BIRTH AND EVOLUTION OF WAVE-FRONT DISLOCATIONS IN A LASER-BEAM PASSEDTHROUGH A PHOTOREFRACTIVE LINBO3-FE CRYSTAL

We report on an experimental and numerical investigation of the proces s of spontaneous optical vortices nucleation in a. wave front of a las er beam passed through a photorefractive LiNbO3 : Fe crystal with self -induced nonlinear lens. The complex lens structure produces mainly de focusing of the beam passing through the crystal due to a negative var iation of the refractive index, whereas side parts of the lens have a positive sign of refractive-index variation and partially focus the be am. The resulting wave-front distortions lead to a phase bifurcation o ccurring at a certain distance after the crystal when the amplitude of the light wave becomes zero. We study in detail the process of edge d islocation 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.