Study On The Crosstalk Mechanism In OAM Multiplexed Optical Fiber Communication System

In recent years,with the rapid rise of new services such as mobile Internet,cloud computing,VR and AR,the data traffic is in an explosive growth and the network capacity gradually reaches the bottleneck.Therefore,the next generation optical communication network must meet the characteristics of ultra-high speed rate,ultra-large capacity and so on.Multiplexing technology is an effective way to increase capacity.At present,the optical field dimension for multiplexing has been developed and applied to the commercial single mode fiber transmission system successively.In order to improve the transmission capacity to meet the future bandwidth requirements,exploring the new multiplexing dimension is the most effective way to solve the current capacity crisis.For this purpose,it is proposed to use space division multiplexing(SDM)technology based on orbital angular momentum(OAM).The OAM modes corresponding to different topological charges are orthogonal to each other,theoretically,they can be multiplexed infinitely.So,combining OAM multiplexing technologies with other multiplexing technologies can greatly improve communication capacity.OAM multiplexing technology brings some problems to be solved while improving communication capacity,especially mode crosstalk,including intra-mode and inter-mode crosstalk.OAM mode crosstalk will cause the performance degradation of the system and increase the bit error rate of the signal at the receiving end,which becomes one of the main factors that restrict the development and application of SDM based OAM technology.This paper focuses on the research of mode crosstalk in OAM multiplexing optical fiber communication system.The main contents are as follows:1.Based on the theory of optical modes carrying OAM in a ring fiber,we use the finite element method to simulate the influence of the ellipticity and misalignment of the non-ideal ring fiber on the crosstalk in the OAM mode.The effective refractive index difference,walk-off length and intensity distribution of the even and odd eigenmodes that form the basis of the OAM modes are studied.The factors leading to the crosstalk in OAM mode are analyzed in detail,which provides a theoretical guidance for the next OAM optical fiber drawing.2.Firstly,the principle of the OAM mode and the working principle of the spatial light modulator(SLM)are introduced in detail.We build the OAM generation system based on SLM and complete the generation experiments of single OAM mode,fractional order OAM mode and superimposed OAM mode.Meanwhile,the interference experiment of the OAM mode and the basic mode Gaussian mode is completed.Then the generation and detection of OAM mode with high purity and low crosstalk are realized.Finally,the compensation method of OAM mode crosstalk is introduced.3.Based on the elliptic cylindrical coordinate system,we first propose a method for analyzing OAM mode crosstalk,and derive the form of the wave equation solution in the elliptic cylinder coordinate system in detail.Finally,the method for analyzing the numerical calculation of crosstalk in OAM mode is introduced,which provides a theoretical guidance for the OAM mode crosstalk analysis.