Breakup and fusion of self-guided femtosecond light pulses in air

We report experiments showing the breakup and the merging of filaments form ed by the modulational instability of femtosecond optical pulses in air. Fo r input powers as high as 25 times the self-focusing threshold, the beams a re shown to split into two spots, which coalesce into a self-guided beam. T his effect occurs in an optically Kerr regime and plays an important role i n the guiding process. Numerical simulations and theoretical estimates both support the comparison with the experimental data.