High-Resolution Optical Fiber Salinity Sensor With Self-Referenced Parallel Fabry–Perot Fiber Microcavity

A high-resolution salinity sensor based on a self-referenced parallel Fabry–Perot fiber microcavity is proposed. The sensor is fabricated by inserting a short piece of exposed-core microstructure fiber (ECF) between multimode fiber (MMF) and single-mode fiber (SMF). Due to the unique exposed-core fiber structure and the specially designed SMF-MMF-ECF-SMF structure, two prominent parallel Fabry–Perot interferometers (FPIs) are constructed. Besides, a self-referenced differential phase-demodulation technology is proposed to eliminate the wavelength uncertainty of the spectra measurement devices. Enhanced demodulation system stability and accuracy are achieved by analyzing the phase shift responses of the reflection spectrum. A high salinity-phase sensitivity of 17.36 deg/ ‰ in the range from 0 ‰ to 35 ‰ is obtained, and the corresponding refractive index (RI) sensitivity is <inline-formula> <tex-math notation="LaTeX">${1.04} \times {10} ^{{5}}$ </tex-math></inline-formula> deg/RIU. The salinity resolution and RI resolution are as low as 0.0058 ‰ and <inline-formula> <tex-math notation="LaTeX">${9.6} \times {10} ^{-{7}}$ </tex-math></inline-formula> RIU, respectively, which gives a great potential for harsh environmental monitoring and label-free biochemical analysis.