FERMI was the first free-electron laser (FEL) facility where external seeding was implemented. It is based on the high-gain harmonic generation (HGHG) seeding scheme. The initially planned FERMI wavelength ranges of 40–100 nm in single-cascade (FEL1) and 10–40 nm in double-cascade fresh-bunch mode (FEL2) have been largely extended. At present, the facility typically operates in the ranges of 17–110 nm (FEL1) and 4–20 nm (FEL2). The laser system providing the external ultraviolet (UV) seed pulses required by the scheme was designed nearly 15 years ago and is entirely based on Ti:sapphire pumped infrared (IR) optical parametric amplifier (OPA) technology, at that time the only laser configuration allowing one to reach the needed parameters. Originally, a single CPA Ti:sapphire regenerative amplifier was used as a pump. The need for continuous improvement of the seed pulses for both FEL lines, and the high flexibility required (i.e., seed parameters optimized individually for nearly every beamtime) has driven the continuous development and improvements of the original configuration. FERMI’s presently unique pump–probe scheme based on a seed-laser-derived pulse delivered to the end-stations for pump–probe experiments allows a very tight synchronization of the FEL pulse to the seed. This scheme provides nearly jitter-free pump–probe pulse pairs and has been an additional driver for constant improvement and increase of the complexity of the seed laser system. Here we describe the layout of the main laser system, including details on the HGHG seeding dedicated part. Timing synchronization, online diagnostics and feedbacks are essential for the successful user operation of the facility and are also addressed. The scheme used for delivering one part of the pulses generated by the seed laser system for pump–probe experiments has already been described elsewhere[4,5]. It is briefly discussed here to the extent needed to understand better the laser system layout. The final part of the paper contains an outlook of the new seed laser system design needed for the implementation of the echo-enabled harmonic generation (EEHG) seeding scheme at FERMI.
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