Integrated and compact fiber-optic conductivity-temperature-depth (CTD) sensor for marine detection

Conductivity, temperature and depth (CTD) are the three most important parameters of the marine environment. In this paper, an integrated and compact fiber-optic CTD sensor for simultaneous measurement of ocean temperature, salinity (conductivity) and pressure (depth) is proposed. A bare fiber Bragg grating (FBG) for temperature measurement, an FBG with lamellar polyimide (PI) coating for salinity measurement and a diaphragm-based extrinsic Fabry-Perot interferometer (DEFPI) for pressure measurement are connected in series with only one single fiber to form a multiplexed sensing system for CTD detection. The salinity sensing mechanism based on the water diffusion model of the lamellar PI coating is presented. A mold for curing the lamellar PI coating is fabricated and the curing process is introduced in detail. Experimental setup is built for temperature, salinity and pressure measurement. The wavelength-division multiplexing (WDM) technology combined with Gaussian fitting method and the cross-correlation demodulation algorithms are employed to demodulate the measurand from the spectrum. The Gaussian elimination method is used to eliminate the cross-sensitivity effects of the three measurands. Experimental results show that a sensitivity of 10 pm/°C, 141 pm/(mol/L) and 520 nm/MPa for temperature, salinity and pressure measurement, respectively, has been realized. Resolution measurements show that a resolution of 0.1 °C, 0.01 mol and 1.5 KPa for temperature, salinity and pressure measurement, respectively, is achieved. The repeatability of the proposed CTD sensor is also be investigated and a repeatability of 0.73 % FS, 0.72 % FS and 0.1 % FS for temperature, salinity and pressure measurement, respectively, has been obtained. The proposed fiber-optic CDT sensor has the advantages of high-sensitivity, high resolution, high reliability, anti-electromagnetic interference, compact size and low cost, which has great application potential in ocean exploration.