We investigate the feasibility of a full-silica transmission grating acting as a quarter-wave plate at the wavelength of 351 nm as an alternative to an anisotropic crystal. We report on the design, manufacturing process and optical characterization carried out. We evidence the possibility to obtain a full-silica component exhibiting at 351 nm a high damage threshold and a phase retardance of /4 associated with a diffraction efficiency above 98%.
We investigate the feasibility of a full-silica transmission metasurface exhibiting optical properties equivalent to a quarter-wave plate at the wavelength of 351 nm as an alternative to anisotropic crystals. We report the design, manufacturing process and optical characterization of full-silica quarter waveplates. We evidence the possibility to obtain a full-silica component exhibiting at 351 nm a high damage threshold and a phase retardance of λ/4 associated with a transmission efficiency higher than 95%. Such an optical component could offer a great alternative to birefringent materials for the manipulation of polarization of high-energy laser beams.
The Laser Megajoule facility, developed by the CEA is based on 176 Nd:glass laser beams focused on a micro-target positioned inside a 10-meter diameter spherical chamber. The facility will deliver a total energy of 1.4MJ of UV light at 0.35 μm and a maximum power of 400 TW. A specific petawatt beam, PETAL, offers a combination of a very high intensity beam, synchronized with the nanosecond beams of the LMJ. This combination allows expanding the LMJ experimental field in the High Energy Density Physics (HEDP) domain. Since September 2021, a major project milestone has been achieved with the commissioning of the half LMJ (88 beams are fully operational with 10 heating bundles of 8 beams and a specific bundle for plasma diagnostics purpose). The installation and the commissioning of new laser bundles and new plasma diagnostics around the target chamber are continuing, simultaneously to the realization of plasma experiments. Another project milestone has been achieved at the end of 2021, with a dedicated laser experiment in the facility to explore the Power-Energy Diagram.
Laser MegaJoule (LMJ) is a high energy laser facility designed for fusion experiments. To track final optics damage, laser damage monitoring is carried out using images acquired by a camera. To prepare for the LMJ full energy/power operation, the damage models based on the phenomenological laws established in the laboratories are validated by experimental campaigns dedicated to performance. The very high quality of LMJ optics surfaces makes damage highly unlikely. In order to take the greater benefit of these performance campaigns, carried out on a reduced number of laser shots and components, a matrix of nearly 1000 damage sites is initiated offline on one optics. Precisely measured on a metrology bench before and after the campaign, this component was on LMJ facility during a performance campaign at the end of 2021. Very useful for the calibration of the LMJ monitoring camera, it also provided data to set LMJ laser damage models at higher energy level.
High energy laser facilities designed for fusion experiments, such as Laser MegaJoule or National Ignition Facility, are limited by laser-induced damage on their final optics. Accurate and early detection of damage growth is required for successful operation of such facilities. Since the image resolution is about the size of damage sites to monitor, diameter measurements are not sufficient to meet the objectives of damage growth quantification. An accurate size quantification of damage sites is based on light scattering measurements after time-consuming calibrations on the facility. An optical model is proposed to perform a simple and fast calibration of the measurements by numerical simulation. The model is based on light scattering measurements of several damage sites combined with optical simulations of the lighting system.
We investigate the feasibility of a full-silica transmission grating acting as a wave plate at the wavelength of 351 nm as an alternative to an anisotropic crystal. We report on the design, manufacturing process and optical characterization carried out. We evidence the possibility to obtain a full-silica component exhibiting at 351 nm a high damage threshold and a phase retardance ranging between lambda/5 and lambda/4 associated with a diffraction efficiency above 94%.
Final fused silica optics of high energy fusion class laser facilities are one of the components that limit the UV laser energy available for experiments. These final optics suffer from laser-induced damage. Some solutions are available to limit laser damage growth and to increase optics lifetime. However, to use them, it is necessary to be able to detect damage initiation as soon as possible, and to follow damage growth efficiently. An imaging system and a lighting source make the observation of laser damage sites possible after each laser shot without removing the optical components. Laser damage detection algorithms exist but they are not sufficiently efficient to provide reliable monitoring of damage growth over time because of small repositioning fluctuations of the optical system. An effective solution based on digital image correlation and brightness/contrast corrections is proposed to detect and follow laser damage sites as soon as they initiate in an automatic way. The effectiveness of the presented method is compared to the widely used method that is based on the analysis of local signal-to-noise ratio.
