Evidence of stimulated Raman scattering occurring in laser filaments in long-scale-length plasmas

A time‐resolved study is presented of stimulated Raman backscatter (B‐SRS) produced in a large‐scale‐length preformed plasma in an intense laser pulse. The first direct experimental results are presented to (i) show that the B‐SRS is generated predominantly in laser filaments and (ii) provide a direct correlation between the spectral and spatial emission profiles of the B‐SRS as a function of interaction irradiance. The ‘‘anomalous’’ B‐SRS spectral features so commonly reported in time‐resolved studies are shown to be the characteristic emission of B‐SRS from filaments. The B‐SRS fraction was also measured as a function of interaction irradiance. The B‐SRS intensity ‘‘threshold’’ and ‘‘saturating’’ behavior recorded can be well described for the present interaction conditions. The onset of B‐SRS is shown to be strongly correlated with the intensity threshold of the filamentation instability. The effect of laser beam ‘‘smoothing’’ techniques on the B‐SRS emission have also been investigated, using either a...