红外玻璃

Infraredglasses

JacquesLucas*

´´deRennes1,35042RennesCedex,France,CNRSUMR6512,CampusdeBeaulieu,UniversiteLaboratoiredesVerresetCeramiques

Abstract

Drivenbyapplicationsinhotfieldssuchasopticalcommunications,lasers,sensors,etc.infraredglasseshavetobeconsideredaskey

componentsinthedevelopmentofdevicesfortelecomsignalamplification,fibre-laseremissionaswellasforpassivefunctionsrelatedtoIRremotespectroscopyorthermalimaging.Stablevitreousmaterialswithlow-phononenergiesarefoundinthefamilyoffluoridesandchalcogenidesglasses;theyoffertheadvantageofexcellenttransparencyinthemid-IRandweaknonradiativerelaxationwhendopedwithrareearthelements.Despitethenumberofcandidatesonlyaverylimitednumberofglasscompositionscanbeshapedintogoodopticalwaveguidessuchaschannelorfibre.Whenpossible,thisledtoremarkableamplificationinthe1.3mmregionandlasingemissionintheblueormid-IR.Non-linearopticalpropertiesofchalcogen-basedglassesarealsoofspecialinterestforfastallopticalswitchingandphoto-inducedeffects.©1999ElsevierScienceLtd.Allrightsreserved.

1.Introduction

Theinsatiableneedforhighcapacitytelecommunicationsystemsresultsinapermanentreconsiderationofthepotentialofanallopticalnetwork.Inordertosatisfythedemandformorebandwidth,wavelengthdivisionandtemporalmultiplexingarenecessarytotakeadvantageofthetelecommunicationwindowofferedbysilicafibresandwhichextendsroughlyfrom1.2mmto1.6mm.Opticalamplificationinthisspectralregioniscriticalinordertoregeneratethetelecomsignalinusingmultiplexingandthemaximumpotentialofalltherareearth(RE)ionsemittinginthewindow.ThreemainobjectiveswhereIRglasses[1]canplayakeyrolecanbeidentified:(1)theobtainingofflatgainamplificationinagivenspectralrangeforexamplearoundthe1.5mmemissionofEr31.(2)TheuseofotherREelements[2]forcoveringtherestofthewindowtakingintoaccountthattheyaresometimestotallyinactiveinsilicaglassbuthaveagoodefficiencyinlowphononmatrices:Pr31emissionat1.3mmgives,inthatrespect,agoodillustrationofthesituation.(3)Theopportunitytotakeadvantageoftheexcellentnon-linearpropertiesofchalcogenideglassesforultrafastallopticalswitching.

Outoftheveryactivetelecomfield,thereisalsoademandfornewlasersourcesoperatingintheUV-blueaswellasinthemid-IRregionwhichisnotwellcoveredbythesemiconductorlasers.Fluoridefibrelasersofferonthis

*Tel.:133-2-9929-6260;fax:133-2-9928-1600.

E-mailaddress:jacques.lucas@univ-rennes1.fr(J.Lucas)

pointarealopportunitybecauseoftheirlargeopticalwindowandthepossibilityofup-conversionphenomena.Finally,passivefunctionsrelatedtothetransparencyoftheatmosphereinthe3–5and8–12mmregionsaswellastherichnessofthemid-IRsignaturesforalltheorganiccompoundshaveenhancedtheinterestofusingIRglassfibresforremotespectroscopyandopticsforthermalimagingsystems.

2.Glassesforopticalamplificationandfibrelaser

2.1.Opticalamplification

Itistraditionallyadmittedthatthetelecomopticalnetworkhadtooperateinthetwomosttransparentwindowsofthesilicafibreslocatedaround1.3and1.5mm;thefirstwavelengthisinterestingduetoitslocalisa-tioninthelowdispersionregionofthefibre,thesecondbecauseitcorrespondstothelowlossregion.TheveryrecentprogressintheSiO2fibrefabricationresultinginlowOHimpuritiesissuchthatacompletelytransparentwindowlyingfrom1.2to1.6mmcannowbeconsidered.TheneedforahighcapacitynetworkledalsotoincreasetheopticalamplificationefficiencyinthisspectraldomaininusingalltheREelementsemittinginthiswindowsuchasPr31,Nd31[3],Dy31[4]around1.35mm,Tm31at1.45and1.65mmandofcoursetheveryefficientEr31ionforthestrategic1.55mmregion.Flatgainamplificationisalsovitalinordertoincreasethetransmissioncapacityofwavelengthdivisionmultiplexing(WDM)systems[5,6].

1359-0286/99/$–seefrontmatter©1999ElsevierScienceLtd.Allrightsreserved.PII:S1359-0286(99)00007-8

J.Lucas/CurrentOpinioninSolidStateandMaterialsScience4(1999)181–187183

Fig.2.Transmissionspectraofseveralinfraredglassescomparedtosilica.ThetwoglassesZBLANandBIGaremulticomponentfluorideglassesbasedonZrF4andBaF2forthefirstoneandBaF2,InF3,GaF3forthesecond.Theglass2SGisaseleniumbasedmaterialcontainingtheelements:Se,Sb,GeandGa.TheTeXglassisbasedonthecombinationofTe,SeandI2whiletheTeXAsisasimilarglasswiththeadditionofAs.

