A Refractometer for Measuring Thermohaline Density Microstructure in the Ocean

Here we report an optical sensor for measuring millimeter-scale thermohaline density variations in the ocean. The instrument is based on a fiber Fabry-Perot white light interferometer which can resolve the refractive index of water to better than 2 x 10 −8 within a sample volume smaller than 1 mm 3 at a sample rate of 500 Hz. This equates to detectable absolute salinity variations of 0.0001 g/kg, temperature variations smaller than 0.0007°C, and density variations of 0.00007 kg/m 3 . Data collected from laboratory characterization and a field deployment suggest the sensor could be useful as a gradiometer for measuring diapycnal mixing down to sub-millimeter scales. While obtaining high absolute accuracy in refractive index and density was not a primary consideration, the sensor was designed with such an eventual goal in mind, and various aspects of achieving it are discussed.