The rasterscan test procedure implemented in order to determine low laser damage density of large aperture UV
fused silica optics was improved in terms of accuracy and repeatability. Tests have been carried on several facilities
using several pulse durations and spatial distributions. We describe the equipment, test procedure and data analysis to
perform this damage test with small beams (Gaussian beams, about 1mm @ 1/e, and top hat beams). Then, beam overlap
and beam shape are the two key parameters which are taken into account in order to determine damage density. After
data analysis and treatment, a repeatable metrology has been obtained. Moreover, the consideration of error bars on
defects distributions permits to compare data between these installations. This allows us to look for reproducibility, a
necessary condition in order to share results and to make reliable predictions of laser damage resistance. For that, a
careful attention has been paid to beam analysis.
Results of a novel X-ray laser application, aimed at understanding the microscopic effects involved in formation of laserinduced
damage in optical materials exposed to sub-ns laser pulses, will be presented. Specifically, we studied thin plane
beamsplitters that are presently the weakest element of the next generation of high-energy lasers (LMJ, NIF), with
permanent damage threshold below 20 J/cm2. Standard fused silica substrates and a model system, containing welldefined
micron grooves as seeding sites to trigger damage when irradiated by 438 nm laser pulses, were in situ probed by
a neon-like zinc X-ray laser delivering up to 10 mJ at 21.2 nm. The probing beamline employed a double Lloyd's mirror
interferometer, used in conjunction with an imaging mirror to provide magnification of ~8. In conjunction with an array
of in-situ optical diagnostics, one of the questions addressed was whether the damage (transient or permanent) on the
rear surface of the beamsplitter occurs during or after the laser pulse, i.e. whether it is due to local electrical fields or to
other processes. Another issue, examined by both the X-ray interferometric microscopy and the optical diagnostics, is
whether a local rear-surface modification is associated with non-linear effects (self-focusing, filamentation) of the laser
beam in the bulk.
A rasterscan test procedure [L. Lamaignère et al, Rev. Sci. Instrumen. 78, 103105 (2007)] has been
implemented in order to determine low laser damage density of large aperture UV fused silica optics. This
procedure was improved in terms of accuracy and repeatability. Tests have been carried on several facilities
using several pulse durations and spatial distributions. We describe the equipment, test procedure and data
analysis to perform this damage test with small beams (Gaussian beams, about 1mm @ 1/e, and top hat beams).
Then, beam overlap and beam shape are the two key parameters which are taken into account in order to
determine damage density. After data analysis and treatment, a repeatable metrology has been obtained.
Moreover, the consideration of error bars on defects distributions permits to compare data between these
installations. This allows us to reach reproducibility, a necessary condition in order to share results and to make
reliable predictions of laser damage resistance.
During the development of the laser megajoule (LMJ), a high power laser facility dedicated to DT fusion, CEA has made
important efforts to understand and improve laser induced damage threshold of fused silica optics at the wavelength of
351 nm. For several years, with various industrials and academics partners, we have focused on optimizing the grinding,
lapping and polishing processes to increase materials performance. In this paper, we describe our efforts in various
fields: subsurface damage characterization, lapping process simulation, diamond grinding and lapping machine
instrumentations, ... Our concern is to control and manage the material removal at each step of the process in order to
reduce the cracks region extension and thus to diminish the damage density.
This paper deals with the relation between fracture mechanics and 355 nm laser damage at the surface of fused
silica. It is organized in 3 parts. First, we discuss about the link between cracks and laser initiation of surface damage. A
1D model was proposed last year to explain how a nanometer wide, clean, uncontaminated crack could trigger a
macroscopic damage event. Here, using the model, we try to express a damage criterion able to reproduce experimental
features.
In a second part, we consider the relationship between laser damage and mechanical damage by indents or
impacts. From Auerbach's law, it is straightforward to derive an energy density threshold for Hertzian crack initiation.
With the laser fracture interaction model, a laser fluence threshold of cone crack formation can be calculated. When cone
cracks are present, a series of shot at moderate fluence will increase their length exponentially. This is a possible
explanation for exponential damage growth at the exit surface of fused silica.
