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.
As part of the LIL (Ligne d'Integration Laser) and LMJ (Laser Megajoule) projects, CEA/CESTA and SAGEM company have carried out an accurate metrology of the back focus length of filtering lenses. These measurements should enable us to make up for a lack of focus adjustment on the laser and to correctly position these lenses. Three different lenses have been characterized: • Lenses L1 and L2 with a back focus length of 10 m (aperture of F/16). Uncertainty is estimated at ±1,3 mm. • Lens L3 with a back focus length of 26 m (aperture of F/43). Uncertainty is estimated at ±5,3 mm. To achieve these measurements, 4 methods have been used: (1) Measurement of the curvature radius of each side on a Coordinate Measuring Machine and index homogeneity characterization before polishing. (2) Interferometric measurement with a reference spherical mirror. (3) Interferometric measurement using a lens side as a reference spherical mirror with a flat mirror. (4) Interferometric measurement by closing the cavity at the top of the lens back side. Uncertainty calculation has been done for each method.
As part of the LMJ (Laser Megajoule) program, CEA is building the LIL laser with full size optics and LMJ requirements. SAGEM has been selected as the supplier of large optical components and coatings with very high laser- induced damage threshold. Including spare parts, about 100 mirrors 610*430 mm2 with LIDT-3ns>25 J/cm2 have to be produced. Using a 5 m3 vacuum chamber and the 100 J/cm2 mirror coating process developed at CEA-LETI, with Hafnium and SiO2 materials, we are now typically in a serial production phase. To date, about thirty mirrors have been delivered. This paper focuses on the acceptance tests performed after coating, at SAGEM then CEA: LIDT measurement and Raster-Scan on samples; reflectance mapping on CEA automatic photometer; reflected wavefront deformation with &nullset; 800 mm/1ω CEA interferometer.
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 of 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 and to help optimizing their production, CEA equipped with a wide range of metrology devices, many of them were developed for the specific needs of LIL and LMJ programs. In the same time CEA also supported the development of specific metrology means at its optics vendors to help in th fabrication process design. After a short description of the Megajoule laser, we will focus on the different metrology devices used in the characterization of its optical components non-exhaustively ranging from interferometry and photometry measurements to focal spot analysis.
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.
This paper reports the beginning of laser amplifier discs and spatial filtering lens polishing activities performed at REOSC for the Laser Megajoule project Ligne d'Intergation Laser prototype facility.
Laser Megajoule is a very large laser system: 240 beams, each comprised of dozens of optical components size 400 X 400 millimeters to 400 X 700 millimeters, covering the area of a football pitch or two. Like any laser system, the various components must be computed and drawn. These drawings include specifications.
Laser Megajoule is a very large laser system: 240 beams, each comprised of dozens of optical components 400 millimeters squares, covering the area of a football pitch or two. Specifying the optics in Laser Megajoule is a complex matter, because of the large range of types of defects, classified by their lateral size or scale.
Surface quality is an important issue for KDP crystals implemented in high power lasers. Single Point Diamond Turning is the only way to get the required transmitted wavefront and damage threshold, especially in the UV band of the laser Megajoule. The aim of the process development was to reach an rms roughness lower than 3 nm using a vertical fly-cutter, an appropriate cleaning equipment and an interferometric microscope for measurements. A Taguchi Design of Experiment method has been used to optimize the fly-cutting process parameters on 100 X 100 mm2 KDP crystals.
