The Theoretical Study Of Nonlinear Effect Estimation Method In High-Speed Optical Transmission Systems

In high speed long haul optical transmission systems, in order to overcome the fiber loss during transmission, and also obtaining a sufficient optical signal noise ratio (OSNR) at the receiving end, a large launch power at the transmit end is needed. However, large launch power may result in serious fiber nonlinear effects, which may hugely impact the optical transmission systems. So it is necessary to study the non-linear effects in ultra-high speed long-haul optical transmission systems.In this paper, we study the estimation method of nonlinear effects in high-speed optical communication system. The main content is divided into two parts:First, the theoretical study of nonlinear effect based on uncompensated transmission (UT) single carrier polarization multiplexing optical communication system, encompassing different optical communication system link parameters and different modulation formats. To validate the accuracy of the Gaussian noise (GN) model in which the fiber nonlinearity is modeled as an additive GN process, we carried out a thorough simulative validation of that GN model in signal carrier systems. The Minimum Mean Square Error (MMSE) algorithm is used for removing the correlation of linear signal and nonlinear signal to get use the simulation results of nonlinear noise power spectral density (PSD). Through function substitution method and interpolation function, we got the theoretical value of nonlinear noise PSD. By comparing the nonlinear noise simulation results and theoretical results, we got the application scope of GN model.Second, the theoretical study of nonlinear effect based on dispersion-managed transmission (DMT) Nyquist wavelength division multiplexing (Nyquist-WDM) polarization multiplexing optical communication system, encompassing different link parameters in optical communication system. Through the comprehensive in-depth research we got the following conclusions:First, the nonlinear effect estimation method based on the Gaussian noise (GN) model has certain limitation. The reason maybe that the transmitted signal statistically behaves as stationary Gaussian noise is the fundamental hypothesis of the GN model.Second, the initial transient signal will become Gaussian distribution through applying pre-dispersed (PD) at the TX. The results consistently indicated that the GN model analytical approximation for the NLI PSD with pre-dispersion is rather accurate and dependable in all the addressed systems scenarios.Third, through the analysis of the GN model expression in the UT system, we provided revised analytical expressions for estimating nonlinear noise by curve fitting. Simulation results show the simple closed-form analytical expressions can provide an effective tool for the quick and accurate prediction of system performance in DMT coherent optical systems.