由于海底应用环境复杂，海底光缆会受到海水的压力、冲击和暗流等作用，具有随机性的偏振模色散(PMD)成为系统极限传输的限制因素。文章对横向应力、弯曲等外界因素对PMD的影响进行了理论分析，根据海底光缆的实际工作情况，模拟4×2.5 Gbit/s系统取3 dB的功率冗余度，进行了PMD仿真，得出系统正常工作时PMD系数不超过1.8 ps/km的结论。为避免PMD补偿，当弯曲半径为0.3 m时，要求横向应力不得大于8.9 N/m。

As submarine cables applied in usually complicated environment are subject to water pressure, impact and subterranean flow, the resulted stochastic Polarization Mode Dispersion (PMD) has become a limiting factor for the system extreme transmission. In this paper, the effects of such external factors as lateral stress and bending on PMD are theoretically analyzed and simulation of the 4×2.5 Gbit/s communication system is performed in accordance with the actual work conditions of the cable, taking 3 dB as the power redundancy. Simulation results show that when the communication system works normally, the PMD coefficient does not exceed 1.8 ps/km. In order to avoid PMD compensation, the lateral stress is not to be greater than 8.9 N/m when the bending radius is 0.3 m.