The Megajoule laser (LMJ) and its first prototype, the Laser Integration Line (LIL), is equipped with a specific final optics assembly involving two diffraction gratings instead of a classical focusing lens. Both gratings have a dimension of 420 X 470 mm2 and are working at an incidence of 25 degree(s).. Gratings are plano transmission holographic gratings directly engraved into fused silica substrates. The 1(omega) grating is working at the wavelength of 1.053 micrometers , its grooves are straight and equispaced. The 3(omega) grating, is a focusing grating working at the wavelength of 0.351 micrometers . Its grooves are curved and non equispaced. The gratings were designed and manufactured to present efficiencies superior to 90% on the whole clear aperture and an improved damage threshold. After Jobin Yvon's selection by CEA in 1999, specific equipment and facilities were put in place to manufacture these large gratings. The aim of this contribution is to present the early results of the development of this 1(omega) and 3(omega) gratings. After a short introduction to the 1(omega) and 3(omega) gratings specifications, manufacturing process, efficiencies result and AFM profiles of the first manufactured gratings will be detailed.
|