Experimental investigations into diffractive optics and optomechanical systems for future gravitational wave detectors

In 1916 Einstein published his General Theory of Relativity, from which the existence of gravitational waves was predicted. Gravitational waves are considered to be ripples or fluctuations in the curvature of space-time, propagating isotropically from their source at the speed of light. However, due to the weak nature of gravity, observing this phenomenon presents a great challenge to the scientific community. Small deviations in the apparent positions of stellar objects were measured by Eddington during a solar eclipse in 1919, which confirmed the curvature of space-time and its effect on light, and there have since been many astronomical observations of gravitational lenses. In 1993 Hulse and Taylor were awarded the Nobel Prize in Physics for their observations of a pulsar in a binary system, providing strong evidence for energy loss by emission of gravitational waves. However, the quest for a direct detection of gravitational waves is ongoing through the development of ever more sensitive technology. The development of laser interferometry, based on Michelson topologies, pro- vides the most encouraging route to observing gravitational radiation. There is currently a global network of first generation interferometric gravitational wave detectors in operation, including GEO600 (UK/Germany), Virgo (Italy/France) and TAMA (Japan) as well as several second generation detectors under construction such as Advanced LIGO (USA) and LIGO-Australia (Australia). In the coming years GEO600 will also undergo a series of small sequential upgrades to GEO-HF, while Virgo aims to become an order of magnitude more sensitive across the entire frequency band, as Advanced Virgo. The Institute for Gravitational Research (IGR) at the University of Glasgow has for many years been in strong collaboration with the Albert Einstein Institute in Hanover and Golm, the University of Hanover, the University of Cardiff and the University of Birmingham. The Glasgow group have been involved with developments on GEO600 since its initial construction in 1995, from which a lot of technology has been subsequently adopted for use in other large baseline detectors. There is a 10m prototype interferometer housed in the JIF laboratory at Glasgow, which is utilised for testing new technology and optical configurations of interest to this and the wider collaboration. The research contained in this thesis has been carried out on the Glasgow prototype to investigate novel technology of potential importance to future generations of gravitational wave detectors. In Chapter 1 the history of gravitational radiation is discussed, along with a summary of Einstein’s General Theory of Relativity to reveal the nature of gravitational radiation production. From this analysis several potential sources of astronomical origin are detailed for which the design of ground based detectors are optimised. Various interferometric solutions for detecting gravitational waves are described in Chapter 2, beginning with the most fundamental Michelson topology and thereupon key enhancements, such as Fabry-Perot cavities, power recycling and signal recycling are outlined. The Pound-Drever-Hall scheme used to sense and control the relative distances between each optical component is detailed, including modifications to this technique for controlling significantly more complex systems with many optical elements. The most important attribute in the overall design of an interferometric gravitational wave detector is the total noise limit to the sensitivity, which is comprised of both technical noise and fundamental noise. A summary is provided of the seismic, thermal, and laser noise contributing to technical noise as well as the fundamental quantum noise, consisting of photon shot noise and radiation pressure noise. From this discussion, the author introduces the current global network, and proposed future generations of ground-based detectors intended to open a new field of gravitational wave astronomy. In all proposed upgrades and future detectors the input power must be increased to improve detector sensitivity. Two experiments were designed, con- structed and completed at the Glasgow prototype interferometer related to separate issues of concern for high power regimes. In the first experiment, one of the arms of the Glasgow prototype was commissioned as an all-reflective optical cavity, whereby the partially transmissive input mirror was replaced with a three-port diffraction grating mounted on the bottom stage of a triple pendulum. This investigation was designed to characterise the performance of the grating compared to the conventional input mirror of a Fabry-Perot cavity, whilst revealing issues related to the dynamics of suspended grating input couplers on the control signals. The realisation of grating devices for use in interferometric systems would open a pathway to mitigating the otherwise limiting thermal noise associated to the mirror coatings. The other arm of the Glasgow prototype was chosen to investigate the modified dynamic behaviour of suspended cavity mirrors when signifiant radiation pressure forces are incident. The experiment involved replacing one of the suspended cavity mirrors with a light-weight counterpart designed specifically to increase the overall sensitivity to radiation pressure. By probing the system response for different cavity detunings, it was possible to observe and char- acterise the opto-mechanical resonance, commonly termed an optical spring, which induces optical rigidity at lower frequencies and enhanced sensitivity around the resonant feature. Although optical rigidity suppresses the system response, which is otherwise undesired within gravitational wave detectors, it does however enable systems, which under the right conditions can be self-locking, i.e. the mirror control turned off. Furthermore, the enhanced detector sensitivity at the optical spring frequency can be optimised for different frequencies of interest, and could potentially be used to beat the limit imposed by the Heisenberg Uncertainty Principle for independent cavity mirrors. Together, these experiments may provide information useful to the design of future interferometric gravitational wave detectors