Long-period fiber grating moisture sensor with nano-structured coatings for structural health monitoring

Abstract

In this article, we inscribe long-period gratings on an SMF-28 optical fiber by a focused CO2 laser beam to develop a humidity sensor with high sensitivity and selectivity for moisture detection for civil infrastructures health monitoring. We propose two types of nano-structured film to deposit on surface of the grating region by electrostatic self-assembly deposition process. The primary nanofilm coating (PAH+/PAA−) does not affect long-period grating properties such as resonance wavelength and transmission intensity; however, it increases the sensitivity by changing the refractive index of the surrounding analytes. The secondary nanofilm coating (Al2O3+/PSS−), due to its molecule structure compatibility, can selectively adsorb analyte molecules of interest (in this case H2O only). The experimental results reveal that the nanofilm-coated long-period gratings are more sensitive to moisture in terms of both wavelength shift and resonance intensity change as of 0.019%/pm and 0.01%/10−3 dBm, respectively, while the bare LPG coefficients are 0.021%/pm and 0.0603%/10−3 dBm. The proposed sensor shows excellent thermal stability as well.