Fig. 1. Experimental setup.

Fig. 2. Fluorescence image ((a), (b)) and the on-axis intensity ((e), (f)) of the filament formed by Gaussian beam ((a), (e)) and flattened beam ((b), (f)) respectively, with an incident energy of 672 μJ; the intensity distributions in the cross sections of (b) Gaussian beam and (d) flattened beam. The inset arrow indicates the laser propagation direction.

Fig. 3. Fluorescence image (a) and the on-axis intensity of the filament (b) formed by flattened beam with incident energy of 1.319 mJ.

Fig. 4. Supercontinuum spectra from filamentation of the Gaussian beam with an incident energy of 672 μJ and flattened beam with an incident energy of 672 μJ and 1.319 mJ, respectively.