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1、Chapter5Singlemodefibers--basicsFiberOpticCommunications5.1HowasinglemodefiberworksTherearemanydifferencesbetweenmultimodeandsinglemodefibers.Thesedifferencesstartwiththeprincipleofactionandrunthroughallthefibercharacteristics.TheprincipleofactionMakingafiberthatsupportsonlyonemodeManufacturersuseal
2、lthesemeanstoachievetheirgoal.Thecorediameterofasinglemodefiberisaround10umandless,therangeofoperatingwavelengthstypicallystartsat1300nm,andtherelativeindexislessthan0.4%.HowasinglemodefiberconductsonebeamGaussianbeamHowcanwecomparedifferencebeams?Wehavetointroducesomemeasurebyconvention,knownasthem
3、ode-fielddiameter(MFD),isequalto2w0.Fig.2-23:Mode-fielddiameterMFDisthecross-sectionaldimension2w0,wherethebeam’sintensitydropsto1/e2=1.135Fig.2-23:Mode-fielddiameterCore,cladding,andmode-fielddiameterHerewediscussthestep-indexfiber.Manufacturersusethemode-fielddiameter,MFD,ratherthanthecorediameter
4、asaparameterthatdescribessinglemodefibers.Thisisincontrasttomultimodefiber,wherethegeometricsizesofboththecoreandthecladdingaregiveninthedatasheetsInmultimodefibers,weneedtoknowthecorediameterbecauseallthelightisconfinedwithinthecore.Insinglemodefibers,however,thisstatementisnottrue,asfig5.3,theesse
5、ntialportionofthelight(typically,about20%)iscarriedbythecladding.Somanufacturersdonotevenspecifythecorediameteratall.Effectiverefractiveindex,neffisthecombinationofthecoreandcladdingrefractiveindexes,thisisanothersignificantcharacteristicofasinglemodefiber.TheMFDandOperatingWavelengththeshorterthewa
6、velength,thelesstheMFDtypicalgraphsofMFDasafunctionofwavelengthareshowninfigure5.4.TheMFDisimportantcharacteristicofasinglemodefiber.Whenyouneedtoconnecttwofibers,ifthesetwofibershavedifferentmode-fielddiameters,youwillhaveextrainsertionloss.Thisinsertionlosscanbecalculatedbythefollowingformula:Cuto
7、ffwavelengthFromtheformulaswecanseethecutoffwavelengthInpractice,thefibersaredesignedspecificallyforeithermultimodeorsinglemodeoperation;forthelatter,theoperatingwavelengthisalwayslongerthanλcIntheoth
