Rotation of the wavefront of an optical vortex in free space

It is shown experimentally and theoretically that when an optical vortex pr opagates 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 op tical vortex light flux and the amplitude zero of the perturbed optical vor tex 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 prope rties: 1) the circulation of the Poynting vector on the surface does not de pend on the longitudinal coordinate z; 2) the evolution of the light flux a nd a pure screw dislocation takes place along straight lines of this surfac e; 3) the Poynting vector on the ray surface is always perpendicular to the wavefront surface. (C) 1999 American Institute of Physics. [S1063-7850(99) 01603-1].