The mechanism of laser induced damage in optical materials under high power nanosecond laser irradiation is commonly
attributed to the presence of precursor centers. Depending on material and laser source, the precursors could have
different origins. Some of them are clearly extrinsic, such as impurities or structural defects linked to the fabrication
conditions.
In most cases the center size ranging from sub-micrometer to nanometer scale does not permit an easy detection by
optical techniques before irradiation. Most often, only a post mortem observation of optics permits to proof the local
origin of breakdown.
Multi-scale analyzes by changing irradiation beam size have been performed to investigate the density, size and nature of
laser damage precursors. Destructive methods such as raster scan, laser damage probability plot and morphology studies
permit to deduce the precursor densities. Another experimental way to get information on nature of precursors is to use
non destructive methods such as photoluminescence and absorption measurements.
The destructive and non destructive multiscale studies are also motivated for practical reasons. Indeed LIDT studies of
large optics as those used in LMJ or NIF projects are commonly performed on small samples and with table top lasers
whose characteristics change from one to another. In these conditions, it is necessary to know exactly the influence of the
different experimental parameters and overall the spot size effect on the final data.
In this paper, we present recent developments in multiscale characterization and results obtained on optical coatings
(surface case) and KDP crystal (bulk case).
We describe in this paper a model to link laser damage initiators properties (nature, size distribution, density) to measured Laser Induced Damage Threshold (LIDT). It is based on calculation of light absorption in nanoabsorbers and subsequent heating, coupled to laser damage statistics in order to obtain the laser damage probability as a function of laser fluence.
Applications to the case of optical coatings are then presented. We study the influence of laser irradiation parameters and coatings properties on LIDT measurements. By coupling this multiscale study to our model, we show that information on the initiating defects properties and the physical damage mechanisms involved can be obtained: discrimination between different defects, estimation of densities, size and nature of defects, evolution of the defects under multiple irradiation.
Implication of this approach for physical understanding and metrology applications are discussed.
A comparative study is made on the laser damage resistance of monolayers coatings made with different technologies.
HfO2 and SiO2 thin films have been deposited on fused silica substrates with Dual Ion Beam Sputtering,
Electron Beam Deposition (with and without Ion Assistance) and Reactive Low Voltage Ion Plating technologies.
The laser damage thresholds of these coatings have been determined at 1064nm and 355nm using a nanosecond
pulsed YAG laser, and a 1-on-1 test procedure.
The laser resistance of large optical components remains an important limitation for the performances and the maintenance costs in LMJ or NIF projects. For practical reasons LIDT studies are commonly performed with small samples and table top lasers whose characteristics change from one to another. In these conditions, it is necessary to know exactly the influence of the different experimental parameters (wavelength, spot size, ...) on the final data. These considerations are particularly true in heterogeneous materials as KDP crystals. Indeed the use of different laser beam sizes (from μm to hundreds μm) to plot laser damage probability curves had clearly shown that at 355nm in KDP, it is possible to exhibit a limit of irradiated area which permit to distinguish two different LIDT associated with two laser damage precursors densities. This prior result has put in evidence the influence of irradiated beam size in the discrimination of different kinds of defects in KDP. We present in this paper a systematic study of beam size effect in KDP for three different wavelengths: 355nm, 532nm and 1064nm. This study performed in 1:1 and R:1 mode will reveal precursors for each wavelength and their respective evolution under repetitive shots for small and large beams. This multi-parameters study will help us to highlight mechanisms involved in laser-induced damage in KDP crystal.
For large aperture solid state lasers, the laser resistance of the optical component remains an important limitation
for the performances and the maintenance costs. Since decades, laser induced damage has been intensively
studied in order to understand and control the origin of the phenomenon. LID measurements are commonly
performed with table top lasers whose characteristics change from one to another and, sometimes, the scaling
laws do not permit to explain the experimental differences. For example, we have previously demonstrated that,
in KH2PO4 (KDP) crystals, the laser beam size can influence strongly the determination of the damage probability.
Here, we present a systematic study realized on KDP crystal to quantify the influence of the beam size
on the LIDT (Laser Induced Damage Threshold) measurement at 355 nm. The use of an unique Gaussian beam
ranged from micronic to sub-millimetric sizes permits to highlight different types of laser-damage precursor.
LIDT measurements realized with beams of small (lower than 100 microns at 1/e2)or large (upper than 400 microns at
1/e2)dimensions give information about the behavior of material regarding precursor defects.
Contamination by metallic particles has been known to reduce the laser damage threshold on high power laser
optics. To simulate the presence of metallic particle on the Ligne d'Integration Laser optics, silica substrates
were arti.cially polluted by square aluminum dots of 5 × 5 micron2 and 50 × 50 micron2, respectively. The metallic dot
sites were irradiated by a Nd:YAG laser at 1064 nm with different fluences. The sites were analyzed by Nomarski
microscopy, optic profilometry and photothermal microscopy. For both sizes of metallic dots, vaporization of
metal can be observed. We study in this paper the dot size influence on the surface cleaning process and the
effect of the pre-irradiation mode (1 shoot or several shots).
At very high powers the energy for a single shot in the LIL/LMJ laser is today limited among others by the robustness of the KDP-based components used for frequency conversion. Subsequently it is vitally important to improve as much as possible the Laser Induced Damage Threshold (LIDT) of these components to make possible even more powerful shots. The exceptionally large aperture of such lasers (40*40 cm2) required the development of rapid growth methods. Investigations are under way to improve the damage resistance of such materials by implementing more efficient conditioning procedures. In this work we focus on composition heterogeneities induced by the rapid growth method in KDP crystals and we examine the impact on the laser-damage resistance. Two LIDT measurement facilities are used to investigate KDP triplers robustness. Spatially resolved LIDT measurements at 355 nm show that the LID resistance is significantly lower in some regions. The efficiency of the excimer conditioning in the different regions is also addressed.
