技术领域Technical Field
本发明属于水下激光光谱分析技术领域，尤其涉及一种水下激光诱导击穿光谱信号增强方法及系统。The invention belongs to the technical field of underwater laser spectrum analysis, and in particular relates to an underwater laser induced breakdown spectrum signal enhancement method and system.
背景技术Background Art
稀土元素被誉为“工业维生素”和“工业味精”，已成为保障全球经济发展和社会进步的关键矿产资源。由于其独特的物理和化学性质，稀土元素及其化合物在冶金、石油、农业和高科技产业等领域得到了广泛应用。这些元素的主要来源是富含稀土的陆地矿石以及深海中富含稀土的沉积物，即深海富稀土泥。当前，全球中重稀土资源供应紧缺，需求不断增加，迫切需要寻找新的中重稀土资源。而富含稀土元素的深海沉积物的发现为稀土产业带来了新的机遇。目前，诸如电感耦合等离子体质谱法（ICP-MS）、电感耦合等离子体发射光谱法（ICP-OES）、中子活化分析（NAA）和X射线荧光（XRF）等多种技术已被广泛应用于稀土元素的检测分析。然而，这些方法存在需要复杂的样品预处理、无法精确测定复杂样品中的稀土元素、无法多元素同时探测以及无法原位探测等缺点。因此，有必要探索一种化学分析技术，该技术相比于传统分析技术而言具有更快的速度，能够多元素同时探测，并能够进行水下环境中稀土元素的原位探测分析。Rare earth elements, known as "industrial vitamins" and "industrial MSG", have become a key mineral resource to ensure global economic development and social progress. Due to their unique physical and chemical properties, rare earth elements and their compounds have been widely used in metallurgy, petroleum, agriculture and high-tech industries. The main sources of these elements are rare earth-rich terrestrial ores and rare earth-rich sediments in the deep sea, namely deep-sea rare earth-rich mud. At present, the global supply of medium and heavy rare earth resources is in short supply and the demand is increasing. There is an urgent need to find new medium and heavy rare earth resources. The discovery of deep-sea sediments rich in rare earth elements has brought new opportunities to the rare earth industry. At present, a variety of technologies such as inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry (ICP-OES), neutron activation analysis (NAA) and X-ray fluorescence (XRF) have been widely used in the detection and analysis of rare earth elements. However, these methods have the disadvantages of requiring complex sample pretreatment, being unable to accurately determine rare earth elements in complex samples, being unable to detect multiple elements simultaneously, and being unable to detect in situ. Therefore, it is necessary to explore a chemical analysis technology that is faster than traditional analysis technology, can detect multiple elements simultaneously, and can perform in-situ detection and analysis of rare earth elements in underwater environments.
激光诱导击穿光谱（Laser-Induced Breakdown Spectroscopy，LIBS）是一种基于高功率激光与物质相互作用产生瞬态等离子体的光谱技术，通过研究等离子体的发射光谱可以对元素进行定性和定量分析。与目前常规的元素分析手段如ICP-OES、ICP-MS或XRF相比，LIBS技术具有以下优点：（1）无需对样品进行复杂的预处理，能够实现多元素的同时探测；（2）适用于气态、液体、固态样品的分析，理论上可以探测所有元素；（3）设备及操作简单，检测速度快，可以用于在线或原位探测分析。在水下环境中，LIBS技术已经广泛应用于分析液体中的多种元素，然而，对于溶液中稀土元素的信号增强检测方法研究，利用LIBS技术的报道非常有限。传统的单脉冲LIBS（SP-LIBS）技术对稀土元素进行水下探测依然面临着诸多困难和挑战，稀土元素水下LIBS信号较弱，信号质量不佳，脉冲与脉冲之间的重复性较差，检测灵敏度较低。Laser-induced breakdown spectroscopy (LIBS) is a spectral technique based on the interaction of high-power laser and matter to generate transient plasma. By studying the emission spectrum of plasma, the elements can be qualitatively and quantitatively analyzed. Compared with the current conventional elemental analysis methods such as ICP-OES, ICP-MS or XRF, LIBS technology has the following advantages: (1) no complicated sample pretreatment is required, and multiple elements can be detected simultaneously; (2) it is suitable for the analysis of gaseous, liquid and solid samples, and theoretically all elements can be detected; (3) the equipment and operation are simple, the detection speed is fast, and it can be used for online or in-situ detection and analysis. In underwater environments, LIBS technology has been widely used to analyze a variety of elements in liquids. However, there are very limited reports on the use of LIBS technology for the signal enhancement detection method of rare earth elements in solutions. The traditional single-pulse LIBS (SP-LIBS) technology for underwater detection of rare earth elements still faces many difficulties and challenges. The underwater LIBS signal of rare earth elements is weak, the signal quality is poor, the repeatability between pulses is poor, and the detection sensitivity is low.
通过上述分析，现有技术存在的问题及缺陷为：Through the above analysis, the problems and defects of the prior art are as follows:
传统的单脉冲LIBS（SP-LIBS）技术对稀土元素进行水下探测依然面临着诸多困难和挑战，稀土元素水下LIBS信号较弱，信号质量不佳，脉冲与脉冲之间的重复性较差，检测灵敏度较低。Traditional single-pulse LIBS (SP-LIBS) technology still faces many difficulties and challenges in underwater detection of rare earth elements. The underwater LIBS signal of rare earth elements is weak, the signal quality is poor, the repeatability between pulses is poor, and the detection sensitivity is low.
水下LIBS击穿本身就存在诸多困难。由于水具有较高的热传导率，淬灭效应明显，水下等离子体温度低，电离度低、寿命短。此外，水下等离子体屏蔽效应明显，击穿过程容易受水中悬浮颗粒、溶存气泡等杂质的影响。以上问题导致水下LIBS信号较弱，脉冲与脉冲之间的重复性较差。There are many difficulties in underwater LIBS breakdown. Water has a high thermal conductivity and a significant quenching effect. The underwater plasma has a low temperature, low ionization degree, and short life. In addition, the underwater plasma has a significant shielding effect, and the breakdown process is easily affected by impurities such as suspended particles and dissolved bubbles in the water. The above problems lead to weak underwater LIBS signals and poor repeatability between pulses.
发明内容Summary of the invention
为克服相关技术中存在的问题，本发明公开实施例提供了一种水下激光诱导击穿光谱信号增强方法及系统，具体涉及一种水下激光诱导击穿光谱系统和水下稀土元素信号增强方法。本发明目的在于通过改进LIBS的激发方式来实现对水下稀土元素的信号增强。In order to overcome the problems existing in the related art, the embodiments disclosed in the present invention provide a method and system for underwater laser induced breakdown spectroscopy signal enhancement, and specifically relate to an underwater laser induced breakdown spectroscopy system and an underwater rare earth element signal enhancement method. The purpose of the present invention is to enhance the signal of underwater rare earth elements by improving the excitation mode of LIBS.
所述技术方案如下：一种水下激光诱导击穿光谱信号增强方法，利用非偏振平板分束镜对单束脉冲激光进行分束，使得分束后的两束1064nm激光束通过反射镜与聚焦透镜聚焦于样品的同一位置或者相近位置产生两个等离子体，改进水下LIBS技术激发方式完成稀土元素的信号增强；具体包括以下步骤：The technical scheme is as follows: an underwater laser induced breakdown spectroscopy signal enhancement method, which uses a non-polarizing plate beam splitter to split a single pulse laser beam, so that the two split 1064nm laser beams are focused on the same position or a close position of the sample through a reflector and a focusing lens to generate two plasmas, and improves the underwater LIBS technology excitation method to complete the signal enhancement of rare earth elements; specifically, the following steps are included:
S1，优化系统光路结构，利用非偏振平板分束镜对单束脉冲激光进行分束，使得分束后的两束1064nm激光束通过反射镜与聚焦透镜聚焦于样品的同一位置或者相近位置产生两个等离子体；S1, optimize the optical path structure of the system, use a non-polarizing plate beam splitter to split a single pulse laser beam, so that the two 1064nm laser beams after the splitting are focused on the same position or a close position of the sample through a reflector and a focusing lens to generate two plasmas;
S2，基于单光束分裂的水下LIBS在稀土溶液中产生的两个激光诱导气泡的冲击波对等离子体进行信号增强，发射出增强的光谱信号；S2, the shock waves of two laser-induced bubbles generated in the rare earth solution by underwater LIBS based on single beam splitting enhance the plasma signal and emit an enhanced spectral signal;
S3，利用增强的光谱信号对水下稀土元素进行检测限定量分析。S3, using enhanced spectral signals to perform detection-limited analysis of underwater rare earth elements.
在步骤S2中，基于单光束分裂的水下LIBS在稀土溶液中产生的两个激光诱导气泡的冲击波对等离子体进行信号增强中，冲击波经过时压缩等离子体，使等离子体核心区凝聚，等离子体温度升高，发射出增强的光谱信号。In step S2, the shock waves of two laser-induced bubbles generated in the rare earth solution by underwater LIBS based on single beam splitting perform signal enhancement on the plasma. The shock waves compress the plasma when passing through, causing the plasma core region to condense, the plasma temperature to increase, and an enhanced spectral signal to be emitted.
本发明的另一目的在于提供一种水下激光诱导击穿光谱信号增强系统，该系统实施所述的水下激光诱导击穿光谱信号增强方法，该系统包括：Another object of the present invention is to provide an underwater laser induced breakdown spectroscopy signal enhancement system, which implements the underwater laser induced breakdown spectroscopy signal enhancement method, and the system comprises:
第一脉冲激光器，为调Q式Nd:YAG激光器，用于产生等离子体的脉冲激光，激发波长为1064nm；在出光口安装反射镜，将脉冲激光引入激光衰减系统及后续的扩束准直光路中；The first pulse laser is a Q-switched Nd:YAG laser used to generate pulsed lasers for plasma, with an excitation wavelength of 1064nm. A reflector is installed at the light outlet to introduce the pulsed laser into the laser attenuation system and the subsequent beam expansion and collimation optical path.
