Design And Research Of Transmission Fiber With High Order Orbital Angular Momentum Mode

It is important to produce a vortex beam of high quality in optical communication systems.The vortex beam can be generated in free space or in an optical fiber,and the method produced in the fiber not only simplifies the optical structure,but also has a higher phase purity.In this paper,we have studied several fiber structures capable of transmitting vortex modes and designed a novel fiber with a low refractive index layer added between the core and the cladding of the fiber inverted parabolic fiber.The specific work is as follows:The first point is to make a basic theoretical derivation to generate a vortex beam in the fiber.In the fiber-mode coupling theory,the orbital angular momentum model in the fiber uses the odd-mode and even-mode superposition of the same-order vector mode.In the fiber,it provides a theoretic,al basis for the excitation of the vortex beam.The second point is to design a new type of fiber structure,adding a layer of low refractive index between the core and the cladding to form a new type of structure in the inverted parabolic fiber.We can calculate the number of vector modes,the effective refractive index and effective refractive index difference of each order vector mode by normalizing the operating frequency.The simulation can accommodate the intensity and phase of the vortex beam and the mode field distribution of the line bias mode LP.It is proved that the fiber can accommodate the high-order orbital angular momentum mode field,and the fiber can support 9 vector modes and can accommodate the OAM mode with a topological charge of 3.The third point analyzes the influencing factors that produce the vortex beam in the new fiber.The fiber structure is designed to effectively separate the vector modes,and the separation effect is better for the higher order vector modes.After that,the factors affecting the order of the OAM mode of the fiber are analyzed,mainly through the variation of the radius and the refractive index difference in the inverted parabolic fiber.The results show that the new fiber structure is designed by adding a low refractive index layer between the core and the cladding of the inverted parabolic distribution fiber.The fiber increases the number of OAM modes accommodated by increasing the internal and external refractive index difference of the core,and simulates the vector modes and OAM modes;Calculating the effective refractive index difference shows that the higher the order vector mode of the fiber,the better the separation effect;The influencing factors of the vortex beam generated by the new fiber designed are analyzed.The basis for changing the fiber parameters is to study the transmission and generation of high-order vortex beam in the fiber.