Mid-infrared transmission through germanium-core borosilicate glass-clad semiconductor fibers

Semiconductors are promising candidates as fiber materials for transmission in the mid-infrared (IR) region. This paper reports the lowest measured losses for a Ge-core, borosilicate cladded optical fiber produced by the ‘rod-in-tube’ method that is scalable for manufacturability. The fibers were drawn in a mini draw tower at 1000°C. The relatively low drawing temperature and the optimal location of the Ge core in the preform minimize the time the molten Ge interacted with the borosilicate cladding limited diffusion from the cladding to the core. The coefficient of thermal expansion (CTE) match between the core and cladding led to fibers that were crack- and pore- free. Transmission electron microscopy studies showed the Ge core to have good crystalline quality with low impurity content. The optical properties of the fibers were characterized in the mid-IR region with a quantum cascade laser and transmission losses were found to be in the 3.1-9.1 dB/cm range in the spectral range of 5.82-6.28 μm, with an average loss value of 5.1 dB/cm.