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High resolution optical inertial sensors for low frequency seismic isolation

par Amorosi, Anthony
Président du jury Redoute, Jean-Michel
Promoteur Deraemaeker, Arnaud ;Collette, Christophe
Publication Non publié, 2026-09-02

Thèse de doctorat

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This manuscript presents the design and characterisation of various optical sensors for an active seismic isolation system in 3rd-generation gravitational wave detectors. It is part of the E-TEST and SILENT ERC projects, and contributes to the development of a novel prototype of a passive/active seismic isolation system and Newtonian Noise measurement technique for the Einstein Telescope. The isolation system primarily consists of an inverted pendulum sitting on a 6D active isolation platform. The detector penultimate mirror hangs from a platform on top of the inverted pendulum. The Newtonian Noise measurement technique include a enhanced quantum gravimeter to complement classical seismometer networks. Three variations of sensors have been developed, all based on the same optical technology, and are used in several locations in the prototype. The core technology is an optical displacement sensor consisting of a quadrature Michelson interferometer. The optical scheme uses polarising elements to demodulate displacements of several wavelengths. The optical scheme also features a double-pass action in the measuring arm to accommodate rotational motions of the target. Normalisation and ellipse correction algorithms have been implemented to minimise nonlinear effects and provide reliable readings of motion of a few mm. The demodulation process, including the ellipse correction, has been implemented in real-time in a Simulink DSpace environment for active control purposes. The optical sensors have been experimentally characterised and a resolution of 10^10 m/root(Hz) at 10 mHz, 10^12 m/root(Hz) at 1 Hz and 2x10^13 m/root(Hz) Hz above 10 Hz has been demonstrated. These units are used as stand-alone displacement sensors for monitoring and local control at the inverted pendulum level.Two types of optical seismometers have been designed, using the above interferometer for reading the motion of the proof-mass. The seismometers are 1D sensors: one vertical (E-VINS) and one horizontal (E-HINS) sensor. The vertical sensor mechanics consists of a 1 DOF pendulum maintained in a horizontal position with a CuBe2 leaf-spring, and connected to the sensor frame with a clamped-blade hinge, achieving a 1.45 Hz resonance frequency. The horizontal sensor consists of a Watt's Linkage, or folded-pendulum, with a 0.51 Hz resonance frequency. They use the above-mentioned optical technology as readout and reach sub-pm resolution above their respective resonance frequency. The sensors have been characterised for cross-axis sensitivity and nonlinearity in the active stage of the HAM-ISI system at LIGO-MIT. 3 pairs of horizontal and vertical sensors have been produced to record the platform's 6 DOFs.The last instrument is an optical accelerometer (µVINS) for vibration compensation noise in an absolute quantum gravimeter. The µVINS is a 1D vertical sensor, whose design is a compact assembly of the E-VINS optical seismometer. The design is light-weight and compact, so as to fit the head of the quantum gravimeter. The sensor bandwidth is widened using a frequency-compensation filter up with a flat response to 100 Hz, for vibration noise compensation performance. A first implementation of the optical accelerometer inside the quantum instrument shows promising results for future hybridisation.