Springs simplify micromirror arrays

By Kimberly Patch, Technology Research News

Arrays of tiny mirrors that can be precisely positioned are the key components of adaptive optics used in astronomy, biomedical imaging, free-space communications and satellite imaging systems.

Adaptive optics correct light waves that have been distorted, usually by the atmosphere, by bouncing them off a mirror that rapidly changes shape to produce clearer images or signals.

The mirrors in micromirror arrays are typically less than a fifth of a millimeter square and are controlled individually using a mechanism that converts a digital computer signal into a mechanical movement. Large arrays of micromirrors are relatively expensive because they require a large number of electrical digital-to-analog converters to individually address, or control, each micromirror. This also limits the size of an array.

Researchers from the National University of Singapore have found a way make simpler, less expensive mirror controls. At the heart of the researchers' digital-deflection programmable micromirror array is a digital-to-analog converter that works mechanically rather than electrically.

The device controls a large array of micromirrors by addressing them via row and column lines rather than having separate circuits to each mirror. This drastically reduces the number of routing wires needed and allows an array of mirrors to be controlled by off-chip electronics. It also makes it easier to manufacture.

The method promises to reduce the complexity and thus cost and size of devices that use micromirror arrays.

The research could lead to a cost-effective, compact wavefront aberration correction method that uses little power, said Guangya Zhou, a research fellow at the National University of Singapore. "It is also inherently robust and accurate, insensitive to voltage and temperature fluctuations, and suitable for harsh-environment applications where conventional microelectronics might fail," he said.

Each micromirror is moved vertically by a set of microactuators. Each microactuator has two possible positions -- an unpowered, or off, position, and a powered, or on, position. A set of springs of different stiffnesses connect the microactuators to the micromirror. The differences in spring stiffness determines how far the mirrors move.

The researchers' built a prototype 2-bit, 3-by-3 micromirror array with 160- by 160-micron mirrors. Each bit in the control signal controls a pair of actuators that move the mirror a specific amount. The on-off combinations of two bits coupled with the two spring stiffnesses yield four mirror positions. The mirror positions in the prototype are set to alter the phase of a 632.8-nanometer wavelength laser beam by one quarter of the wavelength, three-eighths of the wavelength, or leave it as is.

The micromechanical digital-to-analog converter positions the prototype's micromirrors precisely, with a range of 261 nanometers in increments of 87 nanometers. A nanometer is one millionth of a millimeter.

The prototype changes mirror positions in less than 100 microseconds, according to Zhou. A microsecond is one millionth of a second.

The researchers' next step is to build a 4-bit digital-deflection micromirror array that contains 10 by 10 micromirrors. The ultimate goal is to produce a 6-bit device that contains more than 100 by 100 micromirrors, said Zhou.

Such an array could be used as a deformable mobile mirror for eliminating atmospheric blurring in astronomy and space surveillance applications, controlling the quality of communications laser beams that travel through the air, and correcting aberrations in retinal imaging, said Zhou. It could also be used as the optical element in holographic optical tweezers, he said. Optical tweezers use beams of light to trap and manipulate microscopic objects.

It will be 3 to 6 years before the device use ready for commercial use said Zhou.

Zhou's research colleagues were Logeeswaran VJ, Fook Siong Chau, and Francis E. H. Tay. The work appeared in the November 15, 2004 issue of Optics Letters. The research was funded by the National University of Singapore.

Timeline:   3-6 years
Funding:   University
TRN Categories:  MicroElectroMechanical Systems (MEMS); Displays; Optical Computing, Optoelectronics and Photonics; Telecommunications
Story Type:   News
Related Elements:  Technical paper, "Line-Addressable Digital-Deflection Programmable Micromirror Array," Optics Letters, November 15, 2004


February 23/March 2, 2005

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