Jérémie Capoulade

Multiscale analysis of the laser-induced damage threshold in optical coatings

We have investigated the influence of laser beam size on laser-induced damage threshold (LIDT) in the case of single- and multiple-shot irradiation. The study was performed on hafnia thin films deposited with various technologies (evaporation, sputtering, with or without ion assistance). LIDT measurements were carried out at 1064 nm and 12 ns with a spot size ranging from a few tens to a few hundreds of micrometers, in 1-on-1 and R-on-1 modes. These measurements were compared with simulations obtained with the statistical theory of laser-induced damage caused by initiating inclusions. We show how to obtain information on the initiating defect properties and the related physical damage mechanisms with a multiscale study. Under certain conditions, it is possible with this method to discriminate different defects, estimate their densities, and follow the evolution of the defects under multiple irradiation. The different metrology implications of our approach, particularly for obtaining a functional LIDT of optical components are discussed.

Effect of laser irradiation on silica substrate contaminated by aluminum particles.

A major issue in the use of high-power lasers, such as the Laser Megajoule (LMJ), is laser-induced damage of optical components. One potential damage initiator is particulate contamination, but its effect is hard to distinguish from that of other damage precursors. To do so, we introduced artificial contaminants typical of metallic pollution likely to be present on the optical components of the LMJ chains. More precisely, aluminum particles of two different sizes were placed on a silica sample. These dots were characterized by optical microscopy and profilometry. Then they were exposed to a laser beam with a pulse length of 6.5 ns at 1064 nm and fluences in the range from 1 to 40 J/cm(2). Each dot was characterized again with the same techniques and also by photothermal microscopy. To complete the experimental results, we performed numerical simulations with a one-dimensional Lagrangian hydrodynamics code. We show that the particle removal by laser irradiation produces a modification of the silica surface that does not evolve into catastrophic damage under subsequent irradiation. However, the effect does depend on the size of the dots. We demonstrate that a procedure exists that removes the dot and leaves the site capable of resisting high fluence.

Influence of laser beam size and wavelength in the determination of LIDT and associated laser damage precursor densities in KH2PO4

The laser resistance of large optical components remains an important limitation for the performances and the maintenance costs in LMJ or NIF projects. For practical reasons LIDT studies are commonly performed with small samples and table top lasers whose characteristics change from one to another. In these conditions, it is necessary to know exactly the influence of the different experimental parameters (wavelength, spot size, ...) on the final data. These considerations are particularly true in heterogeneous materials as KDP crystals. Indeed the use of different laser beam sizes (from μm to hundreds μm) to plot laser damage probability curves had clearly shown that at 355nm in KDP, it is possible to exhibit a limit of irradiated area which permit to distinguish two different LIDT associated with two laser damage precursors densities. This prior result has put in evidence the influence of irradiated beam size in the discrimination of different kinds of defects in KDP. We present in this paper a systematic study of beam size effect in KDP for three different wavelengths: 355nm, 532nm and 1064nm. This study performed in 1:1 and R:1 mode will reveal precursors for each wavelength and their respective evolution under repetitive shots for small and large beams. This multi-parameters study will help us to highlight mechanisms involved in laser-induced damage in KDP crystal.

Nondestructive optical characterization of KH2PO4 crystals heterogeneities and adapted excimer laser conditioning process

The high-power Laser MegaJoule (LMJ) for inertial confinement fusion experiments that is currently under construction at CEA-CESTA in France will require a high number of large aperture Pockels cells and frequency converters made of potassium dihydrogen phosphate (KDP) and DKDP (Deuterated KDP). These optical components will be operated several times a year at fluences close to their Laser Induced Damage Threshold (LIDT) which may reduce significantly their lifetime and increase substantially the maintenance costs of the LMJ. In a global effort to reduce these costs we have designed the SOCRATE facility as a complete system for materials characterization, LIDT measurement and optics conditioning by laser to increase their lifetime. In this paper we examine the relevance of adapting the laser conditioning process to the bulk KDP quality. First the existence of heterogeneities in large KDP crystals is stressed; next the LIDTs in the different parts of the crystals using focused or collimated beams are compared. Finally we focus on the efficiency of the excimer conditioning process in the different growth sectors of KDP samples and demonstrate that for the current conditioning process the efficiency depends only weakly on the original material heterogeneities.

