Dependence of multiple filamentation on beam ellipticity.

We investigate the effects of beam ellipticity on the dynamics of multiple filamentation. We find that increasing the ellipticity of the initial beam decreases the power required for multiple filamentation. At lower input ellipticities, the beam breaks into filaments along its widest dimension, whereas for higher ellipticities the pulse breaks into bands and then into filaments as the power is increased. The breakup patterns of the beam along the wider dimension are consistent with the modulational instability, and these patterns are independent of polarization and noise. Numerical simulations are in qualitative agreement with these features of multiple filamentation breakup.