中文说明：
模式传输的光束质量，模式经过增益饱和放大的光束质量因子的模拟主要运用的方法是最小二乘法拟合。其思路是用分步傅里叶法得到多个不同位置的光强分布，然后通过二阶矩方法得到束半宽，用最小二乘法拟合得到传输半径的一元二次方程，形式如下
English Description：
The least square method is mainly used to simulate the beam quality of mode transmission and the beam quality factor of mode after gain saturation amplification. The idea is to use split step Fourier method to get the light intensity distribution at different positions, then use the second moment method to get the beam half width, and use the least square method to fit the quadratic equation of one variable of the transmission radius