Speaker
Description
Seismic noise remains one of the principal challenges to extending the detection bandwidth of gravitational-wave detectors toward lower frequencies. As the Einstein Telescope aims to achieve unprecedented low-frequency sensitivity, the seismic isolation of its test masses must be significantly improved.
A major difficulty in active isolation at very low frequencies arises from the strong coupling between the different degrees of freedom of the suspension chain. The sensors currently employed for low-frequency isolation are sensitive to both translational and rotational motion, making it difficult to distinguish between them. This limits the performance of isolation systems in the frequency range where both effects are significant, typically below 1 Hz. Several approaches exist to mitigate these couplings, ranging from direct mechanical decoupling solutions to digital compensation techniques.
An alternative approach is the direct measurement and control of the suspension's rotational motion. This requires a sensor capable of detecting extremely small rotations, on the order of nrad/√Hz, while completely rejecting translational motion. Fiber-optic gyroscopes (FOGs), based on the Sagnac effect, are attractive candidates due to their intrinsic insensitivity to translation. Their main limitation is sensitivity, with the best commercial devices typically reaching noise floors of about 10 nrad/s/√Hz.
In this work, we investigate the use of FOGs for low-frequency isolation and evaluate their added value on the ET-CRISTAL platform, a full-scale hybrid isolation system prototype for ET test masses.