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.