光纤通信课件：Chapter1-Fiber-Optic-Communications-Systems

善歌者,使人继其声;善教者,使人继其志.其言也，约而达，微而臧，罕譬而喻，可谓继志矣。请关掉手机Chapter1FiberOpticCommunicationsSystems1.1HISTORYEarlyOpticalCommunicationsHandsignals、Smokesignals、Blinkerlights、PhotophoneModernOpticalCommunications1960:Thefirstlaserwasconstructed.1960-1970TimePeriod:Manylaserapplicationswereproposed.Unguidedopticcommunicationssystem1970:Thefirsttrulylow-lossfiberwasdevelopedfiberopticcommunicationsbecamepractical1.2BasicCommunicationsSystemTransmitter

InformationChannelReceiverInformationChannelsUnguidedChannels(Atmosphere) RadioBroadcast TelevisionBroadcast Wireless SatelliteGuidedChannels(ConductingWires,Waveguides,Fibers)cabletelevisiontelephonelocalareanetworks(LANs)GENERALOPTICALSYSTEMMessageOriginCarrierSource

CouplerModulatorCouplerDetectorProcessorDestinationOpticDomainElectricDomainTransmitterReceiverChannel1.2.1 MessageOrigin:Putsthemessageintoelectricalform.1.2.2 Modulator:Converts,ImpressesAnalog

(continuous)

TimeDigital(discrete)10

101Time1.2.3 CarrierSource:Generatesthelightwaveonwhichtheinformationiscarried.Commondevices:laserdiode(LD),lightemittingdiode(LED)ElectricalDomain

OpticalDomainCarrierSourceELECTRICALCURRENT,IinOPTICALPOWER,POUTIntensitymodulation(IM):OpticPower(P)Inputcurrent(I)1.2.4 ChannelCoupler:Coupleslightfromthesourcetothefiberchannel.

LightSourceFiberAcceptanceangleLightray1.2.5 Informationchannel:Glass(orplastic)fibersthatarethetransmissionmedium.Desirableproperties:1)Lowattenuation(losseslimitpathlengths)2)Largelight-acceptanceconeangle3)LowpulsewaveformdistortionPulseDistortion(PulseSpread)InputPowerWaveformafterashorttraveldistanceWaveformafterfurthertraveltttPowerPowerOpticalamplifiersRepeaters(Regenerators)UsedinbothdigitalsystemandanalogsystemUsedindigitalsystemCompensatesignalattenuationReconstitutebothpowerandwaveshapeSimpler,cheaperMorecomplex,moreexpensiveReceiverCoupler:

Transferstheopticpowerfromthefibertothephotodetector.FiberPhotodetectorElectricCurrentOpticPower1.2.6 Photodetector:Convertsopticalpowertoelectriccurrent.OpticalPower(Pin)ElectricCurrent(Iout)Photodetector1.2.7 SignalProcessing

1)Amplification2)Filteringtoimprovesignalquality3)Decisioncircuitsfordigitalsignals

SignalQualityMeasures

Signaltonoiseratio-measureforanalogsignalsBiterrorrate-measurefordigitalANALOGSIGNALQUALITYsignal-to-noiseratio(SNR). S/N=(SignalPower)/(NoisePower)SNR>10,000isrequiredfordecentviewing.

DIGITALSIGNALQUALITYbiterrorrate(BER)isrequiredforvoicechannel1.2.8 MessageOutput:convertthesignaltoanyformweneeded

BandwidthsofCommonAnalogSystemsTypeBandwidthCommentsVoice4kHzTelephoneMusic10kHzAMradiobroadcastMusicTV200kHz6MHzFMradiobroadcastTelevisionbroadcast

1.2.9 SomeNumbers

BandwidthsofCommonDigitalSystemsTypeBWCommentsVoice

64kbps

Telephone

Ethernet

10Mbps

XeroxLAN

FDDI

100Mbps

Fiberdistributeddatainterface

DIGITIZINGVOICEMESSAGESDigitizingSteps

1)Pickthehighestfrequency: fu=4kHzforvoice

2)Sampleattwicethehighestfrequencyyielding:SamplingRate=2fu=8000samples/second3)Usean8-bitcodeforeachsample.8000(samples/s)x8(bits/sample)=64,000b/sTIMEDIVISIONMULTIPLEXING(TDM)multiplexnumerousmessagesbytransmittingathightransmissionratesandinterleavingthebitsfromseparatemessages.

Example:TheT1transmissionleveloperatesatastandardrate=1.544Mb/s.(includessynchronizationandsignalingpulse)1.2.10 ComputingPowerLevelsinDecibels(dB)

P1P2Component(System)ThedecibelscaleDECIBELSCALEFORCASCADEDELEMENTSP1P4Element1

Element2

Element3P2P3Example1:Supposethatthethreeelementshavelossesof-11,-6,and-3dB,respectively.Findthetotallossofthecombination.Findtheoutputpoweriftheinputpoweris5mW.Example2:Asystemhas-23dBofloss.Computeitsefficiency.Solution:thetotallossis:-11+(-6)+(-3)=-20dBThereceivedpowerisSolution:DECIBELABSOLUTEPOWERSCALEIfthereferencepowerissetto1mW,wehavethedBmscaledefinedby dBm=10logP2wherePisinmilliwatts.Example:

ALEDradiates2mW.ComputethedBmvalueofthisradiatedpower.Thispowertravelsthroughagroupofcomponentshavingacombinedlossof23dB.Computetheoutputpower.Solution:P=2mW,then dBm=10log2=3dBmThelossis23dB,theoutputpowerindBmis3+(-23)=-20dBmThecorrespondingoutputpowerisP=10-20/10=0.01mW1.3WaveNatureofLight1.3.1 WaveNatureofLight1.01.52.0 Wavelength(m)

UltravioletVisible

InfraredOpticalSpectrum(Partial)BlueRedarethefibertransmissionwindows1.3.2 ParticleNatureofLightLightismadeupparticlescalledphotons.Eachphotonhasenergy:h

=6.626x10-34Js(Planck’sconstant)(1.4)Thephotonenergyis:Example:HowmanyphotonsaredeliveredeachsecondforawavewithaveragepowerP=1Watwavelength=0.8m?1.4AdvantagesofGlassFibersCost:Thecostoffiberisverylow,becauseglassisplentifulandcheap.

Highinformationcapacity:

Glassfiberca