Study On The Characteristics Of Fiber Gratings Made Of Uniaxial Crystal Materials

A series of fiber gratings with cladding or core made of uniaxial crystal materials whoseoptical axis is parallel to the axis of grating,i.e.,z-axis are proposed.These fiber gratings modelsinclude short period fiber grating with cladding made of uniaxial crystal,long period fibergrating with cladding made of uniaxial crystal,coaxial short period fiber grating with core madeof uniaxial crystal and coaxial long period fiber grating with core made of uniaxial crystal.Thelinear and nonlinear characteristics have been theoretically analyzed using linear and nonlinearcoupled-mode theory respectively.The calculated results indicated that unxiaxial crystalmaterials have strong impact on the characteristics of these gratings.The degree of the impact isclosely related to the value of the ratio of the extraordinary to the ordinary ray refractive indexK_clof uniaxial crystal.The influence of K_clto the characteristics of fiber grating is mostremarkable in the coaxial long period grating with core made of uniaxial crystal.When K_clvariesfrom 0.9 to 1.1,the resonant wavelength has the increases of 65nm.The relationships betweenK_cland grating characteristics,especially the resonance wavelength,can be utilized to measuresome characteristics of the uniaxial material.Numerical results showed that K_clhas also strongimpact on the fiber grating's nonlinear characteristics.When the ratio of index has changed from0.6 to 1.6 the red shift of Bragg wavelength due to 160kW input pulse peak power has increasedfrom 0.22nm to 0.34nm.Meanwhile the transmissivity has increased from 0.25 to 0.95 whenpeak power of input pulse is 1.5kW.It is indicated that it is propitious to develop nonlinearapplication of FBG using uniaxial positive crystal as a grating cladding.Electro-optic effect inthese kinds of fiber gratings are theoretically analyzed for the first time.The influences of theelectric field applied to the uniaxial crystal materials along z-axis on Bragg wavelengthλ_B orresonance wavelength and the reflectivity spectra of this series gratings are examined.Thecurves of Bragg wavelength or resonance wavelength as a function of an external electric fieldare obtained respectively for three kinds of uniaxial crystal materials as the grating cladding orcore.The calculated results indicate that the coaxial long period fiber grating with core made ofuniaxial crystal has been affected most remarkably by electric field.When the axial electric fieldapplied to the grating cladding varies from 1×10⁶ V/m to 8×10⁶ V/m,resonant wavelength hasthe decreases of 3.5nm.This promises the good application foreground of this new kind ofgrating.New-style electric field senor and tunable optical fiber grating filter can be manufacturedbased on the property that resonance wavelength can be changed by electric field.