Orthogonal double-pulse laser-induced breakdown spectroscopy system and signal stability enhancement method

Abstract

The present invention discloses an orthogonal double-pulse laser induced breakdown spectroscopy system and a signal stability enhancement method, wherein a beam expander and a combined lens are sequentially arranged along the direction of the second laser pulse; wherein the beam expander is used to adjust the focusing angle to find the focal plane with the minimum relative standard deviation of the plasma signal along the direction of the second laser pulse at each focusing angle; the focusing angle and focal plane corresponding to the minimum relative standard deviation of the plasma signal are used as the optimal conditions for the detection of the spectral system. The present invention improves the intensity and stability of the reheated plasma signal by optimizing the focusing angle and focal plane position of the second laser beam, thereby obtaining a LIBS signal with a high signal-to-noise ratio and high stability.

Description

Technical Field

[0001] The invention belongs to the technical field of orthogonal double-pulse laser induced breakdown spectroscopy, and in particular relates to an orthogonal double-pulse laser induced breakdown spectroscopy system and a signal stability enhancement method. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Laser-induced Breakdown Spectroscopy (LIBS) is a multi-element atomic spectroscopy technique. The working principle of LIBS is to focus high-energy laser pulses on the surface of the sample to be tested, so that the material on the sample surface is heated and ablated, and plasma is excited to be generated. During the cooling process of the plasma, the excited elements radiate photons outward. By analyzing the frequency and intensity of the spectrum, the elements can be qualitatively and quantitatively analyzed. LIBS technology has many advantages such as fast measurement, simple sample pretreatment, and full element analysis. It has high development potential in the fields of coal online detection, metallurgical detection, environmental monitoring, and space exploration.

[0004] For complex samples such as coal, the repeatability and element detection limit of LIBS technology need to be further improved due to factors such as matrix effects. Reheating double-pulse LIBS is an effective method to improve the intensity of LIBS signals. Compared with the research on signal enhancement, the research on the stability of reheating double-pulse LIBS signals is relatively small. Therefore, it is necessary to find out the factors that affect the signal stability of reheating double-pulse LIBS and find ways to improve the signal stability of reheating double-pulse LIBS. Summary of the invention

[0005] In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides an orthogonal double-pulse laser induced breakdown spectroscopy system and a signal stability enhancement method, which improves the intensity and stability of the reheated plasma signal by optimizing the focusing angle and focal plane position of the second laser beam, thereby obtaining a LIBS signal with a high signal-to-noise ratio and high stability.

[0006] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0007] An orthogonal double-pulse laser induced breakdown spectroscopy system is provided with a beam expander and a combined lens in sequence along the direction of a second laser pulse; wherein the beam expander is used to adjust the focusing angle to find a focal plane along the direction of the second laser pulse at each focusing angle and with the smallest relative standard deviation of a plasma signal; and the focusing angle and focal plane corresponding to the smallest relative standard deviation of a plasma signal are used as the optimal conditions for detection by the spectral system.

[0008] Furthermore, the combined lens includes a double cemented achromatic lens and a plano-convex lens.

[0009] Furthermore, the focusing angle is:

[0010]

[0011] Among them, d represents the beam diameter of the laser pulse after being magnified by the beam expander, and l represents the distance between the focus and the combined lens after the beam passes through the combined lens.

[0012] Furthermore, the spectral system also includes an enhanced charge coupled device for collecting images of plasma signals.

[0013] Furthermore, the method for determining the focusing angle and focal plane corresponding to the minimum relative standard deviation of the plasma signal is as follows:

[0014] At each focusing angle, along the direction of the second laser beam, the focal plane is changed in sequence along the laser forward direction, the plasma is reheated, the relative standard deviation of the plasma signal corresponding to each focal plane position at the focusing angle is obtained, and the minimum relative standard deviation at the focusing angle is found;

[0015] The minimum relative standard deviation at each focusing angle is compared to determine the focusing angle and focal plane position corresponding to the minimum relative standard deviation value.

