Fabrication and characterization of indium fluoride multimode fused fiber couplers for the mid-infrared

Results of the fabrication and characterization of optical fiber couplers made of multimode step-index fluoroindate (InF₃) fibers are presented. The fabrication setup was customized for this type of glass with a constant source of controlled nitrogen flow heated to a target temperature with an accuracy ±1°C. Combined with a novel fast fusion approach and with excellent control of the viscosity throughout the process, the clean gas flow and well-controlled temperature enable the fabrication of fused fiber couplers absent of any noticeable crystallization. A coupling ratio of 45/55 was achieved, with an excess loss of 0.35 dB, at 1.7 µm. To the best of our knowledge, this represents the first low excess loss (<1 dB), multimode, InF₃ fiber couplers.