Inversion of the topological charge of optical vortices in a coil fiber resonator.

In this Letter, we study the propagation of optical vortices (OVs) in a two-coil multimode fiber resonator. We suggest an analytical scheme of solving this problem based on its reduction to one of propagation of OVs in parallel evanescently coupled optical fibers. In the framework of the scalar approximation, we derive expressions for normal modes of the system and study evolution of an OV with an arbitrary topological charge excited at the input end of the resonator. We show that such a coil resonator is able to invert the topological charge of the incoming OV, provided the resonance condition is met.