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17 Jun 2002
Researchers in Spain believe that they have made the first optical sensor to determine the saltiness of the sea.
The sensor measures the refractive index of a water sample and compares this with predicted values from theory to give the salinity. The refractive index varies linearly with typical salinity levels if the temperature is constant.
The cladding of the fibre is polished and a multi layer structure containing a thin metal layer is deposited on it. Light guided in the fibre is attenuated by resonant excitation of surface plasmons in the metal layer.
Salinity is usually measured by determining the electrical conductivity of water and commercial devices based on this are common and relatively cheap. One of the team, Agustin Gonzalez-Cano says, "The novelty of our approach is that it is based in optical measurements. It shares the advantages of any fibre optic sensor, including the possibilty of accessing difficult places, of integrating in complex systems for multi-parameter determination, a high accuracy and a low cost."
Optical fibre sensors can monitor a wide range of chemical and biological factors and the system can be adapted for continuous, real-time monitoring of pollutant concentrations.
The sensor has been developed by Oscar Esteban, Maria-Cruz Navarrete, Eusebio Bernabeu and Agustin Gonzalez-Cano of the Universidad Complutense de Madrid and was reported in the 1 September issue of the journal Applied Optics. Their work is part of a European Union funded programme to design, construct and test a device for detecting pollutants and salinity in seawater. The three-year project will end in March 2000.
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