Fabrication and characterization of semiconductor core optical fibers for mid-infrared transmission
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Transmission in the mid-infrared (2-15 µm) spectrum has many applications, including biomedical surgery, chemical detection, and countermeasures in defense systems, among others. This highlights the necessity of a suitable fiber for the mid-infrared (mid-IR) spectral range. Although fluoride and chalcogenide glasses have shown promise of relatively low transmission losses, they are prone to devitrification at room temperature leading to performance degradation. Semiconductors, such as germanium and silicon have low theoretical losses in the mid-IR spectral range, and are stable at room temperature, making semiconductor-core fibers worthy of exploration for mid-IR transmission. In this study, germanium (Ge)-core, borosilicate glass-cladded; silicon (Si)-core, silica-cladded; and Si-Ge alloy-core silica-cladded fibers were drawn in laboratory-made mini draw towers using the rod-in-tube method at a relatively low temperature of 1000◦C for borosilicate glass drawing and 1760◦C for silica drawing. 3 mm outer diameter core-drilled germanium and silicon rods were placed in borosilicate and silica glass tubes as preforms for the germanium-core and silicon-core fibers, respectively. 1.9 mm outer diameter core drilled germanium rods and 2 mm inner diameter, 3 mm outer diameter core drilled silicon tubes were placed concentrically in silica tubes as preforms of silicon-germanium alloy fibers. The core/cladding area ratio was controlled by adding concentric borosilicate/silica tubes to increase the preform diameter. Depending on the drawing speed and initial core/cladding diameter ratio, fibers with core diameters of 10-200 µm with cladding diameters of 130-500 µm, as well as canes with core diameters of 300-350 µm and cladding diameters of 1.3-1.4 mm were drawn. The drawn fibers were characterized by scanning/transmission electron microscopy (S/TEM), energy dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD) and electron backscatter diffraction (EBSD). It was found that there was minimal diffusion of oxygen and silicon from the cladding to the core in the Ge-core fibers. In the Si-core and Si-Ge alloy-core fibers, around 3 at % oxygen were found in the core, presumably due to enhanced diffusion at the higher drawing temperature of the silica-clad fibers. Optical characterization of the Ge canes, carried out using Fourier transform infrared spectroscopy (FTIR) in the 1.3-16 µm wavelength range, showed similar transmission characteristics, albeit with increased losses, over the entire wavelength range as the core drilled unprocessed germanium rod, even though the germanium core had undergone melting and re-solidification during the fabrication process. The transmission losses in the fibers were measured using two quantum cascade lasers, and were found to average 5.1 dB/cm for Ge-core fibers and 18.3 dB/cm for Si-core fibers in the 5.8-6.2 µm range. Transmission loss of Si-Ge alloy fibers was found to be 75 dB/cm at 6.1 µm. The higher losses of Si-Ge fibers can be attributed to compositional fluctuation in the core, due to the rapid cooling rate during fiber drawing. High temperature annealing of the fibers following by slow cooling homogenized the fiber core, and reduced the transmission losses to 28 dB/cm, but also introduced cracks. Non-linear properties of Ge-core fibers and canes were investigated using femtosecond pump-probe spectroscopy. Unprocessed 3 mm diameter rods exhibited the same detuning oscillations as 770 and 358 µm Ge-core canes and a 132 µm Ge-core fiber, indicating that the non-liner properties of the semiconductor cores were preserved during processing.