Air-Core Ring Fiber Guiding >400 Radially Fundamental OAM Modes Across S + C + L Bands

In this paper, we propose and design a Ge-doped air-core ring fiber, which can support a large amount of OAM modes for mode-division multiplexing (MDM) in optical fiber communications. By varying the mole fraction of GeO₂ and adjusting the structure parameter, including the air-core radius and the GeO₂-doped ring width, we investigate the influence of different fiber parameters on the total supported OAM mode number. The hollow silica fiber with a 50-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> air core and a 1.5-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> thickness of Ge-doped ring is designed in simulation to support fiber eigenmodes up to HE_112,1 and EH_107,1. This provides 436 OAM modes at 1550 nm while maintaining radially single mode condition. Moreover, it can support more than 400 radially fundamental OAM modes for the wavelength from 1460 nm to 1625 nm, covering entire S, C and L bands. The optical parameters of the guided OAM modes in the fiber are also numerically analyzed, including effect of material loss, optical field distribution, effective refractive index profile and chromatic dispersion, etc. The simulation results show that the higher-order OAM modes have longer <inline-formula> <tex-math notation="LaTeX">$2\pi $ </tex-math></inline-formula> and 10-ps walk-off length in the air-core ring fiber with ellipticity or bending compared with low-order modes.