Water salinity measurement using a long-period grating and optical time-domain reflectometry

Abstract

Salinity is a key variable in understanding several issues, from public health to food security. Measuring salinity in situ is traditionally done using conductivimetric methods, and can be challenging due to extreme temperatures, corrosive environment, and oxidation. Fiber-based methods and other alternatives proposed to date have brought a number of advantages, but present low sensor strength, complex or expensive setups, cross-influence of temperature, lack of portability, or prohibitively long response times. This work presents a simple, compact salinity sensing system that, associated with a modern interrogation technique, is capable of achieving good accuracy even in remote sensing and low salt concentrations. The sensor is a long-period grating fabricated using the point-by-point electric arc method. The interrogator is based on optical time-domain reflectometry aided by signal processing techniques inspired by audio processing. Experimental data show that the system is capable of estimating salinity in the range from 0 g/L to 80 g/L within 0.49 g/L on average, with the sensor 4 km away from the light source.