Hänsch-Couillaud locking of a large Sagnac interferometer: advancing below the flicker floor

Large Sagnac interferometers in the form of active ring lasers have emerged as unique rotation sensors in the geosciences, where their sensitivity allows to detect geodetic and seismological signals. The passive laser gyroscope variant, however, is still at a stage of development, and adequate laser frequency stabilization techniques are explored to reach the desired levels of sensitivity and stability. Here, we present the first Hänsch-Couillaud locked passive laser gyroscope. We employ a lock-in scheme to overcome the flicker floor to achieve a sensitivity of 3.1 nrad/s, corresponding to a fraction of $7.7 \cdot 10^{5}$ in the Earth's rotation rate.