Laser Measurement Of Underwater Temperature And Salinity Based On Range Gating

The temperature and salinity of seawater are basic parameters for the study of the physical and chemical processes of seawater.They are of great significance in areas such as aquaculture,environmental safety,and weather forecast.Efficient and accurate measurement methods of temperature and salinity are one of the important guarantees for marine development strategies in the new era in China.Noncontact seawater temperature and salinity remote sensing technology based on 532 nm laser Raman scattering can quickly and widely measure seawater temperature and salinity because its small underwater attenuation coefficient.It has the advantages of simple operation,perfect structure,stable performance,etc.Therefore,it has become a research hotspot in recent years.Backscattering is one of the main interference factors in underwater detection,which affects the detection accuracy and the further improvement of the detection depth.Distance gating is one of the main methods to suppress the backscattering of seawater and also a necessary method to measure water temperature and salt at different depths.The underwater Raman detection system based on the distance gating will effectively improve the salinity and temperature detection capability of seawater.This paper has studied the basic principles of temperature and salinity measurement by Raman spectroscopy,and the modeling methods of the spectral characteristics and temperature salinity of water are analyzed.In addition,the effect of temperature salinity on the eigenvalues of Raman spectrum of water is studied theoretically,and the Raman spectrum is summarized in this paper.It provides a good theoretical basis for the detection of seawater temperature salinity.An underwater temperature and salinity measurement system based on distance gating was designed and built,and a software system for real-time acquisition and data processing of Raman spectrum based on LabVIEW software platform was written.The spectral processing methods such as space-time accumulation algorithm,noise processing,normalization processing,displacement calibration,and laser attenuation calibration effectively improve the sensitivity and signal-to-noise ratio of the detection system.Secondly,30 W high frequency laser was compared with 100 mJ high energy narrow pulse laser on the detection ability.The water Raman scattering spectra of different temperature salinities in short tanks were obtained.In addition,the linear relationship between the spectral characteristics such as ratios of positive and negative area ratios and logarithmic area ratios and temperature salinity of seawater was established.And using the obtained linear relationship to measure the temperature and salinity of the solution real time and continuously.The experiment has verified the temperature salinity measurement accuracy and stability of the system.Experiments show that the narrow-pulse lasers have good experimental results fitted with temperature salinity.The real-time temperature error is about ±1°C,and the salt error is about ±1.5‰.Finally,the underwater temperature measurement was carried out with this detection system.Realtime Raman shift spectra from 10 meters to 15.2 meters were acquired.Raman spectra data with different depths were obtained,considering that the attenuation coefficients of lasers with different wavelengths under water are different,the Raman spectrum was calibrated by applying the light attenuation coefficient,and the temperature at different distances under water was measured by the above determined temperature measurement method.At the same time,the system has the advantages of high degree of integration,simple structure,good stability,and high measurement speed.