A heterodyne frequency-stabilization method for large ring laser gyroscopes with Sub-nW output power

We present the characteristics and the performances of a heterodyne detection scheme allowing a robust frequency lock of the G-Pisa ring laser [1,2] to a He-Ne Doppler shift stabilized reference laser. The system demonstrated a continuous unattended control of the laser frequency for several days of operation of the gyroscope with a single mode output power of about 500 pW.