Journal of Applied Research and Technology
www.jart.ccadet.unam.mx JournalofAppliedResearchandTechnology13(2015)586–590
Original
Beam profile analyzer for CO 2 lasers
Rubén López
∗, Ricardo Villagómez
CICESEMonterrey,ParquedeInvestigacióneInnovaciónTecnológica,Apodaca,NuevoLeónCP66629,Mexico Received1June2015;accepted18September2015
Availableonline21November2015
Abstract
ThedevelopmentofanoptoelectronicsystemtoanalyzethebeamintensityprofileofCO2lasersispresentedherein.Thedevicecollectsthe beamprofilewithaLiTaO3pyroelectricdetectorandusesasamplingtechniquebasedontheacquisitionofhorizontalsectionsatdifferentlevels.
Thedigitalsignalprocessingincludessubroutinesthatdropdowntwodimensionalandthreedimensionalbeamprofiledisplaystodeterminethe laserbeamparametersofopticalpower,peakpixellocation,centroidlocationandwidthofthelaserbeam,withalgorithmsbasedontheISO11146 standard.Withthesystematiccalibrationoftheanalyzerwasobtainedinthemeasurementofpoweranerrorunder5%,fora20–200Wrangeand anerrorunder1.6%forspatialmeasurementsofaTEM00laser.Bydesign,theanalyzercanbeusedduringthelaserprocess.
AllRightsReserved©2015UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisan openaccessitemdistributedundertheCreativeCommonsCCLicenseBY-NC-ND4.0.
Keywords: Laserbeamanalysis;CO2laser;Pyroelectricdetector;Infrared
1. Introduction
The greatest application of lasers in the industry is for materialsprocessing.Amongthelaser systemsmost usedfor manufacturing isfound the CO2 laser. Thisismainly dueto thehighabsorptionofitsinfraredradiation bymaterialssuch aswood,plasticandceramics,inadditiontoitsrelativelyhigh efficiency(greaterthan10%)(DeMaria&Hennessey,2010).
Beforecarryingoutalaserprocess,itisrecommendedthatthe distributionofitsintensity (profile)bechecked,toverifythat its transversal electromagnetic mode (TEM) is not distorted, sincethisreducestheperformanceofthelaser.Theinspection ofthe profilecanbecarriedoutwithnon-electronicmethods, suchasflorescentcardsandburningpaper,however,theseonly allowonetodistinguishareducednumberofintensitylevelsby simpleobservation.Anothernon-electronictechniqueconsists of markingthe profile onan acrylicblock toobtain its three dimensionalrepresentation,however,thisproducestoxicfumes duringthesample(Green,2001).Addedtothatpreviouslymen- tioned,thesetechniquesareconsideredtobestatic,sincetheydo
∗Correspondingauthor.
E-mailaddress:[email protected](R.López).
PeerReviewundertheresponsibilityofUniversidadNacionalAutónomade México.
notrevealinstantaneousfluctuations.Ontheotherhand,there are electronic devices for analyzingthe profile thatmeasures itsattributesandthatpresentitinatwodimensional(2D)and three dimensional (3D) mannerwith the helpof a computer.
Thisequipmentvariesindesign,dependingonthewavelength of thelaserstudiedandthetechniquesusedfor samplingand attenuationofthebeam(Roundy&Kirkham,2014).
In thisrespect,thedevelopmentofabeamprofileanalyzer ispresentedalongwithitsgraphicaluserinterface(GUI).The devicewas designedfor thespecificstudyof CO2lasers that haveanopticalbandwidthbetween9mand11m(infrared).
To collect the intensity profile,the analyzeruses an alterna- tivesamplingtechniquebasedonobtaininghorizontalsections atdifferentlevels(López&Villagómez,2014),whichcanbe observedinFigure1.Forhorizontalsamplingamirrormounted on arotating armis used,witha frequencyof 240rpm.This mirror deviatesthe enteringlaser beamtoward apyroelectric detectorfor12.5ms,limitingtheirradianceto5%andobstruc- ting thebeam only5%ofthe time,whichmeans thatit does notsignificantlyinterferewiththelaserprocess.Asfarasverti- calscanningisconcerned,thedetectorisplacedonanelevator that changes itslevel everytimethe mirrorfinishessampling ahorizontalsection.ThesignalsuppliedbytheLiTaO3detec- torisconditionedanditislaterdigitizedandprocessedbythe computer.
http://dx.doi.org/10.1016/j.jart.2015.10.012
1665-6423/AllRightsReserved©2015UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopen accessitemdistributedundertheCreativeCommonsCCLicenseBY-NC-ND4.0.