The Laser Megajoule facility, developed by the CEA is based on 176 Nd:glass laser beams focused on a micro -target positioned inside a 10-meter diameter spherical chamber. The facility will deliver a total energy of 1.4MJ of UV light at 0.35 μm and a maximum power of 400 TW. A specific pétawatt beam, PETAL, offers a combination of a very high intensity beam, synchronized with the nanosecond beams of the LMJ. This combination allows expanding the LMJ experimental field in the High Energy Density Physics (HEDP) domain. Since October 2019, 56 beams are fully operational (7 bundle of 8 beams). The installation and the commissioning of new laser bundles and new plasma diagnostics around the target chamber are continuing, simultaneously to the realization of plasma experiments. A major project milestone has been achieved at the end of 2019, with the first experiment in the facility involving neutron production, through D-D reaction in a D2 capsule inside a gold rugby cavity. The next major milestones for LMJ will take place at the end of 2021 with the commissioning of the half LMJ (10 heating bundles of 8 beams and a specific bundle for plasma diagnostics purpose). The full presentation will describe the software environment used for the laser operation, the first results on the laser damages using our 3w optical components inspection system, the laser damages analysis software, the system of spot blocking, and the last performances obtained with the PETAL beam.
A novel original method is presented to detect and track laser damage sites on vacuum windows of the Laser MegaJoule (LMJ) facility. The method is based on spatial registration by Digital Image Correlation (DIC). It also involves corrections for gray level variations induced by variable lighting conditions. Using the present method, an efficient way is achieved to detect and follow laser damage sites as soon as they appear on the optical component. The developed tools offer the possibility of characterizing and predicting damage growth as a function of laser shot features.
The Laser MegaJoule (LMJ) is a 176-beamlines facility, located at the CEA CESTA near Bordeaux (France). It is designed to deliver about 1.4 MJ of ultraviolet laser energy on targets set in vacuum chamber, for high energy density physics experiments, including fusion experiments. The commissioning of the seven first bundles of height beams is achieved since November 2019 and the commissioning of next bundles is on the way. For performance requirements, it is important to follow final optics behavior. Moreover, for questions of manufacturability, ease of maintenance and cost, the understanding and the improvement of vacuum windows laser damage resistance are of main importance. The MDCC (Center Chamber Diagnostic System) is thus operating since November 2018 on the LMJ facility. It consists in a high resolution CCD camera combined with a predefined focus set of optics. The resolution of this system is about 100μm with a working distance of 8 m. This system can perform 3 functions: damage detection on the vacuum window surface, the measurement of the spatial profile on the vacuum window plane and of final optics transmission.
In this work, we propose a method to discriminate between upper and lower side material removal during double side polishing of fused silica parts. It consists in engraving cone-shaped craters on the two sides, then measuring the profile of each crater before/after a polishing run. The comparison of the profiles leads to the thickness removed on each side during the run. The craters have been engraved using a CO2 laser and their profiles measured thanks to a nano-scratcher. We have evaluated that this method can determine material removal with an accuracy of about 1μm, is insensitive to a part repositioning error under the tip of about 35μm, and has a repeatability of 0.5μm. Finally, we have been able to measure effective removal differences of 2μm between the two sides.
Laser damage resistance is a key factor for the improvement of high power laser system. The PETAL laser, developed by the CEA-CESTA (France), uses meter scale reflective optics to compress, transport and focalize sub-picosecond laser pulses at 1053nm with high-energy [1]. In the case of defect-free material, laser-induced damage in the sub-picosecond regime is known to be deterministic since the threshold depends only on the electronic structure of the irradiated materials, the pulse duration and the enhancement of the electric fields in thin film coatings. Based on this consideration, a mono-shot technique has been investigated to assess the intrinsic damage resistance of optical component with only one laser shot. On the other hand, while considering real optical components, manufacturing processes included nanoscale defects in the functional coating. These defects can be ejected when irradiated and strongly reduce the laser damage resistance of optics: rasterscan procedure has then been developed to determine defect-induced damage densities. These densities are found to be high even for fluences well below the intrinsic Laser-Induced Damage Threshold and they increase with the fluence. These experiments bring new information on the operating characteristics of optics in short pulse regime. Once damage is triggered, its evolution under subsequent irradiations has also been studied. Growth experiments have been compared to numerical simulations. The investigations on growth behavior allow a better estimation of the functional lifetime of an optic in its operating conditions. The whole of results, damage initiation and damage growth, is discussed to the light of the laser damage observed on PETAL optics.
