Experimental Investigation of the Self-Absorption Effect in Laser-Induced Breakdown Spectroscopy Based on Fiber Laser Ablation

Fiber laser is a stable light source for improving the stability of laser-induced breakdown spectroscopy (LIBS) under long-time operation. However, LIBS based on fiber laser ablation (FL-LIBS) also suffers from the self-absorption effect. In this paper, the self-absorption effect in FL-LIBS was experimentally investigated. The influence of the fiber laser ablation process on self-absorption was studied. The law of ablation on the surface of micro-alloy steel samples based on high pulse repetition rate (PRR) fiber laser ablation was qualitatively analyzed. The spectral data were explored based on the discrete wavelet transform and the spectral internal standard method. The result was an R² of the calibration curve increasing from 0.955 to 0.995. The influence of the laser output power and defocus amount (DA) on the self-absorption effect was investigated. Through the adjusted selection of laser output power and single pulse ablation area (SPAA), compared with the situation when the laser was in focus, the system self-absorption factor can be reduced from 1.29 to 0.61, and the performance index of the calibration curve such as root mean squared error of cross validation (RMSECV) and relative standard variation (RSD) had been improved from 0.18 to 0.13 wt%, 6.61 to 5.88%, respectively. Finally, it was concluded that self-absorption would increase with the increase of laser output power and SPAA in high PRR FL-LIBS system, contrary to conventional LIBS based on low PRR laser ablation. This work investigated the mechanism and suppression of the self-absorption effect in FL-LIBS, aiming to promote the industrial application in the future.

Elemental analysis is an important method of material characterization. Conventional elemental analysis methods (including, but not limited to, chemical colorimetry, atomic absorption spectroscopy, inductively coupled plasma spectroscopy-mass spectrometry [ICP-MS]) are lengthy, complicated, and material-consuming, only used in offline operation, which increases the analytical cost and undoubtedly prolongs the engineering cycle. In contrast, a rapid and online technique for elemental determination can not only evaluate material quality but also provide real-time feedback in manufacturing. In past decades, researchers and engineers have made efforts to develop analytical methods to realize this goal.

Laser-induced breakdown spectroscopy (LIBS) has received extensive research and attention worldwide in recent years as an emerging rapid element composition analysis technology. It is an atomic spectroscopy technology based on laser ablation, in which a laser beam is used to vaporize (ablate) the sample surface, generating s surface plasma. The plasma emission spectrum of the ablated sample is then collected through the light collector (1). The elemental information of the sample is finally obtained by analyzing the plasma emission spectrum through computer data processing. With the attractive characteristics of no or minimal sample preparation, real-time, online, in-situ, non-destructive, long-distance non-contact and multi-element detection, LIBS has become a popular analytical method, widely used in the detection of coal (2,3), biological and food safety (4), solutions (5), Mars exploration (6), metallurgy (7), and other fields.

In LIBS, the bremsstrahlung is generally avoided by the synchronous delay method (8), where the spectrometer has to be synchronized with the laser pulse. Therefore, the laser sources with low pulse repetition rate (PRR, ≤100 Hz) are conventionally used in LIBS. However, these laser sources (such as Nd: YAG Q-switched lasers) generally suffer from unstable output power during long-term operation (9), in an electromagnetic environment (10), and due to impurities in the environment (11). LIBS is therefore still unsatisfactory in stability and accuracy compared with other conventional elemental analysis methods (12), resulting in greatly limited further development of LIBS in industrial fields.

