Dual Concentric-Ring-Core Fiber With Four Zero-Dispersion Wavelengths for Beyond Three-Octave OAM Supercontinuum Generation

Orbital angular momentum (OAM) beams, featuring with unique spatial field distributions, have been extensively investigated and applied into a wide range of fields. In most applications, OAM beams covering a certain frequency band are usually required, and supercontinuum (SC) generation is a feasible method to provide broadband OAM source. As a short pulse is incident into a specially designed optical fiber with flat dispersion over broad bandwidth, a broadband SC could be generated along propagation due to the nonlinear spectral broadening. In this work, we propose a dual concentric-ring-core fiber design to achieve near-zero and flat dispersion profile over 3007-nm wavelength range with four zero-dispersion wavelengths for the OAM mode. The precise position of the zero-dispersion wavelengths can be tailored by varying the structural variables and the germanium-doped concentration of the silica-based fiber. The performance of the spectral broadening in terms of flatness and bandwidth is investigated under different input pulse and propagation conditions. Simulation results depict that the OAM3,1 mode supercontinuum spectrum can achieve beyond three-octave spanning from 445 to 3942 nm at −40 dB level by pumping a 50-fs 600-kW Gaussian pulse at the central wavelength of 1900 nm into a 2-mm designed fiber. By further optimizing the proposed fiber structure for the OAM1,1 mode with <60 ps/(nm·km) dispersion variation over a 3210-nm bandwidth, the generated supercontinuum can cover nearly three octaves.