Sensors for seismic isolation in gravitational wave detectors

Ubhi, Amit Singh ORCID: 0000-0002-3240-6000 (2022). Sensors for seismic isolation in gravitational wave detectors. University of Birmingham. Ph.D.

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Abstract

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.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):