Herve Bercegol

Comparison of laser-induced surface damage density measurements with small and large beams: toward representativeness

Pulsed laser damage density measurements obtained with diverse facilities are difficult to compare, due to the interplay of numerous parameters, such as beam area and pulse geometry, which, in operational large beam conditions, are very different from laboratory measurements. This discrepancy could have a significant impact; if so, one could not even pretend that laser damage density control is a real measurement process. In this paper, this concern is addressed. Tests with large beams of centimeter size on a high-power laser facility have beam performed according to a parametric study and are compared to small beam laboratory tests. It is shown that laser damage densities obtained with large and small beams are equal, within calculated error bars.

Observation of laser-induced damage on fused silica initiated by scratches

A major issue in high power lasers for fusion is laser-induced damage on optics and its evolution in time after a large number of shots. Since damage is often characterized by an initial surface crack, its surface usually increases, following an exponential law. Surface scratches have been made on silica samples in order to get calibrated fractures. Then, to test different experimental conditions, we made a variety of scratches in terms of length and depth. The samples are then irradiated by a Nd:YAG laser first at 1064 nm (1w) then at 355 nm (3w). They are successively tested with the scratches facing the laser beam or placed with the scratches on the back surface.

Characterization of KDP crystals used in large aperture doublers and triplers

We report on laser-induced damage threshold (LIDT) and UV-laser excited defect formation measurements in large aperture KDP crystals developed as doublers and triplers for mega-Joule laser. Measurements of LIDT were performed according to the ISO 11254-2 standard for repetitive pulses with duration ~ 4 ns and repetition rate of 10 Hz. The results for different laser wavelengths (1064, 532 and 355 nm) and polarizations are presented. The largest LIDT was observed for 532 nm pulses and the 1064 nm wavelength had a strong dependence on laser polarization. The LIDT values at 532 nm and 355 nm also depended on the crystal cutting angle, which is different for doublers and triplers. A comparison of LIDT with earlier reported crystal absorptance at different wavelengths is also performed. The UV-laser induced defect formation was investigated by the means of pump-probe technique. The excitation was performed with a single pulse of ns Nd:YAG laser (355 or 266 nm wavelength) and probing with another Nd:YVO4 laser system (532 nm) operating at 1kHz. This gave us a temporal resolution of 1ms. The transient absorption of defect states relaxed non-exponentially and fully disappeared in ~10 s. A comparison is made between crystal grown by distinct growth methods and between different laser polarizations. An influence of laser conditioning on UV induced defect state formation is also revealed.

Computer-controlled measurements of laser-induced damage statistics on large optics

This paper presents data reduction on an experimental set-up that we have recently developed at CESTA, France. It has been implemented to analyze laser-induced damage on optics dedicated to the Megajoule laser project. Our goal is to measure the damage fluence on samples under tests, using a statistical approach on a very large number of sites. The laser-induced damage density is accurately plotted as a function of laser fluence, by measuring the fluence of every single shot during the scan. This improvement of accuracy modifies dramatically the interpretation of the results that can be made, compared to raw data, considering the mean fluence only.

Advanced optical damage studies using x-ray laser interferometric microscopy

We present early results of an application of X-ray laser, aimed at understanding the effects involved in formation of laser-induced damage in optical materials exposed to sub-ns laser pulses. For the purpose of the experiment, a novel interferometric microscopy technique was designed and tested. The interferometric beamline employed a double Lloyds mirror interferometer, used in conjunction with an imaging mirror to provide magnification of ~8 along a plane inclined with respect to the propagation direction of the X-ray beam. The objects investigated were thin plane beamsplitters made of fused silica (SiO2), irradiated by damaging laser light at 438 nm and in situ probed by the developed technique of interferometric microscopy. The soft X-ray beam was emitted by neon-like zinc laser, delivering up to 10 mJ at 21.2 nm. In conjunction with an array of in-situ optical diagnostics, one of the questions addressed was whether the damage of the rear surface of the beamsplitter occurs approximately during of much after the laser pulse. Another issue examined by the X-ray interferometric microscopy technique was whether the surface perturbation seen shortly after the impact of the damaging pulse is associated or not with the pattern of permanent surface modifications.

