Noneffect of electronegative gases on pulsed laser-induced optical surface damage thresholds

Abstract

The pulsed laser-induced damage threshold of several optical surfaces has been measured as a function of the pressure and type of ambient gas. Measurements were made in vacuum and various pressures of nitrogen, Freon 14, (CF4), and SF6 in an attempt to decrease the growth rate of the avalanche which accompanies pulsed laser-induced damage. Test samples included conventional dielectric coatings of ZrO2, diamond-turned copper, bare surfaces of fused silica and ARG-2 glass, and porous surface layers of both leached ARG-2 glass and some solgel coatings. From this preliminary study, it is apparent that the ambient gas has no observable effect on 1-on-1 surface damage thresholds (although some improvement in N-on-1 damage levels had been reported). The present finding is in agreement with previous observations that damage sites appear to be initiated on or just below the surface of optical elements. For uncoated surfaces, both conventional and porous initiation sites seem to be isolated impurities embedded near the surface resulting in the formation of pits. In either case, such damage initiation levels are unmodified by the presence of electronegative gases, and no change in threshold is observed.