Low-energy laser-induced breakdown spectroscopy device and use thereof

Abstract

The invention relates to a fast-analysis low-energy laser-induced breakdown spectroscopy device optimised by a spectrum controller in real-time, and to a spectroscopy method implemented by means of such a device. In particular, the invention relates to an atomic emission laser-induced breakdown spectometry device comprising: a low-energy pulsed laser (100) with a high pulse repetition rate, a focusing lens (104) configured to focus said laser on the surface of a sample (106) so as to create a series of micro-plasmas (108), an optical spectrometer (114) allowing the measurement of the spectrum of a microplasma (108) generated by one or more laser pulses at a time, and a controller (117) with an integrated digitiser and a processor configured to control the clock of the spectrometer (114), to digitise said measurement of the spectrum, and to validate each measurement in real time if the intensities of certain reference lines of the spectrum are within a predefined interval.

Description

[0001] LOW ENERGY LASER-INDUCED PLASMA SPECTROSCOPY APPARATUS AND ITS USE

[0002] FIELD OF THE INVENTION TO WHICH THE INVENTION RELATES

[0003] The present invention is in the field of laser-induced plasma spectroscopy (LIBS), and more particularly of laser-induced plasma spectroscopy using low energy, high repetition rate laser pulses.

[0004] PREVIOUS ART

[0005] Laser-induced scaling spectroscopy (LIBS) devices or apparatus [1,2] are used to accurately detect the elemental concentration of elements. These devices typically include a laser that heats a portion of the sample sufficiently to produce plasma. When the plasma cools, the electrons return to their ground state. In this process, photons are emitted at specific wavelengths, characteristic of the elements constituting the sample. Laser parameters have been shown to strongly influence the interaction between the sample material and the laser and consequently the analytical capabilities of the method. The influence of laser wavelength [3], laser fluence [4], pulse duration [5], and pulse burst [6,7] on plasma properties and the analytical capabilities of LIBS have been extensively studied in the literature.

[0006] The classical approach to LIBS equipment uses a high-power, low-repetition-rate laser to improve its sensitivity, with a synchronized spectrometer to avoid collecting the plasma continuum. However, efforts have been made to be able to use a low-energy laser, since low-power laser diodes have a rather high beam quality, while high-power laser diodes always have, fundamentally, a worse beam quality. Essentially, this is due to the fact that high powers require large emission apertures that make the waveguides used highly multimode. A laser-induced shunt spectroscopy (LIBS) device based on a high-repetition-rate pulsed laser has been presented in patent application US20160069745A1 [8]. In this document, it is noted that the spectrometer collects the LIBS signal from thousands of micro-plasmas.

[0007] By adjusting the integration time of the spectrometer to cover a plurality of periods of the laser pulse train, the spectrometer integrates the LIBS signal produced by this plurality of laser pulses, thereby improving the signal-to-noise ratio (SNR). In addition, the influence of pulse energy variation is minimized, because the resulting LIBS spectrum is that of a plurality of micro-plasmas produced by a plurality of laser pulses.

[0008] A low energy laser with a high repetition rate has also been proposed to be used in conjunction with a movable focusing lens controlled by a controller, to ensure an optimal focusing condition by analyzing the intensity observed at the spectrometer output [9]. It is also discussed how to improve the fluence of low energy lasers by using a small focal length lens, which allows a small laser spot on the material surface. However, it has been shown experimentally that the laser spot on the material surface determines the ablation rate and the efficiency of plasma generation [10-13].

[0009] Small spot dimensions induce micro-localized ablations and form micro-craters where the plasma profile is disturbed. It is experimentally observed that the intensities of atomic or ionic lines in the spectrum vary considerably, even for the main elements, mainly due to the formation of these ablation craters inducing laser defocusing on the material surface. The crater induced by the ablation of materials on the surface does not allow to maintain optimal laser focusing and a constant plasma profile during the collection of the multitude of microplasmas. This phenomenon is all the more noticeable for low energy lasers, where the slightest defocusing seriously degrades the plasma intensity. Therefore, the reproducibility performances remain below the expectations of some applications that require low variability from one measurement to another, for the same sample.

[0010] Furthermore, the use of spectrometers without a controller integrated into the video signal digitization process does not allow the spectrum to be analyzed in real time, as the spectrometer camera pixels are digitized, but only at the end of the camera scan, which causes significant delays in the analysis, especially in the case where the plasma is generated by a plurality of laser pulses.

[0011] There is therefore a serious need for improvement of a method and apparatus for performing laser-induced plasma atomic emission spectroscopy on a targeted sample using low-energy lasers.

