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).
Correctly determining the lifetime of optical components is a major issue in the operation of high power laser facilities such as the Laser Megajoule developed by the Commissariat a l'Energie Atomique (CEA). Laser damage that occurs at the surface is a main cause of optical aging, and may lead to dramatic degradation of the focal spot. To estimate the effect of such defects, we measured and calculated the distortion of the focal spot induced by "model defects." These "model defects" are circular silica dots randomly distributed on a silica substrate. The experiments were conducted in the ANTALIA facility at the Centre d'Etude Scientifique et Technique d'Aquitaine (CESTA). We performed numerical calculations of beam propagation with the Miro software, developed by the CEA. We obtain a remarkable correlation between measurements and simulations in the central part of the focal spot for large defects. However, experimental noise and measurement dynamics become serious problems when we confine our attention to smallerdefects (<500 micron) or to the diffuse light around the central part of the focal spot. We present some modifications of the ANTALIA experimental setup designed to overcome these problems.
This paper presents recent studies of the propagation of high-power laser beams like Laser Integration Line (LIL) and Laser Megajoule (LMJ) beams when interactions occur with environmental pollution particles. The studies are mainly achieved with the CEA-DAM MIRO beam propagation code. The highest intensifications in the downstream propagation are obtained for phase objects such as dielectric particles rather than for amplitude objects such as metallic particles. Dramatic amplifications of Kerr nonlinear effects both inside the component and at its rear-surface can occur depending on the particle size.
In this paper we report several studies of the propagation of the Laser Integration Line (LIL) and Laser Megajoule (LMJ) beams when interactions occur with optical components defects. These studies are mainly achieved on a numerically predictive basis with the CEA MIRO beam propagation code. The flaws that we considered are located at the front or rear surface of the optical components. These surface flaws correspond to engineered defects such as scratches and to surface damage resulting from laser-induced growth process, from mitigation process or from target interactions debris. Results account for the possible downstream impacts of flaws at the rear-surface of the optics and from one component to another along the laser chain. In particular, the influence on the LIL/LMJ end-of-line focal spot intensity and size is predicted.
Several French research laboratories set up goniometers allowing BRDF measurements at different laser wavelengths in the infrared. On the effor of the Delegation Generale de l'Armement (DGA/STTC), a round robin set of painted targets BRDF measurements was undertaken, under the ONERA expertise. The laboratories participating in this round robin were the Aerospatiale Matra CCR Suresnes, The IPN SMA-Virgo Lyon, the Institut Fresnel Marseille, and the CEA DAM CESTA Le Barp. The goniometers of the four laboratories are firstly described. The targets studied are seven 5cm diameter painted disks of aluminum or steel, a spectralon reference sample, and a sandpaper sample. We have first demonstrated that the pollution of painted targets with dust has a very weak influence on the BRDF. Before and after each measurement series, the directional-hemispherical reflectance of the samples was measured at ONERA. The measurements have been achieved according to a protocol specifying the sample position and laser probe size. Chosen wavelengths for the inter-comparison are 1.064 micrometers . For both wavelengths, the characteristics of the different goniometers are compared in term of noise and repeatability. The difference between the painted targets BRDF measured with the various devices are relatively limited at 1.06 micrometers , and mainly induced by speckle. More important differences are obtained at 10.6 micrometers , particularly for a BRDF measurement device using an absolute calibration method. In order to explain these differences, few hypotheses are advanced. Information on the absolute accuracy is obtained by the comparison of the measured directional-hemispherical reflectance and the one computed from BRDF measurements.
The study deals with coherent and incoherent optical scattering from rough surfaces. The purpose is to analyze the evolution of the properties when the angle of incidence varies. A numerical study is led for one-dimensional randomly rough surface by means of both surface integral method and approximate Rayleigh random grating methods. The evaluation of the surface coherent intensity reflection factor is detailed for both numerical techniques. The problem is considered for a general dielectric homogeneous material. Numerical results are presented in the case of different absorbing dielectric materials, and for reflecting materials as metals in the infrared range or as SiC around 11 micrometer. The angular domain concerned goes from normal incidence to the mean surface up to very oblique angles such as 70 degrees. Results are presented for the different materials for the coherent intensity reflection factors and for the diffuse reflection patterns in relation to the angle of incidence and to the roughness parameters. Comparisons are given with the Kirchhoff Approximation model.
In this paper, we present a numerical study of optical waves scattered by rough surfaces. The calculations are based on the use of plane-wave expansions to describe the reflected and transmitted fields on the surface (also known as the Rayleigh hypothesis). This theory is applied to one-dimensional randomly rough dielectric surfaces, by using a random grating numerical generation. Two methods are recalled for the straightforward numerical implementation of the theory: the Point-Matching Method (PMM) and the Fourier Series Method (FSM). Examples of results for a metallic and a dielectric surface, obtained with the latter method, are in good agreement with calculations by an exact numerical method. It is reported that the FSM has a wider domain of validity but that the PMM has a faster computation time.
A fully automated scatterometer, designed for BRDF measurements in the IR at about 10 micrometers , is described. Basically, it works around a reflecting parabola (464 mm diameter, F/0.25) and permits measurements in and out of the plane of incidence. Optical properties of the parabolic mirror are emphasized by a ray-tracing technique which permits determination of the correct illumination on the sample and detection conditions of scattered light. Advantages and drawbacks of such an instrument are discussed, as well as calibration procedures. As a conclusion, we present experimental results to illustrate the instrument capabilities.
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