Spin-dependent OAM flipping in multihelical optical fibres

Abstract

We study the propagation of light beams through optical fibres that possess a multihelical distribution of the refractive index. Analytical expressions for the higher-order modes with the azimuthal numbers and the corresponding propagation constants of low-bi multihelical fibres are obtained. It is shown that the resonance modes are weighted superposition of optical vortices (OVs) with the same sign of circular polarization but opposite topological charges. We unveil the effect of spin-dependent flipping of topological charge (or, equivalently, orbital angular momentum) of an incoming OV. This paves the way to the implementation of the all-fibre stable logical element controlled-NOT gate, in which the circular polarization carries the control qubit and the topological charge carries the target. Such a gate should produce optical beams with entanglement between polarization and orbital degrees of freedom in the regime of linear optics.