Highly dispersive Ge-doped coupled ring fiber for high-order OAM modes

Light-carrying orbital angular momentum (OAM) has recently drawn extensive attention from researchers due to its unique field distribution. As a result of the intrinsic orthogonality among OAM modes with different topological charge values, they can be used as a modal basis in the mode-division multiplexing (MDM) optical communications systems. For fiber-based optical systems, chromatic dispersion induces temporal optical pulse broadening, which seriously limits the rate of information transmission. Consequently, dispersion compensation fiber is promising for mitigating the chromatic dispersion of the complex beam in the optical fiber. We propose and design a novel germanium-doped silica ring fiber composed of two high refractive index ring regions that can support high order OAM modes with large negative dispersion. We numerically investigate the high-order OAM modes guiding property in the proposed fiber by using a full-vector finite-element mode (FEM). Since Ge-doped silica has similar physical properties to silica, they can be easily combined with a tunable mole fraction of GeO2. Through varying the mole fraction of GeO2 and optimizing the structure parameter, we obtain a large negative dispersion of up to -99,685 ps/(nm·km) for OAM11,1 mode at the wavelength of 1614.2 nm. Furthermore, we engineer the chromatic dispersion of some other OAM modes and investigate the effects of fiber parameters on the dispersion, which indicates that the fiber we design is able to support all the OAMe,1 modes (∣l∣≤11) with highly negative dispersion. The designed fiber with tailorable negative dispersion can be applied to compensate for positive dispersion in the OAM-based optical systems.