Levitation technique extends utility of fiber lasers

Levitation technique extends utility of fiber lasers

R. Winn Hardin

A group of researchers has developed a method of pulling neodymium- and erbium-doped YAG composites into fiber, potentially increasing the power of fiber amplifiers at telecommunications wavelengths and extending the range of fiber lasers beyond the 2-µm cutoff of conventional silica-based fiber lasers. Paul Nordine and his associates at Containerless Research Inc. (Evanston, IL) used a conical nozzle levitation technique to float molten yttrium-aluminum-garnet (Y3Al5O12) on a bed of cooling argon gas. In place of silica (SiO2), which strongly absorbs infrared (IR) light at wavelengths beyond 2 µm, the group doped the YAG with different amounts of alumina (Al2O3), yttria (Y2O3), and erbia (Er2O3). These materials absorb significantly less IR light, while making the glass "harder" for pulling. Specific absorption coefficients are unknown, said Nordine; however, he estimates that at 7.1 µm, yttria absorbs the same amount of IR light as silica absorbs at 4 µm and sapphire at 5.1 µm.

"This could be of use for medical and other fiber laser applications. [Also] because the coefficient rises at longer wavelengths in the spectrum, there`s a potential for having less absorption . . . at telecom wavelengths," Nordine explained. Compared with silica, which has four valent bonds, the trivalent structure of neodymium, ytterbium, and erbium oxides should improve the distribution and increase the solvency of erbium and other dopants used in fiber amplifiers, he said. This could result in fiber amplifiers and lasers with higher gains in the IR.

The containerless technique has already been used to study liquid structures from the scattering of synchrotron light. Building on this experience, the group faced several problems in pulling the fiber, such as identifying the correct temperature to make the YAG-glass transition, obtaining the proper viscosity, and keeping the sample from crystallizing.

Conventional container-based ap proaches result in heterogeneous nucleation and do not allow the fragile glass to reach the proper viscosity. Nordine said the experimental oxides required a viscosity similar to that of conventional glass fiber pulls between 101.5 and 102.5 pascals per second.

Containerless melt

The containerless system uses a 250-W-output continuous-wave carbon dioxide (CO2) laser (Synrad; Mukilteo, WA) or a 500-W CO2 laser (Lumonics; Kanata, Ontario, Canada) to heat the material as it floats above the argon gas jet (see photo). Nordine said that 500 W is actually too much optical power for a production machine, but does get the job done fast, taking only a few seconds to heat the material to 2600 K. The argon gas cools the 3-mm, 50-mg sample to the proper point for pulling--between 1600 K and 1660 K--at a rate of approximately 250 K/s. To pull the fiber, a stinger thrusts into the molten sample and withdraws within a few milliseconds to avoid premature crystallization of the molten sample.

Operating at 30 Hz, a custom-built pyrometer monitors the sample temperature and provides feedback to a computer-controlled pulley that draws the fiber from the sample. Nordine and his colleagues discovered that pure YAG rarely resulted in a successful pull. By ensuring that the YAG material and erbia-doped samples contained 1 mol% of alumina, or by substituting 1 mol neodymia (Nd2O3) for yttria, the group could successfully and repeatedly pull the fibers.

A video camera with automatic iris control recorded the reaction of the sample during the experiment. "During the process, you can`t look at it, it`s too bright. A video is a way to research the behavior during the levitation. Sometimes [the fiber] can flatten somewhat from gravity if the liquid surface tension is small, or--if its viscosity is too low--it can vibrate."

Nordine said that within a 15-minute period a technician can draw several fibers at up to 0.5 m long at rates of 1 to 1.5 m/s. Although the beginning and end of the fiber do have small nodules from the pulling process, which Nordine explained is a result of the viscosity transition within the temperature window, the central portion of the fiber is of a uniform diameter between 5 and 30 µm.

"If you take say 10% of a 3-mm specimen and calculate a 10-µm fiber, you`d have an awfully long fiber," Nordine remarked. He expects that the system could be improved to draw from 10% to 50% of the 3-mm sample, but doubts that a larger sample could be used due to the levitation method. Other containerless methods do exist for larger samples, but they would require further development to accommodate the fiber-pulling process.

Part of the improvement to the system could come from temperature control during the stinger-pulling process. "Without pulling the fiber, one can maintain the liquid at 1600 K indefinitely. One only has to make arrangements in fiber pulling to maintain the thermal condition," Nordine said. "It`s the pulling method that limits the [amount] of time."

"The nice thing about the laser fiber applications is that you don`t need pounds to tons of material, you only need meters to kilometers of material," he adds.

R. WINN HARDIN is a science and technology writer based in Fairbury, NE.