Fiber bragg grating based devices英文英文学习教学教案.doc

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1、FiberbragggratingbaseddevicesInrecentyearsfiberBragggratingbasedhavebecomepopularduetheirextremelystablecharacteristics.TheFiberBraggGrating(FBG)isaperiodicstructurefabricatedinsidethecoreoftheopticalfiber.Theperiodicitycouldbemechanicallikevariationofthecorediameterorit

2、couldbeelectricallikevariationoftherefractiveindexofthecore.IntheperiodicstructureliketheFBGthecouplingofenergybetweendifferentco-propagatingandcounter-propagatingmodesofthefibertakesplace.Themodecouplingphenomenonisastrongfunctionofwavelength.InFBGtwoidenticalcounterpro

3、pagatingmodesgetcoupledandtheenergyistransferredfromtheforwardtravelingtothebackwardtravelingmode.Consequentlywegetreflectionofthemodalenergywhichiswavelengthdependent.TheFBGthereforereflectscertainwavelengthskeepingpropagationofotherwavelengthspracticallyun-affected.The

4、FBGsfindapplicationsinvarietyofdevices.ApplicationsofFiberBraggGrating•Narrowbandfiltering•WDMAdd/DropMux/DeMux•Dispersioncompensation•GainEquilization•Fiberlaser•RamanAmplifier•Phaseconjugator•Wavelengthconverter•ModeconverterPrincipleTheFigShowsthebasicFBGfabricationpr

5、ocess.Ifahydrogenloadedfibercoreisexposedtointenseultraviolatebeam,therefractiveindexofthefibergetsmodifiedpermanently.Now,makingtwocoherentultraviolatebeamsinterfereonthecoreofthefiber,thefiberisexposedtoperiodicultraviolateintensityvariation.Thisthencausesapermanentper

6、iodicvariationoftherefractiveindexofthecore.Thepeakchangeinrefractiveindexisoftheorderof.ThemodecouplingphenomenonisweekandoneneedasubstantiallengthoftheFBGtoachievegoodreflectionofthesignal.ThesizeoftheFBGistypicallyfewthousandwavelengths.Accordingtothemodecouplingpheno

7、menonthetwomodesshowstrongcouplingiftheysatisfytheBraggconditionWhere,andarethephaseconstantsofthetwomodes,istheperiodofthevariationoftherefractiveindex(itisassumedthatthevariationissinusoidal),andisanintegerwhichdefinestheorderofdiffraction.Forfirstorder,.Nowifwetaketwo

8、identicalcounterpropagatingmodes,AndtheBraggdiffractionconditionbecomesNowiftheeffectivemodalindexis,,w