CN114894776B

CN114894776B - Orthogonal double-pulse laser-induced breakdown spectroscopy system and signal stability enhancement method

Info

Publication number: CN114894776B
Application number: CN202210396534.5A
Authority: CN
Inventors: 卢鹏; 卢景琦
Original assignee: Shandong University
Current assignee: Shandong University
Priority date: 2022-04-15
Filing date: 2022-04-15
Publication date: 2024-12-24
Anticipated expiration: 2042-04-15
Also published as: CN114894776A

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 along the direction of the second laser pulse at each focusing angle and with the minimum relative standard deviation of the plasma signal; 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

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

Technical Field

The invention belongs to the technical field of orthogonal double-pulse laser-induced breakdown spectroscopy, and particularly relates to an orthogonal double-pulse laser-induced breakdown spectroscopy system and a signal stability enhancement method.

Background

The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.

Laser-induced breakdown spectroscopy (Laser-induced Breakdown Spectroscopy, LIBS) is a multi-element atomic spectroscopy technique. The LIBS works on the principle that high-energy laser pulses are focused on the surface of a sample to be detected, so that substances on the surface of the sample are heated and ablated, and plasma is generated by excitation. During the cooling process of the plasma, the excited elements radiate photons outwards, and the elements can be qualitatively and quantitatively analyzed by analyzing the frequency and the intensity of the spectrum. The LIBS technology has the advantages of quick measurement, simple sample pretreatment, full element analysis and the like, and has higher development potential in the fields of coal on-line detection, metallurgical detection, environmental monitoring, space exploration and the like.

For complex samples such as coal, the repeatability and the element detection limit of the LIBS technology need to be further improved due to the influence of factors such as matrix effect. Reheating double-pulse LIBS is an effective method of increasing the intensity of the LIBS signal. There are also relatively few studies on the stability of reheat double pulse LIBS signals compared to the signal enhancement studies. Therefore, it is necessary to find out factors affecting the signal stability of the reheat double pulse LIBS and find out a method of improving the signal stability of the reheat double pulse LIBS.

Disclosure of Invention

In order to overcome the defects in the prior art, the invention provides an orthogonal double-pulse laser-induced breakdown spectroscopy system and a signal stability enhancement method, which improve the strength and stability of a reheating plasma signal by optimizing the focusing angle and the focal plane position of a second laser beam, so that a LIBS signal with high signal-to-noise ratio and high stability can be obtained.

To achieve the above object, one or more embodiments of the present invention provide the following technical solutions:

An orthogonal double-pulse laser-induced breakdown spectroscopy system is provided with a beam expander and a combined lens sequentially along the pulse direction of a second beam of laser, wherein the beam expander is used for adjusting focusing angles to find a focal plane which is along the pulse direction of the second beam of laser under each focusing angle and has the minimum relative standard deviation of a plasma signal, and the focusing angle and the focal plane which correspond to the minimum relative standard deviation of the plasma signal are used as optimal conditions for detection of the spectroscopy system.

Further, the combined lens includes a doublet achromat and a plano-convex lens.

Further, the focusing angle is:

wherein d represents the beam diameter of the laser pulse amplified by the beam expander, and l represents the distance between the focal point and the combined lens after the beam passes through the combined lens.

Further, the spectroscopic system further comprises an enhanced charge coupled device for acquiring an image of the plasma signal.

Further, the focusing angle and focal plane determining method corresponding to the minimum relative standard deviation of the plasma signals comprises the following steps:

Under each focusing angle, along the second beam laser direction, the focal plane is sequentially changed along the laser advancing direction, and the plasma is reheated to obtain the relative standard deviation of the plasma signals corresponding to the positions of the focal planes under the focusing angles, and the minimum relative standard deviation under the focusing angles is searched;

And comparing the minimum relative standard deviation under each focusing angle, and determining the focusing angle and the focal plane position corresponding to the minimum relative standard deviation.

