Research On Characteristics Of Scattering In Few-Mode Fibers

With the rapid development of communications technology,especially in recent years,the Internet plus,broadband China proposed and the continuous development of 5G construction have made the demand for bandwidth in the global network soaring.The communication capacity of single mode fiber has gradually approached its Shannon limit.Mode division multiplexing?MDM?technology based on few-mode fibers?FMF?has attracted attention of optical workers because it can support multiple orthogonal modes parallel transmission and multiply the system capacity.Light scattering is the intrinsic characteristic of FMFs.However,the scattering in FMFs will affect the performance of MDM systems,in which Rayleigh scattering is the main cause of spatial modes loss in FMFs;stimulated Brillouin scattering?SBS?limits the incident power of each spatial mode.Therefore,it is very important to research Rayleigh scattering and SBS threshold characteristics of spatial modes in FMFs.It is of great significance to quantify the relationship between impairments and parameters of FMFs,optimize the structure of FMFs,and improve the transmission performance of MDM system.In this paper,we focus on the measurement of Rayleigh scattering and SBS thresholds in FMFs.The measurement of Rayleigh backscattering and SBS thresholds in FMFs are investigated based on the research of Rayleigh backscattering theory and the Brillouin scattering characteristics in FMFs.The specific work of this paper is as follows:Firstly,the mode theory and the orthogonality between modes are deduced in detail by using optical waveguide theory.The cut-off frequencies and modal simulation diagrams of low-order linear polarization modes in fibers are given.The mode coupling in fibers is introduced.Then three kinds of scattering in fibers are introduced.Secondly,the theory of Rayleigh backscattering in FMFs is analyzed.An experimental system for measuring Rayleigh backscattering in FMFs is built.The Rayleigh backscattering curves of each mode of FMFs are successfully measured.The relationship between Rayleigh backscattering power and the pulse width of incident light is researched.Based on the Rayleigh backscattering theory,the mode coupling theory in FMFs is deduced.The coupling coefficients between high-order modes and fundamental mode of 4km six-mode fiber are calculated by Rayleigh backscattering power.The mode coupling coefficients between LP₀₁1 and LP_11a,LP_11b,LP_21a,LP_21b,LP₀₂2 are-23.34dB/km,-23.65dB/km,-26.04dB/km,-26.35dB/km,-20.37dB/km,respectively.The experimental results agree well with the measured results of transmission method.Thirdly,Brillouin scattering characteristics in FMFs are analyzed theoretically.Then the theory of SBS threshold is deduced,and the curves of SBS thresholds of two kinds of FMFs with respect to the length of the fibers are given.After that based on the two photon lanterns and the FMF circulator,an experimental system for measuring the thresholds of SBS in each mode of FMFs is built.The various parts of the experimental system are introduced in detail,and the parameters of the key components are given.The SBS thresholds of 9.8km two-mode fiber and 11km four-mode fiber are measured by the experimental system.The deviations between the measured results and the theoretical analysis are within 0.7dB.Then the factors that may cause deviations are analyzed.Finally,the main work of this paper is summarized.The measurement and application of Rayleigh backscattering in FMFs and the theoretical analysis and the experimental system of SBS thresholds of each mode are objectively evaluated.The future research directions of Rayleigh backscattering and SBS in FMFs are prospected.