Spallation caused by shock waves in optical components such as those used in the Laser MegaJoule facility during laser operation leads to material fracture during a Laser-Induced damage event. One solution may be to use a viscoelastic thin film on these components to mitigate spallation, but it must have excellent optical, mechanical, and resistance to laser damage properties. Among the viscoelastic materials investigated were Nafion and polydimethylsiloxane-based Ormosil. These materials, as thin films deposited on a fused silica substrate, were studied under nanosecond pulsed lasers at 1064 and 532 nm with different diagnostics in situ and post-mortem. In particular, the effect of the films on spallation was studied using the laser shock technique. Preliminary results showed that these thin films have interesting properties that could help to reduce mechanical damage to optical components.
Spallation effects caused by shock waves in optical components such as those used in the Laser MegaJoule facility during laser operation can lead to material fracture. One solution could be to use a viscoelastic thin film on these fused silica components to reduce the reflection of shock waves from the rear surface, but it must have excellent optical, mechanical, and power-handling properties. Among the viscoelastic materials investigated were Nafion and polydimethylsiloxane-based ormosil, with ormosil synthesized using a sol-gel process. The materials were characterized optically and especially tested for acoustic attenuation. These materials, as thin films deposited on a fused silica substrate, were studied under shock wave propagation using the laser shock technique. Preliminary results showed that these thin films have interesting properties that could help reduce mechanical damage to optical components.
The MegaJoule Laser (LMJ) for inertial confinement fusion experiments is currently in operation at CEA-CESTA in
France. All the lenses are coated by an antireflective (AR) layer to optimize the light power transmission. This AR layer
is manufactured by sol-gel process, a soft chemical process, associated with a liquid phase coating technique to realize
thin film of metal oxide. These optical components are hardened into ammoniac vapors in order to mechanically
reinforce the AR coating and to make them more handling. This hardening induces a thickness reduction of the layer so
an increase of the stiffness and sometimes a crazing of the layer. As these optical components undergo a high-power
laser beam, so, it is important to verify if the AR properties (optical and mechanical) influence the value of the threshold
laser damage. A series of coated samples have been manufactured having variable elastic moduli to discuss this point. In
that purpose, a homemade Laser Induced Damage Threshold (LIDT) setup has been developed to test the layers under
laser flux. We describe the used methods and different results are given. Preliminary results obtained on several coated
samples with variable elastic moduli are presented. We show that whatever are the elastic stiffness of the AR coating, an
overall decrease of the threshold appears with no noticeable effect of the mechanical properties of the AR coatings.
Some possible explanations are given.
The dynamics of electrons and holes in potassium dihydrogen phosphate ( KH2PO4 or KDP) crystals and its
deuterated analog (KH2PO4 or DKDP) induced by femtosecond laser pulses is investigated at λ = 800nm. To
do so, experiments based on a femtosecond time-resolved interferometry technique have been carried out. It
is shown that two relaxation dynamics exist in KDP and DKDP crystals. In particular, it appears that one
dynamics is associated with the migration of proton/deuteron in the crystalline lattice. Both of the dynamics
correspond to physical mechanisms for which the multiphoton order required to promote valence electrons to
the conduction band is lower than the one of a defect-free crystal. These results suggest the presence of states
located in the band gap that may be due to the presence of defects existing before any laser illumination or
created in the course of interaction. In order to interpret the experiments, a model based on a system of rate
equations has been developed. Modeling results are in good agreement with the experimental data, and allow
one to obtain fundamental physical parameters governing the
laser-matter interaction as multiphoton absorption
cross sections, capture cross sections, recombination times, and so forth. Finally, it will be shown how these
results can be used to the understanding of laser-induced damage by nanosecond pulses in inertial confinement
fusion class laser aperture.
The controls by optical mean of coatings deposited on optical components are generally made with flat witnesses. When
the components are spherical or aspherical, like lenses or mirrors, the spectral response may vary because of the
nonuniformity of thickness that is really linked to the deposition process. For large radius of curvature, control can be
achieved even with classical spectrophotometers. However, control becomes more and more difficult when the radius of
curvature decreases or when the optical device has a complex shape. Thus special devices are needed to perform this
kind of measurement. ZEISS and CEA Le Ripault use spectral reflection as a mean of measurement, which enables to
investigate optical coatings on curved parts. Two different devices have been implemented and used to measure the same
antireflective coating deposited on an aspheric lens. In this work, we show the obtained results and we compare these
results to theoretical simulation.
The controls by optical mean of coatings deposited on optical components are generally made with flat witnesses. But
when the components are spherical or aspherical, like lenses or mirrors, the spectral response can vary because of the
nonuniformity of thickness that is really linked to the deposition process. For large radius of curvature, control can be
achieved even with classical spectrophotometers. However, control becomes more and more difficult when the radius of
curvature decreases or when the optical device has a complex shape such as slicers for example. So to perform this kind
of measurement, special devices are needed.