[0012] BRIEF DESCRIPTION OF THE INVENTION

[0013] The aim of the invention is to provide a compact and fast LIBS laser-induced plasma spectroscopy apparatus, which allows high performance in reproducibility of results, comprising a low-energy, high-repetition-rate Q-switched laser, focused using a lens, also comprising a spectrometer, the clocks of the spectrometer camera being controlled by a controller, the high and low times of the clock cycle being defined by the digitalization time of the camera signal which integrates within it a process of real-time analysis of the intensities of some predefined spectral lines and which validates the measurement if these line intensities are in a predefined intensity interval.

[0014] In the case of invalidation, the controller interrupts the spectrometer camera scan by reducing the clock time and the integration time. Thus, scan times are reduced to a strict minimum during spectra invalidation, ensuring fast operation suitable for high repetition rate lasers. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to better understand the invention, its embodiments will now be described in detail by way of example, with reference to the appended drawing (Fig. 1) which schematically represents one of the embodiments of a LIBS apparatus according to the present invention.

[0016] DETAILED DESCRIPTION OF THE INVENTION

[0017] Before describing in detail embodiments that are in accordance with the present invention, it should be noted that the embodiments reside primarily in combinations of method steps and the interaction of apparatus components related to a laser-induced plasma spectroscopy (LIBS) apparatus, based on a pulsed laser together with a spectrometer and a spectrum controller and analyzer. Accordingly, the apparatus components and method steps have been represented where necessary by conventional symbols in the drawings, showing only the specific details relevant to understanding the embodiments of the present invention, so as not to obscure the disclosure with details that will be readily apparent to those skilled in the art.

[0018] An exemplary embodiment of a laser-induced plasma spectroscopy (LIBS) apparatus is shown in Fig. 1. The LIBS apparatus comprises a pulsed laser 100 as an excitation light source. The pulsed laser 100 is a passively or actively switched laser or a mode-locked laser, or preferably a Q-switched diode-pumped solid-state (DPSS) laser, capable of producing a train of laser pulses at a high repetition rate, preferably greater than 1 Khz. The pulse width of the laser is preferably less than 10 nanoseconds (ns). The laser beam 102 from the pulsed laser 100 is focused by a focusing lens 104 onto a surface of the sample 106. The laser pulse produces a plasma emission, i.e., a LIBS signal 108 at the surface of the sample 106 collected by a collection lens 110, and is then focused into a light guide 112, such as an optical fiber or a fiber bundle.

[0019] The light guide 112 then delivers the LIBS signal 108 to an optical spectrometry device 114 for spectral analysis. The optical spectrometer 114 is typically a CCD, CMOS, or NMOS linear camera-based spectrometer whose operation is governed by a controller 117 that controls all the clocks required by the camera, as well as the integration time through a processor, microprocessor, or microcontroller. The integration time of the spectrometer is set so that the collected plasma is generated by a single laser pulse or by multiple laser pulses.

[0020] In order to enable fast and real-time analyses, the controller 117 consists of a processor integrating a digitizer. The controller 117 controls the clock cycles of the camera of the spectrometer 114, and where the clock frequency is determined by the digital conversion time of the analog video signal recovered pixel by pixel. At each clock cycle, the intensity of the digitized pixel is analyzed by the controller 117 which compares the intensities of certain predefined lines with respect to threshold values ​​predefined in advance. The measurement is validated by the controller 117 when the intensities of certain predefined atomic or ionic lines are included in a range of values ​​defined in advance.In the case where the intensity of the reference line is not included in the predefined intensity interval, the scan of the spectrometer camera 114 is interrupted by reducing the clock time to its strict minimum in accordance with the specifications of the camera and by decoupling the clock cycle from the digitization time. This is achieved by avoiding the digitization of the analog video signal and the storage of the data, which increases the clock frequency and decreases the integration time and the scan time.

[0021] The analyses of the intensities of the reference lines could be carried out either as the clock cycles that allow switching from the analog reading of one pixel to the next, or after the camera scan is completed, the former case being more optimal. The LIBS apparatus further comprises an optical window 116 positioned in front of the lens 104 to protect its optical components from contamination. In a minor variation of the LIBS apparatus, the collection lens 104 and the focusing lens 110 can be replaced by other types of optical focusing elements, such as concave mirrors, to avoid chromatic aberration of the optical lenses. The optical spectrometer 114 is coupled to a controller 117 which allows the measurement to be analyzed and validated or invalidated.

[0022] INDUSTRIAL APPLICATION

[0023] The present invention is in no way limited to the embodiments described and shown, but those skilled in the art will be able to make any variation in accordance with its spirit.

