Modelling a High Precision Fibre Optic Gyroscope for Low-frequency Active Seismic Isolation Université de Liège

Abstract

The Einstein Telescope (ET) requires unprecedented low-frequency seismic isolation to extend its gravitational wave detection bandwidth. A key challenge is the tilt-horizontal coupling of standard inertial sensors, which makes rotational motion indistinguishable from horizontal translation, necessitating a dedicated rotation sensor with a noise floor of 10−10 rad/s/Hz^(1/2) in the 10 mHz to 10 Hz band.

This thesis presents the design and theoretical evaluation of a large-scale Fibre Optic Gyroscope (FOG) to meet these requirements. Through a progressive architectural evolution, a comprehensive noise budget is developed addressing both fundamental and environmental noise sources. Key implementations include a broadband ASE source to suppress Coherent Rayleigh Backscattering and the optical Kerr effect, a Multifunction Integrated Optical Chip (MIOC) for splitting and phase modulation, Lyot depolarisers, a quadrupole winding to cancel the Shupe effect, and a μ-metal enclosure against the Faraday effect. A large sensing coil D=2.5 m, L=20 km) maximises the Sagnac scale factor, alongside active RIN subtraction.

The final design achieves a theoretical noise floor of ∼10−6 rad/s/Hz​^(1/2), four orders of magnitude above the ET target but a marked improvement over the baseline. This work establishes a rigorous theoretical foundation and a clear roadmap for future experimental optimisation.