Azimuthal instabilities of intense femtosecond pulses propagating in air

Summary form only given. The propagation of intense femtosecond pulses in air and their spatial filamentation has attracted much interest in recent years. In order to understand how rotationally symmetric initial beams change their azimuthal shape, we numerically perform a linear stability analysis. Our model accounts for dispersive and diffractive broadening as well as for the action of a cubic nonlinearity and of the generated plasma. We have shown that rotationally symmetric light distributions become azimuthally unstable due to the onset of plasma generation. With increasing input power the maximum of the instability gain shifts towards perturbations with a higher order of v/sub m/. We suppose that the m with the highest gain is correlated with the number of filaments that will emerge.