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VCSEL shrinks position sensor

24 Sep 2008

Combining a VCSEL and a photodetector array creates a compact and highly accurate position sensor.

US researchers have unveiled a clever way to measure position, which combines an integrated vertical-cavity surface-emitting laser (VCSEL) and photodetector array. According to the group, its high-precision system requires no complex signal processing and the compact design opens up new potential applications (Applied Optics 47 4555).

"Our technology allows the incorporation of compact, non-interfering, and non-contact sensors into spaces that other accurate position sensors cannot fit," Antonios Giannopoulos, a researcher at the University of Illinois, told optics.org. "The VCSEL and detector chips are less than 1 mm2 in area and can be thinned down to a few hundred micrometers."

While VCSELs and photodectors have already found use in optical encoders and laser mice, such systems do not offer very high resolution and require complicated signal processing to measure position. The Illinois team, however, proposes a straightforward technique that provides a resolution of 850 nm, which is on the order of the wavelength of light emitted by the VCSEL.

The sensor is composed of two parts: the integrated VCSEL and detector component, and a periodic reflection grating, which is attached to the surface to be measured. Light emitted from the VCSEL is reflected off of the grating and measured by the photodectector. As the grating moves, the intensity of the reflected light varies and the change in position can be deduced.

"Our sensors measure the relative translation of two parallel surfaces and in theory, the sensor can offer an infinitely long range of motion," explained Giannopoulos. "Any mechanical system that requires high precision measurement of parts moving parallel to one another would benefit from this design."

In the setup, the VCSEL emits at 850 nm and has a maximum output power of 6.7 mW. Each VCSEL measures 25 µm in diameter and the detectors range from 100 µm2 to 10,000  µm2 in area.

The next steps for the team will be to focus on experimental verification of its predictions as well as gaining an understanding of which VCSEL and detector geometries are optimal for position measurement applications.

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