Exploitation of Fiber Laser Induced Breakdown Spectroscopy on the Quantitative Analysis of Aluminum Alloys

Abstract

With fiber laser induced breakdown spectroscopy, we explored the influence of pulse width (2–200 ns) and repetition rate (1 Hz to 1 MHz) on the quantitative results of elements in aluminum alloys. Moderately long pulse width was found to be beneficial to the improvement of detection sensitivity under the circumstance of limited pulse energy. Up to 40.4% enhancement of sensitivity and a 27.0% improvement in limit of detection (LOD) were achieved under high laser repetition rate, resulting in an agreeable LOD as low as 17.9 ppm considering the low laser irradiance applied here. The ranking of Mg > Mn > Cu > Fe > Zn was observed in terms of detection sensitivity, which was altered to Mg > Zn > Mn > Cu > Fe as the enhancement level was considered. The melting and boiling points of these elements were found to be highly correlated with the enhancement, which was ascribed to the efficient heat accumulation process under shortened inter-pulse delay that facilitates the melting and evaporation of elements having low melting and boiling points. Though helpful for improving detection sensitivity, high repetition rate may induce elemental fractionation and should be applied cautiously.

Keywords: fiber laser, laser induced breakdown spectroscopy, quantitative analysis, pulse width, repetition rateIntroduction

Introduction

As the power source of laser-induced breakdown spectroscopy (LIBS), the laser plays a vital role in the establishment of LIBS methods, whose rapid development greatly boosts innovation in LIBS instrumentation, methodology, and application. Among them, the gradual maturing of fiber laser technology in robustness, high beam quality (M² ≈ 1), adjustability in pulse width (up to hundreds of nanoseconds) and repetition rate (up to the level of MHz) is drawing attention from analytical chemists.¹

The applicability of fiber laser in LIBS analysis has already been assessed by several research groups. Compared to the routinely used Nd:YAG laser LIBS, fiber laser LIBS (FL-LIBS) has been evaluated to feature similar analytical figures of merit in terms of signal-to-background ratio, calibration curve linearity, sensitivity, and limit of detection,^1-4 and even better spectral simplicity with a higher signal-to-background ratio and dominant atom or monovalent ion emission lines has been observed.⁵

Additionally, the ease of tuning pulse width in the nanosecond range and repetition rate up to the MHz level makes fiber laser an ideal source to investigate the underlying mechanism of LIBS and the effect of the parameters on analytical results, as both of these parameters are quite difficult to be manipulated in other types of lasers. However, the relevant study has been rather scarce. For example, Elnasharty et al. (40 ns to 200 ns)⁶ and Gravel et al. (30 ns and 200 ns)¹ studied the influence of pulse width on line intensity, background emission, ablation rate, pit morphology, plasma absorption coefficient, excitation temperature, and density; Parker et al. (up to 2 MHz)⁷ and Riedel et al. (1 kHz to 1550 kHz)⁸ investigated the effect of laser repetition rate on metal ablation threshold and signal intensity. Generally in these studies the commercial fiber laser worked incessantly with repetition rate of more than 1 kHz without precise control of the number of laser shots, which makes it difficult to investigate the effect of repetition rate.

In this research, we established a FL-LIBS apparatus with the availability of achieving single-shot working mode and precisely controlling the number of laser shots, which enabled the dependent investigation of the effect of laser repetition rate. To fully exploit the merits of fiber laser, we carried out an integrated laser parametric study of pulse width and repetition rate on aluminum alloys. The possible underlying mechanism was tentatively discussed. Finally, the influence of repetition rate on quantitative result was evaluated and discussed.

Materials and Methods

Materials

Eight certified aluminum alloy standard samples (ZBY523a, ZBY524a, ZBY528, ZBY531, ZBY533, ZBY534, ZBY538, ZBY540) were purchased from Jinan Zhongbiao Technology Co., Ltd.

