Super-efficient control of angular momentum and mode conversion in snake-type fiber resonators

In this paper, we have theoretically studied transformations of higher-order modes and angular momentum of light in the optical snake, a coil resonator composed of two evanescently coupled coils of a multimode fiber. To this end, we have developed a full-vectorial theory of normal modes of coupled parallel multimode fibers that allows for the spin–orbit interaction and goes beyond the limits of weak orthogonality approximation. We have studied the evolution of optical vortices and circularly polarized scalar-approximation fiber modes in a two-coil optical snake. We have shown that this system allows a parametric control over the topological charge of the outcoming optical field, as well as its orbital and spin angular momenta.