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1、1LowcoherencedefmationsenssfthemonitingofcivilengineeringstructuresD.InaudiaA.ElamaribL.PflugaN.GisinbJ.BreguetbS.Vurpillota“IMACLabatyofStressAnalysisSwissFederalInstituteofTechnologyCH1015LausanneSwitzerl‘GAPGroupofApp

2、liedPhysicsOpticalSeciionGenevaUniversityCH1205GenevaSwitzerlRcccivcd25January1993inrevisedfm8March1994accepted25March1994AbstractAnopticalfiberdefmationsenswitharesolutionof10pmanoperationalrangeof60mmhasbeenrealized.Th

3、esystemisbasedonlowcoherenceinterferometryinstardsinglemodetelecommunicationfibers.Itallowsthemonitingoflargestructuresoverseveralmonthswithoutnoticeabledrift.Nocontinuousmeasurementisneededthesystemisinsensitivetovariat

4、ionsofthefiberlosses.Thistechniquehasbeenappliedtothemonitingofa20mX5mX0.5m120tonconcreteslaboversixmonths.Itispossibletomeasuretheshrinkageofconcreteitselasticcoefficientduringprestraininggivingreproducibleresultsingood

5、agreementwiththeeticalcalculationsmeasurementsperfmedonsmallconcretesamples.Thispaperdescribestheopticalarrangementtheproceduresusedtoinstallopticalfibersinconcrete.KeywCivilengineeringstructures1.IntroductionBoththesecu

6、rityofcivilengineeringwksthelawrequireaperiodicmonitingofstructures.Themethodsusedfthispurposesuchastriangulationwaterlevelsvibratingstringsareoftenoftediousapplicationrequireonemanyspecializedoperats.Thiscomplexitythere

7、sultingcostslimitthefrequencyofthemeasurements.Furthermethespatialresolutionisoftenpotheobservationisusuallyrestrictedtothesurfaceoftheobject.Thereisthusarealdemfatoolallowinganinternalautomaticpermanentmonitingofstructu

8、reswithhighaccuracystabilityoverperiodstypicallyofthederof100yearsfbridges.Inthisframewkfiberopticsmartstructures(i.e.structureswithselftestingcapabilities)aregaininginimptanceinmanyfieldsincludingaeronauticscompositemat

9、erialmoniting.Thistechnologycanbeappliedincivilengineeringinparticularftheshtlongtimeobservationoflargestructuressuchas3wherezzristheeffectiverefractiveindexofthefiberzzgthegrouprefractiveindex(about1%higherthannefrinsil

10、ica)AthecentralvacuumwavelengthofthelightzitheautocrelationfunctiontakingthespectralacteristicsoftheemissionintoaccountALthephysicalpathdifferencebetweenthetwointerferingpaths.FurthersimilarinterferencetermsappearinEq.(1

11、)inthespecialcaseswhenALLALL.Whentheopticalpathdifferencebetweenthearmsinthereferenceinterferometercrespondstotheoneinducedbythetwofibersinstalledinthestructure(withinthecoherencelengthofthesource)interferencefringesappe

12、ar.ScanningALwiththemirrofthereferenceinterferometeritispossibletoobtainAL=0eitherwithAL=ALwithAL=ALthustwointerferencefringepacketsasdescribedbyEq.(1).ThemirrpositioncrespondingtoAL=0alsoproducesaninterferenceisusedasar

13、eference.Thesethreefringepacketsarcdetectedbymeansofalockinamplifiersynchronizedwiththemirrdisplacements.Themirrdisplacementsthedigitalizationofthelockinoutputarecarriedoutbymeansofaptablepersonalcomputer.Sincethereferen

14、cesignalisgcneratedseparatelydoesnothaveaconstantphaserelationtotheinterferencesignalonlytheenvelopeofthedemodulatedsignalhasaphysicalmeaningcrespondstotheenvelopeofthefringepattern.Alockinplotshowingthethreetypicalpeaks

15、isshowninFig.2.Eachpeakhasawidthofabout30pm.Thecalculationofitscenterofgravitydeterminesitspositionwithaprecisionbetterthan10pm.Thisprecisionisthelimitingfactofthewholemeasurementtechnique.SinceALisknownwithmicrometerpre

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