光纖布拉格光柵解讀

光纖布拉格光柵FiberBraggGratings國立中山大學電機工程探討所林武文教授2006.11.11Ref:AndreasOthonos&KyriacosKalli,FiberBraggGratings,ArtechHouse,Norwood,MA.U.S.A.199927.FiberBraggGratingSensors7.1Introduction7.2SensingExternalFields7.3WavelengthDemodulationofBraggGratingPointSensors7.4SimultaneousMeasurementofTemperatureandStrain7.5PolarizationStabilityofInterrogationSchemes37.FiberBraggGratingSensors7.6MultiplexingTechniques7.7SensorsBasedonChirpedBraggGratings7.8DistinguishingBraggGratingStrainEffects7.9BraggGratingFiberLaserSensors7.10BraggGratingsasInterferometerSensorsandReflectiveMarkers41.1FiberBraggGratings7.11OtherBraggSensorTypes7.12ApplicationsofBraggGratingSensors57.1IntroductionFBGissuitableformeasuring-Temperature-strain-pressure67.1IntroductionTheprincipaladvantageisthatthemeasuredinformationis-wavelengthencoded-flexibilitytonetwork-linearity(fromppb~%)-lowinsertionloss&narrowbandwavelength-reflection-easilyembeddedintomaterials77.1IntroductionThekeydetectionissueforFBGis-thedeterminationofsmallmeasurand-inducedwavelengthshifts.CategoryofFBGsensors-pointsensor-array(WDM/TDM/SDM)ordistributed-areflectivemarker(OTDR)-extremelylonggrating87.2SensingExternalFieldsInch.3welearned(1)Strainresponseofthegratingarisesfromthephysicalelongationofthesensor,leadingtoafractionalchangeinthegratingpitchwithacorrespondingchangeinthefiberindexbecauseofthephotoelasticeffect.97.2SensingExternalFields(2)Thermalresponse-inherentthermalexpansionofthefibermaterial.-thetemperaturedependenceoftherefractiveindex.107.2SensingExternalFieldsEq(3.9)&(3.10)&(3.11)worksformeasuring-acceleration-ultrasonicwaves-forces117.2SensingExternalFields7.2.1PressureSensitivity7.2.2DynamicMagneticFieldSensitivity127.2.1PressureSensitivity(7.1)(7.2a)(7.2b)Apressurechangeofwavelengthshif

ForSMFWhereEisYoung’smodulus137.2.1PressureSensitivity(7.3a)(7.3b)(7.4)Given,thenormalizedpitchpressureisgivenbyFortherefractive-pressurecoefficientsaregivenbyTherefore147.2.2DynamicMagneticFieldSensitivity(DMFS)Faradayeffect-Alongitudinalmagneticfieldappliedtothegratingchangestherefractiveindexforthetwocp(circularpolarization,CP)2Braggconditions:Where“+”refertorightcp.“”refertoleftcp.157.2.2DynamicMagneticFieldSensitivity(DMFS)(7.5)FBG’shavebeenusedforDMFSthrutheFaradayeffect.167.3WavelengthDemodulationofBraggGratingPointSensors7.3.1Quasi-StaticStrainMonitoring7.3.2DynamicStrainSensing7.3.3SimultaneousInterrogationofBraggGratingsandInterferometricSensors177.3WavelengthDemodulationofBraggGratingPointSensorsForFBGpointsensors1pmresolutionisrequiredtoresolveByOSA+Tunablelaser.187.3WavelengthDemodulationofBraggGratingPointSensors197.3.1Quasi-StaticStrainMonitoringPassiveBroadbandInterrogationRatiometricDetectionwithaWDMFiberCouplerInterrogationviaScanningOpticalFilterBraggGratingInterrogationUsingWavelengthTunableSource207.3.1Quasi-StaticStrainMonitoringRecoveryofBraggGratingWavelengthShiftUsingCCDSpectrometerAnalysisofBraggGratingWavelengthShiftUsingFourierTransformSpectroscopyMode-LockedFiberLaserOtherLasers217.3.1Quasi-StaticStrainMonitoringAlotofschemesusedforrecoverythewavelength-shiftinformationisrequiredforsmartstructureapplication.ThemostfundamentalmeansforinterrogatingaFBGreliesonbroadbandilluminationofthedevice.Thegratingusedinsensorapplicationshavebandwidthof0.05to0.3nm.227.3.1PassiveBroadbandInterrogationBroadbandoredgefiltersprovidedawavelength-dependentlosswhenthecutoffislosetothesignalwavelength.