第二脉冲激光器，用于提供照明光源，在拍摄激光诱导气泡时负责照亮CCD拍摄范围；The second pulse laser is used to provide an illumination light source and is responsible for illuminating the CCD shooting range when shooting laser-induced bubbles;
激光衰减系统，由零级二分之一波片和偏振棱镜组成，用于调节激光能量，以满足探测不同浓度稀土溶液和不同类型光谱时的能量需求；The laser attenuation system, which consists of a zero-order half-wave plate and a polarizing prism, is used to adjust the laser energy to meet the energy requirements when detecting rare earth solutions of different concentrations and different types of spectra;
扩束准直光路，由一个凹透镜和一个凸透镜组成，位于激光衰减系统和非偏振平板分束镜之间，通过调节扩束准直光路中两个透镜间的距离，改变激光聚焦于水槽溶液中的位置，并且改变聚焦角度；The beam expansion and collimation optical path is composed of a concave lens and a convex lens, and is located between the laser attenuation system and the non-polarizing plate beam splitter. By adjusting the distance between the two lenses in the beam expansion and collimation optical path, the position where the laser is focused in the water tank solution is changed, and the focusing angle is changed;
激光扩束后由非偏振平板分束镜进行分束，分为透射激光束与反射激光束，反射激光束由一系列反射镜与聚焦透镜聚焦于水槽中产生等离子体，透射激光束直接由聚焦条件聚焦产生等离子体，反射激光束产生的等离子体和透射激光束产生的等离子体产生在相同或相近位置，产生的两个激光诱导气泡的冲击波对两个等离子体发生相互作用；After the laser beam is expanded, it is split by a non-polarizing plate beam splitter into a transmitted laser beam and a reflected laser beam. The reflected laser beam is focused in a water tank by a series of reflectors and a focusing lens to generate plasma. The transmitted laser beam is directly focused by a focusing condition to generate plasma. The plasma generated by the reflected laser beam and the plasma generated by the transmitted laser beam are generated at the same or similar position. The shock waves of the two laser-induced bubbles generated interact with the two plasmas.
聚焦透镜安装于位移平台，调整透射激光束与反射激光束的激光相对焦点位置；The focusing lens is installed on the displacement platform to adjust the relative focal positions of the transmitted laser beam and the reflected laser beam;
样品平台为二维移动平台，上面放置水槽；分束激光通过衰减、扩束、反射、聚焦后，经聚焦透镜汇聚于水槽内的稀土溶液中；The sample platform is a two-dimensional moving platform with a water tank placed on it; after attenuation, beam expansion, reflection and focusing, the split laser is converged into the rare earth solution in the water tank through a focusing lens;
光谱信号接收系统包括两个收集透镜和光纤探头，位于样品平台的侧向，通过一根光纤连接光谱探测系统的光谱仪，光谱仪和ICCD受控于计算机控制系统；光谱探测系统由光谱仪和ICCD组成；等离子体和激光诱导气泡拍摄采集系统由ICCD、CCD组成，同时受控于计算机控制系统；时序控制装置，数字延时脉冲分别控制第一脉冲激光器以及第二脉冲激光器的Flash和Q-Switch，以及ICCD的延时采集。数字信号延时发生器，作为连接第一脉冲激光器和第二脉冲激光器与光谱仪的桥梁，通过连接，由数字信号延时发生器改变探测延时，同时连接控制第一脉冲激光器以及第二脉冲激光器并且控制第一脉冲激光器以及第二脉冲激光器的触发与Q开关，连接光谱仪。The spectral signal receiving system includes two collecting lenses and a fiber optic probe, which are located on the side of the sample platform and connected to the spectrometer of the spectral detection system through an optical fiber. The spectrometer and ICCD are controlled by a computer control system. The spectral detection system is composed of a spectrometer and an ICCD. The plasma and laser-induced bubble shooting and acquisition system is composed of an ICCD and a CCD, and is also controlled by a computer control system. The timing control device and the digital delay pulse respectively control the Flash and Q-Switch of the first pulse laser and the second pulse laser, as well as the delayed acquisition of the ICCD. The digital signal delay generator, as a bridge connecting the first pulse laser and the second pulse laser with the spectrometer, changes the detection delay through the connection, and simultaneously connects and controls the first pulse laser and the second pulse laser and controls the triggering and Q-switch of the first pulse laser and the second pulse laser, and connects to the spectrometer.
进一步，光谱仪连接计算机控制系统采集光谱数据；ICCD通过连接第一脉冲激光器、第二脉冲激光器、计算机控制系统，拍摄等离子体图像。Furthermore, the spectrometer is connected to a computer control system to collect spectrum data; and the ICCD takes plasma images by connecting to the first pulse laser, the second pulse laser, and the computer control system.
进一步，所述计算机控制系统分别连接CCD、ICCD和光谱探测系统，采集水下稀土元素LIBS光谱，拍摄等离子体及激光诱导气泡对的图像，通过数据处理分析验证信号增强效果以及规律；光谱数据由计算机控制系统采集为excel文件，通过Matlab预处理光谱数据，随后采用origin绘制光谱图像；等离子体图像由Matlab处理平均图像导出，激光诱导气泡图像由Matlab裁剪，而后通过整理展示数据。Furthermore, the computer control system is respectively connected to the CCD, ICCD and spectral detection system to collect underwater rare earth element LIBS spectra, take images of plasma and laser-induced bubble pairs, and verify the signal enhancement effect and rules through data processing and analysis; the spectral data is collected by the computer control system as an excel file, the spectral data is preprocessed by Matlab, and then the spectral image is drawn using origin; the plasma image is exported by Matlab processing the average image, the laser-induced bubble image is cropped by Matlab, and then the data is displayed by sorting.
进一步，在激光衰减系统中，二分之一波片用于旋转线偏振光的偏振方向；空气隙零级波片中两片石英波片中间由垫片隔开形成空气隙；偏振棱镜利用晶体的双折射现象而制成的偏振器件；激光先后经过波片与偏振棱镜，通过旋转零级二分之一波片，调节入射激光的能量大小变化。Furthermore, in the laser attenuation system, the half wave plate is used to rotate the polarization direction of linear polarized light; the air gap is formed by separating two quartz wave plates in the air gap zero-order wave plate by a gasket; the polarization prism is a polarization device made by utilizing the birefringence phenomenon of crystals; the laser passes through the wave plate and the polarization prism successively, and the energy change of the incident laser is adjusted by rotating the zero-order half wave plate.
进一步，在扩束准直光路中，凹透镜和凸透镜间的距离通过旋转扩束准直光路中扩束镜的旋钮实现。Furthermore, in the beam expanding and collimating optical path, the distance between the concave lens and the convex lens is achieved by rotating the knob of the beam expander in the beam expanding and collimating optical path.
进一步，在非偏振平板分束镜处更换其他分束镜，使激光分为两束，同时聚焦于样品溶液内部，产生LIBS光谱；聚焦透镜替换为不同焦距的透镜，实现聚焦方式与激发参数条件的双重优化。Furthermore, other beam splitters are replaced at the non-polarizing plate beam splitter to split the laser into two beams, which are simultaneously focused inside the sample solution to generate a LIBS spectrum; the focusing lens is replaced with lenses of different focal lengths to achieve dual optimization of focusing mode and excitation parameter conditions.
进一步，该系统用于对稀土元素Yb、Eu、Y水下LIBS光谱的分析。Furthermore, the system is used to analyze the underwater LIBS spectra of rare earth elements Yb, Eu, and Y.
结合上述的所有技术方案，本发明所具备的有益效果为：为了实现水下LIBS技术对稀土元素的信号增强以及定量分析能力方面的提高，并且通过改变较少的实验条件实现不同激发方式，本发明搭建了一套水下激光诱导击穿光谱信号增强系统。采用了一种简单、成本低的方案来提高水下稀土元素的LIBS信号强度，相比传统SP-LIBS技术，水下LIBS的信号质量得到了明显的改善。在信号增强方面，相比双脉冲LIBS技术，本方法搭建的水下LIBS系统更紧凑，成本更低，实验装置与操作简便；对于长脉冲LIBS技术，目前还没有商业化的长脉冲激光器，且长脉冲激光稳定性较差，激光能量较低，也不利于LIBS信号的稳定性，而本方法的系统采用商业化短脉冲激光器，能在最优激光能量条件下提高稀土元素水下LIBS信号的质量，提高检测灵敏度，实现信号增强。本发明在原有的水下SP-LIBS系统的基础上，完成了探测系统各模块的结构设计、整体光路的设计与搭建。利用非偏振平板分束镜对单束脉冲激光进行分束，使得分束后的两束1064nm激光束通过反射镜与聚焦透镜聚焦于样品的同一位置或者相近位置产生两个等离子体，改进水下LIBS技术激发方式实现稀土元素的信号增强。研究了分束激光相对焦点位置对稀土元素Yb、Eu、Y水下LIBS光谱的影响，以及对激光诱导气泡和等离子体的影响，总结了信号增强机理。通过本方法搭建了一套水下LIBS系统，可实现不同激发方式的水下LIBS技术对稀土元素的信号增强以及定量分析能力的提高，以最佳实验条件降低了稀土元素检测限（LOD）。Combined with all the above technical solutions, the beneficial effects of the present invention are as follows: in order to achieve the signal enhancement and quantitative analysis ability of rare earth elements by underwater LIBS technology, and to achieve different excitation modes by changing less experimental conditions, the present invention builds a set of underwater laser induced breakdown spectroscopy signal enhancement system. A simple and low-cost solution is adopted to improve the LIBS signal intensity of underwater rare earth elements. Compared with the traditional SP-LIBS technology, the signal quality of underwater LIBS is significantly improved. In terms of signal enhancement, compared with the double-pulse LIBS technology, the underwater LIBS system built by this method is more compact, lower cost, and simple experimental device and operation; for long-pulse LIBS technology, there is currently no commercial long-pulse laser, and the long-pulse laser has poor stability, low laser energy, and is not conducive to the stability of LIBS signals. The system of this method uses a commercial short-pulse laser, which can improve the quality of underwater LIBS signals of rare earth elements under optimal laser energy conditions, improve detection sensitivity, and achieve signal enhancement. On the basis of the original underwater SP-LIBS system, the present invention completes the structural design of each module of the detection system and the design and construction of the overall optical path. A single pulsed laser beam was split by a non-polarizing plate beam splitter, so that the two 1064nm laser beams after the splitting were focused on the same position or close position of the sample through a reflector and a focusing lens to generate two plasmas, and the excitation method of underwater LIBS technology was improved to achieve signal enhancement of rare earth elements. The influence of the relative focal position of the split laser on the underwater LIBS spectra of rare earth elements Yb, Eu, and Y, as well as the influence on laser-induced bubbles and plasma, was studied, and the signal enhancement mechanism was summarized. An underwater LIBS system was built by this method, which can realize the signal enhancement of rare earth elements and the improvement of quantitative analysis capabilities of underwater LIBS technology with different excitation methods, and reduce the detection limit (LOD) of rare earth elements under the optimal experimental conditions.
