摘要：
At present, underwater temperature measurement of seawater is a hot research topic, because knowledge about seawater temperature is of great importance in many fields. The laser Raman spectroscopy is a feasible method for measuring the vertical profiling of seawater temperature in large water areas. However, the real-time remote sensing of underwater temperature has not been reported. In this paper, a low-cost and practical Raman Lidar seawater temperature remote sensing system is constructed, and a real-time spectra acquisition and temperature determining software system is developed. Firstly, a background subtraction algorithm which combines the spatial accumulation of the array CCD with the exposure time integral is used to effectively enhance the signal to noise ratio of Raman spectra and improve the detection sensitivity of this Raman Lidar system. Usually the Raman spectra measured on-site are in low signal to noise ratio and baseline drift conditions. In this case, the "area ratio" (i. e. the ratio of the integrated Raman spectrum at low wavelength to the integrated Raman spectrum at higher wavelength) is a good temperature indicator. In this paper, we comprehensively studied the influence of Raman spectra area ratios split positions and fitting methods on the temperature measurement accuracy. More than 500 groups normalized Raman spectra at different temperatures are experimentally measured in the process of water temperature rising continuously. The area ratio SHB/SNHB and SNHB/SHB are used as the spectra characteristics to relate with the water temperature respectively, and both linear and second-order polynomial fitting algorithm are analyzed. The results show that the split positions have a great influence on area ratio variation range, and the fitting order has a great influence on the accuracy of fitting relationship between area ratio and seawater temperature. Both of them will eventually affect the water temperature measurement error. In order to objectively and directly reflect the influences of different area ratio methods, split position and fitting order on the water temperature measurement error, we further analyze the temperature measurement error at different conditions. The results show that the temperature measurement error is less affected by the split position, while is greatly influenced by the area ratio method and the fitting order. For the same split position and the same area ratio method, the results using order polynomial fitting are better than that using linear fitting. The results also show that linear fitting thearea ratios SHB/SNHB with water temperatures is a good choice, because it can obtain good measurement accuracy, and at the same time it has the advantage that the fitting parameters are simple and easy to be adjusted. Furthermore, the influences of different area ratio method and split position on the anti-interference of the system are studied. The results show that the anti-interference of SHB/SNHB method reduces with the decrease of the split wavelength, while the anti-interference of SNHB/SHB method enhances with the decrease of the split wavelength. The research results are used to inform the parameter setting of water temperature determining method, and improve Raman Lidar system temperature measurement accuracy. Considering all these results above, we choose the large wavelength 649.3 nm as the split location to calculate the Raman spectra area ratios SHB/SNHB, and linear fitting them with the water temperatures. Finally, the continuous temperature measuring performance of this Raman Lidar seawater temperature remote sensing system is verified experimentally. The experiment results show that the temperatures measured by Raman Lidar system are in good agreement with that by synchronous temperature sensor which is dipped in the sample tap-water and connected to the computer. The maximum measurement error is about ±0.5 °C, and the standard deviation of measurement error is about 0.21 °C.%海水水下温度测量对研究海洋环境和气候监测及自然灾害的早期预报等十分重要.利用蓝绿激光在海水中良好的透射性, 拉曼光谱技术可应用于大面积海水水下温度的快速遥感监测.但目前能够实现现场连续水温监测功能的实用化拉曼光谱水温遥测系统还尚未见报道.研制了实用化低成本的拉曼光谱水下温度遥测系统, 开发了光谱实时采集和数据处理软件.数据处理中结合了面阵CCD的空间累加与时间积分及本底扣除算法, 有效增强了拉曼光谱的信噪比和光谱系统的探测灵敏度.为了提高实际测温精度, 以短波段拉曼谱的面积 (SHB) 与长波段拉曼谱的面积 (SNHB) 之比作为光谱信标与水温建立关系, 研究了测温精度与光谱积分范围和拟合阶次的关系.实验测量了五百多组不同水温的拉曼光谱, 分别选用比值SHB/SNHB和SNHB/SHB与水温进行线性拟合和二阶多项式拟合.研究结果显示, 分界波长对面积比值变化范围影响很大, 而拟合阶次对面积比与温度的拟合关系的准确度影响很大, 两者最终都影响水温测量误差.为了更客观地反应不同面积比法、分界波长和拟合阶次对水温测量误差的影响, 分析了温度测量误差与不同分界波长的关系.结果显示, 温度测量误差受分界波长影响较小, 受面积比法和拟合阶次影响较大;相同情况下2阶多项式拟合结果优于相应的线性拟合结果;而采用比值SHB/SNHB与水温进行线性拟合时测温精度较高, 且拟合参数易于调整.进一步研究了不同面积比方法和分界波长对系统抗干扰性能的影响.研究结果显示, 比值SHB/SNHB法抗干扰能力随分界波长减小而减小, 而比值SNHB/SHB法抗干扰能力随分界波长减小而增大.上述研究结果提高了温度反演算法参数设置的合理性和拉曼散射系统测温精度及系统抗干扰能力.综合考虑上述研究结果, 数据处理中设定649.3 nm作为分界波长计算拉曼光谱面积比SHB/SNHB与水温进行线性拟合.最后通过实验检验了拉曼光谱水下温度遥测系统的连续实时测温能力和测温精度.结果显示, 拉曼光谱系统测温值与高精度同步温度传感器测量温度一致, 最大测温误差为±0.5°C, 测温误差的标准差约0.21°C.
