High damage threshold hafnia/silica HR mirrors were damage tested. The transient reflectivity of these mirrors was studied during laser irradiation and particularly during catastrophic damage by top layer chipping. The tests were performed in R/1 mode on 50 sites in order to correctly assess the statistical behavior of the damage threshold. During the ramp, we observed the formation of a first plasma coupled to a modification of the reflected pulse without formation of any pit or chipping damage. For further irradiations at higher fluences, the optical properties of the mirror were unchanged up to the creation of a catastrophic chipping damage coming with a more intense plasma. The statistical distributions of fluences obtained in the two cases were different. For the moment, there is no proven correlation between first plasma and chipping: plasma detection cannot be used as a nondestructive quality control.

A laser damage measurement campaign was realized on PHEBUS high power laser on two different high reflecting HfO2/SiO2 mirrors centered at the wavelength of 1.053 micrometers at 45 degrees incidence. The two tested mirrors were deposited using e-beam technology: a large 620 X 440 mm2 LIL mirror was made with an oxide HfO2 target, and a 100 mm-diameter mirror with a metal Hf target. The test were performed with a 40 mm wide beam. Damages were detected by light scattering on a separate facility. Macroscopic and microscopic images of the damages were taken. A statistical analysis of these data is proposed to compare the mirrors. It is also interesting to compare large beam damage data to small beam laboratory statistics.

By direct observation of individual damage sites on the surface of a component, the local fluence of every damage site was obtained. This more accurate measurement was used to evaluate the real defect density at each fluence. These results were compared to some others, calculated from damage fluences distributions, obtained from R/1 tests. Results presented here concern fused silica with a regular polish. The experimental set up used for this work was equipped with a 3.7ns Nd:YAG pulsed laser at 1 omega. The laser beam diameter was 1.1 mm and the angle of incidence was close to 0 degrees.

This study was carried out in collaboration with the CEA within the framework of the Megajoule laser project.

Large components typically 620 mm x 440 mm are needed to build up the LMJ (Laser Megajoule) which belongs to the future generation of the fusion laser. Today the laboratory laser damage facilities allow to test small samples : generally witnesses with 50 am in diameter. However, more of the test procedures are manually and small areas are examined. It means that usually just 20 and 50 sites are tested with a small Gaussian beam ( about 2 mm at 1/e(2)). Therefore, it is essential to verify if the large components needed for the fusion laser have the threshold above the requirements (LIDT > 25 J/cm(2)). It is also important to determine which law exits between the threshold of the witnesses sample and the threshold of the final components. In this paper, these problems are addressed in using the most powerful pulsed Nd:YAG laser in France (Phebus). With this particular facility it becomes possible to test full scale optics on a few sites (typically 10) with a beam size diameter of 6 cm. Except the test on the real system, these kinds of experiments allow to observe the large beam damage morphologies. A study is at the beginning to try to find the scale law which linked the results obtained on the small witnesses with various methods. It also should lead to a procedure to qualify the full scale optics during the production time.

It is now accepted that absorption phenomena are not directly responsible for the flux resistance of multilayer optical components exposed to pulsed lasers. However, such studies involved `overall' absorption by components, which is now known does not characterize an intrinsic property of thin film materials. If the absorption of thin film materials is mapped by exciting different zones less than 100 micrometers in diameter, a clear disparity in absorption thresholds is shown up which must be attributed to heterogeneity in the imaginary refractive index or the extinction coefficient. The heterogeneity of absorption may be related to the incorporation of foreign matter in the materials or to dislocations, nodules, etc... Under these circumstances a key issue is whether the damage thresholds can be correlated with these microscopic absorption sites. We present the equipment used for carrying out the different types of characterization in this study: absorption and diffusion mapping, damage threshold measurements, and analysis by atomic force microscopy. An attempt is made to correlate these different results.

Simultaneous absorption and scattering mappings are used to study the influence of local defects on the laser damage threshold of thin films. The same area is mapped for absorption and scatter before and after irradiation at the threshold fluence. The study is performed for an uncoated fused silica substrate and two single-layer films deposited on fused silica substrates at a wavelength of lambda = 1.06 mu m. Initial results seem to indicate that the irradiation can create and enhance absorption and scattering defects.This study is conducted as part of the Megajoule laser project.

This study was carried out in collaboration with the CEA within the framework of the Megajoule laser project.

One of the concerns with the megajoule laser design is the laser-induced damage threshold of the transport mirrors. Earlier studies have shown that the main constraint on the laser damage threshold comes from nodules at the mirror surface. It is therefore important to restrict the number of such nodules. SFIM-ODS, in close collaboration with CEL-V, has initiated a special study to characterize these nodules as precisely as possible. The objective of the study is twofold: (1) to determine the origin of the nodules and subsequently to adapt the mirror fabrication process in order to limit their formation, (2) to analyze their shapes and dimensions in order to ascertain which nodules are critical for laser-induced damage. To understand the origin of the nodules and their effect on the laser damage threshold, the mirrors are characterized using various methods: (1) absorption and scatter mapping: does the presence of nodules result in specific absorption patterns, (2) surface analysis by atomic force microscopy: to characterize nodule shape and dimensions, (3) focused ion beam cutting of nodules: to locate the seed initiating the nodule and to characterize the seed shape and composition, (4) laser damage threshold measurements: to determine the laser damage threshold of them mirror and study the behavior of nodules under laser irradiation depending on their dimensions and shape.

Laser conditioning is a process which consists in illuminating a coating by a laser below damage threshold in order to increase this threshold value. In a previous study we had reported that laser conditioning actually results in minimization or elimination of surface defects present after coating elaboration. The present paper reports a statistical study whose goal is to seek a correlation between defects size and shape, and defect evolution during laser conditioning. In order to achieve this, hundreds of defects are individually analyzed before and after laser conditioning using Atomic Force Microscopy. The observed samples are e-beam deposited highly reflecting multilayers Zr02/Si02 and Hf02/Si02 mirrors, and also single layers of Si02 and Zr02.