A fiber laser is a promising laser source based on rare-earth doped optical fiber as an active medium. Due to the advantages of high electro-optical efficiency, flexible construction, good quality of beam, simple thermal management, and stable power for long-time operation, fiber lasers are widely used in the industrial field and have largely replaced conventional solid lasers. In recent years, fiber lasers have also attracted LIBS researchers to improve long-time operation stability of LIBS. Baudelet and associates (13) performed the FL-LIBS experiments that employ a thulium fiber laser (2 μs) as the ablation source of copper; Zeng and colleagues (14) developed the FL-LIBS system, quantitatively analyzed the Mn, V and Si elements in steel, and compared them with conventional LIBS; Huang and coauthors (15) used femtosecond fiber laser to characterize the plasma time-resolved spectroscopy of aluminum, steel, glass and silicon; Jiang and associates (16) found that spark discharge can be used to improve the analytical sensitivity of FL-LIBS system; Furthermore, He and colleagues (17) used fiber optic high PRR laser-ablation to realize elemental analysis of aluminum alloys, built the calibration curves of Cr, Cu, Mn, Mg, and Zn, and determine the LOD of these elements; Parker and coauthors (18) used a megahertz Yb fiber laser oscillator to perform the ablation threshold analysis; Jean-François and associates (19) evaluated the potential of FL-LIBS, summarized and analyzed the coupling and performance of fiber lasers with different spectrometers; Guo and colleagues (20) used a fiber laser as a re-excitation source, instead of ablation source, to remove the moisture in the slurry samples; Kang and coauthors (21) used a calibration-free method in a high PRR laser-ablation spark-induced breakdown spectroscopy to analyze the elements of aluminum alloy samples quantitatively. The above research results demonstrated the feasibility and advantages of FL-LIBS for rapid elemental analysis.

However, there is little research on the self-absorption effects in FL-LIBS. Self-absorption effect (22) is the main reason affecting the accuracy of the spectrum. The photons emitted from the high-energy particle inthe center of plasmas are reabsorbed by similar particles of low energy in the outer layer (23,24). This leads to a reduction in the spectral intensity and causes the non-linearity of the calibration curve of the element directly. In the spectrum, the observed intensity of the characteristic spectral line is lower than the theoretical value, and even self-reversal is unavoidable. The self-absorption effect also causes inaccurate analysis results of LIBS, which is one of the main bottlenecks of LIBS towards applications. The researchers have proposed many interesting and fruitful solutions to suppress the self-absorption effect of LIBS, such as Hedwig and colleagues (25) utilized a helium (He) metastable excited state (LIPS-He*) to suppress the self-absorption effect; Yi and associates (26) adopted the method of selecting the collecting zones of the plasma carefully to reduce the self-absorption effect and improve the accuracy of LIBS; Tang and coauthors (22) established exponential calibration curves to investigate the temporal evolution of the self-absorption effect; Xiong and associates (27) proposed the fiber laser ablation LIBS (FLA-LIBS) system, which reduced the self-absorption effect and improved the accuracy of quantitative analysis; In the previous research of our team (28), it was also found that using the laser-stimulated absorption LIBS (LSA-LIBS) system can reduce self-absorption effect effectively.

Compared with that in LIBS system with low PRR lasers, the self-absorption effect is quite different under laser ablation with high PRR in FL-LIBS. Up to now, there is still no research focusing on the self-absorption effect in FL-LIBS, as far as we know.

In this work, to reduce the self-absorption effect and improve the linearity of the calibration curve of FL-LIBS, the basic phenomena and law of the self-absorption effect in FL-LIBS was experimentally investigated. This paper mainly studied the influences of fiber laser output power and single pulse ablation area (SPAA) on the self-absorption effect and revealed the self-absorption effect in the plasma spectrum of the ablation of micro-alloy steel samples based on fiber laser. At first, the phenomenon of fiber laser ablating the surface of steel samples was explored qualitatively. Then, the change in the regularity of the laser ablation zone and the index of the calibration curve was characterized.Finally, the influence of the self-absorption effect based on the fiber laser under different laser powers and SPAA was studied.

