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Abstract
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 ${\rm{Si}}{{\rm{O}}_2}$/porous polymethyl methacrylate (PMMA) thin films. With the exposure to moisture, ${\rm{Si}}{{\rm{O}}_2}$/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.
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