Laser Beam
Detector
Mirror
Rotating Elevator arm
Figure1.Functiondiagramofbeamprofileanalyzer.
Detection
A B C D
Amplification Demodulation Filtering
Figure2.Blockdiagramoftheconditioningcircuitandtheevolutionofthe signalineachstage.
2. Signalprocessing
Signalconditioningisperformed withanelectroniccircuit consistingofalownoiseamplifier,ademodulatorandafilter.In Figure2ablockdiagramofitsfunctionisshown.Inthedetec- tionstage,laserradiationisconvertedbyasmallelectricalsignal (A),consistingofalternatevoltagepulseswithdifferentampli- tudes(pyroelectriceffect)(Meledina,1998).Thesevoltagesare dependentuponthepositionoftherotatingmirror,formingan amplitudemodulatingsignal(AM).Inthefollowingstages,the signalisamplified(B)andthemodulatingsignalisobtainedby demodulationofAM(C)andfinally,thesignalpassesthrough afiltering stage toeliminateripple voltageand toobtain the analogicalsignalofthebeamprofile(D).
2.1. Programming
Digitalsignalprocessingcarried outbythe computer uses algorithmsthatcalculatetotalopticalpower,locatingthemax- imum point of irradiance (peak pixel) and determining the centroidandthebeam widththroughthemethodofmoments specifiedby the ISO11146 standard(ISO 11146, 2000;ISO 13694,2000).To processinformationfromthe analyzer,pro- gramming was developed based on the flow diagram that is presentedin Figure3. From thebeginningthe GUIisgener- atedandvariablesareinitialized.Later,theprogramwaitsuntil theelevatorindicatesthatitisfoundatanyofitsends,through someopticalswitches.Uponobtainingapositiveresponsefrom theanalyzer,thesoftwarewaitsuntilithasthesignalwherethe rotatingarmindicatesthatitbegantodeviatethebeam;when thishappensthecomputercommunicateswiththedataacqui- sitionsystemtobegintodigitizethesignalconditioningofthe detector.Horizontalsections aresampleduntil the elevatoris foundattheotherend,thatis,whenithasfinishedcollecting theprofile.Oncethishasbeendone,theprogramprocessesthe informationacquired,showsthegraphoftheprofileandthemea- surements.Finally,itasksifitwillfinishtheprocessorbegin anotherprofile.
Start
End of the elevator?
End of the elevator?
Finished?
End No No
No No
Yes Yes Yes Yes
Mirror deviated?
DAC
Calculations graphs measurements
Figure3.Flowdiagramofsoftwaredevelopedtocollect,analyzeandvisualize thelaserbeamprofile.
2.2. Measurementofpoweranddimensionalparameters
ThemeasurementofpowerPAdeliveredbytheanalyzeris calculatedbyaddingupthe intensityI(x, y)ofall pointsthat makeupthebeamprofilesampled,withtheequation:
PA=α
m x=0
n y=0
I(x,y) (1)
where(x,y)arethecoordinatesofthesamplepoint,mandnare thenumberofsamplestakenandαisascalingfactorobtained upondividingthereadingofanopticalpowermeter(OPM)by PAwithα=1(calibrationstage).
Forthatconcerning thecoordinatesofthemaximumpoint ofirradianceandthecentroid,thefirstislocatedintheposition (xImax,yImax),andforthesecond,thecoordinates(¯x,¯y)aredeter- minedbythediscreteformulasofthefirstmomentofdistribution ofintensity,withtheequations:
¯x=
m
x=0
n
y=0xI(x,y)
m
x=0
n
y=0I(x,y) (2)
¯y=
m
x=0
n
y=0yI(x,y)
m
x=0
n
y=0I(x,y) (3)
In thesamemanner,the secondmoment ofdistributionof intensity provides the beam widthfor bothcoordinated axes, withtheequations:
dσx=4×
m
x=0
n
y=0(x−¯x)2I(x,y)
m
x=0
n
y=0I(x,y) (4)
dσy =4×
m
x=0
n
y=0(y−¯y)2I(x,y)
m
x=0
n
y=0I(x,y) (5)
where(x−¯x)and(y−¯y)arethedistancestothecentroid.
