Topological resonances, superefficient orbital-angular-momentum control, and spin-orbit-interaction enhancement in fiber-loop resonators

Abstract

In this paper we study the propagation of optical vortices (OVs) through the loop resonator (LR) on a multimode fiber. Within the framework of a fully vectorial treatment that allows for the spin-orbit interaction (SOI) we demonstrate the existence in such LRs of a special topological resonance, which is in the inversion of the topological charge (TC) of the transmitted OV. We study the angular momentum (AM) of the transmitted field and show that near resonance points the orbital and spin AMs of the output field are sensitive to wavelength-scale variations of the LR's length parameters. The mechanism that underlies such supersensitivity is the multipass interference assisted by topological charge inversion in the coupling area. We show that this property can be used for superefficient control of the output field's OAM. We also demonstrate that such LRs are suitable for creation of combined wavelength and mode comb filters on their basis. The effect of losses is also discussed. We show the existence in LRs of an additional resonance: the spin-orbit resonance. This type of topological resonance arises due to the effect of the SOI enhancement, which nature we also unveil.