Further, under the condition of setting a focusing angle and a focal plane, the minimum relative standard deviation calculating method comprises the following steps:

continuously acquiring a plurality of plasma images;

Calculating relative standard deviation for each pixel point of the plurality of images;

and (5) averaging the relative standard deviation of each pixel point in the whole image to obtain the relative standard deviation of the whole image.

One or more embodiments provide a signal stability enhancement method based on the spectroscopic system, comprising the steps of:

Respectively acquiring a plurality of plasma images of the sample to be measured under the conditions of a plurality of focusing angles and a plurality of focal planes along the second beam laser direction;

calculating the relative standard deviation of a plurality of plasma images under each condition;

and taking the focusing angle and the focal plane position corresponding to the minimum relative standard deviation as the optimal conditions of the spectrum system.

continuously acquiring a plurality of plasma images;

Further, the method also obtains the optimal double pulse interval time, namely obtaining a plurality of double pulse delay interval times and corresponding spectrum signals, and searching the interval time with the highest spectrum signal to noise ratio as the optimal pulse interval time.

Further, the method also obtains the optimal acquisition delay time of the spectrograph, namely obtaining the acquisition delay time of a plurality of spectrographs and corresponding spectrum signals, and searching the interval time with the highest signal to noise ratio of the spectrum as the optimal acquisition delay time of the spectrograph.

The one or more of the above technical solutions have the following beneficial effects:

Compared with the traditional method for enhancing the signal intensity, the method provided by the invention has the main purpose of enhancing the signal intensity and improving the stability of the signal.

The invention uses the 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 beam laser and the plasma, and improve the stability of signals.

By adjusting the focusing angle and the focal plane position of the second laser beam, uniform and symmetrical heating of the plasma is realized, and further, the stability of the plasma signal is improved.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention.

FIG. 1 is a schematic diagram of an orthogonal double-pulse laser-induced breakdown spectroscopy system according to a first embodiment of the present invention;

Fig. 2 is a flowchart of a signal stability enhancement method based on the orthogonal double pulse laser-induced breakdown spectroscopy system according to a second embodiment of the present invention;

fig. 3 is a schematic view of focal plane adjustment and uniform symmetric excitation in one or more embodiments of the invention.

In the figure, a 1-first time delay pulse generator, a 2-second time delay pulse generator, a 3-first laser, a 4-second laser, a 5-computer, a 6-spectrometer, a 7-enhanced charge coupled device ICCD, an 8-plano-convex lens L1, a 9-beam expander, a 10-combined lens L2, an 11-light receiving lens, an optical fiber and a 12-moving platform, and 13-plasma, 14-second laser pulses, a 15-focal plane 1, a 16-focal plane 2, a 17-focal plane 3, 18-secondary excitation left side and 19-secondary excitation right side.

Detailed Description

It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the present invention. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.

Embodiments of the invention and features of the embodiments may be combined with each other without conflict.

Example 1

The embodiment discloses an orthogonal double pulse Laser Induced Breakdown Spectroscopy (LIBS) system capable of enhancing signal stability, as shown in fig. 1, the system specifically includes:

the first laser device 3 and the second laser device 4 are respectively used for emitting a first beam of laser pulse and a second beam of laser pulse, a plano-convex lens 8 is arranged along the first beam of laser pulse direction, and a beam expander 9 and a combined lens 10 are sequentially arranged along the second beam of laser pulse direction;

the first laser 3 is connected with a first delay pulse generator, and the second laser 4 is connected with a second delay pulse generator;

The first time delay pulse generator 1 is also connected with a spectrometer 6, and the spectrometer 6 is connected with a receiving mirror 11 through an optical fiber and is used for collecting spectrum signals;

The second delay pulse generator 2 is also connected with an enhanced charge coupled device (ICCD) 7 for acquiring images of plasma signals;

The first time delay pulse generator 1 is connected with the second time delay pulse generator 2.