稀土元素被誉为“工业维生素”和“工业味精”，已成为保障全球经济发展和社会进步的重要矿产资源。目前，全球中重稀土资源需求日益增加，深海富稀土泥的发现为稀土产业提供了新的机遇，深海稀土资源总量是已知陆地稀土资源量的1000倍以上。所以，本发明发展稀土元素的水下原位探测技术的信号增强方法具有重要的战略和现实意义。目前尚未有课题组通过水下LIBS技术对水下稀土元素进行信号增强的系统性工作。本发明中水下LIBS技术对实现了对水下稀土元素信号增强，相比于信号增强中常见的双脉冲LIBS技术与长脉冲LIBS技术，本发明有着成本低、操作简便、激光能量控制区间合理等优势。Rare earth elements are known as "industrial vitamins" and "industrial MSG" and have become an important mineral resource to ensure global economic development and social progress. At present, the global demand for medium and heavy rare earth resources is increasing. The discovery of deep-sea rare earth-rich mud has provided new opportunities for the rare earth industry. The total amount of deep-sea rare earth resources is more than 1,000 times the known amount of terrestrial rare earth resources. Therefore, the signal enhancement method for underwater in-situ detection technology of rare earth elements developed in the present invention has important strategic and practical significance. At present, no research group has carried out systematic work on signal enhancement of underwater rare earth elements through underwater LIBS technology. The underwater LIBS technology in the present invention realizes the enhancement of underwater rare earth element signals. Compared with the common dual-pulse LIBS technology and long-pulse LIBS technology in signal enhancement, the present invention has the advantages of low cost, simple operation, and reasonable laser energy control range.
附图说明BRIEF DESCRIPTION OF THE DRAWINGS
此处的附图被并入说明书中并构成本说明书的一部分，示出了符合本公开的实施例，并与说明书一起用于解释本公开的原理；The accompanying drawings herein are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the description, serve to explain the principles of the present disclosure;
图1是本发明实施例提供的水下激光诱导击穿光谱信号增强方法图；FIG1 is a diagram of an underwater laser induced breakdown spectroscopy signal enhancement method provided by an embodiment of the present invention;
图2是本发明实施例提供的水下激光诱导击穿光谱信号增强系统示意图；FIG2 is a schematic diagram of an underwater laser induced breakdown spectroscopy signal enhancement system provided by an embodiment of the present invention;
图3是本发明实施例提供的水下LIBS光谱采集及等离子体图像采集实验系统图；FIG3 is a diagram of an underwater LIBS spectrum acquisition and plasma image acquisition experimental system provided by an embodiment of the present invention;
图4是本发明实施例提供的水下激光诱导气泡投影成像系统光路图；FIG4 is an optical path diagram of an underwater laser induced bubble projection imaging system provided by an embodiment of the present invention;
图5是本发明实施例提供的本发明采集的水下LIBS在30mJ条件下与SP-LIBS在30mJ、15mJ条件下的稀土溶液典型LIBS光谱对比图一；FIG5 is a first comparison diagram of typical LIBS spectra of rare earth solutions collected by the present invention under the conditions of 30 mJ by underwater LIBS and under the conditions of 30 mJ and 15 mJ by SP-LIBS provided in an embodiment of the present invention;
图6是本发明实施例提供的本发明采集的水下LIBS在30mJ条件下与SP-LIBS在30mJ、15mJ条件下的稀土溶液典型LIBS光谱对比图二；FIG6 is a second comparison diagram of typical LIBS spectra of rare earth solutions collected by the present invention under the condition of 30 mJ by underwater LIBS and under the condition of 30 mJ and 15 mJ by SP-LIBS provided in an embodiment of the present invention;
图7是本发明实施例提供的本发明采集的水下LIBS在30mJ条件下与SP-LIBS在30mJ、15mJ条件下的稀土溶液典型LIBS光谱对比图三；FIG7 is a third comparison diagram of typical LIBS spectra of rare earth solutions collected by the present invention under the condition of 30 mJ by underwater LIBS and under the condition of 30 mJ and 15 mJ by SP-LIBS provided in an embodiment of the present invention;
图8为不同激发方式条件下Yb I 398.79nm的光谱峰值强度随激光能量的变化图；FIG8 is a graph showing the variation of the spectral peak intensity of Yb I 398.79 nm with laser energy under different excitation conditions;
图9为不同激发方式条件下Eu I 459.40nm的光谱峰值强度随激光能量的变化图；FIG9 is a graph showing the variation of the spectral peak intensity of Eu I 459.40 nm with laser energy under different excitation conditions;
图10为不同激发方式条件下YO 616.51nm的光谱峰值强度随激光能量的变化图；FIG10 is a graph showing the variation of the spectral peak intensity of YO 616.51 nm with laser energy under different excitation conditions;
图11为不同激发方式条件下Yb I 398.79nm的光谱峰值强度随激光能量变化的信噪比变化图；FIG11 is a graph showing the signal-to-noise ratio of the spectral peak intensity of Yb I 398.79 nm as the laser energy changes under different excitation conditions;
图12为不同激发方式条件下Eu I 459.40nm的光谱峰值强度随激光能量变化的信噪比变化图；FIG12 is a graph showing the signal-to-noise ratio of the peak intensity of the spectrum of Eu I 459.40 nm as the laser energy changes under different excitation conditions;
图13为不同激发方式条件下YO 616.51 nm的光谱峰值强度随激光能量变化的信噪比变化图；FIG13 is a graph showing the signal-to-noise ratio of the spectral peak intensity of YO 616.51 nm as the laser energy changes under different excitation conditions;
图14为本发明在不同激发方式条件下Yb I 398.79nm的LIBS光谱RSD随激光能量的变化图；FIG14 is a graph showing the variation of RSD of the LIBS spectrum of Yb I 398.79 nm with laser energy under different excitation conditions of the present invention;
图15为本发明在不同激发方式条件下Eu I 459.40nm的LIBS光谱RSD随激光能量的变化图；FIG15 is a graph showing the variation of RSD of the LIBS spectrum of Eu I 459.40 nm with laser energy under different excitation conditions of the present invention;
图16为本发明在不同激发方式条件下YO 616.51nm的LIBS光谱RSD随激光能量的变化图；FIG16 is a graph showing the variation of RSD of the LIBS spectrum of YO 616.51 nm with laser energy under different excitation conditions of the present invention;
图17为本发明Yb I 398.79nm峰值强度随随x轴轴向的分布变化图；FIG17 is a graph showing the distribution of the peak intensity of Yb I 398.79 nm along the x-axis of the present invention;
图18为本发明Eu I 459.40nm峰值强度随随x轴轴向的分布变化图；FIG18 is a graph showing the distribution of the peak intensity of Eu I 459.40nm along the x-axis of the present invention;
图19为本发明YO 616.51nm峰值强度随随x轴轴向的分布变化图；FIG19 is a graph showing the distribution of the peak intensity of YO 616.51 nm along the x-axis of the present invention;
图20为本发明Yb I 398.79nm峰值强度随z轴轴向的分布变化图；FIG20 is a graph showing the distribution of the peak intensity of Yb I 398.79 nm along the z-axis of the present invention;
图21为本发明Eu I 459.40nm峰值强度随z轴轴向的分布变化图；FIG21 is a diagram showing the distribution of the peak intensity of Eu I 459.40nm along the z-axis of the present invention;
图22为本发明YO 616.51nm峰值强度随z轴轴向的分布变化图；FIG22 is a graph showing the distribution of the peak intensity of YO 616.51 nm along the z-axis of the present invention;
图23为本发明在x=-0.5mm处反射激光焦点位于透射激光焦点后方0.5mm的气泡演化图像；FIG23 is an image of bubble evolution in the present invention when the focus of the reflected laser is located 0.5 mm behind the focus of the transmitted laser at x=-0.5 mm;
图24为本发明在不同相对焦点位置条件下等离子体图像随延时的变化图；FIG24 is a graph showing the change of plasma image with time delay under different relative focus position conditions of the present invention;
图25为不同延时条件下等离子体辐射强度随x轴相对焦点位置的变化图；FIG25 is a graph showing the change of plasma radiation intensity with the relative focus position on the x-axis under different delay conditions;
图26为本发明Yb I 398.79nm的单变量定标曲线图；FIG26 is a univariate calibration curve diagram of Yb I 398.79 nm of the present invention;
图27为本发明Eu I 459.40nm的单变量定标曲线图；FIG27 is a univariate calibration curve diagram of Eu I 459.40 nm of the present invention;
图28为本发明YO 616.51 nm的单变量定标曲线图；FIG28 is a univariate calibration curve diagram of YO 616.51 nm of the present invention;
图中：1、第一脉冲激光器；2、第二脉冲激光器；3、激光衰减系统；4、扩束准直光路；5、样品平台；6、反射镜；7、聚焦透镜；8、位移平台；9、光谱信号接收系统；10、等离子体和激光诱导气泡拍摄采集系统；11、光谱探测系统；12、时序控制装置；13、计算机控制系统；14、非偏振平板分束镜；15、数字信号延时发生器。In the figure: 1. first pulse laser; 2. second pulse laser; 3. laser attenuation system; 4. beam expansion and collimation optical path; 5. sample platform; 6. reflector; 7. focusing lens; 8. displacement platform; 9. spectral signal receiving system; 10. plasma and laser-induced bubble shooting and collection system; 11. spectral detection system; 12. timing control device; 13. computer control system; 14. non-polarizing plate beam splitter; 15. digital signal delay generator.
