Intensity-Modulated Fiber-Optic Salinity Sensor by Tapered Microcavity Mach–Zehnder Interferometer

In this article, a fiber-optic microcavity Mach–Zehnder interferometer is proposed and experimentally demonstrated for high-precision salinity sensing, based on the tapered multimode-hollow-core-multimode fiber structure. The excited evanescent wave field distribution is analyzed, and the quantitative relation is proved between the light intensity and taper ratio. The comprehensive tests are then performed and the experimental results show that the proposed sensor has obvious intensity modulation with the varied RI of liquid. The sensitivity as high as 796 dB/RIU is gained by the structure with the taper ratio of ~0.22, and the corresponding salinity sensitivity reaches 0.154 dB/‰ in the range from 0‰ to 11‰. Besides, due to low-temperature crosstalk and high stability, the real limit of detection of our sensor is constrained within 0.1‰.