Depth profiling of polishing-induced contamination on fused silica surfaces

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.

Study for Improvement of Laser Induced Damage of 1064 nm AR Coatings in Nanosecond Pulse

“…The ion-beam etching process successfully removed the layer of fractures and associated contaminants, while the appropriate energy ions did not penetrate or disrupt the structure near the surface. At the same time, these results agreed well with the depth profile of embedding polishing compound as suggested by Kozlowski [7].…”

“…It is a fact that laser-induced damage of AR coatings was a limiting factor on reliable operation of such high-power solid-state lasers. Many studies on how to improve transmittance element damage threshold have been carried out over the past decades years [5][6][7][8]. Among these studies, understanding the origin and mechanisms of laser-induced damage was important for the improvement of the laser radiation resistivity of the AR coatings.…”

Effect of subsurface impurities of fused silica on laser-induced damage probability

“…10,11 However, in most cases, the impurities are not identified by modern optical techniques since they are nanoscale size and are distributed at low concentration. 12 It is obvious that the inclusion-initiated damage has a statistical character because of the spatial distribution of inclusions in a sample. 10 The theoretical studies of inclusion-initiated damage were based on the resolution of Fourier equation.…”

Investigation of subsurface damage impact on resistance of laser radiation of fused silica substrates

“…It is well known that the conventional CeO 2 abrasive polishing technique, which relies on a normal load applied to an abrasive particle, cause material removal and generating scratches and subsurface damage (SSD) of fused silica glass [1][2][3]. Subsurface damages are defined as residual digs and scratches, some of which are filled with polishing slurry and covered with so-called Bielby layer (polished layer) [4][5][6].…”