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Snake Eyes

Fiber optic cables that detect light and touch aim to take interactivity to the next level.

Researchers at the Massachusetts Institute of Technology have developed glass fibers that can detect light and touch. The initial prototype has a spherical shape, but the technology could take other forms, including being embedded in projector screens, to enable interactivity.

Researchers at the Massachusetts Institute of Technology have developed glass fibers that can detect light and touch. The initial prototype has a spherical shape, but the technology could take other forms, including being embedded in projector screens, to enable interactivity.

FIBER OPTIC CABLE transmits light, but can it detect it, too? Yes, according to a group of researchers at the Massachusetts Institute of Technology (MIT) who have developed glass fibers that can measure the direction, intensity, and phase of light. The technology eventually could be tweaked to detect sound, too.

Here's how it works: Each of the 1-millimeter photodetecting fibers looks like those found in standard fiber optic cables, except that it has a “gut” of metal electrodes and semiconductor material running the length of its inside. When light hits the glass surface, it produces an electrical signal that triggers the detection process. These electrical signals are then fed into a standard PC that processes that information to determine each light beam's direction. For example, the system could track the movement of a laser pointer as it moves across a projection screen embedded with fibers.

The fibers can detect light beams coming from any direction, so unlike conventional optical systems — such as camera arrays — they have a much larger field of view. However, a single fiber can't detect the light beam's angle of incidence. To overcome that limitation, the MIT researchers fashioned the fibers into a spherical array 30 centimeters in diameter. This design creates points of intersection that are fed into the computer in real time to identify the coordinates of each light beam.

One major benefit of fibers is that they're flexible. That's obvious from the fact that the MIT researchers fashioned them into a sphere, but it's also possible to weave them into a textile or embed them in a projection screen. In fact, the initial sphere shape was chosen because, well, the researchers liked it.

“The fibers are very flexible,” says Ayman Abouraddy, a research scientist who worked on the project with professor Yoel Fink and others in MIT's Department of Materials Science and Engineering and Research Lab of Electronics. “You could make any shape you want. We toyed around with a cube for a while, but we just thought that a sphere looked nicer.”

Shaping perception

Besides the sphere, MIT's researchers also created two flat fiber webs and put them parallel to each other. This set-up generated rough copies of objects, including a backlit capital E stencil. (One way to understand how that process works is to think of it like an X-ray, where bones and other structures are detected by the film on the other side of the patient.) By increasing the density of fibers in the mesh, it's possible to make the image sharper.

Increased density doesn't make the mesh noticeable, mainly because the fibers are so thin: about 1 millimeter in the prototype. In fact, the MIT researchers say that the mesh appears transparent. That architecture enables a wider range of applications, especially those where the web needs to be unobtrusive.

One major difference between photodetecting fibers and existing lens-based systems, such as cameras, is durability. Fiber webs don't have sensitive parts that can get knocked out of alignment, so the technology could be a good fit for applications where cameras might get bumped. The fiber optic material itself also is relatively inexpensive compared to cameras.

Sights for sore eyes

The fiber-web system isn't the first movement-sensing technology to come out of MIT in recent years. One of the better-known projects is “gesture technology,” which was featured in the 2002 movie “Minority Report.”

A gesture technology system consists of an off-the-shelf DLP projector and special gloves embedded with reflective beads. Infrared cameras are suspended from a truss above the screen and monitor the glove's reflections in order to track the wearer's hand movements. A computer collects the tracking information and adjusts the onscreen images accordingly. For example, as in the movie, the user simply waves at an image to move it, or points at it to zoom in.

Gesture technology currently is being developed for government and defense applications by Waltham, MA-based Raytheon. Its creator, MIT grad John Underkoffler, is developing the technology for other commercial applications, such as urban design. In both cases, a major advantage is scale: If a large touchscreen were used instead, the person still would have to be no more than an arm's length away. So if there are a lot of images and text that have to be displayed, the user gets information overload because it's all jammed into the person's immediate field of vision. Gesture technology avoids information overload by letting the user step back to get, literally, the big picture.

Fiber-detecting webs provide the same opportunity to step back — way back. For example, a person making a presentation on a web-embedded screen could use a laser pointer to manipulate objects, advance slides, close the presentation, and open another one. The distance from the screen depends only on how far the laser pointer can project its dot.

But fiber webs also could be used up close —to the point that they can turn a surface into a touchscreen. That's because they can sense minute amounts of heat. “As a result, they can sense the touch of a finger,” Abouraddy says.

The fibers could also be designed to sense other things besides light and heat. For example, in a July 2006 Nature Materials article describing fiber webs, Abouraddy and his colleagues wrote: “Changing the composition of the semiconducting glass will enable sensitivity to other wavelength regions in the electromagnetic spectrum and even to other physical quantities, such as acoustic waves, temperature, or chemical contaminants. Webs constructed of such fibres, using the same principles outlined above, will yield ‘images' in these parameter spaces, and can thus be said to see, hear, sense heat, and smell.”

See you soon?

Although MIT's fibers look like conventional telecom fiber, they're significantly different. Nevertheless, they can be manufactured using the same equipment and techniques. As a result, the fibers can be mass-produced, which in turn helps reduce their cost to the point that an AV vendor might be able to make a business case for embedding the technology in its products, such as projection screens.

Currently there are no firm plans to commercialize fiber-web technology, but one company that's capable of that task is OmniGuide, which Fink co-founded on the basis of his doctoral research. OmniGuide makes fibers for medical applications, such as delivering laser beams to tight places in the human body, and Abouraddy says that its commercial products are based partly on work done on fiber-web technology.

MIT's researchers claim to be the first to create photodetecting fibers. “We're the only ones in the world so far that can produce this,” Abouraddy says. “Over the years, companies have tried to incorporate these different materials but have generally failed, mainly because of the materials.”

For example, past attempts have used the same type of silica glass used in conventional fiber optic cables. “If you stick to glass, you won't be able to make an electronic device because they're insulating materials,” Abouraddy says. “We're able to put semiconductor materials in the fiber.”

MIT's researchers used a different type of glass, doped with tin to create a semiconductor, wrapped that with metal electrodes, and then encased it in polymer to form a strand. Fibers can be manufactured in lengths hundreds of meters long that then can be cut to fit whatever they're made into or embedded in. As a result, the photodetecting fibers could be embedded in screens dozens of feet across — or even larger. “I think these are the largest known photosensitive structures available,” Abouraddy says.

The next step: Figuring out how and where to use the technology in commercial applications.

“It's ready to go,” Abouraddy says. “It's just a matter of finding the killer app.”

For more information about photodetecting fibers and other technologies that enable interactivity, check out:

  • The MIT researchers' “Large-scale optical-field measurements with geometric fibre constructs” article in the July 2006 issue of Nature Materials has more details about fiber-web technology. It's available online for a fee at www.nature.com/naturematerials, or for free at www.fen.bilkent.edu.tr/~mb/docs/MB_NM2.pdf.
  • The MIT Research Laboratory of Electronics website at www.rle.mit.edu provides more information about fiber-web technology, as well as related research.
  • OmniGuide's website is at www.omni-guide.com.
  • What's Next: Nice Gesture – The next interface for information-laden displays could be at the end of your arm, July 2005
  • What's Next: Projecting A Good Image – Shadows are the bane of projector users. They're also a barrier to interactivity, August 2004

Tim Kridel is a freelance writer and analyst who covers telecom and technology. He's based in Kansas City and can be reached at tkridel@kc.rr.com.



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