Laser-induced damage growth of fused silica optics near growth threshold at 351 nm.

Laser-induced damage growth on the exit surface of fused silica optics triggered by nanosecond pulses at 351 nm is widely described with exponential dynamics. In this Letter, a particular experimental setup allowed us to study damage growth with a large beam and fluences near damage growth threshold for a high number of shots. This allowed us to observe and characterize a regime with a slow and linear growth dynamic not documented in the literature and yet fundamental for the operation of high-power laser installations.

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.

Impact of the multilayer dielectric design on the laser-induced damage threshold of pulse compression gratings for petawatt-class lasers.

The peak-power of petawatt-class lasers is limited by laser-induced damage to final optical components, especially on the pulse compression gratings. Multilayer dielectric (MLD) gratings are widely used in compressor systems because they exhibit a high diffraction efficiency and high damage threshold. It is now well established that the etching profile plays a key role in the electric field distribution, which influences the laser damage resistance of MLD gratings. However, less attention has been devoted to the influence of the multilayer design on the laser damage resistance of MLD gratings. In this Letter, we numerically and experimentally evidence the impact of the dielectric stack design on the electric field intensity (EFI) and the laser-induced damage threshold (LIDT). Three different MLD gratings are designed and manufactured to perform laser damage tests. On the basis of the expected EFIs and diffraction efficiencies, the measured LIDTs show how the multilayer design influences the laser resistance of the MLD gratings. This result highlights the impact of the multilayer dielectric design on the electric field distribution and shows how to further improve the laser-induced damage threshold of pulse compression gratings.

Influence of the multilayer dielectric mirror design on the laser damage growth in the sub-picosecond regime.

The peak power of high-power laser facilities is limited by the laser-induced damage to the final optical components. Also, when a damage site is generated, the damage growth phenomenon limits the lifetime of the component. Many studies have been performed to improve the laser-induced damage threshold of these components. The question now arises as to whether improvement of the initiation threshold leads to a reduction of the damage growth phenomenon. To address this question, we performed damage growth experiments on three different multilayer dielectric mirror designs exhibiting different damage thresholds. We used classical quarter-wave designs and optimized designs. The experiments were carried out with a spatial top-hat beam, spectrally centered at 1053 nm with a pulse duration of 0.8 ps in s- and p-polarization. The results showed the impact of design on the improvement of the damage growth thresholds and a reduction of the damage growth rates. A numerical model was used to simulate damage growth sequences. The results reveal similar trends to those observed experimentally. On the basis of these three cases, we have shown that improvement of the initiation threshold through a modification of the mirror design can lead to the reduction of the damage growth phenomenon.

Investigation of the influence of a spatial beam profile on laser damage growth dynamics in multilayer dielectric mirrors in the near infrared sub-picosecond regime.

Laser-induced damage growth has often been studied with Gaussian beams in the sub-picosecond regime. However, beams generated by high-power laser facilities do not feature Gaussian profiles, a property that raises questions concerning the reliability of off-line laser-induced damage measurements. Here, we compare laser-induced damage growth dynamics as a function of beam profiles. Experiments on multilayer dielectric mirrors at 1053 nm have been carried out with squared top-hat and Gaussian beams. The results demonstrate that the laser-induced damage growth threshold does not depend on the incident beam profile. A higher damage growth rate, however, has been measured with the top-hat beam. In addition, three different regimes in the growth dynamics were identified above a given fluence. A numerical model has been developed to simulate a complete damage growth sequence for different beam profiles. The numerical results are in good agreement with the observations, three growth regimes were also revealed. These results demonstrate that a linear description of growth cannot be used for the whole growth domain.

Polarization dependence of laser damage growth features on multilayer dielectric mirrors for petawatt-class lasers.

PETAL (Petawatt Aquitaine Laser) is an ultrahigh-power laser dedicated to academic research that delivers sub-picosecond pulses. One of the major issues of these facilities is the laser damage on optical components located at the final stage. Transport mirrors of the PETAL facility are illuminated under different polarization directions. This configuration motivates a thorough investigation of the dependency of the laser damage growth features (thresholds, dynamics, and damage site morphologies) on the incident polarization. Damage growth experiments were carried out in s- and p-polarization at 0.8 ps and 1053 nm on multilayer dielectric mirrors with a squared top-hat beam. Damage growth coefficients are determined by measuring the evolution of the damaged area for both polarizations. In this Letter, we report higher damage growth threshold in p-polarization together with higher damage initiation threshold in s-polarization. We also report faster damage growth dynamics in p-polarization. The damage site morphologies and their evolution under successive pulses are found to strongly depend on polarization. A numerical model in 3D was developed to assess experimental observations. This model shows the relative differences in damage growth threshold even if it is not able to reproduce the damage growth rate. Numerical results demonstrate that damage growth is mainly driven by the electric field distribution which depends on the polarization.

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.

Temporal dependency in the picosecond regime of laser damage growth.

Based on squared top-hat beam irradiations, we investigate how a change of the pulse duration in the picosecond regime affects the phenomenon of laser damage growth on dielectric mirrors. We first confirm two major previously reported experimental results with a Gaussian beam that are the existence of a growth threshold fluence smaller than the laser-induced damage threshold (LIDT) and the linear evolution, characterized by a growth coefficient, of the damage area with the number of irradiations when growth occurs. We then express the growth coefficient with the fluence and the growth threshold in particular. Changing the pulse duration ultimately allows us to refine this expression a step further which leads us to establish an empirical growth law for the damage area. The temporal dependency displayed within this law appears to be very close to the one found for the LIDT which evidences the deterministic nature of laser damage growth in short pulse regimes.

Large optics metrology for high-power lasers.

The Laser MégaJoule (LMJ) is a high-power laser dedicated to laser-plasma experiments. At the beginning of the project in the mid-1990s, an optical metrology laboratory was created at CEA to help accomplish all the steps in the construction of this laser. This paper proposes an overview of the capabilities of this metrology laboratory in four main fields: surface imperfections, photometry, laser damage measurement, and wavefront measurement. The specificities for high-power laser optics in each domain are highlighted as well as the specific features that make our instruments unique.

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.