(ref.Fig.7.1(a))PassiveBroadbandInterrogationPassiveBroadbandInterrogationComparinglightXmittedthruthefilterwithlightpassedalongareferencepathrecoversthewavelengthshiftofthesensor.(ref.Fig.7.2(a))2PassiveBroadbandInterrogationThisapproachprovides(1)Verylimitedsensitivityforbroadbandfilter.or(2)Limitedrangeforedgefilter.(3)Relyonbulkopticcomponents;alignmentstabilityiscriticalwhichreducesportability.2PassiveBroadbandInterrogation(7.6)Thissystemofferseveraladvantages-lowcost-easeofuse-lowresolution2PassiveBroadbandInterrogation2RatiometricDetectionwithaWDMFiberCouplerAllfiberapproachessuchasfiberWDMfused-taperedcouplersprovidesamonotonicchangeinthecouplingratiobetweenthetwooutputports.(ref.Fig.7.2(b))Advantages:(All-fibertype)-lowpowerloss-lowcost-1%accuracy2InterrogationviaScanningOpticalFilterThedemodulatedoutputistheconvolutionofthetunablefilterspectrumwiththatofthegrating.(ref.Fig.7.1(b)).andtheoutputisoptimizedwhenthespectrumofthetunablefiltermatchesthatofthegrating.30InterrogationviaScanningOpticalFilterDrawbackoftheuseofnarrowbandSOF-samplinganarrowsliceoftheopticalspectrumatagiventime.themeasuredresolutionisstronglydependentonS/NofthereturnsignalandthelinewidthsofthetunablefilterandFBG.3InterrogationviaScanningOpticalFilter<Ex>Repeatedscanningofagratingarrayatafreqfresultsinanenergyreflectedbyeachgratingpersamplingperiodthatisequalto(7.7)WhereR:thegratingreflectivity.I:thespectralbrightnessofthesource:thegrating’sspectralwidth.3InterrogationviaScanningOpticalFilter(7.8)ThescanningfilterfurtherlimitsthedetectionenergyWhere:thefilterbandwidth:thewidthofthescannedwavelengthrange.3InterrogationviaScanningOpticalFilter(1)TunableWavelengthFiberFabry-DerotFilter(FPF)-usedinopticalfibercommunicationsystemtoremoveASE(amplifiedspontaneousemission)noiseemanatingfromEDFA.3InterrogationviaScanningOpticalFilter(2)TunableFiberBraggGratingFilters(3)Acoustic-OpticTunableFilter(4)OtherTunableFilterTypes3InterrogationviaScanningOpticalFilter(1)TunableWavelengthFiberFabry-DerotFilterThecharacteristicsofthefilters-bandpassresonanceofLorentzianlineshapes-bandwidth(BW)~0.3nm.Fig.7.3showshowafunablefilterisused.2operatingschemes-Tracking(closed-loop)mode(for1FBG)-scanningmode(for2or-molFBG)3InterrogationviaScanningOpticalFilter3InterrogationviaScanningOpticalFilter(1)TunableWavelengthFiberFabry-DerotFilterFig7.4Comparesthestrainmonitoredwithascanningfilterdemodulatedgratingandresistivestraingauge(RSG)whenbotharesubjecttoastrainlevel,withresolutionsofforaBWfromDC~360Hz.3InterrogationviaScanningOpticalFilter3InterrogationviaScanningOpticalFilter(1)TunableWavelengthFiberFabry-DerotFilterFig.7.5(inset)showstheoutputofonesensorsubjecttosine-wavemodulationofperiodicityof2minutes.40InterrogationviaScanningOpticalFilter4InterrogationviaScanningOpticalFilter(2)TunableFBGfilters(TBGF)-byJacksonetal.