An Optical Interferometer for the Measurement of Absolute Salinity
Ocean salinity has always been more challenging to measure than temperature, yet is just as important to the density of seawater. At present it is inferred from measurements of the electrical conductivity of seawater, as well as temperature and pressure. An alternative approach is to use the optical refractive index, but this requires very high sensitivity measurements, which here-to-fore have been impractical. The refractive index is especially attractive because it is a more direct measurement of the seawater density and salinity, and would provide a measure of Absolute Salinity, instead of the conductivity-based Practical Salinity, which has systematic errors due to the variations in the concentrations of non-conductive nutrients. Salinity has emerged as an especially important variable for understanding the global water cycle, and since most rain originates by evaporation from the ocean, the salinity signals it leaves behind are proving useful for seasonal predictions of rainfall on land and for identifying long-term trends in the water cycle. Since water availability is an issue of great societal concern, improved salinity measurements have become an oceanographic priority. Here an approach to salinity measurements using the refractive index is described that uses a multi-wavelength interferometer. A new high-speed analysis technique has been invented (patent applied for) that combines the sensitivity of a monochromatic interferometer with the absolute accuracy of a broad spectrum instrument. Extensive use is made of fiber-optic components from the telecommunications industry to achieve very low noise levels. This project will construct and demonstrate a novel and practical sea-going interferometer to directly measure the Absolute Salinity and achieve new levels of accuracy in measuring the density gradients of the ocean.