Part3:Intrinsicandgeometricallaserbeamclassification,propagationanddetailsoftestmethods." ^Ankron."Standarddefinitionofbeamwidth"TechnicalNote,13Sep2008, ^A.E.Siegman,"Howto(Maybe)MeasureLaserBeamQuality,"TutorialpresentationattheOpticalSocietyofAmericaAnnualMeetingLongBeach,California,October1997. ^A.E.Siegman,"Howto(Maybe)MeasureLaserBeamQuality,"TutorialpresentationattheOpticalSocietyofAmericaAnnualMeeting LongBeach,California,October1997,p.9. ^M.BornandE.Wolf,PrinciplesofOptics:ElectromagneticTheoryofPropagation,InterferenceandDiffractionofLight,6thedition,CambridgeUniversityPress,1997. ^StrehlmeterW.M.KeckObservatory. ^MeasuringlaserbeamdivergenceUSLaserCorpsapplicationnote ^Ankron."TechnicalNote5:HowtomeasurebeamjitterwithnanometeraccuracyusingaCCDsensorwith5.6μmpixelsize". ^Aharon."Laserbeamprofilingandmeasurement" ^Aharon,Oren(December2021)."Tomographyandlasersmeetonaknife'sedge:Tomographicbeamprofilersdriveindustriallasermeasurementswithgroundbreakingaccuracyoveralargespectrumoflasers".PhotonicsViews.18(6):50–53.doi:10.1002/phvs.202100064.ISSN 2626-1294. ^Aharon."HighPowerBeamAnalysis" ^abG.Langeretal.,"AwebcaminBayer-modeasalightbeamprofilerforthenearinfra-red,"OpticsandLasersinEngineering,51(2013)571–575. ^Aharon."Widespectralbandbeamanalysis" ^Aharon."Metrologysystemforinter-alignmentoflasers,telescopes,andmechanicaldatum" Laserbeamprofilingmeasurement vteLasers Listoflaserarticles Listoflasertypes Listoflaserapplications Laseracronyms Typesoflasers Carbondioxidelaser Chemicallaser Dyelaser Er:YAGlaser Excimerlaser Free-electronlaser Helium–neonlaser Ionlaser Laserdiode Liquid-crystallaser Nd:YAGlaser Nitrogenlaser Ramanlaser Ti-sapphirelaser X-raylaser Laserphysics Activelasermedium Amplifiedspontaneousemission Continuouswave Dopplercooling Laserablation Lasercooling Laserlinewidth Lasingthreshold Magneto-opticaltrap Opticaltweezers Populationinversion Resolvedsidebandcooling Ultrashortpulse Laseroptics Beamexpander Beamhomogenizer BIntegral Chirpedpulseamplification Gain-switching Gaussianbeam Injectionseeder Laserbeamprofiler Msquared Modelocking Multiple-prismgratinglaseroscillator Multiphotonintrapulseinterferencephasescan Opticalamplifier Opticalcavity Opticalisolator Outputcoupler Q-switching Ramanamplification Regenerativeamplification Category Retrievedfrom"https://en.wikipedia.org/w/index.php?title=Laser_beam_profiler&oldid=1090176955" Categories:MeasuringinstrumentsOpticaldevicesLaserapplicationsHiddencategories:Articlescontainingpotentiallydatedstatementsfrom2002AllarticlescontainingpotentiallydatedstatementsArticlescontainingpotentiallydatedstatementsfrom2008 Navigationmenu Personaltools NotloggedinTalkContributionsCreateaccountLogin Namespaces ArticleTalk English Views ReadEditViewhistory More Search Navigation MainpageContentsCurrenteventsRandomarticleAboutWikipediaContactusDonate Contribute HelpLearntoeditCommunityportalRecentchangesUploadfile Tools WhatlinkshereRelatedchangesUploadfileSpecialpagesPermanentlinkPageinformationCitethispageWikidataitem Print/export DownloadasPDFPrintableversion Languages Addlinks