Laser-produced plasmas of dual-pulse fiber-optic laser-induced breakdown spectroscopy with different target positions relative to the waist region of laser focusing are studied using fast imaging and optical emission spectroscopy (OES). The laser energy of the two laser pulses is both maintained at around 18 mJ (i.e. the total energy is 36 mJ). The inter-pulse delay is kept at 250 ns. Ten spot sizes changing from 947 to 543 μm are obtained by precisely adjusting the distance between the focusing lens and the target surface. The profile of laser beam output from fiber shows a distinct top-hat shape. When approaching the dual-pulse waist region, the self-absorption and self-reversal of matrix iron lines gradually become intense while the plasma emission is enhanced, but the signal-to-noise ratio of minor elements gradually decreases. Under a similar spot size, the emission intensity with the target surface behind the waist region is weaker than that in front of the waist region and also with greater jitters. The target surface position is optimized to deviate from the waist region by ~ 1.3–1.6 mm towards the focusing lens for improving SNR of minor elements, corresponding to the lens-to-sample distances of 11.8–12.1 mm. Plasma morphology has undergone a transformation from stream- to umbrella-like structure using the recorded Intensified Charge-Coupled Device (ICCD) images. The expansion distance of the plasma front is increased from 1.07 to 1.28 mm, and the plasma volume is increased from 0.59 to 1.60 mm³. Besides, by utilization of OES, the maximum variation of plasma temperature and line broadening width rise to 2702 K from 1630 and to 0.0492 from 0.0316 nm along the vertical direction. The significant increase of optical thickness and nonuniformity of plasma temperature and density is the main reason for the intensification of self-absorption.

使用快速成像和光发射光谱法（OES）研究了双脉冲光纤激光诱导击穿光谱的激光产生等离子体，其相对于激光聚焦的腰部区域的目标位置不同。两个激光脉冲的激光能量都保持在18 mJ左右（即总能量为36 mJ）。脉冲间延迟保持在250 ns。通过精确调整聚焦透镜与目标表面之间的距离，可以获得从947到543μm的十个光斑尺寸。从光纤输出的激光束轮廓显示出明显的礼帽形状。当接近双脉冲腰部区域时，基体铁线的自吸收和自反转逐渐增强，同时等离子体发射增强，但微量元素的信噪比逐渐降低。在类似的光斑尺寸下，目标表面在腰部区域后面的发射强度要比腰部区域前面的发射强度弱，并且具有更大的抖动。目标表面位置经过优化，可从腰部区域向聚焦透镜偏离约1.3-1.6 mm，以改善次要元素的SNR，对应于透镜到样品的距离为11.8-12.1 mm。使用已记录的增强电荷耦合器件（ICCD）图像，等离子体形态已从流状转变为伞状结构。等离子前沿的扩展距离从1.07毫米增加到1.28毫米，等离子体积从0.59毫米增加到1.60毫米 目标表面位置经过优化，可从腰部区域向聚焦透镜偏离约1.3-1.6 mm，以改善次要元素的SNR，对应于透镜到样品的距离为11.8-12.1 mm。使用已记录的增强电荷耦合器件（ICCD）图像，等离子体形态已从流状转变为伞状结构。等离子前沿的扩展距离从1.07毫米增加到1.28毫米，等离子体积从0.59毫米增加到1.60毫米 目标表面位置经过优化，可从腰部区域向聚焦透镜偏离约1.3-1.6 mm，以改善次要元素的SNR，对应于透镜到样品的距离为11.8-12.1 mm。使用已记录的增强电荷耦合器件（ICCD）图像，等离子体形态已从流状转变为伞状结构。等离子前沿的扩展距离从1.07毫米增加到1.28毫米，等离子体积从0.59毫米增加到1.60毫米 使用已记录的增强电荷耦合器件（ICCD）图像，等离子体形态已从流状转变为伞状结构。等离子前端的扩展距离从1.07毫米增加到1.28毫米，等离子体积从0.59毫米增加到1.60毫米 使用已记录的增强电荷耦合器件（ICCD）图像，等离子体形态已从流状转变为伞状结构。等离子前沿的扩展距离从1.07毫米增加到1.28毫米，等离子体积从0.59毫米增加到1.60毫米³。此外，通过利用OES，沿垂直方向，等离子体温度和线展宽的最大变化从1630年增加到2702K，从0.0316nm增加到0.0492。光学厚度的显着增加以及等离子体温度和密度的不均匀性是增强自吸收的主要原因。