A ring laser gyroscope with optical subtraction

A prototype passive ring laser gyroscope uses heterodyne detection of optically derived common mode and differential mode error signals. A 7 {dollar}times{dollar} 7 cm{dollar}sp2{dollar} ring cavity demonstrates a rotation rate sensitivity of 1.3 degrees/hr for integration periods from 1 to 30 s. Long-term sensitivity is limited by AM contamination of the phase modulated input beams and phase drifts in the injection loop interferometer.; The alignment of the injected beams is stabilized to the reference cavity by heterodyne detection of misalignment-induced coupling to the off-axis modes. Phase sensitive detection of partially transmitted sidebands produces error signals that are nulled by a control system that steers the input beams. The alignment technique demonstrates sensitivities to tilts of 0.1 nrad/{dollar}sqrt{lcub}rm Hz{rcub}{dollar} and to lateral beam displacements of 0.08 nm/{dollar}sqrt{lcub}rm Hz{rcub}{dollar} in the 1 to 1000 Hz frequency range for a two degree-of-freedom system. Maximum alignment error suppression is 80 dB.; A proposed Earth-based experiment capable of measuring the gravitomagnetic frame-dragging effect to 1% requires a large passive ring laser gyroscope (300 m optical path length) integrated with a long-baseline (100m) stellar interferometer located at the South Pole.