Dual parameter simultaneous sensing of ammonia and humidity utilising dye-encapsulated extrinsic Fabry–Pérot interferometer

This work demonstrates a fiber optic sensing probe functionalized with a single-film for measuring two parameters: gaseous ammonia and humidity. The sensor is constructed with a dye (Tetraphenylporphyrin tetrasulfonic acid hydrate) encapsulated into a porous silica film dip-coated on the distal end of optical fiber forming an extrinsic Fabry–Pérot cavity. This approach combines two popular optical measurement techniques such as white light interferometry and colourimetry measurement in a single fiber and performs spectroscopic analysis of the cavity for dual sensing. Experimental results demonstrate that multiple attenuation bands appearing in the reflection spectrum consist of a mixture of dye absorption and white light interference with spatially resolved bands, allowing sensing of the two-parameter based on colourimetry shift and refractive index change, respectively. The corresponding attenuation bands shift independently in response to ammonia (range: 0–18 ppm) and humidity (0–100%) with negligible crosstalk. The sensor exhibits ~2 nm wavelength shift for 0.7 ppm of gaseous ammonia and a sensitivity of 0.4 nm/% RH for humidity measurement.