At the U.S. Naval Research Laboratory, scientists have invented a new process for using nano-particles to build powerful lasers that are more effective and safer for your eyes.

The process uses “rare-earth-ion-doped fiber.” To put it plainly, it’s laser light pumping a silica fiber that has been filled with rare-earth ions of holmium. According to Jas S. Sanghera, who leads the Optical Materials and Devices Branch, they have achieved 85 percent efficiency with their new process.

“Doping just means we’re putting rare-earth ions into the core of the fiber, which is where all the action occurs,” Sanghera explained. “That is how we’ve produced this world record efficiency, and it’s what we need for a high-energy, eye-safer laser.”

According to Colin Baker, a research chemist with the Optical Materials and Devices Branch, the lasing process depends on a pump source—most often another laser—which stimulates the rare-earth ions, which then release photons to produce a high-quality light for lasing at the preferred wavelength.

“But this process has a disadvantage,” Baker said. “It’s never 100 percent efficient. What you’re putting in is pump energy, not the high-quality light at the wavelength you want. The output is a much higher quality of light at the specific wavelength that you want, but the remaining energy that isn’t converted into laser light is wasted and converted into heat.”

That loss of energy, Baker said, ultimately limits power scaling and the quality of the laser light, which makes efficiency especially important.

With the help of a nano-particle ‘dopant,’ they’re able to achieve the 85 percent level of productivity with a laser that operates at a 2 microns wavelength, which is taken an “eye-safer” wavelength, rather than the usual 1 micron. However, Baker pointed out, no laser can be said to be safe when it comes to the human eye.

The risk of scattered light to be reflected in the human eye during a laser operation is a grave danger. Dispersed light from the path of a 100-kilowatt laser operating at 1 micron can cause substantial damage to the retina, leading to blindness. With an eye-safer laser, functioned at wavelengths beyond 1.4 microns, however, the danger from scattered light is significantly reduced.

According to the researchers, the nano-particle doping is not only useful in this matter but also solves many other problems, such as that it protects the rare earth ions from the silica. At 2 microns, the glassy structure of silica can diminish the light output from the rare-earth ions. The nanoparticle doping also splits the rare earth ions from each other, which is useful as packing them tightly together can also decrease the light output.

Conventional lasers that function at 1 micron, using a ytterbium dopant, aren’t nearly as affected by these factors, said Baker.

“The solution was clever chemistry that dissolved holmium in a nano-powder of lutetia or lanthanum oxide or lanthanum fluoride to create an appropriate crystal environment for the rare earth ions,” Sanghera said. “Using bucket chemistry to manufacture this nano-powder was key in keeping the cost down.”

For a previous project, Sanghera’s team had formerly made the particles of the nanoparticle powder, which are typically less than 20 nanometers. In simple words, it means 5000 times smaller than a human hair.

Adding on, Baker said that it is essential to successfully dope these nano-powders into the silica fiber in quantities that would be right to achieve lasing.

Team of scientists under Sanghara is working with a room-sized, glass-working lathe at the Optical Materials and Devices Branch, where the glass that will eventually become the fiber is cleaned with fluorine gases, shaped with a blow torch and infused with the nano-particle mixture—what the scientists termed as a “nanoparticle slurry.” The final result is a rare-earth-ion-doped, one-inch diameter glass rod, or “optical preform.”

In the next room, scientists use a fiber pulling system–a colossal tower which occupies up two big rooms and; the height of two floors of the building–to unstiffen the preform with a kiln and elongate it, in a process alike to pulling taffy, into an optical fiber as thin as a human hair, which then curls onto a nearby large rod. Sanghera’s team has submitted a patent application for the process. Among the applications, they foresee for the new fiber laser are high-powered lasers and amplifiers for defense, telecommunications, welding, and laser-cutting.

“From a central perspective, the whole process is commercially feasible,” Sanghera said. “It’s a low-budget process to make the powder and integrate it into the fiber. This is very similar to making telecom fiber.”