Ursula Gibson, recently elected 2017 OSA Vice President, holds a sample of optical fiber in her office at the Norwegian University of Science and Technology. [Image: Nancy Bazilchuk, Norwegian University of Science and Technology]
Over the past decade, scientists have investigated semiconductor-core optical fibers for their many possible uses, from sensors to endoscopes. Now, a research team in Norway has devised a new laser-based method for manufacturing glass fibers with cores of two semiconductors—silicon and germanium—with different melting points (Nature Comms., doi:10.1038/ncomms13265).
Getting to a consistent core
Core blends of silicon and germanium have many interesting optoelectronic properties. However, during the fiber-drawing process, germanium (melting point 938 °C) tends to “freeze” in place before silicon solidifies at 1414 °C, resulting in highly inconsistent mixtures of the materials. Ursula Gibson, a physics professor at the Norwegian University of Science and Technology and the incoming OSA vice president, figured out with her colleagues that reheating the fiber after the drawing process results in homogeneous core composition.
The scientists started with a silicon dioxide preform—the basic building block of virtually all optical fibers—that surrounded a silicon rod and germanium shot. “In our method, the semiconductor core melts completely, mixing the silicon and germanium, then the glass is drawn down,” Gibson says. “The core solidifies after the diameter is reduced by the encapsulating glass.”
Thermal Progress
If the process stopped there, the team would end up with dendritic structures and other inhomogeneities in the fiber’s core. The scientists, however, subjected the fiber to recrystallization by moving it through the beam of a carbon-dioxide laser, operating at a wavelength of 10.6 μm and focused into a spot size of 166 μm. The silica cladding absorbs the infrared beam only to a depth of about 20 μm, so thermal conduction from the surface melts the core.
The team experimented with various core diameters, core compositions and fiber velocities to determine critical parameters—since the smaller the core, the higher the critical velocity needed to achieve recrystallization. Fine control over these parameters could allow the building of useful microstructures, such as Bragg gratings, within the fibers. The scientists also tested the semiconductor-core fiber’s transmission abilities with signals at 1500 and 2000 nm.
According to Gibson, team members will further study the interface between the silica and the semiconductor. They also want to explore how recrystallization works in two dimensions by applying their technique to encapsulated thin films of silicon and germanium.
Researchers at other institutions in Norway, Sweden, England and the United States also participated in this study.
