Dynamics of optical vortex solitons

We analyse the drift of an optical vortex soliton created on a slowly diffracting, finite-extend background field. In the framework of the generalized nonlinear Schrödinger equation we derive the motion equation describing the change of the vortex velocity induced by local gradients of the phase and intensity of the background field. We present experimental measurements of the motion of a vortex soliton, created by a phase mask in a diffracting Gaussian laser beam passed through a nonlinear saturable medium. The experimental results are shown to be in good agreement with our theoretical model and corresponding numerical simulations carried out for both Kerr and saturable media with experimentally determined initial conditions.