Gasrefractometrybasedonanall-fiber
spatialopticalfilter
SusanaSilva,1,2L.Coelho,2R.M.André,2andO.Frazão1,*
INESCPorto,RuadoCampoAlegre687,4169-007Porto,Portugal
2
DepartamentodeFísicadaFaculdadedeCiências,UniversidadedoPorto,RuadoCampoAlegre687,4169-007Porto,Portugal
*Correspondingauthor:ofrazao@inescporto.pt
ReceivedMay17,2012;revisedJuly2,2012;acceptedJuly3,2012;postedJuly3,2012(Doc.ID168770);publishedAugust13,2012
Aspatialopticalfilterbasedonsplicemisalignmentbetweenopticalfiberswithdifferentdiametersisproposedforgasrefractometry.Thesensingheadisformedbya2mmlongopticalfiberwith50μmdiameterthatissplicedwithastrongmisalignmentbetweentwosingle-modefibers(SMF28)andinterrogatedintransmission.ThemisalignmentcausesaFabry–Perotbehavioralongthereduced-sizefiberanddependingonthelead-outSMF28position,itispossibletoobtaindifferentspectralresponses,namely,bandpassorband-rejectionfilters.Itisshownthatthespatialfilterdeviceishighlysensitivetorefractiveindexchangesonanitrogenenvironmentbymeansofthegaspressurevariation.Amaximumsensitivityof−1390nm∕RIUforthebandpassfilterwasachieved.Bothdeviceshaveshownsimilartemperatureresponseswithanaveragesensitivityof25.7pm∕°C.©2012OpticalSocietyofAmericaOCIScodes:060.2370,120.3180.
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Fabry–Perot(FP)cavitiesinopticalfibershavebeenmanufacturedformanyyearswithdifferentkindsofcon-figurations.Thefirstfiber-basedFPcavitydevelopedwasasingle-modefiber(SMF)sectioninwhichtheuncoatedcleavedendswereemployedasthemirrorsurfacesofthecavity[1].TheSMFwasplacedinabulkconfigurationandusedasanaccelerometerdevice.Later,itwaspos-sibletoobtainanintrinsicFPcavitybyformingreflectingmirrorsalongcontinuouslengthsofSMFwithafusion-splicingtechnique.EachreflectorwasbasedontwosplicedSMFs,oneofwhichhadadielectricthin-filmcoatingonitscleavedend[2].Itwasalsodemonstratedthatthesensorwassuitableforwavelengthmultiplexing.Recently,differenttypesofFPcavitiesusingphotoniccrystalfibers(PCFs)havebeenachievedfortemperatureandstrainsensing[3,4].AnothersolutionwastocreatealateralholeintotheSMFcorebymeansofafemtosecondlaserpulse[5].TheFPcavitywasusedforthemeasure-mentofliquidrefractiveindices(RIs),andasensitivityof1163nm∕RIunit(RIU)fortheRIchangeofwaterwasachieved.Recently,itwaspossibletoobtainanFPcavitybymisaligninganSMFsectionbetweentwoSMFcoreswiththepurposeofmeasuringtheRIofair[6].Thisconfigurationwasalsoexploredforthedevelop-mentofahighlysensitivetemperatureFPsensor[7]andaMach–ZehnderinterferometerforRImeasurement[8].Dependingoffibergeometry,sensitivitytoRIcanbeachievedintheregionbetween1000and3000nm∕RIU.InthisLetter,aspatialfilterbasedonanFP-typeconfigurationispresentedforgassensingduetoRIvar-iations.Twodevicesweredeveloped:band-rejectionfiltersandbandpassfilters.Thefabricationprocessofthesetwodevicesistouseasimplefusion-splicingmachineinmanualoperation.Thetemperaturesensitiv-ityofbothspatialfilterswasalsoanalyzed.
