Study On Theory For Mode Coupling In Mode Division Multiplexing Optical Communications System

With the rapid development of artificial intelligence,the Internet of Things,big data,online audio and video services,cloud computing,AR + VR,and 5G mobile communication technologies,global data traffic has grown exponentially.At present,single mode fiber adopted in the global backbone communication network has approached the nonlinear Shannon limit after the application of multi-dimensional multiplexing such as time division multiplexing,wavelength division multiplexing and phase modulation,etc.The sharp contradiction between the rapid growth of data traffic and the weak growth of optical fiber capacity will leads up to a crisis of capacity crunch.The concept of mode division multiplexing(MDM)was proposed in order to further increase the capacity of optical fiber communication systems.MDM has two main implementation forms: LP mode multiplexing and OAM mode multiplexing.In an ideal state,the multiplexing modes are orthogonal to each other and do not interfere with each other.Therefore,in theory,the modes can be infinitely multiplexed,which greatly increases the capacity of the optical fiber communication system and is the most promising solution to solve the capacity crunch crisis in the future.However,in practical applications,optical fibers are inevitably perturbed due to various internal factors(such as fiber structure defects caused by incomplete fiber manufacturing processes)and external factors(such as fiber bending,twisting,and compression caused by stress).The orthogonality between the multiplexed modes is destroyed,causing coupling between the modes,which in turn causes crosstalk between the signals loaded on the modes,which seriously affects system performance.Mode coupling is an important factor restricting the application of MDM to long-distance information transmission.Therefore,the research and analysis of mode coupling mechanisms and laws is a very important subject.This article focuses on the problems of mode coupling in mode division multiplexing optical communications systems.The main research contents are as follows:1.Theory for mode coupling in perturbed fibers is proposed.Based on the proposed mode coupling theory and vector mode theory,a numerical solution method is proposed to analyze the mode coupling components and solve the coupling coefficients of each coupling mode.This provides theoretical guidance for quantitative analysis of mode coupling in communication systems,as well as receiver recovery and compensation of signals.2.Based on the proposed mode coupling theory and numerical solution method,the mechanism of mode coupling under the perturbation of ring-core ellipticity and misalignment and its influence on signal transmission are analyzed quantitatively.Based on the selfconsistency of the mode coupling theory,the error analysis of the numerical calculation results is performed.This lays a certain theoretical foundation for the design of new optical fibers that can perform OAM mode transmission with high quality.3.The spin angular momentum(SAM)and the orbital angular momentum(OAM)of the Bessel mode commonly used in optical fiber information transmission is theoretically analyzed,and the purity of the OAM modes in the optical fiber is analyzed.Designed and completed the experiments of OAM mode excitation,reception and interference with the fundamental mode.This provides theoretical guidance for the construction of the OAM mode multiplexing system and its application to long-distance information transmission.