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Abstract
Based on the demand for monitoring conductivity, temperature, and depth (CTD) in the ocean, a hybrid coated long-period fiber grating, tilted fiber Bragg grating, and fiber Bragg grating (LPG-TFBG-FBG) structure is proposed, which could eliminate the mutual interference of CTD in measurements. First, according to coupled mode theory and phase matching conditions, the coupling equation of each segment of the grating is deduced. Next, through the difference iterative method, the total transfer matrix of coated LPG-TFBG-FBG is obtained. Then, to improve the sensitivity of the structure and make the demodulation more accurate and convenient, the structure of the grating is optimized. On this basis, the transmission spectrum of the structure is simulated, and three peaks formed by coupling the forward core mode with the forward cladding mode in LPG ($\lambda _L^{{\rm cl}}$), the backward high-order cladding mode in TFBG ($\lambda _{\textit{TB}}^{{\rm cl}}$), and the backward cladding mode in FBG ($\lambda _B^{{\rm cl}}$) are investigated. Furthermore, the sensing characteristics of the surrounding refractive index (SRI), temperature, and water pressure of the peaks are analyzed, and the three peaks show different sensitivities to environmental parameters. Therefore, a three-parameter matrix equation is established, and the three parameters can be measured simultaneously without interference. The simulation results of the response of the three transmission peaks to the three parameters show good performance in both bandwidth (less than 5 nm) and linearity (all larger than 0.99). The maximum sensitivities of the three transmission peaks to SRI, temperature, and water pressure are ${-}{343}\;{\rm nm/RIU}$, ${-}{1318.7}\;{\rm pm/}^\circ{\rm C}$, and ${-}{9.5}\;{\rm nm/Mpa}$, respectively. Finally, the condition number of the parametric demodulation matrix is 581.5822, indicating that the inversion of the resonant wavelength offset has high precision. To our knowledge, this is the first optical fiber CTD sensor based on fiber gratings that can compensate for each other regarding CDT. The proposed coated LPG-TFBG-FBG structure has good linearity and high sensitivity, which give this structure great application potential in the fields of multi-parameter sensing and segmented sensing.
© 2022 Optica Publishing Group
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