Aluminium-doped zinc oxide sensing membrane integrated fiber-optic Michelson interferometer for trace fluoride-ion detection

A waist-enlarged taper Michelson interferometric trace fluoride-ion sensor based on silver film encapsulated the head face of thin-core fiber (TCF) is proposed and presented. The waist-enlarged taper is spliced between the single-mode fiber and the TCF to realize the optical splitter. The aluminium doped zinc oxide is coated on the outside surface of TCF as the fluoride-ion sensing membrane. Silver film on the head face of TCF is to enhance light reflection. The refractive index of the sensing membrane is changed by the adsorption of fluoride-ion, and then the monitoring interference dip of the sensor changes accordingly. Thus, the fluoride-ion concentration can be determined by the corresponding wavelength shift. The sensor can be used to detect trace fluoride-ion in the range of 0–50.0 ppb with an average sensitivity of 60.02 pm ppb−1. When the concentration is in the range of 0.01–0.10 ppb, the linearity is 0.998, and the sensitivity can be 3341.23 pm ppb−1. In the 0.3–2.0 ppb concentration range, the linearity is about 0.975, and the sensitivity can reach 404.75 pm ppb−1. As the concentration ranging from 5.0 to 50.0 ppb, the linearity is about 0.982 with the good sensitivity (23.75 pm ppb−1). The response time (about 40 s) of the sensor has been determined. The sensor has the advantages of high sensitivity, excellent selectivity, simple structure and rapid response, and can be put into the application for the trace fluoride-ion detection in water.