Employing the Reflected Amplitude of a Fibre Bragg Grating Sensor for High Resolution Salinity and Depth Measurement

Continuous and accurate monitoring of oceanic physical parameters, including salinity and depth, is essential for naval surveillance, communication and environmental monitoring. In this regard, Fibre Bragg Grating (FBG) sensors are being implemented as highly accurate temperature and strain sensors for diverse applications. However, discriminating the individual effects of the two parameters, salinity and depth, is challenging, because both have to be transduced from the strain response. In this paper, we present a innovative approach, by implementing the amplitude of the FBG's reflected spectrum as a second responding element. We have theoretically and experimentally demonstrated that the reflected amplitude of an FBG is inversely proportional to strain, and used this principle for salinity and depth measurement. Using the Bragg wavelength, we obtain a resolution of 0.2 Practical Salinity Unit (PSU) for salinity and 0.25 mm for depth. But by employing the reflected amplitude, the resolution improves to 0.00119 PSU for salinity and 0.00273 mm for depth. The proposed approach allows for the transduction of two variables from a single transducing element without requiring any additional sensor structure modifications, and without compromising accuracy or sensitivity, and is expected to be a breakthrough in the development of a single multi-parameter fibre-optic sensor.