High-power laser systems use hundreds to thousands of large optical components to amplify, filter, transport, sometimes compress and/or frequency-convert, and focus laser beams. Most of these optics are dioptric optical components: mirrors, lenses, windows, laser slabs, crystals, ... Apart from the iconic example of the compression gratings used in the chirped pulse amplification, the use of diffractive optics and in particular transmission gratings is relatively limited. Here we detail the development we carried out to use transmission grating for beam steering and focusing laser beams of the Megajoule laser (LMJ). We describe our early attempts, the first prototype, and the performances finally reached to equip the 176 laser beams of the LMJ. We follow this path by extending the implementation of transmission gratings for beam steering and focusing to the manipulation of the polarization state of highly energetic laser beams. We detail the design and performance of nanostructured silica for achieving linear-to-circular polarization conversion. This full-silica meta-optics acts as a quarter waveplate operating in the UV frequency range at the wavelength of 351 nm. In addition to its effect on polarization, we show how this meta-optics can be used to push back the Kerr filamentation threshold occurring in components of these high-power lasers.
To enhance the PETawatt Aquitaine Laser (PETAL) operation, efforts are directed towards increasing the Laser-Induced Damage Threshold (LIDT) of transport mirrors. Three approaches are being considered : i) changing the design of thin film stacks, ii) the materials, and iii) the deposition process.
Monolayers of pure SiO2, HfO2, Sc2O3 and mixtures of HfO2/SiO2 and Sc2O3/SiO2 were elaborated by magnetron sputtering using oxide targets. Laser damage tests, combined with optical and physicochemical characterizations, revealed that the Sc2O3/SiO2 mixture exhibits the highest LIDT. The introduction of a small amount of oxygen into the plasma reduced the refractive index and improved the LIDT.
A Bragg mirror, designed for PETAL's specifications (R > 99% at 1053 nm for s polarization at 45° incidence) is being manufactured using HfO2 (high refractive index) and Sc2O3/SiO2 (low refractive index). The films thicknesses are finely controlled with the quartz crystal microbalance technique.
KEYWORDS: Optical coatings, Laser microstructuring, High power lasers, Chemical composition, Silica, Resistance, Laser induced damage, Oxygen, Matrices, Laser development
To build quarter-wave plate components for a high-power laser application, the Laboratory for Laser Energetics has developed a 21-layer silica coating fabricated by GLancing Angle Deposition. This stack alternates columnar birefringent layers with isotropic layers. We present a study on the SiO2 matrix state, the sub-stoichiometry and presence of oxygen vacancies that affect robustness and a reduced laser damage resistance. The composition throughout the film thickness is investigated thanks to GD-OES and Tof-SIMS combined with photoelectron spectroscopies for the composition. Anisotropic and isotropic layers exhibit differences in composition, between them and throughout the depth. Photoluminescence measurements show a peak that could represent oxygen vacancies that may reduce the damage threshold. Vibrational characterization further supports our findings. This comprehensive overview is discussed in relation to deposition process and resistance to laser-induced damage and will enable us to improve our current coatings.
Multilayer dielectric (MLD) gratings provide high diffraction efficiency and a high damage threshold. They represent the main solution to compressing a high-power laser beam. However, the laser resistance of MLD gratings limits the power of such facilities. The community devoted a lot of resources to increasing the damage threshold of those components. Today, it is well known that the etching profile plays a key role in the electric field distribution and consequently the laser resistance. In this paper, we focused our optimization on the multilayer dielectric stack to increase the laser-induced damage threshold (LIDT). We numerically and experimentally demonstrated the impact of the MLD stack on the electric field distribution and the LIDT. We manufactured two sets of three samples with identical etching profiles. The calculated electric field intensities were in good agreement with the measured LIDTs. These results demonstrated how to further optimize grating designs through the dielectric stack.
