Generation of optical vortices in spun multihelicoidal optical fibers

We have theoretically studied long-period spun l-helicoidal fibers and their ability to generate singular beams from regular ones. On the basis of perturbation theory in the presence of degeneracy, applied to the scalar waveguide equation, we obtained the structure of coupled modes of such fibers and their spectra. It is shown that the coupled modes consist of the fields, which taken separately bear topological charges that differ by l units. We have numerically studied the process of the passage of a Gaussian beam through such a fiber and demonstrated that long-period l-helicoidal fibers have the ability to change-in a certain wavelength range-the topological charge of the incoming Gaussian beam by l units, generating in this way charge-l optical vortex.