When light gets pushy: radiation pressure effects in interferometric gravitational wave detectors

Advanced LIGO made the first direct observation of a gravitational wave in 2015. This signal, the largest measured so far, had a peak strain amplitude of 1 x 10-21 and frequency range 35-250 Hz. LIGO's sensitivity was achieved after decades of development. Suspended optics made from low-loss materials are employed in an optical configuration with multiple coupled cavities to increase the circulating power and shape the frequency response of the detector. This design results in radiation pressure that substantially alters the detector's behaviour. One challenging consequence is parametric instabilities, an unstable coupling between the optical field and mechanical resonances in the mirrors. My detailed investigations of parametric instabilities in both LIGO and ET, a planned next-generation detector, show that the severity of the instabilities depends on the complete optical configuration. With this model, an optimal operating point for LIGO can be determined and ET design choices can be weighed against potential instabilities. An 'optomechanical filter' has been proposed that uses radiation pressure to enhance the detector bandwidth. I test the analytical models against numerical simulations and outline an experimental research programme that will implement a trial filter