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Long-period gratings (LPGs) have shown their significant promising applications in sensors owing to the attractive
features that they posses such as small size, immunity for electromagnetic interference, geometric versatility,
multiplexing capability, and resistance to corrosive and hazardous environments. Recent researches have revealed that
LPGs written on the standard optical fibers could be used as a powerful sensing platform for structural health
monitoring. In this work, we inscribe LPGs into SMF-28 optical fiber by focused-beam CO2 laser, demonstrating as a
refractive index sensor for nondestructive chemical detections in the civil infrastructures. Although evanescent-field
based LPG sensors have been applied in quantitatively monitoring chemical analytes including moisture, chloride, and
corrosion by-product, etc., the sensitivity, selectivity, and response time as well as thermo-stability of such sensors are
still the issues for some special purposes. In order to improve those characteristics of the sensors, we propose two types
of nano-film to be coated in grating region by electrostatic self-assembly (ESA) deposition processing. The primary
coating does not affect on LPG transmission parameters such as resonance wavelength and its intensity that can be used
for sensing, but it increases the sensitivity to refractive index change of surrounding material. The secondary coating is
for selectively absorption of analyte molecule of interest. Response time of the nanofilm-coated LPG sensor is dependent
on the analyte absorption and de-absorption rates as well as the thicknesses of the coating materials, which is also
investigated. Multi-channel sensor system is being designed to monitor different analytes simultaneously, which is
continuing to further explore the monitoring of structural health conditions through in situ measurements of corrosion in
the concrete structures.
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