The propagation of waves in non-linear media is perturbed by the fact that the index of refraction of such media is modified by the intensity of the wave which, in turn, modifies the shape of the wave which, in turn, modifies their intensity of the wave, and so on. The localized small concavity in the wavefront due, typically, to non-perfect components, also perturbs the wavefront by creating a localized intensity peak further down the line. The computation of such modulation is relatively easy in vacuum, by simple wave summation according to Fresnel, but rather less so in non-linear media. This is because the above- mentioned index variations modify the actual optical path of the rays as compared to vacuum. For shallow defects, a simple but effective method is to proceed via the Fourier transform of the defect shape. Individual spatial modes propagate with accurately-known amplitude variation. Modes are summed up after individual propagation, leading to accurate results. The amplification of such modes can be very great, leading to many-fold amplification of localized intensity peaks and, therefore, to potential damage to optical components. We explain the method, and compare numerical with 'real' simulations performed on MIRO. Additionally, as we show, insight into the behavior of defect amplification is gained, and accurate predictions can be made with little or no computational effort.
KEYWORDS: Crystals, Electroluminescence, Spindles, Wavefronts, Interferometry, Microscopes, Diamond machining, Laser crystals, High power lasers, Single point diamond turning
Surface quality is an important issue for KDP crystal implemented in high power lasers. Single Point Diamond Turning is the only way to get the required transmitted wavefront and damage threshold, specially in the UV band of the laser Megajoule. The aim of the process development was to reach an rms roughness lower than 3 nm using a vertical fly-cutter, an appropriate cleaning equipment and an interferometric microscope for measurements. A Taguchi Design of Experiment method has been sued to optimize the fly-cutting process parameters on 100 X 100 mm2 KDP crystals.
Large aspherical focusing and beam deviating square lenses will be used in the framework of the Laser Megajoules project developed by the French Atomic Energy Commission. In order to validate the associated manufacturing processes a half scale prototype lens has been manufactured and tested by REOSC (SFIM subsidiary). Specific aspherical generating process and computer controlled micro-polishing technology have been used in order to demonstrate the faisability of a mass production on an industrial basis (approximately 1300 lenses in 6 years). A 250 mm square lens in Fused Silica with a convex hyperboloid profile (250 microns difference with respect to the best sphere) has been manufactured and REOSC reached a transmitted wavefront better than 130 nm Peak-peak, 11 nm rms. Moreover residual micro-oscillations amplitudes (spatial frequency 0.5 to 30 mm) have been limited to 60 nm PTV and 8 nm rms. The total work duration for this exploratory lens remained below 60 hours.
Sfim ODS is involved in the French Megajoule Project managed by the Commissariat a l'Energie Atomique. It is a high power UV-laser using 240 square beams (410 mm X 410 mm). Sfim ODS achieved a technical and economical analysis concerning the production of flat-parallel debris-shields in silica (430 X 430 X 6 mm, transmitted wavefront: (lambda) /2, 2 500 shields/year) using large and specific dual-side fine- grinding and polishing machines. This analysis was supported by trials using half-scale windows (same stiffness equals> thickness equals 1.7 mm) showing the ability to reach the specifications with low production costs.
SFIM ODS is involved in the French MEGAJOULE Project managed by the Commissariat a l'Energie Atomique. It is a high power UV-laser using 240 square beams. SFIM ODS achieved a technical and economical analysis concerning the production of flat-parallel debris-shields in silica using large and specific dual-side fine-grinding and polishing machines. This analysis was supported by trials using half-scale windows showing the ability to reach the specifications with low production costs.
Over the past two years, laserdot has developed a new machine vision system for obstacle detection by mobile robots. A pulsed laser illuminates the road in a zone from 100 to 150 meters in front of the vehicle and the backscatter is analyzed by a linear array of photodetectors connected to a computer. Each obstacle is detected and its position is determined. The distance is calculated by measuring the pulse time-of-flight, producing a complete three-dimensional image without scanning. The system was jeep-mounted for testing in a military environment at the Angers Technical Center (ETAS) in France. This article describes the laserdot vision system and its design features, as well as the test results from ETAS. The system is capable of providing information on the shape of the road in front of the vehicle, including slope and banking measurements. Lastly, the future integration of the detector in a mobile robot is detailed. These works have been supported by Direction des Recherches, Etudes et Techniques (DRET).
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