Photonics Spectra - September 2009

Multitasking fibers weave a new
story for imaging systems

A group at MIT has developed a multimaterial optical fiber that can detect the components of any light that strikes it. Woven together, the fibers make a flexible, lensless camera. Courtesy of Fabien Sorin, MIT.

CAMBRIDGE, Mass. – Imagine walking to work one day and, for no obvious reason, you get a feeling that everyone is looking at you. You ignore the feeling for a while, but it persists, despite the fact that none of the people bustling around nearby are casting a single glance at you and you’ve gotten used to the growing number of surveillance cameras filling every city block. Now you feel silly, perhaps even a little paranoid. But you might not be wrong – a new imaging technology has begun to weave itself into the fabric of everyday life.

In the lab of professor Yoel Fink of the materials science and engineering department at MIT reside swaths of woven strands of optical fibers. Not off-the-roll fibers ordered from a catalog, but custom lines drawn from a preform crafted by Fink’s group. Interwoven like a patch of cloth, the fibers combine to form a lensless, flexible camera.

The researchers made the fiber using polyethersulfone as the base material and alternating layers of semiconducting As40Se60 or As40Se54Te6. Contacts made of tin were attached to the semiconductor rings. After deposition, the layers were rolled together onto a tube. After making a series of these tubes, the engineers stacked them, joined the ends by heating them and drew them out into their final fiber diameter (see figure). The preforms were about 3 cm in diameter, while the processed fibers ranged from 100 μm to 1 mm in diameter.

When an external electric field is applied to the contacts, which act as electrodes, the semiconductor layers become responsive to light via the photocurrent effect. A single layer of the material can discriminate the incoming light’s angle of incidence; a second layer distinguishes wavelength. A third layer, in theory, would add RGB information to the mix.

According to Fabien Sorin, a member of Fink’s group representing MIT’s electronics lab, the semiconductor bandgap can be adjusted so that wavelengths from the UV to the visible to the IR can be detected.

The investigators tested the fiber’s ability to form an imaging system by arranging them into a 32 ; 32 grid, with about 1 cm between each pair of strands. Powered up, the optical fiber “fabric” patch could image an object with features as small as 100 nm.

“The size [of the grid] was limited for convenience of use in the lab,” Sorin said, “but could be made much larger, with smaller spacing between fibers.” That would significantly improve image resolution.

Fink’s group sees strong potential for this technology – which its members have dubbed “multimaterial fibers” – in such applications as large-area medical imaging, remote sensing, industrial control and intelligent fabrics.

It may even show up as watchful sew-on patches on clothes and baseball caps, giving their wearers an extra eye with which to watch the world and you something to be wary of on your walk to work.