Three-Wavelength Fiber Laser Sensor With Miniaturization, Integration for the Simultaneous Measurement of Underwater pH, Salinity, Temperature, and Axial Strain

In situ monitoring for complex multiphysical field changes of underwater environment has urgent need for the miniaturized integrated multifunctional sensor. We have proposed and demonstrated a compact three-wavelength fiber laser sensor designed for simultaneous measurement of underwater pH, salinity, temperature, and axial strain, emphasizing miniaturization and integration. The entire sensing probe consists of a 75 mm length, comprising a 5-mm single-mode fiber (SMF)multimode fiber (MMF)-hollow core fiber (HCF)-MMF-SMF (SMHMS) interferometer cascading a polarization-maintaining fiber Bragg gratings (PMFBG) and an etched FBG. The surface of the SMHMS interferometer has been coated with silane coupling agent-polyvinyl alcohol/polyacrylic acid (SCA-PVA/PAA) coating, while the etched FBG is modified with a coating of SCApolyimide (SCA-PI). By monitoring three laser wavelengths and output powers, the proposed sensor enables the simultaneous measurement of underwater pH, salinity, temperature, and axial strain. In our underwater experiment, the intensity of IFBG exhibits a sensitivity of 2.577 dB/pH during pH rising process and 1.930 dB/pH during pH falling process within the pH range of 7.4-8.6. Additionally, the wavelength responses of temperature are 0.040, 0.009, and 0.0098 nm/C-circle, respectively. The characteristics of salinity are obtained with sensitivities of 0.195 dB/wt% and 0.10 nm/wt%. Furthermore, the axial strain sensitivity is determined to be 7.6 x 10(-4) nm/mu epsilon and -7 x 10(-4) dB/mu epsilon, indicating the wavelength and intensity sensitivity to axial strain. The favorable repeatability, stability, and linearity of the fiber laser sensor have been experimentally demonstrated.