In addition to the moment method, there are alternative optomechanical techniques that use a detector and different types of shutters to obtain the beam width. One of them is theknife-edgemethod(Cohen,Little, &Luecke,1984).This consistsofslidingaknifeinfrontofadetectortoobstructthe beaminthe directionofthe Cartesianaxisthat isrequiredto knowthewidth.Withthat,thepowertransmittedasafunction ofthepositionoftheknifeismeasured.TheISO11146standard establishesthatforthismethod,thebeamwidthisdirectlypro- portionaltotwotimesthedistanceexistingbetweenthepositions ofx1andx2oftheknife.Thesepositionsaredeterminedrespec- tivelyby84%and16%ofthepowertransmitted,asisindicated intheequation:
dσ =2|x2−x1|√ K
0.81
1
√K−1
+1
(6)
where K is the beam propagation factor (provided by the manufacturer) and0.81 is the correlation factorbetween the knife-edgemethodandthestandardmethod(moments).Touse thistechnique,thelaserbeamstudymusthaveadiameterthat islessthantheactiveareaofthedetectorandthis,atthesame time,mustbelessthanthediameteroftheknife.
3. Analyzercalibration
Calibration of the power anddimensional parameterswas carried outwiththeexperimentalsetupobservedinFigure4.
Thebeam emittedfromthe laserpasses throughthe analyzer whereitis sampledandthe informationobtainedis senttoa computertobeprocessedandanalyzed.Uponexitingtheana- lyzer, the beam continues its trajectory toward a knife-edge devicewithatwodimensional movementandanOPM.With
CO2Láser Analyzer OPM
Laser beam PC
Knife
Figure4.Experimentalsetuptocalibratethebeamprofileanalyzer.
this,totalpowervaluesareobtained(openknife)aswellasinci- dentalpoweraccordingtotheposition,necessarytodetermine thebeamwidth.
4. Results
TheanalysisoftheprofilewascarriedoutwithaDIAMOND 150K laser from CoherentInc. Thislaser has a TEM00 and a maximum opticalpower of 275W (pulsed operation). The methodologytocalibratethemeasurementofpowerconsisted of settingthe laserpulseperiodat100sandlaterobtaining
0
0 5 10 15 20 25 30 35 40
20 40 60 80 100 120 200 180 160 140
Pulse width (μs)
Optical power (W)
Figure5.Powermeasurementcarriedoutwiththeanalyzer(redblocks)andthe OPM(bluediamonds).
dx=.0128x2 -1.12x+7.0032 R2 =.9996
dy=.0276y2-1.2947y+7.51 R2=.9989
Beam width (mm) Beam width (mm)
Analyzer - knife difference (mm)Analyzer - knife difference (mm)
0.5 1.0 1.5 2.0
2.0 2.5 3.0
3.0 3.5
4.0
4.0 4.5
5.0
6.0 7.0
5.0
2.0 3.0 4.0 5.0 6.0 7.0
0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
0.0
a
b
Figure6.Calibrationcurvesofwidth(a)indirectionxand(b)indirectiony (solidlines).Aswellasthemathematicalexpressionsthatrepresentthem.The bluediamondsshowthevaluesobtainedbytheanalyzer.
Figure7.Graphicaluserinterfaceofthebeamprofileanalyzer.
an average of 10 measurements taken with the analyzer and the OPM in a simultaneous manner, in a pulse width inter- valof between 6sand 34s.The comparison betweenthe powermeasured by the analyzerproposedand the OPM can beobservedinFigure5.Thecurverepresentedbyredblocks belongstotheaveragepowervaluesobtainedwiththeanalyzer andthe bluediamondsrepresentthevaluesobtainedwiththe OPM.AsacalibrationreferenceanOPMmodelFieldMaster GS from Coherent Inc.was used. The error inthe measure- mentsofthe analyzerinthe 20W to200W rangewasunder 5%.Thispercentageis withinthe limitpermitted bytheIEC 61040standard(IEC61040,1990),correspondingtothistype ofdevice.
Concerningthebeam width,the analyzerrequires calibra- tioninthemeasurementofthisparameter,duetothefactthat thelowerthepowerofthelaser,theimageoftheprofileshown inthecomputerwillhavesmallerdimensions.Thisisduetothe factthat thesignalamplifierhasafixedgain. Thelaser man- ufacturerunderstudyindicatesavalueofdiameterof7.0mm, withatoleranceof±0.5mmandaKfactorgreaterthan0.77.
