Fiber optic evanescent wave humidity sensor based on SiO2/TiO2 bilayer films.

A SiO2/TiO2 bilayer thin-film-based fiber optic humidity sensor was fabricated via a modified dip coating process with enhanced sensitivity. SiO2 film was coated on the surface of the fiber core, followed by deposition of the TiO2 layer on SiO2. The relative humidity (RH) is measured by modulation in intensity of the transmitted laser at room temperature. The optical fiber humidity sensor based on SiO2/TiO2 film shows two-segmented linearity in measurement with sensitivities of 5.35 and 1.94 µW/% RH at 15%–50% RH and 50%–95% RH, respectively. The response time and recovery time are 25 s and 50 s, respectively. To our knowledge, the superior response time and recovery time of the sensor in our study were achieved over those fiber optic humidity sensors reported with modulation in intensity. Furthermore, this fiber optic humidity sensor has a good reproducibility and long-term stability. The sensing mechanism is attributed to effects of moisture on the refractive index and the light absorption coefficient of SiO2 film and modulation in the transmission characteristic of evanescent waves in the optical fiber.