Demonstration of Real-Time, Phase-Locked Alignment of Tiled Gratings for Chirped-PulseAmplified Lasers

242 LLE Review, Volume 100 Introduction The OMEGA EP (extended performance) petawatt, multikilojoule, solid-state laser, presently under construction at LLE, includes compressors with four tiled-grating assemblies (TGA’s), where each assembly contains three subaperture diffraction gratings, to compress the pulse before it is focused onto target.1,2 The tiled-grating compressor (TGC) was chosen to stay within the damage-threshold limitations of the largest currently available multilayer dielectric (MLD) gratings, together with the anticipated intensity modulation of the laser output beam. The four-grating compressor is shown schematically in Fig. 100.17. The holographically generated grating tiles3,4 have a line frequency of 1740 grooves/mm and a clear aperture of 400 × 470 mm. The individual BK-7 grating substrates have a thickness of nominally 100 mm and are approximately 50 kg in weight. For the TGC to deliver the compressed pulse with a near-diffraction–limited energy distribution on target, the individual tiles have to be aligned to each other with an optical-path difference (OPD) of less than 50 nm. An active control approach, similar to a three-actuator deformable mirror, has been chosen to maintain the correct alignment between grating tiles.