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Light-Sensing Fibers for Transparent Cameras

Semiconducting fiber webs could transform the way we make images.

You may be about to see the world in a whole new way. MIT researchers, reporting in this month’s issue of Nature Materials, have demonstrated that nearly transparent webs made up of novel semiconducting fibers could replace lenses and sensors in cameras, and, among other things, lead to uniforms or automobile exteriors that give people a continuous view of their surroundings.

MIT materials scientist Yoel Fink poses with a sphere of light-sensitive fibers. A system using these fibers could lead to transparent cameras that need no lenses. (Courtesy of Greg Hren Photography/Research Laboratory of Electronics, MIT)

The fibers are made of a semiconducting glass core, lined along its full length by wires that act as positive and negative electrodes, and surrounded by a transparent polymer (see link to images below). When light hits the photosensitive core, an electrical current in the fiber changes, registering the hit.

[Click here for images of this light-sensing fiber.]

A mesh of these fibers can then be used to identify the location of the light on a surface. In the Nature Materials paper, the researchers, led by materials scientist Yoel Fink and physicist John Joannopoulos, demonstrate that the fibers, in addition to locating a point of light, can be used to determine the direction from which a light beam comes and can also sense light from a scene to form an image. “Here’s a structure that’s close to being invisible – but can see,” says one of the team members, Ayman Abouraddy, a research scientist at MIT.

For direction sensing, the researchers formed a grid of fibers into a sphere. A light beam from a flashlight first hits one side of the sphere and the grid registers the location. The light then passes through the sphere and out the other side, where it is detected again. Then an integrated circuit compares the entrance and exit points to calculate the path of the light.

Using a similar technique, the researchers were also able to record a scene, not just points of light. Light from a scene passes through first one, then another of two flat, parallel fiber grids, which register the intensity of light from the scene. However, because there’s no lens, which in a camera focuses light from a given plane onto a light detector, the grids receive a blurry image. To compensate for the lack of a lens, the researchers wrote algorithms that compare slight differences between the images recorded by the two fiber grids. These differences allow them to trace the light back to its source – and mathematically reconstruct an in-focus image. Because this “focusing” happens after the data has been recorded, it’s also possible to refocus on various objects in a scene after a picture has been taken.

A simple manufacturing process makes the fibers inexpensive, and thus it could be practical to cover large areas with them – even the outside of buildings. The fibers begin as a “preform,” a long cylinder about as thick as a spray-paint can that has the same structure (core, electrode, and polymer) as the finished product, only with a much larger diameter. The preform is then heated and pulled into kilometers-long fibers as thin as a tenth of a millimeter. In developing this method, the researchers had to find materials that could be drawn into thin fibers at the same temperature without coming apart. The metal electrodes actually melt during the process, but stay in place, contained by the polymer and glass.

The fiber grids are ready now for early single-point light detection applications, such as being woven into soldier’s uniforms to detect laser sightings by snipers (see “Material Alert”). They can also be tuned to detect heat, helping medics locate wounds. “There’s no more research that needs to go in for this to go into clothes,” Abouraddy says. “A lot of things you see in papers, there’s a distance between what you see in the paper and the application. With these fibers, there’s no distance, this is how they would look.” Another relatively early application could be incorporating the fibers into computer screens, allowing speakers giving presentations or video-game players to control their computers with a laser pointer.

But Abouraddy says work still needs to be done to make the scene-imaging capabilities practical. For example, the resolution of the images is limited by the need to space the fibers within the grid far enough apart that the first grid does not distort the image received by the second. The grids themselves also need to be separated, which could make the current system difficult to incorporate into some applications, such as on the skin of a car, where keeping the grids at a distance wouldn’t be practical. But the researchers say work is currently being done that could overcome these limitations.

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