Laser-induced damage studies in optical elements using X-ray laser interferometric microscopy

Results of a novel X-ray laser application, aimed at understanding the microscopic effects involved in formation of laserinduced damage in optical materials exposed to sub-ns laser pulses, will be presented. Specifically, we studied thin plane beamsplitters that are presently the weakest element of the next generation of high-energy lasers (LMJ, NIF), with permanent damage threshold below 20 J/cm2. Standard fused silica substrates and a model system, containing welldefined micron grooves as seeding sites to trigger damage when irradiated by 438 nm laser pulses, were in situ probed by a neon-like zinc X-ray laser delivering up to 10 mJ at 21.2 nm. The probing beamline employed a double Lloyds mirror interferometer, used in conjunction with an imaging mirror to provide magnification of ~8. In conjunction with an array of in-situ optical diagnostics, one of the questions addressed was whether the damage (transient or permanent) on the rear surface of the beamsplitter occurs during or after the laser pulse, i.e. whether it is due to local electrical fields or to other processes. Another issue, examined by both the X-ray interferometric microscopy and the optical diagnostics, is whether a local rear-surface modification is associated with non-linear effects (self-focusing, filamentation) of the laser beam in the bulk.

Laser damage densities measurements on fused silica optics: round-robin test at 351-355 nm

A rasterscan test procedure [L. Lamaignère et al, Rev. Sci. Instrumen. 78, 103105 (2007)] has been implemented in order to determine low laser damage density of large aperture UV fused silica optics. This procedure was improved in terms of accuracy and repeatability. Tests have been carried on several facilities using several pulse durations and spatial distributions. We describe the equipment, test procedure and data analysis to perform this damage test with small beams (Gaussian beams, about 1mm @ 1/e, and top hat beams). Then, beam overlap and beam shape are the two key parameters which are taken into account in order to determine damage density. After data analysis and treatment, a repeatable metrology has been obtained. Moreover, the consideration of error bars on defects distributions permits to compare data between these installations. This allows us to reach reproducibility, a necessary condition in order to share results and to make reliable predictions of laser damage resistance.

Characterizations of UV-laser damage on fused silica surfaces

Significant improvement in polishing processes of fused silica optical components, has increased optics lifetime at the wavelength of 351 nm. Nonetheless, for large laser operation facilities like the Laser MegaJoule (LMJ), zero defect optics are not yet available. Therefore a damage mitigation technique has been developed to prevent the growth of initiated damage sites: this technique consists in a local melting and evaporation of silica by CO2 laser irradiation on the damage site. Because of the difficulty to produce efficient mitigated sites with large depth, the initial depth of damage to mitigate is a critical issue. An aim of our work was to determine the real extension of the damage site (including fractures) for different laser pulse durations between 3 ns and 16 ns and at different laser fluences. The fractures are nondetectable in conventional microscopy. The depth of the damage can thus be underestimated. Hence confocal microscopy, was used to observe these sub-surface fractures and to measure precisely the depth of damage. Results show that the damage is 2 to 4 times wider than deeper and this ratio is independent of the pulse duration and of the fluence. With this new information, the mitigation process can now be optimized.

Linear and nonlinear absorption and defects formation in KDP crystals used for large aperture doublers and triplers

We report the measurements of the linear and non linear absorption at 1064, 532 and 355 nm in samples of KDP crystals fabricated with the rapid growth process developed for NIF and LMJ high power lasers. Measurements were performed according to the ISO11551 standard by the pulse or gradient calorimetric method using a pulsed, diodepumped, Q-switched Nd:YAG laser. Time resolved spectroscopy method was used for the investigation of defects formation, responsible for the non linear absorption at 355 nm.

X-ray spectroscopy study of electronic structure of laser-irradiated Au nanoparticles in a silica film

The electronic structure of gold nanoparticles embedded in a silica film is studied, both before and after irradiation at 355nm by a laser. The Au 5d occupied valence states are observed by x-ray emission spectroscopy. They show that before irradiation the gold atoms are in metallic states within the nanoparticles. After irradiation with a fluence of 0.5J∕cm2, it is found that gold valence states are close to those of a metal-poor gold silicide; thanks to a comparison of the experimental Au 5d states with the calculated ones for gold silicides using the density-functional theory. The formation of such a compound is driven by the diffusion of the gold atoms into the silica film upon the laser irradiation. At higher fluence, 1J∕cm2, we find a higher percentage of metallic gold that could be attributed to annealing in the silica matrix.