DOC

Lightwave Modulation Techniques for Advanced Modulation Formats

By Howard Rivera,2014-01-11 15:14
7 views 0
Lightwave Modulation Techniques for Advanced Modulation Formats

    Lightwave Modulation Techniques for

    Advanced Modulation Formats

    OPTICALFIBERCOMM

    新型光通信

    LightwaveModulationTechniques

    forAdvancedM0dulationFormats

    TetsuyaKawanishi

    NationalInstituteofInformationandCommunicationsTechnology(NICT),Japan

    Abstract:Arbitrarywaveformcanbesynthesized bytwoo~hogonalcomponents:in-phaseandquad--

    raturecomponents.Thus.ifwecanmanipulatethe twocomponentsindependently,variouscomplicat

    edmodulationcanbeachieved.Thistechniqueis calledvectormodulation,whichiScommonlyused inwirelessradiocommunicationsystemswhere thecarrierfrequencyiSlowerthanl00GHz.On theotherhand.inlightwavetransmissionsystems, thecarrierfrequencyishigherthan100THz,and thestabilityofthesignalsourceswouldbemuch poorerthaninradiosystems,SOthatthesimplest modulationformat(on.off-keying)wasoftenused incommercialsystems.However,recently,various modulationtechniquessuchasquadratureampli- tudemodulation,etc.,havebeendemonstratedby usingpreciseandrapidlightwavecontro1.Inthis report,wereviewrecentprogressinopticalvector modulationtechnologieswhichcangeneratehigh

    speedandcomplicatedopticalsignals.Integrated opticalmodulatorscansynthesizemultilevelsig

    nalsinphotoniccircuits.

    Keywords:vectormodulation;integratedoptical modulator;photoniccircuit

    I.INTRODUCTION

    Inlightwavetransmissionsystems,wheredigitized info?rmationshouldbedescribedbysomeparam

    etersofelectromagneticwave,suchasamplitude, phase,frequency,etc.,lightwavemodulationtech

    niquesplayimportantrolesinhighspeedoptical

    signalgeneration[1].Aswellknown,arbitrary waveformcanbesynthesizedbytwoorthogonal components:Inphase(I)andQuadrature(Q)corn

    ponents.Thus,ifwecanmanipulatethetwocom

    ponentsindependently,variouscomplicatedmodu

    lationcanbeachieved.ThistechniqueiScalled vectormodulationorIQmodulation,whichiS commonlyusedinwirelessradiocommunication systemswherethecarrierfrequencyiS1owerthan 100GHz.Ontheotherhand.inlightwavetransmis

    sionsystemsthecarrierfrequencyiShigherthan 1OOTHz.andthestabilityofthesignalsources wouldbemuchpoorerthaninradiosystems.Thus, itwasverydifficulttocontrollightwaveprecisely withoutlosingspeedofmodulation,andthesim

    plestmodulationformat(OnOff-Keying:OOK)

    wasoftenusedincommercialsystems.However, 20107UJ

呻国专chinaCOmlTIunications

    recently,variousmodulationtechniquessuchas DifferentialPhaseShiftKeying(DPSK)[3],Dif-

    ferentialQuadraturePhase?ShiftKeying(DQPSK)

    48,amplitudeandphaseshiftkeying(APSK)

    [910,FrequencyShiftKeying(FSK)[10-l5],

    SingleSideBand(SSB)modulation[16l8],quad

    ratureAmplitudeModulation(QAM)[1924],etc.,

    havebeenattractedconsiderableattentiontoob

    tainenhancedspectralefficiencyorreceiversen

    sitivityinopticalfibertransmission.High.speed digitalsignalprocessingforcompensationofopti

    calphasefluctuation.andadvancedmodulation techniquesforpreciseandrapidlightwavecontrol arekeytechnologiestoachievevectormodulation basedlightwavecommunicationsystems[24.26

    311.Inthisreport,wereviewrecentprogressin opticalvectormodulationtechnologieswhichcan generatehigh--speedandcomplicatedopticalsig-- nals.Integratedopticalmodulatorscansynthesize multilevelsignalsinphotoniccircuits.

    1I-COMPONENTSF0RVECT0RMODU.

    LATIONWlTHDlGITALCOHERENT

    IandQcomponentsshouldbeindependentlyma

    nipulatedanddetectedinvectormodulationtrans

    missionsystems.Inaddition.optica1phaserotation duetophasenoiseofthelasersourceshouldbe compensatedbydigitalsignalprocessing[24.29

    3l1.Aswellknown,phaseandamplitudeof lightwavecanbemathematicallyexpressedby

    apointonatwo-dimensionalcomplexplane. Lightwavewithafixedopticalfrequencycanbe describedbyafixedpointonthecomplexplane. Usually,"wavelength"isusedtodescribetheopti

    calfrequency.while"frequency"oftenmeansthe frequencyofelectricsignalfedtoamodulatoror generatedbyaDhot0detector.Figure1showsa setupforvectormodulationanddemodulation. Thephaserotationcanbecompensatedbyusinga highspeeddigitalsignalprocessing.Thisscheme iscalleddigitalcoherent.whosesetupconsistsofa diodelaserasalightsource.anopticalmodulator, a90degreeopticalhybrid,anAnalog-toDigital

