Propagation of Bessel beams carrying optical vortices

The variation of the vorticity of the superposition of two Bessel singular beams under free-space propagation is analyzed. We demonstrate that in the near field the combined beam creates light pattern with much richer vortex content than that of individual beams. Under diffraction the combined beam dynamically evolves into the beam with rather simple vortical structure in the far field. A qualitative agreement of experimental results with theoretical predictions is obtained.