Selection of single-mode radiation from a short-pulse free-electron laser

We demonstrate the selection of narrow-band radiation from the output of an infrared free-electron laser operating with electron pulses of a few picoseconds duration. A continuous output with a linewidth of less than 10^-3 cm^-1 and a powder in the order of 10 W in macropulses with a duration around 10 {μ}s can be obtained. The number of active cavity modes is reduced by a factor of 40 by inducing coherence between separate optical pulses in the cavity. An external Fabry-P\'erot interferometer is used to select one of the remaining modes.