MEMS Adaptive Optics Offer 11,000 Iterations per Second
Mirrors based on microelectromechanical systems (MEMS) promise to meet the requirements of adaptive optics, but standard control-system architectures cannot fully utilize their speed and size advantages. A new integrated MEMS mirror array and very large scale integration (VLSI) control system may yield an adaptive optics setup that corrects for aberration at a rate of 11,000 iterations per second.
Researchers have tested integrated micromirror arrays and a parallel stochastic gradient descent control system with seven very large scale integration chips. The setup may find applications as a compact adaptive optics system that can correct for aberration at a rate of 11,000 iterations per second. Courtesy of Mikhail Vorontsov.
Adaptive optics systems compensate for variable wavefront distortion in an optical path. They are critical components in modern astronomical imaging, for which they mitigate the effects of atmospheric aberration. But to adequately address rapid and small-scale changes in the length of the optical path, the systems must have high-bandwidth, densely spaced actuators.
Researchers use several technologies to construct MEMS mirrors, each of which typically addresses a specific problem and employs a custom control system. To evaluate micromirror systems, a team from the US Army Research Laboratory's Computational and Information Sciences Directorate of Adelphi, Md., Boston University of Brookline, Mass., MEMS Optical Inc. of Huntsville, Ala., and Johns Hopkins University of Baltimore implemented a parallel stochastic gradient descent control system with seven VLSI chips on a 6.5 x 4.5-in. board.
The system sought to maximize the amount of energy transmitted through a pinhole or an optical fiber by randomly varying the actuator voltages in parallel above and below a progressively better mean. The start point for the next set of iterations was a function of the actuator voltages that provided better power throughput in the last iteration.
The researchers have evaluated several MEMS mirrors with the control system, including a 12 x 12 actuator, segmented piston type and a 5 x 5 membrane tip-tilt from Boston University, and a 6 x 6 spiral-spring, piston-only arrangement from MEMS Optical.
<100 µs per iteration
Mikhail A. Vorontsov, a researcher at the Army Research Laboratory who worked on the project, explained that because this approach corrects for distortion in the phase of the wavefront without sensing that wavefront, the MEMS-based adaptive optics systems could be compact, efficient and inexpensive. And because many iterations are necessary to compensate for wavefront distortion, the system must be fast. "But this," he said, "is exactly what micromirror arrays can provide."
Even with the addition of adaptive-gain calculations to speed up the iterative process and to eliminate problems with local extrema, the systems required less than 100 µs per iteration, largely thanks to the parallel on-VLSI-chip computations.
"There is no way," Vorontsov said, "a digital computer can handle 11,000 iterations per second simultaneously for 144 control channels."
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