Lapin B. Propagation of optical vortices in optical fibers with combined anisotropy

Propagation and conversion of optical vortices in periodically perturbed weakly guiding optical fibers are studied. It is shown that optical vortices are unstable in translation-invariant fibers and change the sign of topological charge. In such fibers spin angular momentum is controlled by material anisotropy and orbital angular momentum by the form anisotropy. Coiled fibers with different types of anisotropy are studied. It is shown that l>=1 modes of coiled strongly anisotropic fibers are presented by linearly polarized optical vortices. It is established that l=1 modes of the coiled strongly elliptic fibers are presented by elliptically polarized fields. It is demonstrated that l=1 modes of the coiled fibers with strong combined anisotropy of material and form are represented by linearly polarized modes. Topological effects in such fibers are studied. It is revealed that long period helical layered fibers and bihelical fibers can change topological charge of incoming field by one and two units, correspondingly. It is shown that in Bragg helical fibers a monovortex regime can be implemented, at which only two undamped modes exist represented by optical vortices with circular polarization and topological charge +1. In such fibers the spin-orbit interaction does not affect the possibility of existing of monovortex regime. It is demonstrated that Bragg layered helical fibers can change the topological charge by unity in the reflected light.