Converting Light Wavelengths within Fiber

A novel technique could expand the reach of telecommunications technology.

Fiber-optic networks zip billions of bits of information across the world every day, using light with a wavelength of 1,550 nanometers, which is well suited for snaking through kilometers of glassy fiber. And, because telecommunications uses this wavelength, many important devices, such as light sources, amplifiers, switches, and light detectors, are fine-tuned for that wavelength.

Other applications, though, such as sea and air communications, biomedical lasers, and electronic displays, which operate using different wavelengths, could benefit from advanced telecommunication devices as well. Now researchers have found a way to use fiber to convert wavelengths of light so that the existing, well-developed telecommunications technology can be used for other purposes.

[Click here to view images of the wavelength conversion technique.]

"For the last 20 years, an enormous amount of time and money has been spent developing technology at the telecommunication wavelengths," says Colin McKinstrie of Lucent, a scientist involved in the research. "People would like to be able to generate, transmit, and detect electromagnetic radiation at different wavelengths. This experiment is exciting because it shows that you can convert radiation efficiently between widely different wavelengths."

The research group, led by Stojan Radic, a professor of electrical engineering at the University of California in San Diego (UCSD), showed that wavelengths of light between 1,541 and 1,560 nanometers could be used to generate visible green light with wavelengths between 515 and 585 nanometers -- all within the confines of an optical fiber. Their results were presented last month at the Optical Fiber Communications Conference in Anaheim, CA.

Traditionally, wavelength conversion occurs outside the optical fiber, in electronic devices called modulators, explains Robert Boyd, professor of optics at the University of Rochester. By keeping the signal within a fiber, however, wavelength conversion can be more reliable, faster, and ultimately cheaper, says Boyd (who was not involved with the research).

For the experiment, the researchers converted light within photonic crystal fiber, a bundle of glass tubes with a diameter of a couple of micrometers. Radic explains that the conversion relied on mixing two different wavelengths of laser light in the fiber: one beam with a wavelength of about 1,550 nanometers and another beam of 800 nanometers.

When the beams mix in the small confines of a photonic crystal fiber, they produce incredibly intense light, explains Prem Kumar, professor of electrical engineering at Northwestern University (who was also not associated with the research). This high intensity in tiny fiber cavities -- hundreds of kilowatts per square centimeter -- forces the light waves to interact with each other and with the fiber in counter-intuitive ways, he says. When the 1,550 nanometer beam mixes with the 800 nanometer beam, the outcome is an amplified 1,550 nanometer beam and an entirely new beam with a wavelength of about 515 nanometers.