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Employing functional and structured thin films on fiber optic sensors has tremendously improved capabilities in humidity sensing applications. In this paper, we demonstrate fabrication of a fiber optic evanescent wave humidity sensor based on S i O 2 /porous polymethyl methacrylate (PMMA) thin films. With the exposure to moisture, S i O 2 /porous PMMA thin films absorb water molecules. The refractive index and absorption coefficient of thin films change with ambient humidity, resulting in modulation of the light intensity transmitted in fiber. A good linearity is determined between the logarithm of output light intensity and relative humidity (RH). An optimal average sensitivity of 188.3 lux/%RH is achieved with an increase of 11.7 fold in the RH range of 5% to 95%. The response and recovery times are 8 s and 23 s, respectively. Furthermore, the sensor exhibits low hysteresis, and excellent stability and repeatability.

A S i O 2 / T i O 2 bilayer thin-film-based fiber optic humidity sensor was fabricated via a modified dip coating process with enhanced sensitivity. S i O 2 film was coated on the surface of the fiber core, followed by deposition of the T i O 2 layer on S i O 2 . The relative humidity (RH) is measured by modulation in intensity of the transmitted laser at room temperature. The optical fiber humidity sensor based on S i O 2 / T i O 2 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 S i O 2 film and modulation in the transmission characteristic of evanescent waves in the optical fiber.