A rasterscan test procedure [L. Lamaignère et al, Rev. Sci. Instrumen. 78, 103105 (2007)] has been implemented
in order to determine low laser damage density of large aperture UV fused silica optics. This procedure was improved in
terms of accuracy and repeatability and is now used for the determination of bulk damage density for KDP crystals. The
large area (volume) scanned during tests permits to measure very low damage density. On small samples, small area are
tested using the normalized 1/1 test procedure consisting on the irradiation of few sites at several fluences. The classical
damage probability plot is converted in terms of damage density. The two testing procedures are complementary: the 1/1
mode is practical to test a wide fluence range while the rasterscan mode allows exploring low damage densities with
higher accuracy.
Tests have been carried out on several facilities using several pulse durations and spatial distributions. We
describe the equipment, test procedure and data analysis to perform this damage test with small beams (Gaussian beams,
about 1mm @ 1/e, and top hat beams). Then, beam overlap and beam shape are the two key parameters which are taken
into account in order to determine damage density. After data analysis and treatment, a repeatable metrology has been
obtained. Moreover, the consideration of error bars on defects distributions permits to compare data between these
installations. This allows us to reach reproducibility, a necessary condition in order to share results and to make reliable
predictions of laser damage resistance.
Other tests are realized with larger beams (centimeter sized) and with a single shot. Due to a large beam contrast,
a large fluence range is then covered. Then after data treatment, we find a good correlation between tests realised with
small and large beams. This allows us to make tests with different laser characteristics (spectral modulations, pulse
duration, laser polarisation) and then to study their influences on laser damage.
Significant improvement in polishing processes of fused silica optical components, has increased optics lifetime at the
wavelength of 351 nm. Nonetheless, for large laser operation facilities like the Laser MegaJoule (LMJ), zero defect
optics are not yet available. Therefore a damage mitigation technique has been developed to prevent the growth of
initiated damage sites: this technique consists in a local melting and evaporation of silica by CO2 laser irradiation on the
damage site. Because of the difficulty to produce efficient mitigated sites with large depth, the initial depth of damage to
mitigate is a critical issue. An aim of our work was to determine the real extension of the damage site (including
fractures) for different laser pulse durations between 3 ns and 16 ns and at different laser fluences. The fractures are nondetectable
in conventional microscopy. The depth of the damage can thus be underestimated. Hence confocal microscopy, was used to observe these sub-surface fractures and to measure precisely the depth of damage. Results show that the damage is 2 to 4 times wider than deeper and this ratio is independent of the pulse duration and of the fluence. With this new information, the mitigation process can now be optimized.
Campaigns of laser damage tests at 1w of Nd-YAG laser (1064 nm), 3w and with a combination of these two
wavelengths, were conducted to complete previous existing data on damage growth in fused silica output surface.
It is known that UV light is very effective in inducing preexisting damage craters to grow. When both
wavelengths are present, the effect of 1w beam on damage growth depends on the delay between the IR and the UV
beam. When the 1w reaches the sample before the 3w, it has nearly no consequence on growth rate. On the opposite,
when the IR beam is delayed and strikes the sample after the 3w pulse, its energy simply adds to the UV in enhancing
damage growth.
Damage initiation is much more affected by 3w than 1w pulses. However, the number of surface damage craters
also increased by the addition of 1w photons to the UV beam.
Numerous experimental and theoretical contributions in the past have stressed the detrimental effect of fractures
in the generation of surface laser damage sites in fused silica illuminated at 351 nm. However, two very important steps
lack for the moment on the way towards a scientific understanding of the role of fractures.
1. a physical model must be developed to predict damage events starting from real defect sites
2. a reproducible measurement must be obtained and compared with calculations.
Here we present the theoretical work realized to reach the first goal. Contrary to previous discussions on fractures, the
electromagnetic configuration is calculated in the case of a real material, with electronic surface states, bulk defects, and
defects dynamics. Due to electromagnetic field enhancement in the fracture, surface defects absorb a sufficient part of
laser energy, able to heat silica above the vaporization temperature. This is the initial event that triggers production of
more excited states during the pulse, and steep increase of temperature and pressure fields. Comparisons with available
experimental results are positive. Calculated fluences of damage initiation are very near those of measured events on
engineered fractures, or on real defects in polished samples.
