A Safer, More Efficient Fiber Laser from Doping with Nanoparticles

Scientists at the U.S. Naval Research Laboratory (NRL) are using nanoparticles to build fiber lasers that are safer for the eyes and more efficient. The core of the laser’s silica fiber is doped with rare earth ions of holmium. With the aid of this dopant, the researchers were able to achieve an 85% level of efficiency with a laser operating at a wavelength of 2 µm, which is considered safer for the eyes than a traditional 1 µm wavelength.

Jas Sanghera, U.S. Naval Research Laboratory branch head for Optical Materials and Devices, holds up an optical fiber that will be used to produce eye-safer lasers at U.S. NRL. Courtesy of Jonathan Steffen/U.S. Naval Research Laboratory.

The particles of the nanopowder dopant are typically less than 20 nm. “The solution was some very clever chemistry that dissolved holmium in a nanopowder of lutetia or lanthanum oxide or lanthanum fluoride to create a suitable crystal environment [for the rare earth ions],” said Jas S. Sanghera, who heads the Optical Materials and Devices Branch. The researchers had to be able to successfully dope these nanopowders into the silica fiber in quantities that would be suitable to achieve lasing.

Using a room-size, glass-working lathe, the scientists cleaned the glass that would become the fiber with fluorine gases, molded it with a blow torch, and infused it with the nanoparticle slurry. The result was a rare-earth-ion-doped, 1-in. diameter glass rod, called an optical preform, which was then softened with a furnace and elongated to make an optical fiber about as thin as a human hair.

Colin Baker, U.S. Naval Research Laboratory, holds a silica glass rod (optical preform) that will be pulled into an optical fiber suitable for production of an eye-safer laser. Courtesy of Jonathan Steffen/U.S. Naval Research Laboratory.

According to research chemist Colin Baker, the nanoparticle doping not only improves eye safety, but also shields the rare earth ions from the silica. In addition, the doping separates the rare earth ions from each other, which is helpful since packing the ions closely together can reduce the light output.

Scattered light from the path of a 100-kw laser operating at 1 µm can cause significant damage to the retina, leading to blindness. With a laser that is operated at wavelengths beyond 1.4 µm, like this laser doped with nanoparticles, the danger from scattered light is considerably lessened.

Sanghera’s team has already submitted a patent application for the process. Among the potential applications the team envisions for the new specialty fiber laser are high-powered lasers and amplifiers for defense, telecommunications, and even welding and laser-cutting.

“From a fundamental perspective, the whole process is commercially viable,” Sanghera said. “It’s a low-cost process to make the powder and incorporate it into the fiber. The process is very similar to making telecom fiber.”