Water salinity fiber sensor with selectable sensitivity using a liquid-fillable composite in-fiber Fabry-Perot cavity

We propose and experimentally demonstrate a fiber-optic sensor for monitoring water salinity in desalination processes with selectable sensitivity. The proposed sensor structure is formed by drilling an air-gap near the cleaved end of a single-mode optical fiber using Focused Ion Beam (FIB) milling. Utilizing the cleaved fiber end as an additional reflective plane, the proposed structure can be simplified as a three-wave interferometer because there are three cavities formed by different pairs of reflective planes. The interference spectrum exhibits three groups of fringes with different sensitivities with respect to the variation of dissolved sodium chloride content in the liquid surrounding the sensor. Therefore, the proposed sensor can measure water salinity with selectable sensitivity depending on the wavelength windows used for spectral interrogation. The temperature effect on the sensor operation was also measured and discussed.