光纤布拉格光栅

1光纖布拉格光柵FiberBraggGratings國立中山大學電機工程研究所林武文教授2006.11.12Ref:AndreasOthonos&KyriacosKalli,FiberBraggGratings,ArtechHouse,Norwood,MA.U.S.A.199937.FiberBraggGratingSensors7.1Introduction7.2SensingExternalFields7.3WavelengthDemodulationofBraggGratingPointSensors7.4SimultaneousMeasurementofTemperatureandStrain7.5PolarizationStabilityofInterrogationSchemes47.FiberBraggGratingSensors7.6MultiplexingTechniques7.7SensorsBasedonChirpedBraggGratings7.8DistinguishingBraggGratingStrainEffects7.9BraggGratingFiberLaserSensors7.10BraggGratingsasInterferometerSensorsandReflectiveMarkers51.1FiberBraggGratings7.11OtherBraggSensorTypes7.12ApplicationsofBraggGratingSensors67.1IntroductionFBGissuitableformeasuring-Temperature-strain-pressure77.1IntroductionTheprincipaladvantageisthatthemeasuredinformationis-wavelengthencoded-flexibilitytonetwork-linearity(fromppb~%)-lowinsertionloss&narrowbandwavelength-reflection-easilyembeddedintomaterials87.1IntroductionThekeydetectionissueforFBGis-thedeterminationofsmallmeasurand-inducedwavelengthshifts.CategoryofFBGsensors-pointsensor-array(WDM/TDM/SDM)ordistributed-areflectivemarker(OTDR)-extremelylonggrating97.2SensingExternalFieldsInch.3welearned(1)Strainresponseofthegratingarisesfromthephysicalelongationofthesensor,leadingtoafractionalchangeinthegratingpitchwithacorrespondingchangeinthefiberindexbecauseofthephotoelasticeffect.107.2SensingExternalFields(2)Thermalresponse-inherentthermalexpansionofthefibermaterial.-thetemperaturedependenceoftherefractiveindex.117.2SensingExternalFieldsEq(3.9)&(3.10)&(3.11)worksformeasuring-acceleration-ultrasonicwaves-forces127.2SensingExternalFields7.2.1PressureSensitivity7.2.2DynamicMagneticFieldSensitivity137.2.1PressureSensitivityPPnnPnnBp)11()(EPvLL)2.1((7.1)(7.2a))2)(21(211122ppvEPnnn(7.2b)Apressurechangeofwavelengthshifp:pForSMFWhereEisYoung’smodulus147.2.1PressureSensitivity)2)(21(21)21(111122ppvEnpnnEvPPppvEnEvBp)]2)(21(2)21([11122(7.3a)(7.3b)(7.4)Given,thenormalizedpitchpressureisgivenby/LLFortherefractive-pressurecoefficientsaregivenbyTherefore/157.2.2DynamicMagneticFieldSensitivity(DMFS)Faradayeffect-Alongitudinalmagneticfieldappliedtothegratingchangestherefractiveindexforthetwocp(circularpolarization,CP)2Braggconditions:22BnBnWhere“+”refertorightcp.“”refertoleftcp.167.2.2DynamicMagneticFieldSensitivity(DMFS)2VHnn(7.5)FBG’shavebeenusedforDMFSthrutheFaradayeffect.177.3WavelengthDemodulationofBraggGratingPointSensors7.3.1Quasi-StaticStrainMonitoring7.3.2DynamicStrainSensing7.3.3SimultaneousInterrogationofBraggGratingsandInterferometricSensors187.3WavelengthDemodulationofBraggGratingPointSensorsForFBGpointsensors1pmresolutionisrequiredtoresolveByOSA+Tunablelaser.~0.11Coro197.3WavelengthDemodulationofBraggGratingPointSensors207.3.1Quasi-StaticStrainMonitoring7.3.1.1PassiveBroadbandInterrogation7.3.1.2RatiometricDetectionwithaWDMFiberCoupler7.3.1.3InterrogationviaScanningOpticalFilter7.3.1.4BraggGratingInterrogationUsingWavelengthTunableSource217.3.1Quasi-StaticStrainMonitoring7.3.1.5RecoveryofBraggGratingWavelengthShiftUsingCCDSpectrometer7.3.1.6AnalysisofBraggGratingWavelengthShiftUsingFourierTransformSpectroscopy7.3.1.7Mode-LockedFiberLaser7.3.1.8OtherLasers227.3.1Quasi-StaticStrainMonitoringAlotofschemesusedforrecoverythewavelength-shiftinformationisrequiredforsmartstructureapplication.ThemostfundamentalmeansforinterrogatingaFBGreliesonbroadbandilluminationofthedevice.Thegratingusedinsensorapplicationshavebandwidthof0.05to0.3nm.237.3.1PassiveBroadbandInterrogationBroadbandoredgefiltersprovidedawavelength-dependentlosswhenthecutoffislosetothesignalwavelength.(ref.Fig.7.1(a))247.3.1.1PassiveBroadbandInterrogation257.3.1.1PassiveBroadbandInterrogationComparinglightXmittedthruthefilterwithlightpassedalongareferencepathrecoversthewavelengthshiftofthesensor.(ref.Fig.7.2(a))267.3.1.1PassiveBroadbandInterrogationThisapproachprovides(1)Verylimitedsensitivityforbroadbandfilter.or(2)Limitedrangeforedgefilter.(3)Relyonbulkopticcomponents;alignmentstabilityiscriticalwhichreducesportability.277.3.1.1PassiveBroadbandInterrogation}{OBRSAII(7.6)Thissystemofferseveraladvantages-lowcost-easeofuse-lowresolution287.3.1.1PassiveBroadbandInterrogation297.3.1.2RatiometricDetectionwithaWDMFiberCouplerAllfiberapproachessuchasfiberWDMfused-taperedcouplersprovidesamonotonicchangeinthecouplingratiobetweenthetwooutputports.(ref.Fig.7.2(b))Advantages:(All-fibertype)-lowpowerloss-lowcost-1%accuracy307.3.1.3InterrogationviaScanningOpticalFilterThedemodulatedoutputistheconvolutionofthetunablefilterspectrumwiththatofthegrating.(ref.Fig.7.1(b)).andtheoutputisoptimizedwhenthespectrumofthetunablefiltermatchesthatofthegrating.317.3.1.3InterrogationviaScanningOpticalFilterDrawbackoftheuseofnarrowbandSOF-samplinganarrowsliceoftheopticalspectrumatagiventime.themeasuredresolutionisstronglydependentonS/NofthereturnsignalandthelinewidthsofthetunablefilterandFBG.327.3.1.3InterrogationviaScan