具体实施方式DETAILED DESCRIPTION
为使本发明的上述目的、特征和优点能够更加明显易懂，下面结合附图对本发明的具体实施方式做详细的说明。在下面的描述中阐述了很多具体细节以便于充分理解本发明。但是本发明能够以很多不同于在此描述的其他方式来实施，本领域技术人员可以在不违背本发明内涵的情况下做类似改进，因此本发明不受下面公开的具体实施的限制。In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific implementation disclosed below.
本发明的创新点在于：本发明通过建立了一种水下LIBS稀土元素信号增强方法。搭建了一套水下LIBS系统，验证了系统对稀土元素的信号增强能力并总结了不同相对焦点位置的信号增强规律，通过研究激光诱导气泡及等离子体解释了信号增强的机理。相较于传统SP-LIBS，SBS-LIBS系统实现了信号增强1.5倍以上，光谱信号稳定性提高。The innovation of the present invention is that the present invention establishes a method for underwater LIBS rare earth element signal enhancement. An underwater LIBS system is built to verify the system's signal enhancement capability for rare earth elements and summarize the signal enhancement rules at different relative focal positions. The mechanism of signal enhancement is explained by studying laser-induced bubbles and plasma. Compared with traditional SP-LIBS, the SBS-LIBS system achieves a signal enhancement of more than 1.5 times and improved spectral signal stability.
本发明建立了一种水下LIBS稀土元素信号增强方法。将非偏振平板分束镜14应用于水下LIBS系统光路中探测水下稀土元素，用一个简便器件完成分束，改变激发参数条件实现了信号增强，光路结构简单。优化光谱信号质量，提高定量分析能力。The present invention establishes a method for enhancing the underwater LIBS rare earth element signal. A non-polarized plate beam splitter 14 is applied to the optical path of the underwater LIBS system to detect underwater rare earth elements, a simple device is used to complete the beam splitting, and the signal enhancement is achieved by changing the excitation parameter conditions. The optical path structure is simple, the spectral signal quality is optimized, and the quantitative analysis capability is improved.
本发明搭建了一套水下LIBS系统，验证了系统对稀土元素的信号增强能力并总结了不同相对焦点位置的信号增强规律，通过研究激光诱导气泡及等离子体解释了信号增强的机理。相较于传统SP-LIBS，水下LIBS系统实现了信号增强2倍，光谱信号稳定性提高。The present invention built an underwater LIBS system, verified the system's signal enhancement capability for rare earth elements, summarized the signal enhancement rules at different relative focal positions, and explained the mechanism of signal enhancement by studying laser-induced bubbles and plasma. Compared with traditional SP-LIBS, the underwater LIBS system achieved a 2-fold signal enhancement and improved spectral signal stability.
具体的，本发明公开了一种水下激光诱导击穿光谱信号增强方法。为了实现水下LIBS技术对稀土元素的信号增强以及定量分析能力的提高，并通过改变较少的实验条件实现不同激发方式，本发明搭建了一套水下激光诱导击穿光谱信号增强系统。采用了一种简单、成本低的方案来提高水下LIBS信号强度，相比传统SP-LIBS技术，水下LIBS的信号质量得到了明显的改善。在原有的水下SP-LIBS系统的基础上，完成了探测系统各模块的结构设计、整体光路的设计与搭建。利用非偏振平板分束镜14对单束脉冲激光进行分束，使得分束后的两束1064nm激光束通过反射镜6与聚焦透镜7聚焦于样品的同一位置或者相近位置产生两个等离子体，改进水下LIBS技术激发方式实现稀土元素的信号增强。分析了分束激光相对焦点位置对稀土元素Yb、Eu、Y水下LIBS光谱的影响，以及对激光诱导气泡和等离子体的影响，总结了信号增强机理。通过本方法搭建的系统可实现不同激发方式的水下LIBS技术对稀土元素的信号增强以及定量分析能力的提高，以最佳实验条件降低稀土元素检测限（LOD）。Specifically, the present invention discloses a method for enhancing underwater laser induced breakdown spectroscopy signals. In order to achieve the signal enhancement of rare earth elements and the improvement of quantitative analysis capabilities of underwater LIBS technology, and to achieve different excitation modes by changing less experimental conditions, the present invention builds a set of underwater laser induced breakdown spectroscopy signal enhancement system. A simple and low-cost solution is adopted to improve the underwater LIBS signal intensity. Compared with the traditional SP-LIBS technology, the signal quality of underwater LIBS is significantly improved. On the basis of the original underwater SP-LIBS system, the structural design of each module of the detection system and the design and construction of the overall optical path are completed. A single-beam pulsed laser is split by a non-polarizing plate beam splitter 14, so that the two beams of 1064nm laser beams after the splitting are focused on the same position or a similar position of the sample through a reflector 6 and a focusing lens 7 to generate two plasmas, and the excitation mode of the underwater LIBS technology is improved to achieve signal enhancement of rare earth elements. The influence of the relative focus position of the split laser on the underwater LIBS spectra of rare earth elements Yb, Eu, and Y, as well as the influence on laser-induced bubbles and plasma are analyzed, and the signal enhancement mechanism is summarized. The system constructed by this method can realize the signal enhancement and quantitative analysis capability of rare earth elements by underwater LIBS technology with different excitation modes, and reduce the detection limit (LOD) of rare earth elements under the optimal experimental conditions.
实施例1，如图1所示，本发明实施例提供一种水下激光诱导击穿光谱信号增强方法，包括：Embodiment 1, as shown in FIG1 , the embodiment of the present invention provides a method for enhancing underwater laser induced breakdown spectroscopy signals, comprising:
S1，优化系统光路结构，利用非偏振平板分束镜14对单束脉冲激光进行分束，使得分束后的两束1064nm激光束通过反射镜6与聚焦透镜7聚焦于样品的同一位置或者相近位置产生两个等离子体；S1, optimize the optical path structure of the system, use the non-polarization plate beam splitter 14 to split the single pulse laser beam, so that the two 1064nm laser beams after the splitting are focused on the same position or a close position of the sample through the reflector 6 and the focusing lens 7 to generate two plasmas;
一束1064nm脉冲激光经过分束镜分为两束，分别为透射激光束与反射激光束，反射激光束可以通过三个不同的反射镜6改变激光的路径，而投射激光束则是正向由聚焦透镜7聚焦产生等离子体，而反射激光束经过三重反射后与透射激光束方向相对，同样聚焦后产生等离子体，反射激光束的聚焦透镜7安装在位移平台8，可以控制焦点位置，实现控制聚焦于样品的同一位置或者相近位置产生两个等离子体。本发明通过DG645（数字延迟发生器）连接激光器与CCD，可以控制激光的延时，延时不同则气泡大小和冲击波位置也不同，而通过控制延时，恰好在300ns延时的时候冲击波经过等离子体，信号增强明显。A beam of 1064nm pulsed laser is divided into two beams by a beam splitter, namely a transmission laser beam and a reflection laser beam. The reflection laser beam can change the path of the laser through three different reflection mirrors 6, while the projection laser beam is forwardly focused by a focusing lens 7 to generate plasma, and the reflected laser beam is opposite to the direction of the transmission laser beam after triple reflection, and also generates plasma after focusing. The focusing lens 7 of the reflected laser beam is installed on a displacement platform 8, which can control the focal position and realize the control of focusing on the same position or a close position of the sample to generate two plasmas. The present invention connects the laser and the CCD through DG645 (digital delay generator), which can control the delay of the laser. Different delays result in different bubble sizes and shock wave positions. By controlling the delay, the shock wave passes through the plasma just when the delay is 300ns, and the signal is significantly enhanced.
S2，基于单光束分裂的水下LIBS在稀土溶液中产生的两个激光诱导气泡的冲击波对等离子体进行信号增强，发射出增强的光谱信号；S2, the shock waves of two laser-induced bubbles generated in the rare earth solution by underwater LIBS based on single beam splitting enhance the plasma signal and emit an enhanced spectral signal;
S3，利用增强的光谱信号对水下稀土元素进行检测限定量分析。S3, using enhanced spectral signals to perform detection-limited analysis of underwater rare earth elements.
在步骤S2基于单光束分裂的水下LIBS在稀土溶液中产生的两个激光诱导气泡的冲击波对等离子体进行信号增强中，冲击波经过时压缩等离子体，使等离子体核心区凝聚，等离子体温度升高，发射出增强的光谱信号。控制与拍摄发现冲击波的位置，证明光谱信号增强则是通过后面实验结果图中的光谱峰值强度对比，信噪比SNR对比，等离子体发射强度对比等来验证。In step S2, the shock waves of two laser-induced bubbles generated in the rare earth solution by underwater LIBS based on single beam splitting are used to enhance the plasma signal. When the shock waves pass through, the plasma is compressed, the plasma core region is condensed, the plasma temperature is increased, and an enhanced spectral signal is emitted. The position of the shock wave is controlled and photographed, and the enhancement of the spectral signal is verified by comparing the spectral peak intensity, signal-to-noise ratio (SNR), and plasma emission intensity in the experimental result diagram below.
具体的，通过优化水下LIBS系统的激发方式，从而实现水下LIBS技术对稀土元素的信号增强。优化系统光路结构，利用非偏振平板分束镜14对单束脉冲激光进行分束，使得分束后的两束1064nm激光束通过反射镜6与聚焦透镜7聚焦于样品的同一位置或者相近位置产生两个等离子体。水下LIBS在稀土溶液中产生的2个激光诱导气泡的冲击波对等离子体产生了信号增强作用。这种增强效应可以解释为冲击波的“压缩”效应，即冲击波经过时会压缩等离子体，使等离子体核心区更凝聚，等子体温度更高，发射出更强的光谱信号被LIBS系统采集到。冲击波除了使等离子体更加凝聚外，还有助于提高等离子体核心位置的稳定性，这表明将获得更好的信号稳定性。搭建了一套水下LIBS稀土元素信号增强系统，可实现不同激发方式的水下LIBS技术对稀土元素的信号增强以及定量分析能力的提高。Specifically, by optimizing the excitation mode of the underwater LIBS system, the signal enhancement of rare earth elements by underwater LIBS technology is achieved. The optical path structure of the system is optimized, and a single pulsed laser is split by a non-polarized flat beam splitter 14, so that the two 1064nm laser beams after the splitting are focused on the same position or a close position of the sample through a reflector 6 and a focusing lens 7 to generate two plasmas. The shock waves of the two laser-induced bubbles generated by underwater LIBS in the rare earth solution have a signal enhancement effect on the plasma. This enhancement effect can be explained as the "compression" effect of the shock wave, that is, the shock wave compresses the plasma when it passes, making the plasma core area more condensed, the plasma body temperature higher, and emitting a stronger spectral signal to be collected by the LIBS system. In addition to making the plasma more condensed, the shock wave also helps to improve the stability of the plasma core position, which indicates that better signal stability will be obtained. A set of underwater LIBS rare earth element signal enhancement system has been built, which can realize the signal enhancement of rare earth elements and the improvement of quantitative analysis capabilities of underwater LIBS technology with different excitation modes.