[0016] Furthermore, under the conditions of setting the focus angle and focal plane, the minimum relative standard deviation calculation method is:

[0017] Continuously acquire multiple plasma images;

[0018] For each pixel of multiple images, the relative standard deviation is calculated separately;

[0019] The relative standard deviation of each pixel in the entire image is averaged to obtain the relative standard deviation of the entire image.

[0020] One or more embodiments provide a method for enhancing signal stability based on the spectral system, comprising the following steps:

[0021] Under the conditions of multiple focusing angles and multiple focal planes along the direction of the second laser beam, a plurality of plasma images of the measured sample are respectively obtained;

[0022] The relative standard deviation of multiple plasma images under each condition was calculated;

[0023] The focusing angle and focal plane position corresponding to the minimum relative standard deviation are taken as the optimal conditions of the spectral system.

[0024] Furthermore, under the conditions of setting the focus angle and focal plane, the minimum relative standard deviation calculation method is:

[0025] Continuously acquire multiple plasma images;

[0026] For each pixel of multiple images, the relative standard deviation is calculated separately;

[0027] The relative standard deviation of each pixel in the entire image is averaged to obtain the relative standard deviation of the entire image.

[0028] Furthermore, the method also obtains the optimal double-pulse interval time: obtains multiple double-pulse delay interval times and corresponding spectral signals, and finds the interval time that makes the spectral signal-to-noise ratio the highest as the optimal pulse interval time.

[0029] Furthermore, the method also obtains the optimal acquisition delay time of the spectrometer: obtains multiple spectrometer acquisition delay times and corresponding spectral signals, and finds the interval time that makes the spectral signal-to-noise ratio the highest as the optimal acquisition delay time of the spectrometer.

[0030] One or more of the above technical solutions have the following beneficial effects:

[0031] Compared with the traditional method which aims to enhance the signal strength, the present invention takes improving the signal stability as the main purpose while taking into account the enhancement of the signal strength.

[0032] The present invention uses a double-cemented achromatic lens to reduce the spherical aberration of the second beam focusing lens, reduce the "electrostatic shielding effect", eliminate the generation of the "dynamic breakdown effect", enhance the coupling between the second laser beam and plasma, and also improve the stability of the signal.

[0033] By adjusting the focusing angle and focal plane position of the second laser beam, uniform and symmetrical heating of the plasma is achieved, thereby improving the stability of the plasma signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0035] Figure 1 This is a schematic diagram of the structure of the orthogonal double-pulse laser induced breakdown spectroscopy system described in Example 1 of the present invention;

[0036] Figure 2 This is a flow chart of the signal stability enhancement method based on the orthogonal double-pulse laser induced breakdown spectroscopy system described in the second embodiment of the present invention;

[0037] Figure 3A schematic diagram of the focal plane adjustment and uniform symmetrical excitation in one or more embodiments of the present invention.

[0038] In the figure, 1-first delayed pulse generator, 2-second delayed pulse generator, 3-first laser, 4-second laser, 5-computer, 6-spectrometer, 7-enhanced charge coupled device ICCD, 8-plano-convex lens L1, 9-beam expander, 10-combined lens L2, 11-light collecting mirror and optical fiber, 12-moving platform; 13-plasma, 14-second laser pulse, 15-focal plane 1, 16-focal plane 2, 17-focal plane 3, 18-secondary excitation left side, 19-secondary excitation right side. DETAILED DESCRIPTION

[0039] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0040] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0041] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0042] Embodiment 1

[0043] This embodiment discloses an orthogonal double-pulse laser induced breakdown spectroscopy system (LIBS) capable of enhancing signal stability, such as Figure 1 As shown, the system specifically includes:

[0044] The first laser 3 and the second laser 4 are used to emit a first laser pulse and a second laser pulse, respectively. A plano-convex lens 8 is provided along the direction of the first laser pulse, and a beam expander 9 and a combined lens 10 are provided in sequence along the direction of the second laser pulse. The directions of the first laser pulse and the second laser pulse are orthogonal, and a movable platform 12 is provided at the intersection for placing the sample to be tested. The combined lens 10 includes a double-cemented achromatic lens and a plano-convex lens.

