All — Fiber Evanescent Wave Sensors for the Mid-Infrared Spectroscopy of Liquids

Abstract

Methods of the mid-IR spectroscopy provide reliable tools for detecting chemical composition of gases and liquids. In the mid-IR spectral range (wavelengths of 3–25 μώ), various molecules and functional groups have fundamental absorption bands with large absorption coefficients. Creation of materials and technologies for the remote sensors consisting of fiber-optic broadband sources of coherent mid-IR radiation, chalcogenide fibers for transmitting light to the sensing elements and spectrally-selective photodetectors is a problem of a high scientific priority. In this work, we develop the fiber-optic analytical spectroscopy for chemical analysis of aqueous solutions of acetone, ethanol, for determination of the fractional chemical composition of hydrocarbons. As a material base for the fiber-optic sensors, chalcogenide fibers with an extremely low content of impurities have been designed and fabricated by single- and double-crucible methods (Fig.1a) (including the REE-doped glass fibers) [1,2]. By using these fibers, technologies of fabrication of permanent fiber bends, tapers, microstructured fibers have been developed for using these structures in fiber lasers, supercontinuum generators, sensing elements [3,4]. A theoretical approach based on electromagnetic theory of optical fibers has been applied for computer-aided design of the evanescent-wave sensors [4]. A sensing element shaped as the U-bent fiber that was applied for chemical analysis of liquid consisting of a diesel oil and a fuel additive is shown in (Fig.1b). For chemical analysis, the absorption band at the wavelength of 7.83 μm, corresponding to esters, has been chosen (shown by an arrow. Output characteristics of the sensor were studied in experiment by variation of the fiber core diameter and the bend radius. In a computer model, it was shown that attenuation coefficients of evanescent modes propagating in the fiber bend grow with increase of the bend radius and with decrease of the fiber core diameter. This allows for enhancing sensitivity and decreasing the minimum detectable amount of an analyte. In fact, with an optimised sensing element, the minimum detected amount of the fuel additive was less than 0.1 vol.%.