Spectroscopy of short, intense laser pulses due to gas ionization effects

We study the effects of ionization of neutral gases by use of short, intense, chirped pulses with various higher-order distortions in the frequency domain. Investigations were carried out with a one-dimensional, fully relativistic fluid code, which included strong field tunneling and collisional ionization as well as quasi-two-dimensional focusing and defocusing. The resulting effects on the spectrum are explained in terms of blue shifting and depletion of different parts of the spectral intensity distribution according to their position within the laser pulse in the time domain. In turn, we point out that qualitative information about the pulse's phase distortion may be gained from the observation of such spectral distributions. A comparison of the experimentally measured spectrum of a strongly distorted laser pulse with our simulations is given. © 1997 Optical Society of America.