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International audience; Correlated magnetic noise from Schumann resonances threatens to contaminate the observation of a stochastic gravitational-wave background in interferometric detectors. In previous work, we reported on the first effort to eliminate global correlated noise from the Schumann resonances using Wiener filtering, demonstrating as much as a factor of two reduction in the coherence between magnetometers on different continents. In this work, we present results from dedicated magnetometer measurements at the Virgo and KAGRA sites, which are the first results for subtraction using data from gravitational-wave detector sites. We compare these measurements to a growing network of permanent magnetometer stations, including at the LIGO sites. We show the effect of mutual magnetometer attraction, arguing that magnetometers should be placed at least one meter from one another. In addition, for the first time, we show how dedicated measurements by magnetometers near to the interferometers can reduce coherence to a level consistent with uncorrelated noise, making a potential detection of a stochastic gravitational-wave background possible.

The recent discovery of merging black holes suggests that a stochastic gravitational-wave background is within reach of the advanced detector network operating at design sensitivity. However, correlated magnetic noise from Schumann resonances threatens to contaminate observation of a stochastic background. In this paper, we report on the first effort to eliminate intercontinental correlated noise from Schumann resonances using Wiener filtering. Using magnetometers as proxies for gravitational-wave detectors, we demonstrate as much as a factor of two reduction in the coherence between magnetometers on different continents. While much work remains to be done, our results constitute a proof-of-principle and motivate follow-up studies with a dedicated array of magnetometers.

In this paper we present preliminary experimental results relative to the application of open loop monolithic folded pendulum sensors to the control of large band multistage suspensions (seismic attenuators) and inertial platforms in the band 0.01 ± 10Hz. In fact, beyond the obvious compactness and robustness of monolithic implementations of folded pendulum, high sensitivity, large measurement band (10−7 ÷ 100Hz), tunability of the resonance frequency (necessary to adapt the sensors to the mechanics), high sensitive integrated laser optical readout (e.g. optical lever, laser interferometer), and very good immunity to environmental noises are some of the main advantages of this class of sensors. The results are presented and discussed in this paper together with the planned further developments and improvements.

This paper describes a new implementation of monolithic horizontal sensor, developed at the University of Salerno, based on the Folded Pendulum architecture, configurable both as seismometer and as accelerometer. The large low-frequency band (10-6 ÷ 10Hz), the high sensitivity ( in the band 0.1 ÷ 10 Hz) and the high quality factor in air (Q > 1500) are largely better than all the previous Folded Pendulum implementations. Moreover its monolithic implementation of the whole mechanics, coupled with a full tunability of its resonance frequency (70 mHz ÷ 1.2 Hz) obtained with a specially designed calibration procedure and with an integrated laser optical readout, guarantees both compactness, robustness and immunity to environmental noises. This makes this sensor suitable for a large number of scientific applications, also in high vacuum and cryogeny. Applications of this sensor are already started in the field of geophysics, including the study of seismic and newtonian noise for characterization of suitable sites for future underground interferometric detectors of gravitational waves.

In this paper we present the control system we implemented for test the control of a suspended mass using monolithic tunable folded pendulum sensors in the band 0.01 - 100 Hz. The main goal of this experiment is to test the performances of monolithic sensors as main acceleration sensors for application in inertial damping control systems. The main advantages of these sensors with respect to the others are the sensitivity (better than 10-11 m/s2/√(Hz) in the band 0.1 ÷ 10Hz), the large band (0.01 ÷ 100Hz), the tuneability of the resonance frequency (necessary to match the sensors to the suspension mechanics) and the high sensitive integrated laser optical readout, consisting of an optical lever and an interferometer, and a very good immunity to environmental noises. Preliminary results are presented and discussed in this paper together with the planned further developments and improvements of monolithic sensors.

