除了激光源、偏振分光镜（PBS）和波片等偏振器件之外，非偏振分光镜（NPBS）也是外差干涉仪中重要的非线性误差源。研究了多层介质膜NPBS的退偏效应和方位角对非线性误差的影响。采用p、s分量透射比、反射比、反射相移和透射相移共同表征NPBS的退偏效应，并逐项分析了其非线性误差模型。如果入射光束为理想线偏振光，则只有NPBS的反射相移和透射相移影响测量精度。如果入射光束存在偏振椭圆化和非正交，则NPBS的方位角、透射比和反射比、相移等参数的变化都会引入非线性误差，误差曲线的斜率受椭圆率和非正交角的影响。因此选用不同的激光源，同一个NPBS产生的非线性误差大小不同，一般可达几个纳米量级。为了实现纳米/亚纳米级精度的外差干涉测量，选择性能稳定的NPBS，特别是NPBS退偏效应与激光源输出偏振态之间的匹配非常重要。

In a laser heterodyne interferometer, nonpolarizing beam splitters (NPBS) are also the important nonlinear error sources besides laser sources, polarizing beam splitters (PBS), wave plates and other polarizing components. The influence of depolarization effect and misorientation of a multi-layer dielectric NPBS on the nonlinear error is investigated. The depolarization effect is characterized as the reflectance, transmittance, reflection-induced-retardance (RIR) and transmission-induced-retardance (TIR) between the p- and s- components. The producing mechanism of the nonlinear error is accordingly analyzed. Only the RIR and TIR have contribution to the nonlinear error while the incident light is in ideal polarization state. Otherwise the variation of the misorientation and the depolarization effect will introduce nonlinear error, and the error slopes are affected by the elliptical polarization and non-orthogonality. Therefore the error introduced by the same NPBS varies with laser sources used in interferometers. Generally it will be up to several nanometers. A NPBS with steady performance should be selected to seek nanometer even sub-nanometer accuracy. The parameters match between the depolarization effect of NPBS and the polarization state of laser source should also be attached more importantly.