THE EFFECT OF SELF-PHASE MODULATION ON STIMULATED BRILLOUIN-SCATTERING IN FILAMENTARY LASER PLASMAS

Self-phase modulation (SPM) in filamentary plasma can explain anomalous large frequency shifts in time-resolved spectra of laser light backscattered from laser plasmas. We have modelled the effect of SPM by assuming a rapid variation in the plasma electron density. This is expected when laser light filaments or self-focuses in plasmas. The first direct experimental observation is presented showing the correlation between laser beam self-focusing and anomalous backscattered stimulated Brillouin (B-SBS) spectra. The B-SBS spectral features recorded are strongly characteristic of those expected when the frequency of the incident laser light is modified by SPM. A consequence of our observation is that SPM should be considered in the interpretation of SBS spectra in laser-plasma interactions where laser self-focusing is occurring. Also, SPM can contribute to the saturation of the SBS process to low levels by effectively destroying the coherence of the SBS interaction.