Birth and evolution of wave-front dislocations in a laser beam passed through a photorefractive LiNbO3:Fe crystal

We report on an experimental and numerical investigation of the process of spontaneous optical vortices nucleation in a. wave front of a laser beam passed through a photorefractive LiNbO3 : Fe crystal with self-induced nonlinear lens. The complex lens structure produces mainly defocusing of the beam passing through the crystal due to a negative variation of the refractive index, whereas side parts of the lens have a positive sign of refractive-index variation and partially focus the beam. The resulting wave-front distortions lead to a phase bifurcation occurring at a certain distance after the crystal when the amplitude of the light wave becomes zero. We study in detail the process of edge dislocation nucleation and its decay in the near field producing a pair of unity-charged opposite-sign screw dislocations; After birth, they spread along dislocation axes as stable objects.