(ref.Fig.7.6)-AreflectrometricFBGbasedtunablefilterscheme.modifiedbyBradyetaltheparalleltopologybyusingaseriesarrayofreceivinggrating.-areflectmetricapproach.-Advantage(1)moreefficient.(2)reducesystemcomponents.4InterrogationviaScanningOpticalFilter4InterrogationviaScanningOpticalFilter(2)TunableFBGfilters(TBGF)-byDaviset.al.(ref.Fig.7.7)-usingthereceivinggratingsinanefficientXmissivemodeMinimizetheeffectoflightloss.Improvingsensorsensitivity.4InterrogationviaScanningOpticalFilter4InterrogationviaScanningOpticalFilter(3)Acoustic-OpticTunableFilters(AOTF)-isasolid-stateopticalfilter-thewavelengthofthediffractedlightisselectedbyRF.-largetuningrange-fastaccesstime,>5kHz-narrowspectralbandwidth.4InterrogationviaScanningOpticalFilter4InterrogationviaScanningOpticalFilter-AOTFisattractiveforwavelengthmultiplexingverylargeBragggratingarrayswiththeprovisothatasuitablebroadbandsource,orarraysourcesisavailable.-Suitableforopticalfiber.4InterrogationviaScanningOpticalFilter(7.9)

ForagivengratingandAOTFbandwidththereisanidealfreqderivationformaximizingthetrackingerrorsignalgivenbyWhereistheAOTFbandwidth.Note>(7.9)isindependentofthefilterXmission,gratingreflectivity,andintensitynoise.4InterrogationviaScanningOpticalFilterOneofthenotableadvantagesoftheAOTFoverallotherfiltersisthepossibilityofdrivingthedeviceatmultipleRFsignalstoallowfortrueparallelprocessingofmultiplewavelengthsignalsusingasinglefilteranddetector.Volanthenefal.demothemonitoringof2FBGwrittenat1300and1550nmusingaATOF.(ref.Fig.7.9)50InterrogationviaScanningOpticalFilter圖7.95BraggGratingInterrogationUsingWavelengthTunableSourceAdvantage-ToimproveS/Nasthemeasurementdeferminesamaxingratingreflectedpoweranditdispenseswiththeneedforopticalfiltering.5RecoveryofBraggGratingWavelengthShiftUsingCCDSpectrometer5RecoveryofBraggGratingWavelengthShiftUsingCCDSpectrometer(7.10)(7.11)Peak-Wavelength-DetectionAlgorithms.Theceutroiddetectionalgorithm(CDA)useEq(710)togetWherearetheintensityandcenterwavelengthoftheCCDpixel.Where(ref.Fig.7.11(a))5RecoveryofBraggGratingWavelengthShiftUsingCCDSpectrometer5AnalysisofBraggGratingWavelengthShiftUsingFourierTransformSpectroscopyFouriertransformspectroscopy(FTS)-byDavis&Kersey-ref.Fig.7.12-AMichelsoninterferometer5AnalysisofBraggGratingWavelengthShiftUsingFourierTransformSpectroscopy5AnalysisofBraggGratingWavelengthShiftUsingFourierTransformSpectroscopy(7.12)(7.13)(7.14)IftheeffectivecoherencelengthofthereflectedlightexceedtheOPDThegratingcenterwavelengthcanbefoundfromthePeriodicityoftheinterferogram,whichcanbemeasuredbyfindingananalyticsigualH:HilberttransformWhereisthesuperpositionofsignalsfromtheIndividualgratings.5AnalysisofBraggGratingWavelengthShiftUsingFourierTransformSpectroscopy5Mode-LockedFiberLaser