The CEA Ripault has designed a new facility of measurements of spectral reflection. This reflectometer can be used to
measure optical coating with a very high accuracy on steeply curved parts. The aim of this paper is to enhance the limits
of this device by studying measurement uncertainty and giving some examples of measurement. One of our most
relevant measurements is the study of an aspheric condenser having a 38 mm focal length. Furthermore, the obtained
reflectivity on an angle iron will be achieved and commented. Soon, WINLIGHT SYSTEM Company will manufacture
this device.
Potassium dihydrogen phosphate (KH2PO4 or short KDP) is one of the major nonlinear optical crystals for frequency
conversion and electro-optic switching in high power lasers. In particular, this material has been chosen for the
frequency converters of the Laser Mega Joule in France and the National Ignition Facility in the US. These laser work
close to the damage threshold of the crystals and large efforts have been provided to improve the laser induced damage
threshold for KDP at different wavelength.
We present in this paper first results of a new setup dedicated to the correlation of non destructive luminescence
spectroscopy and destructive laser damage tests. We concentrate on the differences between conventionally grown KDP
and KDP-crystals that have been produced by the rapid growth method that has been developed in the last years
especially for the large laser installations LMJ and NIF. Different photoluminescence spectra are obtained from
conventionally and rapidly grown KDP for both pump configurations: (i) pulsed pumping by the forth harmonic of a
Nd:YAG laser at 266nm, and (ii) continuous pumping using a frequency doubled Argon ion laser at 244nm.
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.
In this paper, we present different procedures of laser conditioning realized on KDP doubler crystals. First, components
are treated either with an excimer laser (SOCRATE facility, 351 nm, 12 ns) or a Nd: YAG laser (MISTRAL facility,
355 nm, 7 ns). Then damage tests are performed at 2ω (532 nm - 5 ns BLANCO facility) and 3ω (355 nm - 2.5ns
LUTIN facility) in order to estimate the conditioning gain for these two wavelengths.
For the best procedures, results show that it is possible to increase laser damage threshold at 532 nm so that it becomes
compatible with the nominal specifications of the LMJ. Moreover, tests realized at 355 nm highlight also an
encouraging improvement for the laser conditioning of tripler crystals.
This study is concerned with the identification of the defects responsible for laser damage observed on
KDP/DKDP frequency triplers used in high power lasers. We reported at BDS 2005 a non destructive high energy X-ray
topographic setup able to characterize lattice imperfections in optics. Results obtained using this technique on KDP and
DKDP crystals are reported and discussed. The influence of each type of defect, observed or likely to exist in optics, is
discussed in light of damage mechanisms recently published. Finally, an experimental setup presumably able to reveal
those defects is proposed.
A thermal model is considered to better understand Laser-Induced Damage and conditioning mechanism in
KH2PO4 (KDP) and D2xKH2(1-x)PO4(DKDP) crystals. We mainly focus on two points, the probed volume of
the laser beam and the optimization of the conditioning process. Our predictions are in agreement with recent
experimental data.
In this paper we examine how optical techniques can be used for impurities and defects detection in KH2PO4 (KDP)
components. This is important in so far as some of these defects are responsible for a weaker than expected laser-induced
threshold in these materials. Photothermal deflection, polariscopy, fluorescence and photoexcitation are
investigated with the aim of localizing and identifying the laser-induced damage precursors. Impurities concentration
is measured directly by ICP-AES and Fe is accordingly checked to be at the origin of a higher absorption in the
prismatic sectors of rapidly grown KDP crystals. We also exhibit a fluorescence signal in the near-ultraviolet range
by pumping at 248 nm; in rapidly grown crystals, in the same way as iron, the incorporation rate of the fluorescent
centers is shown to depend on the growth sector.
The reflector's specifications of amplifying section of LMJ need to have spectral high-precision reflectance measurements. The innovative solution proposes to increase the precision of reflectance measurements, and to enable measure of pieces in form like spherical mirrors. This activity has been beginning for months and improvements have been performed.
Laser diodes are not normally able to cut paper, since plain paper does not absorb light in the visible and in the Near Infrared. However, it is possible to make paper locally absorbing to the Near Infrared laser light by ink-jetting an ink formulated from Near Infrared absorbing dyes. Different inks are found suitable for the ink-jetting process using commercial printers. The absorbing characteristics of inked paper are reported and discussed. Absorption characteristics are checked to allow laser cutting using a 1 W diode. Several applications of this technology are presented and discussed, with cutting speeds up to several tens of meters per minutes.
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.
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.
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.
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.
