Photonic crystal fiber

Overview

PhotonicCrystalFibers(PCF),alsoknownasMicro-StructuredFibers(MSF),haveattractedwidespreadattentioninrecentyears.Itscross-sectionTherearemorecomplexrefractiveindexdistributions,usuallycontainingporesindifferentarrangements.Thedimensionsoftheseporesareroughlythesamemagnitudeasthewavelengthofthelightwaveandrunthroughtheentirelengthofthedevice.Thelightwavecanbeconfinedtopropagateinthefibercorewithalowrefractiveindex.

Theconceptionoftheconcept

Theconceptofphotoniccrystalsfirstappearedin1987.Atthattime,itwasproposedthattheelectronicbandgapofsemiconductorshasaperiodicmediumstructuresimilartooptics.Oneofthemostpromisingareasistheapplicationofphotoniccrystalsinopticalfibertechnology.Themaintopicitdealswithistheperiodicmicrostructureofhigh-indexfibers(theyareusuallycomposedofairholeswithsilicaasthebackgroundmaterial).Thefibersinquestionareoftencalledphotoniccrystalfibers(PCFs),andthisnewtypeofopticalwaveguidecanbeeasilydividedintotwodistinctgroups.Thefirsttypeoffiberhasahighrefractiveindexcorelayer(usuallysolidsilicon)andissurroundedbyatwo-dimensionalphotoniccrystalcladding.Thesefibershavepropertiessimilartoconventionalfibers,andtheirworkingprincipleistoformawaveguidebytotalinternalreflection(TIR);comparedtotraditionalrefractiveindextransmission,theeffectiverefractiveindexofthephotoniccrystalcladdingallowsahigherrefractiveindexofthecorelayer.Therefore,itisimportanttonotethattheseso-calledtotalinternalreflectionphotoniccrystalfibers(TIR-PCFs)areactuallycompletelyindependentofthephotonicbandgap(PBG)effect.Anotherfiber,whichiscompletelydifferentfromTIR-PCFs,itsphotoniccrystalcladdingshowsthephotonicbandgapeffect,whichusesthiseffecttocontrolthebeaminthecore.Thesefibers(PBG-PCFs)exhibitconsiderableperformance,themostimportantofwhichistheabilitytocontrolandguidethebeamtopropagateinacorewithalowerrefractiveindexthanthecladding.Incontrast,totalinternalreflectionphotoniccrystalfibers(TIR-PCFs)werefirstmanufactured,andtruephotonicbandgaptransmissionfibers(PBG-PCFs)haveonlyrecentlybeenexperimentallyproven.

Fabricationofphotoniccrystalfiber

Thefocusofthedesignanddevelopmentofnewopticalfibersisthecontrolofthedrawingprocessandtheselectionofmaterialsused.Thetraditionalsingle-modefiberrequiresthecoreandcladdingmaterialstohavesimilarrefractiveindexes(generally,therefractiveindexdifferenceisabout1%),whilethephotoniccrystalfiberrequiresalargerefractiveindexdifference,reaching50%to100%.Thesmallrefractiveindexdifferenceinordinaryopticalfiberisoftenusedtoobtaintherequiredpreformbyvapordepositiontechnology,whilethelargerefractiveindexdifferencerequiredbyphotoniccrystalfiberisusuallyusedtomakethepreformbytubestackingtechnology.

