(ed.). Gravitational waves (IOP, 2001)(422s).

h(sqrt(1/Hz))10 −610 −810 −1010 −1210 −1410 −1610 −1810 −2010 −2210 −2410 −2610 −28Introduction 107Horizontal SeismVert. Seism * 10 −2Thermal NoiseQuantum LimitShot NoiseNewtonianSensitivity curve10 −1 10 0 10 1 10 2 10 3 10 4frequency (Hz)Figure 9.3. Virgo sensitivity curves.Orsay, LAPP-Annecy, INPN-Lyon for CNRS and Firenze, Frascati, Napoli,Perugia, Pisa, Roma for INFN. The aim is to reach, in a useful detectionbandwidth between a few hertz up to a few kilohertz, a sensitivity which allowsus to detect gravitational waves emitted by coalescing binary stars, gravitationalcollapses, spinning neutron stars or constituting the stochastic background of thegravitational radiation. The Virgo sensitivity curve is shown in figure 9.3, asa function of frequency; the vertical scale shows the linear spectral density ofthe minimum detectable dimensionless gravitational amplitude: at low frequencythere is the pendulum thermal noise, then the mirror substrate thermal noise andat high frequency the shot noise of the laser light, the interferometer sensitivitydecreases at high frequency for the high storage time of the 3 km Fabry–Perotcavity. Figure 9.3 shows the thermal and shot noise and the other important noiseof the Virgo antenna.The laser has to be frequency stabilized at the level of 10 −1 Hz/ √ (Hz) inthe frequency band of interest [8], the mirror losses have to be below 1 ppmin order to obtain the required recycling factor and cavity finesse; moreover,power stabilization is required, and a mode cleaner 144 m long, with finesse1000, is necessary in order to have a good quality TM 00 mode injected into theinterferometer, and to contribute to the amplitude stability, since such a long modecleaner has a pole at 10 Hz. All the optical components will be suspended invacuum by anti-seismic suspensions called Super Attenuators (SA); the opticalpath will be completely under vacuum. The vacuum system will consist of: twoorthogonal vacuum tubes, 3 km long, 1.2 m in diameter, containing the arm opticalpaths; one vacuum tube, 144 m long, 0.3 m in diameter for the mode cleaner