Gravitational waves are ripples in the curvature in the space-time which propagate at the speed of light, emitted by accelerating massive bodies. Predicted by Einstein in 1918 as a consequence of the general relativity, their existence was indirectly proven by Hulse and Taylor in 1974. Since then, gravitational waves haven’t been directly detected but efforts have been made worldwide in order to build Gravitational wave detectors and to improve their performances; because gravitational wave detection will provide a new tool for investigating the universe, opening a new era for astronomy.

The European detectors is called Virgo, it is a Michelson interferometer with 3 km long arms in the plain of Arno, Italy. The effect of a gravitational wave which reaches the interferometer is to compress one arm and to stretch the other; this change in the relative length of the arms can then be detected as a variation of the interferometer output fringe. Because of the very weak signal, all noise sources must be under control. One of the key elements is the laser source which must reach high levels of stabilization in power and in frequency. In this talk I will present the techniques that allow to obtain the Pre-Stabilized Laser fulfilling the VIRGO requirements.