Thetypicaldrawingprocessofphotoniccrystalfiber:First,completethedesignandproductionofthepreform.Thepreformcontainsthedesignedstructure;thenthepreformisplacedinthefiberdrawingtower,usingordinaryThedrawingmethodoftheopticalfiberdrawsaphotoniccrystalfiberthatmeetsthesizerequirementsundermoreprecisetemperatureandspeedcontrol.Inthedrawingprocess,byadjustingthepressureoftheinertgasinsidethepreformandthedrawingspeedtomaintainthesizeratiooftheairholesinthefiber,aseriesofphotoniccrystalfiberswithdifferentstructuresareobtained.Someresearchgroupsalsoreportedsomespecialpreformfabricationmethods,whichcanbeusedtodrawphotoniccrystalfibersofspecialmaterialsorstructures.Forexample,becausethesoftglassmaterialisnotaseasytoformatubeassilicon,theordinarymethodofmakingpreformsbystackingtubesisnotsuitable,andthenewtechnologyofdirectlysqueezingthepreformscanproducephotoniccrystalfiberpreformsofthistypeofmaterial.Thephotoniccrystalfiberpreformofpolymermaterialcanbemadewellbythemethodsofstacking,punchinganddrilling.Throughauniquerollingcigartechnology,polymerandglassaresynthesizedintoaphotoniccrystalfiberwithaBraggstructure.Ontheotherhand,P.Falkensteinetal.insertglassmaterialsthatcanbecorrodedbyacidintotheglassrodsthatconstitutethepreform,arrangethemaccordingtothedesignrequirementsandmeltthem,andthenuseacidtocorrodetheunwantedpartstoformairholes.Thepreformformedbythemethodcandrawaphotoniccrystalfiberwithamoreperfectstructureandmoreinlinewiththedesignrequirements.

Classificationofphotoniccrystalfibers

Photoniccrystalfiberscanbedividedintotwocategoriesaccordingtotheirlightguidemechanism:refractiveindexlightguidetype(IG-PCF)andbandgapguidedtype(PCF)).Band-gapphotoniccrystalfiberscanconstrainlighttopropagateinacorewithalowrefractiveindex.Thefirstphotoniccrystalfiberwasbornin1996.Itwasasolidcoresurroundedbycylindricalholesinaregularhexagonalarray.Thiskindoffiberquicklyprovedtobebasedontherefractiveindexoftotalinternalreflectiontoguidelight.Thetruebandgapguidedphotoniccrystalfiberwasbornin1998.Inthebandgapphotoniccrystalfiber,therefractiveindexofthelightguidecenterislowerthantherefractiveindexofthecladding.Hollow-corePCF(HC-PCF)isacommonbandgapphotoniccrystalfiber.Photoniccrystalfibersaremainlydrawnbystacking,andinsomecases,hardmolds(die)areusedtoassistinmanufacturing

Refractiveindex-guidedphotoniccrystalfiberscanbedividedinto:non-stopsinglemodel,enhancedNon-lineareffecttypeandenhancednumericalaperturetype,etc.Thephotonicbandgapphotoniccrystalfibercanbedividedinto:spiderwebvacuumtypeandBraggreflectiontype.

Animportantparameteroftheopticalfiberisthepowerlosswhentheopticalsignalistransmittedintheopticalfiber.Inthepast30years,duetothegradualimprovementoftechnology,thelossinordinaryopticalfiberhasbeendecreasing,andnowithastendedtointrinsicloss.Thewavelengthwiththelowestlossinthefusedsilicafiberisabout1550nm,andthelossatthiswavelengthisabout0.12dB/km.Forthephotoniccrystalfiber,thelossinthesolid-corephotoniccrystalfiberisbelow1dB/km,andthelowestlosshasreached0.28dB/km,whichisequivalenttoordinaryfiber.Sincethetransmissionmechanismisthesameasthatofordinaryfiber,thelossofsolid-corephotoniccrystalfiberisunlikelytobegreatlyreduced.Forphotonicbandgapphotoniccrystalfibers,thelowestlossreportedrecentlyis1.2dB/km.Thehollowstructuremakesthistypeofphotoniccrystalfiberhavealowerintrinsiclosslimit[,sothereportedvalueisfarfromreachingtheintrinsiclossvalue.

