The Laser MégaJoule (LMJ) is a French high power laser project that requires thousands of large optical components. The wavefront performances of all those optics are critical to achieve the desired focal spot shape and to limit the hot spots that could damage the components. Fizeau interferometers and interferometric microscopes are the most commonly used tools to cover the whole range of interesting spatial frequencies. Anyway, in some particular cases like diffractive and/or coated and/or aspheric optics, an interferometric set-up becomes very expensive with the need to build a costly reference component or a specific to-the-wavelength designed interferometer. Despite the increasing spatial resolution of Fizeau interferometers, it may even not be enough, if you are trying to access the highest spatial frequencies of a transmitted wavefront for instance. The method we developed is based upon laser beam diffraction intermediate field measurements and their interpretation with a Fourier analysis and the Talbot effect theory. We demonstrated in previous papers that it is a credible alternative to classical methods. In this paper we go further by analyzing main error sources and discussing main practical difficulties.
The French Laser MégaJoule (LMJ) is a high power laser project, dedicated to fusion and plasma experiments. It will
include 176 square beams involving thousands of large optical components. The wavefront performances of all those
optics are critical to achieve the desired focal spot shape and limit the hot spots that could damage the components. The
CEA has developed experimental methods to qualify precisely the quality of the large optical components manufactured
for the project and measure the effect of various defects. For specific components (coated or parabola mirrors, lenses or
gratings), classical techniques like interferometric setups may fail to measure the wavefront highest spatial frequencies
(> 1 mm-1). In order to improve the measurements, we have proposed characterization methods based upon a laser beam
diffraction interpretation. They present limits and we need to improve the wavefront measurement for high spatial
frequencies (> 1 mm-1). We present in this paper the intermediate field measurement based upon the Talbot effect theory
and the Fourier analysis of acquired intensity images. The technique consists in a double pass setup: a plane wave is
transmitted through the component twice, to simplify the setup and improve the measurement. Then, intensity images are
acquired at different distances with a CCD camera and lead to the wavefront power spectral density. We describe the
experimental setup to measure the wavefront of large specific components. We show experimental results. Finally, we
discuss about the advantages and the limits of such a method, and we compare it with our previous measurement
The French Laser MégaJoule (LMJ) will include 176 square beams involving hundreds of large optical components. Wavefront performances of all these components are critical to achieve the desired focal spot shape and to limit hot spots that could damage the components. These specifications are usually checked with interferometric setups. This can be uneasy to achieve for specific components such as multi-dielectric mirrors or gratings because one has to use the exact nominal configuration (wavelength, incidence, geometry of the incident beam) to perform the measurement. For the smallest spatial periods, classical techniques like interferometric microscopes fail to measure the wavefront and propose a "surface" measurement that can lead to misinterpretations. We present in this paper measurement methods based on a laser beam diffraction interpretation that can efficiently replace the usual techniques. The first technique consists in measuring intensity level of the dim scattered "corona" around the focal spot. The second one is based upon image processing of near-field acquisitions by the means of Fourier analysis and the Talbot effect theory. Those techniques do not lead to a phase map as classical techniques do but they give access to the Power Spectral Density of wavefront defects over a large spatial frequency bandwidth. For many applications, this is enough information to estimate the component performance. We present results obtained by this way on LMJ components and a comparison with Fizeau interferometer measurement.
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).
LIL and LMJ are two French high power lasers dedicated to fusion and plasma experiments. These laser beams involve
hundreds of rather large optical components, the clear aperture of the beams being 400x400 mm2. Among these
components are multi-dielectric mirrors designed to reflect more than 99% at the wavelength of 1053 nm.
Measuring the phase effects due to slight thickness defects in thin films is a difficult problem when one cannot achieve
the phase measurement at the wavelength for which the mirror is designed. We believe this problem to be general in the
world of thin films. Despite the fact that we have an interferometer that can achieve wavefront measurements at the
correct wavelength, we performed measurements with another standard 633 nm Fizeau interferometer. Indeed, this
second interferometer has a much higher spatial resolution. The effect of the wavelength difference can be strongly
dependent on the layer design; that is why we achieved spectrophotometric measurements in order to have the most
accurate knowledge we could get for the coating parameters. The phase effects for different kinds of defects have been
simulated at both wavelengths and have been compared to experimental results. This study leads to a better understanding
of the limits and the trust we can have in such measurements performed at the "wrong" wavelength.
LIL and LMJ are two French high power laser facilities dedicated to laser-plasma interaction experiments. In order to
control the flatness requirements of their optics in a wide spatial periods bandwidth, the CEA has several Fizeau
interferometers of different diameters. We use special phase objects to qualify their spatial resolutions. A few papers
already dealt with the determination of a Fizeau interferometer transfer function. This was achieved by using either a
phase step object or a "virtual" sinusoidal phase object (made of the superposition of two wavefronts with different
amplitudes and a small tilt). For practical reasons, we chose to use true sinusoidal phase objects to qualify our
instruments. Sinusoidal profiles were then eroded in silica plates. Three different periods are available: 10 mm, 2.5 mm
and 1 mm, with two different amplitudes for each period. These phase plates are used to qualify the interferometers
performance in terms of spatial resolution in the different configurations (wide or narrow field of view, reflection or
transmission) used for LIL/LMJ optics inspection. A comparison to the transfer functions obtained using steps of
different widths is also proposed. An experimental verification of the Talbot effect is achieved with the 1-mm plate to
investigate propagation effects, as well as contribution of the depth of field.
High power laser systems such as the LMJ laser or the LIL laser, its prototype, require large optical components with
very strict and various specifications. Technologies used for the fabrication of these components are now usually
compatible of such specifications, but need the implementation at the providers' sites of different kind of metrology like
interferometry, photometry, surface inspection, etc., systematically performed on the components. So, during the
production for the LIL and now for the LMJ, CEA has also equipped itself with a wide range of specific metrology
devices used to verify the effective quality of these large optics. These various systems are now used to characterize and
validate the LMJ vendors' processes or to perform specific controls dedicated to analyzes which are going further than
the simple "quality control" of the component (mechanical mount effect, environment effect, ageing effect,...).
After a short introduction on the LMJ laser and corresponding optical specifications for components, we will focus on
different metrology devices concerning interferometry and photometry measurements or surface inspection. These
systems are individually illustrated here by the mean of different results obtained during controls done in the last few
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.
The design of high power lasers such as the MEGAJOULE laser (LMJ) and its first prototype, the Laser Integration Line (LIL) requires optical components with very strict and diverse specifications over large apertures. Though technologies used for the fabrication of these components may be usually compatible with such specifications, fabrication processes are often restricted by our ability to measure the effective performances. In order to determine the effective quality of its components, CEA is equipped with a wide range of metrology devices, many of them were developed for the specific needs of LIL and LMJ programs. After a short description of the Megajoule laser, we will focus on two different metrology devices used in the characterization of its optical components: interferometry and photometry.