Biochemical sensor based on a novel all-fibre cavity ring down spectroscopy technique

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Biochemicalsensorbasedonanovelall-fibrecavityringdownspectroscopytechniqueincorporatingatiltedfibreBragggrating

KaimingZhoua,Ã,DavidWebba,MarkFarriesb,NeilHayesc,LinZhanga,IanBennionaaPhotonicResearchGroup,AstonUniversity,B47ET,UnitedKingdom

bGooch&HousegoTorquayLtd.,TQ27QL,UnitedKingdom

cEvanesCoLtd,NewtonAbbot,TQ124ADUnitedKingdom

articleinfoKeywords:CavityringdownLong-periodgratingTiltedfibreBragggratingOpticalfibresensorabstractAnovelall-fibrecavityringdownspectroscopytechniqueisproposedwhereatiltfibreBragggrating(TFBG)orlong-periodgrating(LPG)inthecavityprovidessensitivitytosurroundingmedium.SuchconfigurationwithanLPGastherepresentativewastheoreticallyanalyzed.TwospectralbandswereidentifiedemployableforsensingofsurroundingrefractiveindexforaweakLPGwhileonlyonebandexistedforastrongLPG.ATFBG,withenhancedsensitivitycomparedtousualLPGs,wasusedinaringdowncavityof1mconstructedwith2fibreBragggratingsasthereflectorsandthedecaytimechangedfrom220to450nswhentheTFBGwasimmersedintowaterfromair.&2009ElsevierLtd.Allrightsreserved.

1.IntroductionCavityringdownwasfirstdevelopedtoevaluatemirrorsofhighreflectivitythatareotherwisedifficulttocharacterize[1].Nevertheless,itwaslaterfoundtobeasuitablemethodtodetectveryweaklossesinacavitythatcouldbecausedbyabsorptionofmoleculesoftracegases[2],andsincethencavityringdownspectroscopy(CRDS)hasbecomeanimportanttooltoanalyzegas-phasematerials[3].ManyaspectsofCRDShavebeendeveloped,suchastheworkingwavelengths,theuseabletargetsandthecavityconfiguration,etc.Asfarasthelatterpointisconcerned,theliteraturecoverscavitiesformedbymirrors[2],prism[4,5],andalsoinopticalfibres[6–11].Recently,CRDSschemesconstructedfromopticalfibreshavebecomemoreandmoreofinterestbecausealignmentdifficultiesareremovedandlonginteractionlengthsarepossible.Opticalfibre-basedCRDSsystemswereinitiallyconstructedbyplacingmirrorsformedbydielectricfilmsatthetwoendsofthefibrebutthedesirabilityofconnectingthecavitytootherfibrecanbringunwantedeffectstothesefilms,suchaspressure-inducedrefractiveindexchangeofthefilm[11].WiththeemergenceoffibreBragggratings(FBGs),dielectricfilmscouldthenbereplacedwithapairofFBGsthatactaswavelengthselectivemirrorsmadeinsidethefibre[9].Afibreloop-basedringdownschemetomeasurepressurehasalsobeenreportedrecently[7].However,sofarapplicationsoffibre-basedCRDShavebeenlimitedbecausethelightinthecoreofthefibreiswellscreened

fromthetargetmaterialsurroundingthefibre.Somesolutionstothisproblemhavebeenbroughtforwards.Stewartandcoworkersusedamicro-opticalcellinthefibrelink[8],butbydoingsotheadvantagesofanallfibredevicewerecompromisedandactivefibrehadtobeusedtocompensatefortheextralossincurred.Etchingortaperingthefibreinthecavitytoexposetheevanescentwavewerealsoreported,buttheconfigurationthenbecomesfragile[11].Inthispaper,wereportonanovelmethodinwhichsensitivitytosurroundingrefractiveindexisprovidedbyincorporatingalong-periodgrating(LPG)ortiltedfibreBragggrating(TFBG)withinthecavity.

2.TheoreticalanalysisAnLPGisafibregratingstructurethatcoupleslightofcertainwavelengthsfromthecoremodeintoforward-propagatingcladdingmodesfromwherethelightislostbyscatteringandabsorption.LPGsrevealthemselvesasaseriesofattenuationbandsinthetransmissionspectrumofthecoreguidedlight.Althoughbackward-propagatingcladdingmodescouldalsobeexcitedbyFBGs,wewillconcentrateontheforwardcouplingregimeinthispaperandleavethebackwardcouplingtofuturework.BesidesLPGsandFBGs,anotherlessfamiliargratingtype,theTFBG,isalsocapableofcouplinglightfromthecoremodeintobothforward-andbackward-propagatingcladdingmodes,de-pendingontheangleoftilt;thesestructurespossesssomeuniquepropertiesthatmakethemattractiveforsomesensingtasks[12].Whenthetiltangleofthegratingisbigenough,modecouplingtoforward-propagatingcladdingmodesisenabled,andthusthedevicecanbeconsideredtobeatypeofLPG.

ContentslistsavailableatScienceDirectjournalhomepage:www.elsevier.com/locate/optlasengOpticsandLasersinEngineering

0143-8166/$-seefrontmatter&2009ElsevierLtd.Allrightsreserved.doi:10.1016/j.optlaseng.2009.04.001

ÃCorrespondingauthor.

E-mailaddress:k.zhou@aston.ac.uk(K.Zhou).

OpticsandLasersinEngineering](]]]])]]]–]]]Pleasecitethisarticleas:ZhouK,etal.Biochemicalsensorbasedonanovelall-fibrecavityringdownspectroscopytechniqueincorporatingatiltedfibreBragggrating.OptLaserEng(2009),doi:10.1016/j.optlaseng.2009.04.001