Experimental Setup and Methodology

Experimental Setup

The schematic of the FL-LIBS setup, which was used in this work, is shown in Figure 1. The laser beam was emitted by a fiber laser (Raycus Laser P30Q, 30 kHz, 126 ns, range of power 3 W-30 W) and focuses on the sample surface. In this study, the output power range of the laser was set from 15.9 W to 26.7 W. The defocus amount (DA) of the fiber laser can be adjusted by fine-tuning the height of the laser system up and down. The sample was fixedly placed on an X-Y movable platform, which was controlled by computer software. In our experiments, the horizontal movement adopted a serpentine movement path. The horizontal distance can be adjusted according to the size of the sample, and the vertical path spacing was fixed to 0.1 mm. The moving speed and the acceleration were set to 5 mm/s and 20 mm/s², respectively. The vertical height was adjusted according to the height of the sample so that the absolute height of the upper surface of the sample remained constant. At the same time, considering the difference of the flatness of the upper and lower surfaces of the sample, the horizontal indicator and the horizontal adjustment knob of the movable platform were used to adjust the flatness of the upper surface until the upper surface of the sample was parallel to the table. After each sample was replaced, the vertical height of the movable platform was adjusted and was consistent with the preset value. This process was detected by the displacement sensor (Panasonic, HG-C1000) to ensure repeatability in changing the samples.

Figure 1: Schematic of the experimental setup.

After the laser beam ablated the surface of the sample, the laser-induced plasma was generated on the sample surface. In the dimensions of full space and time resolution, the light emitted from the plasma plume was collected and coupled into an optical fiber with 7 cores. The other side of the optical fiber was connected to the spectrometer (Avantes, AvaSpec-ULS2048CL-EVO-RS, slit: 10 μm, wavelength range: 244-415 nm, resolution: 0.09 nm). Then the spectral data was transmitted to the computer and analyzed.

DA was converted into a quantitative indicator of SPAA, the calculation method of SPAA in this experiment can be obtained according to the area formula of the circle, since the laser beam used in this experiment was circular, ablation width can be used as the diameter of the circle, so the diameter of the ablation area can be directly used to approximately calculate the SPAA. Ablation width can be measured by a confocal microscope. In this work, a confocal microscope (Olympus, Japan, OLS40-SU) was used to observe and measure the laser ablation zone with different DAs under the same laser power, and the data point with the smallest value is defined as DA = 0.

Micro-Alloy Steel Samples

Seven certified steel samples (NCS Testing Technology Co., Ltd.) were used in this work. The concentration information of Mn is given in Table I.

Data Processing Method

The bremsstrahlung cannot be avoided by the synchronous delay method under laser ablation with high PRR. Therefore, the continuum background was intensive in the FL-LIBS spectra (14). There were several plasmas generated by high PRR laser pulse during an integration time. In this work, the discrete wavelet transform (DWT) (29) was used to self-adaptively remove spectral background. Furthermore, the Internal reference method (IR method) (30) was used to reduce spectral fluctuation generated by the fluctuation of ablative amount. The first-order exponential fitting method was adopted to obtain the calibration curves, the degree of self-absorption effect was reflected by the system self-absorption factor, and the quantitative analysis result was reflected by indicators including R², RMSECV, limit of detection (LOD), and RSD. It is worth mentioning that the integrated intensity was not used to replace the peak intensity in this experiment, because the continuous background is too strong, which will introduce additional errors.

Results and Discussion

In this work, the integration time was 1.2 ms (corresponding to 360 laser shots) in the experiment. We recorded 12 spectra continuously with 3 ms interval between each two measurements. The maximum and minimum values were removed to improve the accuracy and stability of the result.

Laser Ablation Zone and SPAA

The width of laser ablation zone under different defocus amounts is shown in Figure 2a. To explore whether the laser power had influence on SPAA, the output power of the fiber laser was tuned from 15.9 W to 26.7 W under the condition of DA = 0.

Figure 2: (a) Correspondence between DA and laser ablation zone width; (b) the influence of power on the ablation zone width; (c) laser ablation zone measurement chart.

The result is shown in Figure 2b. Compared with the DA value, the output power of the laser had no significant effect on the laser ablation zone width. Therefore, it can be considered that the width of the laser ablation zone only depends on the value of DA. Figure 2c shows the detail view of the laser ablation zone obtained by the high-speed scanning mode of the confocal microscope (The laser output power was fixed to 21.4 W, and only the case when DA = 0 was shown here).

According to these measurement charts, there is a corresponding relationship between the DA and the ablation zone width in the optical system. As shown in Figure 3, the SPAA values were deduced from the laser ablation zone width under the conditions of different DA values (Figure 2a). The laser-induced plasma was too weak to observe elemental lines in a spectrum when DA > 2.5 mm, where these data were ignored.