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.
Laser damage at 3ω, 351 nm, of fused silica optical components is a major concern for LMJ maintenance.
Indeed, even a low density of damage sites is unacceptable due to the exponential growth of surface damage with a series
of laser shots. A technique is now used to prevent the growth of initiated damage sites : this mitigation technique consists
in a local melting and evaporation of silica by CO2 laser irradiation on the damage site. Even if the growth is stopped in
most cases, we showed previously that some of the mitigated sites re-initiate on their peripheral area, where most of redeposited
debris are located. To further increase the efficiency of mitigation technique, the treatment was improved by
varying the spatial profile of the CO2 laser beam. We present here the new set-up and the results obtained in terms of
laser damage resistance: about 98% of the mitigated sites sustained 200 shots of a 10 J/cm2 3ω YAG laser without
damage.
We report on laser-induced damage threshold (LIDT) and UV-laser excited defect formation measurements in large
aperture KDP crystals developed as doublers and triplers for mega-Joule laser. Measurements of LIDT were performed
according to the ISO 11254-2 standard for repetitive pulses with duration ~ 4 ns and repetition rate of 10 Hz. The results
for different laser wavelengths (1064, 532 and 355 nm) and polarizations are presented. The largest LIDT was observed
for 532 nm pulses and the 1064 nm wavelength had a strong dependence on laser polarization. The LIDT values at 532
nm and 355 nm also depended on the crystal cutting angle, which is different for doublers and triplers. A comparison of
LIDT with earlier reported crystal absorptance at different wavelengths is also performed.
The UV-laser induced defect formation was investigated by the means of pump-probe technique. The excitation was
performed with a single pulse of ns Nd:YAG laser (355 or 266 nm wavelength) and probing with another Nd:YVO4 laser
system (532 nm) operating at 1kHz. This gave us a temporal resolution of 1ms. The transient absorption of defect states
relaxed non-exponentially and fully disappeared in ~10 s. A comparison is made between crystal grown by distinct
growth methods and between different laser polarizations. An influence of laser conditioning on UV induced defect state
formation is also revealed.
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.
We studied filamentation, front surface damage and rear surface damage at 1064 nm and 351 nm with nanosecond pulses on a fused silica optical window. With temporally single-mode pulses, self-focusing occurs together with front surface damage, which is attributed to a Stimulated Brillouin Back Scattering (SBS) wave. The use of temporally multi-mode pulses suppresses the occurrence of front surface damage, and increases self-focusing. With single-mode pulses, the observation of filaments seems coherent with standard Kerr self-focusing effect, and can be understood according to the numerical treatment by Marburger et al, using non linear index values measured in other experiments. However, when multi-mode pulses were used, filaments occurred for much smaller peak intensities, by about a factor of 2. In this case, the non linear index causing self-focusing appears to be twice bigger. This second case is relevant to the situation of vacuum windows in high power laser installations, where the spectrum of light is widened to get rid of SBS. We discuss the physical effects that could be causing the enhancement of self-focusing.
The interaction of intense femtosecond laser pulse with model samples containing gold nanoparticales embedded in dielectrics is studied in order to understand the role played by nanodefects in optical breakdown of dielectrics. A theoretical study of the conduction electrons dynamics in the laser field predicts an efficient injection of carriers from the metallic inclusion to the conuction band of the dielectric, which leads to a strong local increase of the optical
absorption in the initially transparent matrix. This prediction is tested experimentally by using time -resolved spectral interferometry to measure excitation densities as a function of the laser intensity in silica and samples doped with gold nanoparticles, which are compared with similar measurements in pure silica.
We report the measurements of the linear and non linear absorption at 1064, 532 and 355 nm in samples of KDP crystals fabricated with the rapid growth process developed for NIF and LMJ high power lasers. Measurements were performed according to the ISO11551 standard by the "pulse" or "gradient" calorimetric method using a pulsed, diodepumped, Q-switched Nd:YAG laser. Time resolved spectroscopy method was used for the investigation of defects formation, responsible for the non linear absorption at 355 nm.
