Multiple birth of nonlinear optical vortices

The spontaneous formation of optical vortex quadrupoles in an initially smooth elliptical Gaussian beam was observed in the near-field regime within a self-defocusing medium. Two characteristically different nonlinear phenomena were observed, namely the appearance of large-core vortices on the periphery of the beam for a weakly nonlinear system and small soliton-like vortices in the beam interior for a high power beam. The former case is attributed to nonlinear lensing, and we attribute the latter to a Kelvin-Helmholtz type optical instability. To our knowledge, all experimental observations of this instability introduced perturbed beams containing, for example, a dark stripe or stripes across the beam to initiate the process. We describe means of achieving this instability on a smooth beam. The observed phenomenon may occur in self-defocusing medium such as slightly absorbing liquid or gas, and thus may be important in the control of laser propagation through the air or sea