Low-coherence interferometric fiber optic sensor for humidity monitoring based on nafion® thin film

The main aim of this work was the design and development simple fiber optic Fabry-Perot interferometer (FPI) sensor devices for relative humidity (RH) sensing with emphasis on high sensitivity and good stability. The RH fiber FPI sensor is fabricated by coating the end of a cleaved standard multi-mode (MM) fiber with hydrophilic Nafion® sensing film. The Nafion® thin film acts as an active resonance cavity of the low-coherence interferometric sensing structure. The fringe pattern, which is caused by interfering light beam in the Nafion® thin film will shift as the RH changes because the water molecules will swell the Nafion® film and thus change optical pathlength of the sensing structure. The operating principle of a FPI sensor based on the adsorption and desorption of water vapour in the Nafion® and the limitations of this sensor type are discussed in this work. The fiber optic hygrometer was tested in the visible (400-900 nm) region of spectra for measurement of relative humidity (RH) in the range of 5.5-80% at room temperature (RT) in air. The fiber optic humidity sensor has a very short response time (t90 = 5-80 s) and a fast regeneration time (t10 = 5-12 s) as good as commercial sensors.