[en] The advanced LIGO detectors are currently in their final design stage, and the installation phase will start at the end of 2010: they will have about 10 times better sensitivity than initial LIGO, with a sensitive band ranging from 10 Hz to 10 kHz. As compared with previous LIGO detectors, there will be increased complexity in the optical configuration, improved seismic isolation system and significantly higher power circulating in the arm cavities. In the new detectors, the control of the angular orientation of the mirrors will be particularly challenging. The advanced LIGO (aLIGO) mirrors need to have a residual angular motion of the order of 1 nrad RMS in order to achieve high sensitivity. In the high power regime, the torque induced by radiation pressure effects will be comparable with the restoring torque of the mirror suspension, such that we must think of the opto-mechanical response, instead of just the mechanical response. These modifications have to be considered in order to design the control strategy for keeping the mirrors well aligned. Moreover, to meet the sensitivity target the alignment control noise coupled to the gravitational-wave channel must be well below 6x10^-18m/√Hz at 10 Hz. We developed a model of the alignment sensing and control scheme of aLIGO which takes into account radiation pressure effects and meets the noise target.

[en] We present a simple feedback description of parametric instabilities which can be applied to a variety of optical systems. Parametric instabilities are of particular interest to the field of gravitational-wave interferometry where high mechanical quality factors and a large amount of stored optical power have the potential for instability. In our use of Advanced LIGO as an example application, we find that parametric instabilities, if left unaddressed, present a potential threat to the stability of high-power operation.

[en] We report on the stabilization of the laser frequency for the Virgo gravitational-wave detector. We have obtained a frequency noise level, measured in loop, of 1.9x10^-7 Hz/√(Hz) at 10 Hz for the 1064 nm laser; this value is limited by shot noise. The Allan standard deviation for relative frequency noise is 1.0x10^-21 on a 100-ms time scale. The spectral density of the laser frequency noise is negligible in the channel where gravitational waves ought to appear and meets the specifications for the target spectral resolution of the Virgo interferometer in the 10 Hz-10 kHz detection bandwidth.

[en] The mirror relative motion of a suspended Fabry-Perot cavity is studied in the frequency range 3-100 Hz. The experimental measurements presented in this paper have been performed at the Low Frequency Facility, a high finesse optical cavity 1 cm long suspended to a mechanical seismic isolation system like the one of the VIRGO gravitational wave antenna. Because of the radiation pressure between the two mirrors of the cavity, the dynamic behavior of the system is characterized by the optical spring stiffness. In the frequency region above 3 Hz, where seismic noise contamination is negligible, the mirror displacement noise is stationary and its statistical distribution is Gaussian. Using a simplified mechanical model of the suspended system and applying the fluctuation dissipation theorem, we show that the measured power spectrum is reproduced in the frequency region 3-90 Hz. Since the contribution coming from different sources of the system to the total noise budget turns out to be negligible, we conclude that the relative displacement power spectrum of this opto-mechanical system is compatible with a system at thermal equilibrium within its environment. In the region 3-10 Hz this measurement gives so far the best upper limit for the thermal noise of the suspension for a gravitational wave interferometer

[en] Squeezed light has become a standard technique to enhance the sensitivity of gravitational wave detectors. Both optical losses and phase noise in the squeezed path can degrade the achievable improvements. Phase noise can be mitigated by having a high bandwidth servo to stabilize the squeezer phase to the light from the interferometer. In advanced LIGO, this control loop bandwidth is limited by the 4 km arm cavity free spectral range to about ∼15 kHz. Future generation gravitational-wave detectors are designed to employ much longer arm cavities. For cosmic explorer [1], a 40 km arm length will limit the bandwidth to ∼1.5 kHz. We propose an alternative controls scheme that will increase the overall phase noise suppression by using the in-vacuum filter cavity as a reference for stabilizing the laser frequency of the squeezed light source. This will allow for rms phase noise of less than a milliradian—a negligible level for all future generations of gravitational-wave detectors [2]. (paper)

