Low-Loss Fluoride Optical Fiber Coupler for Mid-Infrared Applications

For the first time, a 2 × 2 optical fiber coupler based on indium fluoride optical fibers is designed via coupled mode theory, fabricated via fused biconical tapering technique, and characterized in the mid-infrared spectral range. A conventional glass processing system, Vytran GPX-2400, is employed for the 2 × 2 optical fiber coupler fabrication, overcoming the main limitation related to the narrow temperature range for indium fluoride optical fiber processing. Moreover, the fabricated device exhibits negligible surface crystallizations. The experimental results are in optimum agreement with the simulations, demonstrating that reproducible manufacturing of low-loss fused optical fiber components for the mid-infrared spectral range is possible. In particular, the fabricated 2 × 2 optical fiber coupler is characterized by a measured coupling ratio CR = 72.2:27.8 at the wavelength λ = 3.34 µm, with low excess loss EL = 0.88 dB.