ABSTRACT
Refractometric optical humidity sensors are the subject of analysis in this chapter. In this chapter, you can find a description of the principles of their operation, including evanescent spectroscopy. The sensing mechanism of refractometric optical humidity sensors is based on the change in the effective refractive index (RI) of the porous coating formed on the surface of the fiber or planar waveguide. In refractometric optical humidity sensors, because of the interaction with water vapor adsorbed on the core or on the cladding of the fiber or planar waveguide, a perturbation of the corresponding RI is obtained. As a result, a modification in the intensity of the light transported by the fiber or planar waveguide takes place and the adsorbed species can thus be detected. Thus, the principle of these sensors rely on the use of the humidity-sensitive materials to generate secondary effects such as RI change or strain on the sensing fiber that result in the shift of output spectra or change in the intensity. The change of the RI and polarization in an optical humidity sensor are generally explained by the adsorption water and also by the capillary water condensation within the pores of a humidity-sensitive material. This chapter also describes the approaches used in the development of refractometric optical humidity sensors. In particular, the bend loss-based sensor sensors, microbent sensors, evanescent wave-based humidity sensors, whispering gallery mode-based microresonators, and sensors based on fiber Bragg grating, long period gratings, and tilted fiber gratings are analyzed. Features of their manufacturing and advantages and disadvantages of their use in measuring air humidity are described. Materials acceptable for applications in refractometric optical humidity sensors are also analyzed in detail in this chapter.
