Theoretical And Experimental Research On The Pattern Characteristic Of Structured Light In Few-mode Fiber

With the rapid development of mobile Internet,Internet of things,5G network,cloud computing services,IP Broadband interconnection and other technologies,the demand for bandwidth is growing rapidly,and the requirements for communication rate and capacity of interconnection network are increasing rapidly.Unlike single mode fibers,there exists multi fiber modes in few mode fibers,which interfering with each other and form the complicated field with anisotropic polarization and distribution.This kind of light with specific spatial structure is called structured light,in which the pure fiber mode is the simplest structured light field in few mode fibers.The research on the internal structured light field of few mode fiber is of great significance for the device manufacturing and systematical application on few mode fiber.On the one hand,the manufacturing of few mode optical fiber devices often requires accurate matching of the optical field of the optical fiber to improve the conversion or coupling efficiency.It needs an accurate understanding of the physical mechanism of the field inside few mode fibers.On the other hand,because the complicated spots emitting from few mode fibers consist of eigenmodes supported in the fiber,which inversely reflect the amplitudes and phases of these fiber modes.As a result,it’s possible to obtain the mode components in few mode fibers based on the detection of the emitting spots,so as to achieve the purpose of mode demultiplexing.At present,the academic understanding of the structured light field of few-mode fibers is still relatively superficial,mainly focusing on the pure fiber mode or some simple fiber mode combination states.Combined with the research contents and objectives of the National Key Research and Development Program of China and the National Natural Science Foundation of China,the research work of this paper deeply and systematically carried out the theoretical and experimental research based on the mode spot characteristics of the structured light field of few mode fiber,expanded the relevant theoretical system of optical fiber spatial mode,and also provided theoretical support and solutions for the device manufacture and systematically application of few mode fiber.The main research contents and innovative achievements of this paper are as follows:1、The spatial mode theory of optical fiber is deeply studied,the wave equation of optical fiber is described by Dirac symbol system,and different branches of wave equation are integrated under the perturbation framework,namely spin orbit coupling,coupled mode equation and nonlinear Schrodinger equation.The scalar wave equation is served as the unperturbed equation,whose solution is linearly polarized(LP)mode or orbital angular momentum(OAM)mode.Based on the unperturbed scalar wave equation,through the transverse refractive index perturbation,the cylindrical vector(CV)mode and the spin orbit coupling term are derived.Through the longitudinal refractive index perturbation,the coupled mode equation is derived.Through the perturbation introduced by the frequency term and nonlinear term of refractive index,the nonlinear Schrodinger equation is derived.2、A four-dimensional complex space theoretical model describing the same order structured light field of few mode fiber is proposed.Based on the theoretical model,the complete conversion relationship among three kinds of fiber modes,namely CV mode,LP mode and OAM mode,is revealed,which is equivalent to the base transformation process in this complex space.The correctness of the conversion relationship is verified by experiments,and the unreported elliptical polarization OAM state is predicted based on this theoretical model.3、An extended Jones complex space theoretical model is proposed,and the bidirectional mapping relationship between fiber mode components and the mode spots emitting from fiber is established.The points in the complex space represent the complex amplitude of the combined fiber modes.Based on the theoretical model,the general expression of arbitrary mode field combination of higher-order modes is derived.The concept of characteristic parameters of optical fiber mode spot is proposed.These characteristic parameters are served as the bridge connecting optical fiber mode components and mode spot emitting from fiber.4、The singularities splitting phenomenon in the structured light field of few mode fiber is proposed and explained,and the corresponding interference method for identifying the splitting singularities is optimized.An arrow analysis method is proposed to analyze the characteristics of splitting singularities,which can quickly distinguish the main mode components in the optical field of six mode fiber.In addition,the amplitude and phase of each eigenmode field of the optical fiber are adjusted to generate a coaxial multi-singularity beam.At the same time,the interference detection method for identifying the splitting singularities is discussed.The results show that compared with the traditional helical interference pattern,the seldom used fork wire interference pattern has higher interference resolution and can identify the splitting singularity caused by 0.1% impurity mode.5、The detection method and recovery algorithm of fiber mode components based on mode spots emitting from fiber are proposed.Firstly,the feasibility of retrieving fiber mode components based on mode spots emitting from fiber is demonstrated mathematically,and the minimum needed information to be extracted from mode spots is analyzed.In the experiment,the algorithm is used to recover the mode components of the two-mode fiber.The recovery spot is in good agreement with the experimental results.However,with the increase of optical fiber modes,it will be difficult to extract information from the mode spots.Therefore,an angular Fourier transform algorithm is further proposed to better extract the mode spot information to recover the mode components.