Laser systems for gravitational wave detection

This paper reports on the status of the Virgo detector, now on its commissioning phase: we describe the setup, the implemented functions, we discuss the spectral resolution measured during the last commissioning data taking, we present some of the practical issues. We show the role of the optical metrology in the instrument. Its first role is on the apparatus design: we show how the optical design of Virgo improves the sensitivity compared to a simple Michelson interferometer. The laser beam needs to be very stable, in the detection band 10 Hz-10 kHz: beam pointing, power stability and frequency stability. We show that this can be achieved using very stable references and enhanced feedback loops. Optical metrology is also necessary to measure the mirror substrates and surfaces and ensure that they have the required properties: roughness, absorption, and local defects, transmission maps. The output of optical simulations using these maps can be compared with in-situ measurement of some global parameters: cavity finesses, mirror curvatures, contrast defect.