Technological Reseach Of Phase Noise Suppression In CO-OFDM Systems

In recent years, coherent optical orthogonal frequency division multiplexing system(CO-OFDM) has attracted widespread attention in large-capacity transmission, for its advantages of OFDM and coherent reception. However, OFDM has a longer symbol length, and thus is more prone to phase noise. Due to inter-carrier interference(ICI) and the common phase error(CPE) caused by phase noise, the transmission performance of the system becomes worse. With the rapid development of digital signal processing(DSP) technology, there are many methods to compensate for linear phase noise caused by the laser linewidth, such as pilot,training sequence and so on. Meanwhile, there are many algorithms that can reduce nonlinear phase noise caused by fiber nonlinear, such as digital pre-compensation, backward propagation methods, to strengthen the system of nonlinear phase noise tolerance.However, the cost is generally higher, or the computational complexity is higher.This paper presents a digital coherent(DCS) superposition to combat the system phase noise at the expense of the spectral efficiency. At the transmitter, we simply use a real-valued OFDM signal to drive a Mach-Zehnder modulator(MZM) biased at the null point and the so-generated OFDM signal is Hermitian in the frequency domain. At the receiver, after the conventional OFDM signal processing, there is DCS for OFDM subcarriers pairs, which requires conjugation and summation. In this paper, for the advantage of easy implement and low computional complexity, the proposed method is applied into CO-OFDM systems, and the main study includes:1. The proposed algorithm is applied to CO-OFDM system.At the transmitter, there is an MZM, which is much easier to implement. This algorithm can effectively reduce the inter-carrier interference(ICI) due to the linear phase noise caused by laser linewidth and the non-linear phase noise caused by optical fiber nonlinear. By simulation and experiment analysis, the CO-OFDM system with DCS has a better transmission performance than the traditional CO-OFDM with I/Q modulation.2. With symmetrical dispersion compensation, the proposed method in this paper can effectively mitigate the first-order nonlinear distortions and hence improve the tolerance of CO-OFDM to fiber nonlinear effects. In simulation, the 71.53 Gbit/s OFDM signal per channel with Hermitian symmetry is transmitted over 400 km single mode fiber(SMF) in WDM-PDM-CO-OFDM with five channels. 4-quadrature amplitude modulation(QAM) is used for symbol mapping. Firstly, the channel spacing is 25 GHz, the maximum signal-to-noise ratio(SNR) improvement is 9.05 dB or 6.02 dB with or without dispersion compensation compared with the conventional OFDM. 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 spacing in WDM scheme, the maximum SNR improvement is 8.75 dB or 4.9 dB with or without dispersion compensation, respectively. In a PDM-CO-OFDM simulation system, the optimum launch power is increased 2 dB or 3 dB after 1600 km or 3200 km, respectively, and the SNR improvement is 7.24 dB and 5.24 dB, respectively.