Recrystallization of silicon core fibers by CO2 laser annealing

Abstract

The innovation of introducing silicon in the core of optical glass fibers has lately been of great interest to the optoelectronics industry. The application areas of silicon core fibers are ranging from medical imaging and diagnosis, sensor technologies and photovoltaics. The polycrystalline structure of the silicon cores has up to date limited the optical performance of the fibers. This study has investigated the possibilities of high-throughput recrystallization of silicon core fibers by using an industrial $CO_2$ laser engraver. The high powers that were available with this machine allowed for annealing studies of fibers with $30-80$ $\mu$m core diameters in this work. There is no published studies on laser-induced recrystallization of pure silicon fibers with this core diameter range. High cooling rates were ensured by increasing the laser scanning speed, which favors single crystalline growth during annealing. Power intensities between $150-300$ W/$\mu^3$ delivered to the exposed fiber volume, and speeds between $2-5$ mm/s resulted in highly oriented silicon cores. One of the fiber cores showed single crystalline results, while for most samples only a few grains were detected. The crystal orientation of the fibers where uniform along the complete treated region. To effectively characterize the fibers after treatment, a conventional powder-in-tube X-ray diffraction instrument was used. A program code was developed to assess the degree of crystallinity of the samples after treatment. Electron backscattered diffraction characterization and computer simulations of the collected X-ray signals served as secondary sources for crystallography analysis, and provided information on the growth direction of two specific samples. A growth direction of $\langle 111 \rangle$ was found for both samples, with a possible small tilt of the crystal orientation. Through this work, it has been demonstrated that the industrial CO$_2$ laser engraver has capabilities allowing for high-throughput fiber annealing, which can serve the industry with large quantities of single-crystalline silicon fibers. The conventional and easy accessible X-ray diffraction setup that is applied for crystallinity assessment opens for the possibility of in-line structural characterization of fiber during laser post-processing.