Laser cutting of paper is widely used in the paper conversion industry. CO2 lasers are well suited for this type of applications. Desktop printing is a large market both for digital photography, document management and graphics applications, but it still lacks advanced cutting and scoring ability, and CO2 lasers seem costly to be integrated in mass-market printers. For that reason, mass-scalable and low-cost semiconductor laser diodes would be very advantageous to add paper cutting and scoring features in desktop printers. However, common paper can not be cut properly using visible or Near Infrared (NIR) laser diode since it has a very poor absorption at these wavelengths. We report here an innovative solution to achieve paper cutting or scoring using a 1 W single emitter NIR laser diode, within an inkjet printer. A special ink that absorbs the NIR light, and that penetrates all through the paper, is first disposed on the lines to be cut. Then, the laser diode goes along the lines to be cut. We show that a cutting speed of 2m/min can be achieved on 80g/m2 conventional paper. The influence of the optical properties of the ink on the cutting speed are discussed, as well as focussing issues. In particular, we show that invisible inks are suitable, and very clear-cut edges can be obtained. The perspective of this technique are discussed.
We investigated the crystals with different non-destructive optical diagnostics during the conditioning of KDP with an excimer laser at 351 nm. We measured in the same time the luminescence, the absorption and bulk scattering of the material. These observations pointed out the defects within the crystal. We demonstrated a correlation of the optical signal intensity with the laser damage threshold of KDP.
We perform thne conditioning of various KDP crystals with a XeF excimer laser working at 351 nm. We determine the maximum available excimer laser fluence for conditioning without damage initiation within the crystal. We demonstrate enhancement of the damage resistance with the increase of the cumulative excimer laser fluence. Using the conditioning parameters we show that the damage resistance is also dependent on the crystalline orientation of the KDP samples.
In order to increase the laser induced damage threshold of KDP crystal, a well-known solution consists in a laser conditioning process. In our case, the irradiation of the crystal is performed with an excimer laser XeF (λ = 351 nm, 16 ns). The improvements in laser damage thresholds measured at CEA/CESTA laboratory (Lutin, Yag facility 2.5 ns, parallel beam) and at CEA/Ripault laboratory (Excimer facility 16 ns, focused beams) are different. A possible reason to explain this difference is the depth of focus between both facilities. In order to minimize the influence of limited depth of focus, a solution consits in a multi-plane conditioning process. By means of a local study, it is possible to exhibit with a high accuracy the Laser Induced Damage Threshold (LIDT) in different planes along sample irradiation axis (z-axis). The laser damage threshold is measured locally (8 μm) at 355 nm with a Nd:Yag (pulse duration 7 ns) at Fresnel Institute Marseille.
Using the local LIDT measurements, the purpose of this paper is to highlight the depth of focus in the excimer conditioning process. We demonstrate that it is possible to exhibit a local increase in the conditioning gain till a maximum value, measured with the excimer laser.
A new facility has been designed to enhance laser damage resistance at 351 nm of large scale 3w KDP and silica optics by laser treatment. This facility is a prototype, and the process will be industrialized as a means of fabrication of the LMJ optics. The first step of the process is a conditioning/initiation step, which consists of a UV laser raster scan of the whole optics; the second step is a step of detection and analysis of damage possibly initiated during the previous step; the third step is a mitigation step, which consists of a local melting of the detected damage on silica surface, in order to stop their growth. The facility is equipped with a 3w Nd:YAG laser allowing the process of both KDP and silica. A CO2 laser is used for damage mitigation. Both the lifetime increase and the reduction of the process duration of large scale optics have been taken into account with a view to industrialize the process.
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.
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.
This paper reports on the fabrication of prismatic microstructures on polymer films by a combination of KrF laser micromachining and hot embossing. Typical values of depth and pitch of prisms lie between 50 and 200 micrometers . Optical properties of such polymer surface sin the mid-IR are based on a geometrical optics mechanism. The UV ablation of V channels is achieved by projecting a (Delta) shaped mask onto the workpiece. Both poly(methylmethacrylate) and polyimide plates were machined. THe dependence of depth of grooves on mask shape, laser fluence, ablation rate of material, positioning table velocity and process gas has been investigated for both polymers. At 248 nm, the higher ablation rate of PMMA as compared to polyimide result in a close to 10-fold reduction in processing time. IR transparent polyethylene, films were replicated by hot embossing of either the polyimide master or a nickel shim electroformed on PMMA. Surface profilometry, optical microscopy and SEM measurements revealed that embossed PE replicas were produced with high fidelity. The angular distribution of IR transmissivity of the PE surface relief has been measured at 10.6 micrometers . Experimental results were compared to those calculated.
Optics used in a plant scale SILVA laser system will support high average power laser beam in the visible spectrum (> 1 kW). The losses and resistance to optical stress of the various dielectric multilayer coatings have to be improved, thermal effects quantified and reduced, so specific test benches have been developed. The applied methods will be briefly commented and the various benches described and some examples reported.
High average power laser systems require optical components with increasingly higher performances: high laser damage resistance, high reflectance or transmittance. We report here some results obtained with TiO2/SiO2 and ZrO2/SiO2 low losses mirrors designed for copper vapor lasers. Very low losses (40 ppm) are obtained with TiO2/SiO2; losses down to 400 ppm and laser damage thresholds up to 35 kW/cm2 are obtained with ZrO2/SiO2.
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