    362010.7

    Converter(ADC),DigitaltoAnalogConverter

    (DAC),adigitalsignalprocessor,apairofbal

    ancedphOtOdetectors,andanotherdiodelaseras alocaloscillator.Conventionaloptica1multiplex. ingtechniquesincludingWavelength.Domain. Multiplexing(WDM)canbeappliedtodigital coherentsystems.Thus,opticalfilters,couplers, etc..designedforconventionaltransmissionsys. temscanbeusedindigitalcoherentsystems. However,weneedsomeparticularcomponents suchas,narrowlinewidthlaserlightsources,opti

    camodulatorsforvectormodulation.90.degree opticalhybridsforvectordemodulation.balanced photodetectors,ADCsformultilevelmodulation

    andDACsformultileveldemodulation.Narrow

    linewidthlaserlightsourcesareindispensablefor

    advancedmodulationformats.Thefluctuation ofsymbolsonthecomplexplanedependsonthe linewidthofthelightsources.Thetypica1linew

    idthoflaserdiodesisafewMHz.but.recently, laserswithnarrowlinewidthslessthanafewkHz arecommerciallyavailable[3234].Theimpacton

    thesignalwaveformdependsontheratiobetween baudrateandlinewidth,sothat,therequirement forthelinewidthwouldberelaxedifthebaudrate iShigherthan10Gb/s.Alaserdiodeisalsoused inadigita1coherentreceive~asa1oca1oscillator. Thephaseorfrequencyoflightwavegenerated bythelocaloscillatorwouldhavesomediflfer. encefromthatofthelightsource,duetothefinite 1inewidthsofthelaserdiodes.A90degreeopti

    calhybridwhichcanmixareceivedsignaland alocaloscillator.deliverstwodifferentialsignal pairsoftheIandQcomponents[3537].Inad

    dition,anopticalhybridwitheightoutputports providespolarizationdiversityoperation[38], wherePolarizationModeDispersion(PMD)can becompensatedbydigitalsignalprocessing[29]. BalancedDhotodetectorscanconverttheoptical differentia1signalintoanelectricoutput[391.For highbaudratesystems,highspeedmonolithi

    callyintegratedbalancedDhotodetectorswhich showshighsymmetrywithin0.5dBwerereported. wherethe3dBcutofrfrequencywas40GHz[40].

    Forvectormodulationvarioustypesofmodula

    tiondeviceswerereportedrecently.TheIandQ

    componentscanbecontrolledindependentIvbv usinganintegratedmodulatorconsistingoftwo parallelamplitudemodulators[1].Forhighspeed

    andpreciseIightwavecontro1.thefollowingtwo itemsplayimportantroles:1,highperformance

    modulatorelements(enhancedefficiencyand bandwidth1,and21integrationfornewfunctions. Regardingmodulatorelementsvarioustechniques havebeendevelopedalready.Directmodulationof injectedcurrenttolaserdiodesprovidesasimple transmitterconfiguration,however,thehighspeed

    operationis1imitedbytheresponseofthe1asers. andtheoutputwouldhaveundesiredfrequencyor phaseshift(chirp,henceforth)[26].Opticalexter

    nalmodulatorswithElectro.OpticfEO1orElectro

    Absorption(EA)effectcanprovidehighspeed

    modulation,sothattheyarecommonlyusedin highperformancetransmissionsystems.Thesize ofEAmodulatorscanbemuchsmallerthanthatof EOmodulators.however,theoutputOfEAmodu

    latorsstillwouldhavesomechirp[21.AlthoughEA modulatorscanbasicallycontrolonlyoptica1inten

    sity,complicatedmodulationispossiblebyusing integratedEAmodu1atorsf271.Ontheotherhand, inamaterialhavingtheEOeffect.theRefractive Index(RI)dependsonthevoltageappliedonthe material,sothatanopticalphasemodulatorcanbe constructedwithanopticalwaveguideandanelec

    trodeonEOmateria1.Varioustypesofmodulators f0radvancedmodulationformatscanbebuiltwith

opticalphasemodulators.Mach.Zehnderinter

    ferometerbasedmodulators(MZMs)consisting oftwohighspeedopticalphasemodulatorscan controllightwaveamplitude,precisely[28].Dual

    ParallelMZMs(DPMZMs).whichcancontrol therealandimaginarycomponentsindependently, OPTICALFIBERCOMM

    新毙通信

    arecommonlyusedforhighspeedDQPSKsignal

    generation[6,8].AQuadParallelMZM(QPMZM)

    cansynthesize4leve1signalopticallybothinthe realandimaginarycomponents,sothata16QAM

    signalcanbegeneratedfromfourelectricbinary datastreams[201.