[0024] REFERENCES

[0025] [1] Handbook of Laser-Induced Breakdown Spectroscopy ed. DA Cremers, LJ Radziemski, John Wiley & Sons, England, 302 p. 2006

[0026] [2] Laser Induced Breakdown Spectroscopy ed. AW Miziolek, V. Palleschi, I. Schechter, Chambridge University Press., United Kingdom, Cambridge, 638p., 2006

[0027] [3] LM Cabalin, JJ Laserna, “Experimental determination of laser induced breakdown thresholds of metals under nanosecond Q-switched laser operation”, Spectr. Acta B, 1998, 53, 723 - 730

[0028] [4] Y.-l. Lee, J. Sneddon, “Spatial and temporal characteristics of an excimer laser induced lead plasma emission”, Spectrosc. Lett., 1996, 29, 1157-1171

[0029] [5] V. Margetic, A. Pakulev, A. Stockhaus, M. Bolshov, K. Niemax, R. Hergenrôder, “A comparison of nanosecond and femtosecond laser induced plasma spectroscopy of brass samples”, Spectrochim. Acta, Part B, 2000, 55, 1771 -1785

[0030] [6] F. Colao, V. Lazic, R. Fantoni, S. Pershin, “A comparison of single and doublepulse laser-induced breakdown spectroscopy of aluminium samples. “, Spectrochim. Acta. B, 2002, 57, 1167-1179 [7] V.l. Babushok, F.C. De Lucia Jr, J.L. Gottfried, C.A. Munson, and A.W. Miziolek, “Double pulse laser ablation and plasma: Laser induced breakdown spectroscopy signal enhancement”, Spectrochim. Acta, Part B, 2006, 61 , 999-1014

[0031] [8] US20160069745A1 Laser Induced Breakdown Spectroscopy (LIBS) Apparatus Based on High Repetition Rate Pulsed Laser, Sean Xiaolu Wang, Qun Li B&w Tek C / o Blank Rome LLC LLP

[0032] [9] US20140204377A1 Handheld libs spectrometer, David R. Day, Konstantin Derman, John Francis Egan, Paul Edward Soucy, Sciaps Inc.

[0033]

[0010] . D. Bâuerle, Laser Processing and Chemistry, Springer- Verlag, Berlin Heidelberg New York (2000).

[0034]

[0011] , M. von Allmen, A. Blatter, Laser-Beam Interaction with Materials, Springer- Verlag, Berlin (1995).

[0035]

[0012] , T.E. Itina, B.J. Garrison, L.V.Zhigilei, Limit of overheating and the threshold behavior in laser ablation, Physical Review E 68, p. 041501 .1 - 041501 .4 (2003)

[0013] . M. Hasida, A.F. Semerok, O. Gobert, G. Petite, Y. Izawa, J.F. Wagner, Ablation threshold dependence on pulse duration for copper, Applied Surface Science 197- 198, p. 041501.1 -041501.4 (2002)

Claims

AMENDED CLAIMS received by the International Bureau on April 17, 2025 (17.04.2025) 1. Laser-induced plasma atomic emission spectrometry apparatus comprising: - A low energy pulsed laser (100) with a high repetition rate greater than 1KHz, A focusing lens (104) configured to focus said laser on the surface of a sample (106) so as to create a series of micro-plasmas and an optical spectrometer (114) allowing the measurement of the spectrum of a micro-plasma generated by one or more laser pulses at a time, and - A controller (117) consisting of an electronic circuit, said electronic circuit consisting of a digitizer and a processor, said electronic circuit allowing communication between the digitizer and the processor, said processor being configured to control the clock of the spectrometer camera, and at each clock cycle said processor initiates the digitizer to digitize the intensity of the first pixel of the spectrometer, said processor validates the intensity of the first pixel by verifying that it belongs to a predefined intensity interval, and, in the event of validation of pixel i, then proceeds to digitize the next pixel i+1 and then to validate it, this digitization and verification process being iterated until the last pixel of the spectrometer; in the event of validation of all the pixels, all the digitized digital values ​​are transmitted to an external interface;whereas when the intensity of a pixel is not validated the controller sets the clock frequency of the digitization of all subsequent pixels of the spectrometer camera to match the maximum frequency allowed by the camera and does not transmit any data; 2. A spectrometry apparatus according to claim 1, wherein the focusing lens (104) has a fixed or variable focal length.

3. Spectrometry apparatus according to claim 1, wherein the sample (106) or the focusing lens (104) are movable.

4. The spectroscopy apparatus of claim 1, wherein the controller (117) is a processor, a microprocessor or a microcontroller.

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