Influence Of Water Parameters On Frequency Shift Of Stimulated Brillouin Scattering

The stimulated Brillouin scattering(SBS)is a third-order nonlinear scattering caused by the interaction between the intense incident laser and the acoustic wave field in the medium.As an important spectral parameters,the frequency shift of SBS is closely related to characteristics of the medium.Therefore,the retrieval of characteristic parameters of the medium can be obtained according to the frequency shift of SBS.SBS lidar is an application based on SBS,which could be applied to detect underwater targets and measure seawater parameters(temperature,salinity,sound velocity,etc.).For the ocean remote sensing detection by employing SBS lidar,it is necessary to study the influence of seawater parameters on SBS.In the actual ocean environment,seawater parameters change with increasing depth,which can affect the detection results of SBS lidar.In this research,the influence of temperature,salinity and pressure of seawater on SBS frequency shift and the measurement of sound velocity in seawater are studied.The research results have an important significance for the actual ocean remote sensing detection by employing SBS lidar.In this research,the frequency shift of SBS affected by temperature,salinity and pressure is investigated both theoretically and experimentally.Theoretically,the frequency shift of SBS at different temperatures,salinities and pressures is calculated.According to the data of temperature and salinity at different depths in the South China Sea and East China Sea from the World Ocean Atlas 2018(WOA18),profile distributions of temperature,salinity and frequency shift at different depths in the South China Sea and East China Sea are obtained,and effects of temperature,salinity and pressure on the frequency shift of SBS are theoretically analyzed.Experimentally,by designing a simulation system for ocean environmental parameters,the frequency shift of SBS at different temperatures,salinities and pressures is measured,and the relationship between water parameters and the frequency shift of SBS is quantitatively analyzed.The results show that compared with salinity and pressure,temperature is the most important factor affecting the frequency shift of SBS,and at the same salinity and pressure,the frequency shift increases by about 14 MHz when the temperature increases by every 1 ℃.On the research basis of frequency shift of SBS,this paper further investigate the measurement of sound velocity in seawater and its influencing factors.Theoretically,the profile distribution of sound velocity in the South China Sea is simulated based on the Del Grosso algorithm.Experimentally,the sound velocity is obtained from the experimental measurement of frequency shift,and the inversion formula of sound velocity with respect to temperature,salinity and pressure is fitted to retrieve the profile distribution of sound velocity in the South China Sea.The results show that the sound velocity is proportional to the frequency shift of SBS,which is also influenced by the temperature,salinity and pressure.The experimental measurement results of sound velocity coincide well with the theoretical simulation results,and the difference of sound velocity between the experimental inversion results and the theoretical simulation results calculated by the sound velocity algorithm is about 1 ～ 2 m/s at the depth 0 ～ 400 m of seawater in the South China Sea.