Laser Megajoule performance status.

The Laser Megajoule (LMJ) is among the most energetic inertial confinement fusion laser facilities in the world, together with the National Ignition Facility (NIF) in the USA. The construction of the facility began back in 2003, and the first photons were emitted by the laser bundle #28 in 2014. Today, 11 laser bundles consisting of 88 large aperture 0.35×0.35m 2 laser beams are in operation, delivering daily up to 330 kJ of energy at the wavelength of 351 nm on a target placed in the center of a 10 m diameter vacuum chamber. In this paper, we describe the laser system and its operational performances. We also detail the first laser campaigns carried out to prepare an increase of energy and power on the target. These campaigns, along with the completion of additional bundles mounting, will bring LMJ performance to 1.3 MJ thanks to 22 bundles in operation.

Linear-to-circular polarization conversion with full-silica meta-optics to reduce nonlinear effects in high-energy lasers.

High-energy lasers have benefited from intense efforts to bring light-matter interactions to new standards and to achieve laser fusion ignition. One of the main issues to further increasing laser energy is the resistance of optical materials to high laser fluences, in particular at the final stage of the laser beamline where nonlinear Kerr effects can occur in optical materials and provoke laser filamentation. One promising way to mitigate this process is to reduce the nonlinear susceptibility of the material by switching the polarization from a linear to a circular state. Here, we report a significant reduction in the laser filamentation effect on glass by using a full-silica metamaterial waveplateable to switch the linear-to-circular polarization of high fluence laser beams. This result is achieved through the use of a large size full-silica meta-optics exhibiting nominal polarization conversion associated with an excellent transmission efficiency and wavefront quality, as well as a high laser damage resistance.

Estimation of laser-induced damage depth from surface image features.

In laser damage experiments, damage initiation and growth are typically monitored by imaging the surface of the tested fused silica sample, ignoring their bulk morphology. The depth of a damage site in fused silica optics is considered to be proportional to its equivalent diameter. However, some damage sites experience phases with no diameter changes but growth in the bulk independently from their surface. A proportionality relationship with the damage diameter does not accurately describe the growth of such sites. In the following, an accurate estimator for damage depth is proposed, which is based on the hypothesis that the light intensity scattered by a damage site is proportional to its volume. Such an estimator, using the pixel intensity, describes the change of damage depth through successive laser irradiations, including phases in which depth and diameter variations are uncorrelated.

Full-scale optic designed for onsite study of damage growth at the Laser MegaJoule facility.

Large fusion scale laser facilities aim at delivering megajoules laser energy in the UV spectrum and nanosecond regime. Due to the extreme laser energies, the laser damage of final optics of such beamlines is an important issue that must be addressed. Once a damage site initiates, it grows at each laser shot which decreases the quality of the optical component and spoil laser performances. Operation at full energy and power of such laser facilities requires a perfect control of damage kinetics and laser parameters. Monitoring damage kinetics involves onsite observation, understanding of damage growth process and prediction of growth features. Facilities are equipped with cameras dedicated to the monitoring of damage site growth. Here we propose to design and manufacture a dedicated full size optical component to study damage growth at increased energy, on the beamline, i.e. in the real environment of the optics on a large laser facility. Used for the first time in 2021, the growth statistics acquired by this approach at the Laser MegaJoule (LMJ) facility provides a new calibration point at a fluence less than 5 J cm-2 and a flat-in-time pulse of 3 ns.

Optical model-based calibration of gray levels for laser damage size assessment.

Fused silica is prone to damage under ultraviolet laser irradiation. Because they are key components to achieve fusion on high energy laser facilities, final fused silica optics are analyzed after each laser shot. The quantification of damage sites is limited by the image resolution. Measurements of scattered light by damage sites allow for sub-pixel detection and growth monitoring after a calibration step based on time-consuming measurements at laser facilities. It is proven herein that modeling laser damage size monitoring based on light scattering is efficient to link gray levels to damage diameters, thereby avoiding any experimental calibration based on reference optics at the facility.

Estimating and monitoring laser-induced damage size on glass windows with a deep-learning-based pipeline.

Laser-induced damage is a major issue in high power laser facilities such as the Laser MégaJoule (LMJ) and National Ignition Facility (NIF) since they lower the efficiency of optical components and may even require their replacement. This problem occurs mainly in the final stages of the laser beamlines and in particular in the glass windows through which laser beams enter the central vacuum chamber. Monitoring such damage sites in high energy laser facilities is, therefore, of major importance. However, the automatic monitoring of damage sites is challenging due to the small size of damage sites and to the low-resolution images provided by the onsite camera used to monitor their occurrence. A systematic approach based on a deep learning computer vision pipeline is introduced to estimate the dimensions of damage sites of the glass windows of the LMJ facility. The ability of the pipeline to specialize in the estimation of damage sites of a size less than the repair threshold is demonstrated by showing its higher efficiency than classical machine learning approaches in the specific case of damage site images. In addition, its performances on three datasets are evaluated to show both robustness and accuracy.

