Long-term stabilization of the length of an optical reference cavity
Résumé
To obtain a high degree of length stabilisation 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 N$^{th}$ mode of a reference Fabry-Perot cavity and part of the laser light is shifted in frequency by an acousto-optical modulator (AOM) mounted in double-pass, matching half of the free spectral range of the reference cavity to be in resonance with the (N+1)$^{th}$ mode of the 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.