Most lasers have operated along similar physical principles: a small, precisely dimensioned, closed cavity containing specific materials that will get incoming photons excited enough to band together and form a coherent laser beam. However, this isn't the only way to get a material to lase, as it's possible to get the same effect by taking advantage of the scattering of light. Researchers have now demonstrated a scattering-based laser that relies on light pumped into a 50 mile long fiber optic cable.

A laser needs two things to work: a cavity that traps the light, usually created by two mirrors, and a material that is able to amplify the radiation while it's trapped. This amplification is usually caused by gas particles that are excited by an electric current running through the cavity. The excited particles will catch the photons, become even more excited, and emit additional photons of the same wavelength. This process can take place over and over until there are enough photons to populate a laser beam.

The new work involved making a laser that used fiber as a guide for scattered photons. The long, straight fibers help direct the photons, generally keeping them on a straight course. Light traveling through the fiber can accumulate a certain amount of disorder because of small imperfections in the medium. That disorder turns out to be critical to the operation of the laser.

The fiber laser depends on two photon scattering properties. One is Rayleigh scattering, which occurs when light hits fetures that are smaller than the wavelength of the light itself, usually individual molecules or atoms. In this case, the photon bounces away, leaving with the same amount of energy it came with.

The second is Raman scattering, where particles bounce off a feature and leave with a different amount of energy than they had prior to the collision. Raman scattering happens significantly less often than Rayleigh but, given enough space and time, Raman effects can accumulate.

In the new laser, two pumps direct photons into the open cavity formed by a fiber. As photons travel along the length of the fibers, they hit imperfections that cause them to scatter. Some of the Rayleigh-scattered photons will scatter backwards, much like the photons that strike a mirror in a closed cavity laser. Others will be Raman scattered, which not only causes them to reverse directions, but also ramps up their energy—this is the equivalent of the excited atoms in a closed cavity laser.

While, individually, these effects are weak at best, the researchers found that, if enough photons are given enough space for these scattering events to happen many times, there are eventually enough excited photons to create a laser beam.

The scattering has to be allowed to happen over a significant distance—the fiber laser tested was over 51 miles long, and scientists estimate that the fiber tunnel needs to be at least 43 miles in order for lasing to happen at all. In some ways, the fiber lasers are cheaper, as they require no mirrors or glass, and use easily acquired materials that need less calibration. Of course, they aren't about to replace the pocket-sized one you use to play with your cat.

Still, it might be possible to find a use for lasers of this sort, given the huge lengths of fiber optic cable that are already out there. The authors also suggest that it could be immensely useful for scientists to take a second look at other already well-known physical anomalies—their new applications could be only 43 miles away.

Nature, 2010. DOI: 10.1038/NPHOTON.2010.4