Few-cycle laser pulses tested with double ionization of H₂

Different compression schemes for the production of sub-10 fs laser pulses are tested using double ionization of H-2. Compression schemes differ in the laser beam guiding modes which are necessary in order to get a spatially-homogeneous spectral broadening through self-phase modulation in argon. Guiding may take place in a hollow fibre waveguide or may be self-induced in a filament. We tested four set-ups: a single hollow fibre, two hollow fibres, a hollow fibre then a filament, and two filaments. The shortest pulses around 8 fs were recorded with the set-ups involving one or two filaments. Double ionization of H2 and the associated proton spectra give results in good agreement with the spectral and interferometric autocorrelation measurements. The two-filament set-up gives the shortest pulses. In addition, the H2 experiment reveals that the spectral phase induced with two filaments cannot be fully compensated with second-order chirped mirrors as for the other tested set-ups.