Research On Methods Of Characteristics Measurement Of The Ocean Brillouin Scattering Spectrum

In recent years, Brillouin lidar system using Fabry-Perot etalon(FPE) and intensified Charge-coupled Device(ICCD) is widely used in remote sensing the ocean characteristics such as the temperature, sound speed, salinity, etc. In this system, the interference spectrum formed in the FPE is recorded by the ICCD and shown in fringe pattern. Brillouin spectrum characteristics including Brillouin shift and linewidth can be retrieved from the interference spectrum, which can be used to obtain the ocean water characteristics based on the relations between Brillouin scattering characteristics and ocean water characteristics. This method is a non-scanning measurement and takes very short time. And the collimation of the incident light, solid angle and monochromatic of the laser is not strict. Hence this system can fit in complex and harsh physical environment in measurement application. However, the current pixel resolution restriction of ICCD limits the resolution of the interference spectrum recorded by ICCD and the distinction between the interference spectrum and the original Brillouin spectrum is to be studied. As a consequence, the measurement accuracy is restricted and the scattering spectrum accuracy of current methods is about tens of MHz, corresponding to 1oC of temperature's measurement accuracy, which can't satisfy the need of high-accuracy applications.In order to improve the measurement accuracy of Brillouin spectrum characteristics in this system, a lot of research work has been done in this paper. Firstly, the basic theory of Brillouin scattering in ocean water and the retrieval theory of temperature through Brillouin scattering is analyzed. These work is theoretical foundation of the next researches. Then based on the researches on the working principle of FPE and the processes of interference spectrum generation through FPE, the distinction between the interference spectrum and the original Brillouin spectrum is shown in theory. Afterwards, a summary of traditional methods on signal process of the interference pattern is made. A pre-process on the interference pattern is taken to improve the resolution of the obtained interference spectrum and then a new fitting model is proposed. By the least square error fitting using Levenberg-Marquardt(L-M) algorithm, the actual Brillouin spectrum can be retrieved and the measurement accuracy of spectrum characteristics can be improved. Particularly, the setting of parameters in L-M algorithm is analyzed in advance and optimized. Finally, experimental results show that the fitting method using the new model can archive high-accuracy measurement of the spectrum characteristics. The accuracy is improved to MHz magnitude, corresponding to about 0.1oC accuracy in ocean water temperature, which is significantly improved compared to the accuracy of current mothods.