Tiled Gratings

October 2003

Tiled-Grating Pulse Compressor: An important element of the new OMEGA EP (extended performance) laser system is the grating compressor used to form short pulses of light at the output of the system. The most-promising technology is a holographically formed, etched grating that, combined with a multilayer dielectric (MLD) coating, forms a highly efficient grating used in reflection. Gratings with high damage threshold (approximately = 1 J/cm²) and a large aperture (~1 m) are required for OMEGA EP an alternative to meter-sized MLD gratings, LLE is investigating the coherent addition of multiple small gratings to form a larger grating — an approach referred to as grating tiling. Recent experimental work at LLE demonstrates that grating tiling is a viable approach toward energy scaling of pulse-compressed laser systems. Two gratings with a groove density of 1740 g/mm, manufactured by Jobin–Yvon, were independently mounted within a precision assembly to achieve coherent addition. Accurate control of the relative position and orientation of the grooves of each grating — a significant but achievable challenge — was accomplished by locating the tiled grating assembly within a diagnostic package consisting of a Fizeau interferometer and a focal-plane sensor. This diagnostic package provided precision control of the phase front, thus allowing controlled studies of the focal-plane irradiance.

Based on extensive first-hand experience with grating tiling, including laser pulse compression, a design approach is being established for the OMEGA EP system. A complete system prototype, including hardware and software, will be developed in advance of the OMEGA EP activation date.

High-fidelity pulse compression of a CPA laser was achieved with tiled gratings.

Flexures offer optimal characteristics for tile-grating mounts. The flexures carry the weight of the gratings with high mechanical stiffness.