Parametric study of laser-induced damage growth in fused silica optics with large beams at 351 nm. Part 1: stochastic approach.

Both the rate and probability of the growth of laser-induced damage sites in fused silica depend on several parameters. In this two-part paper, we investigate the impact of the laser parameters on damage growth. In Part I, we present statistical measurements of damage growth at different energy densities, pulse durations, and initial damage sizes. In Part II, we use fractal analysis to quantify the evolution of the damage morphology as a function of the laser energy density and pulse duration. Damage initiation is performed using phase masks. These phase masks allow for the initiation of evenly spaced damage sites that can then be exposed to the same laser beam, and, therefore, the same pulse duration. This configuration allowed the study of damage growth in a large population of more than 5000 damage sites. The results clearly indicate that both the probability and the rate at which a damage site will grow strongly depend on the laser pulse duration. These differences can be explained by hypotheses that we have developed from an observation of the bulk damage morphology. Such observations will be presented in detail in the second part of this article.