Seed level requirement for improving the temporal coherence of a Free-Electron Laser

The temporal coherence of Free-Electron Laser (FEL) sources, which exhibit, in the self-amplified spontaneous-emission mode, spiking spectral and temporal distributions, can be drastically improved by seeding with an external laser or high-order harmonics. Here, experiments at 160 nm put in evidence that the improvement of spectral properties (and thus temporal coherence) of the FEL radiation takes place for a larger seed intensity than the one required to overcome the shot noise. Copyright (C) EPLA, 2009