Grating monochromator with ultrafast response for FLASH2 at DESY

We discuss the design of grating-based monochromators for coherent ultrafast pulses in the extreme-ultraviolet. The main application of such instruments is the monochromatization of ultrafast high-order laser harmonics and freeelectron-laser pulses. We present the conditions to be fulfilled by a grating monochromator that doesn't increase the pulse duration significantly longer than the Fourier limit. A full correction of the pulse-front tilt requires the use of two gratings in a time-delay compensating configuration. The grating-monochromator configuration is applied to the design of the monochromatic beamline for FLASH2 at DESY. The monochromator has to be tunable in the 50-1000 eV energy range with a resolving power higher than 1000 and an instrumental response shorter than 100 fs in the whole energy range. Given the actual parameters of the FLASH2 radiation and the restrictions in the positioning of the optical elements, the tilt of the pulse-front given by a single grating would give an unacceptable temporal stretching of the pulse. This has to be corrected by a second grating in the compensated configuration. The residual distortion of the pulse-front after the second grating is well below 10 fs.