Design and simulation of a cascaded-coated LPG-FBG sensor structure for simultaneous monitoring of CTD in ocean

Based on the demand for monitoring conductivity, temperature and depth (CTD) in the ocean, a cascaded coated long-period fiber grating and fiber Bragg grating structure (LPG-FBG) is proposed, which could eliminate the mutual interference of CTD in the measurement. Firstly, according to the coupled mode theory and phase matching conditions, the coupling equation of each segment of grating is deduced. Next, through the difference iterative method, the total transfer matrix of cascaded LPG-FBG is obtained. Then, to improve the sensitivity of the sensor and make the demodulation more accurate and convenient, the structure of the grating is optimized. On the basis of the above, 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(cl)(L)), the backward cladding mode in FBG (A(B)(cl)) and the backward core mode in FBG (lambda(co)(B)) are investigated. Furthermore, the sensing characteristics of surrounding refractive index (SRI), temperature and pressure of the peaks are analyzed, and the three peaks show different sensitivity to marine environmental parameters. Therefore, a three-parameter matrix equation is established, and the three parameters can be measured simultaneously without interference. Simulation results indicated that the transmittances of all transmission peaks are lower than 0.5, the full width at half maxima of all transmission peaks is less than 7 nm, and the linearity of the three transmission peaks to three parameters is larger than 0.999. The maximum sensitivities of three transmission peaks to SRI, temperature and water pressure are - 440 nm/RIU, - 1375.67 pm/degrees C and - 10.87 nm/Mpa, respectively. Compared with the other three-parameter sensors used in ocean, the proposed cascaded-coated LPG-FBG structure has simple structure, high sensitivity and good linearity, which makes this structure have great application potential in the fields of multiparameter sensing and segmented sensing in marine environment.

Automated summary

A cascaded coated long-period fiber grating and fiber Bragg grating structure is proposed to monitor the conductivity, temperature, and depth in the ocean. The structure can eliminate the mutual interference of CTD in the measurement. The simulation results show that the proposed structure has a simple structure, high sensitivity, and good linearity, making it suitable for multiparameter sensing and segmented sensing in the marine environment.