Tunable Pulse Shape DUV Photocathode Laser for X-ray Free Electron Lasers at DESY

Modern X-ray Free Electron Lasers (XFEL) enable a variety of scientific applications [1] . For driving their electron gun photocathode, they require a robust deep ultraviolet (DUV) laser source. To enable the capability for generating both 1 ps electron bunches for few femtoseconds, single spike self-amplified stimulated emission (SASE) X-ray pulses and high charge 20 ps electron bunches for standard operation, we demonstrate a 257.5 nm photocathode laser with widely tunable output pulse durations. The schematic of the set-up is shown in Fig. 1 . The laser front-end consists of a pm-Yb-fiber oscillator and 3-stage amplifier, which generates a 20 mW, 1 MHz pulse train at 1030 nm center wavelength to seed a 4-stage Yb:YAG amplifier chain. An additional nonlinear Yb-fiber amplifier generates 100 mW, 150 fs for cross-correlation reference [2] . The Yb:YAG amplifier chain boosts the laser pulse energy to more than 180 µJ. The front-end contains an additional fiber stretcher and a programmable high-resolution spectral shaper for tuning the amplifier output spectrum. This allows after passing a fixed dispersion transmissive grating compressor to tune the NIR output pulse duration between 1 ps and 20 ps.