3.2 - Acoustic wave amplitude modulation of a multimode ring laser

Synchronous intercavity amplitude modulation of a ring laser operating well above threshold causes the numerous axial modes in each oppositely directed (OD) wave to lock into fixed amplitudes and phases. The light in the cavity is composed of two oppositely directed pulses of roughly 30-cm spatial extent. Saturation of the active medium dictates the location of the modulator for symmetric treatment of the OD pulses. If the modulator is located symmetrically (natural crossing), the pulses cross at the modulator and have identical amplitudes. If the modulator is located asymmetrically, the OD pulses generally have different amplitudes, experience different net population inversions in the active medium, and do not cross at the modulator. A frequency offset between OD pulses also appears when the modulator is located asymmetrically. Other phase-locking phenomena including extinction of one of the OD waves are observed. When the modulator is located symmetrically, the rotation sensing is improved with multimode phase-locked operation over that of the same ring laser operating near threshold. Frequency locking still occurs, however, at about three times the earth rate, or approximately 200 Hz. The locking frequency is found to depend upon modulator depth and frequency. The frequency locking may result from internally reflected pulses which are necessarily coincident with the proper pulses when the modulator is located symmetrically, and a modulation scheme has been devised to reduce such effects.