Gas laser mode-locking using an external acoustooptic modulator with a potential application to passive ring gyroscopes

The fundamental pulsation frequency of a mode-locked laser is shown to be multipliable by an integer through a change of the length of a secondary cavity formed by an acoustooptic modulator (AOM) and an external mirror. A He-Ne laser with an optical path length of 177.5 cm with an intermode frequency separation of 84.5 MHz was used, with the frequency monitored by a scanning Fabry-Perot interferometer. A mirror reflected the diffracted beam back on itself to expose the injected beam to a Doppler shift to produce a total shift at the intermode beat frequency. Mode-locking occurred when the reflecting mirror was set at a distance from the laser mirror equal to the length of the laser cavity. Smaller lengths of reflection also produced waveforms equal to the mode-locked frequency. Using the beam folding technique to fabricate compact gyroscopes.