Todemonstratetheproposedconfiguration,thesen-singheadstructuresshowninFigs.1and2wereimple-mented.Itconsistsofa2mmlongsilicarodwith50μmdiametersplicedbetweentwostandardSMF28(with8.2μmand125μmofcoreandcladdingdiameters,respectively),whereastrongmisalignmentwasimposed
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sothatthesilicarodcouldnottouchthecoreofbothSMFs.ThefiberdevicebehavessimilarlywithanFPin-terferometer,eitherintransmissionorinreflection,inagreementwiththemisalignmentpositionbetweenthesilicarodandthelead-outSMF[seethesensingheadschematicofFigs.1and2(a)].Experimentshaveshownthat,usingasilicarodwithlengthsbelow1mm,allowsFabry–Perotcavitiestoappearintheairwithatwo-waveinterferenceresponse[6,7].However,forlengthsgreaterthan5mm,nospectralresponseisattainedbecausethefiberistoolongandthereisnotenoughpowertoreachthelead-outSMF.
Fig.1.(Coloronline)(a)Schematicand(b)photoofthesensinghead;(c)opticalspectrumoftheband-rejectionfilter.©2012OpticalSocietyofAmerica
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Fig.2.(Coloronline)(a)Schematicand(b)photoofthesensinghead;(c)opticalspectrumofthebandpassfilter.
Inthefabricationprocess,afusion-splicingmachineinmanualoperationwasusedwiththefollowingpara-meters:electricarcof70mAanddurationtimeof400ms.ThedischargewasappliedintheSMFregionwithanoffsetof20μm.Thesplicingprocessisveryimportantbecauseitmustbeensuredthatthesilicaroddoesnottouchthecoreofbothlead-inandlead-outSMFs.Thefirstsplice(betweenthelead-inSMFandthesilicarod)wasmonitoredinrealtimewithanopticalspectrumanalyzer(OSA),andtheFresnelreflectionatthefiberendwasobserved.Thesecondsplice(betweenthesilicarodandthelead-outSMF)wasmonitoredintransmissionduringthemisalignmentofthefiberstoobtainthede-siredopticalspectrumoftheintendednewdevice.Inthisapproach,weusedabroadbandsourceinthe1550nmspectralrangewithabandwidthof100nmandanOSAwith5pmresolutiontointerrogatethesensingstructuresintransmission.Theopticalspectrumandsen-singheaddetailofeachfiberdeviceimplementedintheexperimentisshowninFigs.1and2,forband-rejectionandbandpassfilters,respectively.
Theresultsshowthat,inagreementwiththemisalign-mentbetweenthesilicarodandthetwoSMFs,afilteringspectralbehaviorcanbeattained.Theband-rejectionfilterpresentsfourwell-definednotchresonancesdis-tancedby22.4nm,whereeachonehasaFWHMof∼0.5nmandafinesseof44.8;thebandpassfiltercon-tainsfiveresonancesdistancedby21nm,eachonewithaFWHMof∼3nmandamuchlowerfinesseof7.
RecallingthattheSMFcoredoesnottouchthesilicarod,thelightthatcomesfromthelead-inSMFenterslat-erallyintothefiberwithanincidenceangleθ.However,
Fig.3.(Coloronline)Media1:Intensitydistributionofelectricfieldonthe(a)xzplaneandthe(b)–(e)xyplaneatdifferentfiberlengths.
theinputlightfieldwillnotbeenoughtoexcitethelong-itudinalmodesalongthesilicarod;instead,somebeamsarecoupledintothesilicarodandtravelwithmultipleinternalreflections[seeFigs.1and2(a)].Thesetofwavesthatleavethesilicarod,eitherbyreflectionortransmission,areparallelwitheachotherandonlyafewbeamsarecoupledintothelead-outSMFduetothenumericalapertureoftheSMF.Thefinesseattainedforeachspatialfilterishigherthanatwo-waveinterferom-eter.However,whencomparedwitheachother(44.8fortheband-rejectionfilterand7forthebandpassfilter),thedifferenceissignificantbecausethelightcouplingintothelead-outSMFishighlydependentontheSMFcorepositionandthesilicarod.
Therefore,anSMFlongitudinalcore-to-corealignment(withlargelateraloffsetofthesilicarod)originatesaspectralbehaviorsimilartoanFPinreflection—i.e.,band-rejectionfilter[seeFig.1(a)]—whiletheoppositelongitudinalmisalignmentofthetwoSMFcausesanFP-likeopticalspectrumintransmission,i.e.,bandpassfilter[seeFig.2(a)].
