Objective Orbital angular momentum (OAM) theoretically has an infinite number of topological charges and can take an infinite number of mutually orthogonal modes, making it a high- capacity information carrier. Mode division multiplexing (MDM) is a crucial application of OAM, where each OAM mode acts as an independent channel to transmit optical signals, significantly improving the channel capacity of optical fiber communication. Increasing the number of OAM modes that an optical fiber can support is particularly important. In recent years, helical structures have been applied to optical fibers. Spirally twisted photonic crystal fibers (PCFs) further enhance the flexibility of design, offering greater potential for applications in OAM generation. However, existing helical twisted fibers cannot support enough OAM modes, the number of OAM modes generated by a single structure is not high enough, and their optical characteristics have not been comprehensively analyzed, hindering the effective transmission of high- order OAM modes. To address this issue, we design a twisted PCF OAM generator. Through simulation analysis, the maximum topological charge of 17 can be achieved, allowing stable transmission of 66 OAM modes. This generator exhibits excellent optical performance, enhancing the transmission and application in modular division multiplexing. This twisted PCF can stably transmit OAM modes and is suitable for long-distance- distance transmission, providing a possibility to improve the capacity of optical communication systems. Methods The design of the fiber structure plays a crucial role in the generation of OAM and its optical properties. Accurate transmission of the OAM mode in the fiber can be achieved when the refractive index distribution and the mode field distribution of the OAM mode are both annular and the effective mode refractive index difference between the mixed modes exceeds 10(-4). To meet these requirements and improve optical properties, we design a novel spiral twisted PCF. The fiber's spiral distortion rate is 7391.983 rad/m. The twisted PCF features multiple layers of circular air holes arranged in a ring structure. The circular air holes, with varying radii, form concentric rings filled with highly nonlinear As2Se3, divided into inner and outer layers with a refractive index of 2.808. The rest of the fiber is composed of Schott SF2. We employ COMSOL Multiphysics software to study the characteristics of the twisted optical fibers. However, constructing a three-dimensional- dimensional model of twisted PCFs is complex and challenging. To address this, we utilize a two-dimensional- dimensional modeling approach to simulate the twisted PCF. This simulation is achieved by converting between the spiral coordinate system and the rectangular Cartesian coordinate system, leveraging the translational invariance along the fiber axis. Using the finite element method, we obtain the eigenmode of the proposed PCF, allowing us to observe and calculate the supported OAM mode. Results and Discussions The simulation results demonstrate that the twisted PCF can support 66 OAM modes, surpassing current twisted fiber OAM generators. Figures 4-9 illustrate the optical properties of the twisted PCF. Within the C- band, the mode of the twisted PCF surpasses 10(-4), mitigating coupling during all vector mode transmission. This underscores the fiber's effectiveness in transmitting the OAM mode within the C- band reliably. The purity of all modes exceeds 99. 4%, indicating high- quality OAM mode conditions, which are conducive to signal coding and multiplexing in optical communication systems. In addition, the effective mode field area of all modes measures less than 48 mu m(2), suggesting concentrated light field energy within the ring core region, facilitating stable transmission of the OAM mode. Furthermore, the nonlinear coefficients of all modes are below 36 W-1<middle dot>km(-1),indicating reduced nonlinear effects. Consequently, the designed twisted PCF exhibits improved transmission performance in optical communication and is conducive to the application of MDM for enhancing the capacity of optical fiber communication systems. The dispersion coefficients of different modes are all less than 35 ps/(km<middle dot>nm), indicating favorable dispersion characteristics of the OAM generator for stable transmission of all supported modes. The limiting loss value remains consistently within the order of 10(-10) -10(-9)dB/m, suggesting minimal loss during optical signal transmission, enhancing the effective long-distance- distance transmission of OAM. Conclusions In this paper, we propose an OAM generator based on twisted PCF. The generator exhibits a topological load of up to 17, allowing for the stable transmission of 66 OAM modes. Within the C- band, the effective refractive index difference between the two modes of the same order OAM mode of the twisted PCF exceeds 10(-4),preventing coupled crosstalk. Within all modes boasting purity levels higher than 99.4%, the OAM generator facilitates signal coding and multiplexing in optical communication systems. Moreover, the effective mode- field areas and nonlinear coefficients of all modes are sufficiently small, enhancing transmission performance and supporting MDM application. In addition, the OAM generator demonstrates low stationary dispersion characteristics, ensuring stable transmission for all supported modes. The excellent optical properties of the twisted PCF meet the requirements for long-distance- distance transmission of OAM mode, offering a promising avenue for enhancing the capacity of optical communication systems.