We present early results of an application of X-ray laser, aimed at understanding the effects involved in formation of laser-induced damage in optical materials exposed to sub-ns laser pulses. For the purpose of the experiment, a novel interferometric microscopy technique was designed and tested. The interferometric beamline employed a double Lloyd's mirror interferometer, used in conjunction with an imaging mirror to provide magnification of ~8 along a plane
inclined with respect to the propagation direction of the X-ray beam. The objects investigated were thin plane beamsplitters made of fused silica (SiO2), irradiated by damaging laser light at 438 nm and in situ probed by the developed technique of interferometric microscopy. The soft X-ray beam was emitted by neon-like zinc laser, delivering up to 10 mJ at 21.2 nm. In conjunction with an array of in-situ optical diagnostics, one of the questions addressed was whether the damage of the rear surface of the beamsplitter occurs approximately during of much after the laser pulse. Another issue examined by the X-ray interferometric microscopy technique was whether the surface perturbation seen shortly after the impact of the damaging pulse is associated or not with the pattern of permanent surface modifications.
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.
We report the measurements of the linear absorption at 1064, 532, and 355 nm and non linear absorption at 355 nm in samples of KDP crystals fabricated with the rapid growth process developed for NIF and LMJ high power lasers.
In the field of laser-induced surface damage, it has been shown that localized re-fusion of silica can be used as a mean to mitigate the damage and therefore stop its growth before the use of the optical component is impaired. In this paper, this localized re-fusion was produced using a continuous CO2 laser. As the damage is reshaped, we observed that a ring of evaporated silica is systematically deposited around the mitigated damage. This evaporated silica is likely to be non-stoechiometric and therefore to present absorption and luminescence properties.
Thus we decided to perform photoluminescence measurements in order to analyse the mitigated damages. We performed fluorescence imaging and spectroscopy using 351nm continuous laser excitation. Different experimental conditions were used for the re-fusion process and the consequences on the photoluminescence properties were studied. We also compared these properties to the properties of non-mitigated damages.
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.
The occurrence of filaments in fused silica irradiated by UV laser light is well described by the product of light Intensity by Length of propagation in the material. For a spatially gaussian peak, in the well-known treatment by Marburger et al, this product is predicted to depend upon input power and non linear index. At a wavelength of 355 or 351 nm, the compilation of our past and present measurements give a smaller critical intensity by length product, i.e. a higher non linear index, than previously measured.
These values of non linear parameters allow for the prediction of rear surface damage on thick windows. The predictions compare well with damage probability measurements. Even when the intensity is not high enough to generate filaments, self-focusing is still the cause of damage, due to the increase of output intensity and fluence.
For high power laser applications like the "Laser Megajoule" facility under construction in France, laser-induced damage threshold (LIDT) in fused silica is a limitation. CEA has made efforts to improve LIDT at the wavelength of 351 nm. Polishing and post polishing processes have been optimized. Laser damage sites density was decreased by several orders of magnitude by combining different fabrication steps. In order to further enhance optical laser resistance and to remove damaged sites on full-size optics, several small-beam raster scanning techniques have been studied and developed to condition fused silica optics. To stop the growth of damage sites, a continuous CO2 laser was used to re-melt them. Laser induced damage tests, performed on instrumented and automated facilities, are reported in order to check and illustrate the effectiveness of these treatments. Damage initiation studies as well as damage growth measurements are presented.
The purpose of this paper is to gather experimental elements allowing for the prediction of laser damage on full size components installed on high power Nd-glass laser lines. Damage can initiated on material defects, which aren’t known in their nature, but the density of which can be measured. On transmissive optics, depending on the component thickness, and on the intensity distribution at the front surface, rear surface damage can also appear due to self-focusing of hot spots. These two contributions produce damage sites that are prone to grow. The growth rate has been shown to be proportional to the damaged area. The resulting exponential growth is the major limitation to the lifetime of optics. A representation of these phenomena in the plane Intensity/Fluence gives a practical description of the impact of laser damage on the lifetime of optical components. It also enlightens the comparison between different operating conditions.