Multiple laser irradiations induce a critical issue as regards the time of life of optical components. The problem can appear either in high repetition rate lasers or in high power systems even at low frequency. Two opposite behaviors are commonly observed under repetitive irradiations. A "fatigue effect" of materials under subsequent shots is generally observed and results in a decreasing of laser induced damage threshold (LIDT), but in some cases an improvement of LIDT can be noticed. This second effect linked to the pre-irradiation is well known as "conditioning" of the material. In most cases the LIDT in optical components is specified in 1:1, S:1 or R:1 modes, whatever the application of the real system. The aim of this paper is to show that the LIDT is strongly dependant on the parameters of irradiation such as shot number, shot frequency, wavelength and location in the material (surface or bulk). Therefore in order to approach a "true" value of LIDT it is necessary to test the component in the conditions of use, considering all the influential parameters. To illustrate this purpose the influence of previous parameters is studied for KDP and silica. This study shows that we can define a "functional laser damage threshold" in repetitive shot mode and also that the time of life could be deduced for each component. Furthermore these results can be useful to optimize the parameters involved in the conditioning processes.
The analysis of modifications induced by laser damage in optical materials is important for understanding the damage process. In this work, we study the morphological and structural modifications induced by a nano-second pulsed laser (355 and 1064nm) on fused silica samples (Suprasil, Herasil) and silica thin films (deposited by IP, IAD, EBD, IAD). The morphological changes are analyzed using optical microscopy, optical 3D surface profiler. The absorption modifications are measured by photothermal microscopy with a micronic resolution. Luminescence (excited at 244nm) mappings are also performed on damage sites and the luminescence spectra analyzed. Based on these measurements, we study the different laser damage step that we have identified: initiation by absorbing nanoscale defects, heating of the defect and the surrounding matrix, modification of the surrounding material that becomes absorbing, and macroscopic damage at final. We identify and spatially resolve several kinds of defects induced by catastrophic breakdown and we also discuss the observed structural changes on damage sites, taking into account the physical processes involved.
KDP crystals are currently used for frequency conversion and Pockels cells in large aperture laser systems such as the LMJ and NIF. These different functions are obtained by cutting the KDP crystals with different orientations. We show by measuring the LIDT with three different facilities, that the cut angle plays a key role in the damage mechanism. Consistently with the three measurement set-ups, we demonstrate that the doublers have a weaker LIDT value than the triplers. The z-cut KDP samples have a LIDT higher than both the doublers and the triplers. These results are analyzed in terms of probed volumes and pulse duration.
One of the major issues met in the operating of high power lasers concerns the cleanliness of laser components. In this context, in order to assess laser-induced damage in presence of metallic particulate contamination, we study the behaviour of aluminum on a silica substrate. Model samples containing calibrated aluminum square dots of 50 x 50 μ2 have been deposited by photolithography on a silica substrate. The sample was irradiated by a Nd:YAG laser at 1064 nm with different fluences and also different numbers of shots on each dot. Then the initial aluminum dot zone and the surrounding silica were analyzed using Nomarski microscopy, profilometry and photothermal microscopy. Laser fluence is revealed to be a very important parameter for the behaviour of aluminum dots. For example, it is possible to find a fluence of irradiation where aluminum dots are blown off the substrate and only small modifications occur to silica. In this case, increasing the number of shots doesn't significantly affect the silica surface.
The high-power Laser MegaJoule (LMJ) for inertial confinement fusion experiments that is currently under construction at CEA-CESTA in France will require a high number of large aperture Pockels cells and frequency converters made of potassium dihydrogen phosphate (KDP) and DKDP (Deuterated KDP). These optical components will be operated several times a year at fluences close to their Laser Induced Damage Threshold (LIDT) which may reduce significantly their lifetime and increase substantially the maintenance costs of the LMJ. In a global effort to reduce these costs we have designed the SOCRATE facility as a complete system for materials characterization, LIDT measurement and optics conditioning by laser to increase their lifetime. In this paper we examine the relevance of adapting the laser conditioning process to the bulk KDP quality. First the existence of heterogeneities in large KDP crystals is stressed; next the LIDTs in the different parts of the crystals using focused or collimated beams are compared. Finally we focus on the efficiency of the excimer conditioning process in the different growth sectors of KDP samples and demonstrate that for the current conditioning process the efficiency depends only weakly on the original material heterogeneities.
The laser damage resistance of optical coatings is a critical point for a large number of applications. However improving this resistance is often hard to obtain because of the large number of parameters in the deposition processes than can modify the laser damage threshold and the lack of detailed and exploitable studies published on this subject. Then, the aim of this work is to test and analyze the laser damage resistance of a usual material for high power applications (silica) deposited in various conditions. The thin films of different thicknesses were specially deposited using different techniques available at the Institut Fresnel: Dual Ion Beam Sputtering, Electron Beam Deposition, Ion Assisted Deposition and Ion Plating. The laser-induced damage thresholds of these coatings were determined at 1064nm and 355nm using nanosecond pulsed YAG lasers, with a 1-on-1 test procedure. Other diagnostic tools were used to complete the study and make potential correlations with laser damage: photothermal techniques, luminescence spectrocopy, optical profilometry, dark field and Nomarski microscopy. The comparative study of these results highlight different laser damage behaviors of the silica coatings that we correlate to the density and the nature of the defects.
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