Rear-surface laser damage on 355-nm silica optics owing to Fresnel diffraction on front-surface contamination particles

Light intensity modulations caused by opaque obstacles (e.g., dust) on sili ca lenses in high-power lasers often enhance the potential for laser-induce d damage. To study this effect, particles (10-250 mu m) with various shapes were sputter deposited on the input surface and irradiated with a 3-ns las er beam at 355 nm. Although a clean silica surface damages at fluences abov e 15 J/cm(2) a surface contaminated with particles can damage below 11.5 J/ cm(2). A pattern that conforms to the shape of the input surface particle i s printed on the output surface. Repetitive illumination resulted in catast rophic drilling of the optic. The damage pattern correlated with an interfe rence image of the particle before irradiation. The image shows that the in cident beam undergoes phase land amplitude modulations after it passes arou nd the particle. We modeled the experiments by calculating the light intens ity distribution behind an obscuration by use of Fresnel diffraction theory . The comparison between calculated light intensity distribution and the ou tput surface damage pattern showed good agreement. The model was then used to predict the increased damage vulnerability that results from intensity m odulations as a function of particle size, shape, and lens thickness. The p redictions provide the basis for optics cleanliness specifications on the N ational Ignition Facility to reduce the likelihood of optical damage. (C) 2 000 Optical Society of America OCIS code: 140.3380.