This paper describes a new mechanical architecture of the monolithic tunable folded pendulum, developed at the University of Salerno, configurable both as seismometer and, in a force-feedback configuration, as accelerometer. Typical applications are the remote monitoring of seismic and newtonian noises for geophysical applications. This sensor, shaped with precision machining and electric-discharge-machining, like the previous versions, is a very compact instrument, very sensitive in the low-frequency seismic noise band, with a very good immunity to environmental noises. Important characteristics are the large band (10-6 - 10mHz), the tunability of the resonance frequency and the laser optical readout, that integrates an optical lever and a laser interferometer. The theoretical sensitivity curves, largely improved due to a new design of the pendulum arms and of the electronics, are in a very good agreement with the measurements. In particular, a very good sensitivity (10-12 m/Hz1/2) has been obtained in the band 0.1 - 10 Hz). Prototypes of monolithic seismometers are already operational in selected sites around the world both to remotely acquire data for scientific analysis of seismic noise and to collect all the useful information to understand their performances in the very low frequency band. The results of the monolithic sensor as accelerometer (force feed-back configuration) are also presented and discussed.

A tunable mechanical horizontal monolithic seismometer/accelerometer, developed for applications in the fields of geophysics and interferometric detection of gravitational waves of second and third generation, is described. The large measurement band (10−3 ÷ 10 Hz) with sensitivities of ≈ 10−12m/, as seismometer, and better than 10−11 m/s2/, as accelerometer, have been obtained with an optimised mechanical design and the introduction of a very sensitive laser interferometric optical readout, the latter aimed also to ensure a very good immunity to environmental noises. Prototypes of seismometers are operational in selected sites both to acquire seismic data for scientific analysis of seismic noise and to collect all the useful information to understand their performances in the very low frequency band (10−6 ÷ 10−3 Hz).

In this paper we describe the architecture and the performances of a hybrid modular acquisition and control system prototype for environmental monitoring and geophysics. The system, an alternative to a VME-UDP/IP based system, is based on a dual-channel 18-bit low noise ADC and a 16-bit DAC module at 1 MHz. The module can be configured as stand-alone or mounted on a motherboard as mezzanine. Both the modules and the motherboard can send/receive the configuration and the acquired/correction data for control through a standard EPP parallel port to a standard PC for the real-time computation. The tests have demonstrated that a distributed control systems based on this architecture exhibits a delay time of less than 25 us on a single channel, i.e a sustained sampling frequency of more than 40 kHz (and up to 80 kHz). The system is now under extensive test in the remote controls of seismic sensors (to simulate a geophysics networks of sensors) of a large baseline suspended Michelson interferometer.

In this paper we describe the architecture and the performances of a hybrid modular acquisition and control system prototype developed for the implementation of distributed monitoring and control systems. The system, an alternative to the VME-UDP/IP based system, is based on a dual-channel 18-bit low noise ADC and 16-bit DAC module at 800 kHz, managed by an ALTERA FPGA. Experimental tests have demonstrated that this architecture allows the implementation of distributed control systems with delay time t < 30µs, on single channel, using a standard laptop PC for the real-time computation. The system was used for the longitudinal control of the end mirror of a suspended Michelson Interferometer, performed through an electrostatic actuators, giving effective performances. The preliminary results are also reported.

This paper describes a mechanical monolithic tunable sensor prototype with elliptical hinges, shaped with electric-discharge-machining, that can be used both as seismometer and, in a force-feedback configuration, as accelerometer in the control of mechanical suspensions of interferometric gravitational waves detectors. The monolithic mechanical design and a laser optical readout make it a very compact instrument, very sensitive in the low-frequency seismic noise band and with a very good immunity to environmental noises. The theoretical sensitivity curves and the simulations show a very good agreement with the measurements. Very interesting scientific result its measured natural resonance frequency of ≈ 70mHz with a Q ≈ 140 in air.

In this paper we describe the architecture and the performances of a hybrid modular acquisition and control system prototype for environmental monitoring and geophysics. The system, an improvement of a VME-UDP/IP based system we developed for interferometric detectors of gravitational waves, is based on a dual-channel 18-bit low noise ADC, a 16-bit DAC module at 1MHz, and a 20-bit slower ADC necessary for the acquisition of an external calibration signal. The module can be configured as stand-alone or mounted on a motherboard as mezzanine in parallel with other modules. Both the modules and the motherboard can send/receive the configuration and the acquired/correction data for control through a standard EPP parallel port to a standard PC, where the real-time computation is performed. Experimental tests have demonstrated that the distributed control systems implemented with this architecture exihibit a delay time of less than 25 μs on a single channel, that is a sustained sampling frequency of more than 40kHz. The system is now under extensive test in two different experiments: the remote control and data acquisition of a set of seismometers, velocimeters and accelerometers to simulate a geophysics networks of sensors and the remote control of the end mirrors of a suspended Michelson interferometer through electrostatic actuators for interferometric detectors of gravitational waves.

