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.

Refractive twisted microaxicons.

Complex-shaped light fields with specially designed intensity, phase, and polarization distributions are highly demanded for various applications including optical tweezers, laser material processing, and lithography. Here, we propose a novel (to the best of our knowledge) optical element formed by the twisting of a conic surface, a twisted microaxicon, allowing us to controllably generate high-quality spiral-shaped intensity patterns. Performance of the proposed element was analyzed both analytically and numerically using ray approximation and the rigorous finite difference time domain (FDTD) solution of Maxwell's equation. The main geometric parameters, an apex cone angle and a degree of twisting, were considered to control and optimize the generated spiral-shaped intensity patterns. The three-dimensional structure of such a microaxicon cannot be described by an unambiguous height function; therefore, it has no diffraction analogue in the form of a thin optical element. Such an element can be produced via direct laser ablation of transparent targets with structured laser beams or direct laser writing via two-photon photopolymerization and can be used in various micro- and nano-optical applications.