Seed Laser Upgrade for EEHG Operation of FERMI FEL-1

Echo Enabled Harmonic Generation (EEHG) is a promising scheme to extend the wavelength range of externally seeded FELs to shorter wavelengths [1]. The FERMI FEL-1 facility is being upgraded to include the EEHG option in order to reach wavelengths down to 10 nm by 2023, about two times shorter compared to the current ~20 nm limit in High Gain Harmonic Generation (HGHG) mode. This requires the generation, propagation and insertion of a second tunable Deep UV (DUV) seed pulse in a second modulator, and a DUV seed at fixed wavelength inserted in the first modulator. The original FERMI seed laser system design [2] has to be therefore strongly modified for generating the two seed pulses, while still providing the possibility for easy switch to HGHG mode of both FEL1 and FEL2. I addition, the new design has to preserve the distinct feature of the FERMI pump-probe scheme, namely to generate the IR pulses sent to the experimental stations for pump-probe experiments.