A Hybrid Cladding Ring-Core Photonic Crystal Fibers for OAM Transmission with Weak Spin–Orbit Coupling and Strong Bending Resistance

Abstract

A hybrid cladding ring-core photonic crystal fiber (PCF) for transmitting orbital angular momentum (OAM) modes is proposed, which breaks the circular symmetry of the fiber structure to suppress the spin–orbit coupling and promotes bending resistance. Through the optimization of fiber structure parameters, the designed fiber can support 22 OAM modes (6 OAM mode groups) over a 200-nm wide bandwidth (covering the whole C + L band) with large effective refractive index separation between adjacent modes (>10−4) and mode groups (>3.6 × 10−3), low confinement losses (<3.5 × 10−9 dB/m), and high mode purity (>98.3%). Meanwhile, the phase of the OAM modes varies periodically and uniformly with an increase in the azimuth angle, and the polarization of OAM modes maintain nearly circular polarization in the designed fiber, which also demonstrates that the fiber has weak spin–orbit coupling. Moreover, the confinement losses of all vector modes are less than 10−7 dB/m when the bending radius is larger than 0.8 mm, indicating strong bending resistance. Furthermore, the fiber also exhibits large differential group delay, relatively low and flat dispersion, and low nonlinear coefficients (<2.0 W−1/km). Therefore, the novel fiber structure has great potential in the application of mode division multiplexing (MDM) based on OAM modes.