Rotation of the wavefront of an optical vortex in free space

It is shown experimentally and theoretically that when an optical vortex propagates in free space, its wavefront rotates through an angle numerically equal to the Gouy phase. It is found that both the energy maximum of the optical vortex light flux and the amplitude zero of the perturbed optical vortex field propagate along the ray surface. It is shown that the ray surface, which is a consequence of the relativistic constraints on the beam group velocity, forms an unparted hyperboloid of revolution and has various properties: 1) the circulation of the Poynting vector on the surface does not depend on the longitudinal coordinate z ; 2) the evolution of the light flux and a pure screw dislocation takes place along straight lines of this surface; 3) the Poynting vector on the ray surface is always perpendicular to the wavefront surface.