Design of compressors for FEL pulses using concave deformable gratings

Chirped pulse amplification (CPA), which allows the generation of extremely powerful ultrashort laser pulses in the infrared and visible spectral ranges, has recently being demonstrated for the first time on a seeded FEL in the soft X-rays [1]. The technique relies on stretching the seed pulse in time by means of a linear frequency chirp before amplification. The frequency chirp of the seed is transmitted to the FEL harmonic pulse generated at the end of the radiator and is compensated by an optical compressor, consisting of two plane diffraction gratings at grazing incidence and two plane mirrors, which steer the FEL beam back to its original propagation axis. The chirped FEL pulse, that was centered at 37.4 nm with duration of ≈140 fs, has been compressed down to 50 fs, only a factor of 1.1 above the transform limit.