光通訊基礎(chǔ)1課件

Light:1.Electromagnetic(EM)wave----Inthewaveview2.Arayorbeam----Inthegeometricopticsview3.Astreamofphotons----Inthequantumview1.lELECTROMAGNETICWAVES

1.Wavelength:λ2.Velocity:c3.Frequency:

4.Theperiod:T5.Spectrum:Visible:λ=400nm----700nm=1/T

λ=c(1.l)6.Interference

Thereisinterferencebetweenlightwavesofbotharms.Iflightwavesofbotharmshavethesamephase,theywillinterfereconstructivelyanddestructivelyiftheyarereverse.

7.diffractionSnell'slaw:θ1=θ3n1sinθ1=n2sinθ2(1.3)Totalinternalreflection:

Whenlighttravelsfromamediumwithahigherrefractiveindextoamediumwithalowerrefractiveindexanditstrikestheboundaryatmorethanthecriticalincidentangle,(θ1>θ1C)alllightwillbereflectedbacktotheincidentmedium,meaningitwillnotpenetratethesecondmedium.

Theincidentangleatwhichtheangleofrefractionequals900iscalledthecriticalincidentangle,θ1C.Totalinternalreflectionisanecessaryconditiontomakeopticalfiberworkasacommunicationslink.1.3ASTREAMOFPHOTONSAnenergy-LevelDiagram

Bohr'smodelassumesthatelectronsrotateonstationaryorbitsandthereforepossessastationaryvalueofenergy.Anatomcanbeatanyoftheselevels,orotherwords,itcanchangeitsenergystates,anditcanchangeitsenergyonlybyjumpingfromoneleveltoanother;InlevelonlydiscretelyRadiationandAbsorptionAphoton'senergy,EP=handaphotonwascreatedwhenanatomjumpedfromE3toE2andreleasedenergy(E3–E2).Therefore,EP=ΔE=E3–E2(1.5)ButEP=h;hence,h=E3–E2and=(E3–E2)/h.Ontheotherhand,=c/,Therefore,

=ch/(E3–E2)(1.6)Conclusion:thewavelength(thecolor)ofradiatedlightisdeterminedbytheenergylevelsoftheradiatingmaterial.Theseenergylevelsaregivenbynatureandwecannotcontrolthem.Butwecanchooseanothermaterialtoachievedifferentcolorsofradiatedlight.Atomswanttoexistatthelowestpossibleenergylevels.Toraisethemtohigherlevels,wemustenergizethemfromanexternalsource.Whenatomsabsorbexternalenergy,theyjumptothehigherenergylevelsandthendroptothelowerlevels,radiatingphotons---thatis,light.Theprocessofmakingatomsjumptothehigherlevelsbyfeedingthemexternalenergyiscalledpumping.

Whathappensifanexternalphoton(light)strikesamedium?Ifitsenergy,EP=h,isequaltotheenergygap,ΔE,thephotonwillbeabsorbedbyanatomandtheatomwilljumptotheappropriatehigherlevel.IfEPisnotequaltoΔE,thephotonwillpassbythematerialwithoutinteraction.1.ThevalueofEgdeterminestheconductive(resistive)propertiesofthematerial.Goodconductorshavenogapbetweenthevalenceandconductionbands,goodinsulatorshaveabigenergygap,andsemiconductorshaveagapsomewhereinbetween.

2.

Ifanexcitedelectronfallsfromaconductionbandtoavalenceband,itreleasesaphotonwhoseenergy,isequaltoorgreaterthantheenergygap.3.Tomakeasemiconductorradiate,itisnecessarytoexciteasignificantnumberofelectronsattheconductionband.Thiscanbedonebyprovidingexternalenergytothematerial.Themostsuitableform