During the life of a high-power laser chain, optical components may be damaged due to local high fluence levels in the
inhomogeneous beam. The origin of the laser damage can be impurities, surface defects or flaws and cracks resulting
from polishing, or it may be produced by self-focusing in the component. The aim of this study is to better understand
the correlation between a surface crack on a silica optical component and laser damage. To accomplish this, calibrated
indentations were made on silica samples. Observations of the sites were made with an optical microscope, and three
different morphologies were recognized. Then the zones containing the indentations were irradiated (single shot mode)
with a Nd Yag laser at 355 nm for various fluences. Subsequent observations of the sites were made with an optical
microscope, with the aim of correlating site morphology and laser-induced damage. Some sites were believed to have
undergone laser conditioning. They were further irradiated (raster scan mode) at high fluence, and some evidence for a
laser conditioning effect was obtained.
A major issue in high power lasers for fusion is laser-induced damage on optical components. Since damage is often initiated by a surface crack resulting from polishing, it is important to understand the physics involved in this phenomenon. In this study, calibrated surface scratches have been realized on two silica samples using an indenter-scratcher apparatus. A variety of scratches have been tested by applying different speeds and forces on the scratcher needle. Optical microscope observations show that the scratches made at highest speed create irregular dashed lines. In addition, we have observed, at intermediate speed, an evolution in time of the scratches due to local stresses in silica.
One of the samples was irradiated by a Nd:YAG laser beam at 355 nm with the scratches on the exit surface. Microscope observations were made before and after irradiation.
Strong dependence on the scratch speed was observed on the local laser damage. Again, temporal evolution of the damage has been observed.
In this paper we report several studies of the propagation of the Laser Integration Line (LIL) and Laser Megajoule (LMJ) beams when interactions occur with optical components defects. These studies are mainly achieved on a numerically predictive basis with the CEA MIRO beam propagation code. The flaws that we considered are located at the front or rear surface of the optical components. These surface flaws correspond to engineered defects such as scratches and to surface damage resulting from laser-induced growth process, from mitigation process or from target interactions debris. Results account for the possible downstream impacts of flaws at the rear-surface of the optics and from one component to another along the laser chain. In particular, the influence on the LIL/LMJ end-of-line focal spot intensity and size is predicted.
This paper presents data reduction on an experimental set-up that we have recently developed at CESTA, France. It has been implemented to analyze laser-induced damage on optics dedicated to the Megajoule laser project. Our goal is to measure the damage fluence on samples under tests, using a statistical approach on a very large number of sites.
The laser-induced damage density is accurately plotted as a function of laser fluence, by measuring the fluence of every single shot during the scan. This improvement of accuracy modifies dramatically the interpretation of the results that can be made, compared to raw data, considering the mean fluence only.
A major issue in high power lasers for fusion is laser-induced damage on optics and its evolution in time after a large number of shots. Since damage is often characterized by an initial surface crack, its surface usually increases, following an exponential law.
Surface scratches have been made on silica samples in order to get calibrated fractures. Then, to test different experimental conditions, we made a variety of scratches in terms of length and depth. The samples are then irradiated by a Nd:YAG laser first at 1064 nm (1w) then at 355 nm (3w). They are successively tested with the scratches facing the laser beam or placed with the scratches on the back surface.
To obtain better understanding of particulate contamination, chromium dots (50 x 50 μm2) were deposited on a silica substrate by photolithography. The aim in using this sample is to observe the mechanism of damage initiation that can be attributed to surface contamination of micro-metric size. A Nd:YAG laser irradiated the sample at 1064 nm for different fluences and also different numbers of shots. Several methods were used to characterise the laser effects on the chromium dots and the silica substrate: "Nomarski", "atomic force" and photothermal microscope observations.
The laser fluence is found to be the most important parameter for the behaviour of the chromium dots. At low fluence (<1 J/cm2), they become cracked (fractured). At medium fluence (around 1 J/cm2) chromium fusion is reached and chromium oxide appears. Finally at higher fluence (3 J/cm2), although chromium dots are blown off the substrate and small damage to silica occurs on the first shot, the subsequent shots do not lead to a dramatic increase in the damage.
Experiments have been performed to measure the rate of laser-induced damage growth at the rear surface of fused silica windows at 1064, 1053 and 351 nm. One test bench delivered 9 ns monomode gaussian pulses at 10 Hz and 1064 nm. The size of the focused beam on the sample was a few mm2. Another test bench delivered 2.5 ns single or multimode pulses at 1053 and 351 nm. The focused spot on the sample was a few cm2. We compare and discuss our laboratory experimental results, the larger scale ALISE laser data and other results obtained at LULI.
The functional lifetime of large aperture optical components used in high power lasers, like LIL and LMJ facilities, is mainly determined by laser damage measurements. We present experimental procedures and statistical analysis, made on small samples with mm-size beams, to determine damage densities and damage growth laws. The tests and analysis are compared to other results obtained with larger beams (few cm2) on large aperture components.