Manipulation of Fiber Laser

YDFLP-E-30-M7 series pulsed fiber laser (JPT Opto-Electronics, China) with master oscillator power amplifier (MOPA), central wavelength of 1064 nm, maximum average delivering power of 30 W, and beam quality factor M² < 1.4 was utilized in this study. When a pulse width is fixed, there exists a threshold trigger frequency at which the maximum pulse energy can be achieved, as shown in Table 1. Hence, the laser parameters were set according to Table 1 in this study.

Table 1: Features of the fiber laser utilized in this study.

To precisely control the laser repetition rate, first the trigger pulses should be delivered at the desired frequency, which is typically no less than 1 kHz, and the Master Oscillator signal (MO) should be on. Afterward, the laser pulses will be fired as soon as the Power Amplifier (PA) signal is on. In this research, we manufactured a homemade electronic card, which is capable of setting the laser repetition rate at a desired value (less than the trigger frequency) by adjusting the time delay between two consecutive PA pulses.

Instrumental Setup

Figure 1. Experimental setup of the fiber laser LIBS.

The schematic instrumental setup is illustrated in Figure 1. The average output power of the fiber laser was measured by an optical power meter (Fieldbest, China). The fired laser was reflected by a mirror and then focused onto the sample by a quartz lens with a focal length of 63 mm, resulting in an ablation pit diameter of about 50 μm.

The sample was placed on an electrically driven 2D moving stage (Jiangyun Opto-electronics, China) to refresh the ablation spot. To maximize the collected plasma emission light, an optical fiber (core diameter: 600 μm, NA: 0.22) was placed 5 mm horizontally and 2 mm vertically away from the ablation spot without focusing lenses. The collected light was detected by a two-channel compact Czerny-Turner spectrometer with a slit of 10 μm, detection range of 153–412 nm, and spectral resolution better than 0.20 nm (Oceanhood Opto-Electronics, China). No gating operations were applied to the spectrometer. The synchronization between the laser and spectrometer was realized by the homemade electronic card controlled by a program built in LabVIEW (version 2020, USA).

Experimental Methods

For each measurement in this study, two consecutive laser shots were delivered on the same spot to acquire one spectrum. For each pulse width (Table 1), the repetition rate was arranged in the pattern of “1, 2, 5” in each order of magnitude until reaching the corresponding threshold trigger frequency. Each spectrum was acquired with an integration time of 2 s, and 50 consecutive measurements were executed in each case.

To estimate the plasma lifetime, a high-speed photodiode detector (Xunmiao Optoelectronics, China) was deployed to sense the undispersed plasma emission light directly collected by the optical fiber, the electric signal of which was further measured by an oscilloscope (DS1202Z-E, RIGOL, China) with a sampling rate of 1 GSa/s and bandwidth of 200 MHz.

For the evaluation of the effect of laser repetition rate on quantitative results, 1 Hz and 37 kHz were compared under the pulse width of 200 ns. The peak intensities of Mg I 383.88 nm, Mn I 403.07 nm, Cu I 324.75 nm, Fe I 373.50 nm, and Zn I 334.50 nm were picked and background-corrected by the corresponding background emissions at nearby wavelengths.

Results and Discussions

Estimation of Plasma Characteristics

Figure 2: Estimation of plasma characteristics. (a) Spectrum, (b) plasma lifetime, (c) Boltzmann plot of Al I 305.05 nm, 305.79 nm, 306.67 nm, 309.29 nm, and Al II 358.72 nm and (d) Lorentz fitting of Al II 281.62 nm under one laser shot of 200 ns, 0.792 mJ, 1 Hz.

An example of the acquired spectrum is shown in Figure 2a (200 ns, 0.792 mJ). Through 50 repetitive measurements, the relative standard deviation of Al I 309.29 nm was estimated to be 15.2%, which is an agreeable stability result for LIBS measurements. Moreover, the duration of overall plasma emission is exhibited in Figure 2b, from which the plasma lifetime can be deduced as about 50 μs. This is relatively long compared with those commonly observed in LIBS experiments,⁹ which can be ascribed to the different light collection method and long pulse width applied herein.

