Fiber grating sensors for high temperature measurement.docx

Fiber grating sensors for high temperature measurement.docx

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Fibergratingsensorsforhightemperaturemeasurement

Fibergratingsensorsforhightemperaturemeasurement

YageZhana,,HuaWua，QinyuYanga，ShiqingXiangb,HongHeb

aDepartmentofAppliedPhysics,CollegeofScience,DonghuaUniversity,Shanghai201602,China

bShanghaiInstituteofOpticsandFineMechanics,ChineseAcademyofSciences,Shanghai201800,China

AbstractTwofibergratingsensorsforhightemperaturemeasurementareproposedandexperimentallydemonstrated.Theinterrogationtechnologiesofthesensorsystemsareallsimple,lowcostbuteffective.Inthefirstsensorsystem,thesensorheadiscomprisedofonefiberBragggrating（FBG）andtwometalrods.Thelengthsoftherodsaredifferentfromeachother.Thecoefficientsofthermal-expansionoftherodsarealsodifferentfromeachother.TheFBGwillbestrainedbythesensorheadwhenthetemperaturetobemeasuredchanges.Thetemperatureismeasuredwithbasisofthewavelength-shiftsoftheFBGinducedbystrain.Inthesecondsensorsystem,alongperiodfibergrating（LPG）isusedashightemperaturesensorhead.TheLPGisvery-high-temperaturestableCO2-laser-inducedgratingandhasalinearfunctionofwavelength-temperatureintherangeof0-800℃.Adynamicrangeof0-800℃andaresolutionof1℃havebeenobtainedbyeitherthefirstorthesecondsensorsystem.Theexperimentalresultsagreewiththeoreticalanalyses.

Keywords:

opticalfibersensor,fiberBragggrating,longperiodfibergrating,hightemperature

1．Introduction

Insomefields,suchastunnelmonitoring,materialprocessing,minemonitoring,structurehealthmonitoringandoilwellmonitoring,reliablehightemperaturesensorsarenecessaryandimportant.1,2Traditionalelectricalhightemperaturesensorshavesomedisadvantages,includinglargetemperaturefluctuation,latentdangeroffireaccidentandlowreliability.Opticalfibergratingsensorshavenumerousadvantagesovertraditionalelectricalsensors,suchashigherstabilityandsensitivity,immunitytoelectromagneticinterference,beingcompetentforapplicationinharshenvironments,“smartstructures”andon-sitemeasurements.3,4So,fibergratingsensorsarethemostappropriatesensorsforapplicationsinthefieldsmentionedabove.

FiberBragggrating（FBG）andfiberlongperiodgrating（LPG）areusuallyusedastemperaturesensorhead.ButneithercommonFBGnorcommonLPGcanbeuseddirectlyashightemperaturessensorhead,becausetheywillbedecayedwhenthetemperaturehigherthan200℃andwillbedestroyedwhenthetemperaturehigherthan350℃.5,6So,untilnow,onlyaveryfewkindoftechnologiesonFBGorLPGhightemperaturemeasurementhavebeenresearched.7,8G.BrambillaetchaveresearchedthehightemperaturemeasurementcharacteristicsofFBGsthatwithspecialdopants（suchasSnand/orNa2O）.TheydiscoveredthattheseFBGsexhibitunusualoscillationsinreflectivity.9Thesemethodsarenotsuitedforhightemperaturemeasurement.

Thispaperproposestwofibergratinghightemperaturesensors,basedonaFBGwithanoveldesignedstructureassensorheadandaspecialhigh-temperaturestableLPGassensorhead,respectively.Theexperimentalresultsandthecharacteristicsofthesensorsystemsarealsodiscussed.Thetwohightemperaturesensorheadshavebeendesigned,preparedandusedinexperimentssuccessfully.Adynamicrangeof0-800℃andaresolutionof1℃havebeenexperimentallyachievedbyeitherthefirstorthesecondsensorsystem.Experimentalresultsagreewiththeoreticalanalyses.

2.Theoreticalanalyses

2.1Principleofthefirsthightemperaturesensorhead

Fig.1Schematicdiagramofthefirstsensorhead

CommonFBGcannotbeusedashightemperaturesensorheaddirectly,soanovelhightemperatureFBGsensorheadhasbeendesigned.ThesensorheadismainlycomprisedofaFBG（FBG1）andtwometalrods,asshowninfig.1.Thetwometalrodshavedifferentlengthsanddifferentcoefficientsofthermal–expansion（CTE）.Thelengthsofthetwometalrodsare

and

respectively.TheCTEsofthetwometalrodsare

and

respectively.Therodsarefixedintooneadiabaticplate.Adiabaticcylinder1andadiabaticcylinder2areusedtoprotectthetwometalrods,inorderthatthereisnotransversethermalradiation.Theleftendsofthetwometalrodsconnecttwoadiabaticrods.FBG1ispre-strainedandgluedtotheendsurfaceoftheadiabaticrodsonpointAandpointB.FBG1isprotectedbyadiabaticcylinder3inorderthatFBG1isnotbemodulatedbytheenvironmenttemperatureandthethermalradiationoftheadiabaticplate.

Thesensingends（seefig.1）touchtheobjectwhosetemperaturetobemeasured.Whenthetemperaturetobemeasuredchanges,thetwometalrodswillhavedifferentelongation,whichwillmakeLchange（thedistancebetweenthetwoadiabaticrods）andFBG1bestrained.Thetemperatureismeasuredwiththebasisofwavelength-shiftsofFBG1.