We have selected and characterized 2 cerium oxide slurries. We have then modified their pH to polish fused silica samples. The material removal rate, roughness, surface defects density and morphology have been observed as a function of pH. We noticed that while roughness and surface defect density don’t seem to be very affected by slurry pH, the latter has an influence on material removal rate and width of the scratches generated during polishing.
Laser damage resistance is a key factor for the improvement of high power laser system. The PETAL laser, developed by the CEA-CESTA (France), uses meter scale reflective optics to compress, transport and focalize sub-picosecond laser pulses at 1053nm with high-energy [1]. In the case of defect-free material, laser-induced damage in the sub-picosecond regime is known to be deterministic since the threshold depends only on the electronic structure of the irradiated materials, the pulse duration and the enhancement of the electric fields in thin film coatings. Based on this consideration, a mono-shot technique has been investigated to assess the intrinsic damage resistance of optical component with only one laser shot. On the other hand, while considering real optical components, manufacturing processes included nanoscale defects in the functional coating. These defects can be ejected when irradiated and strongly reduce the laser damage resistance of optics: rasterscan procedure has then been developed to determine defect-induced damage densities. These densities are found to be high even for fluences well below the intrinsic Laser-Induced Damage Threshold and they increase with the fluence. These experiments bring new information on the operating characteristics of optics in short pulse regime. Once damage is triggered, its evolution under subsequent irradiations has also been studied. Growth experiments have been compared to numerical simulations. The investigations on growth behavior allow a better estimation of the functional lifetime of an optic in its operating conditions. The whole of results, damage initiation and damage growth, is discussed to the light of the laser damage observed on PETAL optics.
Effects of deep wet etching on the surface quality and the laser induced damage probability have been studied on fused
silica samples. Results obtained with a HF/HNO3 solution and a KOH solution were compared on both polished pristine
surface and scratched surfaces. The hydrofluoric solution radically deteriorated the surface quality creating a haze on the
whole surface and increasing considerably the roughness. For both solutions, neither improvement nor deterioration of
the laser damage performances has been observed on the etched surfaces while the laser damage resistance of scratches
has been increased to the level of the surface. We conclude that laser damage performances are equivalent with both
solutions but an acid etching induces surface degradation that is not experienced with basic etching.
Based on an experimental system that can be used for simultaneous laser damage testing and time-resolved acquisition of intensity and phase images, we describe different experiments related to the study of laser damage process in the sub-picosecond regime. We report firstly on quantitative measurement of the Kerr effect in a fused silica substrate at fluences closed to the Laser Induced Damage Threshold. Then we study the damage initiation process in optical coatings, linked to intrinsic properties of the materials, and the dynamics of free electron generation and relaxation. At last, damage growth sequences are analyzed with time-resolved microscopy in order to understand laser damage growth in the case of High Reflective mirrors.
Secondary Ion Mass Spectroscopy (SIMS), Electron Probe Micro Analysis (EPMA) and X-Ray Photoelectron
Spectroscopy (XPS) were used to analyze the polishing induced contamination layer at the fused silica optics surface.
Samples were prepared using an MRF polishing machine and cerium-based slurry. The cerium and iron penetration and
concentration were measured in the surface out of defects. Cerium is embedded at the surface in a 60 nm layer and
concentrated at 1200 ppmw in this layer while iron concentration falls down at 30 nm. Spatial distribution and
homogeneity of the pollution were also studied in scratches and bevel using SIMS and EPMA techniques. An
overconcentration was observed in the chamfer and we saw evidence that surface defects such as scratches are specific
places that hold the pollutants. A wet etching was able to completely remove the contamination in the scratch.
A rasterscan procedure is set to determine laser-induced damage densities in sub-picosecond regime at 1053nm on high-reflective coatings. Whereas laser-induced damage is usually considered deterministic in this regime, damage events occur on these structures for fluences lower than their intrinsic Laser-Induced Damage Threshold (LIDT). Damage densities are found to be high even for fluences as low as 20% of the LIDT. Scanning Electron Microscope observations of these “under threshold” damage sites evidence ejections of defects, embedded in the dielectric stack. It brings a new viewpoint for the qualification of optical components and for the optimization of manufacturing processes of coatings.
We have selected three colloidal silica slurries and compared their ability for fused silica finishing, by varying the load and slurry concentration. We found that the process parameters can affect differently the finishing efficiency, roughness and surface defects density, depending on the slurry considered.