[en] An optical spring effect has been observed in the motion of a Fabry-Perot cavity suspended in the Low-Frequency Facility, R and D experiment of the VIRGO Collaboration. The experimental setup consists of a 1-cm-long cavity hanging from a mechanical isolation system, conceived to suppress seismic noise transmission to the optical components of the interferometer. The observed radiation pressure effect corresponds to an optical stiffness k_opt ranging between 2.5x10⁴ and 6.5x10⁴ N/m. This paper reports several measurements of the mirror relative displacement carried out in different working conditions. The measured error signal spectra show broad resonances at frequencies compatible with the optomechanical system. In other runs cavity detuning oscillations have been observed at subhertz frequencies. In these cases the power spectrum of the control loop error signal exhibited a broad resonance with superimposed uniformly spaced peaks. These observations, validated by a simple model, prove that the fluctuations of the optical spring effect can become so large as to exhibit nonlinear features

[en] In the framework of the expected association between gamma-ray bursts and gravitational waves, we present results of an analysis aimed to search for a burst of gravitational waves in coincidence with gamma-ray burst 050915a. This was a long duration gamma-ray burst detected by Swift during September 2005, when the Virgo gravitational wave detector was engaged in a commissioning run during which the best sensitivity attained in 2005 was exhibited. This offered the opportunity for Virgo's first search for a gravitational wave signal in coincidence with a gamma-ray burst. The result of our study is a set of strain amplitude upper limits, based on the loudest event approach, for different but quite general types of burst signal waveforms. The best upper limit strain amplitudes we obtain are h_rss=O(10^-20) Hz^-1/2 around ∼200-1500 Hz. These upper limits allow us to evaluate the level up to which Virgo, when reaching nominal sensitivity, will be able to constrain the gravitational wave output associated with a long burst. Moreover, the analysis presented here plays the role of a prototype, crucial in defining a methodology for gamma-ray burst triggered searches with Virgo and opening the way for future joint analyses with LIGO

[en] The VIRGO interferometer is the largest ground based European gravitational wave detector operating at the EGO Laboratory in the Pisa, Italy; countryside. During the last commissioning period relevant progress have been done in approaching its design sensitivity all over the detection bandwidth. Thanks to the effort of the whole Collaboration a long scientific run has been done collecting data for more than 4 months in conjunction with the LIGO detectors. The results obtained from the detector point of view are: a very good stability and a duty-cycle as high as 81% in science mode. In this paper we present the status of the VIRGO interferometer giving an overview of the experimental apparatus together with its most relevant features

[en] We present a methodology of network data analysis applied to the search for coincident burst excitations over a 24 h long data set collected by AURIGA, EXPLORER, NAUTILUS and Virgo detectors during September 2005. The search of candidate triggers was performed independently on each of the data sets from single detectors. We looked for two-fold time coincidences between these candidates using an algorithm optimized for a given population of sources and we calculated the efficiency of detection through injections of templated signal waveforms into the streams of data. To this end we have considered the case of signals shaped as damped sinusoids coming from the galactic center direction. Our method targets an optimal balance between high efficiency and low false alarm rate, aiming at setting confidence intervals as stringent as possible in terms of the rate of the selected source models

[en] Interferometric gravitational-wave detectors are complex instruments comprised of a Michelson interferometer enhanced by multiple coupled cavities. Active feedback control is required to operate these instruments and keep the cavities locked on resonance. The optical response is highly nonlinear until a good operating point is reached. The linear operating range is between 0.01% and 1% of a fringe for each degree of freedom. The resonance lock has to be achieved in all five degrees of freedom simultaneously, making the acquisition difficult. Furthermore, the cavity linewidth seen by the laser is only ∼1 Hz, which is four orders of magnitude smaller than the linewidth of the free running laser. The arm length stabilization system is a new technique used for arm cavity locking in Advanced LIGO. Together with a modulation technique utilizing third harmonics to lock the central Michelson interferometer, the Advanced LIGO detector has been successfully locked and brought to an operating point where detecting gravitational-waves becomes feasible. (paper)