Abstract
We have observed damage to 80-cm-diam fused-silica disks and lenses subjected to high-fluence pulses (up to 2.3 J/cm2) from an upgraded Nova laser beamline (wavelength 351 nm; pulse duration 2.35 ns; beam diameter 70 cm; energy up to 8 kJ). Damage occurred in the center of each element, where a 6-cm-wide obscuration prevented direct illumination. We believe that light strongly scattered by transverse stimulated Brillouin scattering (SBS) interacts with the surface and with the bulk of the substrate, producing two kinds of acoustic waves that propagate to its center, where they become strong enough to do damage. In the surface interaction, scattered light is absorbed by an 0-ring near the perimeter of the optic, creating a Rayleigh wave that propagates along the surface to the center of the optic. The resulting damage takes the form of crater-shaped fractures about 8 mm in diameter and 4 mm deep. In the bulk interaction, transverse SBS strongly compresses the optic in large regions transverse to the direction of beam polarization at the perimeter of the beam. The compression may result from electrostriction: the SBS intensity is several times that of the incident beam. Compressive waves resulting from the relaxation of these regions propagate to the perimeter of the optic, where they are reflected as bulk tensile waves. The focusing of these tensile waves in the center of the optic results in cracks along the direction of polarization. Up to 25 percent of the incident beam energy is lost to SBS at these high fluences. Frequency chirping of the laser beam by 45 GHz strongly suppresses the SBS, and reduces the amplitude of the stress waves by about an order of magnitude; no energy loss, cratering, or cracking occurs under these conditions. We propose design rules for avoiding acoustic damage in large optics and compare observed thresholds for transverse SBS with predictions in the literature.