While considering long pulse or short pulse high power laser facilities, optical components performances and in particular laser damage resistance are always factors limiting the overall system performances. Consequently, getting a detailed knowledge of the behavior of these optical components under irradiations with large beam in short pulse range is of major importance. In this context, a Laser Induced Damage Threshold test facility called DERIC has been developed at the Commissariat à l’Energie Atomique et aux Energies Alternatives, Bordeaux. It uses an Amplitude Systemes laser source which delivers Gaussian pulses of 500 fs at 1053 nm. 1-on-1, S-on-1 and RasterScan test procedures are implemented to study the behavior of monolayer and multilayer dielectric coatings.
Laser induced damage of optical components is often a limiting factor for the development of high power lasers. Indeed,
for many years, organic contamination is identified as a factor decreasing the laser induced damage threshold of optical
surfaces, limiting the use of high fluencies. Also, for the development of its laser facilities, Laser MégaJoule and
PETawatt Aquitaine Laser, the Commissariat à l’Energie Atomique et aux Energies Alternatives investigates the
influence of organic contamination on the performances of the optical components. Actually, although great care is
provided on the cleanliness of the optics, organic volatile compounds outgassed from surrounding materials can be
adsorbed by the sensitive surfaces during its timelife. Thus, for this study, performances of clean and contaminated
multilayer dielectric mirrors are compared. Contamination is intentionally realized either by controlled protocols or by
exposing optics inside the laser facilities. Qualification and quantification of the organic contamination is realized by
automated thermal desorption and gas chromatography coupled with mass spectrometry. Laser induced damage
threshold of clean and contaminated mirrors are then investigated by 1053 nm laser at 670 fs.
We investigate the capacity of magnetorheological finishing (MRF) process to remove surface and subsurface defects of fused silica optics. Polished samples with engineered surface and subsurface defects were manufactured and characterized. Uniform material removals were performed with a QED Q22-XE machine using different MRF process parameters in order to remove these defects. We provide evidence that whatever the MRF process parameters are, MRF is able to remove surface and subsurface defects. Moreover, we show that MRF induces a pollution of the glass interface similar to conventional polishing processes.
The MegaJoule laser being constructed at the CEA near Bordeaux (France) is designed to focus more than 1 MJ of
energy at 351 nm, on a millimetre scale target in the centre of an experiment chamber. The final optic assembly of this
system operating at a wavelength of 351 nm is made up of large fused silica optics, working in transmission, that are
used to convey and focus the laser beam. Under high fluences (i.e. more than 5 J/cm2 for 3 ns pulses), the limited lifetime of final optical assembly is a major concern for fusion scale laser facilities. Previous works have shown that surface finishing processes applied to manufacture these optical components can leave subsurface cracks (SSD), pollution or
similar defects that act as initiators of the laser damage. In this work, we used epi-fluorescent light scanning microscopy
(ELSM) and Stimulated Emission Depletion (STED) in confocal mode with fluorescent dye tagging to get a better
knowledge of size and depth of these subsurface cracks. Magnetorheological fluid finishing technique (MRF) was also
used as a tool to remove these cracks and thus assess depths measured by confocal microscopy. Subsurface cracks with a
width of about 120 nm are observed up to ten micrometers below the surface.
Relationships between subsurface damage (SSD) depth and peak to valley surface roughness (Rt) have been widely studied and present a major interest for an easy assessment of the SSD depth. We seek the relation between SSD depth and other surface roughness parameters using the Abbott-Firestone curve on a large campaign of grinding tests (with different abrasive grain size, grinding speed and grinding mode). The results reveal that the Abbott-Firestone parameter Mr2, which can be linked to the volume fraction of valley in the roughness profile, is more accurate than Rt for an assessment of the SSD depth and that the relationship between Mr2 and the SSD depth varies when changing the grinding mode.