This paper describes a mechanical monolithic sensor for geophysical applications developed at the University of Salerno. The instrument is basically a monolithic tunable folded pendulum, shaped with precision machining and electric-discharge-machining, that can be used both as seismometer and, in a force-feedback configuration, as accelerometer. The monolithic mechanical design and the introduction of laser interferometric techniques for the readout implementation make it a very compact instrument, very sensitive in the low-frequency seismic noise band, with a very good immunity to environmental noises. Many changes have been produced since last version (2006), mainly aimed to the improvement of the mechanics and of the optical readout of the instrument. In fact, we have developed and tested a prototype with elliptical hinges and mechanical tuning of the resonance frequency together with a new laser optical lever and laser interferometer readout system. The theoretical sensitivity curve for both laser optical lever and laser interferometric readouts, calculated on the basis of suitable theoretical models, shows a very good agreement with the experimental measurements. Very interesting scientific result is that the measured natural resonance frequency of the instrument is ≈ 70mHz with a Q ≈ 140 in air without thermal stabilization, demonstrating the feasibility of a monolithic FP sensor with a natural resonance frequency of the order of 5 mHz with a more refined mechanical tuning.

In this paper we describe the architecture and the performances of a hybrid modular acquisition and control system prototype we developed in Napoli for the implementation of geographycally distributed monitoring and control systems. The system, an improvement of a VME-UDP/IP based system developed by our group for interferometric detectors of gravitational waves, is based on a dual-channel 18-bit low noise ADC and 16-bit DAC module at 1 MHz, managed by an ALTERA FPGA, that can be used standalone or mounted as mezzanine (also in parallel with other modules) on a motherboard. Both the modules and the motherboard can send/receive the configuration and the acquired/correction data for control through a standard EPP parallel port to an external PC, where the real-time computation is performed. Experimental tests have demonstrated that this architeture allows the implementation of distributed control systems, using a standard laptop PC for the realtime computation, with delay time Dt < 30 ms on a single channel, that is a sustained sampling frequency fc > 30kHz. Each module is also equipped with a 20-bit slower ADC necessary for the acquisition of an external calibration signal. The system is now under extensive test in two different experiments, i.e. the control of a Michelson Interferometer to be used as Velocimeter for Seismic Waves in Geophysics and the control of the end mirrors a suspended Michelson Interferometer through electrostatic actuators, a prototype for mirror control for Interferometric Detectors of Gravitational Waves.

Laser interferometry is one of the most sensitive methods for small displacement measurement for scientific and industrial applications, whose wide diffusion in very different fields is due not only to the high sensitivity and reliability of laser interferometric techniques, but also to the availability of not expensive optical components and high quality low-cost laser sources. Interferometric techniques have been already successfully applied also to the design and implementation of very sensitive sensors for geophysical applications. In this paper we describe the architecture and the expected theoretical performances of a laser interferometric velocimeter for seismic waves measurement. We analyze and discuss the experimental performances of the interferometric system, comparing the experimental results with the theoretical predictions and with the performances of a state-of the art commercial accelerometer. The results obtained are very encouraging, so that we are upgrading the system in order to measure the local acceleration of the mirrors and beam splitter of the velocimeter using an ad hoc designed monolithic accelerometers for low frequency direct measurement of the seismic noise.

