中文摘要：

相干光正交频分复用（COOFDM）是目前光传输领域的研究热点之一。COOFDM系统采用马赫-曾德尔光调制器（MZM）实现射频（RF）信号到光的转换,而正交频分复用（OFDM）信号对非线性十分敏感,所以在系统设计上最关键的就是实现信号的线性传输。从理论上分析了COOFDM系统中MZM对OFDM信号的非线性影响,通过对MZM偏置点及调制指数优化,实现COOFDM系统的最佳传输,同时对COOFDM系统品质因子Q与偏置点及调制指数关系进行了仿真。结果表明,为了实现最佳线性传输,不同于传统的基于强度调制/直接检测系统（偏置点选在积分点）,COOFDM系统中MZM最佳偏置点选在零点;MZM调制指数的选择也会对系统性能产生影响,当COOFDM系统中MZM调制指数为0 dB时,系统性能达到最佳。

英文摘要：

Coherent optical orthogonal frequency-division multiplexing（COOFDM） is being considered as a promising technology for future optical transmission.In COOFDM systems,optical Mach–Zehnder modulators（MZM） are used to up-convert orthogonal frequency-division multiplexing（OFDM） signal from radio frequency（RF） domain to optical domain.However,OFDM is very sensitive to nonlinearity,so subsequently study of nonlinearity impact from MZM is of great interest.The impact of COOFDM systems introduced by the nonlinearity of MZM is focused on and a numerical simulation to identify system Q-value under various MZM bias points and modulation index is performed.Results show that in contrast to intensity-modulation/direct-detected system（bias point is quadrature point）,optimal modulator bias point is zero point and optimal modulation index is 0 dB for COOFDM system.