实施例2，如图2所示，本发明实施例提供一种水下激光诱导击穿光谱信号增强系统，包括：Embodiment 2, as shown in FIG2 , an embodiment of the present invention provides an underwater laser induced breakdown spectroscopy signal enhancement system, comprising:
第一脉冲激光器1为调Q式Nd:YAG激光器，用于产生等离子体的脉冲激光，激发波长为1064nm，脉冲重复频率10Hz。The first pulse laser 1 is a Q-switched Nd:YAG laser used to generate pulsed laser for plasma, with an excitation wavelength of 1064 nm and a pulse repetition frequency of 10 Hz.
第二脉冲激光器2，用于提供照明光源，在拍摄激光诱导气泡时负责照亮CCD拍摄范围，在第一脉冲激光器1出光口安装反射镜6，将脉冲激光引入激光衰减系统3及后续的扩束准直光路4中。The second pulse laser 2 is used to provide an illumination light source and is responsible for illuminating the CCD shooting range when shooting laser-induced bubbles. A reflector 6 is installed at the light outlet of the first pulse laser 1 to introduce the pulse laser into the laser attenuation system 3 and the subsequent beam expansion and collimation optical path 4.
激光衰减系统3由零级二分之一波片和偏振棱镜组成，用于调节激光能量，以满足探测不同浓度稀土溶液和不同类型光谱时的能量需求。二分之一波片常用于旋转线偏振光的偏振方向。空气隙零级波片中两片石英波片中间由垫片隔开形成空气隙，该结构适用于较高功率激光器应用。偏振棱镜是利用晶体的双折射现象而制成的偏振器件，无论是自然光还是偏振光通过偏振棱镜后就变成振动方向由棱镜偏振方向所决定的线偏振光。激光先后经过波片与偏振棱镜，通过旋转零级二分之一波片，可以精确调节入射激光的能量大小变化。The laser attenuation system 3 consists of a zero-order half-wave plate and a polarizing prism, which are used to adjust the laser energy to meet the energy requirements when detecting rare earth solutions of different concentrations and different types of spectra. Half-wave plates are often used to rotate the polarization direction of linearly polarized light. In the air-gap zero-order wave plate, two quartz wave plates are separated by a gasket to form an air gap. This structure is suitable for higher-power laser applications. The polarizing prism is a polarization device made using the birefringence phenomenon of crystals. Whether it is natural light or polarized light, after passing through the polarizing prism, it becomes linearly polarized light whose vibration direction is determined by the polarization direction of the prism. The laser passes through the wave plate and the polarizing prism successively. By rotating the zero-order half-wave plate, the energy change of the incident laser can be accurately adjusted.
扩束准直光路4由一个凹透镜和一个凸透镜组成，位于激光衰减系统3和非偏振平板分束镜14之间，通过调节扩束准直光路4中两个透镜间的距离，可改变激光聚焦于水槽溶液中的位置，并且改变聚焦角度，凹透镜和凸透镜间的距离可以通过旋转扩束准直光路4中扩束镜的两部分旋钮来实现；The beam expansion and collimation optical path 4 is composed of a concave lens and a convex lens, and is located between the laser attenuation system 3 and the non-polarizing plate beam splitter 14. By adjusting the distance between the two lenses in the beam expansion and collimation optical path 4, the position where the laser is focused in the water tank solution can be changed, and the focusing angle can be changed. The distance between the concave lens and the convex lens can be achieved by rotating the two knobs of the beam expander in the beam expansion and collimation optical path 4.
激光扩束后由非偏振平板分束镜14进行分束，分为透射激光束与反射激光束，反射激光束由一系列反射镜6与聚焦透镜7聚焦于水槽中产生等离子体，透射激光束直接由聚焦条件聚焦产生等离子体，反射激光束产生的等离子体和透射激光束产生的等离子体产生在相同或相近位置，产生的两个激光诱导气泡的冲击波对两个等离子体发生相互作用。这种增强效应可以解释为冲击波的“压缩”效应，即冲击波经过时会压缩等离子体，使等离子体核心区更凝聚，等离子体温度更高，发射出更强的光谱信号被LIBS系统采集到。冲击波除了使等离子体更加凝聚外，还有助于提高等离子体核心位置的稳定性，这表明将获得更好的信号稳定性。After the laser beam is expanded, it is split by a non-polarizing flat beam splitter 14 into a transmitted laser beam and a reflected laser beam. The reflected laser beam is focused in a water tank by a series of reflectors 6 and a focusing lens 7 to generate plasma. The transmitted laser beam is directly focused by the focusing condition to generate plasma. The plasma generated by the reflected laser beam and the plasma generated by the transmitted laser beam are generated at the same or similar positions. The shock waves of the two laser-induced bubbles generated interact with the two plasmas. This enhancement effect can be explained as the "compression" effect of the shock wave, that is, the shock wave compresses the plasma when it passes, making the plasma core area more condensed, the plasma temperature higher, and emitting a stronger spectral signal to be collected by the LIBS system. In addition to making the plasma more condensed, the shock wave also helps to improve the stability of the plasma core position, which means that better signal stability will be obtained.
聚焦透镜7安装于位移平台8，以便调整透射激光束与反射激光束的激光相对焦点位置。The focusing lens 7 is mounted on the displacement platform 8 so as to adjust the relative focal positions of the transmitted laser beam and the reflected laser beam.
样品平台5为二维移动平台，上面放置水槽。分束激光通过衰减、扩束、反射、聚焦后，经聚焦透镜7汇聚于水槽内的稀土溶液中。The sample platform 5 is a two-dimensional moving platform, on which a water tank is placed. After attenuation, beam expansion, reflection and focusing, the split laser beam is converged into the rare earth solution in the water tank through the focusing lens 7.
光谱信号接收系统9包括两个收集透镜和光纤探头。两个收集透镜实现将LIBS光谱信号汇聚到光纤探头中。两个收集透镜间安装磁吸镜架，方便拆卸或安装，用于采集LIBS光谱。光谱信号接收系统9位于样品平台5的侧向，通过一根光纤连接光谱仪，光谱仪和ICCD受控于计算机控制系统13。The spectral signal receiving system 9 includes two collecting lenses and a fiber optic probe. The two collecting lenses realize the convergence of the LIBS spectral signal into the fiber optic probe. A magnetic lens holder is installed between the two collecting lenses for easy disassembly or installation for collecting LIBS spectra. The spectral signal receiving system 9 is located on the side of the sample platform 5 and is connected to the spectrometer through an optical fiber. The spectrometer and ICCD are controlled by a computer control system 13.
光谱探测系统11由光谱仪和ICCD组成。The spectrum detection system 11 is composed of a spectrometer and an ICCD.
等离子体和激光诱导气泡拍摄采集系统10由ICCD、CCD组成，同时受控于计算机控制系统13。The plasma and laser-induced bubble shooting and collection system 10 is composed of an ICCD and a CCD, and is controlled by a computer control system 13 .
时序控制装置12中的数字延时脉冲分别控制第一脉冲激光器1以及第二脉冲激光器2的Flash和Q-Switch，以及ICCD的延时采集。The digital delayed pulses in the timing control device 12 respectively control the Flash and Q-Switch of the first pulse laser 1 and the second pulse laser 2, as well as the delayed acquisition of the ICCD.
实验中，数字信号延时发生器15（DG645）作为连接第一脉冲激光器1和第二脉冲激光器2与光谱仪的桥梁，通过连接，由数字信号延时发生器15（DG645）改变探测延时，同时连接控制第一脉冲激光器1以及第二脉冲激光器2，并且控制第一脉冲激光器1以及第二脉冲激光器2的触发与Q开关，连接光谱仪；In the experiment, the digital signal delay generator 15 (DG645) serves as a bridge connecting the first pulse laser 1 and the second pulse laser 2 with the spectrometer. Through the connection, the digital signal delay generator 15 (DG645) changes the detection delay, and at the same time connects and controls the first pulse laser 1 and the second pulse laser 2, and controls the triggering and Q switch of the first pulse laser 1 and the second pulse laser 2, and connects the spectrometer;
光谱仪连接计算机控制系统13采集光谱数据。ICCD通过连接第一脉冲激光器1、第二脉冲激光器2、计算机控制系统13，拍摄等离子体图像。The spectrometer is connected to the computer control system 13 to collect spectrum data. The ICCD is connected to the first pulse laser 1, the second pulse laser 2 and the computer control system 13 to take plasma images.
计算机控制系统13分别连接CCD、ICCD和光谱探测系统11，采集水下稀土元素LIBS光谱，拍摄等离子体及激光诱导气泡对的图像，通过数据处理分析验证信号增强效果以及规律。光谱数据由计算机控制系统13采集为excel文件，通过Matlab预处理光谱数据，随后采用origin绘制光谱图像。等离子体图像由Matlab处理平均图像导出，激光诱导气泡图像由Matlab裁剪，而后通过整理展示数据。The computer control system 13 is connected to the CCD, ICCD and spectral detection system 11 respectively, collects underwater rare earth element LIBS spectra, takes images of plasma and laser-induced bubble pairs, and verifies the signal enhancement effect and regularity through data processing and analysis. The spectral data is collected by the computer control system 13 as an excel file, and the spectral data is preprocessed by Matlab, and then the spectral image is drawn using origin. The plasma image is exported by Matlab processing the average image, and the laser-induced bubble image is cropped by Matlab, and then the data is displayed by sorting.
