With the development of fiber communication, traditional multiplexing techniques cannot meet the demands for capacity. The utilization of orbital angular momentum (OAM) modes to carry information is a way to implement space division multiplexing (SDM) technology. This approach can greatly increase the capacity and spectral efficiency of fiber communication and show broad application prospects due to its unique advantages. Currently, transmitting OAM modes based on photonic crystal fiber (PCF) structures face problems such as difficulty in preparation and high loss. As research deepens, scholars attempt to solve these problems by utilizing anti-resonance fiber (ARF) to transmit OAM modes. The Fresnel reflection of the negative curvature tube in ARF cladding can enhance the confinement of the fiber core to the beam, further reducing the confinement loss (CL). Additionally, the preparation method of ARF is simpler than that of PCF, and fewer structural parameters make ARF easier to optimize for enhancing fiber performance. However, there is still a low number of transmission OAM modes in the current ARF design. Based on this, we propose an ARF composed of two sets of negative curvature anti-resonance tubes with different sizes. By introducing high refractive index materials in the ring core, more OAM modes can be transmitted, thereby improving the capacity of the fiber communication system.

An ARF is designed by introducing high refractive index materials into the ring core to organically combine total internal reflection and anti-resonance guiding mechanisms. In this ARF, the OAM mode is limited by total internal reflection in the ring core and the Fresnel reflection from the negative curvature tube of the cladding, which realizes the stable transmission of OAM modes in the fiber. Its structure includes a central air hole, an inner layer of high refractive index amethyst glass tube, an intermediate layer silica (SiO₂) glass tube, and an outer layer of negative curvature tube. As the OAM mode number determines the capacity of the fiber communication system, and the radius of the central air hole, the thickness of Amethyst and SiO₂ glass tubes are key factors affecting the OAM mode number. First, the control variates are adopted to optimize the three key parameters of ARF at the 1.55 μm wavelength. Considering the influence of dispersion on stable OAM mode transmission, the optimal structural parameters of ARF are determined by optimizing the thickness of the SiO₂ glass tube, with the fundamental mode HE_1,1 as the observation term. The optimization takes into account both the OAM mode number and dispersion. Based on this, the fiber bending resistance is analyzed. Secondly, considering the practical fiber applications, it is necessary to have a certain operating bandwidth. Therefore, the stable OAM mode transmission and fiber transmission characteristics in 1.5-1.7 μm are analyzed and discussed. Additionally, the effective refractive index, effective refractive index difference, dispersion, CL, mode purity, effective mode-field area, nonlinear coefficient, and numerical aperture (NA) are included. Finally, the effect of preparation errors on the fiber properties is analyzed and discussed.

First, in ARF, total internal reflection and anti-resonance are organically combined to support stable transmission of 130 OAM modes in 1.5-1.7 μm bands, which can greatly increase the capacity of the fiber communication system. Secondly, the transmission characteristics are analyzed, and the results show that the introduction of high refractive index materials in the core results in a larger effective refractive index difference ?n_eff between adjacent hybrid modes, with a maximum value of 6.08×10^-3 (Fig. 9), which can inhibit the degradation of the OAM modes into linearly polarized (LP) modes. By optimizing the thickness of the intermediate layer SiO₂ glass tube, the dispersion changes of the hybrid modes all exhibit a flat state, with a minimum dispersion change of 0.43 ps/(nm·km) (Fig. 10). A flat dispersion is beneficial for dispersion compensation. The negative curvature tubes in the outer layer are all in the anti-resonance state, which further enhances the restriction on photonic energy of the ring core and reduces the CL. The CL of hybrid modes maintains between 10^-14-10^-8 dB/m (Fig. 11). It is shown that this design ensures a large OAM mode number and has good fiber transmission characteristics and simplifies fiber preparation.

We design an ARF that supports stable OAM mode transmission and features excellent transmission characteristics and relatively simple preparation. Based on the finite element method, the ARF is modeled and simulated. The results show that 130 OAM modes can be stably transmitted within the range of 1.5-1.7 μm, a maximum effective refractive index difference is 6.08×10^-3,^{and the minimum dispersion change rate is only 0.43 ps·nm-1·km-1. The CL maintains in the range of 10-14-10-8 dB/m and the highest mode purity reaches 99.26%. The maximum effective mode-field area is 187.38 μm2, the minimum nonlinear coefficient is 0.87 W-1·km-1, and NA is concentrated between 0.064-0.086. The proposed ARF applied to SDM has higher communication capacity and spectral efficiency and provides references for the study of transmitting OAM modes by anti-resonant structures.}