Laser damage resistance qualification of large optics for high power laser

Abstract

The rasterscan procedure, developed to test large components, is an efficient method that allows measuring extremely low surface damage density (until 0.01 site/cm² for large optics). This procedure was improved in terms of accuracy. The equipment, test procedure and data analysis to perform this damage test of large aperture optics are described. The originality of the refined procedure is that a shot to shot correlation is performed between the damage occurrence and the corresponding fluence by recording beam parameters of hundreds of thousands of shots during the qualification. Because tests are realized with small Gaussian beams (about 1mm @ 1/e), beam overlap and beam shape are key parameters which have to be taken into account in order to determine damage density. After complete data analysis and treatment, a repeatable metrology has been reached. The measurement is destructive for the sample. However the consideration of error bars on defects distributions allows us to compare data obtained on a same batch of optical components. This will permit to reach reproducible metrology. Then this procedure provides a straightforward means of comparing the experimental results obtained from several facilities using different lasers. Recently, an additional step has been added to the procedure, a growth step that permits considering only growing damage sites. To the end the lifetime of large optics on high power laser can be predicted.