通过上述实施例可知，本发明在原有的水下SP-LIBS系统的基础上，通过优化设计系统的光路结构，利用非偏振平板分束镜14对单束脉冲激光进行分束，使得分束后的两束激光通过反射镜6与聚焦透镜7聚焦于样品的同一位置或者相近位置产生两个等离子体，从激发方式上实现稀土元素的信号增强。It can be seen from the above embodiments that the present invention, based on the original underwater SP-LIBS system, optimizes the optical path structure of the system and uses a non-polarizing flat beam splitter 14 to split a single pulse laser beam, so that the two split laser beams are focused on the same position or a close position of the sample through a reflector 6 and a focusing lens 7 to generate two plasmas, thereby realizing signal enhancement of rare earth elements from the excitation mode.
本发明相比传统SP-LIBS技术，信号质量得到了明显的改善与提升；相比双脉冲LIBS技术，本方法搭建的水下LIBS系统更紧凑，成本更低，实验装置与操作简便；对于长脉冲LIBS技术，目前还没有商业化的长脉冲激光器，且长脉冲激光稳定性较差，激光能量较低，也不利于LIBS信号的稳定性，而本方法的系统采用商业化短脉冲激光器，能在最优激光能量条件下提高稀土元素水下LIBS信号的质量，提高检测灵敏度，实现信号增强。Compared with the traditional SP-LIBS technology, the signal quality of the present invention has been significantly improved and enhanced; compared with the dual-pulse LIBS technology, the underwater LIBS system constructed by this method is more compact, lower in cost, and simpler in experimental setup and operation; for the long-pulse LIBS technology, there is currently no commercial long-pulse laser, and the long-pulse laser has poor stability and low laser energy, which is not conducive to the stability of the LIBS signal. The system of this method uses a commercial short-pulse laser, which can improve the quality of the underwater LIBS signal of rare earth elements under the optimal laser energy conditions, improve the detection sensitivity, and achieve signal enhancement.
可以理解，为了确保后续实验的准确性和信噪比良好，必须选择合适的激光能量。通过对不同聚焦透镜7条件和不同能量下目标稀土元素信号峰值强度及信噪比的分析，确定了20mJ的激光能量作为定量分析实验的理想选择。当激光能量达到20mJ时，稀土元素的四个峰位的光谱信噪比达到最高，因此本发明选择20mJ作为后续定量分析实验的最佳能量。It is understood that in order to ensure the accuracy and good signal-to-noise ratio of subsequent experiments, appropriate laser energy must be selected. Through the analysis of the peak intensity and signal-to-noise ratio of the target rare earth element signal under different focusing lens 7 conditions and different energies, a laser energy of 20mJ is determined as an ideal choice for quantitative analysis experiments. When the laser energy reaches 20mJ, the spectral signal-to-noise ratio of the four peaks of the rare earth element reaches the highest, so the present invention selects 20mJ as the optimal energy for subsequent quantitative analysis experiments.
本方法可实现水下稀土元素的信号增强以及定量分析能力的提高，通过改变较少的实验条件来完成不同的激发方式。该方法还可以研究分束激光相对焦点位置对水下LIBS光谱、激光诱导气泡及等离子体的影响，阐明信号增强机理，以最佳实验条件实现信号增强。本发明适用性强，应用成本较低，操作简单。This method can achieve underwater rare earth element signal enhancement and quantitative analysis capability improvement, and complete different excitation modes by changing a few experimental conditions. This method can also study the influence of the relative focus position of the split laser on underwater LIBS spectrum, laser-induced bubbles and plasma, clarify the signal enhancement mechanism, and achieve signal enhancement under optimal experimental conditions. The present invention has strong applicability, low application cost, and simple operation.
实施例3，本发明提供的水下激光诱导击穿光谱信号增强系统，Example 3, the underwater laser induced breakdown spectroscopy signal enhancement system provided by the present invention,
一个优选方案中，可在非偏振平板分束镜14处更换其他分束镜，使激光分为两束，同时聚焦于样品溶液内部，产生LIBS光谱，可以作为上述改变激光脉冲激发方式的一种替代方案。In a preferred embodiment, other beam splitters can be replaced at the non-polarizing plate beam splitter 14 to split the laser into two beams and simultaneously focus them inside the sample solution to generate a LIBS spectrum, which can serve as an alternative to the above-mentioned change in the laser pulse excitation mode.
另一个优选方案中，分束的两束激光的聚焦透镜7替换不同焦距的透镜，在信号增强的基础上进一步提升效果，可实现聚焦方式与激发参数条件的双重优化，可作为上述系统聚焦方式的替代方案。In another preferred embodiment, the focusing lens 7 of the two split laser beams is replaced with lenses of different focal lengths, which further improves the effect on the basis of signal enhancement, and can achieve dual optimization of focusing mode and excitation parameter conditions, which can serve as an alternative to the focusing mode of the above-mentioned system.
在上述实施例中，对各个实施例的描述都各有侧重，某个实施例中没有详述或记载的部分，可以参见其它实施例的相关描述。In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.
为进一步说明本发明实施例相关效果，进行如下实验。To further illustrate the effects of the embodiments of the present invention, the following experiment was conducted.
图3为水下LIBS光谱采集及等离子体图像采集实验系统。其中，Plasma为等离子体，M为反射镜6，HW+GP为激光衰减器（由零级半波片和偏振棱镜组成），BS为非偏振平板分束镜14，LEM为能量计，LBE为扩束镜，MO为显微物镜，F为滤光片，DG645为数字脉冲延时发生器；Figure 3 shows the underwater LIBS spectrum acquisition and plasma image acquisition experimental system. Among them, Plasma is plasma, M is reflector 6, HW+GP is laser attenuator (composed of zero-order half-wave plate and polarization prism), BS is non-polarizing plate beam splitter 14, LEM is energy meter, LBE is beam expander, MO is microscope objective, F is filter, DG645 is digital pulse delay generator;
其中，图3中的（a）部分为等离子体图像采集系统，图3中的（b）部分为实验光路；Part (a) in FIG3 is the plasma image acquisition system, and part (b) in FIG3 is the experimental optical path;
激光光源为Nd:YAG调Q激光器（Beamtech，Dawa-200），输出波长为基频1064nm，脉冲宽度为10ns，重复频率均固定为10Hz。为了减小聚焦像差、提高击穿概率，激光光束经一组用双胶合透镜（焦距60mm）和正弯月透镜（焦距100mm）组成的聚焦透镜7聚焦到石英样品池的稀土溶液（REE stand solution），该双透镜的等效焦距为36mm。脉冲激光由一个非偏振平板分束镜14（BS）调整为两束激光，实现单光束分裂，随后两束激光被同样的双透镜聚焦到样品水槽的稀土溶液中产生等离子体。本发明使用含有三种稀土元素Yb、Eu、Y的标准溶液作为工作样品，浓度均为500ppm，所用溶剂为5% HNO3溶液，实验样品每次提取100 mL。等离子体时间分辨图像的采集由采用的ICCD（Andor,iStar DH 734i）完成。实验过程中，为了防止ICCD相机感光面的过曝损伤，本发明需要将中性密度滤波片（F）安装于ICCD前方，这是因为等离子体刚产生时的辐射较强。对于LIBS光谱探测部分，为了将等离子体的辐射光汇聚至光纤中，本发明采用了一组由焦距分别为60mm和40mm的石英平凸透镜组成的收集透镜组，使辐射光汇聚到光纤中。光纤的另一端耦合到光谱仪（Avantes,Avaspec-ULS2048）中，在260-820nm波长范围内分析光谱，采集水下稀土元素LIBS光谱数据。The laser light source is a Nd:YAG Q-switched laser (Beamtech, Dawa-200), with an output wavelength of 1064nm, a pulse width of 10ns, and a repetition frequency of 10Hz. In order to reduce focusing aberration and increase the probability of breakdown, the laser beam is focused to the rare earth solution (REE stand solution) in the quartz sample pool by a focusing lens 7 composed of a double glued lens (focal length 60mm) and a positive meniscus lens (focal length 100mm), and the equivalent focal length of the double lens is 36mm. The pulsed laser is adjusted into two laser beams by a non-polarizing plate beam splitter 14 (BS) to achieve single beam splitting, and then the two laser beams are focused by the same double lens to the rare earth solution in the sample water tank to generate plasma. The present invention uses a standard solution containing three rare earth elements Yb, Eu, and Y as a working sample, with a concentration of 500ppm, and the solvent used is a 5% HNO3 solution. 100 mL of the experimental sample is extracted each time. The acquisition of plasma time-resolved images is completed by the ICCD (Andor, iStar DH 734i). During the experiment, in order to prevent overexposure damage to the photosensitive surface of the ICCD camera, the present invention needs to install a neutral density filter (F) in front of the ICCD, because the radiation is strong when the plasma is just generated. For the LIBS spectrum detection part, in order to converge the radiation light of the plasma into the optical fiber, the present invention uses a group of collecting lenses composed of quartz plano-convex lenses with focal lengths of 60mm and 40mm, respectively, to converge the radiation light into the optical fiber. The other end of the optical fiber is coupled to a spectrometer (Avantes, Avaspec-ULS2048), and the spectrum is analyzed in the wavelength range of 260-820nm to collect underwater rare earth element LIBS spectrum data.
