光纖通信課件：Chapter3-Light-Fundamentals

Chapter3LightFundamentalsPropagationFactor傳播因子Attenuationcoefficient吸收系數(shù)Dispersion色散SpectralWidths譜寬Pulsespread脈沖展寬Soliton光孤子Informationrate信息速率Polarization偏振ResonantCavity諧振腔CriticalAngle全反射Reflectance反射率3.1ElectromagneticWavesThisexpressionrepresentsawavetravelingwithzeroloss.Istheattenuationcoefficient

PropagationFactorWaveTravelinginaLossyMediumt2t1Distance(z)ElectricFieldSpectralWidthsforTypicalLightSourcesSource SpectralWidth(nm) LED 20-100 LaserDiode 1-5 Nd:YAG-Laser 0.1 HeNeLaser 0.0023.2Dispersion,PulseDistortion,InformationRatef=sourcebandwidth(rangeoffrequenciesemittedbythesource).1.5fFrequencyNormalizedPowerff1f21.5WavelengthNormalizedPower=linewidthorspectralwidth

213.2.1MaterialDispersionandPulseDistortionDispersion:

PropagationvelocityvarieswithWavelength.MaterialDispersion:causedbythematerial.WaveguideDispersion:causedbythestructureofthewaveguide.InputPowerOutputPowerttttttt1tT12233T+FastestwavelengthSlowestwavelengthArriveslastArrivesfirstInputPowerOutputPowerPac,inPac,out12Slowerwavelength12 Pac,out<Pac,in

ttRefractiveIndexVariationforSiO2

1.45oInflectionPoint0non’dn/dlFirstDerivative0oSecondDerivativedn2/dl20n’’materialdispersionM:(3.14)M()isinpicosecondsofpulsespreadpernanometerofsourcespectralwidthandperkilometeroffiberlength.

FurtheranalysisshowsthatDefinedpulsespread

perunitlength

1.31.550.82110-20M(ps/nm.km)(m)Discussion:Example:ConsideranLEDat=0.82m,L=10km,and=20nm.Find(/L).Fromthegraph,at0.82mm,M=110ps/nm·km.Changethewavelengthto=1.5m,=50nm.At1.5mm,M=-15ps/nm·km.ThenAsolitonisapulsethattravelwithoutspreading.Therefractiveindexofglassdependsuponthepulseintensity.Thisfibernonlinearityisusedtocountertheeffectsofdispersion.3.2.2SolitonsFiberAmplifierconsderBlue:l1Red:l2TimePowerT/2Dt3.2.3InformationRateThismodulationfrequencyturnsouttobethe3-dBbandwidth.Thesignalisactuallyreducedbyhalf(3-dB)atthismodulationfrequency.Therelationshipofopticalbandwidthandelectricalbandwidthis:(3.19)ConsideraReturn-to-Zero(RZ)digitalsignal.tp=T/2 pulsewidthR datarate,b/sttpPower111010T2T3T4T5T6T7TFrequencyPowerSpectralDensity(Watts/Hz)0ttp11101

T2T3T4T5T6T7TConsidertheNon-Return-to-Zero(NRZ)digitalsignal.Power0FrequencyPowerSpectralDensity(Watts/Hz)

0Examples:LEDLDCalculate:thepulsespreadperkilometer,3-dBopticalBandwidth-lengthproduct,3-dBelectricalbandwidth-lengthproduct,rate-lengthproductofRZ,andrate-lengthproductofNRZBANDWIDTHDATARATESUMMARY3.3PolarizationyzxEvxyEunpolarizedLinearlypolarized3.4ResonantCavitiesLEzMirrorMirrorTheresonantwavelengthsare:Thecorrespondingresonantfrequenciesare:longitudinalmodesFrequency(3.24)Thefreespacewavelengthspreadcorrespondingtofcisccalculated

from:(3.26)Example:ConsideranAlGaAslasercavity.L=0.3mm=300m;n=3.6;o=0.82m.FindthecavityresonantwavelengthspacingcExample:SupposetheAlGaAs,LDhasaspectralwidthof2nm.Determinethenumberoflongitudinalmodesintheoutput.2nm0.82mGain(AlGaAs)0.82mCavityResonances

c2nm0.82mcLaserOutput3.5REFLECTIONATAPLANEBOUNDARYReflectionCoefficient:

n1n2IncidentWaveTransmittedWaveReflectedWaveBoundaryExample:Forair-to-glass,computethetransmittedpower.4%powerreflected.96%powertransmitted.IndB,thetransmittedpoweris:10log(0.96)=-0.177dBTypicallyweroundthisoffto0.2dB.Considerarbitraryincidence:

Fresnel’sLawofRefractionForparallelpolarizationtheresultis(3.29)(3.30)Forperpendicularpolarization,theresultisPlotsofpandsforn1=1(air),n2=1.5(glass)Parallel(rp)Perpendicular(rs)Angle(q)rsrpn1n2n3l/4AntireflectioncoatingClearly,thereflectancebecomeszeroif:Example:foranair-toglassinterface,assumethatthecoatingmaterialisMgF2,computethefractionalamountoflightreflectedWithoutthecoating,theair-to-glassinterfacewouldhaveareflectanceof4%.TheARcoatinghasreducedthereflectanceto1.4%.Callthesolutionc,thecriticalangle.conlyexistsifn1>n2