Direct spectral measurements of a quasi-cw free-electron laser oscillator

Mode competition in a far-infrared, free-electron laser operating in the quasi-cw regime has been investigated by means of direct spectral measurement with a heterodyne receiver system. About 30% of the spectra had a single longitudinal mode. The observed data compare reasonably well with theory. It is found that even in a regime of multimode operation the laser signals are only weakly modulated in time, as observed with a video detector. Such an observation is consistent with a phase locking of multiple modes, as predicted by theory.