饱和长度对少模光纤光栅影响的数值仿真

对折射率剖面非均匀的少模光纤Bragg光栅(FM-FBG)的反射特性进行了数值仿真，研究了光纤侧面紫外曝光法制作的光纤光栅中饱和长度的影响.用试射法（shooting）数值求解了耦合模方程，得到FM-FBG的反射谱.仿真结果表明，在纤芯和包层边界的最大光致折射率变化量和纤芯吸收因子不变的情况下，随着纤芯折射率变化的饱和长度的增大，LP01和LP11模间的互耦合反射峰呈抛物线形变化,当饱和长度增大到一定值后，互耦合反射峰随饱和长度的增大而逐渐减小并最终消失;同阶LP模的自耦合反射峰随饱和长度的增大而增大.

Numerical Simulation on Effect of Saturation Length for Few-Mode Fiber Grating

The reflection characteristics of few mode fiber Bragg grating (FM-FBG) with non-uniform profile of index change were numerically studied. The reflection spectrum of FM-FBG was obtained by solving the coupled mode equation using shooting method. Under the condition that the maximum index change at the core-cladding interface and absorption coefficient were constant, the numerical results indicated that the reflectivity of cross-mode coupling peak between LP_ 01 and LP_ 11 modes varies parabolically with the saturation length of index change. When saturation length was small, the reflectivity of cross-mode coupling increased with saturation length and reached a maximum value. With the further increasing saturation length,the reflectivity of cross-mode coupling decreased gradually to zero when saturation length increased to the diameter of fiber core. It was also indicated that the peak reflectivities, which correspond to forward modes coupled to the same order back-ward modes, increased monotonically with saturation length.