Design And Performance Analysis Of Orbital Angular Momentum Photonic Crystal Fibers

With the rapid development of information technology in today’s society,improving the system capacity of optical communication has always been an important research content in the field of communication.The Orbital Angular Momentum（OAM）multiplexing technology provides a new solution to improve the communication capacity.In theory,the number of OAM topological charges can be infinite,and the OAM modes with different topological charges have spatial orthogonality,showing a new degree of freedom,so using OAM as the carrier of data information in optical communication can effectively improve the communication capacity.Since OAM beams are disturbed by the atmosphere in free space and cannot be transmitted over long distances,optical fibers become a more ideal transmission medium for OAM beams.Photonic crystal fiber（PCF）has been widely concerned because of its flexible structure design and better transmission characteristics,so the combination of OAM and PCF have become a hotspot in the research field of OAM.In order to achieve stable transmission of various OAM modes,three PCF structures are designed in this paper,and the transmission characteristics of the proposed PCF are simulated and analyzed by using the finite element method.Firstly,a novel semi-circular structure photonic crystal fiber is proposed.The semi-circular air hole structure can easily limit the light field in the ring core,and silica is used as the background material to reduce the production cost.The transmission characteristics of the PCF are analyzed in detail.In the wavelength range of 1.2μm to 1.8μm,the fiber structure can support the simultaneous and stable transmission of 114 OAM modes.At the wavelength of 1.55μm,the nonlinear coefficients of all the eigenmodes are less than1.3 W^-1/km,and the confinement loss is less than 3.03×10^-7d B/m.Secondly,taking advantage of the flexible structure design of Photonic quasi-crystal fiber（PQF）,a photonic quasi-crystal fiber based on 12-fold photonic quasi-crystal structure is proposed.The ring core part of the PQF is made of Schott SF₂material.Through the optimization of the ring core thickness of PQF and the radius of the air hole in the cladding,a better transmission performance is obtained.In the wavelength range of 1.4μm～1.7μm,the PQF supports 150 OAM modes.The mode quality of all the eigenmodes is more than 97.6%,the minimum dispersion change is 23.9 ps/（km·nm）,the confinement loss is as low as 10^-9d B/m～10^-12d B/m,and the maximum effective mode area can reach 313μm².Finally,a single layer air hole photonic crystal fiber with simple structure is proposed.The PCF uses pure silica as the background material.Through the optimization analysis of the thickness of the ring core and the radius of the air hole in the cladding,it can transmit 166OAM modes in the wavelength range of 1.4μm～1.7μm,and effective refractive index difference between most of the eigenmodes is greater than 10^-3.The mode quality of all the eigenmodes is higher than 94.1%,the minimum dispersion change is only 12.7 ps/（km·nm）,the confinement loss is in the range of 10^-8d B/m～10^-12d B/m,and the nonlinear coefficients are all less than 0.7 w^-1/km.