Large-frame ring laser gyroscopes are extremely sensitive inertial detectors of rotational motion. When properly coupled to the ground, they provide precise measurements of the Earth rotation rate and give important informations to geodesy and geophysics. Recent advances in this technology led to consider their application to fundamental physics. In this context is GINGER - Gyroscopes IN GEneral Relativity (INFN Pisa, TU-LMU Munich & University of Canterbury), a scientific proposal for testing General Relativity (local observation of the Lense-Thirring effect) with a ground-based array of ring laser gyroscopes [1, 2]. The experimental target is to locally measure the Earth rotation rate with a relative precision better than one part in 109, corresponding to an absolute rotational resolution of 10-14 rad/s.
After a brief dissertation about the ring laser technology, in this talk I will address the problem of the deformations of the ring cavity geometry that, induced mainly by temperature, pressure and aging of materials, affect the path of the laser beams and set up the present limit of the most stable devices. To measure the Lense-Thirring effect requires a very high precision control of the cavity mirrors positions. This goal can be achieved only by means of interferometric methods, taking advantage of modern optical metrology.
I’ll report about the development of a sub-nanometer length metrology for next generation square ring laser gyroscopes, that has been the final result of my PhD studies [3, 4, 5].
 Bosi F et al 2011, Measuring Gravito-Magnetic Effects by Multi Ring-Laser Gyroscope, Phys. Rev. D 84(12), 22-45
 Di Virgilio A et al 2014, A ring lasers array for fundamental physics, Compt. rend. Phys. 15(10), 866-874
 Belfi J et al 2014, Interferometric length metrology for the dimensional control of ultra-stable ring laser gyroscopes, Class. Quantum Grav. 31(22), 225003
 Santagata R et al 2015, Optimization of the geometrical stability in square ring laser gyroscopes, Class. Quantum Grav. 32(5), 055013(11)
 Santagata R 2015, Sub-nanometer length metrology for ultra-stable ring laser gyroscopes, PhD Thesis, University of Siena