Optimized Multifunctional Optical-Fiber Sensor Integrating MZI-FPI Spectra for Simultaneous Salinity-Temperature Measurement

Optical-based salinity measurement schemes are plagued by salinity-temperature cross-sensitivity, whereas multifunctional optical fiber sensors based on misaligned splicing can achieve high-sensitivity measurement and salinity-temperature decoupling. However, due to the limited mode field diameter (MFD) of single-mode optical fiber, sensors based on misaligned splicing impose stringent requirements for high precision in misalignment control. To address this problem, an optimized multifunctional optical fiber sensor is proposed, where multimode fiber (MMF) with a larger MFD is adopted to enhance the divergence and collection capacity of light, and the Mach-Zehnder interferometer (MZI) and Fabry-Perot interferometer (FPI) are integrated in the same sensing structure to solve the cross-sensitivity problem. The proposed sensing structure is fabricated by misaligned splicing of the MMF, where the MMF provides greater flexibility in misalignment control and releases the fabrication difficulty. The fabrication experiment demonstrates that MZI and FPI spectra can coexist well when the misaligned splicing range is 60-64 μm, and the spectrum intensity is about 10 dB higher than the SMF-based sensor. The performance test results indicate the salinity sensitivities of MZI and FPI spectra are -2.67 nm/‰ and 79.3 nm/‰ (in OPD domain), respectively, while their temperature sensitivities are 1.04 nm/°C and -19.55393 nm/°C (in OPD domain) at a temperature of 20°C. The cross-sensitivity between temperature and salinity can be resolved by leveraging the distinct response characteristics of the two spectra. Furthermore, the proposed scheme alleviates the misalignment tolerance and enhances the spectrum intensity, improving the practical applicability of the sensor.