Research On Nonlinear Compensation And Regeneration Technology For Few-mode Optical Fiber Transmission Systems

With the rapid growth of network bandwidth requirements,the capacity of single-mode optical fiber wavelength division multiplexing?WDM?systems has approached the limit of nonlinear Shannon.In order to adapt to the development of future networks,space division multiplexing?SDM?technology has attracted more and more attention.Mode division multiplexing?MDM?system based on few-mode fiber has become one of the important SDM methods.This thesis studies the nonlinear compensation methods and all-optical regeneration technology for few-mode fiber transmission systems.The main contents and innovations are as follows:1.We investigate the influence of self-phase modulation and cross-phase modulation on the propagation characteristics of guided light in few mode fibers,and present the analytic solution to the corresponding nonlinear coupled-mode equations.Taking quadrature phase shift keying?QPSK?systems with two linearly polarized?LP?modes as an example,the transmission performance of the MDM system dependent on mode loss and nonlinear effects is analyzed by simulation,and the compensation effectiveness for inter-mode phase modulation and the adaptability of the compensation algorithm are also discussed in detail.Research results show that,for correct demodulation of QPSK signals after compensation,the maximum allowable misalignment of mode loss is exponentially dependent on its initial value,and the allowable deviation of the inter-mode nonlinear coefficient is 0.146W^-1 km.In addition,the analytical solution to the nonlinear coupled-mode equation can also be used to calculate the nonlinear coefficients of few mode fibers.2.According to the coupled mode equations with four wave mixing?FWM?in MDM systems,the adaptability of the digital backward propagation?DBP?algorithm in a few-mode fiber transmission system is studied.Our research shows that,for correct QPSK decision,the maximum allowable deviation of the modular nonlinear coefficient?_{1 4} and?_{4 1} are 0.184 W^-11 km and 0.185W^-1 km,respectively.For the given nonlinear coefficients,the error vector magnitude?EVM?values of LP_11a,LP₀₁,LP₀₁,and LP_11b modes can reduce by about 85%after compensation using the few-mode DBP algorithm.We also discuss the effectiveness of DBP algorithm used in few-mode fiber transmission systems by means of the multi-mode Manakov equations or the analytic solutions to the nonlinear coupled-mode equations under some specific cases.3.We put forward a novel structure of pulse amplitude modulation?PAM?regenerator based on few-mode nonlinear optical fiber loop mirror?FM-NOLM?for the first time,and theoretically analyze the working principle for few-mode reshaping.The detailed design steps of the PAM regenerator are also given.The highly nonlinear sulfide fiber and the multimode coupler are used to build up the FM-NOLM and their mode characteristics are simulated by the COMSOL software.The parameters of the regenerator are determined by taking the few mode optical fiber supporting LP₀₁,LP₁₁,and LP₂₁ modes as an example,and then the power transfer function?PTF?curve of each mode is optimized for PAM regeneration.We simulate and analyze the noise reduction ratio?NRR?performance of the few-mode regenerator for PAM-4 signals,and compare with the single-mode counterpart.Our simulation shows that,?1?for each spatial mode of PAM signals,all regenerative levels have the same power transfer performance;?2?for the input signal-to-noise ratio?SNR?greater than 20 dB,the NRR for each mode can exceed 3 dB,and increase with the input SNR at the slope of about1.2;?3?the NRR difference between the three modes is less than 1.1 dB for the same input SNR.This scheme proposed here has some advantages over the existing regenerators in the applicability for the long-haul SDM system with high spectral efficiency and for the regeneration of any level number of PAM signals in theory due to its uniform multi-level regeneration function,but also is capable of being extended to the wavelength domain for higher transmission capacity.