Abstract
The coupling between Kerr-induced filamentation and transient stimulated Brillouin scattering (SBS) is theoretically investigated for nanosecond laser pulses propagating in bulk silica. Power thresholds for beam collapse are evaluated by means of three-dimensional numerical simulations. It is shown that the nonlinear self-focusing of powerful pump waves is able to enhance the Stokes component to high fluence levels even if the incident beam exhibits a broad spectral bandwidth. In contrast, pump pulses with a few tens of picoseconds amplitude modulations drastically inhibit SBS.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. The Kerr self-focusing of powerful nanosecond light pulses in dielectrics can trigger multiple filamentation, which breaks the homogeneity of the energy distribution and can cause severe damage. Part of the incident beam is, furthermore, reflected back by stimulated Brillouin scattering (SBS), which initiates front surface damage. Here, the coupling between Kerr-induced filamentation and SBS is numerically investigated for laser pulses propagating in bulk silica, at ultraviolet and infrared wavelengths, for input powers of up to 30 times the critical power for self-focusing.
Main results. For Gaussian pulses, the SBS-driven pump depletion leads to an increase of the self-focusing distance along the optical path (black curve in the figure). Part of the energy is transferred to the Stokes component which can be strongly amplified with fluences close to the damage threshold. To inhibit the Stokes growth, pump pulses with appropriate phase or amplitude modulations are examined next. A phase modulation, which decreases SBS at low input powers, turns out to be less effective at powers above critical. Instead, modulational instabilities occur, leading to a shorter pump self-focusing distance that is in excellent agreement with experimental data (star symbols in the figure). In contrast, suitable amplitude modulations profiling the pump into picosecond subpulses can definitively eliminate SBS.
Wider implications. Our results open new strategies to avoid material damage in silica devices used in high-energy laser facilities. Future work will address the possible existence of critical spectral bandwidths, beyond which phase and amplitude modulations can provide comparable efficiency.
Figure. Mapping I1(0) × zc vs pump power at 355 nm. The black curve refers to the classical prediction for a Gaussian pulse. Blue squares correspond to data points for unmodulated beams. Pink triangles (red circles) report results from amplitude- (respectively, phase-) modulated pumps. The star symbols recall experimental data. Insets on the left-hand side illustrate beam profiles of the backscattered wave in the (x,t) plane at the entrance of a 5 cm thick silica sample for (top) an unmodulated, (middle) amplitude-modulated and (bottom) phase-modulated pump pulse.