Innovative perspectives for seismic isolation of gravitational-wave detectors

The discovery of gravitational waves opened a new way to look at the Universe and offered new opportunities to shed light on the still unknown aspects of physical sciences. The work presented in this thesis wants to give a contribution to the development of this new type of research: the author chose to focus on the improvement of the instruments able to detect the gravitational waves. This field is important to make the detectors more sensitive, in order to see more gravitational-wave sources and help to complete the mosaic of the astrophysical science. In particular, the detectors currently in use are interferometers, which are especially blind in a range of frequency below 30 Hz: this affects the chance to detect sources emitting in this frequency band. This lack of sensitivity is mainly due to seismic motion, and the work presented in this thesis focussed on new techniques to lower the noise sources and allow the instruments to be sensitive below 30 Hz. During the studies, the development and test of devices capable of potentially reducing the seismic motion have been performed, such as optical levers for tilt motion reduction and laser stabilization for low frequency readout; a new concept of the seismic system on one of the interferometers (LIGO) has also been proposed. The optical levers can in principle reduce tilt motion below 1 Hz; the use of capacitive position sensors in a new software configuration for LIGO can help to suppress ground motion by a factor of 3 in order of magnitude below 0.1 Hz. A competitive frequency stabilization to 3.6 times 10$^{3}$ $Hz\sqrt{Hz}$ at 1 Hz for readout at low frequency is possible with a compact and easy to handle setup. These results are promising to provide suppression of the seismic motion in the bandwidth of interest and show that it is possible for a ground-based instrument to be seismically more stable and capable of detecting gravitational waves where it is now forbidden

Nonlinearities in Fringe-Counting Compact Michelson Interferometers

Compact Michelson interferometers are well positioned to replace existing displacement sensors in the readout of seismometers and suspension systems, such as those used in contemporary gravitational-wave detectors. Here, we continue our previous investigation of a customised compact displacement sensor built by SmarAct that operates on the principle of deep frequency modulation. The focus of this paper is the linearity of this device and its subsequent impact on sensitivity. We show the three primary sources of nonlinearity that arise in the sensor: residual ellipticity, intrinsic distortion of the Lissajous figure, and distortion caused by exceeding the velocity limit imposed by the demodulation algorithm. We verify the theoretical models through an experimental demonstration, where we show the detrimental impact that these nonlinear effects have on device sensitivity. Finally, we simulate the effect that these nonlinearities are likely to have if implemented in the readout of the Advanced LIGO suspensions and show that the noise from nonlinearities should not dominate across the key sub-10 Hz frequency band

High resolution compact vertical inertial sensor for atomic quantum gravimeter hybridization

peer reviewedInertial sensors are devices capable of measuring the absolute motion of the support they are fixed onto. The advance of very high-end scientific instruments such as gravitational wave detectors, always pursuing ever greater sensitivities and performance, puts a large demand on ultra-high-resolution inertial sensors, capable of measuring very low-frequency and small-amplitude motions. Our group has a long experience in the design of low-frequency inertial sensors intended to be used in active isolation systems. The latest horizontal and vertical interferometric inertial sensors that have been designed have performance that competes with industry standards. They were shown to reach a resolution of 2×10−13 m/Hz−−−√ at 1 Hz and are capable of measuring ground motion from 0.1 to 100 Hz. However, they are large and heavy, measuring approximately 20 × 20 × 30 cm3 each, which makes their integration into practical systems tedious. In addition, experimental characterization of these sensors revealed three main limitations to their resolution. They are: (i) thermal noise, (ii) electronic readout noise and (iii) broadband white noise caused by mechanical and optical nonlinearities. The present paper presents a revised version of the vertical sensor, where the size of the device has been made to fit a 10 × 10 × 10 cm3 while simultaneously addressing the aforementioned sources of noise. The mechanics of the compact sensor is made of a leaf-spring supported pendulum, connected to the frame using a flexure hinge. A moving mirror is connected to the mass and guided using a so-called "4-bar" mechanism, providing the moving mirror with linear translation motion (iii). The joints of the mechanics are made of fused silica, allowing to reach a low natural frequency of ≈1 Hz with a compact design, in addition to significantly reducing structural thermal noise (i) due to the low dissipation rate of fused-silica. On the other hand, the readout system used in this sensor is a homemade design of a Michelson interferometer. The optical scheme features numerous polarizing elements that allow the propagation of two laser beams in phase quadrature, and custom hardware is developed for minimizing electronic noise (ii). Lastly, the vertical sensor is operating in closed-loop, using a homemade actuator design, so as to reduce non-linear effects related to either the mechanics or the optical readout (iii). The sensor frequency response is characterized using a test bench that has been specifically developed for testing low-frequency sensor response. The noise floor is extracted using a Huddle Test

Design and sensitivity of a 6-axis seismometer for gravitational wave observatories

We present the design, control system, and noise analysis of a 6-axis seismometer comprising a mass suspended by a single fused silica fiber. We utilize custom-made, compact Michelson interferometers for the readout of the mass motion relative to the table and successfully overcome the sensitivity of existing commercial seismometers by over an order of magnitude in the angular degrees of freedom. We develop the sensor for gravitational-wave observatories, such as LIGO, Virgo, and KAGRA, to help them observe intermediate-mass black holes, increase their duty cycle, and improve localization of sources. Our control system and its achieved sensitivity makes the sensor suitable for other fundamental physics experiments, such as tests of semiclassical gravity, searches for bosonic dark matter, and studies of the Casimir force

Design and sensitivity of a 6-axis seismometer for gravitational wave observatories

We present the design, control system, and noise analysis of a 6-axis seismometer comprising a mass suspended by a single fused silica fibre. We utilise custom-made, compact Michelson interferometers for the readout of the mass motion relative to the table and successfully overcome the sensitivity of existing commercial seismometers by over an order of magnitude in the angular degrees of freedom. We develop the sensor for gravitational-wave observatories, such as LIGO, Virgo, and KAGRA, to help them observe intermediate-mass black holes, increase their duty cycle, and improve localisation of sources. Our control system and its achieved sensitivity makes the sensor suitable for other fundamental physics experiments, such as tests of semiclassical gravity, searches for bosonic dark matter, and studies of the Casimir force

Active platform stabilization with a 6D seismometer

We demonstrate the control scheme of an active platform with a six degree of freedom (6D) seismometer. The inertial sensor simultaneously measures translational and tilt degrees of freedom of the platform and does not require any additional sensors for the stabilization. We show that a feedforward cancelation scheme can efficiently decouple tilt-to-horizontal coupling of the seismometer in the digital control scheme. We stabilize the platform in the frequency band from 250 mHz up to 10 Hz in the translational (X, Y) degrees of freedom and achieve a suppression factor of 100 around 1 Hz. Further suppression of ground vibrations was limited by the non-linear response of the piezo actuators of the platform and by its limited range (5 lm). In this paper, we discuss the 6D seismometer, its control scheme, and the limitations of the test bed

Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo

Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level

Ultralight vector dark matter search using data from the KAGRA O3GK run

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM