GeO₂-doped ring-core photonic crystal fiber for supporting robust orbital angular momentum modes

Orbital angular momentum (OAM)-mode-supported photonic crystal fibers (PCFs) have inspired intensive research in modern fiber optics due to the robust propagation and theoretically unlimited signal-carried channels. In this paper, a dual-cladding GeO₂-doped ring-core PCF is designed, and a strategy for optimizing OAM mode properties is analyzed by structure parameters and GeO₂-doping concentration. Numeric results show that high structural degrees of freedom are available to improve the effective refractive index separation (within the vector modes), chromatic dispersion, effective mode field area, nonlinear coefficient, and OAM mode purity in terms of inner cladding, outer cladding, and ring-core. In particular, the effective refractive index separation and chromatic dispersion can exhibit a high order magnitude of 10^-3 and a low value in the broad band from 1.3 µm to 1.7 µm, respectively. In addition to structural optimization, doping high index material into the ring-core is another way to regulate the fiber performance by controlling the doping concentration. A systematic investigation shows that as the doping concentration increases, the effective refractive index separation and mode purity increase obviously, while the dispersion and mode field area gradually decrease. This flexible manipulation offers a method for customizing the optical properties of OAM-supported PCFs in communication and sensor systems.