Multilayer dielectric gratings (MLDG) are key optical components of Petawatt-class laser that are used to compress short pulses of high intensities. Laser-induced damage can occur on the top area of the components, typically arising in the pillars periodically etched. This phenomenon limits the power yielded by high power laser facilities such as PETAL (PETwatt Aquitaine Laser) laser facility. PETAL is expected to delivery pulses with a wavelength around 1053 nm, an energy around 3 kJ and a pulse duration between 0.5 and 10 ps. Coupled with LMJ (Laser MegaJoule), PETAL aims to study materials in extreme conditions to reproduce the environment in the heart of stars or planets, fusion by inertial confinement, particularly rapid ignition and shock ignition, and nuclear physics for medical proton therapy. In this study, we present a process to improve the laser-induced damage threshold of PETAL pulse-compression gratings in sub-picosecond regime by reducing the electric field intensity in the pillars. PETAL gratings have specific parameters of operation: Transverse Electric polarization, under vacuum, a period equal to 1780 lines per mm and diffraction efficiency higher than 95% for the -1st order. Theoretical designs are calculated with a code developed at the Fresnel Institute. The code solves Fresnel equations by using the differential method, Fast Fourier Factorization (FFF) and S matrix propagation algorithm. As a result, we obtain the distribution of the electric field and diffraction efficiency of any given diffraction order. First, starting with a given MLD mirror, we calculate an etching profile that maximizes the diffraction efficiency at the -1st order by taking into account the manufacturing constraints of future suppliers. Then, we optimize the mirror stack without changing the etching profile. We modify only the first top layers under the grooves. We obtained theoretical designs with the same etching profile and identical diffraction efficiency, associated with different electric field intensity values and expected different laser induced damage thresholds.
The standardization and comparison of laser-damage protocols and results are essential prerequisites for development and quality control of large optical components used in high-power laser facilities. To this end, the laser-induced–damage thresholds of two different coatings were measured in a round-robin experiment involving five well-equipped damage testing facilities. Investigations were conducted at the wavelength of 1 μm in the sub picosecond pulse duration range with different configurations in terms of polarization, angle of incidence, and environment (air versus vacuum). In this temporal regime, the damage threshold is known to be deterministic, i.e., the continuous probability distribution transitions from 0 to 1 over a very narrow fluence range. This in turn implies that the damage threshold can be measured very precisely. These characteristics enable direct comparison of damage-threshold measurements between different facilities, with the difference in the measured values indicating systematic errors or other parameters that were not previously appreciated. The results of this work illustrate the challenges associated with accurately determining the damage threshold in the short-pulse regime. Specifically, the results of this round-robin damage-testing effort exhibited significant differences between facilities. The factors to be taken into account when comparing the results obtained with different test facilities are discussed: temporal and spatial profiles, environment, damage detection, sample homogeneity, and nonlinear beam propagation.
The standardization and the comparison of laser-damage testing are essential prerequisites for development and quality control of large optical components used in high-power laser facilities. To this end, the laser-induced–damage thresholds of two different coatings were measured at four laboratories involved in a round-robin experiment. Tests were conducted at 1 m in the subpicosecond range with different configurations in terms of polarization, angle of incidence, and environment (air versus vacuum). In this temporal regime, the damage threshold is known to be deterministic, i.e., the continuous probability distribution transitions from 0 to 1 over a very narrow fluence range. This in turn implies that the damage threshold can be measured very precisely. These traits enable direct comparison of damage-threshold measurements between different facilities, while the difference in the measured values are not accompanied by large statistical uncertainties.
In this presentation, the results of this comparative experiment are compiled, illustrating the challenges associated with accurately determining the damage threshold in the short-pulse regime. Specifically, the results of this this round-robin damage-testing effort exhibited significant differences between facilities. The factors to be taken into account when comparing the results obtained with different test facilities are discussed: temporal and spatial profiles, environment, damage detection, samples homogeneity and nonlinear beam propagation.
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