Nonlinear Impairments Mechanism Analysis And Compensation Methods Research In High-Speed Fiber Coherent Transmission System

Although the linear impairments have been compensated nearly perfectly based on high-order modulation formats and DSP-based coherent detection technology, the nonlinear impairments of commercial100G optical fiber transmission system are becoming more and more obvious. With the increasing of channel capacity, the order of modulation format, the transmission distance and the fiber input power, the channel spacing is decreasing. As a result, the nonlinear effects will play a more important role in the400G and1G transmission systems in the future:(1) the fiber input power will be larger with the increasing of OSNR. The higher single-channel transmission rate and the order of modulation format, the higher OSNR;(2) the total input power will be improved due to more flexible and diverse channel, the implemention of super-channel which will decrease the channel spacing. In a word, research on nonlinear compensation methods has no time to delay.Recently, study on nonlinear compensation based on Back-propagation(BP) algorithm is in full swing. In order to eliminate or mitigate the effects to system performance caused by the nonlinear impairments, BP algorithm solve the received signal reversely based on the nonlinear Schrodinger equation or Manakov equation of light transmission in fiber. However, the contradictions between the performance and complexity of the algorithm still exist.In this paper, the nonlinear characteristics of fiber and its effects are theoretically analyzed, and the simulation and simplification on complexity of BP algorithm are studied, based on the research topic of mechanism analysis and compensation methods of nonlinear impairments in high-speed optical coherent transmission system. Details are as follows:(1)Nonlinear Schrodinger equation and Manakov equation are derived according to nonlinear polarizability of fiber, which are based on to analyze the origin of several nonlinear impairments and their effects to fiber transmission system;(2)The principle of BP algorithm are elaborated and a PM-16QAM optical fiber transmission simulation platform is set up, obtaining the impact of ADC sampling rate and iteration step size on the performance of BP algorithm;(3)The BP algorithm with simplified filters is employed according the simulation results that the performance of BP algorithm compensating the nonlinear phase noise is very poor. The full-time-domain dispersion and nonlinear impairments compensation can be achieved, when FIR filters using window function are improved to ideal Gaussian filters. The computational complexity can be greatly reduced when the number of FFT transform is decreased. Based on research on the single-channel PM-QPSK and multi-channel PM-16QAM experimental platform, results are obtained that good compensation effect can be achieved when FIR filters with a few taps are employed whose frequency-domain response are similar to Gaussian distribution. In contrast, the fiber input power threshold of linear compensation is2dBm lower, the BER beyond1e-3, and the monitoring range of OSNR is4-5dB larger than before. In addition, the results also validate that the performance of the algorithm can be further improved when the loss of fiber link loss is reduced and the nonlinear phase noise is no longer a Gaussian distribution, which verified the effectiveness of the proposed algorithm on the other hand.