Theoretical Research And Design Of Vortex Fiber

The demand for optical fiber communication capacity is increasing,while the transmission capacity of standard single mode fiber is limited by Shannon's limit.Space-division multiplexing has become an effective way to improve the capacity of optical fiber communication.The vortex beam has the characteristics of phase or polarization singularity,and its wavefront phase is a helical shape distribution.As a vortex beam with phase singularity,the mode with orbital angular momentum(OAM)could theoretically offer infinite orthogonal basis with different quantum states.Such orthogonal,as a multiplexing mechanism,can effectively enhance the fiber transmission capacity.Stable transmission of OAM mode in fiber requires solving the problem of degenerate mode in fiber,that is,degrading the vector mode in the same scalar LP mode group to obtain a vortex mode that can be transmitted over long distances in the fiber.In this thesis,based on the comparison among the three types of optical fiber structure including the step-index fiber structure,the air core fiber structure and the inverse-parabolic graded-index fiber,a high-index-ring assisted inverse-parabolic fiber is proposed and analyzed systematically.First,the influence of the core layer parameters of the inverse-parabolic graded-index structure on the mode characteristics is analyzed comprehensively.Within a certain range,for the LP11 mode group and the LP21 mode group,the effective refractive index of each mode decreases with the decrease of the core radius,and the effective refractive index difference between the modes will reach the maximum near the critical value of the core radius tends to be cut off.The refractive index difference has a complex dependency on the invertse-parabolic characteristic parameters of the core.Then,the high refractive index layer and the low refractive index layer are added to the outside of the invertse-parabolic core structure,and the influence of the parameters on the mode characteristics is analyzed.The obtained optimal parameters allow the fiber structure to achieve a large effective refractive index difference,while ensuring a large effective area of mode field.At a wavelength of 1550 nm,the effective refractive index difference of LP11 mode group is up to 7.2× 10-4,and the effective refractive index difference of LP21 mode group can reach 3.1 × 10-4,which effectively achieves the degenerate separation of vector mode.Finally,the OAM beam is generated by simulation of the high-index-ring assisted inverse-parabolic fiber structure,and the calculation shows that the purity can reach 0.982.The conclusion of this paper is of great significance to the design and manufacture of vortex fiber.