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Laser damage phenomena in fused silica are currently under study because of numerous related high power laser applications. Nanosized defects are believed to be responsible for some laser damage initiation. In order to predict and to quantify this initiation process, engineered submicronic gold defects were embedded in silica. The study of these samples by localized pulsed irradiation of isolated gold particles coupled with Nomarski, atomic force and photothermal microscope observations permits us to discriminate between two distinct stages of material modification: one detectable at the surface and the second in the neighbourhood of the embedded particle. Comparison between the observations and simulations results in good agreement if we assume that inclusion melting initiates the damage.

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In the range of nanosecond pulse lengths, the mechanisms of surface laser damage to dielectric materials are still unclear. A large amount of experimental and theoretical work has been performed over recent years. In order to test theoretical predictions and compare experimental results, reproducibility is essential whatever the beam parameters and experimental conditions. The rasterscan procedure, previously developed to test large components, is an efficient method that allows measuring extremely low surface damage site density (until 0.01 site/cm2 for large optics). In this paper, we show that by suitable data reduction, error bar calculation, and attention paid to beam analysis, laser-induced surface damage density of fused silica optics can be measured with high accuracy and repeatability in the range of pulse durations from 2 to 16 ns. This procedure provides a straightforward means of comparing the experimental results obtained from several facilities using different lasers.

We analyze laser damage precursor evolution under multiple irradiations by changing test parameters such as shot number, wavelength, shot frequency, and test location (bulk or surface). The experimental data exhibit different behaviors under repetitive shots regarding the damage precursor densities and thresholds. The results provide new information for understanding the laser damage initiation process in silica. Furthermore, the data permit us to predict the lifetime of optical components under multiple irradiations.

The laser damage community has long been searching for the reproducibility of damage measurements in the nanosecond (ns) regime. Here we show that laser-induced bulk damage density of frequency conversion crystals can be measured with high accuracy and repeatability in the range from 2 to 20 ns. The rasterscan test procedure (Lamaignère et al 2007 Rev. Sci. Instrum. 78 103105), previously developed in order to determine laser damage density of large aperture UV fused silica optics, has been adapted to bulk damage measurement. The large volume scanned during tests permits us to measure very low damage densities. For smaller optical components, small volumes are tested using the normalized 1/1 test procedure. Whatever test procedures, accuracy and repeatability are obtained by means of a suitable data reduction. For comparison between different procedures, the classical damage probability plot has to be converted in terms of damage density. The consideration of error bars on damage site distributions is compulsory to compare experimental data. A special emphasis is put on damage detection tools. When tests are carried out on diverse facilities, pulse duration, spatial distribution and beam overlap are the key parameters which are to be taken into account to compare experimental data. We describe the equipment, test procedures and data analysis to perform these damage tests with small beams (Gaussian beams, about 1 mm @1/e, and top-hat beams). Other tests are realized with larger beams (cm sized) which are compared with small beam results. The consistency of all the results gives confidence in the measurements. Reproducibility of measures is discussed in connection with current theoretical understanding of laser damage.

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Significant improvement in the polishing process of fused silica optical components has increased their lifetimes at 351 nm. Nevertheless, for large laser facilities like the LaserMegaJoule (LMJ), zero defect optical components are not yet available. Therefore, a damage mitigation technique has been developed to prevent the growth of the laser-initiated damage sites. Because of the difficulty to produce mitigated sites with sufficiently large depth, the initial morphology of damage to mitigate is a critical issue. The aim of this work is to determine laser parameters (pulse duration, fluence) which permit us to initiate damage sites in accordance with our mitigation process. Confocal microscopy is used to observe damage sites that have sub-surface cracks and consequently to measure precisely the diameter and the depth of the area to mitigate.