光纤的色散与非线性效应ppt

光纤的色散与非线性效应随着脉冲在光纤中传输，脉冲的宽度被展宽劣化的程度随数据速率的平方增大决定了电中继器之间的距离光纤的色散模间色散(ModeDispersion)色度色散(CromaticDispersion)偏振色散(PolarizationModeDispersion)10000100010010600km12345678910调制速率（Gbps）小色散光纤－理论上小色散光纤－实际上传统光纤－理论上传统光纤－实际上色散对传输的限制Material(Chromatic)Dispersion•Thisiscausedbythefactthattherefractiveindexoftheglassweareusingvaries(slightly)withthewavelength.Somewavelengthsthereforehavehighergroupvelocitiesandsotravelfasterthanothers.Sinceeverypulseconsistsofarangeofwavelengthsitwillspreadouttosomedegreeduringitstravel.GroupVelocityDispersion”(GVD)•NormalDispersionRegime:thelongwavelengthstravelfasterthantheshortones!Thusaftertravellingonafibrewavelengthsattheredendofthepulsespectrumwillarrivefirst.Thisiscalledapositivechirp!•AnomalousDispersionRegime:theshortwavelengths(blueendofthespectrum)travelfasterthanthelongwavelengths(redend).Aftertravelonafibretheshorterwavelengthswillarrivefirst.Thisisconsideredanegativechirp.PolarisationModeDispersion(PMD)•ThereisusuallyaveryslightdifferenceinRIforeachpolarisation.Itcanbeasourceofdispersion,usuallylessthan.5ps/nm/km.•Theeffectistocauseacircularorellipticalpolarisationtoformasthesignaltravelsalongthefibre.•Dispersionresultingfromthebirefringentpropertiesoffibreiscalled“PolarisationModeDispersion”(PMD).WaveguideDispersion•Theshape(profile)ofthefibrehasaverysignificanteffectonthegroupvelocity.Thisisbecausetheamountthatthefieldsoverlapbetweencoreandcladdingdependsstronglyonthewavelength.Thelongerthewavelengththefurtherthetheelectromagneticwaveextendsintothecladding.•sinceagreaterproportionofthewaveatshorterwavelengthsisconfinedwithinthecore,theshorterwavelengths“see”ahigherRIthandolongerwavelengths.Thereforeshorterwavelengthstendtotravelmoreslowlythanlongerones.传输使用的三种不同类型的单模光纤G.652单模光纤（NDSF）G.653单模光纤（DSF）G.655单模光纤（NZ-DSF）常规G.655大有效面积G.655大多数已安装的光纤低损耗大色散分布大有效面积色散受限距离短2.5Gb/s系统色度色散受限距离约600km10Gb/s系统色度色散受限距离约34kmG.652+DCF方案升级扩容成本高结论:不适用于10Gb/s以上速率传输，但可应用于2.5Gb/s以下速率的DWDM。G.652单模光纤（NDSF）低损耗零色散小有效面积长距离、单信道超高速EDFA系统四波混频（FWM）是主要的问题，不利于DWDM技术结论:适用于10Gb/s以上速率单信道传输，但不适用于DWDM应用，处于被市场淘汰的现状。G.653单模光纤（DSF）在1530－1565nm窗口有较低的损耗工作窗口较低的色散，一定的色散抑制了非线性效应（四波混频）的发生。可以有正的或负的色散——海底传输系统正色散SPM效应压缩脉冲，负色散SPM效应展宽脉冲。为DWDM系统的应用而设计的G.655单模光纤（NZ-DSF）结论:适用于10Gb/s以上速率DWDM传输，是未来大容量传输，DWDM系统用光纤的理想选择。1550nm1310nm色散ps/nmkm普通光纤(SMF)非色散位移光纤（NDSF，G.652）已有光纤的95%波长色散位移光纤（DSF,G.653）非零色散位移光纤（NZDSF,G.655）180DWDM波长范围三种光纤色散情况比较正常色散区反常色散区CalculatingDispersion•inatypicalsingle-modefibreusingalaserwithaspectralwidthof6nmoveradistanceof10km:Dispersion=17ps/nm/km×6nm×10km=1020ps•At1Gbpsapulseis1nslong.Sothesystemwouldnotwork.