Excitation of optical vortices in a waveguide with a helical depressed cladding

Helices with a pitch of the order of the wavelength of light in the IR range, in which the refractive index is depressed compared to unexposed glass, were inscribed in the volume of phosphate glass by a femtosecond laser beam. Thus, the written helices represent the cladding of a few-mode waveguide with a built-in Bragg grating. The reflection spectrum of the obtained Bragg structures is studied. It is shown that the beam reflected from the Bragg gratings has a helical wave front and carries an orbital angular momentum.