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.

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.

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.

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.

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.