光纤通信课件：Chapter7-Light-Detectors

Chapter7OpticalDetectorsandReceivers光电检测器的要求-能检测出入射在其上面的光功率，并完成光/电信号的转换常用的半导体光电检测器：光电二极管和雪崩光电二极管-足够高的响应度，对一定的入射功率能输出足够大的光电流-具有尽可能低的噪声，以降低器件本身对信号的影响-具有良好的线性关系，以保证信号转换过程中的不失真-具有较小的体积、较长的工作寿命等Photo-detectorsincommunicationneedsThepredominanttypesoflightdetectorusedincommunicationssystemsrelyontheprincipleofionisationinasemiconductormaterial.1.DetectorResponsivity(探测响应度）Fourimportantparametersofphoto-detectors:2.SpectralResponseRange（频谱响应范围）3.ResponseTime（响应时间）4.NoiseCharacteristics（噪声特性）Modelofdigitalopticalreceiver

OpticalsignalDetectionAmplifyingDecisionReshapingElectricsignalp-n

Photodiodes:HowTheyWork光电二极管实际上是一个加了反向偏压的pn结n型p型耗尽层耗尽层当反向偏压足够大时耗尽区自由载流子被完全耗尽pnTheseparatedelectronandholeareattractedbythepositiveandnegativepotentialsofthedepletionvoltage,respectively.CarriersandcurrentgenerationpnE光生电流

IhvWhenaphotonstrikesadepletionregion,itsenergyseparatesanelectronfromitshole外加反向偏压抽取光能量的过程Input-outputcharacteristic

ThemorephotonsthatstriketheactiveareaofaPD,themorechargecarrierswillbecreatedTheinputislightpowerPTheoutputisphotocurrentIP

Thus,IP

isproportionaltoPResponsivityofaphotodiode4.2PowerRelationshipInanycommunicationsdevices,weareinterestedinitsresponsetotheamplitudeandthefrequency(orwavelength)oftheinputsignal.Responsivityversuswavelength

Photocurrentisthenumberofelectrons,perunittime,thatisLightpowerislightenergyperunitoftimeResponsivityversuswavelengthquantumefficiencyηThus,theoretically,thegraph"Responsivityvs.Wavelength"shouldbeastraightlineWhyisresponsivityproportionaltowavelength?BecauseresponsivityisinverselyproportionaltolightpowerLightpowerisproportionaltothenumberofphotons(NP)andtheenergyofanindividualphoton(EP=hc/λ)EPisinverselyproportionaltothewavelength(λ)Therealcurvevstheexpectedstraightline.Thereasonsforthisdiscrepancyisacutoffwavelengthλc,whichisdeterminedbytheenergygapEg.Forwave-lengthslongerthanλcwilltravelthroughthismaterialPartiallyabsorbedandpartiallytransmitted

P(w)wDepletionregion在半导体材料中光功率的吸收呈指数规律：其中aabs(l)为材料对波长l的吸收系数，P0是入射光功率，P(x)是光在耗尽区中经过距离x后被吸收的功率光损耗AbsorptioncoefficientThus,quantumefficiencyisnottheconstant,aswaspreviouslyassumed,butthevariable.BandwidthTwobasicmechanismsrestrictingbandwidth:Thefirstrestrictionstemsfromthefactthatchargecarrierscreatedbyaphotonneedsometimetobecollected.Thesecondrestrictiononbandwidthderivesfromtheinherentcapacitanceofap-nphotodiode.Typicalvaluesofw~10umandυsat~105m/s,wecancompute~100ps.Theequivalentcircuitofap-nphotodiode.Thejunction(shunt)resistanceRj

~tensofMΩResistanceofthepandnlayersandoftheelectricalcontactsactsasaseriesresistanceRs~5to10Ω

