Analysis Of Transmission And Coupling Of Orbital Angular Momentum Mode In Optical Fiber

The rise of mobile internet,big date,cloud computing,5G and other information wave has brought great demand of network capacity,and the capacity crunch of optical fiber communication is more and more serious.Space division multiplexing(SDM)is considered to be one of the most promising ways to increase the capacity of optical fiber communication.Orbital angular momentum(OAM)mode multiplexing,as a kind of SDM,has attracted the attention of researchers because it can avoid the complex multi input and multi output(MIMO)digital signal processing technology at the receivers.The most critical part of OAM multiplexing system is optical fiber,which directly determines the transmission distance and the complexity of digital signal processing,thus it is of great importance to find an OAM fiber owning good properties and study the mode transmission characteristics.This paper focuses on the design of OAM fibers and the analysis of mode coupling mechanism in OAM fibers.The main research achievements are as follows1.The design tool for the family of circular photonic crystal fiber(C-PCF)supporting orbital angular momentum(OAM)modes is proposed.The calibrated effective normalized parameters for the C-PCF family are presented.Based on the proposed effective normalized parameters,the modal map defined by the cutoff conditions of vector modes as a function of effective normalized parameters,the color map defined by the effective index difference,confinement loss and effective mode field area as a function of effective normalized parameters,and the spin orbital coupling as a function of effective normalized parameters are also proposed.Aiming at the problem of designing OAM fibers with many factors and complicated design,a convenient and easy-to-use design guidance solution is given.At the end of paper,we also give some fiber design examples by using the design tool,which proves the effectiveness of the design.2.Analysis of mode coupling mechanism in elliptical OAM fibers.For the elliptical deformation of OAM fibers,based on the coupling mode theory and numerical simulation,the coupling in the same OAM mode group is analyzed in depth.It is found that the elliptical deformation does not simply introduce a time delay between even and odd modes,but also change the structure of eigenmodes,and the structural damage of the eigenmodes are different under different strength of elliptical deformation.When the elliptical deformation is relatively small,the structural damage of the eigenmode can be neglected,coupling occurs between the two OAM modes composed of the same vector mode.When the elliptical deformation is relatively large,the structural damage of the eigenmode cannot be ignored,coupling occurs among the four OAM modes in the same mode group,and the coupling between the two OAM modes composed of the same vector mode is serious.The study of mode coupling mechanism in OAM fibers paves the way for better simulation of mode transmission in OAM fibers.3.Analysis of OAM fiber birefringence modal dispersion.When the internal defects and external disturbances are relatively small,or the effective refractive index difference between HE and EH modes in the same mode group is large enough,it can be reasonably assumed that the OAM modes composed of HE and EH modes are independent of each other.At this time,mode coupling in OAM fibers is similar to polarization mode dispersion(PMD)in single-mode fiber.This paper draws on the analysis method of PMD in single-mode fiber,gives the Jones space,Stokes space and Poincare sphere representation of the modes in OAM fibers,and uses the Jones matrix transmission method to establish the mathematical model of OAM fiber birefringence modal dispersion.Compared with the polarization,the mode is related to the spatial azimuth angle,so that the representation of the mode in the OAM fiber and the birefringence mode dispersion modeling include the parameter of the spatial azimuth angle.