Long-Range Coherence in a Short-Pulse Fel

In the free electron laser for infrared experiments (FELIX), a radio-frequency linear accelerator is used to deliver electron micropulses with a duration of about 3 ps. The micropulses are repeated at a frequency of 1 GHz. The length of the optical cavity is 6 m, so that 40 independent optical micropulses are simultaneously circulating in the cavity. Almost complete coherence has been induced between the multiple pulses in FELIX by means of an intracavity Fox-Smith interferometer. A large fraction of the cavity modes is suppressed in this way, and the remaining lines have a relative width on the order of 1:10(6). The stability is in the same range. The gain and saturated power of the laser are almost unaffected. A single mode can in principle be selected from the output beam. This makes the resulting radiation useful for high-resolution spectroscopy applications. Coherence between successive micropulses has also been observed, quite unexpectedly, without the Fox-Smith arrangement.