The Laser Megajoule (LMJ)
Gratings
The Laser Megajoule (LMJ) is a high energy laser facility under construction in Bordeaux for the French nuclear research agency (Commissariat à l’Energie Atomique, CEA). At completion, 240 pulsed laser beams will be focused on a 2 mm target, delivering 2 MJ and producing the high density, pressure and temperature conditions where nuclear fusion triggers.
An original feature of the LMJ is the use of large diffractive optic components, where the only comparable system in the world (the American National Ignition Facility at Lawrence Livermore Laboratories in California) uses classical dioptric components.
Thanks to a close cooperation between CEA and HORIBA Jobin Yvon scientists, the feasibility of these unique components (400×400 mm focusing gratings) was confirmed and production started in 2000 for the demonstration prototype which confirmed the high performance of the design.
The profile figure presents the SEM profile of the gratings produced at HORIBA Jobin Yvon: the groove depth is 2 times the period which was a real challenge. The uniformity over the 420x470 mm surface is also a technological achievement.
Specifications of the Transmission Gratings for High Energy Lasers
To produce the LMJ-type transmission gratings, we produce first a holographic mask, then we transfer the mask modulation directly into the fused silica substrate. So the grating is made only of fused silica (without any photoresist on epoxy layer). As a result the laser-induced damage threshold is as high as a fused silica plate.
We are producing two types of transmission gratings for high energy lasers:
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gratings 1ω: gratings with straight and equidistant
lines, 800 l/mm, optimized for 1.053 µm, 92 to 94% average efficiency
on TM polarization.
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gratings 3ω: focusing grating with curved
and non- equidistant lines, 2400 l/mm optimized for 351 nm, 90 to 92%
average efficiency on
TM polarization.
This focusing grating acts as a stigmatic focusing lens.
Measured damage threshold =
25 J/cm² at 1.053 µm
for ns pulse
12 J/cm² at 351 nm for ns pulse
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| SEM Groove Profile of 2500 l/mm Transmission Grating | The efficiency map of a 1ω LMJ Transmission Grating demonstates the largescale ion etching uniformity |
