Instruments for seismic isolation: Improving performance of advanced displacement and inertial sensors for active suppression of seismic noise in Gravitational Wave detectors

Laser interferometric Gravitational Wave telescopes are limited in sensitivity by seismic noise in the low frequency region (below a 10 Hz). The aim of this research is to improve performance of an advanced displacement sensor and an ultra-low noise inertial sensor, that can help measuring and correcting for seismically induced motion of the GW detetctor’s test masses. In a Rasnik, light from a back-illuminated encoded ChessField mask is projected onto a pixel image sensor, and the relative position is tracked between mask and sensor. In past experiments, Rasnik was modelled to have resolution of below ∼0.5 nm in X and Y, while RMS resolutions of ∼25 nm were measured [1], a longstanding discrepancy central to this research. It was found that, analyzing spectral information of Rasnik response data, Rasnik did in fact reach its sub-nm modelled noise floor. A resolution floor of 퐴፧ = 270 pm/√Hz was measured, shown to be limited by SNR of the pixel content and white noise aliasing. What is more, with a different setup, an optical magnification trick was shown to result in RMS resolution gain. For the forced-feedback accelerometer with interferometric readout, Heijningen [2] measured displacement sensitivities of up to 8 fm/√Hz above 30 Hz, while the instrument was modelled to be shot noise limited at 3 fm/√Hz - another discrepancy this research attempted to explain. A significant reduction of the thermal noise of the instrument’s mechanics was achieved by re-designing the feedback actuator. After that, the accelerometer’s performance was re-evaluated on a ultra-low vibration platform. A resolution of 50 fm/√Hz was measured, a factor 30 above the modelled shot noise. Sub-optimal relative intensity fluctuations in the laser, as well as laser frequency noise, were found to be responsible