BIRTH, EVOLUTION AND ANNIHILATION OF PHASE SINGULARITIES IN THE PROPAGATION OF A LASER-BEAM PASSED THROUGH A SELF-FOCUSING STRONTIUM BARIUMNIOBATE CRYSTAL

We report the first observations and the mechanism of natural nucleati on, propagation and annihilation of optical vortices in a laser beam w ith initially smooth Gaussian wave front passed through self-focusing photorefractive strontium barium niobate crystal. Wave-front distortio ns due to the formation of thermal-induced nonlinear lens lead to the origin of phase singularities. Pairs of opposite topological charge sc rew wave-front dislocations nucleate and annihilate through a formatio n of limited edge dislocations. In a free-space propagation the altern ating sign vortices array nested in the beam has the form of quadruple .