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

Abstract

Large frame ring laser gyroscopes are top sensitivity inertial sensors able to measure absolute angular rotation rate below prad/s in few seconds. The GINGER project is aiming at directly measuring the Lense-Thirring effect with an 1% precision on an Earth based experiment. GINGER is based on an array of large frame ring laser gyroscopes. The mechanical design of this apparatus requires a micrometric precision in the construction and the geometry must be stabilized in order to keep constant the scale factor of the instrument. The proposed control is based on square cavities, and relies on the length stabilization of the two diagonals, which must be equal at micrometric level. GP2 is the prototype devoted to the scale factor control test. As a first step, the lengths of the diagonals of the ring cavity have been measured through an interferometric technique with a statistical accuracy of some tens of nanometers, and they have been locked to the wavelength of a reference optical standard. Continuous operation has been obtained over more than 12 hours, without loss of sensitivity. GP2 is located in a laboratory with standard temperature stabilization, with residual fluctuations of the order of 1 • C. Besides the demonstration of the control effectiveness, the analysis of the Sagnac frequency demonstrates that relative small and low-cost ring lasers (around one meter of side) can also achieve a sensitivity of the order of nrad/s in the range 0.01−10 Hz in a standard environment, which is the target sensitivity in many different applications, such as rotational seismology and next generation gravitational waves detectors.

Noise Analysis of a Passive Resonant Laser Gyroscope

“…The residual cavity length fluctuation noise is not a limitation at this moment, which benefits from the sufficient primary loop gain shown in Figure 5 a. If we implement an active stabilization scheme, like in [ 16 , 32 , 34 ], both of them can be suppressed significantly.…”

Sagnac Gyroscopes and the GINGER Project

“…This is the required stability for the 1% test of the Lense-Thirring effect (Di Virgilio et al, 2017). The GP2 prototype has been installed in a laboratory of the basement of the INFN Pisa building with the purpose of studying the experimental details of these procedures, and the main parts of the above explained procedure have already been tested (Beverini et al, 2020).…”

Analysis of ring laser gyroscopes including laser dynamics

“…Data acquired by our prototypes GINGERINO and GP2 are utilised. GP2 is an apparatus with comparatively low quality mirrors and located in a noisy environment [25,26], while GINGERINO is located in a very quiet place [24,27], and is presently equipped with state of the art mirrors. We remark that GINGERINO is free running, the geometry is not controlled and long time operation and high duty cycle (> 90%) are possible since it is located in the underground Gran Sasso laboratory, which exhibits high natural thermal stability (typically ∼ 0.01 • C in one day).…”

“…Figure 5 compares the fringe contrast (TOP) and the relative phase (BOTTOM) for GP2 data taken during the geometry control test. 1 This was achieved by keeping constant the length of the two diagonals within 80 nm [26]. Figure 5 shows that mode jumps occur not only to compensate changes of the perimeter, but also in order to keep close to ± π 2 , keeping the relative phase close to a certain range, as required in the dissipative coupling regime.…”

“…GP2 has been designed to test the geometry control developed for GINGER and based on diagonal length measurements; data with and without geometry control have been compared and it has been checked that, with adequate analysis, sensitivities are comparable[26].…”