Radially anisotropic ring-core optical fiber: towards vector-vortex guided transmission using the full modal space

The radially anisotropic ring-core fiber with cylindrical birefringence is theoretically and numerically investigated as a novel platform for the transmission of vector-vortex beams with unique modal properties. First, we elucidate the parametric conditions where such fiber enables modal substitution in which either the donut-shaped azimuthal TE01 or radial TM01 mode replaces the normal Gaussian-like HE11 mode as the fundamental mode of the waveguide. We also demonstrate that it is possible to significantly engineer the waveguiding properties of the fiber via the addition of small radial birefringence (∼10−4) so as to make the (hitherto non-degenerate) TE0m and TM0m modes fully degenerate. The latter property is used to create stable vortex modes of high purity (>99%) with the newly degenerate modal pair – a feat not possible with standard few-mode fibers—all without affecting the co-propagating hybrid HE/EH modes that remain available as an independent basis set to produce vortex beams of similarly high purity. These new insights are relevant to the topical fields of mode-division multiplexing (MDM), structured light, fiber modelling and fabrication. With respect to MDM applications, the newly available vortex modes created with the degenerate TE/TM basis set can now be concurrently used with the more common vortex modes created via the HE/EH modal basis set.