Length measurement and stabilisation of the diagonals of a square area laser gyroscope

Large frame ring laser gyroscopes are top sensitivity inertial sensors able to measure absolute angular rotation rate below $\rm \mathbf{prad/s}$ in few seconds. The development of ring laser based on a simple mechanical structure, usually called hetero lithic structure, requires to control the geometry of the apparatus. Our prototype GP2 is a middle size ring laser, whose main purpose is the geometry control with opto-mechanical means. The first tests have been performed, and the data analysed. The lengths of the diagonals of the ring cavity have been measured with $\pmb{\mu {\rm m}}$ accuracy, and continuous operation has been obtained, without loss of sensitivity. GP2 is located in a standard laboratory, with a temperature stabilisation around 1 degree Celsius. The analysis shows that middle size ring lasers can obtain nrad/s sensitivity also in a standard environment.