Twisted anisotropic fibers for robust orbital-angular-momentum-based information transmission

We study the light propagation in the twisted anisotropic optical fibers endowed with torsional mechanical stress by obtaining the analytical solution of the vector wave equation. We show that at certain interplay between fiber parameters optical vortex beams of topological charge $\ell =0,\pm 1,\pm 2,...$ become the modes of the fibers in question. To explain the splitting of the optical vortex propagation constants we introduce the notions of orbital birefringence and optical Zeeman effect. Moreover, we unveil that induced by torsional stress circular birefringence makes the vortex beams with the well-defined orbital angular momentum robust against small perturbations characterized by both constant and spatially varying orientation of a director. We believe that such fibers can be successfully utilized for the long-range robust transmission of information encoded in the light's orbital degrees of freedom.

Figure 1

Schematic view of a twisted anisotropic (a) and twisted elliptical (b) fiber. (XYZ) is the laboratory Cartesian coordinate frame; $H$ is the twist pitch. In the inset (a) the blue arrows indicate the orientation of the rotating director of material anisotropy. The red arrows (directed along the $X$ axis) specify the orientation of the director of constant external perturbation.