Research On Transmission Characteristics Measurement Of Few-mode Fiber Based On Rayleigh Backscattering

With the rapid development of information technology,various bandwidth-consuming services have been refurbished.The demand for network bandwidth has increased from 100 Gb/s to 100 Tb/s.At present,the capacity of single-mode fiber(SMF)communication systems has approached its Shannon limit,and a breakthrough and new technology is urgently needed to break the capacity crisis.In the mode-division multiplexing(MDM)technology over few-mode fiber(FMF),the orthogonality between modes is applied and different modes as independent channels carrying information are transmitted at the same time,increasing the communication capacity manyfold and fundamentally breaks the capacity limit of single-mode optical fibers.However,although the advantages of MDM technology are prominent in the improvement of the capacity of the optical communication network,it has shortcomings.Some of the obvious problems are caused by FMF,such as mode coupling,mode-dependent loss(MDL),differential mode group delay and other impairments,destroy the orthogonality between the modes,making the MDM system difficult to obtain the expected transmission performance.In order to study the optimization solution of FMF deeply and compensate the signal impairments of the MDM system,the first thing to do is to perform detailed,accurate and comprehensive measurement analysis on the transmission characteristics of FMF,specifying the specific processes and distribution of various impairments.Therefore,the measurement of transmission characteristics of FMF has become a hot research issue in academia.Backscattering measurement technology,using the Rayleigh backscattering effect generated when light pulses are transmitted in the fiber,visually demonstrate the transmission characteristics of the under test fiber.The transmission and reception operations can be completed at the same end of the measured fiber,which is relatively simple to implement,and the result is accurate.In this paper,we mainly study the transmission characteristics measurement method based on backscattering in FMF.The measurement study of mode coupling and MDL of FMF is performed from the aspects of theoretical derivation and experimental measurement.Firstly,the mode theory of FMF is relatively comprehensively explained based on the optical waveguide theory and the Maxwell equations.The orthogonality between the modes is deduced in detail using the Lorentz reciprocity theorem,and on this basis,the further analysis on mode coupling and MDL characteristics of FMF andthe description methods are performed.The effects of mode coupling and MDL on the MDM system are analyzed deeply.Secondly,a mathematical model for the backscattering power in FMF is established using the backscattering capture theory.Based on the coupled power equations,the basic principle of measuring MDL in FMF based on the backscattering principle is elaborated.An experimental system for backscattering measurement on FMF is designed and built based on this principles.Then the influence of the device and the pulse parameter setting are analyzed.The MDL of 9.75 km three-mode fiber and 4 km six-mode fiber are measured using the experimental system.The present method is compared with traditional transmission measurement experiment and the conclusions are given.Then,on the basis of the coupled mode theory,the basic principle of mode coupling of FMF based on the backscattering principle is further elaborated.The mode coupling of the 9.75 km three-mode fiber and the 4 km six-mode fiber is performed using the built backscattering measurement system.The present method is compared with traditional transmission measurement experiment and the conclusions are given.Finally,the main work of this thesis is summarized.Then the measuring method of the transmission characteristics of FMF based on the backscattering principle is evaluated objectively,and the further research direction is prospected.