光纖通信課件：Chapter7-Light-Detectors

Chapter7OpticalDetectorsandReceivers光電檢測器的要求-能檢測出入射在其上面的光功率，并完成光/電信號的轉(zhuǎn)換常用的半導體光電檢測器：光電二極管和雪崩光電二極管-足夠高的響應度，對一定的入射功率能輸出足夠大的光電流-具有盡可能低的噪聲，以降低器件本身對信號的影響-具有良好的線性關系，以保證信號轉(zhuǎn)換過程中的不失真-具有較小的體積、較長的工作壽命等Photo-detectorsincommunicationneedsThepredominanttypesoflightdetectorusedincommunicationssystemsrelyontheprincipleofionisationinasemiconductormaterial.1.DetectorResponsivity(探測響應度）Fourimportantparametersofphoto-detectors:2.SpectralResponseRange（頻譜響應范圍）3.ResponseTime（響應時間）4.NoiseCharacteristics（噪聲特性）Modelofdigitalopticalreceiver

OpticalsignalDetectionAmplifyingDecisionReshapingElectricsignalp-n

Photodiodes:HowTheyWork光電二極管實際上是一個加了反向偏壓的pn結(jié)n型p型耗盡層耗盡層當反向偏壓足夠大時耗盡區(qū)自由載流子被完全耗盡pnTheseparatedelectronandholeareattractedbythepositiveandnegativepotentialsofthedepletionvoltage,respectively.CarriersandcurrentgenerationpnE光生電流

IhvWhenaphotonstrikesadepletionregion,itsenergyseparatesanelectronfromitshole外加反向偏壓抽取光能量的過程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在半導體材料中光功率的吸收呈指數(shù)規(guī)律：其中aabs(l)為材料對波長l的吸收系數(shù)，P0是入射光功率，P(x)是光在耗盡區(qū)中經(jīng)過距離x后被吸收的功率光損耗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)層：低摻雜n型高摻雜n+型高摻雜p+型加反向偏置電壓后形成一個很寬的耗盡層10GHzBandwidthPINPhotodetector

Features:

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

HighbitrateorhighbandwidthopticalnetworksSpecifications(T=25oC):

AvalanchePhotodiodes(雪崩光電二極管)設計動機：在光生電流尚未遇到后續(xù)電路的熱噪聲時已經(jīng)在高電場的雪崩區(qū)中得到放大，因此有助于提高接收機靈敏度工作過程拉通型雪崩二極管(RAPD)“拉通”來源于其工作情況：增加偏壓，使耗盡區(qū)正好拉通到整個本征p區(qū)，以增加高阻材料帶有少量p摻雜的本征材料p+ppn+結(jié)構光電二極管中所有載流子產(chǎn)生的倍增因子M定義為：IM

：雪崩增益后輸出電流的平均值Ip：未倍增時的初級光電流V：反向偏壓VB：二極管擊穿電壓n：常數(shù)，一般為2.5~7

類似于pin，APD的性能也由響應度來表征：GainandResponsivity(倍增因子和響應度)APDbandwidth

ForatypicalAPD,thegain-bandwidthproductcanbeevaluatedbykAistheratioofholestoelectronsinvolvedintheionizationprocess.APDPhotodiode-2.0GHzFeatures:

HighreliabilityInGaAsplanarAPDstructure55umtopilluminatedactiveareaLowdarkcurrentLowcapacitanceApplications:

Opticalreceiversupto2.5Gbps

Specifications:

OpticalReceiver-數(shù)字信號傳輸系統(tǒng)中的各種干擾最終產(chǎn)生誤碼誤碼源：碼間串擾光纖色散以及非線性效應導致展寬，使比特能量向其它比特泄漏，導致接收機0和1的判決誤碼誤碼源：非線性損傷光纖傳輸中的非線性效應：1.布里淵散射2.拉曼散射3.交叉增益調(diào)制4.交叉相位調(diào)制5.四波混頻造成：

WDM系統(tǒng)中多信道之間的串擾>40Gb/s單波長高速信道內(nèi)的碼間串擾系統(tǒng)的各個環(huán)節(jié)都會帶來噪聲，噪聲對光信號會產(chǎn)生干擾。如鏈路中：由接收機引入的噪聲源如下圖所示：誤碼源：噪聲的干擾主要包括光檢測器、前置放大器、主放大器、均衡器、自動增益控制(AGC)電路、時鐘提取電路以及取樣判決器。基于強度調(diào)制的數(shù)字接收機模型接收機工作的基本原理前端線性通道時鐘提取數(shù)據(jù)再生光接收機的核心部分：前端前端：由光電二極管和前置放大器組成作用：將耦合入光電檢測器的光信號轉(zhuǎn)換為時變光生電信號， 然后進行預放大，以便后級作進一步處理要求：低噪聲、高靈敏度、足夠的帶寬光檢測器的選擇：A. pin光電二極管具有良好的光電轉(zhuǎn)換線性度，不需要高的工作電壓，響應速度快B. APD最大的優(yōu)點是它具有載流子倍增效應，探測靈敏度高，但需要較高的偏置電壓和溫度補償電路常用搭配：EDFA光放大器+pin前置放大器 主要作用是保持探測的電信號不失真地放大和保證噪聲最小，一般采用基于場效應晶體管(FET)的放大器器件的選擇線性通道對主放輸出的失真數(shù)字脈沖進行整形和補償，使之成為有利于判決的碼間干擾最小的波形根據(jù)輸入信號大小自動調(diào)整放大器增益，使輸出信號保持恒定，以擴大接收機動態(tài)范圍提供高的增益，放大到適合于判決電路的電平判決再生與時鐘提取任務：把線性通道輸出的波形恢復成數(shù)字信號對某時隙的碼元進行判決，恢復原始信息為精確確定“判決時刻”，需要從信號碼流中提取準確的時鐘信息作為標定，以保證與發(fā)送端一致入射到光檢測器的二進制數(shù)字脈沖序列表示為：其中P(t)是接收光功率Tb是比特周期