The functional lifetime of large aperture components used in high power lasers, like LIL and LMJ facilities, is mainly determined by laser damage measurements. Automatic damage test benches allow to obtain more data in less time than traditional tests. We present, first experimental procedures and statistical analysis made on small samples with mm-size beams, to determine damage densities and damage growth laws. The presented methods are the usual 1on1, Non1, Ron1 and Son1 tests and more specially the raster scan procedure. The tests and analysis are compared to other results obtained with larger beams (few cm2) on large optics. We show that the exact knowledge of each shot parameters (energy, surface and pulse duration) permits to determine the damage growth rate (and then to predict the lifetime of each optics), to precisely study self-focusing phenomenon and more to finely observe pre-damage-levels. In this way, the main parameters like fluence or intensity are associated to the observed phenomenon. Moreover laser beam diagnostics, many diagnostics used for the detection and the observation of damage occurrence are equally very important. It is also necessary to develop test procedures entirely computed which permit to scan all the surface of a component and to acquire in real time the beam parameters and the results of laser-matter interaction. Experimental results are reported to illustrate what could be achieved on an instrumented and automated facility.
The growth of damage sites on the rear surface of fused silica plates was studied as a function of fluence and angle of incidence. At 1053 nm, a 70 J beam, 3 ns in pulselength, was directed to a 5 cm2 zone on a bare fused silica window. Initiation and growth was observed. The growth of previously initiated sites was also studied. Growth is exponential in nature. The experiments allow for the determination of the growth coefficient as a function of fluence. At 355 nm, damage sites were irradiated at various angles of incidence, with a tripled Nd:Yag laser, spatially Gaussian, 2.5 ns in pulselength. By fitting growth with an exponential law, it was determined that the relevant fluence for growth was that taken inside the material.
Samples of Neodymium doped laser glass were irradiated by the 1ω beam of a Nd:glass laser that delivers up to 80 J during a 3-ns pulse duration. Prior to this experiment, platinum inclusions were revealed by a systematic scanning with a lab-scale Nd:Yag laser. The damage sites due to impurity inclusions were subjected to tens of shots of the centimeter-size beam. Several inclusions were irradiated by a series of shots, at a fixed fluence comprised between 10 and 20 J/cm2. The incidence on the optical component was taken at Brewster-angle. In each case, the damage zone began to grow, then the growth rate slowed down and finally stopped. Thus, a stabilization of the growth was obtained for this bulk damage as opposed to steady growth observed in the case of silica surfaces.
High damage threshold hafnia/silica HR mirrors were damage tested. The transient reflectivity of these mirrors was studied during laser irradiation and particularly during catastrophic damage by top layer chipping. The tests were performed in R/1 mode on 50 sites in order to correctly assess the statistical behavior of the damage threshold. During the ramp, we observed the formation of a first plasma coupled to a modification of the reflected pulse without formation of any pit or chipping damage. For further irradiations at higher fluences, the optical properties of the mirror were unchanged up to the creation of a catastrophic chipping damage coming with a more intense plasma. The statistical distributions of fluences obtained in the two cases were different. For the moment, there is no proven correlation between first plasma and chipping: plasma detection cannot be used as a nondestructive quality control.
We present the experimental results of the detection of subsurface defects which show that the memory effect (or the far-field correlation function) of speckle patterns is sensitive to a small local change under the rough surface geometry. We envision this property can be applied to inspection of a target buried in the background by speckle mapping.
We report the experimental results which show that the far- field correlation function is sensitive to a small local change of the rough surface geometry, where the speckle spatial correlation is adopted rather than the sample ensemble average. The angular cross-correlation function of the far field speckles scattered by one-dimensional random rough surfaces is measured, when a polarized beam of light is incident on the rough surface from vacuum, where one part of the surface used is a thin dielectric film deposited on a glass substrate and the other part is identical to the first one except for a localized defect on it.
An optical method of fabricating one- and two-dimensional diffraction gratings is described. As opposed to conventional gratings which feature regularly spaced grooves, these gratings have variable profiles which represent mathematical functions previously defined. These surface profiles are produced by a laser beam exposing photoresist-coated plates with a raster scanner under computer control. Two kinds of 1-D profiles for the gratings have been made: (1) Deterministic: the profile represents a mathematical function; and (2) Random: gratings possess random profile with certain statistical distributions. Several surfaces have been fabricated and characterized with a stylus profilometer. By changing surface deterministic and statistical properties, the light-scattering in the far-field can be controlled for many optical applications.
The fabrication of characterized rough surfaces on photoresist is described. The technique used here can be separated into tow parts: first, a photoresist coating, generally 5 to 15 microns thick, is deposited on a flat glass substrate and baked in an oven; second, the plate is exposed, one line at a time, by a laser beam in a raster scanning method. Plate displacement is controlled by a PC computer in order to create the desired structure on the photoresist. Several deterministic and randomly rough gratings have been fabricated and characterized with a stylus profilometer.
A cooled Fabry-Perot spectrometer working in the far-infrared the 350 and 450 micrometers wavelength atmospheric windows is presented. It is designed for low temperature operation (1.5 K), in vacuum. The reflecting surfaces (gold inductive grids) are deposited on monocrystalline quartz substrates. Three push-pull motors perform the scanning of the spectrometer.
A fully automated scatterometer, designed for BRDF measurements in the IR at about 10 micrometers , is described. Basically, it works around a reflecting parabola (464 mm diameter, F/0.25) and permits measurements in and out of the plane of incidence. Optical properties of the parabolic mirror are emphasized by a ray-tracing technique which permits determination of the correct illumination on the sample and detection conditions of scattered light. Advantages and drawbacks of such an instrument are discussed, as well as calibration procedures. As a conclusion, we present experimental results to illustrate the instrument capabilities.
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