Design, optimization and development of pulse compression gratings for the MPW-HE LIL

A multi-peta-watt laser is being developed inside the Laser Integration Line (LIL), prototype of the future Megajoule Laser (LMJ). This peta-watt laser line will deliver impulsions of 7.2 PW, 500 fs at the wavelength of 1053 nm. The chirped pulse amplification (CPA) method is used. We work on the design and the fabrication of gratings used in the CPA scheme. Since damage threshold of more than 3 J/cm2 for 500 fs are needed, we retained multi-dielectric gratings (MLD) instead of classical gold gratings. These gratings, developed in mid 90's, consist in a multi-dielectric mirror stack with its top layer engraved with a grating structure. We have been working to optimize the groove geometry of the MLD grating to minimize the E field maximum in the structure. We have showed that it is possible to construct gratings with a diffraction efficiency of 99 per cent. It is possible to decrease the value of E2 by increasing the depth the modulation and decreasing the width of the pillars of the structure. In our configuration, a reduction of E2 from 2.7 to 1.1 is attainable. We also propose an innovative design using a mixed metal/dielectric mirror stack: a gold insert is set between the substrate and the multi-dielectric mirror stack in order to reduce the stack thickness while preserving optical properties and damage threshold