PHASE SINGULARITIES VIA NONLINEAR BEAM-PROPAGATION IN SODIUM VAPOR

The creation of phase singularities in a nonlinear optical system with out feedback is observed experimentally after a single-pass of a focus ed laser beam through a high-density sodium vapor. Their appearance is reproduced in a numerical model which allows for a slight astigmatism of the incoming beam and can be qualitatively understood by the astig matic deformation of the phase fronts, to first order by means of self -induced astigmatic lensing. Alternatively, the phase singularities ca n be interpreted to result from the interference of symmetry broken mo des whose mutual phases are determined by the astigmatic lens.