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1、InvitedPaperMultiplexingfiberbragggratingsensorsww.Morey,J.R.Dunphy,andG.MeltzUnitedTechnologiesResearchCenterEastHartford,CT06108ABSTRACTBraggreflectiongratingsandout—couplingtapsforsensorscanbewrittenholographicallywithinthecoreofmanycommercialfil,ersavailabletoday.Th
2、egratingsappeartobepermanentandhavebeentestedtotemperaturesinexcessof500°C.Quasi-distributedtemperature,strain,pressure,chemical,andinterferometrictypesensorscanbemadewiththewavelengthselective,reflectiongratingsandtaps.Thefibergratings,andthedifferenttypesofsensorsthey
3、canmake,convenientlylendthemselvestoWDM,1DM,andFDMtypesofmultiplexingschemes.Instrumentationtodetectthemultiplesensorsandmeasuretheirspectralshiftforlocalizedandquasi-distributedsensingiscurrentlyunderdevelop-ment.1.INTRODUCTIONPermanentholographicgrátmgshavebeenwritten
4、inmanycommercialopticalfibertypesthathavegermaniadopedcores.Theseincludethestandardtelecommunicationsfibers,polarizationmaintainingfibers,bendinsensitiveorpayoutfibers,andrareearthdopedamplifierorlaserfibers.ThegratingsaremadewithatechniqueusinginterferingbeamsofUvlaser
5、illuminationonthesideofthefiber1'2.Aperiodicmodulationoftheindexofrefractionisproducedintheger-maniadopedcoreofthefiberfromtheinterferingUVleamfringesbleachingtheoxygenvacancydefectabsorptionband.Thistechniqueisnoninvasiveandrequiresonlythatthebuffercoatingbetemporarily
6、removedfortheexposure.Fraction-a!indexmodulationchangesupto3xi0havebeenproducedtodate.Thegratingformsastop-bandfilter,asshowninFig.1,byreflectingopticalsignalswhosewavelengthinthelightguidingcoreistwicethegratingspacing.ThiswavelengthmatchingconditionisknownastheBraggco
7、nditionorBraggwavelength.AtransmissionspectrumofatypicalgratingisshowninFig.2.Gratingreflectivitiesapproaching100%havebeendemonstrated.1ipicalgratinglengthswehavemadetodaterangefrom1.5mmto15mm,withthelongergratingsgivinganarrowerspectralbandwidth.Thegratingspacingandhen
8、cetheBraggwavelengthiscontrolledbytheangleoftheexposingUVbeamsonthesideofthefiberandbytheexpos-ingUVwavelength