The MegaJoule laser being constructed at the CEA near Bordeaux (France) is designed to focus more than 1 MJ of energy of UV light, on a millimeter scale target in the centre of an experiment chamber. After amplification and transport at the wavelength of 1053 nm, frequency conversion at 351 nm is done with KH2PO4 crystals. The final optic assembly of this system is made up of large fused silica optics, working in transmission, that are used to convey, focus or shape the laser beam. When exposed to fluences of some joules per square centimeter at 351 nm within nanosecond pulse duration, fused silica optics can exhibit localized damage. Damage sites grow exponentially after further laser exposition and therefore dramatically limit the optic lifetime. The nature of the surface finishing process has been established to determine the lifetime of these components under high UV fluences (i.e. more than 5 J/cm2 for 3 ns pulses). Being able to reduce or eliminate the damage initiators such as subsurface cracks present in subsurface damage (SSD) layer of conventionally polished optical components aims this study. Magneto-rheological fluid finishing (MRF) is chosen as a final polishing tool to remove layers of material without inducing further damages. MRF enables to process optics with very small normal stresses applied to the surface during material removal and thus permits the elimination of the residual subsurface cracks. This study offers a better understanding of the efficiency of MRF polishing on the elimination of subsurface cracks in SSD layers.
The Laser Mégajoule (LMJ) facility has about 40 large optics per beam. For 22 bundles with 8 beams per bundle, it will contain about 7.000 optical components. First experiments are scheduled at the end of 2014. LMJ components are now being delivered. Therefore, a set of acceptance criteria is needed when the optical components are exceeding the specifications. This set of rules is critical even for a small non-conformance ratio. This paper emphasizes the methodology applied to check or re-evaluate the wavefront requirements of LMJ large optics. First we remind how LMJ large component optical specifications are expressed and we describe their corresponding impacts on the laser chain. Depending on the location of the component in the laser chain, we explain the criteria on the laser performance considered in our impact analyses. Then, we give a review of the studied propagation issues. The performance analyses are mainly based on numerical simulations with Miró propagation simulation software. Analytical representations for the wavefront allow to study the propagation downstream local surface or bulk defects and also the propagation of a residual periodic aberration along the laser chain. Generation of random phase maps is also used a lot to study the propagation of component wavefront/surface errors, either with uniform distribution and controlled rms value on specific spatial bands, or following a specific wavefront/surface Power Spectral Distribution (PSD).
The lifetime of optical components submitted to high laser fluences is decreased under organic contaminated
environment. Our previous studies have shown that chemical species outgassed from materials present in the laser
environment of the Ligne d’Intégration Laser (LIL) and in the optics packaging (phthalates, silicones, and aromatic
compounds) are potential contaminants for optics. In order to avoid the presence of such molecules in the Megajoule
Laser (LMJ) environment, a new comprehensive program is started up using a qualified Micro-chamber/Thermal
Extractor (M-CTE250 Markes International) for controlled contaminations of optics. The final target is the development
of a qualification procedure to determine the compatibility of materials used for the building of the LMJ with the LMJ
optics. First results of this program will be presented.
Photoluminescence excited by 325 nm laser light is used to investigate defect populations existing in different surface
flaws in high purity fused silica and to achieve a better understanding of laser damage mechanisms. Luminescence bands
peaking at 1.9, 2.1, 2.3, 2.7 and 3.1 eV have been detected in the spectral area ranging from 1.6 up to 3.6 eV. According
to the literature, the 2.3 eV band would be due to STE's (Self Trapped Excitons) relaxation. In order to study this
hypothesis, temperature dependent experiments have been driven in the 90 K-300 K range. For indentations as well as
laser damage, we show the evolution of luminescence spectra with temperature. Contrarily to the well known behavior
of STE's, which shows a change of several orders of magnitude for luminescence intensity, the 2.3 eV band is weakly
influenced by temperature decrease, from the ambient down to 90 K. The Gaussian decomposition of spectra allows
dividing the five luminescence bands in two categories. The first one corresponds to bands showing a significant
intensity enhancement with temperature decrease, and the second one to bands remaining insensitive to the fall in
temperature. That classification may provide helps in order to establish links between luminescence bands and defects.
A Petawatt facility called PETAL (PETawatt Aquitaine Laser) is under development near the LMJ (Laser MegaJoule) at
CEA Cesta, France. PETAL facility uses chirped pulse amplification (CPA) technique. This system needs large pulse
compression gratings exhibiting damage threshold of more than 4 J/cm2 in normal beam at 1.053μm and for 500fs
pulses. In this paper, we present our recent progress and developments of such pulse compression gratings. We have
shown in previous works that the enhancement of the near electric field inside the pillars of the grating drives the damage
threshold. This was evidenced from a macroscopic point of view by laser damage testing. We herein demonstrate that
damage morphology during damage initiation at the scale of the grating groove is also consistent with this electric field
dependence. Some recent grating designs will also be detailed.
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.
