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Interferometers

Gruppi locali

Virgo Firenze
Virgo Pisa
Virgo Perugia
Virgo Roma
Virgo Frascati
Napoli/Salerno

The Virgo Project
By F.Vetrano, National Responsible for Virgo-INFN

Virgo is a French-Italian project whose goal is to detect gravitational waves in a large bandwidth. The collaboration involves about two hundred people (scientists, technologists, technicians) from eleven Laboratories: the Ecole Supérieure de Physique et Chimie Industrielles (ESPCI-Paris), the Florence and Urbino Team (INFN-Sezione di Firenze), the Laboratorio Nazionale di Frascati (INFN-LNF), the Laboratoire de l’Accélérateur Linéaire (LAL-Paris), the Laboratoire de Physique des Particules (LAPP–Annécy), the Laboratoire des Matériaux Avancés (LMA–Lyon), the INFN–Sezione di Napoli, the Observatoire de la Côte d’Azur (OCA–Nice), the INFN–Sezione di Perugia, the INFN–Sezione di Pisa, the INFN–Sezione di Roma1. The Virgo antenna, built in Càscina (Italy) on the site of the European Gravitational Observatory (EGO), roughly speaking is a 3 km arm-length Michelson interferometer.

All the optical components of the Michelson are suspended in order to look like as free falling masses: a gravitational wave impinging on the plane of such suspended masses stretches one of its arm and compresses the other one, inducing the opposite effect half cycle later; the amplitude of the mirrors displacement is approximately given by the product between the arm length and the dimensionless amplitude of the GW, h(t). A detection of the gravitational wave can be provided by interferometric measurement of the phase shift between the interfering beams in the Michelson.

Following presently accepted models of astrophysical sources for GW, the signal emitted from (e.g) a Supernova in the Virgo cluster is a pulse of a few ms with a dimensionless amplitude of about 10^-21: this leads in Virgo antenna a displacement of the mirror of about 10^-18m and a phase shift of about 10^-11radians. Since many astrophysical sources (such as pulsars and coalescing binary systems) are expected to emit mainly low frequency GW, i.e. from fraction of Hz to a kHz, it is important to lower the frequency detection threshold as much as possible.

Virgo is currently the only ground based interferometric antenna planned to lower the detection band to a few Hz. This is accomplished by suspending each optical component of the interferometer from a special attenuation chain, the Superattenuator (SA), which reduces the transmission of seismic noise. It has a structure of a compounded pendulum (six mechanical filters, connected by suitable suspension wires). Higher suspension is sustained by a pre-isolation inverted pendulum; the mirror is suspended at the end of the pendulum through the complex structure constituted by the last filter and a marionetta (a rigid body of particular shape) in order to steer the mirror. An hierarchical control (active filtering) is working along the chain. All the components are enclosed in a vacuum tower.

In order to enhance the phase shift induced by GW, the optical path after the splitting and before the recombination on the beam splitter is increased by replacing each arm of the interferometer with a Fabry-Pérot cavity, resonating at the laser frequency: by using this technique the phase shift can be amplified by a factor 2F/π, where F is the Finesse of the cavity (in Virgo F = 50).
In order to define the sensitivity of the antenna, it is necessary to compare the real signal to each relevant fake signal, expressing this comparison, as usual, in terms of the linear spectral density(1/√Hz). The sensitivity curve of Virgo is plotted by adding in an incoherent way the linear spectral density of all the fake signals associated to spurious processes (see the figure below; it is to be stressed that a continuously updated plot is available at the Virgo official web site). In short, in the low frequency range (below few tens of Hz) Virgo is limited by the thermal noise of the pendulum suspensions; between few tens of Hz and few hundreds of Hz the dominant noise is the thermal noise of the mirror internal modes; at higher frequencies, the limiting noise is the shot-noise.
With this sensitivity curve Virgo should be able to detect high frequency signals coming from Supernovae in the Virgo cluster; furthermore, owing the high seismic isolation reached by the SA, Virgo has the possibility to detect several periodic or quasi-periodic signals in the low frequency range (galaxy pulsars and coalescing binary systems).

The Virgo groups are also involved in some R&D activities, considered and managed as sub-projects aiming at insuring that projected sensitivity of Virgo will be reached and preserved (new control strategies, low noise electronics) or else to improve Virgo sensitivity by reducing the noise (monolithic suspensions, cold mirrors, new materials for substrates and suspension fibers, adaptive optics and new beam shape control, new coatings for suspended mirrors). From this point of view, Virgo R&D is presently the most powerful effort to improve the performances of ground-based interferometric gravitational detectors at short- and mid –term.