An Analytic and Experimental Treatment of Fiber Optic Chemical Sensing: Results on Evanescent Wave Spectrometry

Standoff chemical sensing of chemicals and toxic compounds is very desirable for actively monitoring residential, commercial, and industrial spaces. Analytes such as toxic industrial compounds (TICs), chemical warfare agents (CWAs), and environmental pollutants (EPs) can pose serious threats to civilians, warfighters, and the environment. We report on highly sensitive, robust, and inexpensive distributed standoff fiber optic chemical sensors (FOCSs). FOCSs are activated by analytes reaching a sensing region and inducing a change in its optical properties. The information of a spectral response is transmitted by a shift in transmission spectra probed by evanescent waves, corresponding to the detection of an analyte. However, there are many factors that play a role in the implementation and optimization of such fiber sensors. We find it beneficial to construct the theory of general optochemical waveguide sensors to guide the analysis. We apply it to a specific fiber platfom that is robust, broadband, multi-target, scalable, and compatible to texitle fibers and fabrics - a focus of this work. The good agreement between the predicted and measured spectra of FOCSs exposed to Ammonia (NH$_3$) lends support to the analysis. The experiment yields a 1ppm sensitivity, representing the current best in class. The use of NH$_3$ in the test serves as a representative for a larger set of amine-based TICs, CWAs, EPs, and other chemical species, targeted for wide-area distributed coverage and textile-compatible sensors.