相干光正交频分复用系统（CO-OFDM）是未来高速光传输的重要解决方案，而偏振模色散（PMD）与偏振相关损耗（PDL）相互作用严重影响传输性能。分析了CO-OFDM系统中的偏振效应和数学模型，并通过仿真加以验证。结果表明，一阶PMD效应对正交频分复用(OFDM)信号子载波幅度附加余弦因子；二阶PMD效应中，去偏振项（DR）占主要因素，且一定程度上可缓解色度色散带来的相位噪声；另外，由于PMD带来OFDM子载波偏振度随频率变化加强，也对PDL带来的系统损伤有一定抑制作用，在不考虑色度色散，相对损耗因子α=0.5，单模光纤传输距离为720 km条件下，系统Q值约有2 dB提高。

Coherent optical orthogonal frequency division multiplexing (CO-OFDM) system is considered as a promising technology for high speed optical transmission. Polarization dependent loss (PDL) and polarization mode dispersion (PMD) influence system performance seriously in high speed transmission. The mathematical model and mechanism of transmission affected by first-order and second-order PMD and PDL are presented. Analysis and simulation results show that first-order PMD adds cosine factors for different OFDM subcarriers and in second-order PMD, CO-OFDM system performance is mainly influenced by depolarization rate which can moderate phase noise induced by chromatic dispersion. In addition, PMD can not only make subcarrier polarization strengthen with the increase of frequency, but also moderate noise induced by PDL. At the fiber link of 720 km, loss factor of 0.5 and without chromatic dispersion consideration, system Q value is improved about 2 dB over the case of no second-order PMD.