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  • Combs May Have Navigation and Display Applications

Photonics Spectra
Nov 2004
Hank Hogan

Researchers at Sandia National Laboratories in Albuquerque, N.M., have demonstrated a near-field optical device that could have far-reaching applications, from navigation systems to display technologies. Fabricated using standard microelectromechanical systems (MEMS) techniques, the instrument exploits the optical interaction between adjacent gratings to detect nanometer-scale movement.

Sandia researcher Dustin W. Carr explained that one objective of the project was to create a handheld accelerometer that is more sensitive than inertial electrically based MEMS devices. He noted that the Sandia prototype has a noise floor 1000 times lower than its inertial counterparts. This should translate to 1000 times the signal, an improvement that he attributes to the prototype's design and construction. "Our transducer enables this by giving a very high signal-to-noise ratio while reducing the complexity of the signal-processing electronics," Carr added.

Combs May Have Navigation and Display Applications
A newly developed device exploits the optical interaction between adjacent gratings to detect nanometer-scale movement, with potential applications in navigation systems and displays. Fabricated using MEMS processes, the upper grating layer is suspended by four springs and actuated by two side comb drives, and the lower (hidden in this scanning electron micrograph) is fixed. Courtesy of Sandia National Laboratories.

The device consists of two gratings constructed of polysilicon combs, with one suspended approximately 300 nm above the other. The comb teeth are on a center-to-center pitch of approximately 750 nm. The total dimensions of the combs are 10 × 10 µm. The lower one is held in a fixed position while the upper one sits on springs and can be moved side to side via electrostatics. The upper comb also responds to device movement.

The combs act as gratings with a near-field interaction between them. As a result, a subwavelength shift of the upper grating relative to the lower results in a large change in reflectance. When the researchers bounced a 636-nm diode laser beam off the gratings, a visible-wavelength detector easily picked up a lateral movement of a few nanometers.

Combs May Have Navigation and Display Applications
Monitoring the reflectance of the grating with a laser beam reveals fine motion.

As for applications, the group foresees uses in inertial navigation systems for vehicle control. Other applications could involve displays. As with the micromirror devices found in some large-screen flat televisions, the gratings could turn pixels on or off on command. In theory, these dual-comb devices could be mass-produced very inexpensively. That, however, depends on finding interested commercial concerns and solving issues such as packaging. Carr predicted that it might be two to four years before a truly miniaturized chip-size system will be ready.

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