title: The analysis, experimental characterisation and prototyping of technologies for making quantum noise limited detections of gravitational waves
Creators: name: Briggs, Joseph Henry
Publisher: name: University of Glasgow
Year: 2021

description: The detection of gravitational waves is essential for developing our understanding of the Universe. Systems such as inspiralling binaries of black holes and neutron stars produce gravitational waves, and much of the information carried by a gravitational wave cannot be obtained via any other means. Gravitational waves interact weakly with matter, so kilometre-scale interferometers, such as the LIGO detectors, are the only instruments which have directly measured the strain induced in space-time by gravitational waves. To more accurately determine the parameters of individual sources and to refine statistical models of these systems, it is vital that the sensitivity of these interferometers is increased. To reduce the quantum shot noise of the LIGO detectors, they require a low noise, high power laser. This thesis contains experimental characterisation of the prototype for the laser that will be used during LIGO’s fourth observation run. This laser generated over 100W of amplitude stabilised light in the HG00 mode making it is an important step towards reaching the design sensitivity of the LIGO detectors. A current shunt was compared to an acousto-optic modulator (AOM) for use as the actuator in the control loop for stabilising the laser’s amplitude. It was found that the AOM was more reliable and more versatile than the current shunt, and so it was recommended that the AOM was used during LIGO’s fourth observation run. However, the current shunt may allow for ∼ 10W more power to be delivered to the interferometer, so this should be considered when the maximum laser power that is used by LIGO is limited by the power wasted by the AOM. Balanced homodyne detection is a key part of the upgrade from advanced LIGO to LIGO A+. To lower the quantum noise of the detectors by harnessing the quantum nature of light, it is crucial that the balanced homodyne detector has minimal loss. Mode mismatches between the interferometer and the output mode cleaners are a source of loss; therefore, active optics for mode matching between the interferometer and the output mode cleaner will be used. In this thesis, the uncertainty in the radii of curvature of the optics in the signal recycling cavity (SRC) was used to calculate the distribution of modes which may be present at the signal recycling mirror (SRM). For the LIGO Livingston Observatory, LA., USA (LLO), it was found that the uncertainty in the radii of curvature of an optic known as SR3 is the largest source of uncertainty in the beam parameter at the SRM. From a measurement of the SRC’s Gouy phase, the arm mode at the SRM was inferred to have a width of 1.8 mm and a defocus of −0.28 m−1 . Visualisations for the amount of these modes which the active optics should be able to correct forwere created, and it was found that for LIGO A+, a mode mismatch up to 5% can be entirely corrected with the active optics. Third-generation ground-based gravitational wave detectors, such as the Einstein Telescope and LIGO Cosmic Explorer, will be far more sensitive and be able to probe deeper into the Universe than the current generation of detectors. The increase in sensitivity may be achieved with cryogenically cooled crystalline silicon test masses, but the wavelength of light used in current gravitational wave detectors, 1µm, will not be compatible with these test masses due to them being opaque to this wavelength. Instead, these test masses may work with 2µm light. High quantum efficiency photodiodes are required if the detector’s quantum noise is to be minimal, so off-the-shelf extended InGaAs photodiodes that are sensitive to 2µm light were characterised in the context of the unique requirements of a gravitational wave detector. Both quantum efficiency and 1/f dark noise rise as the reverse bias of an extended InGaAs photodiode increases. A maximum reverse bias was found for the eight photodiodes that were tested such that their dark noises were below the shot noise of a typical current (∼ 10 mA) generated by the photodiode used to sense the gravitational wave signals in an interferometer. The effect of temperature on the dark noise was also investigated. It was found that current off-the-shelf extended InGaAs photodiodes will not be suitable for third-generation detectors as they do not have sufficient quantum efficiency while they are biased such that their dark noise is below shot noise in the frequency band of interest in ground-based gravitational wave detection. Cooling may help reduce this noise, but this poses a significant engineering challenge and the quantum efficiency requirement is still unlikely to be met. Significant amounts of research into the optimal conditions for manufacturing extended InGaAs photodiode would be needed before using them in a third-generation detector is viable.