A fluoride fiber optics in-line sensor for mid-IR spectroscopy based on a side-polished structure

We report a side-polished sensor structure for mid-infrared (IR) fiber evanescent wave spectroscopy, produced by means of a mechanical polishing process. By integrating a mid-IR light source into this sensor system, spectroscopic analyses could be performed over the range from 2515 to 3735 nm. Two types of sensor structures were fabricated to examine their characteristics and performance. Spectroscopic analyses of liquid and gas samples were successfully performed using one of the fiber sensors without any further decoration/customization of the sensing portion. Qualitative/quantitative measurements were demonstrated for glycerin and diethylene glycol as liquid samples. The measurement results for CH4 gas samples with concentrations of 1% and 5% allowed the lowest detectable concentration to be estimated at 0.52%.