In this paper, we have theoretically studied an effective amplification of optical activity by a fiber loop resonator. We propose a scheme in which an optically active element is placed in the loop segment of the resonator. Assuming that the coupling in the resonator is polarization-independent, we have shown that initially small polarization plane rotation, which arises due to the optically active element, can be significantly amplified by tuning the resonator’s closed-path phase. We have also studied the influence of losses on the amplification of optical activity. We have shown that the maximal amplification takes place under the condition of critical coupling, at which the attenuation parameter is equal to the resonator’s effective reflection coefficient. We have also studied effective dichroism in such a system and shown the relevance of a critical coupling regime to that effect.

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.

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In this paper we have theoretically shown that two-part and three-part multihelicoidal fibers of heterogeneous type in the presence of twist defects are able to invert the topological charge of incoming optical vortices. We have shown that three-part multihelicoidal fibers of that type can be used as compact comb filters for optical vortices. Also we have studied the emergence of topologically charged fields localized near defects in such fibers. We have established that strongly localized fields can emerge only in three-part multihelicoidal fibers.

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