Interferometric wavefront sensing for extreme adaptive optics

Abstract

Adaptive optics is concerned with the correction of phase distortions in wavefronts which degrade the quality of images produced by optical systems. It was originally developed for both astronomy and the military, where the Earth's atmosphere causes distortions, although other uses are now being developed. As ground based telescopes become increasingly large the size and complexity of adaptive optics systems also increase, creating "extreme adaptive optics”. This thesis deals with such an adaptive optics system. A novel self referenced phase shifting interferometer based on a liquid crystal (LC) waveplate is presented which can measure high spatial frequency phase distortions. This is then coupled to a LC spatial light modulator wavefront corrector. The geometry is matched such that there is no need for a wavefront reconstructor. The performance is measured in two stages. Firstly, spatially where static phase distortions are measured by the interferometer and corrected. Secondly, temporally where a simple analogue feedback is implemented to show correction over a single corrector pixel for fast time varying phase distortions. This work builds on other published research on using point diffraction interferometry in adaptive optics. The novelty lies in the development of a new implementation of a point diffraction interferometer, and in the demonstration of a high-speed closed loop single channel system. This work therefore contributes to the groundwork required to build an extreme adaptive optics system whose complexity scales linearly with the size (area) of the telescope aperture.