Lightdetectors(光探测器）Lightcanbedetectedbytheeye.Theeyeisnotsuitableformodernfiber眼睛可以探测到光。但是眼睛不适合用在现在的光线通信上因为它的反应太慢了。communicationsbecauseitsresponseistooslow,itssensitivitytolow-levelsignalsis它的敏感度对于低频信号来说太不足了，而且对于电子接收器进行调幅解码还有其他信号处理也不是很简单。inadequate,anditisnoteasilyconnectedtoelectronicreceiversforamplification,decoding,orothersignalprocessing.Furthermore,thespectralresponseoftheeyeis而且眼睛的光谱响应仅限于0.4和0.7UM之间的波长，而这也正是光损失最多的波长。limitedtowavelengthsbetween0.4and0.7um,wherefibershavehighloss.Nonetheless,theeyeisveryusefulwhenfibersaretestedwithvisiblelight.Breakand虽然如此，眼睛在用光纤探测可见光时是非常有用的。终止和打断能够通过观察散射的光观察到discontinuitiescanbeobservedbyviewingthescatteredlight.System,suchascouplersandconnectors,canbevisuallyalignedwiththevisiblesourcebeforetheinfraredemitterisattached.Theremainderofthischapterisconfinedtoaninvestigationofdevicesthatdirectlyconvertopticradiationtoelectricalsignals(eithercurrentorvoltage)andthatrespondquicklytochangesintheopticpowerlevel.PrinciplesofPhotodetectionWewilllookattwodistinctphotodetectionmechanisms.Thefirstistheexternalphotoelectriceffect,inwhichelectronsarefreedfromthesurfaceofametalbytheenergyabsorbedfromanincidentstreamofphotons.Thevacuumphotodiodeandthephotomultipliertubearebasedonthiseffect.Asecondgroupofdetectorsaresemiconductorjunctiondevicesinwhichfreechargecarriers(electronsandholes)aregeneratedbyabsorptionofincomingphotons.Thismechanismissometimescalledtheinternalphotoelectriceffect.hreecommondevicesusingthisphenomenonarethepnjunctionphotodiode,the最常用的应用这个现象的手段是pn节光电二极管，pin二极管，还有雪崩二极管PINphotodiode,andtheavalanchephotodiode.Importantdetectorpropertiesareresponsivity,spectralresponse,andrisetime.The重要的探测要素有敏感度，光谱响应，还有回升时间。灵敏度P是探测器的电流输出与光输入的比。它的定义式为。。它的单位是安培每瓦。responsiveρistheratiooftheoutputcurrentofthedetectortoitsopticinputpower.Inequationformitisρ=i/P,theunitsofresponsivityareamperesperwatt.Insomedetectorconfigurations在有些探测器的构造中电学输出是电压。这种情况下，敏感度的是以伏特每瓦特。theelectricaloutputisavoltage.Inthiscase,theresponsivityisg