The mid-IR spectroscopy provides a universal tool for the real-time remote monitoring of various molecular species. In this study, we apply the wave optics based approach to analyze the functionality of spectroscopic sensors with sensing elements based on tapered multimode chalcogenide fibers. By using the method of local modes, the sensing elements transmittance, sensitivity and detection limit have been calculated. These characteristics are shown to depend on the longitudinal profile of the taper diameter, linear or exponential in our case. The ratio of the taper length to the power attenuation length of a local mode is a key parameter for sensitivity optimization. The detection limit of the spectroscopic measurements can be significantly reduced by delivering radiation in the higher-order local modes of the taper.
Fiber-based evanescent wave spectroscopy in the mid-IR is a powerful tool for the remote chemical analysis of liquids and gases in real time. Design of a sensing element of the fiber sensor is important for optimization of its output characteristics. In addition to unclad chalcogenide fibers that were previously used as the sensing elements, we consider core-clad fibers consisting of a multimode core and a ring cladding with the refractive index greater than that one of the core. For numerical analysis, a theoretical approach based on electromagnetic theory of optical fibers has been used. Calculated transmittance of the sensing elements is compared with the measured output characteristics of a sensing element made of an unclad chalcogenide fiber, which was immersed into aqueous acetone solutions.
Chalcogenide fibers are considered as a base for creation of a fiber-optical platform for the mid-IR evanescent wave spectroscopy. In this work, transmittance of a multimode fiber made of Ge26As17Se25Te32 glass, immersed into an aqueous acetone solution was measured in the range of wavelengths 5 - 9 microns at various concentrations of the solution. A theoretical approach based on electromagnetic theory of optical fibers has been applied for analysis of evanescent modes propagation in the fiber. Attenuation coefficients calculated for each HE1m evanescent mode increase with the mode radial order m. This effect can be used for optimisation of the fiber-optic sensing elements for the mid-IR spectroscopy.
An important problem of investigation of the air and water contamination by the mid-IR spectroscopy is discussed. A model of evanescent wave sensor made of a multimode waveguide transparent in the mid-IR spectral range is developed. Transmittance and sensitivity of a sensing element consisting of an input chalcogenide waveguide and a sensing waveguide depend on distribution of the guided modes amplitudes in the sensing waveguide. We have demonstrated that excitation of higher-order modes is important for optimal performance of such a sensor.
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