Adaptive optics systems based on feedback interferometry

Abstract

Adaptive optics systems are commonly employed in astronomical applications. Their complexity and high-cost often prevent their use in other applications such as optical communication links, low-cost imaging systems, high power lasers, and medical applications. This leads to interest in the development of low-cost adaptive optics systems, and hence feedback interferometry is investigated in this thesis as one way of achieving this. A feedback interferometer is a simple, elegant system that can be used for aberration correction. It provides adequate correction performance and has the potential to operate at a very high-speed. When combined with recently developed low-cost wavefront correctors, compact aberration correction systems can be built. In this thesis, the fundamentals of feedback interferometry are described and theory governing limitations on system parameters is developed. A PID control algorithm was introduced into the feedback loop in the systems developed to achieve better system stability and to improve performance. The theory developed was then tested using a tip-tilt only aberration correction system based on a feedback interferometer. Excellent agreement between the predicted and experimental results was achieved. The system was also used to successfully demonstrate tip-tilt correction. Feedback interferometers for correcting high-order aberrations were constructed using custom-made multi-segmented mirrors. Results in the laboratory showed promising correction ability. A practical system based on a common-path, radial shearing, polarisation Sagnac feedback interferometer was developed for correcting atmospheric turbulence-induced aberrations. The system provided significant improvement on aberrated wavefronts and it is believed that this is the first time feedback interferometry has been used for correcting real atmospheric aberrations. Good correction was obtained for a 40 mm diameter HeNe laser (λ = 632.8 nm) beam propagating over a 120 m open path about 1 m above a grassy field. Two other systems were developed for aberration correction and are discussed in this thesis. The first is a feedback interferometer incorporating a micromachined membrane deformable mirror (a modal wavefront corrector). The results obtained demonstrated the possibility of using such a system for aberration correction despite strong spatial coupling from the mirror influence functions. The second correction system is based on high-speed phase stepping. In spite of the system was developed for demonstration purposes only, remarkably good correction of both static aberrations generated inside the laboratory and aberrations induced by the atmosphere in an outside propagation path were achieved.