Detection and characterization of lower LIDT regions in KDP material

At very high powers the energy for a single shot in the LIL/LMJ laser is today limited among others by the robustness of the KDP-based components used for frequency conversion. Subsequently it is vitally important to improve as much as possible the Laser Induced Damage Threshold (LIDT) of these components to make possible even more powerful shots. The exceptionally large aperture of such lasers (40*40 cm2) required the development of rapid growth methods. Investigations are under way to improve the damage resistance of such materials by implementing more efficient conditioning procedures. In this work we focus on composition heterogeneities induced by the rapid growth method in KDP crystals and we examine the impact on the laser-damage resistance. Two LIDT measurement facilities are used to investigate KDP triplers robustness. Spatially resolved LIDT measurements at 355 nm show that the LID resistance is significantly lower in some regions. The efficiency of the excimer conditioning in the different regions is also addressed.

Laser damage of silica and hafnia thin films made with different deposition technologies

A comparative study is made on the laser damage resistance of monolayers coatings made with different technologies. HfO2 and SiO2 thin films have been deposited on fused silica substrates with Dual Ion Beam Sputtering, Electron Beam Deposition (with and without Ion Assistance) and Reactive Low Voltage Ion Plating technologies. The laser damage thresholds of these coatings have been determined at 1064nm and 355nm using a nanosecond pulsed YAG laser, and a 1-on-1 test procedure.

Influence of the laser beam size on laser-induced damage in KH2PO4

For large aperture solid state lasers, the laser resistance of the optical component remains an important limitation for the performances and the maintenance costs. Since decades, laser induced damage has been intensively studied in order to understand and control the origin of the phenomenon. LID measurements are commonly performed with table top lasers whose characteristics change from one to another and, sometimes, the scaling laws do not permit to explain the experimental differences. For example, we have previously demonstrated that, in KH2PO4 (KDP) crystals, the laser beam size can influence strongly the determination of the damage probability. Here, we present a systematic study realized on KDP crystal to quantify the influence of the beam size on the LIDT (Laser Induced Damage Threshold) measurement at 355 nm. The use of an unique Gaussian beam ranged from micronic to sub-millimetric sizes permits to highlight different types of laser-damage precursor. LIDT measurements realized with beams of small (lower than 100 microns at 1/e2)or large (upper than 400 microns at 1/e2)dimensions give information about the behavior of material regarding precursor defects.

Multiscale analysis: a way to investigate laser damage precursors in materials for high power applications at nanosecond pulse duration

The mechanism of laser induced damage in optical materials under high power nanosecond laser irradiation is commonly attributed to the presence of precursor centers. Depending on material and laser source, the precursors could have different origins. Some of them are clearly extrinsic, such as impurities or structural defects linked to the fabrication conditions. In most cases the center size ranging from sub-micrometer to nanometer scale does not permit an easy detection by optical techniques before irradiation. Most often, only a post mortem observation of optics permits to proof the local origin of breakdown. Multi-scale analyzes by changing irradiation beam size have been performed to investigate the density, size and nature of laser damage precursors. Destructive methods such as raster scan, laser damage probability plot and morphology studies permit to deduce the precursor densities. Another experimental way to get information on nature of precursors is to use non destructive methods such as photoluminescence and absorption measurements. The destructive and non destructive multiscale studies are also motivated for practical reasons. Indeed LIDT studies of large optics as those used in LMJ or NIF projects are commonly performed on small samples and with table top lasers whose characteristics change from one to another. In these conditions, it is necessary to know exactly the influence of the different experimental parameters and overall the spot size effect on the final data. In this paper, we present recent developments in multiscale characterization and results obtained on optical coatings (surface case) and KDP crystal (bulk case).

Influence of the Laser Beam Size on the Laser-Induced Damage in Thin Films and Substrates

The influence of the laser beam size on the laser-induced damage threshold (LIDT) in thin films and substrates is investigated. LIDT measurements realized with beam of different dimensions give information on laser damage precursors.

Laser Damage Resistance of HfO 2 Thin Films Deposited by Electron Beam Deposition, Reactive Low Voltage Ion Plating and Dual Ion Beam Sputtering

We study the laser damage resistance at 1064 and 355nm of HfO2 thin films deposited by three different processes in various conditions. We compare their Laser Induced Damage Threshold obtained trough several measurement procedures.