SWAP and Fredkin gates for OAM optical beams via the sandwich of anisotropic optical fibers.

We study the propagation of circularly-polarized optical vortices of higher order topological charges ℓ ≥ 2 in a sandwich of multihelical - anisotropic - multihelical fibers on the basis of the Jones formalism for modes with orbital angular momentum. We demonstrate that such a system can operate as the all - fiber two - bit SWAP as well as universal tree - bit controlled-SWAP (Fredkin) gates over states of optical vortices, in which the mode radial number carries the control bit, while circular polarization and topological charge are the controlled bits.

Experimental demonstration of acoustically induced polarization-dependent fiber optical vortex inversion.

A recently proposed theoretical model of acousto-optic interaction in optical fibers with a traveling flexural acoustic wave of the fundamental order [M.A. Yavorsky, et al., Opt. Lett.44, 598 (2019)10.1364/OL.44.000598] is experimentally examined. We show the effect of inversion of topological charge of optical vortices, which is governed by the direction of incident linear polarization. This vector effect of a coupling of polarization and orbital degrees of freedom proves the inconsistency of the conventional microbending-based model and confirms the recently suggested approach of the description of acousto-optic interaction that is based on the actual displacement vector. In addition, the obtained results demonstrate the realization of a controlled-NOT gate for orbital angular momentum (OAM) states.

Generation of optical vortices in non-parity-time-symmetric chiral-core optical fibers.

In this Letter, we have studied the effect of losses and gains on generation of optical vortices (OVs) in a chiral fiber with an l-fold rotational symmetric core. Studying both unequal attenuations and symmetric loss-gain cases for core and orbital angular momentum (OAM) modes, we show that losses may play a constructive role in generation of OVs from a Gaussian input. We study the processes of field evolution at the exceptional point (EP). We show that at the EP any superposition of the fundamental mode and l-charge OV evolves in the limit into an equal-weighted superposition of such fields that forms a special attractor mode with average l/2 OAM per photon. At the generalized Poincare sphere, such a mode is represented by the OAM black hole in analogy with the spin black hole introduced in [Optica3, 1025 (2016)OPTIC82334-253610.1364/OPTICA.3.001025].

Optical vortices and orbital angular momentum in strongly coupled optical fibers.

We have studied the effect of strong coupling on the propagation of optical vortices (OVs) and evolution of their orbital angular momentum (OAM) in parallel multimode optical fibers. Based on the perturbation theory that goes beyond the limits of weak orthogonality approximation we have established that strong coupling does not lead to alteration of the structure of supermodes as compared to the case of weak coupling. The strong coupling affects only the propagation constants of such supermodes, which we have found analytical expressions for. We have also studied the evolution of OVs and emphasized the difference between the powers stored in partial OVs and powers located at the fiber cores. We have studied OAM in such fibers, as well as corrections to the total OAM due to interference effects and shown that the influence of such effects on forming the total OAM under strong coupling is negligible. We have also demonstrated that in such systems it is sufficient to take account only of the coupling of OVs with equal by modulus topological charges, whereas other types of coupling are negligible.

Amplification of optical activity in a fiber loop resonator.

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.

Transmission of optical vortices through fiber loop resonators.

In this Letter, we study the propagation of optical vortices (OVs) through the loop resonator (LR) on a multimode fiber. We demonstrate the existence of a special resonance, which is in the inversion of the topological charge of the transmitted OV. Near the resonance, the output orbital angular momentum (OAM) is sensitive to wavelength-scale variations of the LR's optical path length, which can be used for super-efficient OAM control. We also show the feasibility of a combined wavelength and OAM division multiplexing in comb filters on such LRs.

Revised model of acousto-optic interaction in optical fibers endowed with a flexural wave.

We propose a novel model of the acousto-optic interaction in circular fibers endowed with the lowest-order flexural acoustic wave that is based on the actual distribution of the acoustically induced displacement vector. The corresponding expression for the fiber's permittivity is derived and compared with the commonly used one. The resonance optical fiber modes and the propagation constants are found. It is predicted that the lengths of the well-known acoustically driven mode conversion LP0,n→LP1,n' should be slightly different for the x- and y-polarized incident fundamental modes. Moreover, we unveil a new polarization-dependent mode conversion in which the azimuthal mode number ℓ, as well as the optical frequency of the generated standard fiber mode LPℓ,n' , is governed by the linear polarization direction of the incident zero-order beam LP0,n.

Transmission of fractional topological charges via circular arrays of anisotropic fibers.

We demonstrate that in circular arrays of anisotropic fibers at certain distribution of anisotropy directors robust transmission of optical fields with half-integer topological charges is possible. We show that this is possible because the supermodes of such arrays may contain in their circularly polarized components half-integer topological charges of opposite values. We also study the structure of singularities in these supermodes.

Inversion of the topological charge of optical vortices in a coil fiber resonator.

In this Letter, we study the propagation of optical vortices (OVs) in a two-coil multimode fiber resonator. We suggest an analytical scheme of solving this problem based on its reduction to one of propagation of OVs in parallel evanescently coupled optical fibers. In the framework of the scalar approximation, we derive expressions for normal modes of the system and study evolution of an OV with an arbitrary topological charge excited at the input end of the resonator. We show that such a coil resonator is able to invert the topological charge of the incoming OV, provided the resonance condition is met.

Resonance optical activity in multihelicoidal optical fibers.

We have studied the effect of optical activity (OA) in optical fibers with multihelical distribution of refractive index profiles near the resonance wavelength, at which the conversion of an incident Gaussian beam into an optical vortex (and vice versa) takes place. We have shown that at such a wavelength the polarization vector of the input Gaussian beam rotates within the fiber at an average rate proportional to the difference in propagation constants of left- and right-circularly polarized optical vortices with the same topological charge. We also show that for certain fiber lengths the magnitude of OA can greatly exceed its average level and reach anomalously high values.

Helical-core fiber analog of a quarter-wave plate for orbital angular momentum.

We have studied the effect of a twist defect on the conversion of the fundamental mode (FM) into an optical vortex (OV) in a helical-core fiber (HCF). We have shown that if such a twist defect is situated in the middle of the HCF, which converts the FM into an OV, such a fiber system can continuously change the orbital angular momentum (OAM) of the output field from 0 to 1 (in a.u.). This control of the OAM is achieved by variation of the twist angle. In this action upon the OAM, this system has analogy with the quarter-wave plate, which is able to change the spin angular momentum. We also introduced the generalized Stokes parameters (SPs) and Poincaré sphere to visualize evolution of the superposition of states with zero and nonzero OAM. Connection of SPs with geometric characteristics of the location of singularity is made.

Vector erf-Gaussian beams: fractional optical vortices and asymmetric TE and TM modes.

We have considered the paraxial vector erf-Gaussian beams with field distribution in the form of the error function that are shaped by the cone of plane waves with a fractional step of the azimuthal phase distribution modulated by the Gaussian envelope. We have revealed that the initial distributions of the transverse electric and transverse magnetic fields have a far from standard form but at the far diffraction field the field distributions recover nearly the symmetric form.

Subwave spikes of the orbital angular momentum of the vortex beams in a uniaxial crystal.

We have theoretically predicted gigantic spikes of orbital angular momentum caused by conversion processes of the centered optical vortex in the circularly polarized components of an elliptic vortex beam propagating perpendicularly to the crystal optical axis. We have experimentally observed the conversion process inside subwave deviations of the crystal length. We have found that the total orbital angular momentum of the wave beam is conserved.

Natural shaping of the cylindrically polarized beams.

We have experimentally and theoretically shown that the circularly polarized beam bearing a singly charged optical vortex propagating through a uniaxial crystal can be split after focusing into the radially and azimuthally polarized beams in the vicinity of the focal area provided that the polarization handedness and the vortex topological charge have opposite signs.

Filter of optical vortices: highly twisted high-birefringence optical fibers.

Modes and bandgap structure of highly twisted high-birefringence weakly guiding fibers are studied in the scalar approximation. It is shown that within the gap the system can serve as a filter of circular optical vortices.