Nanoparticles Used to Boost Power and Improve Eye Safety of Fiber Lasers

Researchers at the U.S. Naval Research Laboratory have formulated a new process for using nanoparticles to develop robust lasers that are more efficient and safer for a person’s eyes.

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 at U.S. NRL, DC on March 26, 2019. (U.S. Navy photo by Jonathan Steffen)

They are realizing it with what is known as “rare-earth-ion-doped fiber.” In other words, it is laser light pumping a silica fiber that has been infused with rare earth ions of holmium. According to Jas S. Sanghera, who heads the Optical Materials and Devices Branch, they have accomplished 85% 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 happens. That’s how we’ve produced this world record efficiency, and it’s what we need for a high-energy, eye-safer laser.

Jas S. Sanghera, Optical Materials and Devices Branch, U.S. Naval Research Laboratory

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

But this process has a penalty. It’s never 100 percent efficient. What you’re putting in is pump energy, not the high quality light at the wavelength you want. What’s coming out 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.

Colin Baker, Research Chemist, Optical Materials and Devices Branch, U.S. Naval Research Laboratory

That loss of energy, Baker said, eventually confines power scaling and the quality of the laser light, which makes efficiency particularly vital.

With the help of a nanoparticle “dopant,” they are able to attain the 85% level of efficiency with a laser that works at a 2 µm wavelength, which is said to be an “eye-safer” wavelength, instead of the traditional 1 µm. Obviously, Baker emphasized, no laser can be considered totally safe with regards to the human eye.

The risk comes from the potential of scattered light to be reflected into the eye during the operation of a laser. Scattered light from the path of a 100-kilowatt laser working at 1 µm can cause substantial damage to the retina, resulting in blindness. With an eye-safer laser, worked at wavelengths beyond 1.4 µm, however, the danger from scattered light is significantly diminished.

According to Baker, the nanoparticle doping also resolves some other issues, such as that it protects the rare earth ions from the silica. At 2 µm, the glassy structure of silica can decrease the light output from the rare earth ions. The nanoparticle doping also keeps the rare earth ions apart from each other, which is useful since packing them closely together can also decrease the light output.

(Traditional lasers that work at 1 µm, using an ytterbium dopant, are not closely as impacted by these aspects, Baker said.)

The solution was some very clever chemistry that dissolved holmium in a nano-powder of lutetia or lanthanum oxide or lanthanum fluoride to create a suitable crystal environment [for the rare earth ions]. Using bucket chemistry to synthesize this nano-powder was key in keeping the cost down.

Jas S. Sanghera, Optical Materials and Devices Branch, U.S. Naval Research Laboratory

The particles of the nanoparticle powder, which Sanghera’s team had initially synthesized for an earlier project, are usually less than 20 nm, which is 5,000 times tinier than a human hair.

“Additionally, we had to be able to successfully dope these nano-powders into the silica fiber in quantities that would be suitable to achieve lasing,” he continued.

At the Optical Materials and Devices Branch, Sanghera’s team of researchers are dealing with a room-sized, glass-working lathe, where the glass that will ultimately become the fiber is cleaned with fluorine gases, molded with a blow torch, and infused with the nanoparticle mixture—what the researchers term a “nanoparticle slurry.” The outcome is a rare-earth-ion-doped, one-inch diameter, glass rod, or “optical preform.”

Next door, researchers use a fiber pulling system—a tower so huge that it occupies two large rooms and; the height of two floors of the building—to soften the preform with a furnace and stretch it, in a process similar to pulling taffy, into an optical fiber about as thin as a human hair, which then spools onto an adjacent large spindle.

Sanghera’s team has filed a patent application for the process. Among the possible applications, they visualize for the new specialty fiber laser are high-powered amplifiers and lasers for telecommunications, defense, and even laser-cutting and welding.

“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.”


Original article written by Emanuel Cavallaro.

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