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C. Kempf, N. Doble, M. Helmbrecht; Control of a MEMS Based Segmented Deformable Mirror for Vision Science Instrumentation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4244.
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© ARVO (1962-2015); The Authors (2016-present)
Recently there has been considerable success applying adaptive optics (AO) to high resolution in vivo imaging of the human retina. The key component for AO is a wavefront corrector having sufficient dynamic range to correct the typical aberrations found in human eyes. We present the first results detailing control of a high stroke segmented microelectromechanical systems (MEMS) deformable mirror (DM) specifically designed for vision science applications. The system described is being incorporated into a confocal laser scanning ophthalmoscope to be delivered to the United States Air Force for laser damage studies.
Iris AO, Inc. has developed a compact MEMS deformable mirror with an inscribed aperture of 3.5mm. The stroke exceeds 5um at the mirror surface, which begins to approach the levels needed for the human eye. The DM consists of an array of independently actuated hexagonal segments. Each segment is electrically controlled to provide piston, tip, and tilt displacements; thus the overall surface can closely approximate well known Zernike modes to provide accurate wavefront correction. This device is distinct from continuous-surface mirrors in which the mechanical stiffness of the mirror surface often limits the ability to achieve high stroke and good spatial frequency correction. The control of segmented MEMS devices has not been widely studied. In this research, closed-loop control of the mirror was achieved using a wave-front sensor (Shack-Hartmann array) which drove a modal wavefront reconstructor. Correction at the mirror was achieved with a combination of a relatively simple update algorithm in conjunction with an accurately calibrated electro-mechanical mirror model.
Results demonstrated on an AO test-bed system show rapid (essentially one or two sample steps) convergence after the introduction of phase plates having aberration profiles typical of those found in human eyes. Even for wavefront errors exceeding the currently available DM stroke (+/- 2 Diopters), the algorithm achieved a high degree of correction and maintained relatively rapid convergence.
A segmented MEMS based DM can be controlled using traditional modal techniques to achieve good correction and rapid convergence. This adaptive optics subsystem is a key technology platform for achieving high-stroke, high-order corrections in applications such as retinal imaging systems and vision simulators. Extensions of this work to achieve higher stroke and higher segment counts is currently underway.Acknowledgements: This work has been supported under NSF SBIR Grant #0611399.
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