(20%isagoodguidelinefortheacceptablelimit.)Butitwouldprobablyworkquitewellatadatarateof155Mbps(apulselengthof6.5ns).•Anarrowspectralwidthlasermightproduceonlyonelinewithalinewidthof300MHz.Modulatingitat1Gbpswilladd2GHz.2,300MHzisjustlessthan.02nm(at1500nm).Sonow:•Dispersion=17ps/nm/km×.02nm×10km=3.4ps•Inthiscase,dispersionjustceasedtobeaproblem.色散补偿技术•控制光源线宽•色散位移光纤•色散补偿光纤•中途谱反转•啁啾光纤光栅ControlofSpectralWidth•SimpleFPlaser:over5nm;•ExternalcavityDBRlaser:.01nm•Modulationaddstothebandwidthofthesignal,bytwicethehighestfrequencypresentinthemodulatingsignal(1Gbps,.04nm)!•UsingmorecomplexsignalcodingratherthansimpleOOK.•UsingWDM(a2.5Gbpssignalhas1/4oftheproblemwithdispersionasa10Gbpssignal).DispersionShiftedFibre•dispersionshiftedfibreisdesignedwithadispersionzeropointataround1550nm.•However,itisnotalwayspossibleorindeeddesirable：•Inmanycaseswecan'thaveDSFbecausethefibrewemustuseisalreadyinstalled.•four-wavemixingeffectivelyprohibittheuseofDSF.DispersionCompensatingFibreBalancingDispersiononaLinkDCF存在的问题高损耗(0.5dB/km)小截面积(DCF:20mm2G-652:80mm2),比标准光纤的非线性系数高2-4个数量级非线性阈值低3-6dB较大的色散斜率(DCF:-15~-20ps/nm2/km;G-652:0.09ps/nm2/km).短波长过补偿，长波长欠补偿。Mid-SpanSpectralInversion•Theconcepthereistouseadeviceinthemiddleofthelinktoinvertthespectrum.Thisprocesschangestheshortwavelengthstolongonesandthelongwavelengthstoshortones.Whenthepulsearrivesithasbeenre-builtexactly-compensatedforbythesecondhalfofthefibre.Principle•Thisspectralinversionisperformedbyaprocesscalled“opticalphaseconjugation”.Devicesthatchangethewavelengthusingeither4-WaveMixingorDifferencerequencyGenerationinvertthespectrumasabiproductoftheirwavelengthconversionfunction.Thesecanbeusedasspectralinvertersifwecantoleratethewavelengthshiftinvolved.ChirpedFibreBraggGratingsChirpedBragggrating•••longshortTocompensatefor100kmofstandard(17ps/nm/km)fibrethechirpedgratingneedstobe17cmlongforeverynmofsignalbandwidth!InthisinstanceaWDMsystemwithchannelsspreadover(say)20nmwouldneedachirpedFBG(20x17)340cmlong!啁啾光栅用作色散补偿啁啾光栅的色散：(2neffL/c)(1/Dc)neff：有效折射率；c：光速Dc光栅两边缘反射波长之差.5cm长的线性啁啾光栅可以补偿300km的10Gb/(光谱宽度0.1nm)传输线的色散(~5100ps/nm)单信道多信道折射率效应光强度波动引起的折射率的调制自相位调制(SPM)交叉相位调制(XPM)四波混频(FWM)散射效应受激布里渊散射(SBS)受激拉曼散射(SRS)光纤的非线性效应Whathappen•Increaseinsignificanceexponentiallywiththelevelofopticalpowerinthefibre.•“Elastic”effects:noenergyexchangebetweentheopticalwaveandthematter(four-wavemixing).•“InelasticScattering”:thereisanenergytransferbetweenthematterinvolvedandtheopticalwave.光纤非线