A Laser Displacement Sensor Using the Self-Mixing Effect for Modal Analysis and Defect Detection

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1、IEEETRANSACTIONSONINSTRUMENTATIONANDMEASUREMENT,VOL.46,NO.4,AUGUST1997847ALaserDisplacementSensorUsingtheSelf-MixingEffectforModalAnalysisandDefectDetectionNoelServagent,ThierryBosch,andMarcLescure¨AbstractÐAlow-cost,simple,noncontactdisplacementsensorusingtheself-mixingeffectinsideacwsingle-m

2、odelaserdiodehasbeendevelopedformodalanalysisanddefectdetection.Theresonancefrequenciesofthinclampedmetallicplateshavebeendeterminedwitharesolutionof0.2Hz.Withsuchtargets,classicspectrumanalyzersgenerateameasurementerrorupto10%becauseoftheweightoftheaccelerometer.Thefrequency-shiftofthefundame

3、ntalresonancefrequencybetweenastandardplateandthesameonewithaknowndefectcanalsobemeasured,themodalvectorassociatedtothisfrequencybeingdistortedattheplaceofthedefect.IndexTermsÐDisplacementsensor,laserdiode,modalanaly-sis,opticalfeedback,self-mixingeffectI.IFig.1.Experimentalsetupformodalanalys

4、isofaclampedplateusinganNTRODUCTIONML64110Nlaserdiode.HEREaremanyindustrialprocessesinwhichanon-Tcontacttechniqueforvibrationsmeasurements,modalII.LASERDISPLACEMENTSENSORanalysis,ordefectdetectionwouldbenecessaryforqualitycontrolpurposesornondestructivetesting.SomeexamplesFig.1representstheblo

5、ckdiagramofanexperimentaloftheseprocessesarefabricationofmicromechanicscom-setupdevelopedfordisplacementmeasurementusingtheself-ponents,semiconductorsensors,orloudspeaker.Laser-basedmixingeffect.AcwML64110Nlaserdiodeisdrivenattechniquesgenerallyusinginterferometricsetupexistinsuchtheopticalpow

6、erof30mWandawavelengthofprocessesbuttheyareexpensive.Moreover,theyoftenneed785nm.ThelaserbeamoutputfromtheLD'srearfacetismirror-likesurfaces.monitoredbyaphotodiodewithintheLDpackage.ByusingSelf-mixinginterferenceinacwsingle-modelaserdiodearoughtarget,onecanconsiderthemultiplereflectionwithin(LD

7、)isawell-knownlow-costopticalmethodextensivelytheexternalcavitytobeneglectedforaweakfeedback[5],usedtomeasuredistances[1],displacementsandvibrations[6].Fordistancesofthetargetsmallerthanthehalf-coherence[2]±[4].Thelaserbeamisreflectedbya