We applied the Saha-Boltzmann plot method (Al I 305.05 nm, 305.79 nm, 306.67 nm, 309.29 nm, and Al II 358.72 nm, Figure 2c) and Stark broadening method (Lorentz fitting of Al II 281.62 nm, Figure 2d) to calculate the plasma excitation temperature and electron density,¹⁰ the maximum values of which were (6668±69) K and (19.9±1.0)×10¹⁶ cm^-3, respectively. By observing the peak shape of the acquired spectra, no self-reversal was witnessed (with an example of Al II 281.62 nm shown in Figure 2d), which indicates the optically thin state of plasma.

Influence of Pulse Width and Repetition Rate

Figure 3: Influence of laser repetition rate and pulse width on (a) intensity of Al I 309.29 nm, (b) excitation temperature, (c) electron density, and (d) relative signal enhancement.

The influence of laser parameters on plasma characteristics is exhibited in Figure 3. Vertically, one can observe the general trend that all the parameters increase with longer pulse width. It should be noted that the maximum achievable pulse energy at each pulse width grows with the lengthening of pulse width according to Table 1. As pulse energy rises, sample temperature increases, leading to greater ablation and higher initial particle density. Moreover, the longer pulse width prolongs the light absorption process in the plasma, which is conducive to further improvement of plasma temperature and hence particle excitation.

Regarding the effect of repetition rate, the line intensity remains largely insensitive to increases in repetition rate at low pulse widths, yet shows an obvious rising trend at higher pulse widths. On the other hand, excitation temperature ramps slowly with increasing repetition rate, while electron density remains almost unchanged. In short laser pulses, the heat dissipation is rapid and the pulse energy is low; therefore, heat can hardly be preserved after the first laser shot. Yet with the rise of pulse width, the surrounding area of the irradiated spot has already gained much heat after a long period of laser-solid interaction, which relatively weakens heat dissipation from the central spot; hence, the slower heat dissipation and higher pulse energy make the heat accumulation effect indispensable as the repetition rate elevates. As a result, the total ablation is magnified without obvious alteration of plasma characteristics, leading to an increase in line intensity.

Interestingly, by carefully examining Figures 3a and 3b, the line intensity enhancement and excitation temperature elevation become quite obvious within the pulse width of 20–45 ns. To view it more clearly, we plotted the enhancement and excitation temperature versus pulse width in Figure 3d, in which a regional maximum occurs at 20 ns. According to Noll, for pulse widths between 1 and 50 ns, the laser energy partitioning for material evaporation and energy coupling into the radiating plasma seems to be in a favorable state.¹¹ It has been reported that ablation rate improves with longer pulse width,^1,12 which delivers more material to the plasma. Meanwhile, the moderately prolonged pulse width lengthens the laser-plasma interaction, which further excites the existing analytes. However, as the pulse width further rises to more than 100 ns, the energy loss due to the long heat dissipation time may lower the laser energy utilization efficiency, which diminishes the enhancement effect of elevating repetition rate. In summary, the conditions of limited pulse energy, moderately high pulse width, and high repetition rate should be preferred to achieve better analytical sensitivity in FL-LIBS.

Influence of Repetition Rate on Quantitative Results

Figure 4: Calibration curves of (a) Mg, (b) Mn, (c) Cu, (d) Fe, and (e) Zn under the repetition rates of 1 Hz and 37 kHz, respectively.

To optimize the detection sensitivity, we selected the pulse width of 200 ns and repetition rate of 37 kHz, at which the maximum pulse energy reaches the largest value of 0.792 mJ (Table 1). Figure 4 shows the calibration curves of Mg, Mn, Cu, Fe, and Zn in aluminum alloy samples, all of which show good fitting linearity. Note that some high concentration points have been removed in the fitting depending on the linear range of the selected line, and none of the low concentration points have been discarded.¹ Comparing the slopes of the calibration curves, the detection sensitivity of Mg, Mn, Cu, Fe, and Zn has been improved by 40.4%, 20.0%, 15.7%, 7.5%, and 33.7%, respectively. Improvement of signal intensity and ablation threshold at high repetition rate has also been reported in other studies,^7,13 but experimental conditions differed considerably in those studies.

