为了在四频差动激光陀螺腔中得到理想的圆偏振光，采用摄动法分析了偏振度随谐振腔参量的变化规律。对平面腔四频差动激光陀螺，考虑了反射镜各向异性以及水晶片的旋光角和线双折射。对非平面腔四频差动激光陀螺，考虑了反射镜各向异性和异面旋光角。以理想谐振腔的琼斯矩阵为0阶近似，推导了近似到1阶的微扰琼斯矩阵，采用矩阵本征问题的摄动法求出了本征矢量的1阶摄动解，从而可求出光场的偏振度。以旋光90°为例讨论了光场偏振度随反射镜各向异性的变化规律。当偏振度小于0.05时，摄动法求出的解析式与严格数值解在较高的精度上一致。非平面腔四频差动激光陀螺腔内光场偏振度随各反射镜各向异性的变化规律不同于平面腔的情况。该结果对四频差动激光陀螺腔的优化设计具有一定的指导作用。

In order to obtain ideal circularly polarized laser in the four-mode differential laser gyro, the influence of cavity parameters on polarization degree is analyzed with perturbation method. The perturbation matrix to the first order approximation is derived. Not only anisotropies of cavity mirrors but also optical activity and linear birefringence of quartz crystal are considered for planar four-mode differential laser gyro, while anisotropies and nonplanar rotation angle of carity mirrors are considered for nonplanar four-mode differential laser gyro. The first order perturbation solution of eigen-vector is obtained with perturbation method for eigen problem of matrix. The variation of polarization degree as a function of anisotropies of cavity mirrors is discussed when the rotation angle of the cavity is 90°. Expressions derived from perturbation method agree well with strict numerical solutions when polarization degree is less than 0.05. Polarization degrees of four-mode differential laser gyros as a function of anisotropies of mirrors are different between palanar and nonplanar cavity. The results have certain instructive effect for optimizing the cavity design of four-mode differential laser gyro.