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Spectrally

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SpectrallySpectr

    Spectrally

SpectrallySlicedPulseSourceBasedonSupercontinuum

    ;GenerationinDispersionFIattenedPhotonicCrystalFiber

    ;XUYongzhao,ZHANGXia,HUANGYongqing.,RENXiaomin.

    ;(1.DepartmentofElectronicEngineering,DongguanUniversityofTechnology, ;Dongguan523808,CHN;2.KeyLaboratoryofOpticalCommunicationandLightwaveTechnologies

    ;MinistryofEducation,BeijingUniversityofPostsandTelecommunications,Beijing100876,CHN)

    ;Abstract:Designedisdispersion-flattenedphotoniccrystalfiber(PCF)withsmallnormaldispersionfor

    ;generatingflatwidebandsupercontinuum(SC),anddemonstratedisspectrallyslicedpulsesourcewhich

    ;utilizessupercontinuumgeneratedindispersionflattenedphotoniccrystalfiber.Theres

    ultsshowthatthefiber

    ;characterizedbyflatteneddispersionwithsmallnormaldispersionissuitableforflatwidebandsupercontinuum

    ;generation.Intheprocessofspectralbroadening,self-phasemodulationplaysadominantrole.Byfilteringthe

    ;supercontinuum,multi-wavelengthpulsescanbeobtainedoverawidespectralrange. ;Keywords:supercontinuum;dispersion;photoniccrystalfiber

    ;CLCnumber:TN253Documentcode:AArticleID:10070206(2008)03016205

    ;1IntrOductiOn

    ;Withthedevelopmentofphotoniccrystalfibers(PCFs),supercontinuum(SC)generationhasbeenthe

    ;focusofintenseresearchinrecentyearsandPCFswereproventobesuccessfulmediaforefficientSC

    ;generation.UltrawidebandlightsourcesbasedonSCgenerationhavealreadyfoundapplicationsin

    ;variousfieldssuchasopticalcoherencetomography,spectroscopy,frequencymetrologyandwavelength

    ;conversion.SCusedinopticalcommunicationisanotherattractiveapplication. ;Thereisincreasing

    ;interestinapplyingSCsourcestowavelengthdivision-multiplexing(WDM)opticaltra

    nsmission

    ;systems,forwhichmultichannelofSCpulsetrainaremodulatedascarrierwaves[5

    .Toenhance

    ;WDMnetworkswithhigherbitratesandlongertransmissiondistances,furtherresearchtoimprovethe

    ;flatnessofSCisneeded.TherequirementcanbemetbyoptimizationofthedesignofSCfiber

andthe

    ;pumpingcondition.

    ;PCFs,alsocalledholeyfibersormicrostructuredfibers,commonlyconsistofafusedsilicacore

    ;surroundedbyaregulararrayofairholesrunningalongthefiberlength. ;Theyhavetheadvantageof

    ;designflexibilityincontrollingthemodepropagationproperties[.Thechromaticdisper

    sionprofile

    ;canbeeasilycontrolledbyalteringthesizeandthearrangementofairholes.ThereforePCFsofferthe

    ;possibledesignofSCfiberwhichissuitableforSCgenerationinthete1ecommunicationwindowand

    ;meettherequirementofWDM.Inthispaper,wefocusonPCFdesignforflatandwidebandSCsource

    ;generationinthe1.55”mregion.Thedesignofadispersion—flattenedPCFispresentedand

    itisshown

    ;throughnumericalresultsthattheproposedPCFissuitableforflatandefficlentSCgenerationin

    ;Receiveddate:20080626;reviseddate:20080816

    ;Foundationitem:NationalBasicResearchProgramofChina(2003CB314906);FoundationforKeyProgramofMinistryof

    ;Education,China(104046)

    ;?

    ;162?

