Marc Loiseau

Parametric study of the growth of damage sites on the rear surface of fused silica windows

The growth of damage sites on the rear surface of fused silica plates was studied as a function of fluence and angle of incidence. At 1053 nm, a 70 J beam, 3 ns in pulselength, was directed to a 5 cm2 zone on a bare fused silica window. Initiation and growth was observed. The growth of previously initiated sites was also studied. Growth is exponential in nature. The experiments allow for the determination of the growth coefficient as a function of fluence. At 355 nm, damage sites were irradiated at various angles of incidence, with a tripled Nd:Yag laser, spatially Gaussian, 2.5 ns in pulselength. By fitting growth with an exponential law, it was determined that the relevant fluence for growth was that taken inside the material.

Self-focusing and rear surface damage in a fused silica window at 1064 nm and 355 nm

Rear surface damage is known to occur preferentially to front surface damage on silica lenses and windows transmitting 1ω or 3ω light of a Nd:Yag or Nd:glass laser. Lab-scale Experiments were performed to assess the contribution of self-focusing to the measurement of damage statistics. The occurrence of filamentation in the bulk and the statistics of rear surface damage were studied at 1064 nm and 355 nm, for a pulse length around 3 ns. The measurements were performed on synthetic fused silica samples, 4 to 5 cm thick. The laser beam had dimensions around 0.6 mm at the sample. The results are interpreted with the help of the calculations made by J. H. Marburger et al. They are also compared with other experimental results.

Filamentation and surface damage in fused silica with single-mode and multi-mode pulses

We studied filamentation, front surface damage and rear surface damage at 1064 nm and 351 nm with nanosecond pulses on a fused silica optical window. With temporally single-mode pulses, self-focusing occurs together with front surface damage, which is attributed to a Stimulated Brillouin Back Scattering (SBS) wave. The use of temporally multi-mode pulses suppresses the occurrence of front surface damage, and increases self-focusing. With single-mode pulses, the observation of filaments seems coherent with standard Kerr self-focusing effect, and can be understood according to the numerical treatment by Marburger et al, using non linear index values measured in other experiments. However, when multi-mode pulses were used, filaments occurred for much smaller peak intensities, by about a factor of 2. In this case, the non linear index causing self-focusing appears to be twice bigger. This second case is relevant to the situation of vacuum windows in high power laser installations, where the spectrum of light is widened to get rid of SBS. We discuss the physical effects that could be causing the enhancement of self-focusing.

Toward a better understanding of multi-wavelength effects on KDP crystals

Laser damage in KDP crystals has been studied since several years and more accurately with emergence of projects like LMJ (Laser MégaJoule, in France) or NIF (National Ignition Facility, in US). Laser damage tests are essentially performed at 351-nm wavelength (3ω), with regards to their optical behaviours on forementioned facilities. But only few data are available at 1064 nm (1ω) and at 532 nm (2ω), and even with wavelength-mixing more representative of operational conditions of KDP crystals. So in a first approach, we tried to carry out an identity chart of the crystal by performing mono-wavelength tests at 1ω, 2ω and 3ω. Then, a campaign of combination of multi-wavelength (typically 3ω and 1ω) tests has been started with several temporal delays between 3ω and 1ω pulses. These first results lead us to improve pre-existing modelling codes developed by CEA, which have proved their robustness to 3ω -experiment results. Foremost interests consist in implementing wavelength dependency and energy deposition mechanism as a consequence of our first observations on KDP.

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.

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.

Excimer laser conditionning of KDP: influence of the laser parameters and crystal orientation on the laser damage threshold

We perform thne conditioning of various KDP crystals with a XeF excimer laser working at 351 nm. We determine the maximum available excimer laser fluence for conditioning without damage initiation within the crystal. We demonstrate enhancement of the damage resistance with the increase of the cumulative excimer laser fluence. Using the conditioning parameters we show that the damage resistance is also dependent on the crystalline orientation of the KDP samples.

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.

Influence of the cut angle on the laser damage threshold of KDP

KDP crystals are currently used for frequency conversion and Pockels cells in large aperture laser systems such as the LMJ and NIF. These different functions are obtained by cutting the KDP crystals with different orientations. We show by measuring the LIDT with three different facilities, that the cut angle plays a key role in the damage mechanism. Consistently with the three measurement set-ups, we demonstrate that the doublers have a weaker LIDT value than the triplers. The z-cut KDP samples have a LIDT higher than both the doublers and the triplers. These results are analyzed in terms of probed volumes and pulse duration.

Self-focusing and surface damage in fused-silica windows of variable thickness with UV nanosecond pulses

Variable experimental conditions were used to measure the occurrence of front surface, rear surface and filamentation damage in synthetic fused silica windows. Experiments were performed at 355 nm with a table-top beam of mm-size, and at 351 nm with ALISE laser, a 100 J installation. The 351 nm beam was about 3 cm wide at the entrance surface; it was single-mode temporally, with or without a frequency modulation which has the function of widening the spectrum to decrease Stimulated Brillouin Scattering. The 355 nm was single-mode temporally. Thin windows showed very scarce front damage and no filament damage at intensities which cause a high density of rear surface damage. Without any spectral widening, the thicker windows (4.3 cm) showed appreciable amount of front surface damage; filaments were observed and but no filaments. When a spectral modulation was added, front surface damage vanished, filaments and rear surface damage were observed.