Inversion Algorithm And Experimental Of Seawater Parameters Measurement Based On Brillouin Scattering

Real time and accurate remote sensing of ocean temperature profile information is of great significance for understanding ocean properties and biodiversity in the ocean.At the same time,it can also predict the change of weather or temperature through the information of ocean temperature profile.At present,the ocean temperature information is mainly sea surface temperature information,which comes from passive remote sensing satellites,and the temperature profile information is relatively lacking.Brillouin scattering makes it possible for active remote sensing of ocean temperature profile because its frequency shift is sensitive to temperature and its scattering spectrum is narrow and not easily affected by background noise.The key of temperature measurement based on Brillouin scattering is to accurately obtain the Brillouin frequency shift information,and then retrieve the temperature of the measured area through empirical formula and assumed salinity information.In the process of temperature profile inversion,it is difficult to accurately assume the salinity information because the salinity profile varies greatly with season and region.In addition,the random jitter of sea surface urgently requires the detection system to have a large receiving field of view.Therefore,the edge detection technology based on iodine molecular absorption cell is adopted in this paper.After splitting the Brillouin scattering light with temperature information,three detection channels are formed through three iodine molecular absorption cells with different pressures,and two signal intensity ratio information are obtained to avoid the influence of excitation intensity noise.In the process of temperature inversion,the temperature and salinity information of the measured area can be obtained simultaneously by iterative algorithm without assuming the salinity information in advance.Finally,the ability of temperature measurement using this technical route is verified by experiments.This paper theoretically shows that using three iodine molecular cells with different pressures can reverse the temperature and salinity with a temperature error of 0.2K without knowing the seawater temperature and salinity in advance;Experiments show that the ocean temperature inversion system based on Brillouin scattering can achieve a temperature resolution of1 ° C.The main research contents of this paper include:1.The edge detection mechanism based on iodine molecular absorption cell is simulated.The absorption spectra of iodine molecules with wave numbers ranging from 18786.40334cm-1 to18793.45934cm-1 were simulated by simulation software;On this basis,the light scattering process is simulated,including Rayleigh scattering and Brillouin scattering.Through the temperature signal intensity normalization curve obtained by simulating the edge detection process,it is determined that the output center wavelength of the laser is 532.2334112 nm.2.A new iterative algorithm is proposed.Assuming that the salinity is in the range of 30 ‰ to40 ‰,the temperature and salinity can be retrieved simultaneously by using three iodine molecular absorption cells with different pressures and the two ratio information obtained.In order to control the temperature inversion accuracy within 0.2K,the algorithm requires the system jitter not to exceed 1.3 ‰.The reliability and stability of the algorithm are analyzed in this paper.3.The absorption spectrum of iodine molecule was measured.Based on the simulation results,the actual measurement system is built.The iodine molecular absorption curve is measured with different wavelength scanning frequency and different iodine molecular cell temperature,and the best scanning measurement frequency and the best iodine molecular cell temperature are selected.At the same time,the effects of laser frequency stability,Rayleigh scattering intensity,half height and full width of Rayleigh scattering peak on the measurement system are analyzed.The experimental and simulation results show that the temperature measurement results are sensitive to the Rayleigh scattering light intensity,and the half height and full width of the Rayleigh scattering peak and the jitter of the laser have little effect.4.Set up the experimental system to calibrate the laser output center wavelength,the placement angle of the adjustable fiber collimator and the focal length of the adjustable fiber collimator.The measurement environment of seawater is simulated under laboratory conditions,and the ocean temperature experiment based on Brillouin scattering is carried out.The experimental results are analyzed,and the experimental improvement method is put forward.The research work of this paper is from the basic theoretical analysis,to the proposed algorithm for simulation,and then to the final experimental verification.An inversion algorithm of seawater parameters measurement based on Brillouin scattering is proposed theoretically.The output center wavelength of the laser,the absorption temperature of iodine molecular cell and the experimental parameters are selected from the experiment.The temperature normalization curve of edge detection technology based on iodine molecular cell is obtained experimentally.The research methods and conclusions obtained in this paper not only provide strong support for the seawater parameter measurement theory based on Brillouin scattering,but also lay a certain foundation for the practical measurement and application of this technology.