Ultrasonic cleaning and laser surface damage threshold

Since ultrasonic cleaning is used in cleaning surfaces for thin-film deposition and since real increases in roughness lead to decreased laser damage resistance in surfaces, a rudimentary experiment was performed to examine the long-term effects of ultrasonic cleaning on optically polished fused silica surfaces. Samples of fused silica were characterized for surface roughness by the techniques of total integrated scattering and/or fringes of equal chromatic order. They were then subjected to ultrasonic cleaning, laser damage tested, and remeasured for roughness. The samples were cleaned in an ultrasonic cleaner using two types of commercial baths: Radiac solution and Micro solution. Damage testing consisted of irradiating a new site for each exposure, and all target sites were shielded from each other against blowoff and resulting UV radiation. The roughness-normalization of threshold electric fields is found to be an effective means for quantitatively comparing the effects of preparation techniques on fused silica