This papers describes a new low-frequency seismic sensor for geophysical applications. The instrument is basically a monolithic tunable folded pendulum with an interferometric readout system, that can be configured as seismometer or as accelerometer. The monolithic mechanical design and the introduction of a laser interferometric technique for the readout implementation make it a very sensitive and compact instrument with a very good immunity to environmental noises. Preliminary tests on the mechanical performances of the monolithic structure and on the optical reaodut have been performed. Interesting result is the measured resonant frequency of the instrument of ≈ 150 mHz obtained with a rough tuning, demonstrating the feasibility of a resonant frequency of the order of 5 mHz with a more refined tuning. The mechanics of the seismic sensor, the optical scheme of the readout system, the theoretical predictions and the preliminary experimental performances as seismometer are discussed in detail, together with the foreseen further improvements.

Laser interferometry is one of the most sensitive methods for small displacement displacement. This technique was successfully used in several fields of physics, giving very good performances also due to the large availability of optical components and high quality and relatively low cost laser sources. At the same time this technique is enough flexible to be effectively used in very different applications. In particular, in this paper, we present a laser interferometric system used to read the position of a free mirror to measure the local seismic noise. The performances of the interferometric system were analyzed in comparison with a standard accelerometer. The result are encouraging also if some problem, mainly connected with the sensibility a with the stability of the interferometric system have to be better studied.

Laser interferometry is one of the most sensitive methods to measure small displacement. This technique was successfully used in several fields of physics, giving very good performances also due to the large availability of optical components and laser source of high quality and relatively low cost. At the same time this technique is enough flexible to be effectively used in very different applications. In particular, in this work, we present a laser interferometric system used to read the position of the sensible element of a standard seismic accelerometer. Seismic accelerometers working mechanism is based on a control system that acts on the sensible element to stabilize its position; by looking at the force needed to perform this feedback it is possible to deduce the acceleration. Usually the feedback system of these instruments is based on magnet and coil, or on piezoelectric actuators, while the signal transducer are usually capacitive device. The performances of the interferometric system were analyzed in comparison with standard one. The result are encouraging also if some problem, mainly connected with the dynamic of the read-out system should be better studied.

Laser interferometry is one of the most sensitive methods for small displacement measurement. This technique is successfully applied in several fields of physics with very good performances. Its large diffusion is mainly due to the availability of high-quality low-cost optical components and laser systems, but not less important is the general versatility of laser interferometric techniques. In this paper, we present a laser interferometric system for one-dimension local seismic noise measurement. After a description of the theoretical model, the experimental results are presented and discussed, also in comparison with a standard mechanic accelerometers, like the Episensor model FBA ES-T from Kinemetrics. The obtained result are encouraging, so that further studies are scheduled to check the performance of this system at the very low frequencies and to measure its intrinsic noise. Finally, although some problems, arosen in connection with the sensitivity and the stability of the interferometric system, have to be more deeply studied, it is already possible to think to design new versions of this system aimed to the measurement of more degrees of freedom.

In this paper we briefly discuss the possibility to use Adaptive Optics in long baseline interferometric gravitational wave detectors. Analisys is carried out to demonstrate the usefulness of Adaptive Optics as a method to integrate double-mode-cleaner systems, presently used or foreseen in the next generation detectors as systems for the reduction of geometrical fluctuations of input laser beam. Finally a prototype of (AO) system for the control of geometrical fluctuations in a laser beam, based on the interferometric detection of phase front, is presented. By comparison with the usual Shack-Hartmann based AO system, we show that this technique is of particular interest when high sensitivity and high band-pass are required for correction of small perturbations like, for instance, the control of the input beam of gravitational waves interferometric detectors.

We present a study and preliminary experimental results on the possibility of using an adaptive optics system for reduction of geometrical fluctuations of input laser beam in long baseline interferometric detectors of gravitational waves. Presently used completely passive systems are expected to reduce fluctuations only at a level that, due to coupling of geometrical fluctuations with interferometer asymmetries, impose requirements on interferometer operation which are at the limit of present technology. Active pre-stabilization could reduce fluctuations and relax these requirements, allowing a safer and more robust interferometer operation on the planned time-scale of years of continuous data acquisition. The system and the methodologies we have developed are going to be adapted to the Mode Cleaner of the IDGW-3P, a prototype of three-meter suspended Michelson Interferometer expressely developed for Seismic Noise measurement, now becoming operational in Napoli.

