Techniques for qualitative and quantitative measurement of aspects of laser-induced damage important for laser beam propagation

Characterizing laser-induced damage in optical materials is important for laser design and operation. Previous methods of evaluating optical materials damage resistance to high-power laser irradiation have typically suffered from shot to shot uncertainties in laser energy output and/or have insufficient sensitivity. More importantly such methods do not address the aspects of laser-induced damage important to laser beam propagation, namely the amount of light scattered by the damage. We present a method for the quantitative correlation of material modification on the surface or in the bulk of optical materials to laser parameters, which deconvolutes the effects of laser output instability. Image analysis, whereby two images, one a fluence spatial profile and the other a visible light scatter image of the damage, are directly compared to extract scatter as a function of fluence. An automated microscope is used to record the location and number of bulk damage sites and determine a calibration factor between the scatter signal observed and damage density pinpoints (ppt)/mm{sup 3}. We illustrate the method with a determination of both bulk damage density as a function of laser fluence and of a representative size distributions in a DKDP crystal. Our method is capable of determining damage densities with an absolute uncertainty of +/- 0.3 pp/mm{sup 3} in the range 1-100 ppts/mm{sup 3} with our minimum detectable density being 0.01 ppts/mm{sup 3}. We also determined the ppt size distribution for 351-nm, 3-ns damage with the average size being 5.5 +/2.5 mm (1/e{sup 2}) diameter