Researchers suggest that putting an adequate amount of pressure on silica glass during the glass manufacturing state will reduce the attenuation of the resulting optical fibers. Researches conducted by Penn State and AGC Inc. in Japan reveal that the silica glass used for the fiber would have less signal loss if it were manufactured under high pressure.
Attenuation is the optical loss happening to the signal as it travels along the length of an optical fiber. The unit of attenuation in dB and for optical fibers, this is often expressed in terms of dB/km (decibel per kilometer). Typical attenuation loss for a single mode fiber at 1310nm is around 0.33 dB/km and that at 1550nm is 0.18 dB/km. For 50um multimode fibers, the attenuation at 850nm is around 2.5 dB/km and that at 1300nm is 0.7 dB/km.
Attenuation limits the transmission span to around 80 to 100 kilometers and the signal needs to be amplified for further transmission. Reducing the attenuation level allows the network builders to design networks with amplifiers placed at longer distances. This will reduce the number of amplifiers if the network runs hundreds of kilometers, which will result in a significant reduction in the overall project cost.
John C. Mauro, professor of materials science and engineering, Penn State said that due to attenuation, after a certain distance of transmission, the signal wouldn’t be detected properly. This phenomenon would be problematic for long haul networks, particularly for transoceanic networks. The main contributor to attenuation or signal loss is the Rayleigh scattering. Rayleigh scattering happens due to the fluctuations in the atomic structure of glass.
Optical fibers for telecommunication are made out of glass. Maruo, a researcher in AGC said that on an atomic level, glass is heterogeneous in nature and has an open porosity randomly. A breakthrough in optical communication happened when scientists could get rid of the water molecules trapped inside the glass. Water molecules (OH ions) absorb light signals and cause significant loss especially at the wavelength region of 1380 nm.
Using the modified chemical vapor deposition (MCVD), the optical fibers could be made free of water. But, like nearly all glass, optical fibers are manufactured at ambient pressure. Mauro and his team used molecular simulations to investigate the effects of pressure when making optical fibers. They reported their results in npj Computational Materials. The simulations showed that using pressure quenching of the glass, the Rayleigh scattering loss could be reduced by more than 50%.
Pressure treatment of the glass would make the material more homogeneous and decrease the microscopic holes in the structure. This would create a higher mean density material with less variability.
Mauro’s work is a molecular simulation, but Madoka Ono of AGC Inc.’s Materials Integration Laboratories, who is an associate professor in the Research Institute for Electronic Science at Hokkaido University in Japan, tested bulk pieces of silica glass and found that the results matched the simulation.
Madoka Ono said that the optimum pressure they found was 4 Gpa (4 Giga pascals) but the process challenges need to be addressed. To manufacture optical fiber under pressure, the glass would need to be formed and cooled under pressure while it is in the glass transition phase. At the glass transition phase, the glass is sticky, not solid, and not truly liquid. In order to bring the glass to this phase, we need a pressure chamber that can apply a 40,000 atmospheric pressure.