[0045] The first laser 3 is connected to a first delayed pulse generator, and the second laser 4 is connected to a second delayed pulse generator;

[0046] The first delayed pulse generator 1 is also connected to a spectrometer 6, and the spectrometer 6 is connected to a light collecting mirror 11 via an optical fiber for collecting spectral signals; the spectrometer 6 is also connected to a computer 5;

[0047] The second time-delay pulse generator 2 is also connected to an enhanced charge-coupled device (ICCD) 7 for collecting images of plasma signals;

[0048] The first delayed pulse generator 1 is connected to the second delayed pulse generator 2 .

[0049] The beam expander arranged along the pulse direction of the second laser beam is used to adjust the focusing angle to find the focal plane along the pulse direction of the second laser beam and with the smallest relative standard deviation of the plasma signal as the optimal condition for the detection of the spectral system. By optimizing the focusing angle and focal plane position of the second laser beam, uniform and symmetrical heating of the plasma can be achieved, the intensity and stability of the reheated plasma signal can be improved, and a LIBS signal with high signal-to-noise ratio and high stability can be obtained.

[0050] The focusing angle refers to: after the pulse laser is magnified by the beam expander, the beam diameter is d, and after the beam passes through the combined lens (double-glued achromatic lens + plano-convex lens), the focal distance of the combined lens is l, and the focusing angle is α. Then the focusing angle is:

[0051]

[0052] After the second laser pulse passes through the beam expander, it first passes through a double-cemented achromatic lens, which helps to reduce the spherical aberration of the second beam focusing lens, reduce the "electrostatic shielding effect", eliminate the "dynamic breakdown effect", and enhance the coupling between the second laser beam and the plasma. While enhancing the signal strength, it also improves the signal stability.

[0053] The working principle of the orthogonal double-pulse laser-induced breakdown spectroscopy system is as follows:

[0054] (1) using a first delayed pulse generator to control a first laser to emit a pulsed laser, and generating plasma on the sample surface through a focusing lens;

[0055] (2) using a second delayed pulse generator to control the second laser to emit a second pulse laser, which is focused on the plasma through a beam expander, a double-cemented achromatic lens, and a plano-convex lens to heat the plasma again;

[0056] (3) Collect plasma signals and images using a spectrometer and an intensified charge-coupled device (ICCD), where the ICCD is turned on after the second pulse.

[0057] In the process of continuously changing the angle and focal plane position, the sample to be tested is tested multiple times, and the focusing angle and focal plane position corresponding to the minimum relative standard deviation of the plasma are found based on the images collected by the enhanced charge-coupled device, which are used as the optimal conditions for detection by the spectral system.

[0058] Embodiment 2

[0059] Based on the spectral system described in the first embodiment, this embodiment provides a method for enhancing signal stability based on the spectral system, which specifically includes the following steps:

[0060] Step 1: Determine the optimal double pulse interval and the optimal acquisition delay time.

[0061] The step 1 specifically includes:

[0062] Step 1.1: Select a national standard coal sample with a known content and prepare N samples to be tested. Grind, pulverize, filter and press the coal into a cake-like sample;

[0063] Step 1.2: Use two delayed pulse generators to change the delay interval of the two laser double pulses, find the interval with the highest spectral signal-to-noise ratio, and determine it as the optimal pulse interval. Use the same method to determine the spectrometer acquisition delay time.

[0064] Step 2: Find the best focus angle and focal plane position.

[0065] In this embodiment, the plasma signal standard deviation RSD value is used as an indicator to measure signal stability.

