A powerful tool for comparing different test procedures to measure the probability and density of laser induced damage on optical materials

Abstract

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.

Effect of THz-bandwidth incoherent laser radiation on bulk damage in potassium dihydrogen phosphate crystals

“…Since the test range is higher than the design fluence of 2.5 J/cm 2 , a calculation of damage density is performed on the data and fit to a power law as a function of fluence, , following Ref. 20 . This fit is then used to extrapolate the expected damage density at operational fluences which are included in Table 1 .…”

Estimation of laser-induced damage depth from surface image features

“…Damage growth sequences were conducted on the laser damage setup MELBA [23]. Both surface and bulk images were acquired during growth experiments with tunable laser parameters.…”

L’endommagement laser sur les lasers de puissance

“…Des traitements mathématiques tenant compte de la distribution de la population de défauts permettent de faire la correspondance entre les densités de dommages et les probabilités d'endommagement estimées par les différentes procédures. Plus de précisions sont données dans les références [3] et [4]. (partie par million), une largeur de bande interdite élevée (autour de 9eV pour la silice UV par exemple) et très peu d'impuretés.…”