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Thursday, November 29, 2012

Moving Toward Cheaper Lidar Sensors

An alternative [to stereoscopic sensors]—and in many ways better—approach is lidar. Light detection and ranging (by analogy with radio detection and ranging, or radar) sweeps a laser beam over the field of view. By recording how quickly reflections return from different places, a lidar knows exactly how far away they are. The result can then be used by, for example, a robot that wishes to move around in that environment.

Lidar, however, has two problems: it is bulky and it is expensive. But engineers at Vescent Photonics of Denver, Colorado, hope to change this. They have developed a lidar that they think can be shrunk to the size of a box of matches and which will, according to Scott Davis, one the firm’s co-founders, cost hundreds, rather than thousands of dollars if mass produced.

Existing lidars sweep the beam mechanically, and this mechanism takes up space. Vescent has managed to replace these moving mechanical parts with a solid-state beam-steering system. The Steerable Electro-Evanescent Optical Refractor (SEEOR), as the company dubs its invention, sends the laser beam along a glass waveguide that has a special liquid-crystal cladding. Most of the light passes through the glass but part of it, known as the evanescent wave, skims the surface of the liquid crystal. And that allows the whole beam to be manipulated.

One property of liquid crystals is that applying a voltage to them changes their refractive index—that is, the speed of light passing through them. This, in turn, causes the light’s angle of travel to shift. Altering the voltage across the liquid crystals in a SEEOR thus instantly shifts the direction of the emerging beam.

Vescent is not the first outfit to come up with the idea of using liquid crystals to manipulate a lidar beam, but previous attempts to do so have managed to move the beam only a degree or so. SEEOR can scan a beam horizontally over an angle of 60°, and vertically over 20°, thanks to the use of a special liquid crystal, whose refractive index is particularly sensitive to changes in voltage, and also by the perfection of the evanescent-wave approach (previous prototypes sent the whole beam through the liquid crystal, which is less efficient).

Dr Davis thinks that is just the start. He is confident the next version of SEEOR will be able to sweep 120°, both vertically and horizontally. Its range will increase, too, from a few hundred metres to several kilometres. And its size, currently that of a paperback book, will shrink to that of a pack of cards – and ultimately to that of a matchbox.

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