Long term stabilization of large frame laser gyroscopes

The operation of a large frame ring laser gyroscope (RLG) with ultimate sensitivity requires the development of a very careful active control of the ring geometry. This can be obtained in a square ring by measuring against a reference laser the diagonals length and eventually implementing a procedure to equalize them. The technique to measure and to stabilize the diagonals has been tested on GP2, a RLG of reduced dimension installed in INFN laboratories in Pisa. A resolution better than 1 ppm, limited by the local anthropic noise, has been obtained.