Ultrafast MHz-rate burst-mode pump-probe laser for the FLASH FEL\n facility based on nonlinear compression of ps-level pulses from an\n Yb-amplifier chain

The Free-Electron Laser (FEL) FLASH offers the worldwide still unique\ncapability to study ultrafast processes with high-flux, high-repetition rate\nXUV and soft X-ray pulses. The vast majority of experiments at FLASH are of\npump-probe type. Many of them rely on optical ultrafast lasers. Here, a novel\nFEL facility laser is reported which combines high average power output from\nYb:YAG amplifiers with spectral broadening in a Herriott-type multi-pass cell\nand subsequent pulse compression to sub-100 fs durations. Compared to other\nfacility lasers employing optical parametric amplification, the new system\ncomes with significantly improved noise figures, compactness, simplicity and\npower efficiency. Like FLASH, the optical laser operates with 10 Hz burst\nrepetition rate. The bursts consist of 800 $\\mu$s long trains of up to 800\nultrashort pulses being synchronized to the FEL with femtosecond precision. In\nthe experimental chamber, pulses with up to 50 $\\mu$J energy, 60 fs FWHM\nduration and 1 MHz rate at 1.03 $\\mu$m wavelength are available and can be\nadjusted by computer-control. Moreover, nonlinear polarization rotation is\nimplemented to improve laser pulse contrast. First cross-correlation\nmeasurements with the FEL at the plane-grating monochromator photon beamline\nare demonstrated, exhibiting the suitability of the laser for user experiments\nat FLASH.\n