Design and simulation of reflective fiber grating sensor for simultaneous monitoring of CTD in marine environment

A fiber grating reflective probe is designed for simultaneous detection of CTD (Conductivity, Temperature, and Depth) in seawater. In this structure, three long period fiber gratings (LPGs) coated with different film thickness are cascaded, and a metal film is coated at the end of the structure as a fiber reflector. 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 the structure is obtained. Then, the parameters of film, grating period, grating length and connecting fiber length are optimized by theoretical simulations to make the demodulation more accurate and convenient. On the basis of the above, the reflection spectrum of this structure is simulated, and three reflection interference fringes are investigated. Furthermore, the sensing characteristics of CTD are analyzed by the shift of central interference peaks of the three interference fringes. Simulation results indicated that the sensitivities of monitoring peaks to SRI, temperature, and water pressure are − 360 nm/RIU, − 460 nm/RIU, − 510 nm/RIU, 0.16 nm/℃, 0.20 nm/℃, 0.26 nm/℃, − 5.1 nm/Mpa, − 6.1 nm/Mpa, − 8.4 nm/Mpa, respectively; the reflectivities of all monitoring peaks are lower than 0.1; the full width at half maxima of monitoring peaks are less than 1.7 nm and the linearity of the monitoring peaks to three parameters are larger than 0.9. This design has great potential in the fields of multiparameter sensing and segmented sensing in marine environment.