Two-grating compressor for sub-exawatt lasers: Optimal design for highest focal intensity

The generation of exawatt-class lasers, as well as high focal intensities, is an important goal in ultra-intense laser physics, and grating compressors are the key devices for achieving this goal. Since large-size diffraction gratings are not perfectly plane, any grating compressor inevitably introduces spatiotemporal coupling phase distortions, which reduce the focal intensity of an ultra-intense laser. In this paper, we show that the dependence of the focal intensity on the root mean square (RMS) grating surface roughness is Gaussian and that for RMS roughness below 10 nm, the focal intensity decrease is negligible, giving an RMS requirement of gratings for ultra-intense lasers. In a two-grating compressor, the impact of the large-scale part of the grating surface profiles may be negligible, because it can be completely eliminated by use of two adaptive mirrors. However, in a four-grating compressor, such elimination is almost impossible. On the basis of a complete analytical model and an improved numerical code, we study the grating compressors of two sub-exawatt laser projects (XCELS and SEL-100PW), and the results show that a two-grating compressor is the best choice for exawatt-class ultra-intense lasers such as XCELS, SEL-100PW, and OPAL. This work also provides a basis for compressor grating fabrication as part of the further development of ultra-intense lasers.