LONG-RANGE COHERENCE IN A SHORT-PULSE FEL

In the free electron laser for infrared experiments (FELIX), a radio-f requency linear accelerator is used to deliver electron micropulses wi th a duration of about 3 ps. The micropulses are repeated at a frequen cy of 1 GHz. The length of the optical cavity is 6 m, so that 40 indep endent optical micropulses are simultaneously circulating in the cavit y. Almost complete coherence has been induced between the multiple pul ses in FELIX by means of an intracavity Fox-Smith interferometer. A la rge fraction of the cavity modes is suppressed in this way, and the re maining lines have a relative width on the order of 1:10(6). The stabi lity is in the same range. The gain and saturated power of the laser a re almost unaffected. A single mode can in principle be selected from the output beam. This makes the resulting radiation useful for high-re solution spectroscopy applications. Coherence between successive micro pulses has also been observed, quite unexpectedly, without the Fox-Smi th arrangement.