E. Calloni

The Virgo interferometer

The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa to be commissioned in the year 2000. Virgo has been designed to achieve a strain sensitivity of a few times at 200 Hz. A large effort has gone into the conception of the mirror suspension system, which is expected to reduce noise to the level of at 10 Hz. The expected signals and main sources of noise are briefly discussed; the choices made are illustrated together with the present status of the experiment.

High-performance modular digital lock-in amplifier

In this paper we describe an efficient and robust digital implementation of a lock‐in amplifier, based on the classic quadrature technique and on a mathematical algorithm for error signal extraction. Both the hardware and the software architecture are modular. The hardware consists of a VME‐bus (IEEE 1014) standard crate, in which commercial VME boards (the CPU, the ADC and DAC) are housed. The software is written in standard C language for portability and easy integration also within complicated software architectures. The software algorithm implementing the lock‐in amplifier can be particularized by the user on the basis of the needed performances and on the available hardware. Numerical and experimental tests on a lock‐in amplifier prototype have shown that it performs as theoretically predicted. The limit of our prototype (50 kHz maximum sampling rate for 16 bit resolution) depends only on the hardware used, and it is not the present technological limit.

The Virgo Project

The Virgo project is a Italian-French collaboration aiming at the construction of a long baseline interferometric antenna for the detection of gravitational radiation signals of cosmic origin. We describe the principles of the system, and high-light the technical challenges we need to overcome for reaching a sensitiity as low as 10−23Hz−1/2.The gravitational clustering of collisionless particles in an expanding universe is modelled using some simple physical ideas. I show that it is possible to understand the nonlinear clustering in terms of three well defined regimes: (1) linear regime; (2) quasilinear regime which is dominated by scale-invariant radial infall and (3) nonlinear regime dominated by nonradial motions and mergers. Modelling each of these regimes separately I show how the nonlinear two point correlation function can be related to the linear correlation function in hierarchical models. This analysis leads to results which are in good agreement with numerical simulations thereby providing an explanation for numerical results. Using this model and some simple extensions, it is possible to understand the transfer of power from large to small scales and the behaviour of higher order correlation functions. The ideas presented here will also serve as a powerful analytical tool to investigate nonlinear clustering in different models.

Vacuum fluctuation force on a rigid Casimir cavity in a gravitational field

We discuss the possibility of verifying the equivalence principle for the zero-point energy of quantum electrodynamics, by evaluating the force, produced by vacuum fluctuations, acting on a rigid Casimir cavity in a weak gravitational field. The resulting force has opposite direction with respect to the gravitational acceleration; the order of magnitude for a multi-layer cavity configuration is derived and experimental feasibility is discussed, taking into account current technological resources.

An improvement in the VIRGO Super Attenuator for interferometric detection of gravitational waves: The use of a magnetic antispring

We present a method of lowering below 2.5 Hz the vertical normal mode frequencies of the Pisa Super Attenuator by using permanent magnets which provide an antispring force. This method allows a more efficient suppression of the seismic noise decreasing the lower limit of the frequency region devoted to gravitational wave detection.

FRINGE-COUNTING TECHNIQUE USED TO LOCK A SUSPENDED INTERFEROMETER

We implement a digital fringe-counting technique to measure in real time the relative mirror displacement of a suspended Michelson interferometer with modulated optical path length for oscillations much larger than the laser wavelength (λ). This provides the proper error signal for a servo mechanism that reduces the relative displacement within λ/2. The implemented technique does not require extra optics or polarizers and thus can be used for interferometric gravitational wave detectors as a starting procedure to get the system locked.

Effects of misalignments and beam jitters in interferometric gravitational wave detectors

Abstract We present a calculation of the phase noise in a recycled interferometer with Fabry-Perot cavities in the arms, induced by the coupling of the geometrical fluctuations of the input laser with geometrical asymmetries of the interferometer. By comparison with the shot-noise limit planned for long-baseline interferometric gravitational wave detectors, upper limits in interferometer misalignments are established.

Sensitivity of a rigid small interferometer in the 10 Hz frequency region

Abstract In the framework of the development of the super attenuators for VIRGO, the French-Italian long arm interferometric antenna for gravitational wave detection, a small interferometer has been built in the Pisa laboratory, to investigate the sensitivity in the low frequency region. The noise measured at 10 Hz is below 10 −13 m/√Hz, about a factor of 10 less than previous measurements in Munich. Calculation shows that the noise between 10 and 100 Hz is compatible with seismic noise.

The VIRGO interferometer for gravitational wave detection.

The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa in Italy. The accessible sources at the design sensitivity and main noises are reviewed. Virgo has devoted a significant effort to extend sensitivity to low frequency reaching the strain level h = 10−21 Hz−1/2 at 10 Hz while at 200 Hz h = 3 · 10−23 Hz−1/2. Design choices and status of construction are presented.

Adaptive optics approach for prefiltering of geometrical fluctuations of the input laser beam of an interferometric gravitational waves detector

In this article we present a preliminary study and experimental results on the use of an adaptive optics (AO) system for the reduction of geometrical fluctuations in a laser beam. The AO system is based on a Shack–Hartmann wave front sensor and a micromachined deformable mirror as a corrective element. The aim of this work is to investigate the applicability of such technologies to improving wave front clean-up conditions in long baseline interferometric gravitational wave detection.