光纤通信(第五版)(英文版)[(美)帕勒里斯著][样章].pdf

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1、PalCh03v3.qxd9/8/041:08PMPage61CHAPTER3LightwaveFundamentalsWavepropagationisimportantinfiberoptics.Inthischapter,wepresentfundamentalaspectsofwavetravelthatareparticularlyvaluable.Forsomereason,theprospectofstudyingelectromagneticwavesfrightensmanypeople.Indeed

2、,expositionsofelectro-magnetictheoryareoftenquiteformidable.Thediscussiontofollowisascheerfulandpainlessaspossible.Importantresultsareexplained,butthelengthyderivationsre-quiredtodevelopthemareomitted.Mathematicsisminimized.Thespecificconceptsdevelopedarevelocit

3、y,power,dispersion,polarization,interference,andreflectionsatboundaries.Alloftheserelatedirectlytofiberoptics.3.1ELECTROMAGNETICWAVESLightconsistsofanelectricfieldandamagneticfieldthatoscillateatveryhighrates(ontheorderof1014Hz).Thesefieldstravelinwavelikefashio

4、natveryhighspeeds.Apictureofanelectromagneticwave[1]travelingalongthezdirectionappearsinFig.3.1.Theelectricfieldisplottedatthreetimes,showingtheprogressofthewave.Atanyfixedlocation,thefieldamplitudevariesattheopticfrequency.Theamplituderepeatsitselfafteroneperio

5、doftheoscillation.Thewaverepeatsitselfinspace,atafixedtime,afteradistancel.Thisdistanceisthewavelength.Itsreciprocal,1/l,isthewavenumber.TheelectricfieldforthewavesketchedinFig.3.1canbewrittenasE=Eosin1vt-kz2(3.1)whereEoisthepeakamplitude,v=2pfrad/s,andfisthefre

6、quencyinhertz.Thefac-toriscalledthevradianfrequency.Thetermkisthepropagationfactor.Itisgivenbyvk=(3.2)vwherevisthephasevelocityofthewave.Thefactorvt-kzisthephaseofthewave,andkzisthephaseshiftduetotraveloverlengthz.Aplanewaveisonewhosephaseisthesameoveraplanarsur

7、face.Inthepresentexample,thephaseisthesameover61PalCh03v3.qxd9/8/041:08PMPage6262Chapter3LightwaveFundamentalsFIGURE3.1Electricfieldforawavetravelinginthezdirec-tion.Thefieldisdrawnatthreedifferenttimestoillustratethemotionofthewaveinthedirectionoftravel.anyplan

8、edefinedbyafixedvalueofz,sothatEq.(3.1)representsaplanewave.Iftimeisheldconstant,thenEq.(3.1)showsthesinusoidalspatialvariationofthefield.Forexample,ift=0,thenE=Eosin