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MEMS micro scanner finds application in surveillance

Commercial application of a MEMS scanning device developed by The Fraunhofer-Institut fur Photonische Mikrosysteme (Fraunhofer IPMS, Dresden, Germany) has been demonstrated at the Smart Systems Integration (SSI) 2009 show. The infrared-optical room surveillance system, which was designed by CTR Carinthian Tech Research AG (Villach, Austria), uses IPMS' 1D micro scanner mirror to deflect light in such a way that it creates a fan-like light curtain over an area under observation. The resulting precisely defined surveillance area suits it to applications such as safeguarding danger zones, or automotive interior surveillance/anti-trap protection, suggests the Institute.

Commented Ines Shedwell, Fraunhofer IPMS' marketing manager: “A main advantage of the system is the highly improved signal to noise ratio: through the movement of the mirror, the intensity of the laser beam is focused on one point. Typical light barriers use a line laser and the intensity of the laser beam is distributed over the whole line.”

In this system, the light signal, back-scattered by the surrounding area, is collected and directed to an optical detector and processed. The penetration of an object into the scanning area is detected by a change within the received signal. Special filter techniques make the system insensitive to optical interference, such as changing ambient light conditions. According to Shedwell, filtering is achieved optically – the detector window has been coated with a filter adjusted to suit the IR laser diode.

The cigarette box sized demonstrator consists of a transmitter, receiver and the signal processing circuitry, and has the potential for further miniaturisation, according to the Institute.

Fraunhofer IPMS has some 40 versions of its micro scanning mirrors, which can be used for one- and two-dimensional deflection of light, as well as optical path length modulation. They consist of an actuator chip, fabricated in CMOS, and an elliptic silicon plate that serves as mirror plate and is suspended using two torsional springs. A gimbal mounting is used in the case of the 2D device. At the Micromachining/MEMS 2008 in Tokyo/Japan, the research group demonstrated a micro scanning mirror with an optical scan range of 120°. Other applications that could benefit from the tiny devices are mirror based laser projection modules for mobile phones, plus miniaturised spectrometer or scanning photon microscopes, explained Dr. Harald Schenk, Fraunhofer IPMS deputy director, at the time.

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