A Petawatt facility called PETAL (PETawatt Aquitaine Laser) is under development near the LIL (Ligne
d'Integration Laser) at CEA Cesta, France. PETAL facility uses chirped pulse amplification (CPA) technique. This system needs large pulse compression grating exhibiting damage threshold of more than 4 J/cm2
normal beam at 1.053μm and for 500fs pulses. In this paper, we study an alternative design to the classic
multilayer dielectric (MLD) grating called "mixed
metal-multilayer dielectric grating" (MMLD). This design
consists in a gold reflective layer coated with a few pairs of HfO2/SiO2. The top low index SiO2 layer of the
stack is then engraved to receive the grating. We evidenced in a previous work that leads to high efficient
pulse compression gratings. We have shown in last Boulder Damage Symposium that mixed mirror is
equivalent to a "classic" MLD mirror. We herein detail the damage performances obtained on the MMLD
gratings and compare them with these of MLD gratings.
A Petawatt facility called PETAL (PETawatt Aquitaine Laser) is under development near the LIL (Ligne d'Integration
Laser) at CEA Cesta, France. PETAL facility uses chirped pulse amplification (CPA) technique [1]. This system needs
large pulse compression gratings exhibiting damage threshold of more than 4 J/cm2 beam normal at 1.053μm and for
500fs pulses. In this paper, we study an alternative design to the classic Multidielectric (MLD) gratings [2] called "mixed
metal-multidielectric grating" (MMLD). In this design, the dielectric mirror stack of the MLD grating is replaced by a
gold reflective layer covered with a few pairs of HfO2/SiO2 [3]. The number of pairs must be high enough to ensure a
sufficient reflection coefficient in order to prevent damage of the gold layer. On the top of the stack, a silica layer is
coated to receive the grating. After some considerations on the grating design and optimization, a comparison between
MLD and MMLD mirrors is also carried out. We finally detail the measured diffraction efficiencies obtained on MMLD
gratings.
The lifetime of silica optics in high power laser facility as the Laser MégaJoule (LMJ) is typically limited by the
initiation of surface damages and their subsequent growth. To prevent this problem, a mitigation technique is used: it
consists in a local melting of silica by CO2 laser irradiation on the damage site. Because of the difficulty to produce
efficient mitigated sites with large depth, the characterization of damage site to mitigate is very important. In this
context, confocal microscopy appears to be an efficient solution to detect precisely cracks present under the damage site.
The development of high power laser for large instruments such as LMJ and NIF, as well as the use of smaller optical structures with high densities of energy lead to consider laser induce damage threshold (LIDT) as a critical criterion in optical components development. Furthermore in the same time some applications need an increase of the lifetime of the laser source (up to 109 shots for spatial applications). In this context to improve optical components numerous studies are undertaken to determine the origin of laser damage
process. Through these studies, it is now commonly admitted that the first stage of the laser damage process in the nanosecond regime is caused by localized defects included in the material. In order to determine the laser damage threshold, it is necessary to perform a statistical study of damage on materials. This destructive technique beyond the determination of damage threshold allows to determine the density of precursor centers and to discriminate different kinds of precursors by using adapted beam sizes. However the nature and
therefore the origin of the defects remain unknown. In order to get information on this nature, non destructive tools have to be involved. Indeed on one hand photothermal microscopy permits to make measurement of the local absorption under irradiation, on the other hand, photoluminescence cartography and spectroscopy gives information on material composition. The coupling of the different techniques on a laser damage test set-up optimizes the chance to have a complete signature of precursor center and information about the mechanism of damage process. Furthermore, nondestructive diagnostic under mutilple irradiations permit to study "fatigue" and conditioning process.
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.
Pollution is known to play an important role in optical performances of high power laser components. In this study, we propose a method to characterize the particulate pollution and to evaluate its impact on the optical component lifetime. We applied it at the Ligne d'Intégration Laser (LIL), which is a prototype installation of the future Laser Mega Joule facility. By systematic examination of each particle, we obtain, in number, size, and chemical composition, an overview of pollution deposited on optical surfaces. Then we deduce the origin of the principal pollutions. After irradiation of the polluted samples, with conditions close to the maximum of the LIL operation, no laser-induced damage is observed. In contrast, irradiation induces an important cleaning.
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.
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).
To evaluate the impact of particulate contamination in laser induced damage of optical material, an
experimental program is established. The first step consists in the Ligne d'Integration Laser (LIL) particle
contamination sampling. Carbonated cellophane tapes, antireflection coated and uncoated silica samples were
inserted in the LIL laser chain, in six different zones to collect particles. The second step is the pollution
characterization. Polluted cellophane tapes are analysed by Scanning Electron Microscopy and Energy
Dispersive Spectrometry. The density and the nature of particles collected in the Amplification Section are
found to be homogenous throughout this section. The pollution collected in the Frequency Conversion and
Focusing system is more complex. One of its features is a larger proportion of silica particles. The last step
consists in the silica samples irradiation. Antireflection coated and uncoated silica samples are examined by
optical microscopy, then irradiated at 1064 nm or 355 nm and examined again. No damage growing under
several irradiations is observed. We show a cleaning effect efficient for particles larger than 20 microns.
For high-tech industries, such as semiconductor or optical ones, controls must be done not only on airborne particle contaminants in cleanroom and associated controlled environments but also on surface particle contamination. Optical components are leading technologies for particle contamination control with atomic scale resolution over large areas. The aim is to enhance production reliability on miniaturized systems. Finding a correlation between such airborne and surface particle contaminations is the challenge we have to meet. This paper presents a methodology in order to answer this specific question. For, the latter depends on the cleanroom activity, operating personnel, cleanroom lay-out and equipment for example. Theoretically, we can calculate the surface particle contamination based on deposition velocities and deposition rates of airborne particle contamination defined in ISO 14644-1 for very simple cases. The reality is much more complex and the whole methodology presented here is based on experimental complementary measurements. The characterization of particle surface contamination is done mainly by light microscopy measurements and by optical particle counters...Complementary chemical data are obtained thanks to electronic microscopy with X-ray spectroscopy.
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.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.