Optical spring parametric interactions in a macroscopic opto-mechanical resonator

    Research output: ThesisDoctoral Thesis

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    [Truncated abstract] The research described in this thesis investigated optical spring interactions and instabilities in a macroscopic opto-mechanical resonator. The thesis describes an experiment designed to model an optical spring 'tranquiliser' cavity which has been proposed to suppress the predicted parametric instabilities in the next generation of interferometric gravitational wave detectors. In a series of experiments, the optical spring effect was observed in macroscopic optical cavities through measured changes in mechanical stiffness, and measured changes in mechanical loss. The optical spring effect was further characterised through investigation of its dependent parameters. Two pairs of identical, low optical loss mirrors were bonded to a mechanical structure using a novel low mechanical loss technique, forming an opto-mechanical composite resonator. The technique uses the naturally occurring resin Yacca gum as a bonding agent. This resulted in the formation of two optical cavities with a length of l = 0.100±0.001m, only one of which was used in experiments. Using finite element modelling, the resonator's two lowest modes, with frequencies of fm1 = 722.8Hz and fm2 = 747.9Hz, and an effective mass 0.0323±0.0001kg, were found to be subject to the optical spring effect. ... The instabilities are expected to have a parametric gain factor of up to 100 in the frequency range of 15-120kHz. Therefore, if optical spring damping can be made large enough to reduce the Q-factor of the Advanced LIGO test-masses by a factor of 100, all parametric instabilities should be eliminated. For a simple servo loop and an optical cavity with the practically achievable finesse of F = 30,000, a tranquiliser cavity length of 1.3cm was found to produce optimum enhanced damping. This configuration only requires 1.47W of input power, resulting in an intra-cavity power of 5.72kW. The cavity mirrors were assumed to have optical coatings with a damage threshold of 1MW/cm2, which limited the spot size to a minimum area of 0.572mm2, or a radial beam waist of w = 0.427mm. This nearly flat-flat cavity has a stability g-factor of 0.9997. Even given these technical challenges, suppression of the parametric instabilities predicted to occur in the next generation of interferometric detectors is possible to achieve practically using enhanced optical spring damping. A possible design for such a tranquiliser cavity is also suggested.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2007

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