trends in electro-optical communication systems：光电通信系统的发展趋势

1TRENDSINELECTRO-OPTICALCOMMUNICATIONSYSTEMSDJANKHOECobraInstitute,TechnicalUniversityEindhovenP.O.Box5135600MBEindhovenTheNetherlandsE-mail:g.d.khoetue.nlHENRIEVANDENBOOMCobraInstitute,TechnicalUniversityEindhovenP.O.Box513,5600MBEindhoven,TheNetherlandsE-mail:h.p.a.v.d.boomtue.nlThispapergivesanoverviewofpresentstatusandfuturetrendsinElectro-opticalcommunicationsystemsandnetworks.WithinthearchitectureofElectro-opticalcommunicationnetworks,threeareaswithadifferenttypeofresearchfocuscanbedistinguished;thelonghaullinks,thecross-connectsandtheaccessnetworkstothesubscribers.IntroductionTelecommunicationsnetworksarethelargestandmostcomplexartificialstructuresthehumanracehaseverbuiltandhavegraduallybecomeanessentialsocialandeconomicinfrastructure.Theworld-widedemandforcommunicationhasconsistentlypushedthetransportspeedfrommerely34Megabit/sintheearlyeightiesto10Gigabitpersecondtoday,i.e.agrowthofafactor10eachsevenyears.Thegrowingdemandforcommunicationismainlyduetothetrendtowardsglobalisationandtheevolutionofthesocialstructureofoursociety.Presently,wewitnessarevolutiontowardsaglobalinformationsociety,whichismostclearlyvisiblebytheveryrapidintroductionoftheInternet,andofwirelesscommunicationequipment.Traditionally,thetelecommunicationsindustryonlyprovidedlinesconnectingusers.Inthenextmillennium,telecommunicationsandinformationservicesareexpectedtoevolvetowardsmultimediaserviceswhichwillbecharacterisedbyintegrationofthetraditionallyseparatedareasoftelecommunications,computers,computernetworks,andconsumerelectronics.Theenvisionedinformation-sharingsystemsforthenextmillenniumwillalmostcertainlyrequireaseveralordersofmagnitudehighernetworkcapacityandflexibilitythancurrentlyavailable.ThisdevelopmentwillpushthedemandfornetworkcapacitywellintotheTerabit/srange.Itisevidentthatthistransportcapacityrequiresexplorationsofnewultimatesinthetechnologicalplatformofthecommunicationinfrastructuretoovercomelimitswhichsoonwillbeimposedbyelectronicstechnology,whichdevelopsataspeedofafactor10in12years,i.e.approximatelyhalfthespeedoftheincreaseinthedemandforcommunicationcapacity.Opticalcommunicationisemergingasoneofthemostimportanttechnologiesofthefuturetoservetheworld-widedemandforcapacityincommunicationbyprovidingthetechnologyforprocessingthehugeamountofsignalsinvolved.Inprinciple,opticaltechniquesofferlow-losstransmissionoverthousandsofkilometres.Moreover,thebandwidthofanopticalfibreallowsthetransmissionofmanydifferentwavelengthssimultaneously.ItisthusenvisionedthatcommunicationnetworkswilleventuallyreachacapacitybeyondtheTerabit/slevel,thuscreatingmanynewchallengesfortherequiredphotoniccomponentsandnetworkinfrastructure.Withinthearchitectureofcommunicationtransportnetworks,threeareaswithadifferenttypeofresearchfocuscanbedistinguished;thelonghaullinks,thecross-connectsandtheaccessnetworkstothesubscribers(seeFig.1).A.B.SmoldersandM.P.vanHaarlem(eds.)PerspectivesonRadioAstronomyTechnologiesforLargeAntennaArraysNetherlandsFoundationforResearchinAstronomy-1999LonghaullinksCross-connectsCross-connectsAccessnetworksFigure1:TelecommunicationnetworkarchitectureThechallengeinthehighestleveloftelecommunicationarchitecture,thelonghaullinks,ishowtoovercomelimitationsimposedbythefibreattenuation,non-linearitysanddispersion.Long-termresearchemphasisesultrafastandhigh-throughputphotonicconceptstoleadthetransmissionandswitchingcapacitytowardstheTerabit/sdomain.Opticalcross-connects,coverstopicsintheintermediatelevelsofthetelecommunicationarchitecture.Thechallengeistodeveloptelecommunicationnodesthathavemorethroughput,aremorereliableandhavemorespeed.Amainchallengeattheleveloftheaccessnetworksisthedevelopmenttowardsbi-directionalbroadbandaccesstothesubscribers.Costaspectsplayaveryimportantrolehere.2.LonghaullinksIntheareaoflonghaullinksopticalamplifiers,WavelengthDivisionMultiplexing(WDM)andOpticalTimeDomainMultiplexing(OTDM)areimportantresearchissues.Moreover,limitationsimposedbythetransmissionmediumarealsosubjectofexplorations.Cancellationofdispersioneffectshasencouragedresearcherstoinvestigatethepotentialofsolitonpropagation.Phaseconjugationhalfwayatransmissionpathisanotherwayofcancellingdispersion.AbetterunderstandingoftheinteractionsbetweenthetransmissionmediumandtheElectro-magneticlightwaveisrequiredtoproceedtowardsaninfrastructure,whichprovidesthebestconditionsforanultrashortpulsetopropagate.2.1WavelengthDivisionMultiplexing,WDMOpticalfibercommunicationlinksarewidelyusednowadaysforlongdistancecommunications,connectingnodesinopticalnetworks.Toincreasethetransmissioncapacityperfiber,WavelengthDivisionMultiplexing(WDM)hasbeenintroduced,usinganumberofdifferentwavelengthstocarrydifferentdatasignals.Atpresent,transmissionsystemswithaspeedof10Gigabitpersecondarebeingpreparedforcommercialuse.Atthesametime,operationalsystemsatthelevelof2.5Gigabitpersecondarealsobeingupgradedbymeansofwavelengthmultiplexing,creatingasystemwitha286capacityof10or20Gigabitpersecondusing4or8wavelengthsinparallel.Researchersareexploringhigherspeedatonewavelengthaswellastheoptionofmorewavelengthsinparallel,includingconceptswherehighspeedsignalscanbetransferredfromonewavelengthtotheother,usingphotonicwavelengthconverters.TypicalexamplesofWDMsystemsaredemonstrationsbyresearchersoftheNipponTelegraphandTelephone(NTT)Companywhoreportedonanexperimentatatotalcapacitylevelof1.4Tb/s(Terabitpersecond),using200Gb/s(Gigabitpersecond)inthetimedomainand7wavelengthsinparallel1anda2.6Tb/susing132channelswith20Gb/stransmissioncapacity2.Intheworld,Japaneseresearchersareleadingintheseareas.AnotherexampleisademonstrationbyresearchersofBellLaboratories,LucentTechnologies,showing206wavelengthchannelsinparallel.Today,thestrategyforWDMlightsourcesistouselaserdiodeswithdifferentwavelengthsforeachchannel.Novelconceptsformulti-wavelengthlightsourcesmaybeexplored.Breakthroughsinplanardeviceconceptsandtechnologiesareneededtoovercometheproblemsofaccumulationofcross-talk,noiseandsignaldistortion,occurringinlargenetworks.Wavelengthconversionanditsroleinsignalregenerationhavetobefurtherexplored.Novelamplifierconceptsandmaterialshavetobedevelopedinordertobroadentheusablewavelengthwindowof30nm,whichispresentlysetbytheErbiumDopedFibreAmplifier(EDFA).Novelhostglasscombinationsshouldbeexploredandcombinedwithsuitabledopingspecies.Generally,novelmethodsforthelightamplificationintheentirewavelengthsareaofferedbythefibreshouldbeexplored.2.2OpticalTimeDomainMultiplexing,OTDMOngoingprogressinphotonictechnologieswillbeexploitedtofurtherexpandcapacityandflexibilityinthewavelengthdomain,involvinghighdensitywavelengthdivisionmultiplexing(HD-WDM)techniquesandwavelengthconversiontechniques.Photonictechnologieswillalsobeexploitedtoenhancetransmissionandprocessingspeedinthetimedomain,involvingallopticalsignalregenerationandopticaltimedomainmultiplexing(OTDM)anddemultiplexingatafemtosecondtimescale.ThiswillleadtowardsthepossiblerealisationofanallopticaltransportinfrastructurewithaterabitcapacityapproachingthefundamentalmaximumfibrecapacityimposedbyHeisenbergslimitonthetime-bandwidthproduct.Thetransportsystembetweenthenodeshastobenefitfromultrafastphotonicdevicesemergingfromresearchinmaterialsanddevices.Optionsforveryhighbitratetransmissionsystemsinwhichthesignalsaremultiplexed,demultiplexedtoalowerbitrateandregeneratedintheopticaldomainshouldbeexplored.Theconceptsshouldaimattransmissionspeedsofatleast40Gigabitpersecond.Whencombinedwithwavelengthdivisionmultiplexing(WDM),thetotalsystemcapacitycaneasilyexceedtheTerabitpersecondlevel.Keyelementsintimemultiplexedsystemsarethegenerationofultrashortpulses,multiplexers,demultiplexersandtimingextractors.Usingthosekeymodules,datacanbetransmitted,multiplexedtoahigherbitrate,regeneratedandreceived.Forthemanipulationofbitratesandtiming,thepotentialsofusingharmonicsandsubharmonicsincombinationwithmodelockedlaserdiodes(MLLD)andfourwavemixing(FWM)insemiconductoropticalamplifierscandeexplored.Inprinciple,ithasbeenshownthattheMLLDcanbelockedtoaninjectedsequenceofshortopticalpulses,whichoperatesinitssubharmonics.IthasalsobeenshownthattheMLLDcanbelockedtothesubharmonicsofaninjectedsequenceofshortopticalpulses.Largesubharmonicnumberswillopenwaystoallowopticalpulseswithhighrepetitionratestobecreatedfrommuchlowerrepetitionratesandalsotoextractanopticalclocksignalatverylowratesfromanincominghighbitrateopticalsignal.Thecombinationofdifferentfrequenciescanalsobeusedtoexploreopticaldomainmultiplexinginsemiconductoropticalamplifiers(SOA).Opticalmultiplexingmethodsusingthefourwavemixing(FWM)effectsinSOAshasbeenshown.ApossibilityistomixthelowbitratesignalofonewavelengthwithahighfrequencypulsetrainofanotherwavelengthintheSOA,usingaWDMdeviceattheinputoftheSOA.TheFWMintheSOA287willcreateamodulatedsignalattherateofthehighfrequencytrain.AsindicatedearlierOTDMtransportsystemsneedgeneratorsforultrashortopticalpulsesandopticaldomaindemultiplexers.Onewaytogenerateultrashortopticalpulsesisbytheexternalmodulationofacontinuouswaveopticalsourcewithamodulator.Fastmodulatorsshouldbeinvestigatedintheareaofsemiconductormaterialsanddevices.OpticaltimedomaindemultiplexingcanberealisedbymeansofaTOAD(TerahertzOpticalAsymmetricDemultiplexer).TheTOADiscommonlyrealisedbymeansofafibrelooptheendsofwhicharecoupledviaanopticalcoupler.Opticalsignalscoupledintothisloopwillbereflected.However,thereflectionpropertiescanbedisturbedbyinsertingaSOAatanasymmetricpositionintheloop.AprobesignalpassingtheSOAwilldisturbthesymmetryoftheloopandaparticularsignalwillthusbepassedandnotreflected.Experimentshavebeencarriedouttodemultiplex40Gbit/stofour10Gbit/sopticalsignals.SemiconductorversionsofthisTOADconfigurationandtocombinemanyTOADslockedtoasingleringlaserclockdevicewillbeinvestigated.3.CrossconnectsOpticalfibercommunicationlinksarewidelyusednowadaysforlongdistancecommunications,connectingnodesinopticalnetworks.Toincreasethetransmissioncapacityperfiber,WavelengthDivisionMultiplexing(WDM)hasbeenintroduced,usinganumberofdifferentwavelengthstocarrydifferentdatasignals.Routingandswitchinginthenodesisperformedelectricallysoineachnodeofthenetworkallopticalsignalshavetobeconvertedtotheelectricaldomainandviseversa.ElectronictelecommunicationnodeswillnotbecapabletoswitchandroutefutureTerabit/sdatastreams.PhotonicroutingandswitchingnodesmustbedevelopedcapabletohandleWDMandOTDMmultiplexedsignals.Photonicswitchingdeviceswillthereforebeusedinmanyfunctionalmodulesinthenetwork,forsignalmodulation,regeneration,multiplexingandrouting.Physicalphenomena,whichcanleadtoultrafastwavelengthconversion,areasubjectforinvestigation.Inparallel,thephysicalpropertiesofthetransmissionmediumhavetobeexploredmoreaccuratelyandhavetobetunedwherepossible.Integratedopticalcross-connectchipswhichconsistsofmultiplexers,switchesanddemultiplexersarebeingdevelopednowandwillbecommerciallyusedoverafewyears.Themanagementofamulti-wavelengthtelecommunicationinfrastructureisaseriouschallenge.Newapproachesshouldrecognisethattheprobleminvolvesgeographicallydistributedandcomplexcontrolpointsandthereforedealswithbasicproblemsinnetworkarchitectureandmathematicalmodellingofthenetworkelements.Whenopticalnetworksarebecomingmoreandmorecomplex,theuseofopticallaserneuralnetworknodescanbeaninterestingsolution.3.1LaserNeuralNodesTheLaserNeuralNetwork(LNN)providesinterestingoptionsfortherealisationofanopticalnetworknode.Neuralnetworksareintrinsicallysuitedforparalleloperation3.Adedicatedhardwareconfigurationoperatingintheopticaldomainandtheuseoftheultrafastphotoniccomponentssectionsisexpectedtoofferfurtherimprovementsinthespeedandcapacityoftelecommunicationnetworks.Generally,afirstadvantageofopticalconfigurationsisthatlightbeamscancrosseachotherinfreespacewithoutinteracting.Inaddition,allthreedimensionscanbeused,whichreducestheproblemofinterconnectivityandallowsforlargerandmorecomplexconfigurations.Finally,opticaldomainsystemsarepotentiallymuchfasterthanelectricaldomainsystems.Weightingandsummationintheopticaldomaincanbedoneveryfast,whilespeedlimitationscausedbychargebuild-up,likeinelectronicbaseddevices,arenotpresent.Wefirstexploredthepotentialsofasemiconductorlaserasakeyelementintheopticalneural288network,seefig2.Thelongitudinalmodesofthelaserareusedtorepresentneurons,andcontrolledopticalfeedbackviaanexternalcavityisimposedtoeachofthelongitudinalmodes.Abulkgratingisusedtoseparatethemodesspatiallyintheexternalcavity.Theopticalpowercontainedineachofthemodesrespondsnonlinearlytothedegreeofopticalfeedbackandtheconfigurationthusbehaveslikeaneuralnetwork.Controlledopticalfeedbackisprovidedviaamatrixofliquidcrystalelements,whichisinsertedintheexternalcavityofthelaser.Onedimensionofthematrixisusedtoinputdataandtheotherdimensionisusedtoinsertweightfactors.Inprinciple,thelongitudinalmodepatternsrespondveryfastonchangesinfeedback,becausethephenomenonisbasedonintrabandeffects.Figure2:LaserNeuralNetwork.Thecurrentexperimentalconfigurationisonlycapableofhandlinginput-outputproblemswithsmalldimensions.Furthermore,theliquidcrystalmatrixlimitsthespeedofoperation.StrategyforfutureresearchistofindwaystofurtherexpandtheLNNwithalargermatrix,toimprovethespeedandtoinvestigatehowtheLNNcanbeimplementedasanodeintelecommunicationnetworks.Afirstchallengingstepistoinvestigatehowthespeedofoperationcanbeimproved.Itisnecessarytoreplacetheliquidcrystalmatrixwithafasterdeviceandtoreducethelengthoftheexternalcavity.Next,ahigh-speedmatrix,whichcanprovidecontrolledfeedbacktothelongitudinalmodesofthelaser,isrequired.Thelengthoftheexternalcavitywillbereducedwhenthebulkgratingusedinthecurrentexperimentisreplacedbyaplanarconfiguration.AshorterexternalcavitywillallowfasteroperationoftheLNN.LNNconceptswherethediodelasercanbeintegratedwithplanargratingsandasemiconductormatrixelementhavetobestudied.Severaloptionsfortherealisationofthesemiconductormatrixmodulatorsaretobeexplored.Inprinciple,highspeedmodulatorscanbeinsertedintheexternalcavityconfiguration.Themodulators289canbebasedonelectro-opticaleffectsoropto-opticaleffects.Aninterestingoptionforimplementationintheopticalnetworkistheheteron-i-p-istructurewithopto-opticalmodulationbecauseofthepossibilitytooperatetheconfigurationfullyintheopticaldomain.ThesemiconductormodulatorscanbecombinedwithaphasedarraystructureandthelaserdiodeitselftoformamonolithicallyintegratedversionoftheLNN.Sinceatwo-dimensionalexternalcavityconfigurationandatwodimensionalmatrixareneeded,novelconceptsintheintegrationmethodhavetobefound.AsecondissueistheexplorationofconceptsofLNNbasedtelecommunicationnodes.Opticalpayload(data)toberoutedorswitchedcanbeprovidedwithanopticalheader.AnopticalheaderprecedingthepayloadcanbeusedtoinputthemodulatorsintheLNN.Theserialinformationintheheaderneedstobetransformedintoaparallelpattern,whichcanbeusedtoinfluencethedegreeofreflectionimposedonthelongitudinalmodes.Oneoptionistouseanopticalgatethatcanselectandroutetheheadertoanopticalseries-to-parallelconverterthatmayconsistoffibredelaylines.Theoutputoftheconvertercanbeusedtosetthefirstdimensionofamodulatormatrix,whichoperates,intheopto-opticalmode.Alternativemethodsneed