Sensors for seismic isolation in gravitational wave detectors

A new window of astronomy was opened via the direct detection of gravitational waves in 2015. Since then, dozens of detections from compact binary sources have been confirmed via the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo observatories; both having peak sensitivities at approximately 100 Hz. Improvement of the low frequency sensitivity of detectors would enable detections of more massive binaries, and provide insight into the evolution of these systems. Operation of these detectors require sophisticated seismic isolation strategies. Despite the variations in their isolation schemes, passive filtering of the motion is achieved via the use of multistage suspensions of the core optics. In the case of LIGO, the suspension chains hang from state of the art passive-active platforms, requiring inertial sensing to stabilise their motion. The control scheme required degrades the sensitivity to gravitational waves below 30 Hz. In this thesis, the principles of detection for ground based observatories are discussed, focusing on a new novel inertial sensor for improving the sensing scheme of the isolated platforms. Analysis of the design and dynamics of this device are described, and predictions of its sensitivity are determined. A derivative of the seismometer was constructed to test the necessary data handling required for successful operation of the device before further investigations of the original design were pursued. The key results of this study were that the device had similar translational sensitivity to that of LIGO’s commercial seismometers such as the T240. Control of a LIGO-inspired six-axis platform was achieved using the original inertial sensor, obtaining over an order of magnitude improved isolation for 5 of 6 degrees of freedom at 1 Hz. Further work was performed on a separate optical shadow sensor to assess their implementation in cryogenic upgrades to the LIGO observatories. Operation of the device below 123 K resulted in a 25% improvement of its shot noise sensitivity, from 6 × 10−11m/√Hz to 4.5 × 10−11m/√Hz. The findings of these investigations are presented and conclusions are made on the viability of these sensors for use in future upgrades

GWTC-2.1: Deep Extended Catalog of Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run

The second Gravitational-Wave Transient Catalog reported on 39 compact binary coalescences observed by the Advanced LIGO and Advanced Virgo detectors between 1 April 2019 15:00 UTC and 1 October 2019 15:00 UTC. We present GWTC-2.1, which reports on a deeper list of candidate events observed over the same period. We analyze the final version of the strain data over this period with improved calibration and better subtraction of excess noise, which has been publicly released. We employ three matched-filter search pipelines for candidate identification, and estimate the astrophysical probability for each candidate event. While GWTC-2 used a false alarm rate threshold of 2 per year, we include in GWTC-2.1, 1201 candidates that pass a false alarm rate threshold of 2 per day. We calculate the source properties of a subset of 44 high-significance candidates that have an astrophysical probability greater than 0.5. Of these candidates, 36 have been reported in GWTC-2. If the 8 additional high-significance candidates presented here are astrophysical, the mass range of events that are unambiguously identified as binary black holes (both objects ≥3M⊙) is increased compared to GWTC-2, with total masses from ∼14M⊙ for GW190924_021846 to ∼182M⊙ for GW190426_190642. The primary components of two new candidate events (GW190403_051519 and GW190426_190642) fall in the mass gap predicted by pair instability supernova theory. We also expand the population of binaries with significantly asymmetric mass ratios reported in GWTC-2 by an additional two events (the mass ratio is less than 0.65 and 0.44 at 90% probability for GW190403_051519 and GW190917_114630 respectively), and find that 2 of the 8 new events have effective inspiral spins χeff>0 (at 90% credibility), while no binary is consistent with χeff < 0 at the same significance

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

Cryogenic optical shadow sensors for future gravitational wave detectors

Displacement sensors have a variety of applications within gravitational wave detectors. The seismic isolation chain of the LIGO core optics utilises optical shadow sensors for their stabilisation. Future upgrades, such as LIGO Voyager, plan to operate at cryogenic temperatures to reduce their thermal noise and will require cryogenic displacement sensors. We present the results of simulations and experimental tests of the shadow sensors embedded in the Birmingham Optical Sensors and Electromagnetic Motors (BOSEMs). We determine that the devices can reliably viability operate at 100 K. We also show that the performance of the BOSEM sensors improves at cryogenic temperatures

A six degree-of-freedom fused silica seismometer: design and tests of a metal prototype

Ground vibrations couple to the longitudinal and angular motion of the aLIGO test masses and limit the observatory sensitivity below 30\,Hz. Novel inertial sensors have the potential to improve the aLIGO sensitivity in this band and simplify the lock acquisition of the detectors. In this paper, we experimentally study a compact 6D seismometer that consists of a mass suspended by a single wire. The position of the mass is interferometrically read out relative to the platform that supports the seismometer. We present the experimental results, discuss limitations of our metallic prototype, and show that a compact 6D seismometer made out of fused silica and suspended with a fused silica fibre has the potential to improve the aLIGO low frequency noise