Smart Fibers

High-powered lasers, snaked through the body inside thin optical fibers, can quickly and precisely burn off tumors lining the esophagus, intestines, or bronchial tubes. But there’s a risk: if the fiber walls fail, the laser light beam can escape and harm healthy tissue.

Now MIT researcher Yoel Fink, associate professor of materials science, and MIT research scientist Mehmet Bayindir have devised optical fibers that are wired with their own heat-sensitive electronics, which can be used to monitor developing defects while the laser is in use – in time to shut it down before a failure.*

That sensitivity could prevent potentially “massive damage to healthy organs,” says Henry Du, an optical fibers researcher and head of chemical, biomedical, and materials engineering at Stevens Institute of Technology in Hoboken, NJ.

“This is one of a few examples of beautiful university research being translated into applications where an enormous difference can be made, has been made, in health care,” Du says.

“Fault-detection, especially with high power, where you have to shut it off if it fails, is absolutely a good idea,” concurs Rutgers University physicist Jim Harrington, former president of the International Society for Optical Engineering.

Optical fibers with integrated electronics could be made sensitive not just to heat, as in laser applications, but also to light, vibration, and perhaps chemicals, says Fink. Farther in the future, “smart” fibers, capable of sensing, information processing, and data storage, could be woven into fabric.

Fink’s fibers channel a high-powered laser through a hollow core lined with a high-quality mirror. To detect incipient breaks, researchers in Fink’s lab have surrounded the mirror with a semiconducting material whose electrical conductivity changes with temperature. These conductivity changes can be detected by metal wires that run the length of the fiber. When the conductivity changes abruptly, the wires signal the fault and a controller can automatically shut down the laser.

To fabricate the fibers, which are just over one millimeter thick, Fink starts with a cylindrical “preform” that has the exact geometry of the completed fiber, but is much thicker. This form is then heated and drawn out into a much longer, thinner fiber. A 30-centimeter-long preform can make a one-kilometer-long fiber.

The challenge in making the fiber, Fink says, was finding or making materials that could be melted and stretched without separating.

So far, the group has only demonstrated that the self-monitoring fiber works with mid-infrared wavelengths of light. Future applications, such as for laser-based drills for dentistry applications or lasers for curing epoxy, will need to use different wavelengths.

Other high-powered lasers, such as those used to cut and weld metal in automobile manufacturing, will require fibers that can handle much more power than the current ones, says Fink.

But the current applications are promising enough, according to Du. “I have no doubt that the fibers will be successful and will be widely adopted,” he says.

*This story was changed on Monday, November 21, 2005. Originally, this sentence read: “Now MIT researcher Yoel Fink, associate professor of materials science, has devised optical fibers that are wired with their own heat-sensitive electronics, which can be used to monitor developing defects while the laser is in use – in time to shut it down before a failure.” In fact, the work, done in Yoel Fink’s lab, was conceived and initiated by MIT research scientist Mehmet Bayindir.