ref.Fig.7.14-Agratingsensorhasbeenimplementedwithbroadbandultrashortpulsesgeneratedwithapassivelymode-lockedfiberlaser.60Mode-LockedFiberLaser6Mode-LockedFiberLaser6OtherLasersYunandco-workersdemoa0.1-nmlinewidth,wavelength-sweptEDFlaserscannedovera28nmrangetodemodulateaBGSarraytodemodulateaBGSarraywithastrainofresolutionofat250Hz.637.3.2DynamicStrainSensingQuasi-StrainMonitoringDynamicStrainSensingSimultaneousInterrogationofBraggGratingsandInterferometricSensors647.3.2DynamicStrainSensingOpticalinterferometer(MZorMichelson)canbeusedtoconvertthewavelengthshiftfromgratingintoaphasechange.6Quasi-StrainMonitoringFig7.16showstheprincipleofGratingsensorsystembyusingfiberinterferometerwavelengthdiscriminatorfordynamicmeasurement.6Quasi-StrainMonitoring6Quasi-StrainMonitoring(7.15)Theunbalancedinterferometerservestofilterthelightwitharaisedcosinefun;whereandsystemlossistheimbalancelengthbetweenfiberarmsisthereflectedwavelengthfromFBGistheinterferencefringevisibilityisabiasphaseoffsetoftheMZIresettingfromslowlyvarying&randomenvironmentalperturbationsactingontheproductnd.6Quasi-StrainMonitoring(7-16)Foradynamicalstrain-inducedmodulationinthereflectedfromtheFBG,thechangeinthephaseshiftisWith,istheOPD(opticalpathdifference)oftheMZIisthedynamicstrainsubjectedtogratingisthenormalizedstrain-to-wavelength-shiftresponsivetyoftheBragggratingwithIfreplacedby(7.16)&(7.17)areequallyapplicable.6Quasi-StrainMonitoring(7.18)(freespectralrange)(7.19)With,theoutputsignalofthedifferenceAmplifierisTheoperationalsystemraugeisoftheNZIandisasetbytheFSRofthescanninginterferometer.Thereisatrad-offbetweensensitivityandoperationalrange.70Quasi-StrainMonitoring7Quasi-StrainMonitoring7DynamicStrainSensing(7.20)Def>AnenhancementfactorMRef.Fig.7.197Quasi-StrainMonitoring747.3.3SimultaneousInterrogationofBraggGratingsandInterferometerSensors757.4SimultaneousMeasurementofTemperatureandStrain7.4.1PrincipleofOperation7.4.2ExtrinsicTemperatureCompensation7.4.3IntrinsicTemperatureCompensation767.4SimultaneousMeasurementofTemperatureandStrainFBGcanmeasurestrainandtempsimultaneously.-forquasi-staticsignalsanytempvariationalongthefiberwillbeindistinguishablefromstrain.-fordynamicstrainmeasurementis,thisisnottrue,sincethethermalfluctuationoccuratlowfreq.Thattendnottocoincidewiththeresonancefrequiciesofintersts.77Table7.2787.4.1PrincipleofOperation(7.21)Foranidealsensor,withrepresentthephaseintuitedbyfor2systemeigenmode(inthiscasethe2FBG,)Here,assumingthatthestrain-andtemp-inducedperturbationarelinear.797.4.1PrincipleofOperationRef.Fig.7.20Inpractice,(7.22)(7.23)807.4.1PrincipleofOperation817.4.1PrincipleofOperationFig7.20(b)showsthelowcorrespondingtoandplanes.Whentheanglebetweenthelinesattheintersectionpoint=0,Eq.