Geometry control and stabilization of a large ring laser gyroscope

Large Ring Laser Gyroscopes (RLGs) are powerful and high sensitivity instruments for rotation measurements. In this thesis, I will show the system developed to stabilize the ring laser geometry. This is a crucial point for RLG that must reach a high level of accuracy and precision. I will show the mathematical model, that describes the ring laser cavity deformations, and the experimental tests on the optoelectronic system, based on this model, along with some test measurements and considerations that validate our control system. I will also describe other upgrades developed for ring lasers to solve some of the well-known issues of large ring laser gyroscopes. To increase the stability of the laser inside the ring cavity, we have realized a power control system. With this device, we can control and stabilize the laser power, and switch on and off the laser itself remotely. Another common problem with RLGs involves a specific part of it, the capillary. This is a pyrex tube that is used to sustain the plasma that supplies the laser, and it is also employed as a transversal mode selector. This function makes difficult the alignment procedure of the ring, and it is a source of power losses. I will describe the first tests on a new mode selector technique that reduces these losses. In the end, RLGs dedicated to high precision measurements need high-quality mirrors. In our case, the mirrors must be "super mirrors", otherwise mirrors with a reflectivity of 99.999%. It is particularly hard to realize this kind of mirrors. To verify their performance, we have designed and developed a test bench able to measure their reflectivity with an error of a few ppm (part per million). I will describe the apparatus, and the results obtained by testing some mirrors.