Crystalline Silicon Optical Fibers with Low Optical Loss

Polycrystalline silicon core optical fibers have been fabricated by modified thermal annealing of amorphous silicon chemically deposited at high pressure. The resulting fibers have small-diameter cores, a geometry advantageous for optical guidance. Moreover, the combination of chemical deposition and annealing avoids difficulties associated with undesired transfer of oxygen impurities to the silicon core from the molten cladding during the drawing process. The high aspect ratio of the amorphous silicon core and the presence of the silica cladding surrounding make the design rules for annealing to optimize their polycrystalline structure different from those of conventional amorphous silicon films. We find that optimization of the annealing allows for an increase in the polycrystalline grain size and decrease in the defects in the silicon core. A low optical loss of less than 1 dB/cm at a wavelength of 2.2 is thus realized, much lower than that reported for small core size (<10 ) crystalline silicon fibers and comparable to the loss in many planar semiconductor waveguides. This loss is just below the threshold of 1 dB/cm often considered necessary for many photonic and optoelectronic applications at near to mid-infrared wavelengths in areas such as nonlinear photonics, lasers, and in-fiber photodetectors. Further reduction in optical losses as deposition and annealing techniques are improved can be anticipated.