Absolute onset of optical surface damage using distributed defect ensembles

The scaling of laser damage thresholds with irradiated spot size is a well-known effect. When damage is defect dominated, the spot-size scaling can be attributed to the conventional definition of the threshold at the 50% level of damage probability. By redefining the threshold at the 0% level (absolute damage onset), one obtains a result that is spot-size independent. A method is presented here for obtaining the damage onset of optical surfaces subjected to pulsed laser radiation. The method involves weighted least-squares fitting of damage frequency data with a three-parameter distributed ensemble representing inherent defect damage characteristics. Spot-size effects in the data are simulated by a scaling transformation applied to the ensemble before fitting. Direct and inverse transformations are derived for Gaussian and top-hat spatial intensity profiles, and advantages of the latter for testing are pointed out. Three examples of applications to 2.7-μm multilayer coatings are presented, and the general inadequacy of the two-parameter degenerate ensemble is demonstrated. Extraction of defect densities, discrimination of different defect classes, and representation of uncertainty in the onset are discussed.