Intracavity filtering of the spectrum of a short-pulse FEL by inducing interpulse phase coherence

The number of active cavity modes in the short-pulse free-electron laser FELIX was reduced by a factor of 40 at a constant level of the saturated power. This was achieved by inducing phase-coherence between the 40 optical micropulses that are independently amplified at 1 GHz in the 25-MHz cavity. A 1-GHz Fox-Smith intracavity etalon was used to this aim. The resulting spectrum consists of a comb of frequencies that are spaced by 1 GHz. Based on a CW frequency analysis of the coupled cavities we predict that the individual frequencies have a linewidth of 170 kHz. The stability of the selected frequencies was analyzed. Mode hopping over 25 MHz was occasionally observed between macropulses of the laser, but not actually during the macropulse. Simulations by means of a simple pulse evolution model corroborated this behavior. We show that the comb of resonant frequencies can be scanned over a range of 1 GHz by scanning the length of the intracavity etalon. The work presented here gives the theoretical and experimental background of single-linewidth experiments that will be described in a separate paper. The latter experiments concern the selective transmission of a single cavity mode from the phase-locked signal by means of external etalons. This single line should be narrow, stable, and continuously tunable for high-power high-resolution experiments in the far-infrared region of the electromagnetic spectrum.