Optical strategies for gravitational-wave detector enhancement

This thesis investigates optical strategies to enhance the performance of the current and future ground‐based gravitational-wave detectors, with a particular focus on the Advanced Virgo Plus (AdV+) detector. Building on the evolution from Einstein’s predictions to recent gravitational-wave detections and the advent of multi-messenger astronomy, the work addresses challenges posed by thermal and quantum noises, which currently limit detector sensitivity. The study explores the injection of higher-order Laguerre-Gaussian (LG) modes—specifically the LG09 mode—together with corrective mirror coatings to mitigate coating Brownian noise. Detailed simulations compare the achievable sensitivity improvements with those of the conventional Gaussian mode. In addition, the thesis presents the implementation and commissioning of a frequency-dependent squeezing system for AdV+ detector aimed at reducing quantum noise across the entire frequency band. Particular attention is given to the design, installation, and control of the filter cavity, as well as the generation and characterization of the squeezed states. Finally, the optical characterization methods developed for the AdV+ detector during the commissioning of the fourth Observing Run are discussed, highlighting their role in optimizing interferometer control and thermal tuning. Overall, the results contribute to the ongoing upgrade of gravitational-wave detectors, paving the way for more precise measurements of cosmic events.