Advanced techniques in laser interferometry for current and future gravitational wave detectors

The current generation of gravitation wave detectors have been highly successful in observing gravitational wave signals from the inspiral and merging of astrophysical compact binary objects such as black-holes and neutron stars. While these gravitational wave detectors remain in operation, plans are being made to improve the sensitivity of these detectors and to build more sensitive detectors. This thesis covers a series of experimental work in the field of laser interferometry for gravitational wave detector instrumentation, including an investigation of polarising optics for a speedmeter interferometry and topics in laser system development for current and future gravitational wave detectors. The concept of the speedmeter interferometer is considered as a gravitational wave detector with lower quantum noise than the Michelson interferometers currently employed. The practical limitations of one such speedmeter scheme; the polarisation Sagnac is investigated with a single arm prototype experiment. For the polarisation Sagnac speedmeter it is important to demonstrate that polarising optics currently available are of a high enough quality to not limit the performance of a full-scale detector. This can be done by individual testing of the polarising beam-splitter (PBS) and quarter waveplate (QWP) optics, as is shown in Chapter 3, and with a prototype scale interferometer demonstration in the Glasgow 10 m prototype facility as described in Chapter 4. In addition to polarising optics, low levels of birefringence in the cavity mirrors can place a significant limitation on the performance of the polarisation Sagnac speedmeter as this scheme relies on a conservation of the polarisation state of the light within the cavities to ensure the optical beams are directed along the desired path through the interferometer. Any light coupling to a different path though the interferometer will degrade the performance of the interferometer as a speedmeter. In Chapter 4 and 5 this cavity birefringence effect is measured with a 10 m suspended linear cavity experiment. The polarisation of the light transmitted by the cavity was measured for different rotations of one cavity mirror to measure the effect of mirror cavity birefringence on the polarisation state of the light circulating in the cavity. In Chapter 6 the combination of polarisation imperfections is considered in terms of the polarisation Sagnac speedmeter to determine if this scheme can be considered a viable option for future gravitational wave detection. It is shown how the cavity birefringence effect can be combined with the birefringence level of the QWP to cancel out unwanted levels of birefringence for each arm of the polarisation Sagnac speedmeter. Under with circumstances the performance of the interferometer in terms of polarisation optics is limited only by the finite extinction ratio of the PBS optic. In Chapter 9, work to upgrade the laser system of the LIGO Livingston detector is described. This work consisted of the installation of an additional laser amplifier to the laser system that would double the power available to the interferometer. Resulting in an estimated factor of root 2 improvement in the detector sensitivity. In Chapter 10, two investigations into the laser stabilisation system of the LIGO Livingston are presented. This includes an investigation of a persistent issue with the frequency stabilisation system that had been limiting the control bandwidth of the stabilisation loop. This issue was found to be related to the analogue servo electronics for this system. Another issue with the intensity stabilisation system that occurred during the second observing run of the Advanced LIGO detectors in 2017 was investigated as dropouts in this control loop were causing the full interferometer to lose lock. Chapter 11 describes the installation and testing of a new laser system with emission at 1550 nm wavelength for the Glasgow 10 m prototype facility. This include the development and troubleshooting of a frequency stabilisation scheme for the laser with significantly higher intrinsic frequency noise when compared to the standard 1064 nm laser used in Advanced LIGO and other gravitational wave interferometry experiments. With the described stabilisation system the noise of the laser is reduced to 0.1 Hz/rt(Hz) between 100 Hz and 1 kHz with a unity gain point at 500 kHz. In Chapter 12 a digital control system for laser frequency stabilisation is considered and an algorithm for filter design optimisation is described and tested