Pushing cavities to the edge for future gravitational wave detectors

Near-unstable cavities have been proposed as an enabling technology for future gravitational wave detectors, as their compact structure and large beam spot can reduce the thermal noise floor of the interferometer. These cavities operate close to the edge of geometrical stability, and may be driven into instability via small cavity length perturbations or mirror surface distortions. They are at risk of suffering from problems such as high optical scattering loss and Gaussian mode degeneracy. The well-defined Gaussian beams can also be distorted through their interaction with the small imperfections of the mirror surfaces. These issues have an adverse impact on the detector sensitivity and controllability. In this article an experiment is designed and has been built to investigate the technical hurdles associated with marginally cavities. A near-unstable table-top cavity is built and accurate control achieved through length and alignment control systems. This experiment provides an account of the behavior of the near-unstable cavity. Additionally, the experiment provides an insight into how far cavity parameters can be pushed towards geometrical instability.