Engineering of a Ge–Te–Se glass fibre evanescent wave spectroscopic (FEWS) mid-IR chemical sensor for the analysis of food and pharmaceutical products

Using an unclad multimode Ge–Te–Se based chalcogenide glass fibre, simple design robust fibre evanescent wave spectroscopic (FEWS) sensor is demonstrated. Methodologies adopted for material development and fibre drawing are discussed in the following steps: purification of raw materials for high spectral purity, fabrication of glass and fibre preform leading to fibre drawing. The fabricated fibre has a minimum loss of 1.4 dB/m at 4.2 μm, and less than 3 dB/m between 1.5 and 6.3 μm. The feasibility of using such a fibre for evanescent wave spectroscopic sensing has been verified by using the finite-element (FE) computation technique. Supported optical modes as well as corresponding penetration depths of evanescent fields from different modes are discussed. Based on the FE computation, a FEWS sensor consisting of a 40 cm Ge–Te–Se fibre, coupled with a Fourier transform infrared (FTIR) spectrometer and a liquid nitrogen cooled mercury–cadmium–tellurium (MCT) detector, is demonstrated. The active length along this fibre employed for sensing is 3 cm. Based on FEWS design, the fabricated fibre sensor was used for the analysis of chemicals, namely the acetone, ethanol, methanol, tocopherol (vitamin E), ascorbic acid (vitamin C), fresh orange and lemon juice.