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. In image analysis, 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. We illustrate the method with a determination of both bulk damage density as a function of laser fluence and of a representative size distribution in a DKDP crystal. Our method is capable of determining damage densities with an absolute uncertainty of +/- 0.3 pinpoint damage sites per cubic millimetre (pp mm(-3)) in the range 1-100 pp mm(-3) with our minimum detectable density being 0.01 pp mm(-3). We also determined the pps produced by laser pulses of 351 nm, 3 ns light to have a mean diameter of 5.5 +/- 2.5 mu m (1/e(2)).