(7.23)Afundamentaltenetisthattheratioofthestrainresponsesof2gratingsbedifferentfromtheratiooftheirtempresponses.827.4.1PrincipleofOperation(7.24)(7.25)Ideally,theerrorintempandtrain,areTheerrorsareincreasedinthenon-idealcasewhereresponsesarenotorthogonal.wherethedomainofuncertaintyisshowninFig.7.20(b)837.4.2ExtrinsicTemperatureCompensation1.Apassivetemp-compensationpackagetonullifythetemptowavelengthcoefficient.thegratingismountedundertensioninapackagecomprisedof2materialwithdifferentthermalexpansioncoefficients;asthetamprises,strainonthegratingisprogressivelyreleased.847.4.2ExtrinsicTemperatureCompensation2.PackagingtheFBGinaliquidcrystallinepolymerhasresultedinanimprovementintempstabilitybyafactorof10.3.Acantileverisusedwith2FBGmountedonoppositesurface,withonegratingstretchedwhiletheotheriscompressed.Here,thedifferenceintheBragggratingwavelengthsistemp.independentbecausebothBragggratingshavethesametempsensitivity.857.4.3IntrinsicTemperatureCompensationReferenceGratingTaperedGratingDual-WavelengthSuperimposedGratingsMultipleBraggGratingOrdersSimultaneousMeasurementofTemperatureandStrainUsingPANDAGratingsinDissimilarDiameterFibers867.4.3IntrinsicTemperatureCompensationHybridBraggGrating/LongPeriodGratingSuperimposedGratingsandPolarization-RockingFiltersBraggGratingsandBrillouinScattering0CombinedGratingandIn-Line/ExtrinsicFiberEtalonSensorMethods1BraggGratingStrainRosettes2Dual-CoreFiberBraggGratings877.4.3IntrinsicTemperatureCompensation(7.26)887.5PolarizationStabilityofInterrogationSchemesEckeetal.havefoundthatlaterallycompressingorbendingafiberleadproducesdynamicalchangesduetopolarization-modeconversion,resultinginseverenoiseandsystematicerrors.Fig.7.21showsthatforasimplebroadbandsourceandspectrometer-CCDarrangement,thepassiveLyondepolarizergivesadramaticimprovementtothesystemstability,reducingwavelengtherrorstolessthen1pm.897.5PolarizationStabilityofInterrogationSchemes907.6MultiplexingTechniques7.6.1WavelengthDivisionMultiplexing(WDM)7.6.2TimeDivisionMultiplexing(TDM)7.6.3SpatialDivisionMultiplexing(SDM)7.6.4CombinedSDM/WDM/TDM917.6MultiplexingTechniquesBysharingthesourceandprocessingelectronics.-thecostpersensorisreduced.-reducestheoverallsystemweightwhileenhancingdurability.ForFOS-SDM(spatialDivisionMultiplexing)-TDM-FDM-WDM-CDM(coherencedomainmultiplexing)927.6MultiplexingTechniquesLimitedto10FOS,dueto-speed-crosstalk-S/N-bandwidth937.6.1WavelengthDivisionMultiplexing(WDM)ParallelandSerialWDMTopologiesWDMSchemeswithTunableFiltersCombinedWDMandInterferometricDetection9ParallelandSerialWDMTopologies-WDMencodeseachBGSwithauniquesliceoftheavailablesourcespectrum,whichdefinesthesensor’soperatingrougeandisalsoassociatedwithaspecificspatiallocationalongtheopticalfiber.Advantage--Thephysicalspacingbetweenindividualgratingsmaybeasshortasdesiredandtheneedforhigh-speedelectricalsignalprocessing,asisoftenrequiredinTDM,isremoved.9ParallelandSerialWDMTopologies9ParallelandSerialWDMTopologies977.6.2TimeDivisionMultiplexing(TDM)CombinedTDMandInterferometricDetectionTDMandWDM987.6.2TimeDivisionMultiplexing(TDM)997.6.3SpatialDivisionMultiplexing(SDM)SDMandWDMSDMandTDM1007.6.3CombinedSDM/WDM/TDM-TheserialmultiplexingschemesofWDM&TDMandcombinationmakeefficientuseofthesourcepower.1017.6.4CombinedSDM/WDM/TDM

