Three-Wavelength Fiber Laser Sensor With Miniaturization, Integration for the Simultaneous Measurement of Underwater pH, Salinity, Temperature, and Axial Strain
Guofeng Sang, Bin Yin, Ran Yan, Muguang Wang, Haisu Li, Yanzhi Lv, Shilin Liu, Qichao Wang, Songhua Wu
Abstract
In-situ monitoring for complex multi-physical field changes of underwater environment has urgent need for the miniaturized integrated multi-functional 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 75mm length, comprising a 5mm SMF-MMF-HCF-MMF-SMF (SMHMS) interferometer cascading a 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 SCA-polyimide (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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I<sub>FBG</sub></i> 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 nm/°C, 0.009 nm/°C and 0.0098 nm/°C 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 7.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> nm/με and -7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> dB/με indicating the wavelength and intensity sensitivity to axial strain. The favorable repeatability, stability and linearity of the fiber laser sensor have been experimental demonstrated.