CO2 laser annealing of Ge core optical fibers with different laser power

Ge core fibers fixed on a SiO2 rod were annealed by a 360° axially symmetric distribution CO2 laser of the different power, respectively. The residual stress distribution, crystalline properties, and optical loss of the Ge cores were investigated. The non-uniform distribution of Raman frequencies at the Ge core cross-section were found after the fiber was annealed, which means that CO2 laser irradiation is one of the key factors determining the uniformity of the Ge core annealed by a CO2 laser. The bonding state between the Ge core and SiO2 cladding was analyzed according to the Raman mapping. Compared with the Si core fiber, there are fewer covalence bonds between the core and the SiO2 cladding in the Ge core fiber. For the Ge core fiber annealed at relatively high CO2 laser power, the tensile stresses transformed to compressive stresses in some areas of the Ge core cross-section, and the splitting of (111) plane X-ray diffraction peak appeared in the X-ray diffraction spectrum. The optical loss measured using a quantum cascade laser with the wavelength range from 4.7 μm to 4.9 μm shows the lowest optical loss of 2.05 dB/cm has been achieved in all samples.