S-Tapered Fluoride Optical Fiber for Refractive Index Sensing in the Mid-Infrared

This paper illustrates the first S-tapered optical fiber obtained by employing fluoride glass, via off-axis pulling and filament heating. The S-tapered optical fiber has been designed via three-dimensional beam propagation method and its performance with respect to the surrounding refractive index has been numerically predicted. The fabrication has been conducted using a commercial glass processor, operating at low power due to the glass thermal properties. The S-tapered optical fiber transmission spectrum has been measured in the mid-infrared when immersed in different surrounding liquids, i.e., methanol, ethanol, and propanol. A red shift of the dip in the transmission spectrum was observed for increasing values of the refractive index. Compared to the conventional wavelengths, expanding the functionality of optical fiber sensors to the mid-infrared spectrum has the potential to improve sensitivity.