In this paper we present a digital control system designed to damp the mechanical motion of a suspended mass. The control system is based on optical levers for reading the position of the mass with respect to some fixed references. The reading system was implemented by using a laser beam, produced by a commercial laser diode, and two position sensing photodiodes as components. The laser light is reflected from the suspended mass and the reflected beam is sent to the position sensing photodiode. Each beam can be used to control more than one degree of freedom of the mass by simply using a suitable optical arrangement to uncouple translation and rotational motions. The digital control system is based on both standard linear signal filtering and on a non linear section. The latter is necessary to solve particular situations, like the very large motion, a situation in which standard control system techniques are often not adequate. Moreover the control system has to take into account all the degree of freedom of the suspended mass, also to avoid the excitation of some extra resonance due to the coupling of the different motions.

Laser interferometry is one of the most sensitive and versatile techniques for small relative displacements measurement. For these reasons it is used or it is being introduced in many different fields of research. One of its possible applications is seismic activity monitoring, obtained by fixing the mirrors of an interferometer to the ground. In order to obtain a preliminary experimental evaluation of the performances of such an instrument, we tested a simple Michelson interferometer in air as a seismic sensor and compared its sensitivity with a standard seismic sensor. We demonstrated that in the low frequency region the interferometric sensor is much more sensitive and therefore it can represent a valid alternative or replacement to a standard sensor. The optical elements needed to build the interferometer were fixed to the ground to couple the detector with seismic activity. The system were continuously monitored also to take into account of misalignments coming from abnormal air fluctuation and displacement of the mirrors. All data coming from this system are collected by a computer for the storage and are real-time pre-analyzed in order to convert the signal coming from the interference pattern into another describing the equivalent mirrors relative displacement.

We present a study and preliminary experimental results on the possibility of using an adaptive optics system for reduction of geometrical fluctuations of input laser beam in long baseline interferometric detectors of gravitational waves. Presently used completely passive systems are expected to reduce fluctuations only at a level that, due to coupling of geometrical fluctuations with interferometer asymmetries, impose requirements on interferometer operation which are at the limit of present technology. Active pre-stabilization could reduce fluctuations and relax these requirements, allowing a safer and more robust interferometer operation on the planned time-scale of years of continue data acquisition.

A digital servo‐loop was implemented to control the temperature of a laser diode. By using a conditionally stable loop we obtained a temperature stability of about ±20 μK over periods of hours.

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.

This paper describes a mechanical monolithic horizontal sensor for geophysical applications developed at the University of Salerno. The instrument is basically a monolithic tunable folded pendulum, shaped with precision machining and electric discharge machining, that can be used both as seismometer and, in a force-feedback configuration, as accelerometer. The monolithic mechanical design and the introduction of laser interferometric techniques for the readout implementation makes it a very compact instrument, very sensitive in the low frequency seismic noise band, with a very good immunity to environmental noises. Many changes have been produced since last version (2007), mainly aimed to the improvement of the mechanics and of the optical readout of the instrument. In fact, we have developed and tested a prototype with elliptical hinges and mechanical tuning of the resonance frequency together with a laser optical lever and a new laser interferometer readout system. The theoretical sensitivity curve for both laser optical lever and laser interferometric readouts, evaluated on the basis of suitable theoretical models, shows a very good agreement with the experimental measurements. Very interesting scientific result is the measured natural resonance frequency of the instrument of 70 mHz with a Q=140 in air without thermal stabilization. This result demonstrates the feasibility of a monolithic folded pendulum sensor with a natural resonance frequency of the order of millihertz with a more refined mechanical tuning.

We discuss the application of digital systems to the automatic control of dual-wave optical interferometers. We show that, if the mechanical-modulation technique is used for error-signal extraction, digital techniques can be used both for error-signal extraction and for control-signal generation. Therefore, apart from two front/end amplifiers that are necessary to match the dynamics of the detectors and actuators to the dynamics of the analog-to-digital converters and digital-to-analog converters, no other analog devices are required. In particular, the mechanical-modulation technique requires the synchronous demodulation of the photodiode output signal. Hence we need to implement a digital lock-in amplifier whose algorithm is described here. Finally, we describe one of the possible applications of this digital control procedure, such as the control of a classic Mach-Zehnder interferometer in air.