Known for more than 40years, laser damage phenomena have not been measured reproducibly up to now. Laser resistance of optical components is decreased by the presence of material defects, the distribution of which can initiate a distribution of damage sites. A raster scan test procedure has been used for several years in order to determine laser damage density of large aperture UV fused silica optics. This procedure was improved in terms of accuracy and repeatability. We describe the equipment, test procedure, and data analysis to perform this damage test of large aperture optics with small beams. The originality of the refined procedure is that a shot to shot correlation is performed between the damage occurrence and the corresponding fluence by recording beam parameters of hundreds of thousands of shots during the test at 10Hz. We characterize the distribution of damaging defects by the fluence at which they cause damage. Because tests are realized with small Gaussian beams (about 1mm at 1∕e), beam overlap and beam shape are two key parameters which have to be taken into account in order to determine damage density. After complete data analysis and treatment, we reached a repeatable metrology of laser damage performance. The measurement is destructive for the sample. However, the consideration of error bars on defect distributions in a series of parts allows us to compare data with other installations. This will permit to look for reproducibility, a necessary condition in order to test theoretical predictions.
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.
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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