LoadresistanceRL~50ΩTimeconstantandbandwidthTypicalvaluesofCinarefrom1to2pF;ForRL=50Ω,τRC

isintherangeof50to100ps.Timeconstant:ThebandwidthofaphotodiodeisAnothermechanismrestrictingthebandwidthofap-nPD——diffusionphotocurrentBandwidthefficiencyvsreverse-biasvoltageReducethediffusioncurrentIncreasingthebandwidthefficiencyWidedepletionregionIncreasethereverse-biasvoltageRCtimeconstantsThekeypointissimplythis:Weneedtoincreasethewidthofadepletionregionwithoutmanipulatingunnecessarilythevalueofthereverse-biasvoltage.Thesolutiontothisdilemmaisap-i-nphotodiode.4.4p-i-nPhotodiodesi(本征)层：低掺杂n型高掺杂n+型高掺杂p+型加反向偏置电压后形成一个很宽的耗尽层10GHzBandwidthPINPhotodetector

Features:

Highbandwidthdigitaloranalogsystems(10GHz)Highresponsivity,planarGaInAs/InPphotodiodeOperatingwavelength1100~1650nmSinglemodeopticalfibercoupledinputApplications:

HighbitrateorhighbandwidthopticalnetworksSpecifications(T=25ºC):

AvalanchePhotodiodes(雪崩光电二极管)设计动机：在光生电流尚未遇到后续电路的热噪声时已经在高电场的雪崩区中得到放大，因此有助于提高接收机灵敏度工作过程拉通型雪崩二极管(RAPD)“拉通”来源于其工作情况：增加偏压，使耗尽区正好拉通到整个本征p区，以增加高阻材料带有少量p掺杂的本征材料p+ppn+结构光电二极管中所有载流子产生的倍增因子M定义为：IM

：雪崩增益后输出电流的平均值Ip：未倍增时的初级光电流V：反向偏压VB：二极管击穿电压n：常数，一般为2.5~7

类似于pin，APD的性能也由响应度来表征：GainandResponsivity(倍增因子和响应度)APDbandwidth

ForatypicalAPD,thegain-bandwidthproductcanbeevaluatedbykAistheratioofholestoelectronsinvolvedintheionizationprocess.APDPhotodiode-2.0GHzFeatures:

HighreliabilityInGaAsplanarAPDstructure55umtopilluminatedactiveareaLowdarkcurrentLowcapacitanceApplications:

Opticalreceiversupto2.5Gbps

Specifications:

OpticalReceiver-数字信号传输系统中的各种干扰最终产生误码误码源：码间串扰光纤色散以及非线性效应导致展宽，使比特能量向其它比特泄漏，导致接收机0和1的判决误码误码源：非线性损伤光纤传输中的非线性效应：1.布里渊散射2.拉曼散射3.交叉增益调制4.交叉相位调制5.四波混频造成：

WDM系统中多信道之间的串扰>40Gb/s单波长高速信道内的码间串扰系统的各个环节都会带来噪声，噪声对光信号会产生干扰。如链路中：由接收机引入的噪声源如下图所示：误码源：噪声的干扰主要包括光检测器、前置放大器、主放大器、均衡器、自动增益控制(AGC)电路、时钟提取电路以及取样判决器。基于强度调制的数字接收机模型接收机工作的基本原理前端线性通道时钟提取数据再生光接收机的核心部分：前端前端：由光电二极管和前置放大器组成作用：将耦合入光电检测器的光信号转换为时变光生电信号， 然后进行预放大，以便后级作进一步处理要求：低噪声、高灵敏度、足够的带宽光检测器的选择：A. pin光电二极管具有良好的光电转换线性度，不需要高的工作电压，响应速度快B. APD最大的优点是它具有载流子倍增效应，探测灵敏度高，但需要较高的偏置电压和温度补偿电路常用搭配：EDFA光放大器+pin前置放大器 主要作用是保持探测的电信号不失真地放大和保证噪声最小，一般采用基于场效应晶体管(FET)的放大器器件的选择线性通道对主放输出的失真数字脉冲进行整形和补偿，使之成为有利于判决的码间干扰最小的波形根据输入信号大小自动调整放大器增益，使输出信号保持恒定，以扩大接收机动态范围提供高的增益，放大到适合于判决电路的电平判决再生与时钟提取任务：把线性通道输出的波形恢复成数字信号对某时隙的码元进行判决，恢复原始信息为精确确定“判决时刻”，需要从信号码流中提取准确的时钟信息作为标定，以保证与发送端一致入射到光检测器的二进制数字脉冲序列表示为：其中P(t)是接收光功率Tb是比特周期