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).
Nanosecond laser-induced damage (LID) in potassium dihydrogen phosphate (KH2PO4 or KDP) remains an issue for
light-frequency converters in large-aperture lasers such as NIF (National Ignition Facility, in USA) and LMJ (Laser
MegaJoule, in France). In the final optic assembly, converters are simultaneously illuminated by multiple wavelengths
during the frequency conversion. In this configuration, the damage resistance of the KDP crystals becomes a crucial
problem and has to be improved. In this study, we propose a refined investigation about the LID mechanisms involved in
the case of a multiple wavelengths combination. Experiments based on an original pump-pump set-up have been carried
out in the nanosecond regime on a KDP crystal. In particular, the impact of a simultaneous mixing of 355 nm and 1064
nm pulses has been experimentally studied and compared to a model based on heat transfer, the Mie theory and a Drude
model. This study sheds light on the physical processes implied in the KDP laser damage. In particular, a three-photon
ionization mechanism is shown to be responsible for laser damage in KDP.
Nanosecond Laser-Induced Damage (LID) in potassium dihydrogen phosphate (KH2PO4 or KDP) remains an
issue for light-frequency converters in large-aperture lasers such as NIF (National Ignition Facility, in USA) and
LMJ (Laser MegaJoule, in France). In the final optic assembly, converters are simultaneously illuminated by
multiple wavelengths during the frequency conversion. In this configuration, the damage resistance of the KDP
crystals becomes a crucial problem and has to be improved. In this study, we propose a refined investigation
about the LID mechanisms involved in the case of a multiple wavelengths combination. Experiments based on an
original pump-pump set-up have been carried out in the nanosecond regime on a KDP crystal. In particular, the
impact of a simultaneous mixing of 355 nm and 1064 nm pulses has been experimentally studied and compared
to a model based on heat transfer, the Mie theory and a Drude model. This study sheds light on the physical
processes implied in the KDP laser damage. In particular, a three-photon ionization mechanism is shown to be
responsible for laser damage in KDP.
We investigate the influence of THG-cut KDP crystal orientation on laser damage at 1064 nm under nanosecond
pulses. This study makes a connection between precursor defects and the influence of their orientation on the
laser damage. Previous investigations have already been carried out in various crystals and particularly for KDP,
indicating propagation direction and polarization dependences. We performed experiments for two orthogonal
positions of the crystal and results clearly indicate that KDP crystal laser damage depends on its orientation.
We carried out further investigations on the effect of the polarization orientation, by rotating the crystal around
the propagation axis. We then obtained the evolution of the damage probability as a function of the rotation
angle. To account for these experimental results, we propose a model based on heat transfer, the Mie theory
and a Drude model. The geometry of the precursor defects is assumed to be ellipsoid-shaped and we numerically
introduce absorption efficiency calculations for this geometry. Modeling simulations are in good agreement with
experimental results.
The rasterscan test procedure implemented in order to determine low laser damage density of large aperture UV
fused silica optics was improved in terms of accuracy and repeatability. Tests have been carried on several facilities
using several pulse durations and spatial distributions. We describe the equipment, test procedure and data analysis to
perform this damage test with small beams (Gaussian beams, about 1mm @ 1/e, and top hat beams). Then, beam overlap
and beam shape are the two key parameters which are taken into account in order to determine damage density. After
data analysis and treatment, a repeatable metrology has been obtained. Moreover, the consideration of error bars on
defects distributions permits to compare data between these installations. This allows us to look for reproducibility, a
necessary condition in order to share results and to make reliable predictions of laser damage resistance. For that, a
careful attention has been paid to beam analysis.
Laser damage in KDP crystals has been studied since several years and more accurately with emergence of projects
like LMJ (Laser MégaJoule, in France) or NIF (National Ignition Facility, in US). Laser damage tests are essentially
performed at 351-nm wavelength (3ω), with regards to their optical behaviours on forementioned facilities. But only few
data are available at 1064 nm (1ω) and at 532 nm (2ω), and even with wavelength-mixing more representative of
operational conditions of KDP crystals. So in a first approach, we tried to carry out an identity chart of the crystal by
performing mono-wavelength tests at 1ω, 2ω and 3ω. Then, a campaign of combination of multi-wavelength (typically
3ω and 1ω) tests has been started with several temporal delays between 3ω and 1ω pulses. These first results lead us to
improve pre-existing modelling codes developed by CEA, which have proved their robustness to 3ω -experiment results.
Foremost interests consist in implementing wavelength dependency and energy deposition mechanism as a consequence
of our first observations on KDP.
The 3D-AFM technique is a very well known technique as a non destructive reference to calibrate CD-SEM and
Scatterometry metrology. However, recent hardware, tip design and tip treatment improvements have offered to the
technique new capabilities that pave the way for multiple applications in the semiconductor industry. The 3D-AFM
technique is today not only a calibrating technique but also a process control technique that can be use either at the R&D
level or in fab environment.
In this paper, we will address the limits of the 3D-AFM technique for the semiconductor industry depending on the
applications by focusing our study on tip to sample interactions. We will identify, test and validate potential industrial
solutions that could extend the 3D-AFM potentialities. Subsequently, we will show some interesting applications of the
technique related to LER/LWR transfer during silicon gate patterning and related to advance multiwires devices
fabrication.
Campaigns of laser damage tests at 1w of Nd-YAG laser (1064 nm), 3w and with a combination of these two
wavelengths, were conducted to complete previous existing data on damage growth in fused silica output surface.
It is known that UV light is very effective in inducing preexisting damage craters to grow. When both
wavelengths are present, the effect of 1w beam on damage growth depends on the delay between the IR and the UV
beam. When the 1w reaches the sample before the 3w, it has nearly no consequence on growth rate. On the opposite,
when the IR beam is delayed and strikes the sample after the 3w pulse, its energy simply adds to the UV in enhancing
damage growth.
Damage initiation is much more affected by 3w than 1w pulses. However, the number of surface damage craters
also increased by the addition of 1w photons to the UV beam.
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