水下双脉冲LIBS光谱信号增强的实验研究

Investigation on the Signal Enhancement of DP-LIBS in Bulk Water

摘要: 为了增强水下LIBS探测的光谱信号强度，实验中使用两台波长为1064 nm的Nd:YAG调Q激光器，对NaCl溶液样品开展了水下正交双脉冲LIBS实验。首先在100 ns~10,000 ns的延时范围内探究了光谱的时间演化规律，实验结果表明，当采集延时在500 ns~700 ns的范围内时谱线的信噪比很高，有利于进行光谱采集。接着探究了脉冲间延时与激光能量对光谱信号增强的影响，得到了优化的实验参数。最后在优化的实验条件下，观察到双脉冲LIBS的光谱噪声水平下降，谱线展宽变窄，光谱信号持续时间更长(大于10 μs)，且两条Na原子谱线(588.9 nm和589.6 nm)的信号强度分别比单脉冲LIBS的增强了19倍和15倍。同时还发现光谱信号强度的相对标准偏差达到2.4%，远小于此前的单脉冲实验以及在液体表面进行的双脉冲实验，这说明采用双脉冲技术能有效提升水下LIBS测量的信号强度与稳定性。

Abstract: In order to enhance the spectral intensity of underwater LIBS, two Q-switched Nd:YAG lasers operating at 1064 nm were used and combined in orthogonal beam geometry to carry out the DP-LIBS experiment in sodium chloride solution. Firstly, the temporal evolution of the spectra has been studied in the time range of 100 - 10,000 ns. The result showed that the signal-to-noise ratios are very high when the acquisition delay is within the range of 500 - 700 ns, and it is very suitable for spectral acquisition. Additionally, the effects of inter-pulse delay and laser energy on the signal enhancement were carefully investigated, and the optimized experimental parameters were obtained. Finally, under the optimized experimental conditions, we observed the narrower DP-LIBS spectral line with lower noise level, the plasma emission lasted longer than 10 μs, and the spectral line intensity of Na 588.9 nm and Na 589.6 nm enhanced 19 and 15 times respectively, compared to that of single-pulse laser induced breakdown spectroscopy. At the same time, we also found that the relative standard deviation of spectral intensity reached 2.4%, which was less than that of the previous single-pulse experiment and the double-pulse experiment on the liquid surface. All of these indicate that using DP-LIBS can effectively improve the signal intensity and repeatability of underwater LIBS detection.