To further evaluate the analytical figures of merit, we calculated the LOD of the elements by the generally accepted 3σ rule:⁹

where σ is the standard deviation of the background emission, estimated at a wavelength close to the analytical line, and s is the slope of the calibration curve. The ranking of Mg (17.9 ppm) > Mn (19.3 ppm) > Cu (37.3 ppm) > Fe (169.4 ppm) > Zn (268.5 ppm) can be observed under the repetition rate of 37 kHz with an enhancement of 27.0%, 14.3%, 7.3%, 4.7%, and 21.0%, respectively. We also collected data from other FL-LIBS applications and compiled them into Table 2. Similarly, the ranking of Mg > Mn > Cu > Fe was also observed in the studies done by Gravel et al.¹ and Xu et al.². The LODs of Fe and Zn in this study were significantly higher than those of Mg, Mn, and Cu, which is also the case in Gravel’s study.¹ Better LOD results were achieved by Gravel et al.¹ and Elnasharty et al.¹² compared with our results. It may be ascribed to the much higher irradiance (2.1 GW/cm²)¹ and wider spectrometer entrance slit (120 μm),¹² which respectively improve ablation and excitation and magnify the spectroscopic light flux onto the detector. Considering the low laser irradiance and narrow spectrometer entrance slit in this study, the LOD results are agreeable.

Table 2: LODs (ppm) of Mg, Mn, Cu, Fe, and Zn in aluminum alloys using FL-LIBS as reported in the references.

Interestingly, we found agreeable correlations between sensitivity enhancement (Mg (40.4%) > Zn (33.7%) > Mn (20.0%) > Cu (15.7%) > Fe (7.5%)) and melting (Y = −0.027X + 50.05, R² = 0.8230) or boiling points (Y = −0.015X + 51.58, R² = 0.9269). The heat accumulation effect is believed to be responsible for this observation. When the repetition rate grows, the interval of two consecutive pulses is shortened, which raises the initial sample temperature when the second pulse begins. As a result, the average sample temperature is elevated more quickly, which is beneficial for the melting and evaporation of elements featuring low melting and boiling points.

Conclusion

In this study, the effect of laser pulse width and repetition rate on the quantitative results of elements in aluminum alloys was reported. Increasing the pulse width facilitates an increase in signal intensity. However, local maxima of signal can still be observed at the moderate pulse width of 20 ns regardless of its relatively low pulse energy, which supports the fact that a moderately long pulse width is conducive to improving the analytical results of LIBS. The rise of laser repetition rate from 1 Hz to 37 kHz markedly improves the detection sensitivity and LOD. Furthermore, the relative enhancement of the elements was found to be highly correlated with their melting and boiling points, which was ascribed to the significant heat accumulation effect induced by high laser repetition rate.

Nevertheless, care should be taken that high repetition rate may induce a certain level of elemental fractionation during LIBS processes, which may deteriorate the analytical results in some applications and necessitates further investigation to prevent this effect from becoming too severe.

Acknowledgement

This work was supported by the Natural Science Foundation of Xiamen City of China (Grant No. 3502Z20227312), the Natural Science Foundation of Fujian Province of China (Grant No. 2022J05321), and the Cultivation Project of Youth Talent of Xiamen Medical College (Grant No. K2023-15).

Conflict of Interest

The authors declare no conflicts of interest.

References

(1) Gravel, J.-F. Y.; Doucet, F. R.; Bouchard, P.; Sabsabi, M. Evaluation of Compact High Power Pulsed Fiber Laser Source Laser-Induced Breakdown Spectroscopy. J. Anal. Atom. Spectrom. 2011, 26 (7), 1354–1361. DOI: 10.1039/C0JA00228C

(2) Xu, Z.; Xu, B.; Peng, X.; et al. Laser-Ablation Dependence of Fiber-Laser-Based Laser-Induced Breakdown Spectroscopy for Determining Cu, Mg, and Mn Elements in Aluminum Alloys. J. Anal. Atom. Spectrom. 2021, 36 (11), 2501–2508. DOI: 10.1039/D1JA00188D