Recent works have shown that for low contaminants level, damage density is independent of the amount of
contaminants. In these conditions, sub-surface defects (cracks), generated along the optical process, is considered as the
main source of damage. Hence, efforts have been made to improve SSD measurement in order to improve its suppression
during industrial process. We have developed a method to measure SSD depth which is detailed in this presentation. This
method is based on successive acid etching steps. The principle is to establish contamination level (ICP-AES
measurement) as a function of etched thickness of SiO2. The experimental setup has been specially designed to minimize
contaminations, reduce hydrofluoric acid quantities and to ease the etch rate determination. SSD depth is given by the
asymptotic impurities. This method has been applied to a grinded fused silica intentionally doped in barium tracer.
Results have been successfully compared to other characterization techniques such as MRF dimpling or empirical law
correlating SSD and surface roughness.
Subsurface cracks in fused silica optics are known to be damage initiators under laser irradiation. Each step of optic
production, from sawing to polishing, creates its own type of cracks. An efficient optic manufacturing process requires
that each production step removes cracks from the previous step. The extent of cracks has to be measured for each
production step. We review and compare different subsurface damage (SSD) characterization techniques applied to
ground and fine ground fused silica samples.
The impact of molecular contamination on the lifetime of fused silica UV optics used in high power laser facility is
studied. In our particular case corresponding to Laser MégaJoule (LMJ) beams, the irradiation conditions are a fluence
higher than 10 J/cm2 at a wavelength of 351 nm (3 ω) for 3 ns pulse duration and a single shot/day frequency. A confine
environment, a long period of exposition and proximity of the optical components with outgassing materials are critical
parameters for the optics contamination. Consequently, experiments were performed in the UV section of the Ligne
d'Intégration Laser (LIL), actual prototype of the LMJ. Moreover, the optics storage conditions were studied. Indeed, to
ensure an efficient replacement of the optics on the laser bundle, many optical components are fabricated long before.
They are stored during months in polypropylene frames put in containers. Then, we intentionally contaminated silica
samples with one sort of polypropylene. We evidenced an important increase of laser induced damage density on
samples contaminated by both bundle and storage environments. Surface analyses have been used to identify the
potential causes of this effect. Various hypotheses of damage mechanisms are proposed.
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. We herein review
various studies made to develop pulse compression gratings for CPA application with high laser induced damaged
threshold. Different multilayer dielectric (MLD) gratings have been manufactured to exhibit different electric field
maximum values in the pillars of the grating. A damage testing facility operating at 1.053μm, 500fs pulse duration is
used to damage test the parts manufactured from these designs. It is evidenced that for fixed incidence and materials the
damage of the grating is directly related to the electric field intensity maximum in the material, which depends on the
groove profile. Laser induced damage thresholds of 5 J/ cm2 is experimentally reached with very high and uniform
efficiencies. New structures are currently under study, gratings with mixed metal/dielectric layers MLD or more exotic
2D and 3D photonic crystals devices. For each case, we detail the design and expected performances. We also give some
diffraction efficiency and laser damage measurements when samples were manufactured.
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.
Spatially-engineered "top-hat" laser beams are used in solid-state high-energy lasers in order to increase the energy
extraction efficiency in the amplifiers. To shape the laser beam, an efficient alternative to serrated apertures is to modify
a laser cavity so that it naturally generates this "top-hat" beam, replacing a mirror of the laser cavity by a graded phase
mirror. Its complex shape can be approached by microlithographic techniques based on an iterative mask and etch
technique, but many steps are required to avoid large phase steps. The broad-beam ion-etching technique is well suited to
manufacture such surfaces, with a good precision and a perfectly smooth surface. We shall present the technique we used
for square top-hat beam generation. We shall detail the mask optimisation, combining simultaneous simulation of the ion
etching and the beam build-up in the front-end laser. We shall present the results of the surface testing and the final test
of the component in the laser.
An optical bench was developed to measure laser induced damage thresholds in 1:1 and S:1 modes on mirror
and grating samples. The laser based on Ti. Sapphire technology delivers Gaussian pulse of 500fs with a
maximal energy of 3mJ at 1057nm. The experimental setup can deliver on the sample a peak fluence of 9J/cm2
in right section of the beam with a spot size of 200μm (diameter at 1/e2).
Laser induced damage thresholds have been measured on several multidielectric samples produced with
different processes. We present in this paper the damage testing setup in details and give some of the obtained
results.
