Design and performance evaluation of polyvinyl alcohol/polyimide coated optical fibre grating-based humidity sensors

Optical fibre grating based sensors make a significant contribution to relative humidity measurement for various applications and a number of different designs and sensor configurations have been produced and discussed in the literature. Recent developments have included using sensitive polymers and indeed other materials that experience a variation of their physical and optical properties as a result of the absorption of moisture and which thus creates a modification of the characteristic spectral features of the gratings used. The strain and the effective refractive index of the gratings are affected by the change induced by the polymer and these effects were calibrated against the relative humidity variations that initially induce variations in the polymer layer. In this work, an evaluation is made of the performance characteristics of two different moisture sensitive polymers, i.e., polyimide and polyvinyl alcohol, as the basis of such sensors, while at the same time using different types of fibre gratings i.e., fibre Bragg gratings and long period gratings. Their performance is discussed in light of the operation of similar types of polymer-coated and other grating based sensors reported in the literature.