On crystallographic orientation in crystal core optical fibers

Abstract

Recently developed glass-clad semiconductor core optical fibers offer potential advantages over present optical fiber materials, including greatly enhanced Raman cross-sections and extended infrared transparency. While fibers have been fabricated that exhibit a high degree of crystallinity there has not been any in-depth analysis of the nature of the crystallographic orientation of the core material relative to the fiber axes. This crystallographic analysis is of important scientific and technological value since optical fiber fabrication is a highly non-equilibrium process; consequently, achieving high degrees of crystallinity is counter-intuitive. In this work, the crystallographic orientation of germanium core optical fibers was analyzed using single crystal X-ray diffraction and electron backscatter diffraction techniques. Over nearly a 100 mm length of fiber the Ge cores were found to be polycrystalline with the 〈1 0 0〉 and 〈1 1 0〉 orientations dominant implying a dendritic growth mechanism. Single crystal regions were observed routinely in lengths greater than 8 mm with the longest being about 15 mm.