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 power in the order of 10 W in macropulses with a duration around 10 mus 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-Perot interferometer is used to select one of the remaining modes.