多程放大系统隔离能力关系到激光器的安全稳定运行。由于系统中反射光学元件膜层引起的退偏会导致系统隔离能力大幅降低，因此无法满足设计要求。以神光III主机装置为例介绍了主放大系统隔离技术，并分析了光学膜层退偏机理，离线实验观测了单个反射元件的退偏现象。研究发现引起光学膜层退偏的主要原因是膜层厚度及折射率设计误差。根据实际光路中采用的角反射器元件反射率指标计算得到光学薄膜退偏将系统隔离能力限制在32，介绍了系统解决措施。

The running safety of a multipass amplifier laser system needs high isolation ratio. The isolation ratio significantly decrease due to light depolarization in optical coatings of the reflecting elements, which does not meet the needs for safe running. Based on the SG-III laser system, the isolation technique of main amplifier is introduced and the optical coating depolarization mechanics is analyzed. Depolarization phenomenon is observed in single reflecting mirror. It is found that the reasons for depolarization are coating thickness error and refraction error is designed. The actual isolation ratio is restricted around 32 for a real beam path, and the resolvent is given in the end.