Characterisation of a Nafion film by optical fibre Fabry–Perot interferometry for humidity sensing

Nafion has been evaluated as a sensing phase of an optical fibre humidity sensor based on a low-finesse Fabry–Perot interferometer. The sensor was constructed by manual deposition of a drop of a Nafion solution on the tip of a single mode optical fibre, forming a Fabry–Perot resonant cavity. The absorption of water by the Nafion film makes it swells, changing its refractive index and the length of the cavity, which produces a phase shift in the interference signal. The sensitivity, stability and response time of the sensor were evaluated in the RH range from 22 to 80% by analysing the correspondent reflection spectra of the interference fringes. As a result, it was obtained that Nafion can be used as sensing phase of an optical fibre humidity sensor based on optical fibre Fabry–Perot interferometry, presenting a response time of 242 ms (3% RH variation) and a sensitivity of 3.5 nm/%RH.