Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 41875034, 41627807)
Received Date:27 January 2021
Accepted Date:16 July 2021
Available Online:16 August 2021
Published Online:20 November 2021
Abstract:Salinity is an important physical parameter in oceanography. The change of seawater salinity is closely related to the change of marine environment and climate. Investigation of seawater salinity is of great significance for marine biology, climate simulation, weather forecast and hurricane path prediction. At present, in the research of seawater salinity detection based on Raman scattering, the influence of temperature change is ignored, which will cause inaccurate detection results. In order to achieve high-precision detection of seawater salinity, in this paper, a method of combining the precision salinity inversion with ocean Brillouin scattering is proposed. According to the influence of temperature and salinity on Raman scattering spectra, the functional relationship between them is established. Raman scattering spectra and Brillouin frequency shift are used to implement the inversion seawater salinity. The Brillouin frequency shift cannot be obtained directly by the lidar remote sensing method. It can only detect the energy of the echo signal through edge detection, and the photon correlation spectroscopy technology is used to detect the spectra width. The Brillouin frequency shift can be calculated by the energy and spectral width of the echo signal. Therefore, the accurate inversion of seawater salinity can be realized by detecting Raman spectra, Brillouin spectra width and energy signal. The experimental results of Raman spectroscopy are used to verify the established functional relationship, and the inversion error of seawater salinity is less than 0.47‰. In the experiment, the influence of seawater temperature control accuracy of ±0.2 ℃ and the detection results of Brillouin spectrum width and energy are analyzed. Through using the error in measurement result of each parameter, the salinity inversion error caused by them is analyzed. Using the Raman spectrum and Brillouin frequency shift, the problem of the accurate inversion of seawater salinity is solved, and the influence of temperature change on salinity inversion is eliminated. This research provides reliable data support for improving the marine environment, early warning of marine disaster and marine meteorological forecast accuracy, and has important research value and significant social benefits. This method also provides a feasible solution for ocean detection lidar used to detect seawater salinity. Keywords:salinity/ Raman scattering/ Brillouin scattering/ lidar
式中, d是常数, λE是激发光波长, λR是散射光波长, ai, Δvi和σi是振动Raman频率重分配参数, 数值可参照文献[22]. 利用(6)式, 计算获得不同温度下纯水的Raman散射光谱如图2所示. 从图2可以看出, 水分子的Raman散射光谱范围在3000—3800 cm–1. 图 2 不同温度下纯水的拉曼散射光谱 Figure2. Raman scattering spectra of pure water at different temperatures.
表1恒定盐度下, 不同温度Brillouin线宽、能量和频移计算结果 Table1.Calculation results of Brillouin spectrum width, energy and frequency shift at different temperatures under constant salinity.
海水盐度反演的误差计算, 需要获得Raman散射光谱低、高频面积比的对数值ln(SHB/SNHB)和海水Brillouin频移的误差. 其中Brillouin频移的误差结果如图7所示, 拟合关系计算出的频移与理论计算结果之间的误差为 ± 0.8 MHz. 图 7 Brillouin频移拟合关系vB (I, ΓB)与理论计算结果误差 Figure7. Difference between fitted Brillouin frequency shift vB (I, ΓB) and theoretical value.
Raman散射光谱低、高频面积比的对数值ln(SHB/SNHB)的误差计算结果如图8所示. 拟合函数关系计算结果与实验结果之间的误差为 ±0.0018. 最终根据Raman散射与Brillouin散射的误差计算结果, 利用(11)式计算海水盐度反演的误差为 ±0.47 ‰. 图 8 Raman光谱低、高频面积比的拟合结果与实验结果之间的误差 Figure8. The error between the fitting results of low and high frequency area ratio of Raman spectra and the experimental results.
24.4.实验误差分析 -->
4.4.实验误差分析
海水盐度反演需要探测的参量包括: Raman散射光谱、Brillouin散射谱宽和Brillouin散射回波信号能量. 由于海水温度变化对探测结果有影响, 因此样品海水温度控制精度为 ± 0.2 ℃. 控温误差对Raman散射光谱的低、高频面积比的对数值ln(SHB/SNHB)造成的影响如图9所示, 温度误差对散射光谱低、高频面积比的对数值ln(SHB/SNHB)造成的误差小于0.001. 图 9 温度误差对Raman散射光谱低、高频面积比的对数值ln(SHB/SNHB)造成的影响 (a) 恒定盐度, 不同温度下对数面积比理论值与拟合值; (b) 温度误差导致对数面积的误差 Figure9. Effect of temperature error on the logarithmic value of the low and high frequency area ratio of Raman scattering spectra: (a) theoretical value and fitting value of log area ratio under constant salinity and different temperatures; (b) error of log area caused by temperature error.
实验中温度误差对Brillouin散射探测的影响结果如图10所示, 其中图10(a)和图10(b)表明在温度误差范围内, 对Brillouin谱宽探测结果造成的误差小于5 MHz; 图10(c)和图10(d)表明在温度误差范围内, 对Brillouin散射探测能量造成的误差小于0.0016. 图 10 温度误差对Brillouin散射探测造成的影响 (a) 恒定盐度, 不同温度下谱宽理论值与拟合值; (b) 温度误差导致谱宽探测误差; (c) 恒定盐度, 不同温度下探测能量理论值与拟合值; (d) 温度误差导致能量探测误差 Figure10. Effect of temperature error on Brillouin scattering detection: (a) Theoretical and fitting values of spectrum width at different temperatures under constant salinity; (b) temperature error leads to spectrum width detection error; (c) theoretical and fitting values of detection energy at different temperatures under constant salinity; (d) temperature error leads to energy detection error.
温度误差对Raman散射光谱、Brillouin散射谱宽和能量探测造成的影响, 会使得海水盐度反演产生误差. 其中Raman散射光谱探测结果得到的低、高频面积比的对数值ln(SHB/SNHB)的误差, 对盐度反演结果的影响如图11所示, 在低、高频面积比的对数值的误差范围内, 海水盐度反演误差小于0.91‰. 图 11 Raman光谱探测误差对盐度探测结果的影响 (a) 盐度反演结果理论值与拟合值; (b) 盐度反演误差结果 Figure11. Effect of Raman spectral detection errors for salinity detection results: (a) theoretical value and fitting value of salinity inversion results; (b) salinity inversion error results.
Brillouin散射探测的误差对海水盐度的反演结果造成的影响如图12所示. 其中图12(a)和图12(b)给出了Brillouin谱宽探测误差对盐度反演的影响, Brillouin谱宽的误差范围内, 海水盐度的反演误差小于0.82 ‰; 图12(c)和图12(d)给出了Brillouin探测能量误差对盐度反演结果的影响, 在探测能量误差范围内, 海水盐度反演误差小于1.34‰. 图 12 Brillouin散射探测结果对盐度反演的影响 (a) 谱宽改变时, 盐度反演结果理论值与拟合值; (b) 谱宽误差导致盐度反演结果误差; (c) 能量改变时, 盐度反演结果理论值与拟合值; (d) 能量误差导致盐度反演结果误差 Figure12. Influence of Brillouin scattering detection results on salinity inversion: (a) Theoretical value and fitting value of salinity inversion results when the spectral width changes; (b) error of spectral width leads to the error of salinity inversion results; (c) theoretical value and fitting value of salinity inversion results when the energy changes; (d) error of energy leads to the error of salinity inversion results.