HoyateaminmovingfromfluoridetochalcogenideglassesanddevelopingacompositionbasedonGa2S3–Na2ScalledGaNaSwhichcanbedopedwithlargeamountofREandespeciallyPr31.Anextrusionmethodhasbeenusedtomakethepreformforsingle-modepropagation,resultingafterdrawinginafibrehavinganumericalapertureNAof0.31andacorediameterof2.0–2.5mmforacladdiameterofabout120mm.

Recently,opticalamplificationhasbeendemonstratedatwavelengthsdifferentfromtheusualtelecomwindowsat1.3and1.55mm.Thusahighgainamplifierwithinthe1.4–1.5mmspectralrangewhichisstillinthelowlossregionwillpermittoextendthebandwidthfortelecom-munications.ATm31dopedZBLANfibreupconversionamplifier[17]withamaximumgainof28dBat1.47mmhasbeenrealisedandthegainisgreaterthan10dBbetween1.44and1.51mm.Opticalamplifiershavealsobeendevelopedinthe1.6–1.65mmrange.Forexampleat1.65mmasignalgainof35dBwasachieved[18]withaZBLANfibreconsistingofaTm31-dopedcoreandaTb31-dopedcladdingtosuppresstheamplifiedsponta-neousemissionat1.75–2.0mm.

Thetechnicalchallengesforbuildingafibrelaseraresimilartothoserequiredforanopticalamplifierexceptthattworeflectingmirrorshavetobeputattheextremitiesofthesinglemodefibreinordertoobtainoscillations.Themostsignificantresultshavebeenalreadydiscussedbytheauthorandthisreviewwillemphasiseonlytheveryrecentcontributions.Inthefieldoffluorideglassestworesultswhicharesituatedatthetwoextremitiesoftheemittingspectralrangeareofspecialinterest.Theemissionof230

mWblueupconvertedlight[19]fromaTm31-dopedfluoridefibreexcitedbyadiodepumpedNd:YAGlaserandthecharacterizationofaHo31-dopedfluoridefibrelaser[20]emittinginthemid-IRregionat3.9mm.

ThefirstlaseractioninanRE-dopedchalcogenideglassfibrewasreported[21].Laseroscillationsat1080nmwereobtainedinaGaLaSglassfibrewithaNd31-dopedcorefabricatedbytherod-in-tubemethod.Someotheremis-sionsofSm31at1.8,2.9and4.3mmobservedinGaLaSglassesarealsoofpotentialinterest[22].

3.PassivefunctionsofIRglassesandsomephoto-inducedeffects

Inthissection,IRglassesareconsideredaspassivematerials,attractivebecauseoftheirexceptionaltrans-parencyinsomestrategicspectraldomainssuchasthetwoatmosphericwindowsinthe3–5mmandespeciallythe8–12mmregioncorrespondingtotheroomtemperaturethermalimaging.AlsothisspectralregionisofspecialinterestbecauseitcontainstheIRfingerprintsoftheorganicmaterialswhichliefromabout3mmtothe12mmregion.Asamatteroffactsomepropertiesarenottotallypassiveinthesensethatwhenirradiatedbylaserillumina-tionsomeglassessuchasthechalcogenidesexhibitnon-linearpropertiesresultinginultra-fastvariationoftherefractiveindex.Sometimestheirradiationcausesanirreversiblesituationresultinginalocal,permanentvari-ationoftherefractiveindexallowingwaveguidephoto-writing.

184J.Lucas/CurrentOpinioninSolidStateandMaterialsScience4(1999)181–1873.1.Third-ordernon-linearity

Againdrivenbypotentialapplicationsintelecommuni-cations,chalcogenideglasseshavefoundanewnicheinallfastopticalswitchingasdemonstratedbyAsobe[*23].Indeedwhencomparedtosilicathechalcogenideglassesexhibitmuchhighernon-linearproperties,illustratedbyathird-ordernon-linear2susceptibilityX3oranon-linearrefractiveindexnabout100timesgreaterthanthatofSiO2glass[24–26].Forapplicationstoultra-fastopticalswitching,single-modepropagationisnecessaryandfibreshavingasmallercoreandhigherrelativerefractiveindexdifferenceareadvantageousinreducingtheswitchingpower.ThebestresultshavebeenobtainedinusingtherodintubemethodratherthanthedoublecrucibleapproachandthefollowingperformanceshavebeenreachedforanAs2S3-basedfibre:switchingpowerof0.4Wforacoresizeof2.5mm,afibrelengthof4mandalossof0.6dB/m[*23].