[0066] The step 2 specifically includes: using a beam expander to change the diameter of the second laser pulse beam to achieve a change in the focusing angle; at each focusing angle, along the direction of the second laser beam, the focal plane is sequentially changed along the laser forward direction to reheat the plasma; and by using an ICCD image analysis method, the focusing angle and focal plane position corresponding to the minimum value of the relative standard deviation RSD of the plasma signal are found.

[0067] Among them, the ICCD image analysis method is: under the conditions of set focusing angle and focal plane, collect N times (N is generally greater than 20) plasma images through ICCD; calculate the relative standard deviation RSD of each pixel point in the image, and then calculate the average value of the RSD of each pixel point in the whole image to obtain the RSD of the whole image as an indicator of the stability of the plasma signal under the current conditions.

[0068] The process of determining the focus angle and focal plane position is as follows:

[0069] (1) For each focusing angle, obtain the relative standard deviation RSD corresponding to each focal plane position at the focusing angle, and find the minimum relative standard deviation RSD at the focusing angle;

[0070] (2) By comparing the minimum RSD values ​​of the plasma signal at different angles, the focusing angle and the corresponding focal plane position corresponding to the minimum RSD value are obtained as the optimal condition.

[0071] From the ICCD images, it can be observed that the second laser beam achieves uniform and symmetrical reheating of the plasma.

[0072] Step 3: Use the optimal double pulse interval as the double pulse delay interval of the two lasers, the optimal acquisition delay time as the spectrum system acquisition delay time, and the optimal focusing angle and focal plane position as the optimal conditions to perform detection. Use the reheating orthogonal double pulse LIBS under this condition for detection, thereby obtaining an improvement in the stability and intensity of the plasma signal.

[0073] As a specific example, the signal stability enhancement method specifically includes:

[0074] 1) Take GBW11111 as an example. After grinding, filter through a 200 mesh filter. Use a cake making machine to maintain a pressure of 40T for 2 minutes to make a 3cm diameter coal cake sample. The pulse energy of laser 1 and laser 2 is 90mj. The time interval between the two laser pulses is 500ns, the ICCD delay time is 20ns after the laser 2 emits a pulse, and the integration time is 100ns. The spectral system delay time is 1000ns, and the integration time is 1ms.

[0075] 2) First, change the focusing angle to perform secondary heating on the plasma.

[0076] 3) Adjust the focusing mirror L2 from the sample position, change the focal plane position, ablate and collect 5 times at the same position, each sample has 5 different ablation positions, and collect 25 spectra and plasma images, which are sent to the computer for processing. Calculate and compare the RSD of the plasma ICCD image signal, and find the focal plane position corresponding to the minimum RSD. Record the plasma relative standard deviation RSD of the signal at the current focusing angle.

[0077] 4) Continuously change multiple angles and adjust the focal plane position of L2, and record the focal plane position with the minimum RSD of the plasma signal at each angle.

[0078] 5) Compare the minimum values ​​of plasma signal RSD at different focusing angles, and find the focusing angle and focal plane position when the signal RSD is the lowest. Figure 3 As shown, at this time, it can be observed on the ICCD image that the second laser beam excites the plasma uniformly and symmetrically.

[0079] Table 1 shows the RSD values ​​of the plasma ICCD signal and the RSD values ​​of the two spectra at different angles under the condition that the minimum RSD value of the signal corresponds to the focal plane.

[0080] Table 1

[0081]

[0082] The uniform and symmetrical reheating of the plasma described in the above scheme refers to the process of plasma reheating by the conventional second laser pulse, which mainly occurs on the plasma surface. During the expansion of the plasma, its internal temperature is high and the material density is low, while the material content at the edge of the plasma is high, so it is more easily excited for the second time. Due to the electrostatic shielding effect, when the focal plane of the second laser beam is at the center of the plasma, reheating can only occur on one side of the plasma. In this case, the energy of the second laser beam cannot be fully coupled with the plasma, and because only one side is excited, the plasma is squeezed by the expansion of one side, resulting in large signal volatility.

