Development of a highly accurate and fast responsive salinity sensor based on Nuttall apodized Fiber Bragg Grating coated with hygroscopic polymer for ocean observation

This paper experimentally demonstrates the salinity sensing characteristic of an apodized Fiber Bragg Grating (FBG) coated with a hygroscopic polymer. Polyimide (PI) was used as the coating material on the FBG inscribed with Nuttall apodization function. PI coating enhances sensitivity by its hygroscopic property and apodization increases detection accuracy by reducing side lobes in the reflected spectrum. Based on experimental results, it was observed that the proposed salinity sensor has a sensitivity of 0.0026 nm/Practical Salinity Unit (PSU) with a resolution of 0.115 PSU in the range of 0 PSU to 40 PSU with an average response time of 60 seconds. Validation results using commercially available ocean sensors indicated that the proposed optical fiber sensor is capable of measuring seawater salinity with a mean error of 0.2015 PSU.