Mid-infrared fused fiber optics: a fluoride coupler for 5000 nm applications

A 2 × 2 optical fiber coupler is manufactured by employing two single-mode indium fluoride optical fibers enclosed within a low-refractive-index indium fluoride capillary. The design leverages the finite element method to calculate the phase constants of the x and y polarized components of the symmetric and antisymmetric supermodes guided through the whole waveguiding region. The fabricated polarization independent coupler is characterized in the mid-infrared wavelength range up to 5000 nm, demonstrating about 3 dB coupling at this record-high wavelength. The numerical investigations show how both the degree of glass fusion and the capillary affect the coupling length and the coupler sensitivity to the polarization. In particular, for the first time, the influence of the capillary is investigated and discussed improving the understanding of the coupling phenomenon. These results pave the way for obtaining optical devices with enhanced performance in applications such as spectroscopy, medical diagnostics and therapy.