title: Advanced techniqes for future generation gravitational wave detectors
Creators: name: Zhang, Teng
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Gravitational wave detectors, quantum noise, speed meters.

description: After the first direct detection of gravitational waves from a system of two colliding black holes, we have stepped into the era of gravitational-wave astronomy. In order to observe the broader and deeper universe, increase the detection rate for various sources and do better source parameter extraction, it is essential to further enhance the sensitivity of gravitational wave detectors. The design sensitivities of the current detectors are limited by quantum noise nearly over the whole detection band. Various quantum non-demolition technologies have been proposed to suppress the quantum noise, one of which is called speed meter. Speed meter aims to improve the detector sensi- tivity at low frequencies. Not only it can beat the standard quantum limit but also gives lots of astrophysics prospects. For example, it gives longer waring time before the merge stage of binaries and it significantly improves the detection rate of massive binary black holds (so far we have not observe any systems with component masses prior to merger greater than 50 solar masses) allowing to uncover the potential existence of black hold population in this range. One speed meter experiment, Sagnac speed meter proof of concept experiment is currently carried out in Glasgow. This experiment aims to prove the superiority of speed meters in terms of quantum radiation pressure noise in the low frequency compared with an equivalent Michelson. One property of the Sagnac interferometer is that the light fields returning from the arms in two directions share the same path and always interfere de- structively at the signal port. This property is unsuitable for conventional DC readout and pushes the utilisation of balanced homodyne readout. Balanced homodyne readout is also planed to be implemented in advanced LIGO upgrade, A+. This thesis introduces several research topics around speed meters and balanced homodyne readout. One problem of implementing balanced homodyne readout is the optical loss because of the misalignment and mismatch between the separate local oscillator and signal beam in balanced homodyne readout. A theoretical framework is built for analysing the static and dynamic optical higher order modes effects. The results are applied to the Glasgow proof of concept experiment. Misalignment is not only a problem in the balanced homodyne readout, but also in the Sagnac interferometer itself. The effect of misalignment inside the Sagnac interferometer on the quantum noise limited sensitivity is calculated with the example of the Glasgow SSM experiment. Strategies for the implementation of an auto- alignment scheme in SSM experiment are investigated. I also investigate several aspects of considerations on implementing balanced homodyne readout in A+, including local oscillator stability requirement, output mode cleaner arrangement, local oscillator backscattering effects and sensing and control for different degrees of freedom in the balanced homodyne readout. For Sagnac speed meter with Fabry-Pérot resonators in the arms, it was shown theoretically that an asymmetry of the main beamsplitter can lead to a reduction of the quantum limited sensitivity at low frequencies. We propose an approach to solve this problem by utilising balanced homodyne readout and choosing a proper local oscillator delivery port. In Sagnac speed meter interferometer, ring cavities are required. Different from linear cav- ities, the circulating beam in the ring cavities and the normal of the input mirror are not on the same line. The backscattering inside the ring cavity due to mirrors micro-roughness can induce coupling between the two counter-propagating modes. I analyse the effect of backscattering on quantum noise of a ring cavity when conducting measurement at one output port. Starting from previous work in [1], I develop here a more in depth analysis of the backscattering mechanism and present the results distinguished into three characteristic levels of backscattering amplitude. Again this is carried out using the Glasgow SSM experiment as an example. In addition to Sagnac interferometer, other more advanced types of speed meter have been proposed by the community, including one based on the principle of Einstein Podolsky Rosen entanglement. I analyse the effect of several imperfections on quantum noise and the potential sensitivity improvement by injecting frequency dependent squeezing. Another new speed meter configuration is based on a standard Michelson interferometer featuring additional polarisation optics in the output port, named as polarisation circulation speed meter. I propose an acceleration meter configuration based on the combination of the Sagnac speed meter and the polarisation circulation speed meter. With no doubt that the loss-less variational readout scheme is better than speed meter, however, it is more susceptible to optical loss compared with speed meter in real environ- ment. Here I develop a comparison between speed meters and position meters with lossy variational readout.