Mode-Locked Traveling-Wave Oscillations of an Internally Loss-Modulated He-Ne Ring Laser

We have investigated novel behavior in a He-Ne ring laser induced by internal loss-modulation. The characteristic behavior is mode-locked unidirectional and bidirectional traveling-wave oscillations which depend upon the fine tuning of the modulation frequency, modulator position, and modulation depth. The mode-locked unidirectional oscillation is bistable when the modulator is placed at specific positions named natural crossing points. We have considered the behavior theoretically using the coupled-mode equations for a multimode gaseous ring laser developed previously. Analytical solutions for a simple case are investigated to find the general behavior, and numerical solutions and detailed experiments for multimode operations are also shown. They are in good agreement with each other.