Impact of compression grating phase modulations on beam over-intensities and downstream optics on PETAL facility.

Chirped pulse amplification has been widely implemented in high power laser chains. It consists of a set of diffraction gratings used to stretch and compress short laser pulses. In the case of high power laser chains, the compression stage is followed by the transport mirror in order to carry the laser beam to its final target. In such laser chains, laser beams propagate over a complex set of optical components and understanding the propagation of phase noise turns out to be of crucial importance. Phase modulation can induce laser damage on the final optical components. Here, we study the impact of phase modulation induced by the different diffraction gratings of the Petawatt Aquitaine Laser (PETAL) compressor on the downstream over-intensities, in particular on the transport mirror. This work allows us to quantify the impact of phase modulation for every single grating element in the compression stage, and to estimate the quantity of laser induced damage sites on transport optics for a specific laser shot.

Sub-pixel detection of laser-induced damage and its growth on fused silica optics using registration residuals.

Fused silica optics are key components to manipulate high energy Inertial Confinement Fusion (ICF) laser beams but their optical properties can be degraded by laser-induced damage. The detection of laser damage sites is of major importance. The challenge is to monitor damage initiation and growth at sub-pixel scale with highly sensitive measurements. The damage diameter is a widely used indicator to quantify damage growth but its accuracy is strongly dependent on the available image resolution. More recently, it was shown that registration residual maps (i.e., gray level differences between two registered images) could also be used to monitor laser-induced damage. In this paper, the performance of both indicators are compared to detect laser damage initiation and growth at high and low image resolutions thanks to a highly instrumented laser setup. The results prove that registration residual maps are more efficient to detect sub-pixel laser damage growth than diameter measurements at a given image resolution. The registration residual maps are therefore a powerful indicator for monitoring laser-induced damage initiation and growth at sub-pixel scale either for laser damage metrology setups, for high energy laser facilities, or other situations where damage is suspected to occur. The accuracy of (laser-induced) damage laws may also be improved thanks to this tool.

Nanosecond laser damage initiation at 0.35 µm in fused silica.

In nanosecond regime, the laser-induced damage density at the exit surface of fused silica optics at the wavelength of 0.35 µm shows a characteristic behavior: in a specific fluence range, the surface damage density begins to grow exponentially as a function of fluence and then tends to saturate at high fluences. Up to now, no satisfactory explanation of these peculiarities could be provided. We herein detail a statistical model based on laser-matter interaction, where two types of absorbing precursors are involved in the energy deposit: subsurface micro-cracks and surface impurities. We show that the reported model predicts this characteristic damage density for a large range of fluences and different polishing processes.

Robust optimization of the laser induced damage threshold of dielectric mirrors for high power lasers.

We report on a numerical optimization of the laser induced damage threshold of multi-dielectric high reflection mirrors in the sub-picosecond regime. We highlight the interplay between the electric field distribution, refractive index and intrinsic laser induced damage threshold of the materials on the overall laser induced damage threshold (LIDT) of the multilayer. We describe an optimization method of the multilayer that minimizes the field enhancement in high refractive index materials while preserving a near perfect reflectivity. This method yields a significant improvement of the damage resistance since a maximum increase of 40% can be achieved on the overall LIDT of the multilayer.

Author Correction: Understanding the effect of wet etching on damage resistance of surface scratches.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Impact of mechanical stress induced in silica vacuum windows on laser-induced damage.

At the interface between vacuum and air, optical windows must keep their optical properties, despite being subjected to mechanical stress. In this Letter, we investigate the impact of such stress on the laser-induced damage of fused silica windows at the wavelength of 351 nm in the nanosecond regime. Different stress values, from 1 to 30 MPa, both tensile and compressive, were applied. No effect of the stress on the laser-induced damage was evidenced.

Understanding the effect of wet etching on damage resistance of surface scratches.

Fused silica optics often exhibit surface scratches after polishing that radically reduce their damage resistance at the wavelength of 351 nm in the nanosecond regime. Consequently, chemical treatments after polishing are often used to increase the damage threshold and ensure a safe operation of these optics in large fusion-scale laser facilities. Here, we investigate the reasons for such an improvement. We study the effect of an HF-based wet etching on scratch morphology and propose a simple analytic model to reflect scratch widening during etching. We also use a finite element model to evaluate the effect of the morphological modification induced by etching on the electric field distribution in the vicinity of the scratch. We evidence that this improvement of the scratch damage resistance is due to a reduction of the electric field enhancement. This conclusion is supported by secondary electron microscopy (SEM) imaging of damage sites initiated on scratches after chemical treatment.