Figure 3: SPAA value corresponding to DA value.

Comparison of Spectral Processing Methods Based on FL-LIBS

In this work, the DWT (29) and IR (31) methods were utilized to reduce spectral background and experimental deviation. The DWT method decomposed the spectral data, then reduced low PRR component (background). The influence of bremsstrahlung was minimized, and the signal to background ratio (SBR) was improved. IR normalized the spectra with the peak value of a line of the matrix element (iron). The influence of the experimental condition fluctuation (such as laser pulse energy, and sample surface roughness) was decreased.

The comparisons of the spectrum and the calibration curve between before and after data preprocessing based on DWT-IR methods were shown in Figures 4 and 5. The DA was fixed at 0.5 mm, corresponding to SPAA approximately 3.28 x 10⁴ μm². The laser output power was set at 17.6 W. Mn Ⅰ 403.08 nm and Fe Ⅰ404.58 nm were used as the analytical and reference lines for IR, respectively. The spectral blank range for noise estimation was 409–410 nm.

Figure 4: Comparison results of the spectrum. (a) No processing; (b) DWT-IR.

Figure 5: Calibration curve for Mn concentration. (a) No processing; (b) DWT-IR.

In quantitative analysis, the calibration curve after adopting the DWT-IR has higher linearity and accuracy. In quantitative analysis, the main analytical criteria are shown in Table II. Using the DWT-IR improves the R² factor from 0.955 to 0.995. Further, the RMSECV, LOD and the RSD are reduced from 0.60 to 0.18 wt%, 1267 to 869.9 ug/g, 11.7 to 6.61%, respectively. These results show that DWT-IR has obvious advantages compared with the direct strength method in FL-LIBS. In general, spectral data were improved with data preprocessing.

The Influence of Laser Output Power and SPAA on the Degree of Self-Absorption

Self-absorption (SA) is the main criterion to quantitatively evaluate the self-absorption effect in LIBS, which is defined as equation 1 (32):

We define I^thick(λ₀) as the self-absorbed line peak intensity and I^thin(λ₀) as the peak intensity value without self-absorption under ideal conditions. However, the values of these parameters cannot be directly measured in the experiment. To satisfy actual measurement need, I and I₀ are defined as the observed spectral intensity and ideal spectral intensity, respectively. The range of SA is between 0~1. The lower SA means greater self-absorption.

Furthermore, the spectral intensity of plasma with elemental concentration C under optical thickness conditions is defined as (22):

A is the relative spectral line intensity here, and more details of the definition can be found in (32). We define αas the system self-absorption factor and I_b as the intensity of the background. These parameters can be obtained by first-order exponential fitting of data points.

According to the definition of SA and considering that 1-e^-αC can be replaced by an equivalent infinitesimal αC under the optically thin condition. The equation can be expressed as (22,28,33):

According to equation 3, the SA value, in general, depends on not only elemental concentration C, but also the αvalue, so it can only reflect the size of self-absorption at the microscopic level, while the α value reflects the self-absorption of the system and the linearity of the calibration curve. The values of SA and α are negatively correlated at the same concentration, and the higher value of α represents more serious self-absorption. In other words, α can be used as a property criterion to evaluate the degree of self-absorption of the LIBS system, which does not depend on the element concentration and can be obtained by calibration curve.

As shown in Figure 6, the α factor was affected by the laser output power and SPAA when the output power and SPAA were set at 26.7 W and 5.5 × 10⁴ μm², respectively. The αvalue elevated abruptly and exceeded 1.8, which indicated a very serious self-absorption effect. There was also a relatively serious self-absorption effect in the rectangular area where the power was 20~23 W and the SPAA was more than the 4.3 × 10⁴ μm². A self-absorption valley appeared (α ≈ 0.61) when the power and SPAA were 16~18 W and 4.1 x 10⁴ μm² respectively.

Figure 6: The influence of SPAA and laser output power on system self-absorption factor.

