Fiber bragg grating based devices

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1、FiberbragggratingbaseddevicesInrecentyearsfiberBragggratingbasedhavebecomepopularduetheirextremelystablecharacteristics.TheFiberBraggGrating(FBG)isaperiodicstructurefabricatedinsidethecoreoftheopticalfiber.Theperiodicitycouldbemechanicallikevariationofthecorediameteroritcouldbeelectricallikevariatio

2、noftherefractiveindexofthecore.IntheperiodicstructureliketheFBGthecouplingofenergybetweendifferentco-propagatingandcounter-propagatingmodesofthefibertakesplace.Themodecouplingphenomenonisastrongfunctionofwavelength.InFBGtwoidenticalcounterpropagatingmodesgetcoupledandtheenergyistransferredfromthefor

3、wardtravelingtothebackwardtravelingmode.Consequentlywegetreflectionofthemodalenergywhichiswavelengthdependent.TheFBGthereforereflectscertainwavelengthskeepingpropagationofotherwavelengthspracticallyun-affected.TheFBGsfindapplicationsinvarietyofdevices.ApplicationsofFiberBraggGrating•Narrowbandfilter

4、ing•WDMAdd/DropMux/DeMux•Dispersioncompensation•GainEquilization•Fiberlaser•RamanAmplifier•Phaseconjugator•Wavelengthconverter•ModeconverterPrincipleTheFigShowsthebasicFBGfabricationprocess.Ifahydrogenloadedfibercoreisexposedtointenseultraviolatebeam,therefractiveindexofthefibergetsmodifiedpermanent

5、ly.Now,makingtwocoherentultraviolatebeamsinterfereonthecoreofthefiber,thefiberisexposedtoperiodicultraviolateintensityvariation.Thisthencausesapermanentperiodicvariationoftherefractiveindexofthecore.Thepeakchangeinrefractiveindexisoftheorderof.Themodecouplingphenomenonisweekandoneneedasubstantiallen

6、gthoftheFBGtoachievegoodreflectionofthesignal.ThesizeoftheFBGistypicallyfewthousandwavelengths.AccordingtothemodecouplingphenomenonthetwomodesshowstrongcouplingiftheysatisfytheBraggconditionWhere,andarethephaseconstantsofthetwomodes,istheperiodofthevariationoftherefractiveindex(itisassumedthatthevar

7、iationissinusoidal),andisanintegerwhichdefinestheorderofdiffraction.Forfirstorder,.Nowifwetaketwoidenticalcounterpropagatingmodes,AndtheBraggdiffractionconditionbecomesNowiftheeffectivemodalindexis,,w