Withtheknife-edgemethodabeamwidthof6.754mminthe xdirectionand6.767mm inthe ydirectionwas obtained for thislaser,whichiswithinthetoleranceindicatedbythemanu- facturer.Tocompensateerrorinthemeasurementofthewidth, calibrationcurvesthatareshowninFigure6wereobtained.In bothgraphsvaluesobtainedbytheanalyzerarerepresentedby bluediamondsandthesolidline isthecalibrationcurvecon- structedwiththeexpressionshown(quadraticregression).With thisadjustment,thedevicepresentedanerrorinthe measure- mentofthewidthoflessthan1.6%forbothdirections.Forthe dimensionsoftherotatingmirror,themaximumdiameterthat theanalyzercanmeasureis15mm.
Thegraphicaluserinterfaceoftheanalyzer,aswellasthepro- grammingofdataacquisitionandalgorithmsforthecalculation oflaserparameterswasdevelopedwithLabVIEW.Thegeneral
screen for theGUIcan beappreciatedin Figure7.Thewin- dowontherightdropdownthethreedimensionalrepresentation (3D)ofthebeamprofileincolormap,inwhichredrepresents the greatest intensity andpurple the minimum intensity. The intensityscaleisnormalizedandspatialscalesarerepresented inmillimeters.Inthesamemanner,theupperleftwindowdrop downthetwodimensionalprofile(2D)withanautomaticcur- soratthemaximumpointof intensity.Theresolutionofboth imagesis64×64pixels.Ontheotherhand,theleftcentralwin- dowshowsthemeasurements madebytheanalyzer.Apower measurementof181.25Wcanbeobservedandbeamwidthsof 6.77mmand6.82mmindirectionsxandyrespectively.Finally, theupperbandshowstheinformationofthelaserunderstudy uploadedpreviouslybytheuser.Theanalyzercollectsaprofile every10sandusesaNationalInstrumentsUSB-6008cardto digitizethesignalofdetectormodel450-3fromEDOCorp.
5. Conclusions
The beam profile analyzer constructed can characterize pulsedCO2lasers upto200Wwithanerrorof lessthan5%
forthemeasurementofpower.Inthesamemanner,itcanmea- surethewidthofthebeamwithanerrorunder1.6%.Ontheother hand,beamprofilemonitoringcanbeperformedinline,since therotatingmirrorinterfereswiththebeamtrajectoryonly5%
ofthetime.Theanalyzercanalsobeusedinthestudyofcarbon monoxidelasers,sincetheir wavelengthisfoundinthe range operationofthepyroelectricdetectorused.Samplingspeedand theresolutionofthe profileacquired,canbeincreased witha fasterdataacquisitionsystem.
Conflictofinterest
Theauthorshavenoconflictsofinteresttodeclare.
References
Cohen,D.K.,Little,B.,&Luecke,F.S.(1984).Techniquesformeasuring1-m diamGaussianbeams.AppliedOptics,23,637–640.
DeMaria,A.J.,&Hennessey,T.V.(2010).CO2 laser.SPIEProfessional Magazine,.http://spie.org/x38563.xmlAccessedMay2015
Green,L.(2001).Monitoringlaserbeamperformance.TheFabricator.http://
www.thefabricator.com/article/lasercutting/monitoring-laser-beam- performanceAccessedMay2015
IEC61040.(1990).Powerandenergymeasuringdetectors,instrumentsand equipmentforlaserradiation.
ISO11146.(2000).Laserandlaser-relatedequipment–Testmethodsforlaser beamparameters–Beamwidths,divergenceangleandbeampropagation factor.
ISO13694.(2000).Opticsandopticalinstruments–Lasersandlaser-related equipment–Testmethodsforlaserbeampower[energy]densitydistribu- tion.
López,R.,&Villagómez,R.(2014).AnalizadordeperfilparaláseresdeCO2. pp.1–6.PuertoVallarta,Mexico:SOMIXXIX.
Meledina,P.(1998).AdvancedIRdetectorsexpanddesigner’spalette.Electron- icsProducts,41,35–37.
Roundy,C.B.,&Kirkham,K.D.(2014).Currenttechnologyoflaserbeam profilemeasurements.InF.M.Dickey(Ed.),Laserbeamshaping(2nded., pp.463–524).BocaRaton,UnitedStates:TaylorandFrancis.