Sub-picosecond laser damage growth on high reflective coatings for high power applications.

Growth of laser damage on High Reflection (HR) thin film coatings is investigated at the wavelength of 1.030µm in the sub-picosecond regime. An experimental laser damage setup in a pump / probe configuration is used to study the growth behavior of engineered damage sites as well as laser damage sites. Results demonstrate that engineered sites and laser damage sites grow identically which indicates that the growth phenomenon is intrinsic to materials and stack design. In order to analyze the experimental results, we have developed a numerical model to simulate growth. Using FEM simulations, we demonstrate that growth is governed by the evolution of the electric field distribution in the mirror stack under the successive laser shots, which is supported by time-resolved observations of damage growth events. Eventually the results are compared to laser damage observations made on of full scale PETAL mirrors, which fully support the approach.

Effects of deep wet etching in HF/HNO3 and KOH solutions on the laser damage resistance and surface quality of fused silica optics at 351 nm.

We investigate the interest of deep wet etching with HF/HNO3 or KOH solutions as a final step after polishing to improve fused silica optics laser damage resistance at the wavelength of 351 nm. This comparison is carried out on scratches engineered on high damage threshold polished fused silica optics. We evidence that both KOH and HF/HNO3 solutions are efficient to passivate scratches and thus improve their damage threshold up to the level of the polished surface. The effect of these wet etchings on surface roughness and aspect is also studied. We show that KOH solution exhibit better overall surface quality that HF/HNO3 solution in the tested conditions. Given the safety difficulties associated with the processing with HF, KOH solution appears as a pertinent alternative to HF deep wet etching.

Laser damage growth with picosecond pulses.

Laser-induced damage growth has been investigated in the subpicosecond regime at 1030 nm. We have herein studied the growth of damage sites initiated on a high-reflective dielectric coating under subsequent laser irradiations at a constant fluence. We show through an experimental approach that growth can be triggered for fluences as low as 50% of the intrinsic damage threshold of the mirror. Moreover, once growth starts, damage areas increase linearly with the number of laser shots. The behavior of defect-induced damage sites has been observed more extensively, and it appears that their growth probability depends on their initiation fluence.

Assessment of mono-shot measurement as a fast and accurate determination of the laser-induced damage threshold in the sub-picosecond regime.

Standard test protocols need several laser shots to assess the laser-induced damage threshold of optics and, consequently, large areas are necessary. Taking into account the dominating intrinsic mechanisms of laser damage in the sub-picosecond regime, a simple, fast, and accurate method, based on correlating the fluence distribution with the damage morphology after only one shot in optics is therein presented. Several materials and components have been tested using this method and compared to the results obtained with the classical 1/1 method. Both lead to the same threshold value with an accuracy in the same order of magnitude. Therefore, this mono-shot testing could be a straightforward protocol to evaluate damage threshold in short pulse regime.

Laser damage density measurement of optical components in the sub-picosecond regime.

A rasterscan procedure adapted to the sub-picosecond regime is set to determine laser-induced damage densities as function of fluences. Density measurement is carried out on dielectric high-reflective coatings operating at 1053 nm. Whereas laser-induced damage is usually considered deterministic in this regime, damage events occur on these structures for fluences significantly lower than their intrinsic damage threshold. Scanning electron microscope observations of these "under-threshold" damage sites evidence ejections of defects, embedded in the dielectric stack. This method brings a new viewpoint for the qualification of optical components and their optimization for a high resistance in the sub-picosecond regime.

Using STED and ELSM confocal microscopy for a better knowledge of fused silica polished glass interface.

Characteristics and nature of close surface defects existing in fused silica polished optical surfaces were explored. Samples were deliberately scratched using a modified polishing process in presence of different fluorescent dyes. Various techniques including Epi-fluorescence Laser Scanning Mode (ELSM) or STimulated Emission Depletion (STED) confocal microscopy were used to measure and quantify scratches that are sometimes embedded under the polished layer. We show using a non-destructive technique that depth of the modified region extends far below the surface. Moreover cracks of 120 nm width can be present ten micrometers below the surface.

Subsurface damage distribution characterization of ground surfaces using Abbott-Firestone curves.

Measurement of subsurface damage (SSD) induced by grinding process is of major interest in the development of high laser damage fused silica optical components manufacturing processes. Most SSD measurements methods give only access to the peak to peak value. We herein report on the benefit of using Abbott-Firestone curves to get an insight of the SSD distribution inside the optical material. We evidence on various diamond wheel ground fused silica substrates that such an approach is complementary to a classical SSD peak to peak measurement and bring useful information to optimize a grinding process.