Objective Orbital angular momentum based multiplexing is a special form of space division multiplexing, different OAM modes are orthogonal to each other, based on which different modes can carry different information, and multiple OAM beams with different topological loads can be used as carriers for information transmission, which can greatly improve the channel capacity of the communication system without the need for additional bandwidth. Compared with the transmission of OAM in free space, the transmission of OAM modes in optical fibers can effectively avoid the interference of external factors, and ordinary optical fibers are unable to meet the requirements for the transmission of OAM modes. Photonic crystal optical fibers, as a kind of special optical fibers with high structural designability, offer the possibility of realizing the transmission of OAM modes. In order to achieve high quality transmission of more OAM modes, it is necessary to design photonic crystal optical fibers with suitable structures that can support the transmission of OAM modes.
Methods In this paper, a novel photonic crystal fiber structure based on a positive hexagonal arrangement of air holes is proposed. The fiber introduces rectangular air holes with high air filling rate and high refractive index materials to fill the ring transmission region, which can effectively improve the refractive index difference between the ring transmission region and the cladding, and the hexagonal arrangement of the air holes is conducive to the improvement of the effective refractive index difference between the modes. Structure optimization and optimal structure verification take into account the number of OAMs that the fiber can support as well as the effective refractive index difference between modes and mode purity. The optimal fiber structure is obtained through structural optimization, and the performance of the fiber is analyzed using finite element analysis.
Results and Discussions The results of the finite element method analyses show that the optimal optical fiber structure is optimized to support 142 OAM modes in the commonly used S+C+L+U band, with the topological charge ordering up to 36. Moreover, the proposed fiber has good transmission characteristics. The confinement loss is below 10-9 for all eigenmodes, which is at least one order of magnitude lower than the typical photonic crystal fibers; The maximum effective mode field area can reach 206.18 μm2 and the minimum nonlinearity coefficient is as low as 0.397 W−1∙km−1; Flat dispersion and minimum dispersion variation are as low as 1.457 8 ps/(nm∙km); And purity are 93.4%-96.8% for all eigenmodes. Based on the effect of manufacturing errors on the performance of the optical fiber, it can be seen that the optical fiber does not require high manufacturing accuracy.
Conclusions In order to achieve high-quality transmission of more number of OAM modes, this paper proposes a design method of photonic crystal fiber based on hexagonal structure by combining the two ways of rectangular air holes and the filling of annular transmission region with high refractive index materials. The introduction of rectangular air holes and high refractive index materials increases the refractive index difference between the ring transmission region and the cladding, which in turn facilitates the stable transmission of a larger number of OAM modes. The performance analysis of the optical fiber by Comsol Multiphysics finite element analysis software shows that the photonic crystal fiber can not only support a larger number of OAM modes, but also has the characteristics of low confinement loss, low nonlinear coefficient, flat dispersion change, and high mode purity, which is valuable for high-capacity optical fiber communication.
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