Low-Cost High-Sensitivity Large Offset Mach-Zehnder Interferometer Optical Fiber Salinity Sensor With Long Sensing Area

A low-cost, high-sensitivity large offset salinity sensor with a long sensing area is proposed and demonstrated in this article. The sensor is composed of three misaligned single-mode fibers (SMFs), in which the middle long-distance SMF is used for sensing, which is the SMF with long sensing area and is misaligned with a pair of SMFs to form SMF-long sensing area SMF-SMF structure, that is, SMF-LSSMF-SMF structure. According to the simulation results, the SMF large offset structure with a sensing area length of $1000~\mu \text{m}$ was prepared and used to measure seawater salinity. The experimental results reveal that the salinity sensitivity of the structure can reach −1.22 nm/‰ (−6263.11 nm/RIU), and it has more spectral periods in the same spectral range than the short sensing area probe. SMF-LSSMF-SMF has the advantages of compact structure, high sensitivity, simple production method, and outstanding repeatability, which can increase spectral information, provide richer period information for subsequent demodulation processing, eliminate the problem that the period cannot be judged after spectral aliasing under high sensitivity, and has great potential in the field of high-precision seawater salinity measurement.