Mitigation of Laser Beam Perturbations in the Virgo and Advanced-Virgo Gravitational Waves Detectors

Abstract

The present work has been carried out in the framework of the interferometric detection of gravitational waves (GWs), specifically in the Virgo detector and in the Advanced Virgo (Adv-Virgo) project. Adv-Virgo project is an overall improvement of the sensitivity for the Virgo antenna, a power recycled Michelson interferometer with 3 km arms length, located within the site of EGO, European Gravitational Observatory, based at Cascina (Pisa, Italy). The advanced detector will be dual recycled and will use high power laser in order to increase the signal-to-noise ratio (SNR). The GWs are perturbations in the curvature of space-time propagating at speed of light. Actually, research is focused on achieving direct detection of GWs but because of the weak nature of gravity only astronomical scale sources are likely to produce waves of sufficient amplitude to be detected on Earth. In the frequency band of a few Hz to a few kHz, the expected strain amplitude for violent sources is of the order of . Virgo is near the design curve of sensitivity; in this condition the interferometer, to be limited by only the fundamental noises, must work in condition of extremely precision, especially in terms of alignments. For example, the fluctuations in position and direction of the laser beam (beam jitter) are a critical technical noise source that can limit the dark fringe sensitivity. The first part of my PhD work has been addressed to model analytically the all chain of beam jitter noise propagation from the laser to the set requirements for various sub systems and propose new improvements and up-grade of the detector. Starting from the Virgo optical layout, it evaluated how the seismic vibrations of the optical injection system are propagated in the main interferometer and affect the dark-fringe carrying the GW signal. Moreover this study has contributed to set parameters and requirements to constrain a beam jitter noise to be compliant with Adv-Virgo sensitivity. The second part of my PhD work, has been dedicated to the effects of thermal deformations of the Adv-Virgo injection optics caused by the use of high power laser (200W) and the non zero absorption coefficient of the optics. It follows a local heating of the optics and consequently a wavefront geometric deformation of the reflected and transmitted beams. In particular an active system to control and correct the wavefront distortions has been performed. The system proposed has been a deformable mirrors with an heater array used as possible actuators in order to compensate these thermal effects. The system has shown performances in terms of efficiency, checking the system stability, linearity and measurements repeatability.