The beam expander is arranged along the second beam laser pulse direction and used for adjusting the focusing angle to find a focal plane which is along the second beam laser pulse direction and has the minimum relative standard deviation of the plasma signal, and the focal plane is used as the optimal condition for the detection of the spectrum system. By optimizing the focusing angle and the focal plane position of the second laser beam, uniform and symmetrical heating of plasma can be realized, the intensity and stability of a reheating plasma signal are improved, and then LIBS signals with high signal-to-noise ratio and high stability can be obtained.

The focusing angle means that after pulse laser is amplified by a beam expander, the beam diameter is d, and after the beam passes through a combined lens (a double-cemented achromatic lens and a plano-convex lens), the focal distance is l, and the focusing angle is alpha. The focus angle is:

After the second laser pulse passes through the beam expander, the second laser pulse passes through the double-gluing achromatic lens, so that the spherical aberration of the second beam focusing lens is reduced, the electrostatic shielding effect is reduced, the dynamic breakdown effect is eliminated, the coupling between the second laser and plasma is enhanced, the signal intensity is enhanced, and meanwhile, the stability of signals is also improved.

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

(1) Using a first time delay pulse generator to control a first laser to emit pulse laser, and generating plasma on the surface of the sample through a focusing lens;

(2) A second time delay pulse generator is used for controlling a second laser to emit a second beam of pulse laser, and the second beam of pulse laser is focused on plasma through a beam expander, a double-gluing achromatic lens and a plano-convex lens to heat the plasma again;

(3) Plasma signals and images were collected using a spectrometer, enhanced charge coupled device (ICCD). Wherein the ICCD on time is after the second beam pulse.

In the process of continuously changing the angle and the focal plane position, aiming at the sample to be detected, detecting for a plurality of times, and searching the corresponding focusing angle and focal plane position when the relative standard deviation of plasma is minimum according to the image acquired by the enhanced charge coupled device, wherein the focusing angle and the focal plane position are used as the optimal condition for detecting the spectrum system.

Example two

Based on the above-mentioned spectrum system according to the first embodiment, the present embodiment provides a signal stability enhancing method based on the above-mentioned spectrum system, which specifically includes the following steps:

And step 1, determining the optimal double pulse interval time and the optimal acquisition delay time.

The step 1 specifically includes:

and 1.1, selecting national standard coal samples with known content, and preparing N samples to be tested. Grinding coal, pulverizing, filtering, and pressing into cake sample;

And 1.2, changing the double pulse delay interval time of the two lasers by using two delay pulse generators, searching the interval time with the highest spectral signal-to-noise ratio, and determining the interval time as the optimal pulse interval time. The spectrometer acquisition delay time was determined by the same method.

And 2, searching the optimal focusing angle and the focal plane position.

In this embodiment, the standard deviation RSD value of the plasma signal is used as an index for measuring the stability of the signal.

The step 2 specifically comprises the steps of utilizing a beam expander to change the beam diameter of a second laser pulse to change the focusing angle, changing the focal plane along the advancing direction of laser in sequence along the second laser direction under each focusing angle, reheating the plasma, and searching the focusing angle and the focal plane position corresponding to the minimum value of the plasma signal relative to the standard deviation RSD through an ICCD image analysis method.

The ICCD image analysis method comprises the steps of collecting plasma images N times (N is generally larger than 20) through the ICCD under the condition of setting a focusing angle and a focusing plane, solving Relative Standard Deviation (RSD) of each pixel point of the image, and then solving average value of each pixel point (RSD) of the whole image to obtain the RSD of the whole image, wherein the RSD is used as an index of plasma signal stability under the current condition.

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

(1) For each focusing angle, acquiring a Relative Standard Deviation (RSD) corresponding to each focal plane position under the focusing angle, and searching for a minimum Relative Standard Deviation (RSD) under the focusing angle;

(2) And comparing the minimum RSD values of the plasma signals under different angles to obtain a focusing angle corresponding to the minimum RSD value and a corresponding focal plane position, and taking the focusing angle and the corresponding focal plane position as optimal conditions.

From the ICCD image it can be observed that the second laser achieves a uniform symmetric reheating of the plasma.

