Eleonora Polini – Phd’s defense – dec. 22, 2023

Gravitational astronomy began following the first direct detection of gravitational waves by the LIGO detectors in the USA. Subsequently, the European Virgo detector joined the observation network, which enabled a much more precise localisation of the sources. The detection of a coalescence of neutron stars initiated the so-called multi-messenger astronomy. In total up to the end of the last O3 observing run, 90 coalescence events were revealed, reaching a rate of one event per week. The detecotors were further improved in view of the next O4 run, to allow an even larger portion of the Universe to be seen. Three of the major upgrades made in Virgo are addressed in this thesis. The first is the implementation of an innovative technique to reduce quantum noise over the entire detection band known as frequency-dependent squeezing. We have successfully implemented this technique up to the first measurement of squeezed states with rotation at a frequency of 25 Hz, which is what is needed to optimise the quantum noise reduction in Virgo. The second contribution concerns the study and mitigation of scattered light from the detector’s suspended benches. The scattered light that re-couples with the interferometer’s main beam worsens its low-frequency sensitivity and risks hiding the benefit introduced by frequency-dependent squeezing. 

We made projections of the scattered light noise from the suspended benches, measuring the contribution of some of the optics with the back-scatterometer we built at LAPP. We also mitigated spurious beams on the benches that can create stray light. The third contribution concerns the change of the two output mode cleaner cavities of the detector with a new low loss cavity. The main motivation is to reduce optical losses that degrade squeezing performance. We successfully characterized, installed and commissioned the new cavity. The upgrades described in this thesis for O4 will allow greater sensitivity and thus reveal more distant events, allowing us to uncover some of the secrets hidden in our Universe.