Index-guidedphotoniccrystalfiber

Thistypeoffiberiscomposedofapuresilicacoreandacladdingwithaperiodicairholestructure.Duetotheadditionofairholes,thecladdinghasasmallereffectiverefractiveindexthanthecore,thatis,becausetheeffectiverefractiveindexofthesilicaaircladdingissmallerthanthatofthecore,thephotoniccrystalfiberwiththisstructureissimilartothetotalinternalemission.Themechanismoflightguideissimilartoordinaryopticalfiber.Therefore,asimpleanalysismethodistoequatethistypeofphotoniccrystalfibertoarefractiveindexsteptypefiber.Aftertheeffectiverefractiveindexofthecladdinglayerisobtained,themethodofrefractiveindexsteptypefibercanbeusedforanalysisandcalculation.Byadjustingthestructuralparametersofthepreform,thephotoniccrystalfiberwiththerequiredstructureandsizecanbeobtained,whichhasaveryflexibledesignfreedom.Differentairholestructuresandarrangementsmaketheindex-guidedphotoniccrystalfiberhavespecificmodetransmissioncharacteristics.Inparticular,thestudyalsofoundthattheperiodicarrangementofairholesinthecladdingoftheindex-guidedphotoniccrystalfiberisnotnecessary.Randomarrangementofenoughairholescanalsoeffectivelyreducetherefractiveindexofthecladdingandachieveanimprovedtotalinternalreflection.Therefore,thiskindoffiberisdifferentfromtheearlyproposedphotoniccrystalfiberwithperiodicarrangementofairholes.Inordertohighlightthefeaturethattheairholesoftheorderofwavelengtharearrangedinthecladding,theindex-guidedphotoniccrystalfiberismoresuitabletobecalledHolefiberormicrostructurefiber.

Photonicbandgapphotoniccrystalfiber

Comparedwithindex-guidedphotoniccrystalfiber,photonicbandgapphotoniccrystalfiberrequiresstrictperiodicityofthecladdingairholestructure.Whenthefibercoreisintroducedtodestroytheperiodicstructure,adefectstateorlocalstatewithacertainbandwidthisformed,andonlylightwavesofaspecificfrequencycanpropagateinthisdefectarea,andlightwavesofotherfrequenciescannotpropagate.,Namelythephotonicbandgapeffect.Underthislightguidingmechanism,thecorecanbedesignedasahollowstructure.Theextremelylownonlineareffectandtransmissionlossofthephotoniccrystalfiberwiththisstructuremakeithavegreatpotentialadvantagesinthetransmissionofhigh-energylaserpulsesandlong-distanceinformationtransmission.

Inthe1960s,beforethedevelopmentofmodernsiliconopticalfibertechnology,capillarytubeswereoncestudiedasasubstituteforcommunicationopticalwaveguides.Nowadays,thecommonhollowfiberistocoataverythincapillarywithareflectivefilmontheinnersurfacetoenhancethereflectivity,andtoguidelightthroughinternalreflection.Thistechnologyiswidelyusedintheinfraredband.Afterall,itisrelativelysimpletomakelargerairholes,andthecoatingiseasiertoimplement.Butbecausethecoatingisafterthefiberisdrawn,thelengthofthisfiberisrelativelyshort,andthetransmissionmodequalityispoor.Forthephotonicbandgapphotoniccrystalfiber,thefiberdrawingprocessreducesthesizeoftheairholeinthetransversedirectionofthepreformtotheorderofthelightwavelength,andnomoreprocessesarerequired.Thistechnologyhasproducedaverylonghollowphotoniccrystalfiberandcanadjustthecharacteristicsoftheguidedwavemodebychangingthecladdingstructure.

Photonicbandgapphotoniccrystalfibershavemoreroomfordevelopment.Itmayhavelowertransmissionlossthanordinaryopticalfibers,makingthemlikelytobecomeanewforceinfuturecommunicationtransmissionsystems;higherdamagethresholdsthanordinaryopticalfibers,makingthemsuitableforstronglasertransmissionforthepurposeoflaserprocessingandwelding;hollowThestructureprovidesmoreexperimentalsolutionsfornonlinearopticsingas,forexample,itcanformasinglegasmicrocavitywithnodiffractionandlosslimit.ItisreportedintheliteraturethataphotonicbandgapphotoniccrystalfiberfilledwithhydrogencanbeusedasamicrocavityforstimulatedRamanscatteringexperiments.ThethresholdofstimulatedRamanscatteringinthisfiberistwoordersofmagnitudelowerthanthepreviousexperiment.Undertheguidanceofsimilarideas,photonicbandgapphotoniccrystalfiberscanbeusedforgasdetectionorcontrol,orasagainmicrocavityforgaslasers.

