Mid-infrared spectroscopy on a fiber tip for molecular monitoring

Mid-infrared (mid-IR) fiber sensors offer highly specific and sensitive detection and analysis of various chemical species due to many molecular vibrations and fundamental absorption bands in this range. In this paper, we present a compact transflection-based mid-IR fiber probe, designed to be potentially suitable for in vivo sensing, with an optical pathlength that can be controlled during fabrication. The optical fiber probe was fabricated using a silver halide polycrystalline fiber positioned in front of a gold-coated short fiber to act as a mirror, and a connector to hold the two parts facing each other at a predetermined distance. The optical fiber probe was tested with a quantum cascade laser (QCL) in the mid-IR region. To investigate the impact of optical pathlength, the reflected signal was recorded at various distances between the cleaved fiber and the mirror in air and water. The influence of gold layer thickness was also examined to optimize the optical fiber probe's performance. To validate its sensing capability, the probe was employed to detect glucose solutions at physiological concentrations, achieving a detection limit of 8.91 mmol/L. The results highlight the potential of the proposed optical fiber probe sensor for molecular detection and analysis, offering a promising platform for in situ monitoring for chemical and biomedical applications.