Two-channel wavefront sensor arrangement employing moire deflectometry

Abstract

A wavefront sensor which takes advantage of the moiré 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 moiré deflectometers. Directions of the grating's rulings are parallel in each moiré deflectometer but are perpendicular in the two beams. Using a suitable array of lenses and mirrors two sets of moiré patterns are projected on a CCD camera. A suitable spatial filter removes the unwanted frequencies. Recording the successive moiré 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 moiré 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 moiré 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 moiré 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.

Formulation of the moiré patterns formed by superimposing of gratings consisting topological defects: moiré technique as a tool in singular optics detections

“…Finally, we consider a case in which the, direction of the vector connecting the center of first grating to the center of the second one lies in the x + direction. As illustrated in figure 9, in this case, branching direction of the generated fork-shaped moiré pattern at the vicinity of the first grating's branch point follows the sign of l 1 + and the other shape follows the l 2 − sign (see equation (12) and figure 1).…”

“…In addition, moiré deflectometry is one of the most accurate tools in the measurements of incident wave deflections and wave-front gradients. Recently, it appeared as a powerful tool in the nonlinear refractive index measurement [6,7], atmospheric turbulence characterization [8][9][10][11] as well as in wave-front sensing [12][13][14][15][16]. Furthermore, moiré patterns are demonstrated in quantum imaging on the spatial correlations between entangled photon pairs [17,18].…”

Atmospheric Turbulence Characterization and Wavefront Sensing by Means of the Moiré Deflectometry

“…Recently, a wide dynamic range two channel wavefront sensor based on moiré deflectometry has been constructed for measuring atmospheric distortions of wavefront (Rasouli et al, 2009;2010). Schematic diagram of the sensor is shown in Fig.…”

“…As a result of these works, due to the magnification of the telescope, the use of moiré technique, and the Talbot effect, measurements of fluctuations in the AA can be up to 2 orders of magnitude more precise than other methods. Also, moiré deflectometry have been used to wavefront sensing in various schemes (Rasouli et al, 2009;2010). In the recent scheme, 2 Will-be-set-by- IN-TECH an adjustable, high-sensitivity, wide dynamic range two channel wavefront sensor was suggested for measuring distortions of light wavefront transmitted through the atmosphere .…”

Measurement and statistical analysis of the wavefront distortions induced by atmospheric turbulence using two-channel moiré deflectometry

“…The most significant results of these works are the improvement of the measurement precision and access to a large volume of 2D data-namely, because of the magnifications of the telescope, the use of the moiré technique, and the Talbot effect, measurements of light-beam deflections can be up to two orders of magnitude more precise than other methods such as differential image motion monitor systems [15], generalized seeing monitor systems [16], and the edge image waviness effect [17]. Also, we have used moiré deflectometry for wavefront sensing in various schemes [18,19]. In this recent scheme, an adjustable, high-sensitivity, wide dynamic range, two-channel wavefront sensor was suggested for measuring distortions of light wavefronts transmitted through the atmosphere [19].…”