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Title: Characterisation and control of coupled optical springs for future gravitational wave detectors
Author: Gordon, Neil Andrew
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2015
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The detection of gravitational waves in the years to come will be the result of decades of work by institutions around the world to design and construct instruments to probe the characteristic strain of space-time on the smallest scales. Once detected, an entirely new field of study will be open to astronomy, allowing probing of the interior structure of massive celestial objects and insight into the very beginning of the universe. However, this will by no means signal the end of this work. Efforts will continue to push the sensitivity limits of detectors ever lower, at the same time widening our gravitational wave horizon to encompass sources at greater distances. Soon, gravitational wave detectors are expected to be operating at the Standard Quantum Limit throughout much of their detection bandwidth. Novel techniques will need to be employed to probe beneath this level. One such method involves the use of opto-mechanical rigidity, or "optical springs". This technique couples the suspended optics of a Fabry-Pérot cavity together using only the radiation pressure force transferred between them, transforming the system into the harmonic oscillator regime and thus increasing sensitivity in a narrow band about the associated resonance. If this technique is to be applied to a large-scale gravitational wave detector, the effect must be well-characterised and robust control methods investigated. Importantly, the optical spring effect is observed to arise in any gravitational wave detector operating with high power, providing further motivation for the study of control systems to cope with them. It has further been suggested that multiple optical springs may be employed in a single system, either for improved optical stability such that electronic feedback may be reduced, or else to favourably re-shape the noise spectrum of the instrument for wider-band sensitivity improvement. We present the design and commissioning of an experiment at the Glasgow 10m Prototype Interferometer Laboratory to investigate characterisation and control methods for coupled optical spring systems. The experimental system consists of two 10m cavities coupled mechanically by a shared end test mass. Each cavity can be detuned to facilitate opto-mechanical coupling, the combined effects of which are studied. The design of this experiment is covered from initial simulations through mechanical design, testing and installation. Robust digital control loops are built, providing techniques for reliable lock-acquisition of high-finesse optical cavities and maintenance of stability in the presence of strong opto-mechanical effects. A number of experiments are performed to examine the interactions between optical springs in both cavities and the control loops which maintain them. We observe marked power stability over significant periods of time to enable precision measurement of optical spring resonant responses in both cavities, and confirm the power-dependence of the optical spring effect. Techniques are described for the maintenance of stable cavity lock in the presence of strong optical springs and anti-springs, and complex responses resulting from couplings of multiple springs to loop gains are characterised. This work will inform the design of future gravitational wave detectors, which are expected to employ optical spring technologies in order to push further beneath the Standard Quantum Limit.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics