High order, reconstructor-free adaptive optics for 6-8 meter class telescopes

Very high order adaptive optics presents many difficulties related to the high speed (~1kHz), high accuracy and high photon efficiency requirements. In this paper we describe an innovative ``reconstructor-free'' high order adaptive optics system using a self referenced Mach-Zehnder wavefront sensor and a liquid crystal phase corrector. Phase measurements are obtained using the two outputs of a Mach-Zehnder interferometer, with λ/4 path length difference between its two arms, resulting in the intensity difference between corresponding pixels on the two cameras outputs being proportional to the phase value at a particular point. The wavefront corrector is a liquid crystal phase corrector with a square geometry which matches the WFS camera's. The Mach-Zehnder wavefront sensor has the advantage of photon efficiency, for very high order adaptive optics, as well as allowing direct pixel based phase estimation, control and correction. In the laboratory we have achieved wavefront retrieval and correction with very small scale sampling (128 phase measurements per aperture diameter) with lambda/30 rms accuracy. Although the current setup lacks dynamic range and speed work is in progress to improve both in order to allow operation on a real telescope. In fact very recent progress in fast liquid crystal phase modulation using dual frequency material will overcome the speed limitation (for example, π/4 switching speeds have been measured in 120μ s). While limited to small wavefront amplitude errors such Adaptive Optics system could be used very effectively as a second stage very high order AO system without computer image processing and high speed digital reconstruction.