And 3, taking the optimal double pulse interval time as the double pulse delay interval time of the two lasers, taking the optimal acquisition delay time as the acquisition delay time of the spectrum system, and simultaneously taking the optimal focusing angle and the focal plane position as optimal conditions to execute detection. The reheating orthogonal double pulse LIBS under the condition is adopted for detection, so that the stability and the intensity of plasma signals are improved.

As a specific example, the signal stability enhancing method specifically includes:

1) Take GBW11111 as an example. Grinding, and filtering to pass through a 200-mesh filter screen. A 3cm diameter briquette sample was made using a cake making machine maintained at 40T pressure for 2 minutes. The pulse energy of both laser 1 and laser 2 was 90mj. The pulse time interval of the two lasers is 500ns, the ICCD delay time is 20ns after the laser 2 emits the pulse, and the integration time is 100ns. The spectral system delay time is 1000ns and the integration time is 1ms.

2) Firstly, changing the focusing angle, and carrying out secondary heating on the plasma.

3) The position of a focusing lens L2 is adjusted to be away from a sample, the focal plane position is changed, the same position is ablated and collected for 5 times, each sample has 5 different ablation positions, 25 spectra and plasma images are collected, and the images are sent to a computer for processing. And calculating and comparing the plasma ICCD image signals RSD, and finding the focal plane position corresponding to the minimum RSD. The plasma relative standard deviation RSD of the signal at the current focus angle is recorded.

4) The angles are changed continuously, and the focal plane position of L2 is adjusted, and the focal plane position at which the plasma signal RSD is minimum at each angle is recorded.

5) And comparing the minimum value of the plasma signal RSD under different focusing angles, and finding the focusing angle and the focal plane position when the signal RSD is lowest. As shown in fig. 3, a uniform and symmetrical excitation of the plasma by the second laser is now observed on the ICCD image.

Table 1 shows the RSD values of the plasma ICCD signal and the RSD values of 2 spectra at different angles under the condition that the signal RSD minimum corresponds to the focal plane.

TABLE 1

In the above scheme, the uniform and symmetrical reheating of the plasma refers to that the conventional reheating process of the second laser pulse on the plasma mainly occurs on the surface of the plasma. In the process of plasma expansion, the internal temperature is high, the density of the substances is low, and the content of the substances at the edge of the plasma is high, so that the substances are more easily excited for the second time. Because of the electrostatic shielding effect, reheating can only occur on one side of the plasma when the second laser focal plane is at the center of the plasma. In this case, the second laser energy cannot be coupled completely with the plasma, and the signal fluctuation is large because only one side is excited, which causes the plasma to be pressed by expansion of one side.

The embodiment can reduce spherical aberration by using the double-cemented achromatic lens, and reduce the electrostatic shielding effect, thereby being beneficial to the coupling between the second beam of laser and plasma, reducing fluctuation caused by dynamic breakdown benefit and being beneficial to the stability of secondary heating.

The second laser focal plane is properly moved along the laser advancing direction from the center of the plasma, so that the secondary excitation on the two sides of the plasma can be realized. The adjustment of the focusing angle and the focal plane is beneficial to the coupling of the second laser beam and the plasma and the improvement of the stability of the plasma, and can realize the enhancement of the plasma signal and the improvement of the stability. Therefore, the focal plane at the lowest value of the plasma signal RSD is taken as the optimal focal plane position at the corresponding angle, and at this time, the second laser beam is observed to uniformly and symmetrically heat the plasma.

While the foregoing description of the embodiments of the present invention has been presented in conjunction with the drawings, it should be understood that it is not intended to limit the scope of the invention, but rather, it is intended to cover all modifications or variations within the scope of the invention as defined by the claims 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 the 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; 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, wherein 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 the spectral system according to any one of claims 1 to 2, characterized in that it comprises the following steps:

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 the focal plane, the minimum relative standard deviation calculation method is:

Continuously acquire multiple plasma images;

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 as described in claim 3 is characterized in that 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.

6. The signal stability enhancement method as described in claim 4 or 5 is 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.