Broadband Supercontinuum Generation for Orbital Angular Momentum Modes in Graded-Index Double-Ring-Core Fiber

Orbital angular momentum (OAM), characterized by its unique spatial field distribution, has been extensively studied and applied in various fields. In many applications, an OAM beam with coverage over a specific frequency band is required, and supercontinuum (SC) generation presents a viable solution to provide a broadband OAM source. The launch of a short pulse into a specially designed fiber with flat dispersion across a broad wavelength range leads to broadband SC generation through nonlinear spectral broadening during propagation. In this study, we propose a dual concentric ring fiber design that achieves a flat and near-zero dispersion profile over a wavelength range extending up to 3435 nm, featuring four zero-dispersion wavelengths for the OAM mode. The zero-dispersion wavelengths can be precisely tuned by adjusting the structural parameters and the germanium doping concentration in the silica-based fiber. The spectral broadening performance, in terms of flatness and bandwidth, is evaluated under various input pulse and fiber conditions. Simulation results indicate that the OAM3,1 mode SC can span nearly three octaves at the -40 dB level when pumped with a 50-fs, 600-kW Gaussian pulse centered at 1500 nm. With further optimization of the fiber structure for different OAM modes, the generated SC extends beyond 2.82 octaves. Additionally, the effects of different shape factors on the SC characteristics are systematically compared.