Peculiarities of the mid-infrared evanescent wave spectroscopy based on multimode chalcogenide fibers

Output characteristics of the evanescent wave sensor have been studied in experiment by measurement of the transmittance of a chalcogenide fiber submerged into an aqueous solution of acetone in the range of wavelengths 5-9 μm. The minimum concentration of 1 mol% of acetone has been detected at the wavelengths of 7.33 μm and 8.18 μm. By using a theoretical approach based on the electromagnetic theory of optical fibers, we have revealed that dependence of the transmittance logarithm versus the length of the fiber immersed into the solution is not linear because of the greater attenuation of the evanescent modes having greater radial orders. As calibration curves of the higher-order modes are steeper they are more suitable for the sensitive detection of chemicals. Water absorption as a factor limiting the sensitivity of spectroscopic measurements in mid-infrared has been discussed.