The control of the Virgo interferometer for gravitational wave detection

VIRGO is a recycled Michelson interferometer where each arm is replaced by a 3 km long Fabry-Perot cavity. It is designed to detect gravitational waves emitted by astrophysical sources. Mirrors are suspended in vacuum by high performance suspensions, so that detection is possibly starting from very low frequencies, a few Hz, up to a few kHz. The work presented in this thesis is focused on the longitudinal control of VIRGO, that is the control of the position of the test masses along the light beam direction. Relative displacements of the mirrors need to be actively controlled in order to bring and keep the interferometer on its working point. By following the evolution of the VIRGO commissioning, suitable strategies have been designed and applied to differ- ent optical confgurations: a single Fabry-Perot cavity, a Michelson interferometer with Fabry-Perot cavities in the arms (the recombined ITF), and finally the full interferometer (the recycled ITF). Once the lock of full VIRGO was achieved, the process of investigating and reducing the longitudinal control noises coupled into the dark fringe signal began