中文摘要：

相干光正交频分复用（CO—OFDM）对光纤链路中的色度色散（CD）和偏振模色散（PMD）具有较强的容忍性，但是OFDM信号高峰均功率比（PAPR）的特点使其对光纤非线性效应非常敏感，严重影响了系统传输性能。本文提出了基于恒包络（CE）调制的方法使得系统中光信号PA—PR降低为0dB，从而提高了CO—OFDM系统的非线性传输性能。仿真结果表明，子载波采用16QAM调制的40Gbit／s单信道CE调制c0～OFDM系统，在经800km无色散补偿、欠色散补偿和周期全色散补偿标准单模光纤（ssMF）链路传输后，虽然较传统CO—OFDM存在约1．8dB的代价，但是系统最大发射光功率分别提高了6．2、9．3和10．9dB。并且，将本文方案应用到CO—OFDM和10Gbit／sNRZ-OOK混合传输WDM系统中，信道最大发射光功率仍获得了5．2dB的提高。因此，本文提出的CE调制方法能有效地提高C0一OFDM系统在不同传输环境中的光纤非线性容限。

英文摘要：

Coherent optical orthogonal frequency division multiplexing （CO-OFDM） can immune to chro- matic dispersion （CD） and polarization mode dispersion （PMD） in fiber links. However, the large peak- to-average power ratio （PAPR） of OFDM signals makes it susceptible to fiber nonlinearities and conse- quently degrades the transmission performance. In this paper, an approach based on constant envelope （CE） modulation,which can lower the PAPR of the optical signal to 0 dB,is proposed to improve the fi- ber nonlinear performance for CO-OFDM systems. Simulation results show that after 800 km standard single-mode fiber （SSMF） transmission, the maximum launch power of the proposed 40 Gbit/s CE-CO- OFDM system with 16-QAM subcarrier modulation is improved by 6.2 dB,9. 3 dB and 10.9 dB com- pared with conventional CO-OFDM for the dispersion map without dispersion compensation fiber （DCF）, the 100 ps/nrn under-compensated and the fully periodic compensation dispersion maps, respec- tively, although an optical signal-to-noise ratio （OSNR） penalty of - 1.8 dB is observed in the linear re- gion of transmissions. When the scheme is utilized in the hybrid wavelength division multiplexing （WDM） transmission of the 40 Gbit/s CO-OFDM channel with 10 Gbit/s NRZ-OOK channels,a 5.2 dB increase of the maximum launch power is also observed. Therefore,the proposed scheme can effec- tively improve the fiber nonlinear tolerance for CO-OFDM systems in different transmission scenarios.