Athree-dimensionalsimulationbasedonthebeampropagationmethodwasusedtoinvestigatethebeambe-haviorinsideofthe50μmfiberoftheproposedfiberstructureintransmission(seeMedia1).Themodelingoflightpropagationwasdoneforapuresilicafibersec-tionwith50μmdiameter,arefractiveindexof1.444andatotallengthof2mm;however,hereitispresentedonlyasa650μmlongsectionforbettervisualizationofthelightpropagationalongthesilicarod.Also,SMFswith
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Fig.4.(Coloronline)Wavelengthshiftversusrefractiveindexvariationofnitrogenforthe(opencircle)band-rejectionand(solidcircle)bandpassopticalfilters.
coreandcladdingdiametersof8.2and125μm,respec-tively,wereusedforbothlightinputandoutput,allhav-ingacircularcrosssection.Thelead-inSMFcorewasassumedtohavealargelateraloffsetrelativelytothesilicarod,i.e.,distanced∼8μm(centeroftheSMFcore)withrespecttothelateralsideofthesilicarod,asshowninFig.3(a).Thisresultclearlyshowsthatthelightfieldleavingthelead-inSMFenterslaterallyintothesilicarod,causingmultipleinternalreflectionsalongthedirectionofpropagation,similarwithanFPbehavior.Figure3from3(b)to3(e)showstheintensitydistributionofanelectricfieldonthexyplaneatdifferentfiberlengths,namely,60,84,230,and620μm[alsomarkedinFig.3(a)].Onecannoticeaseriesofoverlappingbeamsthatoccurinsidethesilicaroddiameterwhilethelightfieldtravelsalongthefibersection;also,duetothecircularfibergeometry,thesimulationpresentsthefocusoflighttothecenterofthesilicarod.
ThebehavioroftheproposedstructuresforRIsensinginagaseousenvironmentwasdulycharacterized.EachfiberfilterwasplacedinaclosedchamberwithanNenvironment(100%)andsubmittedtoincreasingpres-2sureintherangeof0to1MPa.ChangingthepressureofthegasallowstheN2concentrationtoprevailwhiletheamountofnitrogenmoleculespervolumeincreaseswithincreasinggasRI[9].Figure4showstherelation-shipbetweenthewavelengthshiftwithRIofeachsensingheadastheresultofgaspressurevariation.Alinearbehaviorforbothfiberdevicescanbeobservedwhenthegaspressureisincreased.Sensitivitiesof−1375and−1390nm∕RIUfortheband-rejectionandbandpassfilters,respectively,wereachieved.Thepressurevaria-tionishighlydependentofthegasRIused,withaslopeof2.7×10−6RIU∕kPa[9].
Theresponseofthesensingheadstotemperaturevar-iationswasalsocharacterized,asshowninFig.5.Each
Fig.5.(Coloronline)Wavelengthshiftversustemperaturevariationforthe(opencircle)band-rejectionand(solidcircle)bandpassopticalfilters.
structurewasplacedinatubefurnaceandsubmittedtoatemperaturevariationof300°C,with20°Csteps.TheresultsdepictedinFig.5indicatethatbothstructuresexhibitlinearresponsestotemperaturevariationswithsimilarsensitivities,25.6and25.9pm∕°Cforband-rejectionandbandpassfilters,respectively.Consideringthattheresonantlighttravelsalongthesilicarod,thesensitivityresultsaredeterminedbythetemperaturedependenceoftherefractiveindexandthethermalexpansionofthisfiber.
Summarizing,aspatialfilterbasedonaFP-typecon-figurationwasdemonstratedforgassensingduetoRIvariations.Twodevicesweredeveloped:band-rejectionandbandpassfilters.Thesensitivityisapproximately−for1390thenmband-rejection∕RIUforthebandpassfilter.Thefiltertemperatureand−1375responsenm∕RIUwasalsoascertainedwithanaveragesensitivity25.7pm∕°C.
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