In situ salinity measurements in seawater with a fibre-optic probe

Abstract

We have successfully proved the feasibility of an optical salinity meter for marine applications in a two-week measurement campaign, carried out for the realization of in situ salinity measurements in seawater. An optical instrument (optode), in which the main element is a fibre-optic refractive-index sensor based on surface plasmon resonance (SPR), has been developed for that purpose, and has been especially designed to be able to operate in realistic conditions. The performance of the optode has been evaluated on an oceanographic ship in the Baltic Sea, close to the Vistula estuarine area. The obtained results (in different tests, such as depth-profiling, towing and stationary measurements) show good correlation with the data provided by a commercial probe. Although the device is currently a part of a more complex measuring platform and uses an axial spectrograph as detector, the output power measurement used and the simplicity of its conception allow us to conceive a closed, extremely compact set-up which can be in principle commercially competitive with existing sensors.