In order to understand the role played by nanodefects in optical breakdown of dielectrics, the interaction of an intense laser field with model dielectric samples containing metallic nanoparticles is studied both theoretically and experimentally. A theoretical study of the metal conduction electrons dynamics in the laser field predicts an efficient injection of carriers from the metallic inclusion to the conduction band of the dielectric, which leads to a strong local increase of the optical absorption in the initially transparent matrix. This prediction is tested experimentally by using time-resolved spectral interferometry to measure excitation densities as a function of the laser intensity in silica samples doped with gold nanoparticles, which are compared with similar measurements in pure silica.
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.
Variable experimental conditions were used to measure the occurrence of front surface, rear surface and filamentation damage in synthetic fused silica windows. Experiments were performed at 355 nm with a table-top beam of mm-size, and at 351 nm with ALISE laser, a 100 J installation. The 351 nm beam was about 3 cm wide at the entrance surface; it was single-mode temporally, with or without a frequency modulation which has the function of widening the spectrum to decrease Stimulated Brillouin Scattering. The 355 nm was single-mode temporally. Thin windows showed very scarce front damage and no filament damage at intensities which cause a high density of rear surface damage. Without any spectral widening, the thicker windows (4.3 cm) showed appreciable amount of front surface damage; filaments were observed and but no filaments. When a spectral modulation was added, front surface damage vanished, filaments and rear surface damage were observed.
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.
In the context of high power laser applications like the French "Laser Megajoule" or the "National Ignition Facility" in the United States, laser-induced-damage in fused silica appears to be a limitation. Although it has been established that nano-sized defects are suspected to be responsible of the damage initiation in optical
components, the induced-breakdown process is still unknown. The specific apparatus developed at the Institut Fresnel permits to study in real time the laser interaction on an isolated absorbing defect (around 100 nm): Indeed the coupling of a nanosecond focused laser (beam diameter about 10μm) and a high resolution photothermal
microscope (beam diameter 1μm) in a unique facility permits to highlight the evolution of the local absorption of the defect versus irradiation energy density. Furthermore, the morphology of this modification is followed thanks to a "refraction mapping" (photothermal deflection measured at frequency 0). An in-situ
dark field microscope and an ex-situ AFM complete the observations. In order to simulate the presence of the nano-defects, engineered sub-micronic gold particles (100-600 nm) were embedded in silica samples. The whole observations contribute in the understanding of the different stages of the damage initiation and more particularly, a determination of a "pre-damage" threshold can be performed.
Samples of deuterated and hydrogenated KDP were submitted to damage tests at 355 nm and 351 nm, with a Nd:Yag laser and the Xe-F line of a excimer laser. Bulk damage was observed; the statistical occurrence and the phenomenology of this type of damage was studied in various conditions. The crystals were raster-conditioned with the excimer source, which delivered a 16 ns pulselength. Laser-conditioning was performed with increasing fluences. The highest usable fluence was limited by the occurrence of surface damage on the front of the crystals. After this irradiation, a two-fold improvement of the damaging fluences was obtained when testing with the excimer beam. However, damage statistics were almost unchanged on the Nd:Yag installation, where the pulselength is about 3 ns. This result is discussed with respect to the large scale conditioning of crystals for high power lasers.
Rear surface damage is known to occur preferentially to front surface damage on silica lenses and windows transmitting 1ω or 3ω light of a Nd:Yag or Nd:glass laser. Lab-scale Experiments were performed to assess the contribution of self-focusing to the measurement of damage statistics. The occurrence of filamentation in the bulk and the statistics of rear surface damage were studied at 1064 nm and 355 nm, for a pulse length around 3 ns. The measurements were performed on synthetic fused silica samples, 4 to 5 cm thick. The laser beam had dimensions around 0.6 mm at the sample. The results are interpreted with the help of the calculations made by J. H. Marburger et al. They are also compared with other experimental results.
