The commissioning of the central interferometer of the Virgo gravitational wave detector

Virgo is a laser Michelson interferometer with 3-km long Fabry–Perot in the arms, aiming at the detection of gravitational waves emitted by astrophysical sources in the frequency band from 10 Hz to few kHz. The core of the detector, the central interferometer, was installed first. It is a recycled Michelson interferometer, with 6-m long arms, obtained by replacing the 3-km long Fabry–Perot cavities with high reflectivity mirrors and using the same components designed for Virgo. In particular all the mirrors of the interferometer are suspended from the seismic isolators that represent the main unique characteristic of the Virgo detector. The interferometer has been controlled by means of a fully digital control system, capable of monitoring and controlling the relative position of the mirrors with an accuracy of 10 −12 m. The best displacement sensitivity was 10 −13 m/√Hz at 10 Hz and 8 × 10 −17 m/√Hz above 1 kHz. This article describes the detector configuration, the tests performed in view of commissioning the full detector and the main results obtained.