Double-configuration grating monochromator for extreme-ultraviolet ultrafast pulses

We present the design and characterization of a double-configuration grating monochromator for the spectral selection of extreme-ultraviolet ultrafast pulses. Two grating geometries are joined in an instrument with two interchangeable diffracting stages, both used at grazing incidence: one with the gratings in the off-plane mount (OPM), the other in the classical diffraction mount (CDM). The use of two stages gives great flexibility: the OPM stage is used for sub-50 fs time response and low spectral resolution, while the CDM stage is for 100–200 fs time response and high spectral resolution. The monochromator spectral and temporal performances have been experimentally demonstrated on a high-order laser-harmonics beam line.