Two-channel wavefront sensor arrangement employing moire deflectometry

Abstract

A wavefront sensor which takes advantage of the moire deflectometry has been constructed for measuring atmosphere induced wavefront distortions. In this sensor a collimated laser beam propagates through turbulent atmosphere, then a beam splitter splits it into two beams and the beams pass through a pair of moire deflectometers. Directions of the grating's rulings are parallel in each moire deflectometer but are perpendicular in the two beams. Using a suitable array of lenses and mirrors two sets of moire patterns are projected on a CCD camera. A suitable spatial filter removes the unwanted frequencies. Recording the successive moire patterns by the CCD camera and feeding them to a computer, allow temporal fluctuations of the laser beam wavefront phase to be measured highly accurately. Displacements of the moire fringes in the recorded patterns correspond to the fluctuations of two orthogonal components of the angle of arrival (AA) across the wavefront. The fluctuations have been deduced in successive frames, and then evolution of the wavefront shape is determined. The implementation of the technique is straightforward and it overcomes some of the technical difficulties of the Schlieren and Shack-Hartmann techniques. The sensitivity of detection is adjustable by merely changing the distance between two gratings in both moire deflectometers and relative grating ruling orientation. This overcomes the deficiency of the Shack-Hartman sensors in that these require expensive retrofitting to change sensitivity. Besides, in the moire deflectometry, the measurement is relatively insensitive to the alignment of the beam into the device. Hence this setup has a very good potential for adaptive optics applications in astronomy. Since tilts are measured in the Shack-Hartmann method at discrete locations, it cannot detect discontinuous steps in the wavefront. By this method discontinuous steps in the wavefront are detectable, because AA fluctuations are measured across the wavefront.