Liquid Crystal Fibers Provide Optical Protection

April 01, 1997

San Francisco -- The airplane cockpit's heads-up display suddenly blazes white and the pilot, blinded by the laser flash, doesn't see that all the optical sensors are fried.

At this point in a futuristic novel, the computer or the inexperienced passenger takes over, but in the real world, a Penn State engineer has developed an optical switch that would automatically prevent overload.

"These optical fibers made from liquid crystals will allow low levels of laser light to pass through," says Dr. I.C. Khoo, professor of electrical engineering. "But once the intensity reaches a set level, the fibers automatically absorb the light," he told attendees at the Materials Research Society meeting today (April 1) in San Francisco. The liquid crystal acts as a limiting switch, because it absorbs light differently than most light absorbers.

Conventional light limiters, whether for sun glasses, windshields or other uses, only absorb very specific wavelengths -- colors -- of light and only the percentage of light they were created to absorb. A pair of yellow sun glasses manufactured to absorb 50 percent of the light, will absorb half the green light at dusk and half the green light at noon, independent of the intensity of the light. The liquid crystal fibers absorb all colors of light and react non-linearly to intensity.

"As the intensity of the light increases, the liquid crystal absorbs higher and higher percentages of the light," says Khoo. "As a result, the material actually allows very little light to pass through."

Khoo's liquid crystal optical fiber is a cable of glass with tiny holes the shape of wires running through it. These empty channels are filled with liquid crystal mixed with carbon 60 -- a fullerene. The ends of the cable are sealed. The channels of liquid crystal become the optical fiber, passing low levels of light and limiting higher levels.

This work is funded by the U.S. Department of Defense and Khoo holds a patent on this device.

"The use of lasers is becoming much more common, as is the use of equipment with optical sensors," says Khoo. "Laser light hitting the sensor mechanism of an optical telescope, for example, will burn out the sensors."

While direct application of liquid crystal fibers in glasses to protect the eyes is a possibility because the fibers are effective at as little as a tenth of an inch in length, the more useful application is directly in the optical circuit. A piece of optical fiber placed before the sensors in the telescope or in the remote viewing apparatus for a tank, submarine or aircraft, would automatically prevent sensor burnout and protect human eyes. One of the nice properties of this limiting switch is that it is completely non-electronic and has no moving parts.

"If we relied on sensors to detect high levels of laser light and send a signal to a processor to initiate a response, the light will have done its damage before any action takes place," says Khoo. "With this material, the response is immediate."


EDITORS: Dr. Khoo can be reached at (814) 863-2299 or For other Penn State news, please visit our Home Page on the Web at: Also browse this release at EurekAlert!, a comprehensive news server for up-to-date research in science, medicine, and engineering at

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