Themainadvantageofchalcogenideglassesisthatwhentheoperatingwavelengthisratherfarfromtheresonantregion,namelytheelectronicabsorptionedge,thenon-lineareffectispurelyelectronic.Consequentlyultra-fastmaterialresponseduetothirdorderelectronicpolari-sationisguaranteed.Indeediftheexcitationwavelengthislocatedintheresonantregionhighnon-linearityisex-pectedbutessentiallyduetoabsorptionleadingtothermaleffects.Thedrawbacktothistypeofnon-linearityisthattheresponsetimeisgovernedbythermalrelaxation,inotherwords,veryslow.Kanbara[27]andHirao[28]haveexaminedthethird-ordernon-linearopticalpropertiesofAs–S–Seglassesbythirdharmonicgeneration,opticalKerrshutteranddegeneratefourwavesmixingmeasure-ments.Theauthorsconcludethatthischalcogenideglasscompositionwasagoodcandidateforcompactopticalswitchingdevicesandthatultra-fastresponsetimeoflessthansub-picosecondwasattainable.

Itisnowverywellacceptedthattheoriginofthisexceptionalhighelectronicpolarisabilityisduetothepresenceofnon-bondingelectroniclonepairslocatedonthechalcogenatoms.Thissituationdoesnotexistinsilicaglassesbutthepricetopayforthisisthepresenceofanon-bondinglevelinthebondingenergydiagramofthischalcogenide-basedmaterial.Theimmediateconsequenceisasignificantdecreaseofthebandgapenergyduetotransitionsbetweenthisnon-bondinglevelandtheimmedi-ateupperanti-bondinglevel.Itiswellknownthatmostofthechalcogenideglasseshaveapoortransparencyinthevisibleregionbeingsometimestotallyblackandthattheexpansionoftheabsorptionedgeintothetelecommunica-tionwindowrepresentsanintrinsicfactorlimitingseverelytheirinterestforapplicationsinthefield.

Itisclearthatabetterunderstandingofglassformingtendencyaswellastheinfluenceofhalogenatomsonthebandgapedgewouldbeofprimeinterestfordesigningthe

glasshavingthelargestgapandthebestfibringability[16].

3.2.Photo-refractivityinchalcogenideglasses

Theresearchofphoto-inducedeffectsfordevicessuchasBragggratings,microlensedesign,andthreedimension-almemorieshaveattractedalotofinterestbothonthemechanismofthephenomenaandthepotentialapplica-tions.ThestateoftheartforchalcogenideglasseshasbeenrecentlyreviewedbyTanakawhopioneeredthefield[29].ItisinterestingtonoticethatthisirreversiblemodificationofthelocalglassstructureunderlaserirradiationcanbeobtainedeitherbyilluminationinthebandgapregionusingaHe–Nelaserorbyultrashortpulseirradiation[30].Thephotonenergyisusedtomodifylocallythebondingconfigurationresultinginaso-calledphoto-expan-sionoftheirradiatedvolumeleadingtoapermanentmodificationoftherefractiveindex[31,32,*33,34].Thispeculiarphenomenonwhichcorrespondstoakindofstructuralrelaxationraisedthequestionofthethermo-dynamicsofsuchmaterialwhichisoutofequilibriumforthesecondtime,firstduringthecoolingandthenafterthephotonicprocess.Thedependenceofthephenomenonontheglasscomposition[35]andtheshapeoftheglasssample[36]isalsoofinterest.

He–Neilluminationhasbeenusedformicrolensefabrication[37]andBragggratingsinscriptiononasinglemodeAs–Sfibreinusingthetranverseholographicmethod[38].Thechangeintherefractiveindexwasestimatedtobeoftheorderof1024.Thisfibregratingfabricationwasprovedtobesuccessfulfordispersioncompensationintheallopticalswitchingsystemdiscussedbefore.Miura[30]intheframeoftheHiraoglassprojectalsodemonstratedthatpermanentwaveguidescanbeformedinbulkchalcogenideglassesbyphoto-inducedrefractiveindexchangewithanultra-shortpulselaser.ThelasershotswereproducedusingaTisapphirelaseroperatinginthefollowingconditions;wavelength800nm,speed120fs,repetitionrate200kHz.

3.3.IRglassfibresforpassiveapplications

Duringtheintensivesearchperiodinthe1980saimedatdevelopingultra-lowlossfibres,chalcogenideglasseswereconsideredaspotentialcandidatesbecauseofthebroadseparationbetweenthebandgapandthemultiphononedgesleadingtotheoreticaldeepV-shapecurve.Indeed,duetotheexistenceofaso-calledweakabsorptiontailresultinginasignificantspreadingoftheUrbachtailintothenearIRregionithasbeenobservedthattheexperimen-talopticallossinthemid-IRregionwasmoreinthedB/mrangethanintheexpecteddB/kmestimation.Neverthelessdespitetheirmodestopticalloss,IRglassfibresbasedonchalcogencanbeusedforshortdistanceapplications

186J.Lucas/CurrentOpinioninSolidStateandMaterialsScience4(1999)181–187ofpracticaldevices.IRglassfibresarealsopromiseda

richfutureforthermalimagingsystemsaswellasforremoteevanescentwavespectroscopyanditsapplicationsinmedicalandindustrialfields.

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