AC/DC: The FERMI FEL Split and Delay Optical Device for Ultrafast X-ray Science

Free-electron lasers (FELs) are the most advanced class of light-sources, by virtue of their unique capability to lase high-brightness pulses characterized by wavelengths spanning the extreme-ultraviolet, the soft and hard X-ray spectral domains, as well as by temporal lengths lying in the femtosecond (fs) timescale. The next step to push the current standards in ultrafast X-ray science is strongly linked to the possibility of engineering and exploiting time-resolved experiments exclusively for FELs pulses, ideally having different colors tunable at specific electronic resonance of the chemical elements. At the seeded FERMI FEL (Trieste, Italy) this goal is committed to the optical device known as AC/DC, which stands for the auto correlator/delay creator. AC/DC is designed to double the incoming FEL pulse splitting the photon beam by inserting a grazing incidence flat mirror, thus preserving the spectral and temporal properties, and further delaying one of these two pulses in time. It can independently tune the FEL pulses fluence on the two optical paths by means of solid-state filters, too. Here, we present a detailed description about this optical device. Strong emphasis is dedicated to the AC/DC opto-mechanical design and to the laser-based feedback systems implemented to compensate for any mismatch affecting the FEL optical trajectory, ascribable to both mechanical imperfections and paraxial errors rising during a temporal delay scan.

Automated summary

Free-electron lasers (FELs) are advanced light sources that can produce high-brightness, femtosecond pulses in extreme-ultraviolet, soft and hard X-ray spectral ranges. The development of time-resolved experiments using FEL pulses with tunable colors at specific electronic resonances is the next step in ultrafast X-ray science. The AC/DC optical device at the FERMI FEL facility in Trieste, Italy splits the incoming FEL pulse using a grazing incidence flat mirror, preserves its spectral and temporal properties, and delays one of the split pulses. It also allows for independent control of FEL pulse fluence using solid-state filters.