[0083] This embodiment uses a double-cemented achromatic lens to reduce spherical aberration and reduce the "electrostatic shielding effect", which is beneficial to the coupling of the second laser beam and the plasma. At the same time, it can reduce the fluctuation caused by the "dynamic breakdown effect", which is beneficial to the stability of the secondary heating.

[0084] By moving the focal plane of the second laser beam from the center of the plasma appropriately along the direction of the laser forward, uniform and symmetrical secondary excitation of both sides of the plasma can be achieved. The adjustment of the focusing angle and the focal plane is conducive to the coupling of the second laser beam with the plasma and the improvement of the stability of the plasma, which can enhance the plasma signal and improve the stability. Therefore, the focal plane when the RSD value of the plasma signal is the lowest is taken as the optimal focal plane position at the corresponding angle, at which time it can be observed that the second laser beam heats the plasma uniformly and symmetrically.

[0085] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. An orthogonal double-pulse laser induced breakdown spectroscopy system, characterized in that: A beam expander and a combined lens are sequentially arranged along the direction of the second laser pulse; wherein the beam expander is used to adjust the focusing angle to find a focal plane along the direction of the second laser pulse at each focusing angle and with the minimum relative standard deviation of the plasma signal; and the focusing angle and focal plane corresponding to the minimum relative standard deviation of the plasma signal are used as the optimal conditions for detection by the spectral system; The spectroscopy system also includes an enhanced charge-coupled device for collecting images of plasma signals; The method for determining the focusing angle and focal plane corresponding to the minimum relative standard deviation of the plasma signal is: At each focusing angle, along the direction of the second laser beam, the focal plane is changed in sequence along the laser forward direction, the plasma is reheated, the relative standard deviation of the plasma signal corresponding to each focal plane position at the focusing angle is obtained, and the minimum relative standard deviation at the focusing angle is found; Compare the minimum relative standard deviation at each focusing angle to determine the focusing angle and focal plane position corresponding to the minimum relative standard deviation value; Under the conditions of setting the focus angle and focal plane, the minimum relative standard deviation is calculated as follows: Continuously acquire multiple plasma images; For each pixel of multiple images, the relative standard deviation is calculated separately; The relative standard deviation of each pixel in the entire image is averaged to obtain the relative standard deviation of the entire image; The combined lens comprises a double cemented achromatic lens and a plano-convex lens.

2. The orthogonal double-pulse laser induced breakdown spectroscopy system according to claim 1, characterized in that: The focusing angle is: Among them, d represents the beam diameter of the laser pulse after being magnified by the beam expander, and l represents the distance between the focus and the combined lens after the beam passes through the combined lens.

3. A method for enhancing signal stability based on a spectral system according to any one of claims 1 to 2, characterized in that: The following steps are involved: Under the conditions of multiple focusing angles and multiple focal planes along the direction of the second laser beam, a plurality of plasma images of the measured sample are respectively obtained; The relative standard deviation of multiple plasma images under each condition was calculated; The focusing angle and focal plane position corresponding to the minimum relative standard deviation are taken as the optimal conditions of the spectral system.

4. The signal stability enhancement method according to claim 3, characterized in that: Under the conditions of setting the focus angle and focal plane, the minimum relative standard deviation is calculated as follows: Continuously acquire multiple plasma images; For each pixel of multiple images, the relative standard deviation is calculated separately; The relative standard deviation of each pixel in the entire image is averaged to obtain the relative standard deviation of the entire image.

5. The signal stability enhancement method according to claim 3, characterized in that: The method also obtains the best double-pulse interval time: obtains multiple double-pulse delay interval times and corresponding spectral signals, and searches for the interval time that makes the spectral signal-to-noise ratio the highest as the best pulse interval time.

6. The signal stability enhancement method according to claim 4 or 5, characterized in that: The method also obtains the optimal acquisition delay time of the spectrometer: obtains multiple spectrometer acquisition delay times and corresponding spectral signals, and finds the interval time that makes the spectral signal-to-noise ratio the highest as the optimal acquisition delay time of the spectrometer.

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