中文摘要：

光正交频分复用系统中的光纤非线性效应制约着系统进一步的扩容.针对此问题,提出一种数字相干叠加的方法,用于提高相干光正交频分复用系统对光纤非线性的容忍性.仿真中,5通道的波分复用下偏振复用相干光正交频分复用系统的每个通道传输四进制正交振幅调制映射的71.53 Gbit/s信号在光纤中传输400km.首先,通道间隔为25 GHz,与传统相干光正交频分复用系统相比,色散补偿前后,使用数字相干叠加的相干光正交频分复用系统的信噪比分别提升了6.02 d B和9.05 d B,最佳入纤光功率均增大了2 d B;其次,通道间隔为50 GHz,色散补偿前后,信噪比分别提升了4.9 d B和8.75 d B.通过理论推导及仿真,验证了所提方法能有效消除相干光正交频分复用系统的一阶非线性失真,进而提高系统对光纤非线性的容忍性.

英文摘要：

Fiber nonlinearity of optical orthogonal frequency division multiplexed （OFDM） system restricts the capacity im- provement of optical fiber transmission. In this paper, we propose a novel digital coherent superposition （DCS） scheme to improve the tolerance to fiber nonlinearity in a coherent optical orthogonal frequency division multiplexed system. In simulation, 71.53 Gbit/s orthogonal frequency division multiplexed signal per channel with Hermitian symmetry is trans- mitted over 400 km standard single mode fiber in a wave division multiplexed-polarization-division multiplexed-coherent optical orthogonal frequency division multiplexed system with five channels. The 4-quadrature amplitude modulation is used for symbol mapping. For the receiver, after the conventional OFDM signal processing, we conduct DCS for OFDM subcarrier pairs, which requires only conjugation and summation in the x, y polarization direction, respectively. Firstly, the channel spacing is 25 GHz, the maximum signal-to-noise ratio improvement is 9.05 dB or 6.02 dB with or without symmetric dispersion compensation compared with a conventional orthogonal frequency division multiplexed system. The optimum launch power is increased by 2 dB. Secondly, the channel spacing is changed to 50 GHz to investigate the nonlinearity tolerance at different channel spacings in the wave division multiplexed system, the maximum signal-to-noise ratio improvement is 8.75 dB or 4.9 dB with or without symmetric dispersion compensation, respectively. Theoretical and simulation analysis show that the proposed method in this paper can effectively mitigate the first-order nonlinear distortions and hence improve the tolerance of coherent optical orthogonal frequency division multiplexed system with different channel spacings to fiber nonlinear effects.