    ;

    ;VOI.14No.3XUYZ.eta1.SpectrallySlicedPulseSourceBasedoilSupercontinuumGenerationinDFPCF

    ;telecommunicationwindows,andthemultiwavelengthpulsesourcesbasedonspectrals

    licingofSCare

    ;alsodemonstrated.

    ;2DispersiOnnattenedPCFsDesignandSCSourceGeneration

    ;ThedecisiontousePCFsratherthanconventiona1singlemodefiberstodesignSCfiberis

    duetothe

    ;greatcontroloverthedispersionpropertiesandsimpledesigntechniqueoftheformer.Hereweutilize

    ;theadvantageouspropertiesofPCFtodesignadispersionflattenedPCFwithsmallnorm

    aldispersion.

    ;WeconsideraPCFcharacterizedbyatriangulardistributionofairfilledholeswithacentr

    a1fusedsilica

    ;core.Wehavesimulatedanumberofdifferentfiberdesignsbychangingthepitch(A)andtheairhole

    ;diameter(d)ofPCF.Afull-vectoria1solverEisusedtocalculatethefiberdispersion. ;Fig.1(a)and(b)presentsomeexamplesofflatteneddispersiondesigns.InFig.1(a),wekeepA

    ;2.42ttmwhilechangingdslightly,whereasinFig.1(b),wechangeAwhilekeeping0.59ttm.The

    ;calculationresultsshowthatthedispersioncurvemoveupwardwhilekeepingAconstantan

    dincreasing

    ;d,whereas,whilekeepingdconstantandincreasingAthedispersioncurvealsomoveupwar

    d.Fig.1

    ;indicatesthatwecanobtainaPCFwithflatteneddispersionstructureandsmallnormaldispe

    rsion

    tuningofbothAandd.Bychangingthefiberparameters,wec;throughanappropriatefine

    anadjustthe

    ;characteristicofthedispersioncurve. ;4

    ;.

    ;2

    ;

    ;0

    ;

    ;

    ;乏,2

    ;

    ;

    ;毒,4

    ;1

    ;-

    ;fh1

    ;d=05gpm

    ;{A=240um

    ;2.A241um

    ;3.A.242”m

    ;‘f}}}ll

    ;}li

    ;Fiber3Fiber2Fiber{

    ;_

    ;+

    ;iI.It’

    ;Wavelength/nmWavelength/nm ;Fig?1ExamplesofchromaticdispersionbehaviorofdispersionflattenedPCFs:(a)以一2.

    ;42m,0.584m,0.587m

    ;and0.59m(b)d=0.59m,A2,40m,2.4tmand2.42m ;PropagationofultrashortpulsesinPCFcanbedescribedbythenonlinearSchrodingereq

    uation

    ;(NSE),writtenasF1]

    ;O

;aU+a一薹P0(+)z,T):..dT’R(T);(1)

    ;WhereU(z,T)isthenormalizedenvelopeoftheelectricfield,zisthelongitudinalcoordinate

    a1ongthe

    ;fiber,Tisthetimeinareferenceframetravelingwiththepumppulse,isthe

    thorderdispersion

    ;coefficientatthecentralfrequency0,isthefiberloss,yisthenonlinearcoefficientandP

    ;nisthepeak

    handsideofEq.(1)inc1udesthechromat;poweroftheinputpulse.Thefirstitemoftheright

    ic

    ;dispersioncontributionsandlinearattenuation.Thesecondandthirditemsareresponsiblef

    ornonlinear

    ;effectsincludingself-phasemodulation(SPM),self-steepening(SST)andstimulatedRa

    manscattering

    ;(SRS).Eq.(1)canbenumericallysolvedby’usingsplit—stepFourieralgorithmandthepulse

    ;propagationissimulated.

    ;Weselectfiber3inFig.1(b)asaSCfiber,andconsiderthepropagationofthehyperbolicse

    cant

    ;inputpulsewithdurationof0.5ps(TFwHM)andacentralwavelengthof1550nm.Thevalue

    sare

    ;?

