  When a phase singularity is suddenly imprinted on the axis of an ordinary
Gaussian beam, an optical vortex appears and starts to grow radially, by effect
of diffraction. This radial growth and the subsequent evolution of the optical
vortex under focusing or imaging can be well described in general within the
recently introduced theory of circular beams, which generalize the
hypergeometric-Gaussian beams and which obey novel kinds of ABCD rules. Here,
we investigate experimentally these vortex propagation phenomena and test the
validity of circular-beam theory. Moreover, we analyze the difference in radial
structure between the newly generated optical vortex and the vortex obtained in
the image plane, where perfect imaging would lead to complete closure of the
vortex core.
