Silicon-based photonics has become a very active field of study over the past few years. Of particular interest to the sensing community is that fact that silicon exhibits low intrinsic loss out to about 7 µm and that it has a strong Raman cross-section. A silicon based optical fiber could be valuable for sensing applications in the mid-wave IR either for direct light transmission or for Raman shifting near-IR light into the mid-IR as an efficient new source.

Recently, glass clad fibers possessing a highly crystalline silicon core has been produced. Perhaps most important feature is that the silicon core fiber was fabricated using conventional fiber draw techniques that would permit higher volume manufacturing.

Crystalline materials melt, rather than soften like a glass, and cannot therefore be drawn directly into fibers. In this work, one uses a glass cladding tube essentially as a crucible to confine a core material that melts at the temperature at which the cladding is soft enough to draw. The generalized approach to make the silicon core fiber was to sleeve the silicon core inside a glass cladding tube. At the temperature at which the cladding softens and draws, the silicon core is a fluent liquid. This molten core draws with the cladding and solidifies as the fiber cools.

The silicon fibers were drawn at about 1950C using an industry-grade optical fiber draw tower at Clemson University. X-ray diffraction clearly showed the core to be highly crystalline silicon as was corroborated by Raman spectroscopy. The measured transmission at both 1306 nm and 1532 nm was approximately 2.7 dB/cm. Transmission measurements at ~ 3 micrometers yielded an attenuation of 4.3 dB/m. Although this is low loss for a proof-of-concept fiber, losses are expected to be greatly reduced with the subsequent development of a thermally and mechanically matched cladding glass for the silicon core.

Source: Dr. John Ballato, Clemson University, http://www.clemson.edu/ 

Optics Express, Vol. 16 Issue 23, pp.18675-18683 (2008)