In the context of high power laser systems, the laser damage resistance of fused silica surfaces at 351 nm in the nanosecond regime is a major concern. Under successive nanosecond laser irradiations, an initiated damage can grow which can make the component unsuitable. The localized CO2 laser processing has demonstrated its ability to mitigate (stopping) laser damage growth. In order to mitigate large damage sites (millimetric), a method based on fast microablation of silica has been proposed by Bass et al. [Bass et al., Proc. SPIE 7842, 784220 (2010)]. This is accomplished by scanning of the CO2 laser spot with a fast galvanometer beam scanner to form a crater with a typical conical shape. The objective of the present work is to develop a similar fast micro-ablation process for application to the Laser MegaJoule optical components. We present in this paper the developed experimental system and process. We describe also the characterization tools used in this study for shape measurements which are critical for the application. Experimental and numerical studies of the downstream intensifications, resulting of cone formation on the fused silica surface, are presented. The experimental results are compared to numerical simulations for different crater shape in order to find optimal process conditions to minimize the intensifications in the LMJ configuration. We show the laser damage test experimental conditions and procedures to evaluate the laser damage resistance of the mitigated sites and discuss the efficiency of the process for our application.
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.
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.
We report on the development of a mitigation process to prevent the growth of UV nanosecond laser-initiated damage sites under successive irradiations of fused silica components. The developed process is based on fast microablation of silica as it has been proposed by Bass et al. [Bass et al., Proc. SPIE 7842, 784220 (2010)]. This is accomplished by the displacement of the CO2 laser spot with a fast galvanometer beam scanner to form a crater with a typical conical shape to mitigate large (millimetric) and deep (few hundred microns) damage sites. We present the developed experimental system and process for this application. Particularly, we detail and evaluate a method based on quantitative phase imaging to obtain fast and accurate three-dimensional topographies of the craters. The morphologies obtained through different processes are then studied. Mitigation of submillimetric nanosecond damage sites is demonstrated through different examples. Experimental and numerical studies of the downstream intensifications, resulting in cone formation on the surface, are presented to evaluate and minimize the downstream intensifications. Eventually, the laser damage test resistance of the mitigated sites is evaluated at 355, 2.5 ns, and we discuss on the efficiency of the process for our application.
In the context of high power laser applications, we study the effect of a heat treatment on CO2 laser mitigation of laser damage sites on fused silica samples. The isothermal annealing in a furnace is investigated and then compared to the local annealing by CO2 laser irradiation that is applied to enhance laser damage resistance on mitigated sites. Before and after isothermal annealing, we study the sites morphology, the evolution of residual stress and the laser-induced damage threshold measured at 355nm, 3ns. The results show that the initial laser damage probabilities were significantly improved after annealing at 1050°C for 12 hours. These results are compared to simulations with a thermo-mechanical model based on finite-element method.
Scratches at the surface of fused silica optics can be detrimental for the performance of optical systems because they initiate damage on the optic but also they perturb the amplitude or phase of the transmitted laser light. Removing scratches by conventional polishing techniques can be time consuming as it is an iterative and long process, especially when hours of polishing time are required to obtain very high surface accuracy. So we have investigated ways to remove them with local laser processing. The silica is then heated at temperature higher than the softening point to heal the cracks.
Localized CO2 laser heating of silica glass has demonstrated its ability to mitigate surface damage on optics used for
high power laser applications. The parameters for this process such as the power, the beam size and the exposition time
are however critical and some fundamental studies on the silica behavior under CO2 laser irradiation are required to
develop the processes. It is necessary for instance to understand the silica transformation, the material ejection and the
thermo-mechanical stresses induced by the laser heating and subsequent cooling. A thermo-mechanical model based on
finite-element method has been used to calculate the temperature of silica heated by CO2 laser irradiation and the
residual stress after cooling of the samples. The model, as the different parameters used for calculations, are detailed in
this paper and the numerical results are compared to different dedicated experimental studies.
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.
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).
Various scratches on fused silica optics after polishing have been characterized with confocal microscopy and then tested
with nanosecond UV laser. Scratches are identified as a major contributor to laser damage even if they are only a few
micrometers wide. We propose a process in order to remove these scratches whose depth ranges from 2 to 16 μm. We
use a CO2 laser to heat the scratched areas at high temperature which will heal fractures due to viscous flow. The
characterizations were completed by laser damage tests that finally prove the effectiveness of the repair. We conclude
also that this repair process proves to be fast, localized to the scratch and clean because no debris are generated.
CO2 laser is used to prolong the lifetime of large optics for high power lasers such as the NIF and LMJ.
