光纖通信課件：Chapter7-Light-Detectors

Chapter7OpticalDetectorsandReceivers光電檢測(cè)器的要求-能檢測(cè)出入射在其上面的光功率，并完成光/電信號(hào)的轉(zhuǎn)換常用的半導(dǎo)體光電檢測(cè)器：光電二極管和雪崩光電二極管-足夠高的響應(yīng)度，對(duì)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(duì)信號(hào)的影響-具有良好的線性關(guān)系，以保證信號(hào)轉(zhuǎn)換過程中的不失真-具有較小的體積、較長(zhǎng)的工作壽命等Photo-detectorsincommunicationneedsThepredominanttypesoflightdetectorusedincommunicationssystemsrelyontheprincipleofionisationinasemiconductormaterial.1.DetectorResponsivity(探測(cè)響應(yīng)度）Fourimportantparametersofphoto-detectors:2.SpectralResponseRange（頻譜響應(yīng)范圍）3.ResponseTime（響應(yīng)時(shí)間）4.NoiseCharacteristics（噪聲特性）Modelofdigitalopticalreceiver

OpticalsignalDetectionAmplifyingDecisionReshapingElectricsignalp-n

Photodiodes:HowTheyWork光電二極管實(shí)際上是一個(gè)加了反向偏壓的pn結(jié)n型p型耗盡層耗盡層當(dāng)反向偏壓足夠大時(shí)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(dǎo)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(duì)波長(zhǎng)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(gè)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(shè)計(jì)動(dòng)機(jī)：在光生電流尚未遇到后續(xù)電路的熱噪聲時(shí)已經(jīng)在高電場(chǎng)的雪崩區(qū)中得到放大，因此有助于提高接收機(jī)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(gè)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(gòu)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(shí)的初級(jí)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(yīng)度來表征：GainandResponsivity(倍增因子和響應(yīng)度)APDbandwidth

ForatypicalAPD,thegain-bandwidthproductcanbeevaluatedbykAistheratioofholestoelectronsinvolvedintheionizationprocess.APDPhotodiode-2.0GHzFeatures:

HighreliabilityInGaAsplanarAPDstructure55umtopilluminatedactiveareaLowdarkcurrentLowcapacitanceApplications:

Opticalreceiversupto2.5Gbps

Specifications:

OpticalReceiver-數(shù)字信號(hào)傳輸系統(tǒng)中的各種干擾最終產(chǎn)生誤碼誤碼源：碼間串?dāng)_光纖色散以及非線性效應(yīng)導(dǎo)致展寬，使比特能量向其它比特泄漏，導(dǎo)致接收機(jī)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(yīng)：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?dāng)_>40Gb/s單波長(zhǎng)高速信道內(nèi)的碼間串?dāng)_系統(tǒng)的各個(gè)環(huán)節(jié)都會(huì)帶來噪聲，噪聲對(duì)光信號(hào)會(huì)產(chǎn)生干擾。如鏈路中：由接收機(jī)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(cè)器、前置放大器、主放大器、均衡器、自動(dòng)增益控制(AGC)電路、時(shí)鐘提取電路以及取樣判決器?；趶?qiáng)度調(diào)制的數(shù)字接收機(jī)模型接收機(jī)工作的基本原理前端線性通道時(shí)鐘提取數(shù)據(jù)再生光接收機(jī)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(cè)器的光信號(hào)轉(zhuǎn)換為時(shí)變光生電信號(hào)， 然后進(jìn)行預(yù)放大，以便后級(jí)作進(jìn)一步處理要求：低噪聲、高靈敏度、足夠的帶寬光檢測(cè)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(yīng)速度快B. APD最大的優(yōu)點(diǎn)是它具有載流子倍增效應(yīng)，探測(cè)靈敏度高，但需要較高的偏置電壓和溫度補(bǔ)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(cè)的電信號(hào)不失真地放大和保證噪聲最小，一般采用基于場(chǎng)效應(yīng)晶體管(FET)的放大器器件的選擇線性通道對(duì)主放輸出的失真數(shù)字脈沖進(jìn)行整形和補(bǔ)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(hào)大小自動(dòng)調(diào)整放大器增益，使輸出信號(hào)保持恒定，以擴(kuò)大接收機(jī)動(dòng)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(shí)鐘提取任務(wù)：把線性通道輸出的波形恢復(fù)成數(shù)字信號(hào)對(duì)某時(shí)隙的碼元進(jìn)行判決，恢復(fù)原始信息為精確確定“判決時(shí)刻”，需要從信號(hào)碼流中提取準(zhǔn)確的時(shí)鐘信息作為標(biāo)定，以保證與發(fā)送端一致入射到光檢測(cè)器的二進(jìn)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