Fused silica optics are key components to manipulate high energy Inertial Confinement Fusion (ICF) laser beams but their optical properties can be degraded by laser-induced damage. The detection of laser damage sites is of major importance. The challenge is to monitor damage initiation and growth at sub-pixel scale with highly sensitive measurements. The damage diameter is a widely used indicator to quantify damage growth but its accuracy is strongly dependent on the available image resolution. More recently, it was shown that registration residual maps (i.e., gray level differences between two registered images) could also be used to monitor laser-induced damage. In this paper, the performance of both indicators are compared to detect laser damage initiation and growth at high and low image resolutions thanks to a highly instrumented laser setup. The results prove that registration residual maps are more efficient to detect sub-pixel laser damage growth than diameter measurements at a given image resolution. The registration residual maps are therefore a powerful indicator for monitoring laser-induced damage initiation and growth at sub-pixel scale either for laser damage metrology setups, for high energy laser facilities, or other situations where damage is suspected to occur. The accuracy of (laser-induced) damage laws may also be improved thanks to this tool.

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.

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The determination of the laser damage resistance of optics in the nanosecond regime is based on statistical tests and approaches because the response of the components is mainly related to the presence of defects randomly distributed in the optics and is therefore probabilistic in nature. For practical reasons, the tests are mostly carried out with beams of small dimensions (several tens of micrometer), that make it possible to determine a damage probability from which a laser damage threshold is extracted. This threshold is, however, highly correlated with the size of the test beam and the sampling of the test procedure. Some measurements are also made with beams of large dimensions (several millimeters) from which a damage density is determined. However, the relationship between the damage probability and the damage density is not trivial. It is based on assumptions that are difficult to verify because the experimental validations are carried out on different laser installations. In order to study accurately the coherence between these tests with small and large beams, as well as the link between damage probability and damage density, it is necessary to perform measurements on the same laser installation. We propose here, to compare for the first time, the results obtained with the same laser source with a large beam and also with small beams. The small beams are shaped from phase objects specifically implemented to obtain several small beams from a single larger beam. The consistency of the laser damage that results from both sets of measurements is demonstrated here. It validates the assumptions made and the specific mathematical treatment implemented to establish the link between the two approaches. In fine, it also validates and strengthens the approach previously developed from the rasterscan procedure [Lamaignère et al., Rev. Sci. Instrum. 78, 103105 (2007)] used to measure damage densities from the scanning of optics with beams of small dimensions. The reported original work based on phase objects thus makes it possible to replicate small beam tests with a large beam facility. The comparison between the results from the small beams and the results from the large beam experiments definitively makes the link between damage probabilities and damage densities. This also shows that small beam tests are reasonable representative of tests carried out with large beams.

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 1, we present statistical measurements of damage growth at different energy densities, pulse durations, and initial damage sizes. In Part 2, 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.