Briggs, Joseph Henry (2021) The analysis, experimental characterisation and prototyping of technologies for making quantum noise limited detections of gravitational waves. PhD thesis, University of Glasgow. http://theses.gla.ac.uk/82330/ Copyright and moral rights for this work are retained by the author A copy can be downloaded for personal non-commercial research or study, without prior permission or charge This work cannot be reproduced or quoted extensively from without first obtaining permission in writing from the author The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the author When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given Enlighten: Theses https://theses.gla.ac.uk/ [email protected] The Analysis, Experimental Characterisation and Prototyping of Technologies for Making Quantum Noise Limited Detections of Gravitational Waves Joseph Henry Briggs Wednesday 14th July, 2021 Submitted in fulfillment of the requirements for the Degree of Doctor of Philosophy School of Physics and Astronomy College of Science and Engineering University of Glasgow Abstract 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 100 W of amplitude stabilised light in the HG00 mode making it is an important step towards reaching the design sensitivity of the LIGO detectors. i 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 ∼ 10 W 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 quan- tum 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 inter- ferometer 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 for were created, and it was found that for LIGO A+, a mode mismatch up to 5% can be entirely corrected with the active optics. ii Third-generation ground-based gravitational wave detectors, such as the Ein- stein 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 re- search into the optimal conditions for manufacturing extended InGaAs photodiodes iii would be needed before using them in a third-generation detector is viable. iv Acknowledgements I would like to thank Ken Strain for his supervision and advice during the course of my PhD. He set a great example of how to be a scientific thinker, and he was an invaluable source of experience and knowledge. Ken always made time to answer my questions, explain things to me and give me feedback on the work that I was doing. I am grateful to Bryan Barr for his assistance and insights. Many times through- out my PhD, Bryan offered his time and energy to help me understand how I could solve a problem I was having, and his practical advice was indispensable. I also want to thank Stefan Hild for his guidance, enthusiasm, and encouragement during the first two years of my PhD. Regarding the production of this thesis, I would like to thank Ken Strain, Bryan Barr, Harry Ward, Giles Hammond and Peter Veitch. They highlighted numerous places where I could improve its clarity, spotted typos and suggested structural changes. Of course, any remaining typos are solely my responsibility. v Thanks to my friends for all the walks up mountains, pub trips, the coffee breaks and all the fun conversations. I couldn’t know a better bunch of people to spend my free time with. I would like to thank Dwayne Spiteri for being a great friend and housemate. The writing of this thesis happened during an unusual year, and it was certainly made easier by having him around. I would also like to thank him for his (not always solicited) opinions about what I was thinking about and working on. I would like to thank my family for their never-ending love. Mum, Dad, James and Isabelle have encouraged and supported me at every stage of my life so far. They are always there for me and have helped me at many times to get the best out of life. I cannot be more grateful for my family and I would not be the person I am today without them. To Catriona, thank you for your love, care and support. You bring so much vibrancy, warmth and joy to the lives of all the people you know, and I feel so incredibly lucky to be with you. Your companionship has given me a great sense of happiness and fulfilment, and my life has been made so much more wonderful by sharing it with you. vi Declaration I declare that the work described in this thesis was carried out by me and has not been presented in any previous application for a degree at the University of Glasgow or any other institution. The chapters in this thesis and the contributions of others to them are described below. Chapter 1 This chapter contains introductory information pertaining to ground- based gravitational wave interferometry and the LIGO detectors. Chapter 2 In this chapter, experiments to do with the prototype of the laser that will be used for LIGO’s fourth observation run are described. My contribution to this work was in the setup of the optical table and the taking of measurements. These tasks were completed with help from Nina Bode, Matthew Heintze and Michael Fyffe. Rick Savage and Maik Frede contributed to discussions of how this should be done. The diagnostic breadboard was supplied by the AEI. The design of the prototype was undertaken by others. The outcome of this project was published in [1]. The photographs of the laser were taken by Michael Fyffe. This work was carried out at the LIGO Livingston Observatory as part of the LSC fellows programme and was funded by the STFC long-term attachment scheme.