The Brillouin linewidth in water as a function of temperature and salinity: the missing empirical relationship

Abstract

AbstractBrillouin scattering in water has been proposed as a method to measure temperature profiles in the ocean. Spontaneous Brillouin scattering can be understood as light scattering on phonons in the water originating from thermal fluctuations. The scattering signal usually features three peaks in its spectrum, of which one is attributed mainly to scattering from suspended particles and shows no frequency shift from the incident light, while the other two are shifted by the phonon frequency leading to a linear dependency of this Brillouin shift to the speed of sound. The speed of sound is dependent on temperature and salinity. To measure temperature and salinity independently, a second measurement parameter is necessary. The spectral width of the Brillouin lines constitutes a measure for the phonon lifetime and depends on various macroscopic water properties. This linewidth was investigated in a laboratory using a scanning FPI (Fabry-Perot interferometer) over a temperature range from 1.8 °C to 35 °C and 13 salinities from 0 ppt to 36 ppt. A strong temperature dependence was confirmed and a weaker, nonlinear dependency on salinity was found. An empirical model was created to describe the data for the purpose of enabling remote sensing of water temperature profile and salinity simultaneously in the future.