中文说明：
四阶龙格库塔数值方法，用于求解光纤光栅非线性耦合模方程，可求解正向，反向传输波幅度
English Description：
The fourth-order Runge Kutta numerical method is used to solve the nonlinear coupled mode equation of fiber Bragg grating, which can solve the forward and reverse transmission wave amplitude