    Inthisreport.wereviewEOeffectbasedoptical modulatortechnologies.EOmodulatorscancon. trolthephaseandfrequencyoflightwave,aswell astheintensity.Firstly,basicconceptofvector modulationschemeisdescribed.todiscussrequire

    mentofopticalcomponentsintransmissionsys- temswithadvancedmodulationformats.Theprin

    cipleofopticalphasemodulatorsusingEOeffect isprovidedtodiscussmathematicalexpressionof MZMswhichcancontro1intensityoramplitudeof lightwaves.Theconceptofparalle1MZMsisde. scribedbriefly.Thetotalexpectedbitrategenerated byanintegratedmodulatordependsonthenumber OfMZMelementsandthefrequencyresponse. .BASlCSoFEOMODULATORS

    ElectricalcontroIofopticaIphasecanbeachieved

    byusingEOmaterials,suchasLithiumNiobate (LN)[8,4850],LithiumTantalate(LT)[51],Gal

    liumArsenide(GaAs)[6],IndiumPhoshite(InP) f52,53],etc.Figure2showsaschematicofan optica1modulatorusingEOeffect.Anelectrode isplacedalongwithanopticalwaveguide.By applyingelectricvoltageontheelectrode,we canchangetheRIinthewaveguide.sothatthe phaseofthelightwavecanbecontrolled.For simplicity.theinputlightwaveisassumedtobe monochromaticandbedescribedbyE,e.The outputlightwaveRcanbeexpressedby

    =KEP.)

    flO=E,)

    (1)

    (2)

    2010.737

    呻西逼{ChinaCom1]3u;cations

    whereandErespectively,denotetheelec

    tricvoltageontheelectrodeandcouplingcoef- ficientbetweentheelectricsignalandthelight

    wavesigna1.Kistheopticaltransmittancein thewaveguide.Whenf)hashigh?frequency components,thefrequencyresponseofEshould betakenintoaccount.ThecoecientEcanbeas

    sumedtobeaconstantifthewavelengthsofthe highfrequencycomponentsontheelectrodeare muchlongerthanthe1engthoftheelectrode.The responseofEOmaterialsismuchfasterthanthe frequencyresponseofE.whichisdominatedby

    thatoftheelectrodeandbythephasemismatch betweentheelectricandlightwavesignals[48501.

    Thehalfwavevoltagedefinedbyy.{Esthe

    voltagefor7gopticalphaseshift.

    MachZehnderfMZ1structuresarecommonly usedtoachieveIntensityModulationfIM1orAm

    plitudeModulationfAM1byEOeffects.MZstruc

    turesforIMarecalledMZmodulators(MZMs,. AsshowninFigure3theMZMconsistsoftwo opticalphasemodulators.Wecancontro1theout. putintensitybyapplyingapairofelectricsignals tothetwophasemodulators.Balancedpushpull

    schemeisidealforIM.wheretheinducedphaseof themodulatorAisg(t)/2.andthatofthemodulator Bis-g(t)/2.g(f)describestheopticalphasedif- ferencebetweenthetwoarlTiSoftheMZstructure. Whenthelightwavesinthetwoarinsareinphase g(=2mr,胛一',l,O,+1,,themodulatorisin"on"

    state.Atthejunctionoftheoutputthelightwaves interfereconstructively.Ontheotherhand,when thelightwavesinthetwoarmsareinopposite phaseg()=(2n+1)Jv,thelightwavesareconverted intohigh..orderradiativemodewaveswhichcan.. notpropagatethroughthewaveguideforoutput. Opticalwaveguide

    Fig.3MZstructureconsistingoftwoopticalphasemodu- lators(AandB).Eachopticalphasemodulatorhasan electrodeforopticalphasecontrol

    QUU20107

    Thus.weturnthemodulator''on''and''off'by

    changingthevoltageontheelectrode.Theoutput lightwaveintensityfromtheMZMcanbegivenby lel=IKEcos[g(t)/2]1.oftheMzMisdefinedby thevoltagerequiredtoswitchthemodulatorfrom the"on"statetothe"off"state.andisequaltoa halfofofeachphasemodulatorintheMZM. wheng(f)=2mr,n',l,0,+1,,IR{becomesthe

    maximum,whereIcos[g(t)/2]l1.However,the polarityofR,i.e.,COS(/,dependson.When

    isanoddnumber,thepolarityisplus.When,zis anevennumber.thepolarityisminus.Thus.am

    plitudeoftheopticaloutputcanbecontinuously controlledbychangingg(t)from2mrto(2mr+1)~r. Forexample,wheng(=0,cos[g(t)/2]=+l,while itbecomes1with(f)=2.Thus,wecanflip

    thepolarityofthewaveform,andachieveBinary PSK(BPSK)withl80degreephasedeviation.

    ThistechniqueitselfisveryusefulforDPSK.In addition.morecomplicatedopticalsignalscan besynthesizefrombinaryPSKsignalsbyusing integratedparallelMZMs.Wenotethatbalanced pushpulloperationisveryimportanttoachieve pureamplitudeorintensitymodulation.Theoutput lightwavewouldhavesomephaserotationwhen theopticalphaseshiftsinphasemodulatorsAand BinFigure3.

    1V焱鼹LELT?SFOR

    VT0Dl

    ADPMZMconsistsoftwoMZMs(MZandMZ)

    embeddedinamainMZMasshowninFigure4.

Report this document

For any questions or suggestions please email
cust-service@docsford.com