Improvement of POF humidity sensor with swelling polymer cladding via bending

Abstract

A plastic optical fiber (POF) sensor with a water-swellable polymer cladding (such as polyvinylpyrrolidone (PVP)) can be used to realize a humidity sensor that exploits the change in refractive index associated with the swelling of the polymer. When the refractive index of the cladding of such a POF sensor is slightly higher than that of the core, the POF can easily switch from the leaky mode to the waveguide mode depending on whether water is present. Accordingly, the transmitted light intensity of the POF sensor varies greatly, thus enabling highly sensitive sensor capabilities. Furthermore, the variation in the transmitted light intensity becomes more apparent when the POF sensor is curved. Therefore, in this study, the bending effect of the proposed POF sensor was verified by conducting simulations using a ray-tracing method and by taking measurements. We found that a POF sensor with a curvature of 0.2 cm−1 is 30 times more sensitive than a non-curved POF sensor.