  To obtain a high degree of long-term 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 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.