Photoniccrystalfiber(4sheets)

Characteristicsandapplicationofrefractiveindexlightguidetypephotoniccrystalfiber

RefractiveindexguidetypeThephotoniccrystalfiberhasthecharacteristicsofnon-stopsinglemode,largemodefieldsize/smallmodefieldsize,anddispersionadjustablecharacteristics(adjustmentofd,Λ,etc.,withoutdoping).Itiswidelyusedindispersioncontrol(flatdispersion,zerodispersionshiftcanreach800nm),nonlinearoptics(highnonlinearity,supercontinuumgeneration),multi-corefiber,activefiberdevice(double-cladPCFeffectivelyconfinesthepumplight)Andopticalfibersensingandotherfields.

Characteristicsandapplicationsofbandgapphotoniccrystalfiber

Thegapbandgapphotoniccrystalfiberhaseasycoupling,noFresnelreflection,lowbendingloss,lownonlinearityandspecialwaveguideDispersionandothercharacteristicsarewidelyusedinhigh-powerlightguides,opticalfibersensingandgasopticalfibers.Thedevelopmentofphotoniccrystalfibershasopenedupabroadspaceforopticalfibersensing,especiallyinbiosensingandgassensing,bringingnewdevelopmentstoopticalfibersensingtechnology.

Developmentandapplicationprospects

Photoniccrystalfiberovercomesthelimitationsoftraditionalfiberopticsandbringsnewpossibilitiesandopportunitiesformanynewscientificresearches.Althoughonlyasmallpartoftheresearchteamcanmanufacturethiskindofphotoniccrystalfiber,theextremelyfastdevelopmentspeedandveryeffectiveinternationalscientificcooperationhaveenabledtherapiddevelopmentoftheapplicationofphotoniccrystalfiberinmanydifferentfields.ThemosttypicalexampleisacollaborationinvolvingresearchersfromtheUniversityofBathintheUnitedKingdom.Thephotoniccrystalfibertheyproducedwassuccessfullyusedinthehigh-precisionopticalmeasurementconductedbytheresearchteamledbyProfessorT.HanschoftheGermanPlanckInstituteforQuantumPhotonics.middle.Itisworthmentioningthatthetimeintervalfromthediscoverythatthephotoniccrystalfibercangeneratesupercontinuumspectrumtoitsapplicationinphotometricsisonlyafewmonths,andProfessorT.Hanschisduetothemeasurementofultra-precisionspectroscopy.Ithasmaderemarkableachievements,especiallyformakingimportantcontributionstoperfectingthe"lightcomb"technologyandwonthe2005NobelPrizeinPhysics.Photoniccrystalfibersareaffectingmanyfieldsofmodernscienceatanextremelyfastspeed.Usethephotonicbandgapstructuretosolvesomebasicproblemsinphotoniccrystalphysics,suchasstrengtheningthelocalfield,controllingthetransmissionofatomsandmolecules,enhancingnonlinearopticaleffects,studyingelectronsandmicrocavities,andradiationmodecouplinginphotoniccrystalsTheelectrodynamicprocessandsoon.Atthesametime,experimentalandtheoreticalresearchresultsshowthatphotoniccrystalfiberscansolvemanynonlinearopticalproblems,generatebroadbandradiation,ultra-shortopticalpulses,improvetheefficiencyofnonlinearopticalfrequencyconversion,andbeusedforopticalswitching.Itisnotdifficulttoimaginethatinthenearfuturewewillfindmorepropertiesandmoreapplicationfieldsofphotoniccrystalfibers.