Bistable traveling-wave oscillations of ion ring laser

The dc-excited argon ion laser in an optical ring resonator has been found to oscillate spontaneously in one circulatory direction, for all but a relatively narrow range of gain levels above threshold. Near threshold the TW oscillations occur in the direction opposite that of the ionic drift motion, whereas at high excitation levels they can occur in either direction. The direction of TW oscillations at any level can be controlled or switched, either by means of an external mirror or a weakly resonant loss element which can be tuned to favor either direction. The tendency of the ion ring laser to oscillate in a single direction s attributed in part to the Doppler shift associated with the ion drift velocity, and chiefly to its large natural linewidth. Hole widths burnt in the gain-velocity profile, which are significant in mode competition, usually exceeded the mode spacing of the resonator by large margins. Physical arguments have been advanced to show that, when such large hole widths prevail, and spatial cross-relaxation depends only on ion motions, only running waves can oscillate in a ring resonator, and moreover they will all oscillate in the same direction. Close to threshold, the preferred direction for TW oscillations corresponds to that for which the gain profile is Doppler-shifted to longer wavelengths, possibly because of slightly smaller scattering losses.