No abstract

Laser-induced damage on optical surfaces is often associated with absorbing contaminants introduced by the polishing process. This is particularly the case for W optics. In the present study, secondmy ion mass spectroscopy (SIMS) was used to measure depth profiles of finishing-process contamination on fused silica surfaces. Contaminants detected include the major polishing compound components (Ce or Zr from CeOz or Z@z), Al present hugely because of the use of AlzO~in the final cleaning process, and other metals (Fe, Cu, Cr) incorporated during the polishing step or earlier grinding steps. Depth profile data typically showed an exponential decay of contaminant concentration to a depth of 100-200 nm. This depth is consistent with a polishing redeposition layers formed during the chemo-mechanical polishing of fused silica. Peak contaminant levels are typically in the 10-100 ppm range, except for Al which often exceeds 1000 ppm.A strong correlation has been shown between the presenee of a "gray haze" damage morphology and the use of Ce02 polishing compound. It has not been proven, however, that linear absorption by CeOz, or any other contaminant, is the relevant damage mechanism. Simple thermomechanical calculations show that for the contaminant levels present, temperatures high enough to cause damage m only likely if the contaminant was present as particles with diameters of 10-30 nm. We are not able to prove or disprove the presenee of such particles. No strong correlation between high levels of Ce, or any other contaminant, ad low damage threshold is observed. In fact one of the strongest indications of a correlation is between increased damage thresholds and inaeasd Zr contamination. This suggests that the connection between redeposition layer contamination and laser damage threshold is not simply an absorbing contaminant issue.

No abstract

For the aggressive fluence requirements of the NIF laser, some level of laser-induced damage to the large (40x40 cm) 35 1 nm final optics is inevitable. Planning and utilization of NIF therefore requires reliable prediction of the functional degradation of the final optics.Laser damage tests are typically carried out with Gaussian beams on relatively small test areas. The tests yield a damage probability vs. energy fluence relation. These damage probabilities are shown to depend on both the beam fluence distribution and the size of area tested. Thus, some analysis is necessary in order to use these test results to determine expected damage levels for large aperture optics. : We present a statistical approach which interprets the damage probability in terms of an underlying intrinsic surface density of damaging defects. This allows extrapolation of test results to different sized areas and different beam shapes (NIP has a flattop beam). The defect density is found to vary as a power of the fluence (Weibull distribution).

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