Characterization and comparison of the relative humidity response of hydromorphic polymers in Long-Period Fiber Grating structures

Relative humidity monitorization is a process of extreme importance on both scientific and industrial applications, and while many types of sensors have been developed, the typical low-cost capacitive or resistive structures may display some flaws, such has not being immune to electromagnetic radiation, and not fit to extreme and harsh environments (such as underwater applications). In order to solve some of these limitations, optical fiber sensors have been developed, consisting of optical structures associated with a humidity responding polymer, resulting in a measurable change in the optical spectrum.

In this work, a preliminary study of three different humidity responding polymers in association with Long period Fiber Gratings (LPFG) is reported, along with a complete description of the fabrication methods, relative humidity performance testing and result interpretation. The polymers used are Poly(vinyl alcohol), Poly(ethylene glycol) and Hydromed™ D4, resulting in sensors with different properties and working ranges. The interpretation of the results presented is aided by the usage of numerical simulations of the LPFG structures, enabling a better understanding of the acquired data and defining future work to be carried out.