A new era of pollution requires an important focus on the conservation of archaeological sites and monuments. In the last years great efforts were required to develop various sensors for different tasks. The fiber Bragg grating (FBG) was one of the most studied thanks to the multitude of applications and the surprising performances. An original fiber optic sensor that combines the fiber Bragg gratings<strong> </strong>with a pH responsive polymer coating for monitoring the pH of the rains on critical and prestigious monuments is proposed. The core consists of four different materials, which makes the sensor very innovative and allowing it to reach a high sensitivity. In this study, the setup arrangement of the optical sensor is modeled with COMSOL Multiphysics (Wave Optics Module), based on the FEM (Finite Element Method) solver. Monitoring the pH of water can be used by experts to predict and control the corrosion phenomenon of specific materials, especially limestone and marble, thus scheduling the timely restoration.

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In this paper, a rigorous theoretical approach, adopted in order to generalize the Vectorial Cylindrical-Harmonics (VCH) expansion of an inhomogeneous elliptically polarized plane wave, is presented. An application of the VCH expansion to analyze electromagnetic field scattered by an infinite circular cylinder is presented. The results are obtained using the so-called complex-angle formalism reaching a superposition of Vectorial Cylindrical-Harmonics. To validate the method, a Matlab code was implemented. Also, the validity of the methodology was confirmed through some comparisons between the proposed method and the numerical results obtained based on the Finite Element Method (FEM) in the canonical scenario with a single cylinder.

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