Research On Several Key Devices Of Orbital Angular Momentum Optical Communication System

In recent years,with the rapid development of the Internet of Things,cloud computing,6G mobile communication,artificial intelligence and other emerging big data technologies and the rapid development of business,the transmission capacity of the optical communication system has been put forward higher requirements.However,the transmission capacity of the traditional single-mode optical fiber communication system is close to the nonlinear Shannon limit.In order to cope with the coming"capacity crisis",researchers begin to explore the spatial dimension resources of light waves.By increasing the number of parallel channels in space,the capacity and spectral efficiency of communication systems can be greatly improved.In the spatial dimension resources,the mode multiplexing technology based on the orthogonality of orbital angular momentum（OAM）modes is expected to be one of the most promising schemes to improve the communication capacity and spectral efficiency.This paper focuses on several key devices involved in OAM mode optical communication system,including OAM mode generators,transmission fibers and multiplexing/demultiplexing devices.The main research contents and innovations are as follows:（1）According to the mode rotation theory of asymmetric waveguides,the principle of OAM mode generation in rectangular double-trench waveguides is analyzed.The design principle of OAM transmission fiber is analyzed by using perturbation theory,which provides a theoretical basis for designing the structural parameters of OAM transmission fibers.The principle of OAM mode synthesis in ring fiber is deduced by solving the intrinsic mode of the ring fiber.（2）The double-trench high-order OAM mode generator based on silicon-based waveguides is proposed.Using the finite difference time domain method（FDTD）,the device structure parameter optimization and performance are analyzed.The simulation results show that the device can not only generate high-purity OAM_±1,but also generate high-order OAM_±2 modes.Based on the coupled mode theory,the mode conversion in waveguide is studied.By cascading the mode converter and OAM mode generator,a compact on-chip multiplexer/demultiplexer of five OAM modes is designed,and the crosstalk between most modes is about-30 d B.（3）Two kinds of two-dimensional circular photonic crystal OAM transmission fibers of the RC-PCF and SC-PCF with rectangular air holes and square air holes are proposed.By constructing an equivalent M-type step ring fiber model of circular photonic crystal fiber,the equivalent refractive index of the M-type fiber outer layer is derived and substituted into the full vector wave equation of the equivalent model.The cutoff frequency equation of each mode is obtained.Then the cutoff frequency V_effof each mode is plotted as a function of the inner and outer diameter ratioρof the ring,and the number of OAM modes supported by the two structures can be analyzed.Two-dimensional finite element method（2D-FEM）is used to optimize the structural parameters and analyze the transmission performance.The simulation results show that RC-PCF can support up to 82 OAM modes,the confinement loss is as low as 10^-9d B/m,and the nonlinear coefficient of SC-PCF is less than 1.9 km^-1·w^-1.Compared with the existing circular photonic crystal fiber（C-PCF）structure,the two designed OAM transmission fibers have great improvement in the number of modes and transmission performances.（4）The one-dimensional PCF,i.e.a special Bragg fiber structure that can transmit OAM modes is proposed.Through reasonable design of cladding structure parameters,a high-refractive index ring region is set between the fiber core and the cladding for transmitting OAM modes.The simulation results show that the Bragg-OAM fiber has excellent transmission performances,especially the nonlinear coefficient is only 1.45 km^-1·w^-1,and the variation of ellipticity and eccentricity caused by the geometric deformation has relatively little influence on the dispersion and the confinement loss of the OAM mode transmission.Therefore,the proposed Bragg fiber is a new type of the effective and feasible OAM transmission fiber,and provides a new design scheme for the OAM transmission fiber.