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
