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Optical mouse saves space

26 Jun 2002

Researchers at Philips integrate a space-saving optical mouse into a mobile phone for the first time.

Dutch researchers at the Philips Center for Industrial Technology have developed a compact, optical interface for portable electronic equipment. The team says that its new input gadget can be easily integrated in cellphones, laptops and PDAs, providing an space-efficient alternative to a mechanical trackball or touch-pad for example.

Traditional interface solutions such as mechanical switches and touch screens are not ideal as they either consume too much space ruining the feel of the phone or are expensive. In contrast, the Philips researchers say their optical scrolling device does not suffer these disadvantages.

The device is made up of a low-power red laser diode and a detector placed beneath a lens. Light from the 650 nm laser is focused on an object, such as a fingertip, creating an external cavity. It is not necessary to touch the lens. A small portion of this light is then reflected back and mixes with the light within the internal laser cavity.

"Our scrolling device is based on this so-called laser self-mixing technique", explains Philips researcher Ren Duijvé. "Light emitted by the diode laser re-enters the cavity and affects its gain and frequency. This self-mixing effect can be used to measure the velocity or position of objects."

According to Duijvé, the design works well with surfaces ranging from human skin to paper and even black cardboard.

Duijvé and colleagues have successfully integrated a prototype device that recognises up-down movements and clicks into a cellphone. "The demonstrators were sized some 15 x 10 x 8 mm3," says Duijvé. "Within Philips, technology is available to integrate the device down to a few mm3.

A version using three lasers for up-down, left-right and clicking functions is now being prototyped. This uses a technique called laser self-mixing range finding, where Duijv explains that the self-mixing signal is used to determine the distance between the laser and the external object.

"Our research opens up new opportunites for contactless measurement of speed and distance," he told Optics.org. The team has several patents pending and is currently working with several other parties to commercialize the device.

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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