Prepulse effect on intense femtosecond laser pulse propagation in gas

The propagation of an ultrashort laser pulse can be affected by the light reaching the medium before the pulse. This can cause a serious drawback to possible applications. The propagation in He of an intense $60-fs$ pulse delivered by a Ti:sapphire laser in the chirped pulse amplification (CPA) mode has been investigated in conditions of interest for laser-plasma acceleration of electrons. The effects of both nanosecond amplified spontaneous emission and picosecond pedestals have been clearly identified. There is evidence that such effects are basically of refractive nature and that they are not detrimental for the propagation of a CPA pulse focused to moderately relativistic intensity. The observations are fully consistent with numerical simulations and can contribute to the search of a stable regime for laser acceleration.