Optical Fiber Design For The Photonic Orbital Angular Momentum Wave Modes

With the popularization of intelligent terminal equipments and the rapid development of APP, artificial intelligence and Internet of Things,the amount of Internet data transmission has increased dramatically and the network bandwidth has gradually reached the bottleneck. Therefore, the communication capacity of communication system has become a large problem to be solved in the optical communication field. At present, optical communication systems have adopted WDM technology, quadrature amplitude modulation (QAM), orthogonal frequency division multiplexing(OFDM), time division multiplexing (TDM) and partial differential multiplexing (PDM) to expand the communication capacity. However, due to the nonlinear effect of optical fiber, the transmission capacity of a single fiber is limited by the nonlinear Shannon limit, so the transmission capacity of the system can not increase at the expected speed. Therefore, it is imperative to study the new technology of expanding the communication capacity. The orbital angular momentum (OAM) multiplexing technology is a special form of space division multiplexing. The vortex beams carrying OAM are orthogonal to each other and can not interfere with each other theoretically. Therefore, OAM multiplexing technology working with high order modulation as well as the traditional multiplexing technology, can greatly improve the information capacity of communication systems, and this will be a great solution for the bottleneck problem of communication capacity. At present, optical fiber communication is still the backbone of communications infrastructure, and the application of OAM multiplexing technology in optical fiber communication is an inevitable trend. However,OAM mode has special requirements for the fiber structure that supports its stable transmission. Ordinary optical fiber structures cannot meet these requirements. Therefore, it is necessary to design a new type of fiber structure suitable for the stable transmission of OAM mode.In this paper, we focus on the design of optical fiber supporting the photonic orbital angular momentum mode. Based on the existing circular photonic crystal fiber structure, a novel optical fiber structure capable of supporting the transmission of 26 OAM modes is proposed, and the optical fiber performances have also been in-depth researched and analysed. The main contents are as follows:(1) The design principle of ring fiber structure and photonic crystal fiber structure which could support the transmission of OAM modes were analysed theoretically. And the advantages and disadvantages of the two optical fiber structures were explored by analyzing the performances of typical optical fiber structures.(2) Based on the existing photonic crystal fiber structure, a novel ring-shaped photonic crystal fiber (C-PCF) structure which could stably support 26 OAM modes in the wavelength range of 1.25?m to 1.9?m (650?m) was supposed. The cladding material of the novel fiber structure was silicon,which did not need doping compared with the existing ring fiber structure,and reduced the complexity of the fabrication process. Compared with the existing structure of the photonic crystal fiber which could support the transmission of OAM modes, the structure proposed in the paper could support more OAM modes. In addition, the structure had more tunable parameters, which could be further optimized to obtain a fiber with better performances.(3) The effective refractive index, dispersion, confinement loss and nonlinear characteristics of the proposed fiber were analysed by simulation.The results showed that the new C-PCF structure could not only satisfy the condition that the effective refractive index difference of all vector modes should be larger than 10-4, but also had better preformances compared with the existing photonic crystal fiber structure, such as more flat chromatic dispersion, lower confinement loss, smaller non-linear coefficients.Overall, the performances of the structure were better. In addition, the influence of some parameters on the performance of the fiber and the effect of the fiber geometry on the mode birefringence were discussed, which provided a theoretical basis for the further optimization of the fiber structure.