The Virgo 3 km interferometer for gravitational wave detection

Virgo, designed, constructed and developed by the French-Italian VIRGO collaboration located in Cascina (Pisa, Italy) and aiming to detect gravitational waves, is a ground-based power recycled Michelson interferometer, with 3 km long suspended Fabry -Perot cavities. The first Virgo scientific data-taking started in mid-May 2007, in coincidence with the corresponding LIGO detectors. The optical scheme of the interferometer and the various optical techniques used in the experiment, such as the laser source, control, alignment, stabilization and detection strategies are outlined. The future upgrades that are planned for Virgo from the optical point of view, especially concerning the evolution of the Virgo laser, are presented. Finally, the next generation of the gravitational wave detector (advanced Virgo) is introduced from the point of view of the laser system.