光纤的概念和技术简介(英)

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1、AnIntroductionToFiberOpticConceptsAndTechnologiesGregoryB.NollCableConstructionTherearetypicallyfiveelementsthatmakeuptheconstructionofafiberopticcable:theopticcore,opticcladding,buffer,strengthmemberandouterjacket.Theopticcoreisthelight-carryingelementatthecenteroftheopticalfiber.Itisco

2、mmonlymadefromacombinationofhighlypurifiedsilicaandgermania.Surroundingthecoreistheopticcladdingmadeofpuresilica.Thecombinationofthesematerialsmakestheprincipleoftotalinternalreflectionpossible,asthedifferenceinmaterialsusedinthecoreandthecladdingcreatesanextremelyreflectivesurfaceatthep

3、ointinwhichtheyinterface.Lightpulsesenteringthefibercorereflectoffthecore/claddinginterfaceandthusremainwithinthecoreastheymovedowntheline.Surroundingthecladdingisabuffermaterialwhichactsasashockabsorbertoprotectthecoreandcladdingfromdamage.Astrengthmember,typicallyAramid,surroundsthebuf

4、feraddingcriticaltensilestrengthtothecabletopreventdamagefrompullforcesduringinstallation.Theouterjacketprotectsagainstabrasionandenvironmentaldamage.Thetypeofjacketusedalsodefinesthecable'sdutyandflammabilityrating.Raysoflightpassingthroughafiberdonottravelrandomly.Rather,theyarechannel

5、edintomodes-thethousandsofpossiblepathsalightraymaytakeasittravelsdownthefiber.Afibercansupportasfewasonemodeandasmanyastensofthousands.Thenumberofmodesinafiberissignificantbecauseithelpsdeterminethefiber'sbandwidth.Multimodefiberhasamuchlargercorethansinglemodefiber,allowinghundredsofra

6、ysoflighttopropagatethroughthefibersimultaneously.Singlemodefiberhasamuchsmallercore,allowingonlyonemodeoflighttopropagatethroughthecore.Paradoxically,thehigherthenumberofmodes,thelowerthebandwidthofthecable.Thereasonisdispersion."Modal"dispersioniscausedbythedifferentpathlengthsfollowed

7、bylightraysastheybouncedownthefiber(someraysfollowamoredirectroutedownthemiddleofthefiber,andsoarriveattheirdestinationwellbeforethoserayswhichwastetheirtimebouncingbackandforthagainstthesides)."Material"dispersionoccurswhendifferentwavelengthsoflighttravelatdifferentspee