Roger Courchinoux

An accurate, repeatable, and well characterized measurement of laser damage density of optical materials.

Known for more than 40 years, 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 10 Hz. We characterize the distribution of damaging defects by the fluence at which they cause damage. Because tests are realized with small Gaussian beams (about 1 mm at 1e), 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.

Parametric study of laser-induced surface damage density measurements: Toward reproducibility

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.

Influence of longitudinal mode beating on laser-induced damage in fused silica

In our study, the laser-induced damage densities on a fused silica surface produced by multiple longitudinal mode (MLM) pulses are found to be higher than those produced by single longitudinal mode pulses at 1064 nm. This behavior is explained by the enhancement of the three-photon absorption due to the intensity spikes related to longitudinal mode beating. At 355 nm, the absorption is linear and an opposite behavior occurs. It can be explained with the help of a process involving thermomechanics coupled with the fine time structure of MLM pulses, leading to the possible annealing of part of the absorbent defects.

Study of the evolution of mechanical defects on silica samples under laser irradiation at 355 nm

During the life of a high-power laser chain, optical components may be damaged due to local high fluence levels in the inhomogeneous beam. The origin of the laser damage can be impurities, surface defects or flaws and cracks resulting from polishing, or it may be produced by self-focusing in the component. The aim of this study is to better understand the correlation between a surface crack on a silica optical component and laser damage. To accomplish this, calibrated indentations were made on silica samples. Observations of the sites were made with an optical microscope, and three different morphologies were recognized. Then the zones containing the indentations were irradiated (single shot mode) with a Nd Yag laser at 355 nm for various fluences. Subsequent observations of the sites were made with an optical microscope, with the aim of correlating site morphology and laser-induced damage. Some sites were believed to have undergone laser conditioning. They were further irradiated (raster scan mode) at high fluence, and some evidence for a laser conditioning effect was obtained.

LIL laser performance status

The Laser Integration Line (LIL) was first designed as a prototype to validate the concepts and the laser architecture of the Laser MegaJoule (LMJ). The LIL facility is a 4-beam laser representing a quad structure of the LMJ. A set of test campaigns were conducted to safely ramp up laser performance. The main goal was to measure quad-specific features such as beam synchronization and focal spot (size, smoothing contrast ratio or irradiation nonuniformity) versus the LMJ requirements. Following the laser commissioning, the LIL has become a major instrument dedicated to the achievement of plasma physics experiments for the French Simulation Program and was also opened to the academic scientific community. One of the attributes of the LIL facility is to be very flexible to accommodate the requests of plasma physicists during campaigns. The LIL is constantly evolving to best meet the needs of target physicists. Changes made or planned are either to improve the quality of laser beams, or to increase the LIL Energy-Power operating space. To optimize preparation and design of shot campaigns, the LIL performance status has been elaborated. It gives information about the characteristics of the laser in terms of near field and far field, defines the steps to maintain performance, explains how the facility responds to the request, details settings (smoothing, shaping of the focal spot, energy, temporal pulse shaping, beam pointing) and gives the limits in energy and power. In this paper, an overview of the LIL performance is presented.

Laser-induced damage growth with small and large beams: comparison between laboratory experiments and large-scale laser data

Experiments have been performed to measure the rate of laser-induced damage growth at the rear surface of fused silica windows at 1064, 1053 and 351 nm. One test bench delivered 9 ns monomode gaussian pulses at 10 Hz and 1064 nm. The size of the focused beam on the sample was a few mm2. Another test bench delivered 2.5 ns single or multimode pulses at 1053 and 351 nm. The focused spot on the sample was a few cm2. We compare and discuss our laboratory experimental results, the larger scale ALISE laser data and other results obtained at LULI.

Laser damage resistance qualification of large optics for high power laser

The rasterscan procedure, developed to test large components, is an efficient method that allows measuring extremely low surface damage density (until 0.01 site/cm2 for large optics). This procedure was improved in terms of accuracy. The equipment, test procedure and data analysis to perform this damage test of large aperture optics are described. 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 qualification. Because tests are realized with small Gaussian beams (about 1mm @ 1/e), beam overlap and beam shape are key parameters which have to be taken into account in order to determine damage density. After complete data analysis and treatment, a repeatable metrology has been reached. The measurement is destructive for the sample. However the consideration of error bars on defects distributions allows us to compare data obtained on a same batch of optical components. This will permit to reach reproducible metrology. Then this procedure provides a straightforward means of comparing the experimental results obtained from several facilities using different lasers. Recently, an additional step has been added to the procedure, a growth step that permits considering only growing damage sites. To the end the lifetime of large optics on high power laser can be predicted.

Automatic damage test benches: from samples to large-aperture optical components

The functional lifetime of large aperture components used in high power lasers, like LIL and LMJ facilities, is mainly determined by laser damage measurements. Automatic damage test benches allow to obtain more data in less time than traditional tests. We present, first experimental procedures and statistical analysis made on small samples with mm-size beams, to determine damage densities and damage growth laws. The presented methods are the usual 1on1, Non1, Ron1 and Son1 tests and more specially the raster scan procedure. The tests and analysis are compared to other results obtained with larger beams (few cm2) on large optics. We show that the exact knowledge of each shot parameters (energy, surface and pulse duration) permits to determine the damage growth rate (and then to predict the lifetime of each optics), to precisely study self-focusing phenomenon and more to finely observe pre-damage-levels. In this way, the main parameters like fluence or intensity are associated to the observed phenomenon. Moreover laser beam diagnostics, many diagnostics used for the detection and the observation of damage occurrence are equally very important. It is also necessary to develop test procedures entirely computed which permit to scan all the surface of a component and to acquire in real time the beam parameters and the results of laser-matter interaction. Experimental results are reported to illustrate what could be achieved on an instrumented and automated facility.

Presentation and comparison of damage test procedures for fused silica and KDP crystals

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 and is now used for the determination of bulk damage density for KDP crystals. The large area (volume) scanned during tests permits to measure very low damage density. On small samples, small area are tested using the normalized 1/1 test procedure consisting on the irradiation of few sites at several fluences. The classical damage probability plot is converted in terms of damage density. The two testing procedures are complementary: the 1/1 mode is practical to test a wide fluence range while the rasterscan mode allows exploring low damage densities with higher accuracy. Tests have been carried out 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. Other tests are realized with larger beams (centimeter sized) and with a single shot. Due to a large beam contrast, a large fluence range is then covered. Then after data treatment, we find a good correlation between tests realised with small and large beams. This allows us to make tests with different laser characteristics (spectral modulations, pulse duration, laser polarisation) and then to study their influences on laser damage.

Effects of scratch speed on laser-induced damage

A major issue in high power lasers for fusion is laser-induced damage on optical components. Since damage is often initiated by a surface crack resulting from polishing, it is important to understand the physics involved in this phenomenon. In this study, calibrated surface scratches have been realized on two silica samples using an indenter-scratcher apparatus. A variety of scratches have been tested by applying different speeds and forces on the scratcher needle. Optical microscope observations show that the scratches made at highest speed create irregular dashed lines. In addition, we have observed, at intermediate speed, an evolution in time of the scratches due to local stresses in silica. One of the samples was irradiated by a Nd:YAG laser beam at 355 nm with the scratches on the exit surface. Microscope observations were made before and after irradiation. Strong dependence on the scratch speed was observed on the local laser damage. Again, temporal evolution of the damage has been observed.