The extreme ultraviolet split and femtosecond delay unit at the plane grating monochromator beamline PG2 at FLASH.

An extreme ultraviolet split and femtosecond delay unit based on grazing incidence Mach-Zehnder geometry has been designed and implemented on the plane grating monochromator beamline PG2 at FLASH, the Free Electron Laser at DESY. This device splits the FLASH radiation into two beams, which can independently be steered, filtered and temporally delayed between -5.1 and +5.1 ps with uncertainty in the temporal accuracy of 210 as. To demonstrate the performance of this device, we have performed longitudinal coherence studies of FLASH radiation as well as measured the pulse length by nonlinear two-photon double-ionization in helium.