Figure 7 shows the comparison of the calibration curves under different degrees of self-absorption.The labels on the bottom right corner correspond to the data points in Figure 6, and the comparison parameters and quantitative analysis indicators are shown in Table III. The result of Figure 5b from earlier in this paper is also attached here.

Figure 7: Calibration curve under different laser parameters. (a) Power 15.9 W, SPAA 4.11 x 10⁴ μm² (b) Power 26.7 W, SPAA 5.53 x 10⁴ μm²; and (c) Power 19.7 W, SPAA 4.29 x 10⁴ μm²

Table III shows more details about the quantitative analysis index of Figure 7 and Table II. Compared with Figure 7c, the first case illustrated in Figure 7a shows the obvious reduction of the self-absorption effect, where the linearity of the fitted curve is better. The higher R² value represents closer scatters of data points to the fitted calibration curve, and the error bars were shorter, in quantitative analysis. All data in the first case are significantly better than the others, except LOD, because to laser energy is the lowest.

According to equation 1, optimizing the laser parameters in this work reduced the optical thickness of the high PRR laser-induced plasma from optical thick to optical thin, thus the self-absorption effect was weakened. Self-absorption effect was enhanced following the increase of laser output power, from the overall trend in Figure 7, which is contrary to conventional LIBS based on low PRR (Such as 10 Hz) laser ablation (34). The reason for this difference may be that the uncooled plasma produces a shielding effect on the next laser pulse, resulting in a decrease of the laser energy that reach the sample surface and causing a decrease in the plasma temperature. in addition, the shielding effect is enhanced faster than the laser energy. For the general trend of SPAA, the higher SPAA caused a serious self-absorption effect. This is because a higher SPAA will result in larger volume of plasma, in other words, plasma has a larger contact area with air, which was cooled easier.

Conclusions

In this work, a simple FL-LIBS system was adopted to experimentally investigate the self-absorption effect in manganese determination in micro-alloy steel. Ablation law based on fiber laser was studied, and the influence of DA value and laser output power on the laser ablation zone was explored. Furthermore, the DWT-IR was used to process the spectral data. Then, the system self-absorption factor α was used to quantitatively evaluate the self-absorption effect. The influence of SPAA and laser output power on the α valuewas explored.Comparing the difference of self-absorption under different parameters, it was found that using appropriate DA values (or SPAA) and laser output power can significantly reduce the influence of the self-absorption effect, which can be reflected in the linearity of the calibration curve.At the same time, fitting reliability, analysis accuracy, and stability havealsobeen improved. Finally, it was concluded that the self-absorption effect was enhanced following the increase of laser output power in high PRR FL-LIBS, contrary to conventional LIBS based on low PRR laser ablation. Moreover, the higher SPAA also caused a serious self-absorption effect.This work realizes the suppression of the self-absorption effect and serves LIBS technology to the practical industrial application in the future through the reasonable selection of laser parameters.

Acknowledgments

Key-Area Research and Development Program of Guangdong Province (2020B090922006); National Natural Science Foundation of China (62005081); Guangdong Basic and Applied Basic Research Foundation (2021A1515011932, 2020A1515110985, 2019A1515111120); Science and Technology Program of Guangzhou (202002030165); Featured Innovation Project of Guangdong Education Department (2019KTSCX034); Special Funds for the Cultivation of Guangdong College Students' Scientific and Technological Innovation (pdjh2020b0153).

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Yuzhi Qin, Zhiying Xu, Dongsen Cai, Jiaming Li, Qiongxiong Ma, Nan Zhao, Liang Guo, Qingmao Zhang, and Qitao Lue are with the Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices at the School of Information and Optoelectronic Science and Engineering of South China Normal University, in Guangzhou, China, and the State Key Laboratory of Optic Information Physics and Technologies of South China Normal University, in Guangzhou, China. Qitao Lue is with theGuangdong Provincial Key Laboratory of Industrial Ultrashort Pulse Laser Technology, Shenzhen 518055, China. Direct correspondence to jmli@m.scnu.edu.cn or zhangqm@scnu.edu.cn