With the aim of observing and simulating laser initiated damage, ultra-pure silica "model" samples, seeded with gold nanoparticles (diameters 2 - 5 nm), were prepared and exposed to 3 ns laser pulses at wavelength 355 nm. These samples enable us to study the mechanism of damage initiation caused by inclusions of nanometric size, which is the typical size of defects occurring in optical quality glass. The samples were studied in a series of experiments using a time-of-flight mass spectrometer at Argonne National Laboratory. This installation is of great interest because it enables us to combine the laser irradiation of the sample with the chemical identification of material ejected from the exposed surface. An evaluation of the quantity of gold atoms emitted during irradiation can thus be obtained from the experimental results. These experimental data are completed with “Nomarski” and “atomic force” microscope observations, and then interpreted. In particular, a comparison is made to numerical simulations obtained with our DELPOR code. An encouraging result is the existence of a pre-damage phase at very low fluences that is not detectable by classical optical devices.
The validation of numerical simulations of laser induced damage of fused silica requires detailed knowledge of the different parameters involved in the interaction. To approach the problem, we have performed simulations of laser energy deposition in spherical metallic defects and the surrounding fused silica. Our code DELPOR takes into account various laser/defect induced absorption mechanisms of SiO2, such as radiative ionization, avalanche and multiphotonic ionization. We have studied crater formation produced by the absorber explosion with a 2-D Lagrange-Euler code taking into account crack formation and propagation in the brittle material. To validate our simulations, we have made and tested samples of ultra-pure silica thin film, containing gold nanoparticles of diameter 0.6 μm. The fused silica coating could have three different thickness. We compare experiment and simulations for two laser irradiations at wavelengths 0.351 and 1.053 μm.
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.
A photothermal microscope has been combined with an experimental
set-up allowing damage threshold measurements at the same
wavelength. The microscope is based on photothermal deflection of
the transmitted probe beam: the CW pump beam (1.06 μm
wavelength) and the probe beam are collinear and focused through
the same objective. The diameter of the pump beam on the sample
surface is 1 μm. Laser damage thresholds are measured thanks
to a pulsed beam (1.06 μm wavelength and 6 nanosecond pulse)
and the spatial position of the pulsed beam is controlled by a CCD
camera. This experimental setup has been used to study the
behavior of metallic inclusions in dielectric materials in laser
damage processes. Results are presented with gold inclusions of
about 600 nm in diameter in silica.
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.
Simulations of laser-fused silica interactions at 0.351 μm are a key issue in predicting and quantifying laser damage in large laser systems such as LIL and LMJ. Validation of numerical simulations requires detailed knowledge of the different parameters involved in the interaction. To concentrate on a simple situation, we have made and tested a thin film system based on calibrated gold nanoparticles (0.2-0.8 μm diameter) inserted between two silica layers. The fused silica overcoat was either 2 or 10 microns thick. We have performed simulations of laser energy deposition in the engineered defect (i.e. nanoparticle) and the surrounding fused silica taking into account various laser/defect induced absorption mechanisms of SiO2 (radiative ionization, avalanche and multiphotonic ionization). We have studied crater formation produced by the absorber explosion with a 2D Lagrange-Euler code taking into account crack formation and propagation in the brittle material. We discuss the influence of the defect depth (with respect to the surface) on the damage morphology. The simulations are compared with our experimental results.
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.
Contamination of surfaces prior to or during deposition of dielectric multilayers has been a classical subject of technological studies, but post deposition contamination has not been reported as often. Previous results typically dealt with sealed optics and exposition to laser or UV conditions, or on-orbit optical systems: we report here observations on HfO2/SiO2 mirrors designed for the Megajoules project, with very high laser resistance. Performances in terms of laser damage thresholds at 1.06 micrometers have been measured periodically over periods of several months, on different sets of samples. Comparison of R/1 LIDT distributions are presented versus time and storage conditions. Degradation of LIDT is clearly observed. Recovery methods are investigated.
Laser energy deposition and redistribution in metal nanoparticles embedded in SiO2 glass is studied by a kinetic model, which takes into account photon absorption, electron-electron and electron phonon interactions, as well as heat transfer to the glass matrix. The collision operators are usually written in an integral form. In this work, we transform those in differential operators with the use of Landau approximation. This approach allows to perform kinetic calculations beyond the nanosecond time scale. For a laser intensity relevant to high power lasers, the energy deposition on the electron population can lead to a significant Fermi smearing within very short times. An important part of the electron population is driven beyond a typical 10 eV energy, and consequently this can result in the creation of a plasma around the particle.