激光束通过一个半波片（HWP）和一个格兰棱镜（GP），这种组合可以调整激光在聚焦到稀土溶液前的能量。每个激光脉冲的一部分通过分束镜（BS）传输到激光能量计（LEM），以实时监测激光能量。将能量计的接收部分的圆心对准发射的脉冲激光束，以此接收光信号完成激光能量监测。对于本发明的等离子体发射探测部分，由于稀土标准溶液中的等离子体在水中的尺寸较小，因此使用显微物镜（MO，10x）将其放大。等离子体图像直接使用ICCD相机（ANDOR，iStar DH 720）拍摄，相机前方配有中性密度滤光片（F），以避免探测器饱和。ICCD拍摄的等离子体图像可提供等离子体形态、等离子体发射强度分布和脉冲间等离子体波动等信息，空间分辨率可达3.6mm。激光器、ICCD、光谱仪的延时由数字脉冲延时发生器（DG645）控制。The laser beam passes through a half-wave plate (HWP) and a Glan prism (GP), and this combination can adjust the energy of the laser before focusing on the rare earth solution. A portion of each laser pulse is transmitted to the laser energy meter (LEM) through a beam splitter (BS) to monitor the laser energy in real time. The center of the receiving part of the energy meter is aligned with the emitted pulsed laser beam to receive the optical signal to complete the laser energy monitoring. For the plasma emission detection part of the present invention, since the plasma in the rare earth standard solution is small in water, it is magnified using a microscope objective (MO, 10x). The plasma image is directly taken using an ICCD camera (ANDOR, iStar DH 720), and a neutral density filter (F) is provided in front of the camera to avoid detector saturation. The plasma image taken by the ICCD can provide information such as plasma morphology, plasma emission intensity distribution, and plasma fluctuations between pulses, with a spatial resolution of up to 3.6 mm. The delay of the laser, ICCD, and spectrometer is controlled by a digital pulse delay generator (DG645).
本发明由非偏振平板分束镜14（BS）完成的单激光分束。为了调节两束激光间的相对焦点位置，在分束后，由三个反射镜6反射后的激光束的聚焦透镜7处安装了位移平台8，通过改变反射激光束的聚焦位置来研究透射激光束聚焦后的LIBS光谱信号以及等离子体、激光诱导气泡的图像数据与相对焦点位置变化之间的关系。通过调节位移平台8上聚焦透镜7的位置，可以改变反射激光束的焦点位置。位移平台8的初始位置设置为0mm，恰好是两束激光聚焦后产生的等离子体重合的位置。通过控制位移平台8的移动，可以改变反射激光束的聚焦位置，从而调节两束激光脉冲之间的相对焦点位置。位移平台8的移动范围为-1.5-1.5mm。根据两束激光焦点的相对位置，本发明定义反射激光束聚焦在透射激光束焦点位置之前为正值，聚焦在透射激光束焦点位置之后为负值。透射激光束的聚焦位置保持不变。The present invention performs single laser beam splitting by a non-polarizing plate beam splitter 14 (BS). In order to adjust the relative focal position between the two laser beams, a displacement platform 8 is installed at the focusing lens 7 of the laser beam reflected by three reflectors 6 after beam splitting, and the relationship between the LIBS spectrum signal after the transmission laser beam is focused and the image data of plasma and laser-induced bubbles and the change of the relative focal position is studied by changing the focal position of the reflected laser beam. The focal position of the reflected laser beam can be changed by adjusting the position of the focusing lens 7 on the displacement platform 8. The initial position of the displacement platform 8 is set to 0 mm, which is exactly the position where the plasma generated after the two laser beams are focused overlap. By controlling the movement of the displacement platform 8, the focal position of the reflected laser beam can be changed, thereby adjusting the relative focal position between the two laser pulses. The movement range of the displacement platform 8 is -1.5-1.5 mm. According to the relative position of the focal points of the two laser beams, the present invention defines that the reflected laser beam is focused before the focal position of the transmitted laser beam as a positive value, and is focused after the focal position of the transmitted laser beam as a negative value. The focal position of the transmitted laser beam remains unchanged.
示例性的，这些调整中，通过对比分束与不分束的传统LIBS数据，通过小型螺旋的位移平台8改变反射激光束聚焦后的焦点位置，位移平台8的移动范围为-1.5-1.5mm，对比了光谱图像、信噪比结果、RSD结果以及等离子体还有激光诱导气泡的情况，证明了LIBS光谱信号更强，信噪比更高，等离子体强度更强，并且通过改变焦点位置研究了相对焦点位置变化的信号增强规律，确定最佳的信号增强的点。Exemplarily, in these adjustments, by comparing the traditional LIBS data with and without beam splitting, the focal position of the reflected laser beam after focusing is changed by a small spiral displacement platform 8, and the movement range of the displacement platform 8 is -1.5-1.5mm. The spectral images, signal-to-noise ratio results, RSD results, plasma and laser-induced bubbles are compared, which proves that the LIBS spectral signal is stronger, the signal-to-noise ratio is higher, and the plasma intensity is stronger. In addition, by changing the focal position, the signal enhancement law relative to the change of the focal position is studied to determine the optimal signal enhancement point.
两束激光聚焦后焦点位置重合以及等离子体重合的位置，是通过CCD拍摄气泡以及ICCD拍摄等离子体来呈现，以激光诱导气泡重合以及等离子体头部重合作为焦点重合的评判标准，通过微调位移平台8以及观察实时拍摄图像来控制实现。The overlap of the focal positions and the plasma positions after the two laser beams are focused are presented by CCD photographing the bubble and ICCD photographing the plasma. The laser-induced bubble overlap and plasma head overlap are used as the criteria for focus overlap, which is controlled and achieved by fine-tuning the displacement platform 8 and observing the real-time captured images.
图4为本发明水下激光诱导气泡投影成像系统光路图。为了分析分束激光相对焦点位置对激光诱导气泡特性的影响以及气泡与水下稀土元素LIBS信号增强之间的关系，在实验室搭建了一套针对稀土元素的水下激光诱导空化气泡成像系统实验平台，实验光路如图4所示。1064nm激光首先被分束器分成两部分，其中一部分（~5％）被能量计（LEM）接收，以此来监测激光能量。另一部分激光经扩束镜（LBE）扩束之后被非偏振平板分束镜14（BS）分束，分别为透射激光束以及经过三个反射镜6的反射激光束。两束激光由双透镜组聚焦于石英水槽（3cm×5cm×5cm）中的稀土元素溶液（REE stand solution）击穿产生等离子体和激光诱导气泡。为了照明激光诱导气泡和冲击波，采用了Quantel公司生产的Nd:YAG调Q脉冲激光器（Quantel-QSmart-450）作为背景光源。该激光器的脉冲基频为1064nm，经过倍频晶体处理后能够输出532nm的激光脉冲。激光脉冲的重复频率为1Hz，脉冲宽度为6ns。通过这种方式，背景光可以照射到气泡和冲击波上，使其可被拍摄到。为了观察气泡的演化过程，本发明使用了一个由焦距分别为50mm和150mm的双胶合消色差透镜组成的透镜组，将气泡放大并成像在CCD上。为了应对等离子体初期产生时的强背景辐射，本发明在CCD前放置了一个中心波长为532nm的干涉滤波片，这样只有532nm波长的照明激光能够通过，从而实现对早期气泡和冲击波的成像观察。FIG4 is an optical path diagram of the underwater laser-induced bubble projection imaging system of the present invention. In order to analyze the influence of the relative focal position of the split laser on the characteristics of the laser-induced bubble and the relationship between the bubble and the underwater rare earth element LIBS signal enhancement, an underwater laser-induced cavitation bubble imaging system experimental platform for rare earth elements was built in the laboratory, and the experimental optical path is shown in FIG4. The 1064nm laser is first divided into two parts by a beam splitter, one part (~5%) of which is received by an energy meter (LEM) to monitor the laser energy. The other part of the laser is expanded by a beam expander (LBE) and then split by a non-polarized plate beam splitter 14 (BS), which are a transmitted laser beam and a reflected laser beam after passing through three reflectors 6. The two laser beams are focused by a double lens group on a rare earth element solution (REE stand solution) in a quartz water tank (3cm×5cm×5cm) to break down and generate plasma and laser-induced bubbles. In order to illuminate the laser-induced bubbles and shock waves, a Nd:YAG Q-switched pulse laser (Quantel-QSmart-450) produced by Quantel was used as a background light source. The pulse base frequency of the laser is 1064nm, and after being processed by a frequency doubling crystal, it can output a 532nm laser pulse. The repetition frequency of the laser pulse is 1Hz, and the pulse width is 6ns. In this way, the background light can be irradiated onto the bubbles and shock waves, so that they can be photographed. In order to observe the evolution of the bubbles, the present invention uses a lens group consisting of a double-glued achromatic lens with focal lengths of 50mm and 150mm, respectively, to magnify the bubbles and image them on the CCD. In order to cope with the strong background radiation during the initial generation of plasma, the present invention places an interference filter with a central wavelength of 532nm in front of the CCD, so that only the illumination laser with a wavelength of 532nm can pass through, thereby realizing the imaging observation of early bubbles and shock waves.
本发明采集的水下LIBS在30mJ条件下与SP-LIBS在30mJ、15mJ条件下的稀土溶液典型LIBS光谱对比。如图5、图6、图7所示；与SP-LIBS光谱相比，水下LIBS系统采集的稀土元素光谱强度显著增加。Comparison of typical LIBS spectra of rare earth solutions collected by the present invention under the conditions of 30mJ by underwater LIBS and 30mJ and 15mJ by SP-LIBS. As shown in Figures 5, 6 and 7, the intensity of the rare earth element spectrum collected by the underwater LIBS system is significantly increased compared with the SP-LIBS spectrum.
图8-图10分别为不同激发方式条件下Yb I 398.79nm、Eu I 459.40nm、YO616.51nm的光谱峰值强度随激光能量的变化，图11-图13分别为不同激发方式条件下Yb I398.79nm、Eu I 459.40nm、YO 616.51nm的光谱峰值强度随激光能量变化的信噪比变化图。在300ns延时条件下，对水下LIBS和SP-LIBS在不同激光能量条件下的稀土元素峰值强度以及信噪比进行了对比分析。两种不同激发方式下稀土元素特征峰的信号强度都随着激光能量的增加而增加，Yb I、Eu I的光谱信噪比逐渐提升，YO的光谱信噪比呈现先增大后减小的趋势。光谱背景强度约为600，随着激光能量提高，LIBS条件下的水下稀土元素峰值强度相较于SP-LIBS条件增强了2倍以上，而信噪比提高了约1.5倍。Figures 8-10 show the changes of the spectral peak intensity of Yb I 398.79nm, Eu I 459.40nm, and YO 616.51nm with laser energy under different excitation conditions, and Figures 11-13 show the changes of the signal-to-noise ratio of the spectral peak intensity of Yb I 398.79nm, Eu I 459.40nm, and YO 616.51nm with laser energy under different excitation conditions. Under the condition of 300ns delay, the peak intensity and signal-to-noise ratio of rare earth elements under underwater LIBS and SP-LIBS under different laser energy conditions were compared and analyzed. The signal intensity of the characteristic peaks of rare earth elements under two different excitation modes increased with the increase of laser energy, the spectral signal-to-noise ratio of Yb I and Eu I gradually increased, and the spectral signal-to-noise ratio of YO showed a trend of increasing first and then decreasing. The spectral background intensity is about 600. With the increase of laser energy, the peak intensity of underwater rare earth elements under LIBS conditions is enhanced by more than 2 times compared with SP-LIBS conditions, while the signal-to-noise ratio is improved by about 1.5 times.
