And what is glass made of?
According to the Optical Society of America, U.S. researchers have been able to create a practical optical fiber with a silicon core. As they were able to use the same commercial methods that are used to develop all-glass fibers, this might pave the way for future silicon fibers as viable alternatives to glass fibers. The scientists note that this should help increase efficiency and decrease power consumption in computers and other systems that integrate photonic and electronic devices. Here is a good summary by the lead researcher: ‘In the past, we’ve needed one structure to process light and another to carry it. With a silicon fiber, for the first time, we have the ability to greatly enhance the functionality in one fiber.’ But read more…
The figure above provides “an electron microscopy image of the cross-section of the Si core, silica-clad optical fiber. The core exhibits a degree of ovality, which possibly from a direct and indirect few sources. Firstly, from inspection of the polishing marks the electron micrograph was taken at a slight tilt which facilitates the appearance of added ovality. Secondly, the ovality could result from slight gaps between the three concentric silica tubes that comprised the cladding. As the preform consolidates down during the draw the tubes can adhere to one slide of its neighboring tube causing a non-uniform reduction. This can be corrected in future draws through tighter tolerances.” (Credit: Ballato et al.)
This research work has been led by John Ballato, an associate professor of materials science and engineering and director of the Center for Optical Materials Science and Engineering Technologies (COMSET) at Clemson University, South Carolina. Ballato worked with fiber pioneer Roger Stolen, who was elected to the Russian Academy of Sciences in July 2008, and a dozen of other researchers from various U.S. institutions.
Here are some additional details about these silicon optical fibers. “Usually an optical fiber is made by starting with a glass core, wrapping it with a cladding made from a slightly different glass, and then heating the structure until it can be pulled out into long wires. This works well enough, but for some wavelengths of light, a core made of pure crystalline silicon, like the one developed by the Clemson team, would better carry signals. Additionally, crystalline silicon exhibits certain nonlinear properties (in which the output is not proportional to the input) that are many orders of magnitude larger than for conventional silica glass. This would, for example, allow for the amplification of a light signal or for the shifting of light from one wavelength to another.”
For more information, this research work has been published in the Optical Society of America (OSA)’s open-access journal, Optics Express, under the — austere — title “Silicon optical Fiber” (Volume 16, Issue 23, Pages 18675-18683, November 10, 2008).
Here is the beginning of the abstract. “Described herein are initial experimental details and properties of a silicon core, silica glass-clad optical fiber fabricated using conventional optical fiber draw methods. Such semiconductor core fibers have potential to greatly influence the fields of nonlinear fiber optics, infrared and THz power delivery. More specifically, x-ray diffraction and Raman spectroscopy showed the core to be highly crystalline silicon. The measured propagation losses were 4.3 dB/m at 2.936 µm, which likely are caused by either microcracks in the core arising from the large thermal expansion mismatch with the cladding or to SiO2 precipitates formed from oxygen dissolved in the silicon melt. Suggestions for enhancing the performance of these semiconductor core fibers are provided.”
The full paper is available from the abstract and the above image has been extracted from it. The researchers mention several possible developments and optimizations. Here is a couple of them. “Continuing efforts should develop a more complete understanding on methods to lessen stresses and the uptake of oxygen by the core. The longer interaction lengths enabled by fiberbased geometry may off-set any diminution in nonlinear coefficients brought about by the presence of the oxygen (i.e, the Raman cross-section for Si is ~ 104 times higher than for SiO2). Another area for further attention is reducing the diameter in order to yield fibers of greater flexibility. This will have to be balanced by the effects of diffusion on the optical properties. Better matching of the draw temperature of the cladding to the silicon (or other semiconducting core material) melting point as well as minimizing the tensions due to thermal expansion mismatch should help in this regard.”
Finally, here is an excerpt from the conclusions of the article. “To the best of our knowledge, the first silicon core optical fiber has been fabricated using high speed, high volume fiber draw techniques. X-ray diffraction and Raman analysis indicated that the core was highly crystalline silicon. [...] Optical fibers possessing a silicon core have tremendous potential for Raman and other nonlinear optical fiber devices, mid- and long-wave infrared sensing and power delivery, and terahertz guided wave structures.”
Sources: Optical Society of America news release, October 28, 2008; and various websites
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