Mid-infrared (IR) fibers have been extensively investigated due to their applicability in chemical sensing and remote laser delivery, among others. Materials such as chalcogenides and fluoride glasses transmit mid-IR wavelengths with low practical losses. However, their low glass transition temperatures make them chemically unstable, even at room temperatures, resulting in performance degradation over time. Semiconductors, such as germanium, have a wide transmission window in the mid-IR region, and offer significantly improved chemical stability. In this research, germanium-core, borosilicate-cladded fibers were drawn by a ‘rod in tube’ method using a mini draw tower assembled in-house at 1000°C, which is significantly lower than the drawing temperatures of 2000-2200°C for conventional silica fibers. Typical drawn fibers had a 40 μm core diameter and 177 μm cladding diameter. Transmission electron microscopy (TEM) studies showed that diffusion of oxygen and silicon from the cladding to the core during the drawing process was minimal, with diffusion distances of the order of 10s of nm. This is encouraging for mid-IR transmission, since the presence of oxygen in the fiber core is known to increase transmission losses in the mid-IR spectrum. This low diffusivity is presumably due to the relatively low drawing temperature. Transmission losses through the fibers were measured with a quantum cascade laser (QCL) and the losses were found to be in the 3-9 dB/cm range in the spectral range of 5.75-6.3 μm.
Mustafa Ordu, Jicheng Guo, Boyin Tai, Shyamsunder Erramilli, Siddharth Ramachandran, and Soumendra N. Basu, "Optical properties of semiconductor-core fibers for mid-IR transmission (Conference Presentation)," Proc. SPIE 10109, Optical Interconnects XVII, 101090Q (Presented at SPIE OPTO: January 31, 2017; Published: 28 April 2017); https://doi.org/10.1117/12.2249691.5393348530001.
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