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A Smart, Small Scanner

Photonics Spectra
Jul 2007
Hank Hogan

A team of researchers from Japan has shown that even the smallest optical scanner can pack plenty of smarts, thanks to innovations in fabrication. A group comprising members from the Tokyo-based National Institute of Advanced Industrial Science and Technology, from the University of Tokyo and from Kyushu University in Fukuoka created and demonstrated an optical microscanner that combines a piezoelectric resonator, a sensor and a tuner in one device.

The scientists both shifted the resonant frequency of the device and detected that shift, although the frequency shift was small and the sensor noisy. Nonetheless, they noted that the results indicate the optical microscanner’s potential.

MicroScanner.jpg
Researchers constructed a miniature piezoelectric scanner by supporting a 1-mm2 mirror with 100-μm-wide hinges connected to a resonator. The hinges are coated with a thin film of the piezoelectric material lead zirconate titanate. Reprinted with permission of the American Institute of Physics.


Optical microscanners are of interest because they could be used to steer light in laser microscopes, display systems and bar-code readers, potentially providing an inexpensive and miniaturized solution to beam-steering needs. Devices based on piezoelectric actuators consume less power and operate at lower voltages than those using other techniques.

Piezoelectric-based microscanners consist of a freestanding mirrored surface suspended by hinges from a resonator that is coated with a piezoelectric material. When voltage is applied to the resonator, the piezoelectric effect causes the mirror to twist about the hinges.

Typically, such scanners are operated at their resonant frequency so that the mirror’s rotation angle is large. However, the resonant frequency varies with temperature changes and material fatigue. Therefore, microscanners need sensors to monitor the rotation angle and a tuner to gain precise control. Having these as separate units increases device cost and complexity.

The Japanese group combined all of these functions into one device, using a thin film of lead zirconate titanate (PZT) as a piezoelectric material. They constructed the top and bottom electrodes out of platinum and titanium. With standard lithography and microfabrication techniques, they created a 1-mm2 mirror that was suspended in space on two 100-μm-wide hinges, which they coated with PZT. The coating was a key innovation because, as the hinges moved, the piezoelectric effect generated a changing voltage, and applying a voltage altered the mechanical response of the system. Thus, the researchers had a sensor and tuner after fabricating a top electrode on the hinges.

After building the devices, they used a laser Doppler vibrometer from Graphtec Corp. of Yokohama, Japan, to measure the vibration of the microscanner. By adjusting the tuning voltage from –15 to 15 V, they shifted the resonant frequency from a high of 6472 to a low of 6458 Hz, demonstrating tunability of about 1 Hz/V. They found some hysteresis in the results, with the frequency versus tuning voltage tracing a butterfly curve.

The sensor also worked, responding to the resonant frequency shift caused by the tuner. However, the signal-to-noise ratio was low. The researchers noted that the sensor would need a noise shield to measure the scanning angle. A less noisy sensor could reduce the tuning hysteresis through feedback. The institute’s Takeshi Kobayashi reported that device improvements are now under way.

Applied Physics Letters, April 30, 2007, Vol. 90, 183514.


GLOSSARY
sensor
1. A generic term for detector. 2. A complete optical/mechanical/electronic system that contains some form of radiation detector.
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