Design And Research Of Wideband Mode Multiplexer In Large Capacity Optical Fiber Communication System

With the development of modern society,people’s demand for the capacity of optical fiber communication system is increasing day by day.As a potential scheme to improve the capacity of optical fiber transmission,space division multiplexing technology has attracted the wide attention of researchers all over the world.Space division multiplexing technology is to use the spatial dimension of optical fiber,through the multiplexing mode division mul-tiplexing technology and multiplexing core division multiplexing technology scheme,can realize the double growth of optical fiber transmission capacity,which is of great signifi-cance for the future generation of large capacity optical fiber communication.The research of space division multiplexing technology mainly involves multi-core fiber,few mode fiber,space division multiplexer,mode converter and erbium-doped fiber amplifier.This paper mainly focuses on the mode multiplexing/demultiplexing devices in the field of mode divi-sion multiplexing technology in the current space division multiplexing technology.In view of the problems existing in the current mode multiplexing/demultiplexing devices such as low spatial integration,narrow working band width,large insertion loss and low mode cou-pling efficiency,several all-fiber mode multiplexing/demultiplexing devices are designed from two aspects of mode selective coupler and photon lantern.The details are as follows:Firstly,the wave equations in optical waveguides are solved from Maxwell’s equations,the properties and characteristic equations of different vector eigenmodes are discussed,the vector wave equations are simplified to scalar form by using the weak derivative approx-imation,the correspondence between vector eigenmodes and linear polarization modes is studied,and the mode field distribution of the modes in the optical waveguides under linear polarization modes is given.The solution steps of finite element method and beam propaga-tion method are also described.The design and research of mode multiplexer/demultiplexer will be based on the above mode theory and numerical calculation method.Then,a six mode broadband mode selective coupler based on multi-core fiber structure is designed.The design of tapered fiber core in the coupling area can greatly reduce the impact of phase mismatch between modes,thus effectively reducing the wavelength sensi-tivity of the device.In the S+C+L band,the six mode broadband mode selection coupler can simultaneously multiplex LP₀₁,LP_11a,LP_11b,LP_21a,LP_21band LP₀₂six modes,and the average coupling efficiency of each mode is greater than 90%.A four mode broadband mode selective coupler based on micro structure fiber post-processing technology is also designed.Through the special photonic crystal fiber core structure design,the high fitting effect of refractive index between the desired conversion modes is achieved,so the modes can be matched under different incident light wavelengths.In the S+C+L band,the four mode broadband mode selective coupler can simultaneously multiplex LP₀₁,LP₁₁,LP₂₁and LP₃₁four modes,and the coupling efficiency of each mode is greater than 80%,and the extinction ratio of each mode is greater than 5 d B.Finally,a differential phase injection method was proposed to reduce crosstalk between angular asymmetric and angular symmetric modes in circular core mode selection group photonic lanterns;We also designed an elliptical core eight mode photonic lantern,which combines an elliptical core outer sleeve with eight different core parameters input fibers to achieve LP₀₁,LP_11a,LP_11b,LP_21a,LP_21b,LP₀₂,LP_31aand LP_31beight modes are multi-plexed at the same time,and the modes at the output end are completely separated from each other without coupling.The mode selection group photon lanterns of step input fiber and grade input fiber are respectively used in the S+C+L band,and the coupling efficiency of each mode is greater than 85%and 71%,respectively.Although the performance of the lat-ter is slightly lower than the former,the length of the device packaging cone is only half of the former.