Sensing and control in dual-recycling laser interferometer gravitational-wave detectors

We introduce length-sensing and control schemes for the dual-recycled cavity-enhanced Michelson interferometer configuration proposed for the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO). We discuss the principles of this scheme and show methods that allow sensing and control signals to be derived. Experimental verification was carried out in three benchtop experiments that are introduced. We present the implications of the results from these experiments for Advanced LIGO and other future interferometric gravitational-wave detectors

Frequency dependence of thermal noise in gram-scale cantilever flexures

We present measurements of the frequency dependence of thermal noise in aluminum and niobium flexures. Our measurements cover the audio-frequency band from 10 Hz to 10 kHz, which is of particular relevance to ground-based interferometric gravitational wave detectors, and span up to an order of magnitude above and below the fundamental flexure resonances. Results from two flexures are well explained by a simple model in which both structural and thermoelastic loss play a role. The ability of such a model to explain this interplay is important for investigations of quantum-radiation-pressure noise and the standard quantum limit. Furthermore, measurements on a third flexure provide evidence that surface damage can affect the frequency dependence of thermal noise in addition to reducing the quality factor, a result which will aid the understanding of how aging effects impact on thermal noise behavior.Australian Research Counci

Thermal noise and optical cooling

Thermal noise and optical quantum noise place fundamental limits on the displacement sensitivity of interferometric gravitational wave detectors. Projects such as Advanced LIGO employ a wide range of techniques to reduce thermal noise and use very high optical powers to reduce quantum noise. Thermal noise is Brownian motion caused by the thermal energy in each mode of oscillation. It is fundamentally linked with mechanical loss via the fluctuation-dissipation theorem. The thermal energy of, for example, the fundamental resonance of the suspension of a mirror can be concentrated into the resonant peak by using a very high-Q oscillator. This reduces the spectral density of the fluctuations away from resonance. The shape of the thermal noise spectrum depends on the mechanical loss mechanisms, and can have important implications for interferometer design This thesis presents, to the best of my knowledge, the first off-resonance thermal noise measurement of a high-Q suspension which includes both above-and below-resonance regions. The measurements do not conform to the accepted 'structural' damping model, but rather seem to suit a model with both structural and viscous damping. A number of potentially spurious loss mechanisms were investigated, but none were found to substantially alter the spectral shape of the measured noise. A lower quality factor suspension material was then employed to see if structural damping was dominant, but again a mixed damping model fitted the data better than structural damping. A coating-free mirror was designed and experimentally characterised. Removing the optical coating removes a significant source of mechanical loss from the mirror, potentially improving thermal noise. The combination of high optical powers and high-Q oscillators can lead to strong opto-mechanical interactions. The light inside an optical resonator can act as a complex spring, modifying both rigidity and damping. For a very low-loss mechanical system, a small amount of optical anti-damping can lead to instability. Conversely, it is possible to optically damp, or 'cool', an oscillator by extracting thermal energy. Results are presented showing optical cooling of the fundamental mode of a mirror suspension down to an effective temperature of 70 mK. This cooling is measured by the direct observation of the thermal noise spectrum. The measured traces are in agreement with a prediction of the thermal noise spectrum based on the input laser power, optical configuration, and feedback control system used. Results from the suspended gravitational wave detector prototype, the Caltech 40m interferometer, show how a strong optical spring creates an opto-mechancial resonance which alters the frequency response of the interferometer