Long-term stabilization of the length of an optical reference cavity

To obtain a high degree of long-term length stabilization of an optical reference cavity, its free-spectral range is locked by means of an accurate and stable frequency synthesizer. The locking scheme is twofold: a laser is locked on the Nth mode of a reference Fabry-Perot cavity and part of the laser light is shifted in frequency to be in resonance with the (N+1)th mode of the cavity. This shift is generated by an acousto-optical modulator (AOM) mounted in a double-pass scheme, matching half of the free-spectral range of the reference cavity. The resulting absolute stabilization of the length of the cavity reaches the 10(-11) level per second, limited by the lock transfer properties and the frequency stability of the AOM control synthesizer.