Microstructure ring fiber for supporting higher-order orbital angular momentum modes with flattened dispersion in broad waveband

We design and numerically simulate a microstructure ring fiber, which supports 146 orbital angular momentum (OAM) modes at 1.1 mu m and 70 OAM modes at 2.0 mu m with flattened dispersion and low nonlinear coefficient. The fiber consists of an air hole at the center and a high refractive index ring between two well-ordered air hole rings in the cladding. It is found that the number of well-separated OAM modes decreases linearly with the increasing wavelength. Moreover, the waveguide dispersions of the modes are flat with wavelengths, which has a minimum variation of 2.92416 ps/nm/km over 900 nm bandwidth from 1.1 to 2.0 mu m for HE13,1 mode. In addition, the nonlinear coefficient keeps lower than 1.8/W/km. The designed fiber may pave the way to applications in fiber-based OAM mode-division-multiplexing systems.