(3) Huang, H.; Yang, L.-M.; Liu, J. Qualitative Assessment of Laser-Induced Breakdown Spectra Generated with a Femtosecond Fiber Laser. Appl. Opt. 2012, 51 (36), 8669–8676. DOI: 10.1364/AO.51.008669

(4) Zeng, Q.; Guo, L.; Li, X.; et al. Quantitative Analyses of Mn, V, and Si Elements in Steels Using a Portable Laser-Induced Breakdown Spectroscopy System Based on a Fiber Laser. J. Anal. Atom. Spectrom. 2016, 31 (3), 767–772. DOI: 10.1039/C5JA00453E

(5) Baudelet, M.; Willis, C. C. C.; Shah, L.; Richardson, M. Laser-Induced Breakdown Spectroscopy of Copper with a 2 μm Thulium Fiber Laser. Opt. Express 2010, 18 (8), 7905–7910. DOI: 10.1364/OE.18.007905

(6) Elnasharty, I. Y. Study on the Influence of Laser Pulse Duration in the Long Nanosecond Regime on the Laser Induced Plasma Spectroscopy. Spectrochim. Acta Part B: At. Spectrosc. 2016, 124, 1–15. DOI: 10.1016/j.sab.2016.08.004

(7) Parker, G. J.; Parker, D. E.; Nie, B.; Lozovoy, V.; Dantus, M. Laser-Induced Breakdown Spectroscopy and Ablation Threshold Analysis Using a Megahertz Yb Fiber Laser Oscillator. Spectrochim. Acta Part B: At. Spectrosc. 2015, 107, 146–151. DOI: 10.1016/j.sab.2015.02.011

(8) Riedel, J.; Hufgard, J.; You, Y. LIBS at High Duty-Cycles: Effect of Repetition Rate and Temporal Width on the Excitation Laser Pulses. Front. Phys. 2023, 11, 1241533. DOI: 10.3389/fphy.2023.1241533

(9) He, X.; Li, R.; Chen, Y. Application of Fiber Optic High Repetition Rate Laser-Ablation Spark-Induced Breakdown Spectroscopy on the Elemental Analysis of Aluminum Alloys. Appl. Opt. 2019, 58 (31), 8522–8528. DOI: 10.1364/AO.58.008522

(10) Wang, Y.; Chen, Y.; Li, R.; Kang, J.; Gao, J. Quantitative Elemental Analysis of Aluminum Alloys One-Point Calibration High Repetition Rate Laser-Ablation Spark-Induced Breakdown Spectroscopy. J. Anal. Atom. Spectrom. 2021, 36 (2), 314–321. DOI: 10.1039/D0JA00398K

(11) Noll, R. Laser-Induced Breakdown Spectroscopy, 1st ed.; Springer, 2012; pp 17–46.

(12) Elnasharty, I. Y.; Doucet, F. R.; Gravel, J.-F. Y.; Bouchard, P.; Sabsabi, M. Double-Pulse LIBS Combining Short and Long Nanosecond Pulses in the Microjoule Range. J. Anal. Atom. Spectrom. 2014, 29 (9), 1660–1666. DOI: 10.1039/C4JA00099D

(13) Leme, F. O.; Godoi, Q.; Kiyataka, P. H. M.; et al. Effect of Pulse Repetition Rate and Number of Pulses Analysis of Polypropylene High Density Polyethylene by Nanosecond Infrared Laser Induced Breakdown Spectroscopy. Appl. Surf. Sci. 2012, 258 (8), 3598–3603. DOI: 10.1016/j.apsusc.2011.11.122

(14) Scharun, M.; Fricke-Begemann, C.; Noll, R. Laser-Induced Breakdown Spectroscopy with Multi-kHz Fibre Laser for Mobile Metal Analysis Tasks — a Comparison of Different Analysis Methods and with a Mobile Spark-Discharge Optical Emission Spectroscopy Apparatus. Spectrochim. Acta Part B: At. Spectrosc. 2013, 87, 198–207. DOI: 10.1016/j.sab.2013.05.007