A Mach–Zehnder Interferometer Salinity Sensor Based on a Twin-Core Fiber With C-Type Microcavity

A highly sensitive Mach-Zehnder interferometer (MZI) salinity sensor is demonstrated, which is fabricated by the sequential splice of single-mode fiber (SMF)–multimode fiber (MMF)–twin-core fiber-MMF–SMF (SMTMS). A C-type microcavity is engraved in one of the cores of the twin fiber by a femtosecond laser to facilitate the entry and exit of the seawater solution. The simulation analysis and experimental verification confirm that the SMTMS salinity sensor is feasible in the E-band to the U-band. The results demonstrate that the sensitivity of the proposed SMTMS salinity sensor is up to -2.041 nm/‰ (equivalent to -10561 nm/RIU) with good repeatability and stability. The proposed highly sensitive SMTMS salinity sensor provides a new approach for high-performance seawater salinity parameter monitoring, which can be utilized in marine resources exploration, marine environment monitoring, marine meteorological prediction, and so on.