图14-图16分别为本发明在不同激发方式条件下Yb I 398.79nm、Eu I 459.40nm、YO 616.51nm的LIBS光谱RSD随激光能量的变化。光谱是在延时300ns、门宽1.05ms的条件下采集的。对三种稀土元素在不同激发方式下的光谱信噪比进行了比较，并计算了不同激光能量条件下的RSD。RSD的计算公式为：相对标准偏差（RSD）=标准偏差（SD）/计算结果的算术平均值（X）。相对标准偏差（RSD）就是指：标准偏差与测量结果算术平均值的比值，该值通常用来表示分析测试结果的精密度。通过将不同激光能量条件下的LIBS光谱数据经过谱线归属，确定目标峰位强度，运用excel完成RSD的计算与导出，最后通过origin软件完成柱状图绘制。研究结果表明，在激光能量变化的情况下，采用水下LIBS技术的条件下，稀土元素的光谱RSD相较于SP-LIBS条件更低，光谱信号的稳定性更高。采用水下LIBS系统来检测和分析稀土元素，能够显著提高光谱信号强度并提高光谱信号的稳定性。Figures 14-16 show the changes of RSD of LIBS spectra of Yb I 398.79nm, Eu I 459.40nm and YO 616.51nm with laser energy under different excitation conditions of the present invention. The spectrum was collected under the conditions of delay 300ns and gate width 1.05ms. The spectral signal-to-noise ratios of the three rare earth elements under different excitation modes were compared, and the RSD under different laser energy conditions was calculated. The calculation formula of RSD is: relative standard deviation (RSD) = standard deviation (SD) / arithmetic mean (X) of the calculated results. The relative standard deviation (RSD) refers to the ratio of the standard deviation to the arithmetic mean of the measurement results, which is usually used to indicate the precision of the analysis and test results. The LIBS spectral data under different laser energy conditions are assigned to the spectral lines, the target peak intensity is determined, the RSD is calculated and exported using excel, and finally the bar graph is drawn using origin software. The research results show that under the condition of laser energy change, the spectral RSD of rare earth elements under the conditions of underwater LIBS technology is lower than that under SP-LIBS conditions, and the stability of spectral signals is higher. Using underwater LIBS system to detect and analyze rare earth elements can significantly improve the intensity of spectral signals and improve the stability of spectral signals.
图17-图19分别为本发明Yb I 398.79nm、Eu I 459.40nm、YO 616.51nm峰值强度随x轴轴向的分布变化图，图20-图22分别为本发明Yb I 398.79nm、Eu I 459.40nm、YO616.51nm峰值强度随z轴轴向的分布变化图，光谱是在延时300ns，门宽为1.05ms的条件下采集的。为了确认对水下稀土元素信号增强的最佳相对焦点位置，本发明在x轴为-1.5-1.5mm、z轴为-0.5-0.5mm之间采集了水下LIBS光谱，分析研究随反射激光焦点处等离子体的位置变化对透射激光焦点处等离子体的光谱信号影响情况。峰值强度与信噪比的最低点位于焦点位置重合区域附近，即0点处；最大值点为x轴相对焦点位置±0.5mm处，且-0.5mm数值最佳。Figures 17-19 are respectively the distribution change diagrams of the peak intensity of Yb I 398.79nm, Eu I 459.40nm, and YO 616.51nm along the x-axis, and Figures 20-22 are respectively the distribution change diagrams of the peak intensity of Yb I 398.79nm, Eu I 459.40nm, and YO 616.51nm along the z-axis. The spectra were collected under the conditions of a delay of 300ns and a gate width of 1.05ms. In order to confirm the optimal relative focal position for underwater rare earth element signal enhancement, the present invention collected underwater LIBS spectra between -1.5-1.5mm on the x-axis and -0.5-0.5mm on the z-axis, and analyzed the influence of the position change of the plasma at the focus of the reflected laser on the spectral signal of the plasma at the focus of the transmitted laser. The lowest point of the peak intensity and signal-to-noise ratio is located near the focal position overlap area, that is, at point 0; the maximum value is at ±0.5mm relative to the focal position on the x-axis, and -0.5mm is the best value.
图23为本发明在x=-0.5mm处反射激光焦点位于透射激光焦点后方0.5mm的气泡演化图像。不同相对焦点位置信号增强光谱实验选在了300ns延时进行，可以发现，在最佳增强位置x=-0.5mm的条件下，两个冲击波恰好经过两个气泡，如图23所示；包括x=0.5mm的条件下，冲击波也在300ns时位于两个气泡相近的位置，如图23。本发明推测水下LIBS在稀土溶液中产生的两个等离子体发生了相互作用，且在300ns条件下，两个冲击波同时掠过气泡对的位置，对等离子体发射光谱产生了一定影响。FIG23 is a bubble evolution image of the present invention when the reflected laser focus is located 0.5 mm behind the transmitted laser focus at x=-0.5 mm. The signal enhancement spectrum experiment at different relative focus positions was carried out at a delay of 300 ns. It can be found that under the condition of the optimal enhancement position x=-0.5 mm, the two shock waves just passed through the two bubbles, as shown in FIG23; including the condition of x=0.5 mm, the shock waves were also located at a position close to the two bubbles at 300 ns, as shown in FIG23. The present invention speculates that the two plasmas generated by underwater LIBS in the rare earth solution interacted with each other, and under the condition of 300 ns, the two shock waves simultaneously passed the position of the bubble pair, which had a certain impact on the plasma emission spectrum.
图24为本发明在不同相对焦点位置条件下等离子体图像随延时的变化，以及图25不同延时条件下等离子体辐射强度随x轴相对焦点位置的变化。这些图像是在40mJ激光能量和不同门宽在累计50次激光脉冲平均的条件下记录的，标星处是同延时情况下最亮的等离子体。在延时300ns之前，等离子体在相对焦点位置靠近x=0处的发射强度最大，即等离子体重合的位置附近。在10-200ns期间，靠近0点处两等离子体重合的位置，等离子体明显最亮，并且发射强度最大。在延时200ns时，在x=-0.25mm的情况下，观察到的等离子体最亮，发射强度最大。在300ns延时条件下，在相对焦点位置x=±0.5mm处等离子体最亮且强度最强，x=-0.5mm处增强效果最佳，此时此刻冲击波掠过等离子体以及激光诱导气泡。400ns延时条件的增强规律与300ns相类似，而500ns时，等离子体发射强度最大的位置又回到了0点附近，也是等离子体重合的位置附近。FIG24 shows the change of plasma image with delay under different relative focal position conditions of the present invention, and FIG25 shows the change of plasma radiation intensity with relative focal position of x-axis under different delay conditions. These images were recorded under the conditions of 40mJ laser energy and different gate widths with the average of 50 laser pulses. The star-marked area is the brightest plasma under the same delay condition. Before the delay of 300ns, the emission intensity of the plasma is the largest near x=0 at the relative focal position, that is, near the position where the plasma overlaps. During 10-200ns, the plasma is obviously the brightest and has the highest emission intensity near the position where the two plasmas overlap at point 0. At a delay of 200ns, the plasma observed is the brightest and has the highest emission intensity at x=-0.25mm. Under the condition of a 300ns delay, the plasma is the brightest and has the highest intensity at the relative focal position x=±0.5mm, and the enhancement effect is the best at x=-0.5mm, at which time the shock wave passes through the plasma and the laser-induced bubbles. The enhancement law of the 400ns delay condition is similar to that of 300ns, while at 500ns, the position of the maximum plasma emission intensity returns to near the 0 point, which is also near the position of plasma overlap.
图26-图28为本发明Yb I 398.79nm、Eu I 459.40nm、YO 616.51nm的单变量定标曲线，对比了LIBS条件以及SP-LIBS条件。在水下LIBS条件下，计算得出Yb I 398.79nm、EuI 459.40nm、YO 616.51 nm的LOD分别为53.78ppm、103.29ppm、73.84ppm。在水下SP-LIBS条件下，Yb I 398.79nm、Eu I 459.40nm、YO 616.51nm的LOD分别为106.70ppm、193.87ppm、101.03ppm。结果表明，本发明提供的水下LIBS系统可以明显提高LIBS对水下稀土元素的定量分析能力，并有效的将LOD降低了1.5-2倍。Figures 26-28 are univariate calibration curves of Yb I 398.79nm, Eu I 459.40nm, and YO 616.51nm of the present invention, comparing LIBS conditions and SP-LIBS conditions. Under underwater LIBS conditions, the LODs of Yb I 398.79nm, EuI 459.40nm, and YO 616.51nm are calculated to be 53.78ppm, 103.29ppm, and 73.84ppm, respectively. Under underwater SP-LIBS conditions, the LODs of Yb I 398.79nm, Eu I 459.40nm, and YO 616.51nm are 106.70ppm, 193.87ppm, and 101.03ppm, respectively. The results show that the underwater LIBS system provided by the present invention can significantly improve the quantitative analysis capability of LIBS for underwater rare earth elements, and effectively reduce the LOD by 1.5-2 times.
以上所述，仅为本发明较优的具体的实施方式，但本发明的保护范围并不局限于此，任何熟悉本技术领域的技术人员在本发明揭露的技术范围内，凡在本发明的精神和原则之内所作的任何修改、等同替换和改进等，都应涵盖在本发明的保护范围之内。The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered within the protection scope of the present invention.
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