Indeed, on silica optical components, damaged sites, whose diameter is in the order of tens of microns, appear at
high UV laser fluence, and the size of such sites increases exponentially with each UV laser shot. An intense
heat by CO2 laser ejects the material from the surface of the optical component and removes all fractures around
the damaged site so that this site will not be damaged at fluences of operation of the UV laser. A crater is formed
at the site of initial damage. But the intense heat creates debris and residual stress around this crater. Due to these
debris and stress, the optical component is again weakened. We show here that a second heating process, done
with different settings of the CO2, named here laser annealing, eliminates the debris and reduce stress. The
results presented here establish that annealing significantly improves the resistance of laser optics.
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.
We investigate the residual stress field created near mitigated sites and its influence on the efficiency on the CO2 laser
mitigation of damage growth process. A numerical model of CO2 laser interaction with fused silica is developed that
take into account laser energy absorption, heat transfer, thermally-induced stress and birefringence. Specific photoelastic
methods are developed to characterize the residual stress near mitigated sites in fused silica samples. The stress
distribution and quantitative values of stress levels are obtained for sites treated with the CO2 laser in various conditions
of energy deposition (beam size, pulse duration, incident power). The results obtained also show that the presence of
birefringence/residual stress around the mitigated sites has a critical effect on their laser damage resistance.
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.
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.
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.
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.
The French Commission for Atomic Energy is currently involved in a project which consists in the construction of a
2MJ/500TW (351nm) laser, so called LMJ (Megajoule-class laser) devoted to Inertial Confinement Fusion (ICF)
research in France[1]. For this high power lasers, the sol-gel process[2] has been selected for 95% of laser optical coated
area because of room temperature and atmospheric pressure conditions with guarantee for high optical and laser induced
damage threshold (LIDT) performances at a low cost compared to conventional vacuum deposition processes.
The production rate of sol-gel coatings for the LMJ optical components will require an automated cleaning surface step
during sol-gel process. We are investigating a spraying system and wash cycles compatible with the two sol-gel
deposition methods: dip and laminar-flow coating. The challenge is to achieve the same cleaned optical surfaces as
manual process without using organic solvents. Therefore the main specifications of the cleaning quality are the
following ones: a high surface energy over all optical sides (up to 400×400 mm2 area) and no degradation of polished
sides (surface defects and LIDT).
We present the metrologies carried out and the first results obtained from different wash cycles. These one mainly
consist in measurement of contact angles, defects inspections under specific lighting conditions and LIDT tests. Several
parameters of wash cycles have been investigated such as washing and rinsing temperatures, water quality, type and
concentration of detergents, wettability effects...
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.
The laser system is by far the most expensive part of a laser-produced plasma (LPP) EUV source. Thus LPP source designers have not only to optimize the technical performances of the lasers but also cost, efficiency, reliability, redundancy and industrial availability of their approach. Based on the typical conversion efficiencies of present LPP source targets (CE ~ 1- 3%), the laser system will have to deliver an average power of at least 20 kW. This can hardly be obtained with only few laser chains. Starting from this consideration, the EXULITE consortium has first proposed the concept of a spatially multiplexed and modular LPP source using 10 to 20 identical, moderate power laser units which are simultaneously focused on the same spot. This approach increases system redundancy and reduces the development cost of the laser. We will present an original technical approach which allows both to manage the multi-beam attack of the target using a closed-loop control and to collect a maximum of EUV power. The grazing incidence EUV collector is designed such that optimized vacuum pumping and debris mitigation can be obtained. Finally, we can show that modular LPP sources can satisfy the severe EUV dose stability that is commonly requested.
Within the PREUVE project, the GAP of CEA Saclay has developed an EUV source that should meet (alpha) -tool specifications by the end of this year. In particular, a laser-produced plasma source has been developed that uses a dense and confined xenon jet target. Our technical solution is based on a specific target injector design and the use of well adapted nozzle materials to avoid debris formation by plasma erosion. After injection, the xenon is recycled and highly purified to reach a low cost round- the-clock operation. This source provides both high conversion efficiency and low debris flux. These are necessary conditions for its industrial application in the future EUV microlithography. The conception of the so-called ELSA (EUV Lithography Source Apparatus) prototype allows in principal 2 years full operation on the French lithography test bench BEL (Banc d'essai pour la lithographie) that has been developed during PREUVE. In parallel, the EXULITE consortium that is coordinated by Alcatel Vacuum Technology France (AVTF) has started its activities in the frame of the European MEDEA+ initiative on EUV source development. In collaboration with Thales and the CEA, AVTF develops a prototype power source for EUV lithography production tools by the end of 2004. A low cost and modular high power laser system architecture has been chosen and is developed by Thales and the CEA to pump the laser plasma- produced EUV source.
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