Research On Nonlinear Damage Compensation Technology Of Coherent Optical Transmission System Based On Auto-encoder

With the explosive growth of network service demand,optical fiber communication is developing rapidly in the direction of hypervelocity,high capability and long distance.The application of digital coherent optical transmission technology lead to the enhancement of the transmission capacity and transmission distance of the system.Linear damage in the transmission process,such as chromatic dispersion and polarization mode dispersion,can be effectively compensated by the digital signal processing algorithm of the receiver,but the nonlinear effect of optical fiber can not be effectively compensated with low complexity.At the same time,with the extensive use of higher-order modulation format which has high spectral efficiency,higher optical signal to noise ratio is needed to ensure the signal quality.Therefore,higher launch power is needed in transmission,but this will lead to serious nonlinear effect.Therefore,the nonlinearity of optical fiber has become the primary cause restricting the performance of high capacity long-distance transmission system.In recent years,there is one of the new research directions to improve the performance of the system by using auto encoder to make geometric shaping of constellation.It can learn the optimal constellation under specific channel conditions by the strong learning ability of machine learning,and realize the characteristics of low complexity.This paper will focus on how to use auto encoder to reduce the nonlinear effect of optical fiber and improve the output power of optical modulator.1.One of the key points of nonlinear damage suppression scheme based on auto encoder is to determine the appropriate fiber channel model.In this paper,the mainstream nonlinear effect model of optical fiber are split-step Fourier method,Gaussian noise model,Enhanced-Gaussian noise model and NLIN model.According to the requirements of the scheme for fiber model,we choose NLIN model for its high accuracy and character of reflecting modulation format.2.Directing at the problem of the performance of high-order modulation and high capability long-distance coherent optical transmission system is subject to the nonlinearity of optical fiber,we propose a new scheme of optical fiber nonlinear damage suppression based on auto encoder.It mainly includes the structure of auto encoder that has the NLIN model embedded into it and the damage compensation digital signal processing algorithmic design of the coherent optical receiver which is appropriate for the geometric constellation shaping.The scheme can be used to reduce the nonlinear damage of optical fiber by embedding NLIN model in the auto encoder.By embedding the NLIN model in the auto encoder,the geometric shaped constellation learned the ability to mitigate the nonlinear damage,and by the pilot-based and single side band digital signal processing algorithm,the linear damage can be compensated normally under the geometric shaped constellation.The simulation results of 3 × 32GBaud PM-64QAM wavelength division multiplexing coherent optical transmission system indicate that the presented method can effectively suppress the nonlinear damage of optical fiber,increase the optimal fiber power of the system by 0.5dB,and increase the power range of fiber at the bit error rate 2E-2 by 2dB,and the transmission distance is extended by 80km.3.In order to increase the performance of the high-order modulation system,a joint suppression scheme of nonlinear damage of optical fiber and optical modulator based on geometric shaping using auto encoder is designed.The scheme not only embeds NLIN fiber model but also adds the nonlinear characteristics of optical modulator in the structure of auto encoder,which makes the constellation after geometric shaping can restrain the nonlinear damage of optical fiber,and overcome the nonlinear modulation damage of optical modulator.The simulation results of 3 × 32GBaud 64QAM wavelength division multiplexing coherent optical transmission system show that the proposed scheme can effectively suppress the nonlinear damage of optical fiber and modulator,and can improve the maximum output optical power of optical modulator by 3dB.