Volume holographic gratings and ion-etched dielectric diffraction gratings have been designed with the goal of improving the efficiency and damage threshold when used in CPA laser compression scheme. Two damage threshold measurement techniques have been implemented, including one method based on the statistical distribution of damage fluences. We first tested samples with submillimeter laser spots in the subpicosecond regime. We could demonstrate large sample gratings in a complete CPA configuration with over 96% efficiency per pass and a damage threshold twice the one observed on gold coated gratings.
In this paper we describe the interaction of an intense laser beam with metallic nanoparticles embedded in glass. Energy deposition in the metal is calculated on the basis of Mie's scattering theory, using an accurate model for the dielectric function which involves interband transitions. As is shown by two-temperature modeling of the laser-heated metal, nonequilibrium thermodynamics must be used to describe such systems, even in the nanosecond laser pulse range. Taking into account the particle cooling process by heat diffusion in the glass matrix, the model provides a useful tool for the understanding of laser damage initiation by metallic nanoinclusions.
By direct observation of individual damage sites on the surface of a component, the local fluence of every damage site was obtained. This more accurate measurement was used to evaluate the real defect density at each fluence. These results were compared to some others, calculated from damage fluences distributions, obtained from R/1 test. Results presented here concern fused silica with a regular polish. The experimental set up used for this work was equipped with a 3.7ns Nd:YAG pulsed laser at 1 (omega) . The laser beam diameter was 1.1mm and the angle of incidence was close to 0 degrees.
A laser damage measurement campaign was realized on PHEBUS high power laser on two different high reflecting HfO2/SiO2 mirrors centered at the wavelength of 1.053 micrometers at 45 degrees incidence. The two tested mirrors were deposited using e-beam technology: a large 620 X 440 mm2 LIL mirror was made with an oxide HfO2 target, and a 100 mm-diameter mirror with a metal Hf target. The test were performed with a 40 mm wide beam. Damages were detected by light scattering on a separate facility. Macroscopic and microscopic images of the damages were taken. A statistical analysis of these data is proposed to compare the mirrors. It is also interesting to compare large beam damage data to small beam laboratory statistics.
A simple model is proposed, featuring cooperation of many defects to the damage process. For identical, randomly and independently distributed defects, the response of the optical components to a spatially gaussian laser pulse is calculated. The absorption response of the component is a probability distribution, which depends on the values of mean defect density and beam equivalent area. For a damage threshold expressed as a value of the absorbed energy, the statistical distribution of damage in obtained theoretically, as a function of fluence. The damage fluence distribution resembles the lognormal function found in many recent experimental measurements. Damage statistics were also calculated for a diverse spatial dimensions of the beam. This cooperative model of a damage is compared to the description of defect-related damage developed in particular by Mike Feit et al to correlate small beam damage test to large area irradiation results. In the latter work ,defects react one by one, contrary to the present view.
Laser conditioning is a phenomenon which has been much explore during the last 15 years. Many optical components for high power laser have shown enhancement of their laser- induced threshold after under-threshold pre-irradiation. The improvement brought by laser conditioning is though to be necessary in the building of new NIF and LMF lasers, especially for dielectric multilayers and KDP crystals. However, we lack a complete physical explanation for the observations of conditioning. In particular, we do not know the effect of most physical parameters like pulselength. wavelength, repetition frequency of the laser shot. In this tentative review, we first gather results relevant to the general problem of laser conditioning, especially on dielectric multilayers irradiated with nanosecond pulselength. We find necessary to come back to the basic notion of damage and threshold. As a definition basis, we establish that laser conditioning is a damage with minor optical consequence and no continuing damaging with successive testing. This way of presenting the phenomenon contains a non-threshold description of optical damage. We finally explore some specific questions, that are technologically important for LMJ and NIF construction. Particularly, is laser conditioning necessary once we admit the occurrence of small damages, that is with a functional definition of damage threshold.
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