Search

The impact of laser fluence and pulse duration on both the rate and probability of growth of laser-induced damage sites has been reported and analyzed statistically in a companion paper. In this paper, we report and analyze the volume morphology of damage sites during the growth process in fused silica optical components, at 351 nm, under various laser fluence and pulse durations. Fractal analysis has been used to quantify the bulk damage morphology. A clear link between the damage morphology and laser pulse duration has been observed. The results from fractal analysis allows for a better understanding of the results from the stochastic approach developed in our companion paper. More specifically, fractal analysis shows how the laser parameters such as fluence and pulse duration impact the phenomenology and the dynamics of the growth process.

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.

The Laser MégaJoule (LMJ) is a high-power laser dedicated to laser-plasma experiments. At the beginning of the project in the mid-1990s, an optical metrology laboratory was created at CEA to help accomplish all the steps in the construction of this laser. This paper proposes an overview of the capabilities of this metrology laboratory in four main fields: surface imperfections, photometry, laser damage measurement, and wavefront measurement. The specificities for high-power laser optics in each domain are highlighted as well as the specific features that make our instruments unique.

Lateral and longitudinal laser damage growth under subsequent irradiations at 351 nm in the nanosecond range from micrometric to millimetric scales is presented herein. Atypical behavior has been observed, showing the growth in the longitudinal direction, whereas the lateral growth does not evolve. We propose the use of fractal analysis to describe the evolution of the bulk damage morphology. The results indicate first a dependence between the damage fractal dimension and the laser parameters, such as the fluence and the pulse duration. Next, it seems from observations that the damage morphology modifications drive the growth rate changes.