The effect of spin–orbit coupling on the structure of the stopband in helical-core optical fibres

Abstract

We study the influence of spin–orbit interaction (SOI) on the mode coupling in few-mode helical-core optical fibres with up to a wavelength order values of the helix pitch. Within the framework of a full-vectorial approach, we solve the vector waveguide equation and show that in such fibres at certain values of pitch the fundamental modes strongly couple with l = 1 optical vortices (OVs). We study the spectra of the coupled modes and demonstrate that they feature the presence of forbidden spectrum zones. Within the mutual bandgap the only forward-propagating non-damping modes are represented by two circularly polarized OVs with opposite circular polarization and positive topological charge. We emphasize that within the spectrum gap the fibre maintains the propagation of a solitary circularly polarized OV. We also study the influence of the SOI on the structure of the forbidden zones and show that the presence of this interaction shifts the position of the bandgaps. For fibres with large enough effective curvature radii of the core's helix, the effect of the SOI can lead to the disappearance of the mutual bandgap, thus destroying the fibre's ability to transmit a solitary OV.