    ;163?

    ;【卜g=.gsj/IJ{ds【『Jg

    ;

    ;SemiconductorPhotonicsandTechnologyAug.2008 ;ThecalculatedresultiSshowninFig.2,inwhich ;theoutputSCspectrumwith10dBbandwidthof

    ;126nmoffersabroadenedandflattenedspectrum ;thatcanbeslicedoverawidewavelengthrange. ;Oneofthemostimportantapplicationsoftheflat ;SCgeneratedinthe1550nmbandisthedesignof ;multiwavelengthsourcesforWDMsystems.By ;slicingSCspectrum,wecanobtainmulti- ;wavelengthopticalcarriers.Here,weuseatunable ;Gaussianfiltertosliceoutpulse.Fig.3showthe ;slicedpulsefilteredoutfromSCspectruminthe ;rangefrom1510nmto1590nm.Thepulsewidths ;areapproximately4psoverallrangesandthetime ;bandwidthproductsAvAtarebetween0.43and ;0.45,whichindicatesthatthepulsesarenearly ;transformlimitedGaussianpulses.Theintensity ;variationsamongtheslicedpulsearelessthan3dB ;inthespectrumrangefrom1518nmto1587nm. ;Becausethespectrumisinthenormaldispersion

    ;region,theslicedpulseswithshorterwavelength ;laggedbehindthosewithlongerwavelength.The ;widthoftheslicedpulsedependsmainlyonthe ;Wavelength/nm

    ;FlatwidebandSCgeneratedinfiber3inFig.1(b) ;Fig.3SlicedpulsefilteredoutfromSCspectrumat ;differentwavelength

    ;passbandoftheslicingfilter.Bychangingthepassbandofthefilter,ashorteroflongerpulsetr

    ain

    ;wouldbeobtained.Ifweusearrayed-waveguidegrating(AWG)asaslicingfilter,multiw

    avelength

    ;pulsetrainscanbeobtainedsimultaneouslyandthepowerlevelsofthechannelscanbeequal

    izedwithan

    ;integrateddynamicpowerequalizer.’

    ;Becausethethirdorderdispersion(TOD)isnegligiblysmallandself-steepeningaswe11

    asRaman

    ;scatteringhavelittleinfluenceonthepulse ;propagationundernormaldispersion,thusaSC

    ;spectrumwithgoodsymmetryisgenerated.

    ;?

    ;IntheprocessofSCgeneration,self-phase ;modulation(SPM)isthedominantnonlineareffect ;responsibleforspectrumbroadening.Toinvestigate ;thegenerationofflatwidebandSC,weshowsthe ;evolutionsofeffectivepeakpowerandthe10dB

    ;spectralwidthofthepulsewithpropagation ;distanceinFig.4.Itindicatesthattheeffectivepeak ;powerdecreasesrapidlywhilethespectrumbroaden ;i

    ;{

    ;

    ;

    ;

    ;?

    ;

    ;

    ;Fig.4Evolutionsofeffectivepeakpowerand——10dB

    ;spectralwidthalongthepropagationdistance ;?p/suNLfUz?p/言营_!一日g誉乏g

    ;F

    ;了基ie(1

    ;

    ;VO1.14No.3XUYZ,eta1.SpectrallySlicedPulseSourceBasedonSupercontinuumGene

    rationi_nFP

    ;drasticallvinashortdistance,howeverthespectralwidthtendstobeinvariablejustafter0.2km

    ;becausethepeakpowerhasbeengreatlyreducedandthenonlineareffectshavebeenweakened,

    ;accordinglyalongfiberlengthisnotnecessarytogenerateabroadSC.Nevertheless,thefiberlength

    ;shouldbelongenoughtoobtainaflatSC.Thereasonisthatintheinitialstage,SPMdominatesover

    ;thedisDersionbecauseofsmalldispersion.Thereforethedramaticspectrumbroadeningisaccompanied