Ottaway, DavidVeitch, PeterKing, Eleanor Jean2021-11-042021-11-042020https://hdl.handle.net/2440/132935This thesis describes a variety of characterization measurements made on the Advanced LIGO (aLIGO) interferometers, which are important for understanding the as-built interferometer and enhancing sensitivity. The aLIGO detectors began detecting Gravitational Waves in 2015 after many decades of research and development, beginning a new age of gravitational wave astronomy. Moving into the future, continued improvements in detector sensitivity will increase the range and detecting capability of the instruments. Improvements to detector sensitivity will be achieved through a variety of efforts, including improving lock stability, improving mode matching, and decreasing power losses in the interferometer. Characterizing the aLIGO interferometers is particularly challenging because the interferometers are enclosed within a large-scale vacuum system, which limits access to the equipment. The techniques described in this thesis illustrate ways that the properties of the cavities and optics may be probed when the components of the cavities are not easily accessible for individual measurement. Chapter 1 presents an introduction to gravitational wave detection. Chapter 2 introduces the theoretical background for cavity characterization measurements which underpin many of the measurements presented in later chapters. Chapter 3 describes measurements performed on the interferometer cavities using locked cavity techniques. These measurements yielded accurate results for arm cavity g-factor, power recycling cavity length and Gouy phase, Schnupp asymmetry length, and signal recycling cavity length. The SRC Gouy phase was not accurately measured using this technique due to the low Finesse of this cavity. Chapter 4 discusses a method for measuring the Gouy phase of an unlocked cavity, which may be used to measure the Gouy phase of a low finesse cavity, and describes how this method was used in the signal recycling cavity. Chapter 5 presents in-situ scattering and absorption measurements of the input- and end-test masses and input mode cleaner optics, using cameras and Hartmann wavefront sensors. Chapter 5 also discusses the impact of these properties on power build-up and thermal lensing. Chapter 6 presents a technique which can be used for faster modelling of thermal deformation in optics, which may be used to improve time-dependent interferometer models. The measured properties of the cavities and optics presented in this thesis allow for improved understanding of mode matching and power build-up in the aLIGO interferometers. This information will be used in future commissioning efforts to improve detector sensitivity.enaLIGOgravitational waveslasersthermal compensationinterferometeropticsThesis