N. Blanchot, E. Bignon, H. Coїc, A. Cotel, E. Couturier, G. Deschaseaux, N. Forget, E. Freysz, E. Hugonnot, C. Le Blanc, N. Loustalet, J. Luce, G. Marre, A. Migus, S. Montant, S. Mousset, S. Noailles, J. Néauport, C. Rouyer, C. Rullière, C. Sauteret, L. Videau, P. Vivini
A Multi-Petawatt High-Engergy laser coupled to the LIL (MPWHE-LIL) is under construction in the Aquitaine Region in France. This facility will be open to academic community. Nd:glass laser chain and Chirped Pulse Amplification (CPA)technique makes possible to deliver high energy. Optical Parametric Chirped Pulse Amplification (OPCPA) for pre-amplification and new compression scheme will be implemented. The MPWHE-LIL will deliver output energy of 3.6 kJ in 500 fs on target corresponding to more than 7 PW. The PW laser facility linked to UV-60kJ-ns beam from LIL, will give new scientific research opportunities.
The Megajoule laser, designed for the study of high energy density plasma, is currently being constructed at the CEA Cesta near Bordeaux in France. Constituted of 240 laser beams, this facility will by able to concentrate 1.8MJ of energy on a target placed in the centre of a vacuum chamber in order to obtain fusion. The 240 beams of the LMJ have a right section of 40 x 40 cm2 and are equipped with about 40 optical parts of various types: laser slabs, lenses, mirrors, diffractive optics. All of them have to sustain very high fluence induced by the laser beam. Manufacturing 9000 large laser optics of this type is a real technological and economical challenge. This presentation gives an overview of this activity and details the main recent development realized. In addition, we present results on the current development program made to improve lifetime of fused silica optics at the wavelength of 351 nm.
A multipetawatt laser is in construction in France with a compressor scheme using diffraction gratings. With an original code which takes into account the imprecision range of the geometrical parameters during the fabrication process, we numerically optimize the mirror stack and study different groove profiles in order to reduce the electric field inside the solid materials. It is shown that among all the profiles which lead to good diffraction performances, the profiles with the highest groove depth and width values lead to the smallest enhancement of the electric field inside the solid materials with a decrease by a factor slightly higher than 2.5. Moreover, in view to reduce the mechanical constraints in the stack, an original setup using a metal insert between the substrate and the dielectric mirror is studied.
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.
As part of LMJ project (Laser Megajoule), CEA has built the LIL - Ligne d'Integration Laser - a LMJ prototype. This prototype uses full sized optics (400x400 mm2) with very tight specifications. SESO is one of the suppliers of optical components for this laser, among them filtering lenses - called L3 and L4 - used at 1053 nm (1ω), thin flat plates - continuous phase plates and debrishield - and thick windows, all used at 351 nm (3ω).
All these optics are in fused silica and combine good wavefront specifications, very low roughness and no or few surface quality defects.
Today, including spare parts, about 40 components have been produced.
The purpose of this paper is to describe the facilities for grinding, polishing and finishing these optics.
Computer Controlled Polishing robot for lenses
Double side polishing machine for flat optics
After a brief presentation of the specific metrology used, we give a detailed overview of the performances obtained on the produced components.
This work is related to the LIL - LMJ project directed by CEA, France.
In the field of the development of LIL and LMJ fusion class high power lasers, CEA has made important efforts to understand and improve laser induced damage threshold of fused silica optics at the wavelength of 351 nm. Since several years, we have focused on optimizing the grinding, polishing and post polishing processes to overcome the existing performances with various industrials and academics partners. In this paper, we describe our understanding of the nature of the polished silica interface and our approach to rich our damage threshold goal. Our efforts were mainly put on reducing the cracks region extension and removing or optimizing the polishing top layer. We give also some details on the influences of each of the polishing process from rough material grinding to post processing. We demonstrate that some order of magnitude in laser damage initiation density can be gained by combining appropriate fabrication steps.
Unlike other drive fusion class laser, Megajoule laser (LMJ) and its first prototype, the Laser Integration Line (LIL) are equiped with specific diffractive optical components. All these optics are situated in the final optic assembly.
An high efficiency diffraction focusing grating called 3w grating is used to focus the beam into the center of the target chamber instead of a classical focusing lens. Another large grating called 1w grating is used for optical path compensation purposes. Both gratings have a dimension of 420x470mm2 and are working at an incidence of 25°. Gratings are plano transmission holographic gratings directly engraved into fused silica substrates. The 1w grating is working at the wavelength of 1.053μm, its grooves are straight and equispaced. The 3w grating, is a focusing grating working at the wavelength of 0.351μm. Its grooves are curved and non equispaced.
Jobin Yvon was selected by CEA to manufacture these two types of diffraction graintgs. After processes developpements and facilitization, a complete batch of twelve 1w gratings and sixteen 3w gratings were delivered to CEA for integration.
After a brief presentation of CEA's specification for this diffractive components, we give some details on the manufacturing processes. We also demonstrate good agreement between specified and manufactured component. We give an overview of the global production performances
French Megajoules laser (LMJ) and Ligne d'Integration Laser (LIL) are two high power lasers using large diffractive components for focal spot conditioning. Those components are a 420x470 mm2 focusing grating and a 398x383 mm2 continuous phase plate (CPP), both working at wavelength 0.351 μm. In order to control the tight specifications that have to meet those components, CEA and company Jobin-Yvon, in charge of their manufacturing, have developed specific setups. After a brief introduction describing specifications and requirements, we will present the different setups and analysis software used and finally discuss the different measurements that could be done.
Technological processing of bulk silica is needed at many points of the fabrication of optical components. In the Laser Integration Line (LIL) and Megajoule Laser (LMJ), with the usual but challenging optical constraints of keeping the wavefront quality on large optics the constraint of high flux laser resistance is added. This has led to many technological improvements of silica processing which have been transferred into the industrial tissue. Improved polishing and cleaning processes have been developed which avoid the contamination of surface with polishing agents and are now used for lenses, windows, and substrates preparation of mirrors and polarizers. But some components like the gratings which are to be used on LIL and LMJ need new processing steps which are typical of the semiconductor industry and whose effects are unknown in terms of laser induced damage threshold in silica surface and subsurface. After a summary of the specifications and the performances of these gratings at 1 and 3(omega) wavelength we will focus on the laser induced damage (LID) tests that were performed at different stages of the grating process and see how they impact on the LID threshold of the gratings.
The Megajoule laser (LMJ) and its first prototype, the Laser Integration Line (LIL), is equipped with a specific final optics assembly involving two diffraction gratings instead of a classical focusing lens. Both gratings have a dimension of 420 X 470 mm2 and are working at an incidence of 25 degree(s).. Gratings are plano transmission holographic gratings directly engraved into fused silica substrates. The 1(omega) grating is working at the wavelength of 1.053 micrometers , its grooves are straight and equispaced. The 3(omega) grating, is a focusing grating working at the wavelength of 0.351 micrometers . Its grooves are curved and non equispaced. The gratings were designed and manufactured to present efficiencies superior to 90% on the whole clear aperture and an improved damage threshold. After Jobin Yvon's selection by CEA in 1999, specific equipment and facilities were put in place to manufacture these large gratings. The aim of this contribution is to present the early results of the development of this 1(omega) and 3(omega) gratings. After a short introduction to the 1(omega) and 3(omega) gratings specifications, manufacturing process, efficiencies result and AFM profiles of the first manufactured gratings will be detailed.
To deviate and focus of the beams of the future Laser Integration Line (LIL) and Megajoule laser (LMJ), CEA has chosen an original setup using two large 420 x 470 mm2 transmission gratings. The first grating is an holographic plano transmission master grating with straight and equispaced ruling, 25 degree(s) incidence angle and working at 1.053 micrometers . The second one is an holographic plano transmission master grating, with curved and non equispaced ruling, 25 degree(s) incidence angle which combines both focusing and deviation properties. Groove profile of both gratings is deep laminar. High damage threshold, improved wavefront quality and high efficiencies are the main issues for those two gratings. Jobin Yvon's was selected by CEA in 1999 to develop, industrialize and manufacture gratings reaching LIL/LMJ specifications. A dedicated plant and facilities were built to manufacture the gratings directly engraved into the fused silica substrates provided by CEA. After process developments, Jobin Yvon manufactured the two first 1(omega) and 3(omega) gratings in mid 2001. After a short summary of the specification of these gratings, we present in this paper the production process and the performances of the 1(omega) and 3(omega) gratings manufactured. Wavefront data, efficiency measurements and damage threshold performances are detailed.
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 major preoccupation for the design of the LMJ laser is the mirrors laser damage threshold. SAGEM SA, in collaboration with the CEA, has conducted a study in order to improve the laser induced damage threshold under operational conditions.
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