1027.7SensorsBasedonChirpedBraggGratings7.7.1BroadbandChirpedGratingSensor7.7.2TaperedChirpedGratingSensor7.7.3AsymmetricallyChirpedGratingSensor7.7.4IntragratingSensing1037.7.1BroadbandChirpedGratingSensor1047.7.1(7.27)1057.7.2TaperedChirpedGratingSensor1061077.7.3AsymmetricallyChirpedGratingSensor1087.7.3AsymmetricallyChirpedGratingSensor1097.7.4IntragratingSensingIntensityReflectionSpectrumAnalysisIntragratingSensingThroughPhaseMeasurementCombinedReflectionSpectrumandPhaseMeasurementLowCoherenceReflectivityMeasurement1IntensityReflectionSpectrumAnalysis(7.28)(7.29)1IntensityReflectionSpectrumAnalysis1IntragratingSensingThroughPhaseMeasurement(7.30)(7.31)1IntragratingSensingThroughPhaseMeasurement1CombinedReflectionSpectrumandPhaseMeasurement1CombinedReflectionSpectrumandPhaseMeasurement1LowCoherenceReflectivityMeasurement117

LowCoherenceReflectivityMeasurement1187.8DistinguishingBraggGratingStrainEffects(7.32)(7.33)ThereflectiondistributionfromagratingexperiencingageneralstrainstateisgivenbyWitharegivenbyWherearethetimeaveragedscalarmagnitudesofthesquareoftheelectricfieldsinxandydirection.1197.8DistinguishingBraggGratingStrainEffects(7.34a)(7.34b)Foragratingexperiencingthemostgeneralthermo-mechanicalstrainconditions,thechangeinthereflectionwavelengthisgivenbyWherethecomponentofthelightwithitspolarizationVectorinxandydirection.1207.8DistinguishingBraggGratingStrainEffects1217.9BraggGratingFiberLaserSensors7.9.1SingleandMultipointBraggGratingLaserSensors7.9.2Ultra-HighResolutionBraggGratingLaserSensorsDemodulation1227.9BraggGratingFiberLaserSensorsFBGmayalsobeusedasnarrowbandreflectorforformingin-fiberlasercavities.ThebasicFBGlasersensoremploys2Bragggratingofmatchedwavelengthtocreateanin-fibercavityoronegratingcombinedwithabroadbandreflectorwithEr-dopedfiberastheusualgainmedium.ref.Fig.7.321237.9.1SingleandMultipointBraggGratingLaserSensors1247.9.1SingleandMultipointBraggGratingLaserSensors1257.9.1SingleandMultipointBraggGratingLaserSensors(7.35)1267.9.1SingleandMultipointBraggGratingLaserSensors1277.9.2Ultra-HighResolutionBraggGratingLaserSensorsDemodulation(7.36)1287.9.2Ultra-HighResolutionBraggGratingLaserSensorsDemodulation1297.9.2Ultra-HighResolutionBraggGratingLaserSensorsDemodulation1307.10BraggGratingsasInterferometricSensorsandReflectiveMarkers7.10.1ReflectometricSensingArraysUsingBraggReflectors7.10.2NestedFiberInterferometersUsingBraggReflectors7.10.3BraggGrating-BasedFabry-PerotSensors7.10.4CollocatedFabry-PerotCavitieswithWavelengthAddressableCavityLengths1317.10.1ReflectometricSensingArraysUsingBraggReflectors1327.10.1ReflectometricSensingArraysUsingBraggReflectors1337.10.2NestedFiberInterferometersUsingBraggReflectors(7.37)(7.38)InterrogationoftheThephaseatthejth1347.10.2NestedFiberInterferometersUsingBraggReflectors1357.10.3BraggGrating-BasedFabry-PerotSensors1367.10.4CollocatedFabry-PerotCavitieswithWavelengthAddressableCavityLengths1377.10.4CollocatedFabry-PerotCavitieswithWavelengthAddressableCavityLengths1387.11OtherBraggSensorTypesNewspeciallymodifiedortailoredgratings--phaseshiftdevices-multimodegratings-superstructuregratings1397.12ApplicationsofBraggGratingSens