Multimode Chalcogenide Fibers for Evanescent Wave Sensing in the Mid-IR

Evanescent wave spectroscopy in the mid-infrared (MIR) is a powerful tool for remote real-time sensing. Chalcogenide fibers transparent in MIR are considered as a base for creation of a fiber-optical platform for the MIR sensing. In this paper, a rigorous theoretical approach has been applied for the analysis of evanescent modes propagation in a multimode chalcogenide fiber surrounded by an absorbing medium. A role of particular evanescent mode in power delivering through the fiber has been revealed. Strong absorption of water in this spectral range has been shown to be a main factor limiting sensitivity of the evanescent wave sensor. Possibilities of sensitivity enhancement by using waveguiding properties of the fiber have been discussed. The analysis is supported with an experimental measurement of a [GeSe
₄
]
₉₅
I
₅
glass fiber partially immersed in an aqueous acetone solution, in the wavelength range of 2-5 μm.