Focus: A 75-kilometer-long Laser

Researchers have demonstrated the longest laser ever built, using an optical fiber that could stretch from Washington, DC, to Baltimore. In the 20 January PRL, a team shows that their 75-kilometer laser can transmit light signals with hardly any loss of power over that distance. The laser light provides a uniform source of energy throughout the fiber that prevents the signal from deteriorating. The technique could offer a huge improvement for voice and data transmissions over long distances.

When phone conversations or internet data are converted to light and travel through optical fibers, the signals typically lose five percent of their power for every kilometer they travel. Long fiber cables include amplification systems to periodically restore the signal power at many locations during its trip.

One technique uses the Raman effect: A “pump” laser at each end of a fiber segment injects light that excites some of the fiber’s atoms to higher energy. The atoms use some of that energy for vibration and then emit the rest as photons of a longer wavelength before being excited again. Ordinarily, relatively few atoms participate, but if additional light of the longer wavelength appears at the same time as the pump light–say, from a signal carrying a phone conversation–the Raman process can become much more efficient and produce extra photons to amplify the signal.

But Raman amplification is not uniform throughout the fiber. To prevent the signal from getting too weak in the center of a segment, no segment can be more than some tens of meters long. So a long-distance cable requires a lot of pump lasers.

Juan Diego Ania-Castañón and his colleagues at Aston University in England came up with a simple Raman-based scheme that almost entirely eliminates variations in the signal power along the fiber’s length. With such small variations, they could use an extremely long fiber for a single segment–75 kilometers. At each end of their fiber the team placed a mirror that reflected only light of a single wavelength, 1455 nanometers, which was longer than the pump wavelength but shorter than the signal wavelength. The pump laser light passed right through the mirrors, entered the fiber, and generated many Raman photons with a wavelength of 1455 nanometers. These photons bounced back and forth between the mirrors in a giant laser cavity and stimulated more Raman emission at the same wavelength. This 1455-nanometer laser light then acted as a source of pump light spread throughout the fiber, and it Raman-amplified the 1550-nanometer signal uniformly–the second step of a two-stage Raman amplification process.

“It’s probably the cleanest demonstration of nearly lossless transmission yet,” says David Richardson of the Optoelectronics Research Centre in Southampton, England, and the experiments “could potentially be quite significant for communications work in the future if it can be implemented in a practical fashion.” Besides providing lower noise and better performance, he says, one of these new Raman laser amplifiers might replace a number of conventional amplifiers because the technique works for such a long distance.

–Margaret Putney

Margaret Putney is a freelance science writer in Oberlin, Ohio.

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