The emission spectral lines of underwater laser-induced plasma (LIP) are often influenced by a strong self-absorption effect. It is found that the long-short double-pulse laser-induced breakdown spectroscopy (LS-DP-LIBS) is able to generate the Fraunhofer-type signal from the submerged solid samples. In this work, the manganese concentrations in submerged steel samples are quantitatively analyzed using the Fraunhofer-type signal. We firstly show the Fraunhofer-type signal generated by LS-DP-LIBS. The parameters of the Fraunhofer-type absorption lines are briefly presented and discussed. Then, the Fraunhofer-type line at Mn I 403.307 nm is used to calculate the optical density (OD), which is a parameter for evaluating plasma absorbance. A calibration curve is established using OD values and certified manganese concentrations of ten samples. The results suggest that there is a linear relationship between OD values and Mn concentrations. Therefore, we have further conducted two prediction tests based on the measurement results of ten samples. The results show that the relative error of prediction is less than 10%, which demonstrates that the Fraunhofer-type absorption lines are worthy of study in the quantitative measurement of submerged solid samples.

水下激光诱导等离子体（LIP）的发射光谱线通常受到强烈的自吸收效应的影响。发现长短双脉冲激光诱导击穿光谱法（LS-DP-LIBS）能够从浸没的固体样品中产生Fraunhofer型信号。在这项工作中，使用Fraunhofer型信号对淹没钢样品中的锰浓度进行了定量分析。我们首先显示由LS-DP-LIBS产生的Fraunhofer型信号。简要介绍和讨论了Fraunhofer型吸收线的参数。然后，使用Mn I 403.307 nm处的Fraunhofer型谱线来计算光密度（OD），该光密度是评估血浆吸收率的参数。使用OD值和十个样品的认证锰浓度建立校准曲线。结果表明，OD值和Mn浓度之间存在线性关系。因此，我们根据十个样品的测量结果进一步进行了两个预测测试。结果表明，预测的相对误差小于10％，这表明Fraunhofer型吸收谱线在定量测量淹没固体样品中值得研究。