Filamentation and surface damage in fused silica with single-mode and multi-mode pulses

We studied filamentation, front surface damage and rear surface damage at 1064 nm and 351 nm with nanosecond pulses on a fused silica optical window. With temporally single-mode pulses, self-focusing occurs together with front surface damage, which is attributed to a Stimulated Brillouin Back Scattering (SBS) wave. The use of temporally multi-mode pulses suppresses the occurrence of front surface damage, and increases self-focusing. With single-mode pulses, the observation of filaments seems coherent with standard Kerr self-focusing effect, and can be understood according to the numerical treatment by Marburger et al, using non linear index values measured in other experiments. However, when multi-mode pulses were used, filaments occurred for much smaller peak intensifies, by about a factor of 2. In this case, the non linear index causing self-focusing appears to be twice bigger. This second case is relevant to the situation of vacuum windows in high power laser installations, where the spectrum of light is widened to get rid of SBS. We discuss the physical effects that could be causing the enhancement of self-focusing.