Relationships between subsurface damage (SSD) depth and peak to valley surface roughness (Rt) have been widely studied and present a major interest for an easy assessment of the SSD depth. We look at ...the relation between SSD depth and other surface roughness parameters using the Abbott-Firestone curve on a large campaign of grinding tests (with different abrasive grain size, grinding speed and grinding mode). The results reveal that Abbott-Firestone parameters are better for an assessment of SSD depth and that relationships between SSD depth and surface roughness are not universal but depend on the grinding process.
The impact of storage conditions on laser induced damage density at 351 nm on bare fused polished silica samples has been studied. Intentionally outgassing of polypropylene pieces on silica samples ...was done. We evidenced an important increase of laser induced damage density on contaminated samples demonstrating that storage could limit optics lifetime performances. Atomic Force Microscopy (AFM) and Gas Chromatography -Mass Spectrometry (GC-MS) have been used to identify the potential causes of this effect. It shows that a small quantity of organic contamination deposited on silica surface is responsible for this degradation. Various hypotheses are proposed to explain the damage mechanism. The more likely hypothesis is a coupling between surface defects of optics and organic contaminants.
We analyze the effect of polishing induced contaminants on the laser-induced damage density at the wavelength of 351
nm. Fused silica polished parts were manufactured using various polishing ...processes. If previous works have shown that high polishing induced cerium content leads to high laser-induced damage density, we show that for low cerium content, there is no correlation between the amount of polishing induced cerium contamination in the part interface and the damage density. This can be also extended to other polishing induced contaminants. These results provide new information for the understanding of laser damage initiation.
Using luminescence confocal microscopy under 325 nm laser excitation, we explore the populations of defects existing in or at the vicinity of macroscopic surface flaws in fused silica. We report our ...luminescence results on two types of surface flaws: laser damage and indentation on fused silica polished surfaces. Luminescence cartographies are made to show the spatial distribution of each kind of defect. Three bands, centered at 1.89 eV, 2.75 eV and 2.25 eV are evidenced on laser damage and indentations. The band centered at 2.25 eV was not previously reported in photo luminescence experiments on indentations and pristine silica, for excitation wavelengths of 325 nm or larger. The luminescent objects, expected to be trapped in sub-surface micro-cracks, are possibly involved in the first step of the laser damage mechanism when fused silica is enlightened at 351 nm laser in nanosecond regime.
In this paper we present a self-referenced interferometric single-shot measurement technique that we use to evaluate the longitudinal chromatism compensation made by a diffractive lens corrector. A ...diffractive lens with a delay of 1 ps is qualified for a 60 mm beam aperture. This corrector was implemented on the Alisé Nd:glass power chain. We qualify the corrector and the Alisé power chain chromatism, demonstrating the potential of this measuring principle as well as the interest of diffractive lenses to correct longitudinal chromatism of petawatt-class lasers.
•Controlled adsorption and desorption of molecules on a sol–gel coating is studied.•Molecules outgassed from a material are compared to the ones adsorbed on a surface.•A method to contaminate ...surfaces between 20 and 90ngcm2 is presented.•Affinity between sol–gel coating and semi-volatile organic compounds is highlighted.
Organic contamination may decrease the targeted performances of coated surfaces. To study the contamination induced by surrounding materials, a method using a thermal extractor is presented in the first part of this work. Besides its normal operation (analyses of outgassing compounds from a material), this device is used in an original way to contaminate and decontaminate samples. Efficiency of contamination and decontamination protocols are assessed by automated thermal desorption and gas chromatography coupled with mass spectrometry and by secondary ion mass spectrometry coupled with a time of flight mass analyzer. This enables to study the contamination induced by a bulk material outgassing and to take in consideration the possible competition between outgassed species. This method is then applied to investigate contamination of Laser MégaJoule sol–gel coated optics by a retractable sheath. The impact of the temperature on the outgassing of the sheath has been highlighted. Increasing temperature from 30 to 50°C enables the outgassing of organophosphorous compounds and increases the outgassing of oxygenated compounds and phthalates. Chemical analyses of contaminated optics have highlighted affinities between the sol–gel coating and phthalates and organophosphorous, and low affinities with aromatics and terpens. Finally, samples with increasing levels of contamination have been realized. However a saturation phenomenon is observed at 90ngcm–2.
Owing to their relatively high resistance to laser-induced damage, hafnia and silica are commonly used in multilayered optical coatings in high-power laser facilities as high- and ...low-refractive-index materials, respectively. Here, we quantify the laser-induced-damage threshold (LIDT) at 1053 nm in the short-pulse regime of hafnia and silica monolayers deposited by different fabrication methods, including electron-beam evaporation, plasma ion-assisted deposition and ion-assisted deposition. The results demonstrate that nominally identical coatings fabricated by different deposition techniques and/or vendors can exhibit significantly different damage thresholds. A correlation of the LIDT performance of each material with its corresponding absorption edge is investigated. Our analysis indicates a weak correlation between intrinsic LIDT and the optical gap of each material (Tauc gap) but a much better correlation when considering the spectral characteristics in the Urbach tail spectral range. Spectrophotometry and photothermal absorption were used to provide evidence of the correlation between the strength of the red-shifted absorption tail and reduced LIDT at 1053 nm.
Large high-power laser facilities such as megajoule laser (LMJ) or National Ignition Facility (NIF) are designed to focus about 2
MJ of energy at the wavelength of 351
nm, in the center of an ...experiment chamber. The final optic assembly of these systems, operating at 351
nm is made of large fused silica optics working in transmission. When submitted to laser at the wavelength of 351
nm, fused silica optics can exhibit damage, induced by the high amount of energy traversing the part. The created damage is a set of micro-chips that appear on the optic surface. Current researches have shown that this damage could be initiated on pre-existing sub-surface damages created during the optics manufacturing process. It is then very important to understand, for various set of manufacturing parameters, what are the key parameters for sub-surface damage. The presented work details the development of a simplified model to investigate the polishing process. Both silica (the material to be polished) and the abrasive particles are modeled using a discrete element approach. This numerical tool allows following the evolution of micro-cracks inside the material during the abrasion process. It is shown how the mechanical properties (pressure), the abrasive properties (shape and quantity of abrasive particles) and the system properties (filtration) have an influence on the sub-surface properties at the end of the process.
This paper studies gratings engraved in a multilayer dielectric stack for ultra high intensity laser compressors. A metal layer is inserted between the substrate and the dielectric stack to reduce ...the number of dielectric bilayers and thus the mechanical stress within the stack. A code taking account the fluctuation range of the geometrical parameters during the fabrication process is used to numerically optimize the mirror stack and study different groove profiles to increase the reflected efficiency and the laser induced damage threshold. It is evidenced that of all the profiles leading to good diffraction performances, those with the greatest groove depth and width values result in the smallest enhancement of the electric field square inside the grating with a decrease by a factor close to 2.5.
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