Non destructive evaluation on optical components for high power density applications

The development of high power laser for large instruments such as LMJ and NIF, as well as the use of smaller optical structures with high densities of energy lead to consider laser induce damage threshold (LIDT) as a critical criterion in optical components development. Furthermore in the same time some applications need an increase of the lifetime of the laser source (up to 109 shots for spatial applications). In this context to improve optical components numerous studies are undertaken to determine the origin of laser damage process. Through these studies, it is now commonly admitted that the first stage of the laser damage process in the nanosecond regime is caused by localized defects included in the material. In order to determine the laser damage threshold, it is necessary to perform a statistical study of damage on materials. This destructive technique beyond the determination of damage threshold allows to determine the density of precursor centers and to discriminate different kinds of precursors by using adapted beam sizes. However the nature and therefore the origin of the defects remain unknown. In order to get information on this nature, non destructive tools have to be involved. Indeed on one hand photothermal microscopy permits to make measurement of the local absorption under irradiation, on the other hand, photoluminescence cartography and spectroscopy gives information on material composition. The coupling of the different techniques on a laser damage test set-up optimizes the chance to have a complete signature of precursor center and information about the mechanism of damage process. Furthermore, nondestructive diagnostic under mutilple irradiations permit to study "fatigue" and conditioning process.