Ring laser (RL) gyroscopes are, at present, the most precise sensors of absolute angular velocity. In the near future, their application is foreseen to provide ground based tests of General Relativity. We have recently proposed a tri-axial array of RLs that can reach the sensitivity, accuracy, and long term stability required to measure the inertial frame dragging induced by the rotating Earth, as predicted by General Relativity. The effect, also known Lense-Thirring effect, amounts for the Earth to 1 part in 10 9 of its rotation rate, thus requiring an unprecedented sensitivity and accuracy of experimental apparatus. An array of at least 3 RLs would allow us to measure not only the rotation rate, i.e. the angular velocity modulus, but also the angular velocity vector. In this way, having at disposal the time series of the daily estimate of Earth rotation vector from the International Earth Rotation and Reference System Service, it would be possible to isolate the Geodetic and Lense-Thirring contributions. Our proposal GINGER (Gyro-scopes IN GEneral Relativity) is intended to push the present knowledge of RL physics and technology to achieve an accuracy in the estimation of the Earth rotation rate of 1 part in 10 9. In the experimental apparatus we have to account for systematic errors resulting from non linear dynamics of the active laser medium, and changes of the optical cavity geometry. The redundancy of the array, e.g. the addition of a ring almost parallel to the Earth rotation axis, should allow for the reduction of such errors at the level of the geometry control. In this contribution we describe the intermediate prototypes GP2 and GEMS (GINGER External Metrology System) devoted to control the geometrical fluctuations of a RL cavity and the 3D geometry of the RL array (dihedral angles among RLs), respectively.

He-Ne ring laser gyroscopes are, at present, the most precise devices for absolute angular velocity measurements. Limitations to their performance come from the non-linear dynamics of the laser. Following the Lamb semiclassical theory, we find a set of critical parameters affecting the time stability of the system. We propose a method for estimating the long term drift of the laser parameters and for filtering out the laser dynamics effects from the rotation measurement. The parameter estimation procedure, based on the perturbative solutions of the laser dynamics, allow us to apply Kalman Filter theory for the estimation of the angular velocity. Results of a comprehensive Monte Carlo simulation and results of a preliminary analysis on experimental data from the ring laser prototype G-Pisa are shown and discussed.

GINGERino is a large frame laser gyroscope investigating the ground motion in the most inner part of the underground international laboratory of the Gran Sasso, in central Italy. It consists of a square ring laser with a 3.6 m side. Several days of continuous measurements have been collected, with the apparatus running unattended. The power spectral density in the seismic bandwidth is at the level of 10 −10 (rad/s)/ √ Hz. A maximum resolution of 30 prad/s is obtained with an integration time of few hundred seconds. The ring laser routinely detects seismic rotations induced by both regional earthquakes and teleseisms. A broadband seismic station is installed on the same structure of the gyroscope. First analysis of the correlation between the rotational and the translational signal are presented.

Abstract. Ultra sensitive ring laser gyroscopes are regarded as potential detectors of the general relativistic frame-dragging effect due to the rotation of the Earth: the project name is GINGER (Gyroscopes IN GEneral Relativity), a ground-based triaxial array of ring lasers aiming at measuring the Earth rotation rate with an accuracy of 10 −14 rad/s. Such ambitious goal is now within reach as large area ring lasers are very close to the necessary sensitivity and stability. However, demanding constraints on the geometrical stability of the laser optical path inside the ring cavity are required. Thus we have started a detailed study of the geometry of an optical cavity, in order to find a control strategy for its geometry which could meet the specifications of the GINGER project. As the cavity perimeter has a stationary point for the square configuration, we identify a set of transformations on the mirror positions which allows us to adjust the laser beam steering to the shape of a square. We show that the geometrical stability of a square cavity strongly increases by implementing a suitable system to measure the mirror distances, and that the geometry stabilization can be achieved by measuring the absolute lengths of the two diagonals and the perimeter of the ring.