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.

Scientists have known about the wavelength conversion ability of certain materials for many years. In fact, says Kumar, traditional fiber can also convert wavelengths of light. However, kilometers of common fiber are needed to produce the same effect, whereas the UCSD group needed only about 20 meters of photonic crystal fiber, he says. Kumar also notes that, although wavelength conversion has been done in photonic crystal before, it has not been accomplished “to this broad of a range” – more than 1,000 nanometers, from 1,550 to 515 nanometers. Photonic crystal fiber has been studied for only about five years, and the UCSD group is the first to show that the fibers can convert light over this range.

The researchers’ major motivation for converting wavelengths over this range, explains Radic, was to improve submarine communications, in which signals are sent both under water and through the air to aircraft. Water transmission requires blue and green light, with wavelengths between 490 and 530 nanometers, he says, because water molecules don’t absorb the energy of light at this wavelength and therefore the signal remains strong.

However, Radic says, the current device technology for these wavelengths is not as advanced as it is for the telecommunications wavelengths. His research, he hopes, might provide the basis for a so-called “universal band translator” that could convert any wavelength of light to and from 1,550 nanometers, not just to 515 nanometers. Such a device would make telecommunication technology useful for any wavelength of light.

And the implications of this research extend even further, Radic says. “When we started doing this for submarine applications,” he says, “I didn’t understand the true applications.” The team is now working with a partner to develop a surgical laser, he says, that takes advantage of the capabilities of telecom switches to turn on and off very rapidly. It will use converted green light – a color of laser that can, for instance, “deliver the most damage” to a tumor’s blood supply.

Additionally, this wavelength conversion technology could create better electronic displays. Currently, traditional television and LCD screens rely on varying intensities of red, green, and blue light that together produce a range of colors for each pixel. But with a fast, tunable laser addressing each pixel on the display, Radic says, “one could synthesize any color on the fly” when the laser is paired with a universal band translator. Instead of using a mixture of the three primary colors in each pixel, the tunable laser would supply one pure color per pixel. “In principle,” he says, “the resolution of the display could be tripled.”

It’s early research, though, and the technology needs to be developed further. In order to move the project closer to such applications, custom-designed photonic crystal fibers need to be developed that can effectively confine all wavelengths of light used in the conversion process, says McKinstrie of Lucent. The field of wavelength conversion in photonic crystal fiber is still in its early days, but the research is “an enormous step along the way,” says Boyd.

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