Theadiabaticcylindersareeffective.Thetransversethermalradiationofthemetalrodsisnegligible.Whentherodsareinheatbalance,thetemperatureofeachmetalrodreduceslinearlyfromwhosesensingendtotheotherend.Forbriefness,thelengthchangeof

isgivenby:

（1）

（2）

Where

and

aretheelongationsofthetwometalrodsrespectively.

and

isthelengthchangeofL,namelytheelongationofFBG1sectionoffiber.

and

（

）arethelength,averagetemperatureandaverageCTEofthejthsubsectionofthemetalrod.ThecorrespondingwavelengthshiftofFBG1（

）isexpressedby:

3,10

（3）

Where

istheeffectivephoto-elasticcoefficientoftheglassfiberwithPossionratio

.

and

arethephoto-elasticcoefficientsoffiber.

istheeffectiverefractiveindexoftheguidemodeinthefiber.Foratypicalfusedsilicafiber,

=0.22.

ThetwometalrodsaremadefromanH62brassrodanda45#carbonsteelrodrespectively.TheCTEsofthetwometalrodsare

and

respectively.

and

havebeenmeasuredanddeterminednumericallyby:

（4）

Inthesametemperaturerange,

islargerthan

.ThecurveofthewavelengthofFBG1havebeentheoreticallysimulatedwithsuppositionsofbothL1=20cm,L2=18cmandL1=18cm,L2=20cmintherangeof0-1000℃asshowninfig.2.

Fig.2Thetemperature-wavelengthresponseofFBG1intherangeof0-1000℃

IfL1=20cmandL2=18cm,theBraggwavelengthofFBG1shiftsalmostlinearlywithtemperatureintherangeof0℃--800℃.Whenthetemperatureascendsfrom0℃to800℃,itshifts6.89nm.ThesensitivityofthesensorisenhancedwhenthemetalrodwithlargerCTEislongerthanthemetalrodwithsmallerCTE,whichcanbeconfirmedbythattheslopeofcurve（a）islargerthantheslopeofcurve（b）infig.2.SotheexperimentsareimplementedintheconditionsofL1=20cmandL2=18cm.

2.2Principleofthesecondhightemperaturesensorsystem

BBS:

broadbandsource,C2,C3:

2×2coupler,PD:

photodiode

IMG:

Indexmatchedgel,DAC,dataacquisitioncard

Fig.3Schematicdiagramofthesecondhightemperaturefibergratingsensor

Fig.3showstheschematicconfigurationofthesecondsensorsystem.AnotherLPG（LPG2）isusedashightemperaturesensinghead.LPG2isavery-high-temperaturestableCO2-laser-inducedgratingandhasalinearfunctionofwavelength-temperatureintherangeof0-800℃.FBG2andFBG3areusedasdemodulationelementandreferenceelement,respectively.LightfromBBSismodulatedbyLPG2andthenilluminatesFBG2viaC2,aswellFBG3viaC2andC3.TheopticalisolatorinterdictsthereflectedlightfromFBG3toreturntoLPG2.ThereflectedlightfromFBG2andFBG3aredetectedbyPD2andPD3,respectively.AlltheIMG（☆）armsendatindexmatchinggel.TheanalogelectricsignalsfromallthePDsarecollectedbytheamplifierandthentheDAC,finallyprocessedbyanddisplayedonthescreenofacomputer.

2.3Interrogationprinciplesofthesensorsystems

Fig.4SchematicreflectionspectrumofFBG1andtransmissionspectrumofLPG1

Wavelengthinterrogationtechnologyisveryimportantforfibergratingsensorsystem.Inthefirstsensorsystem,LPG1isusedasalinearresponseedgefiltertoconvertwavelengthintointensityencodedinformationforinterrogation.LPG1hasbeentemperature-compensationencapsulatedandthewavelengthofitwillnotchange.Fig.4showstheschematicreflectionspectrumofFBG1andthetransmissionspectrumofLPG1usedintheexperiments.TheusefulspectrumregionofLPG1isshowntobenearlylinearoverasufficientlywiderange.9Ifaninterrogationsystemisarrangedaccordingasthewayshowninfig.5（b）,lightfromthebroadbandsource（BBS）willbemodulatedbyLPG1andthenilluminatesFBG1.ThereflectedlightfromFBG1isdetectedbyPD1andwillchangewiththeBraggwavelengthshiftofFBG1.ThereforethefilteringmechanismofLPG1yieldsalinearrelationshipbetweenthewavelengthshiftofFBG1andlightintensityPD1detected（

）.TheBraggwavelengthofFBG1（

）islocatedattheintensity-ascendingsideoftheappliedlossbandofLPG1,namelybeingattheLPG1curvewithpositiveslope.So

willincreasewhen

shiftslonger.

Inthesecondsensorsystemasshowninfig.3,LPG2isusedashightemperaturesensingheadandFBG2isusedasinterrogationelement.FBG2hasbeentemperature-compensationencapsulatedandthewavelengthofit（

）willnotchange.Theprincipleissimilartotheprinciplementionedabove,becausewhenaLPGandaFBGarematched,eitherthewavelengthshiftoftheLPGoroftheFBGwillcausearelativewavelengthshiftbetweentheLPGandtheFBG.SotheopticalintensityPD2detected（

）willlinearlychangewiththeshiftof

when

isconstant（supposingthat

isthecentralwavelengthoftheappliedlossbandofLPG