Seed Laser Pulse Shaping for Free-Electron Laser FLASH

External Seeding FEL schemes with harmonic conversion, such as High-Gain Harmonic Generation (HGHG) and Echo-Enabled Harmonic Generation (EEHG), offer the unique possibility of generating fully coherent high-power radiation pulses in the XUV and soft X-ray domain. In particular, those schemes enable a high level of control over the FEL output by tailoring the seed laser pulse properties. It has already been shown that the quadratic phase of the FEL output can be adjusted or reduced to nearly zero with a grating pulse compressor or glass plate in the seed laser [1]. Furthermore, e.g., by seeding with two individual pulses, FEL double pulses with tunable temporal and spectral separation can be produced [2]. Within the FLASH2020+ upgrade of the FEL facility FLASH, external seeding will be implemented [3]. This requires the development of a laser system called SLASH with highly demanding UV pulse properties, which will be achieved using Optical Parametric Amplification (OPA) in combination with two sum frequency generation stages for UV conversion [4].