Interplay between stimulated Brillouin scattering and Kerr filamentation with an inertial plasma response

The filamentation of ultraviolet and infrared nanosecond light pulses in fused silica is investigated theoretically and numerically. Emphasis is put on the action of a dynamical plasma response on two counterpropagating waves, amplified by Kerr self-focusing and stimulated Brillouin scattering (SBS). For a single unperturbed wave, laser filamentation takes place through a quasistationary balance between Kerr self-focusing and plasma defocusing, for which a variational approach reproduces global propagation features. However, such a quasistationary balance cannot hold as temporal modulational instability breaks up the pulse over electron recombination times. For two counterpropagating waves, we report similar instabilities which are justified through a plane-wave stability analysis. These instabilities originate from an intense backscattered wave that induces strong plasma contribution and thus destabilizes the pump near the entrance face of the material. Rapid phase modulations are then applied to suppress backscattering. We show that pump waves with broad enough bandwidths can inhibit SBS and thus prevent instability. The robustness of phase modulations against random fluctuations in the input pump pulse is finally addressed. (authors)