Multi-pass cell capable of improving Raman detection precision, design method and detection method

Abstract

The invention discloses a multi-pass cell capable of improving Raman detection precision, a design method and a detection method, and relates to the technical field of optical detection.The multi-pass cell comprises two concave spherical reflectors, the focal lengths of the two concave spherical reflectors are different, the mirror radiuses of the two concave spherical reflectors are different, and the two concave spherical reflectors are coaxially placed in a confocal mode; the focal length and the mirror surface radius of the concave spherical reflector relatively far away from the first laser incident light in the two concave spherical reflectors are large, the center distance between the two concave spherical reflectors is the sum of the focal lengths of the concave spherical reflectors, and a quartz cuvette is placed at the focal points of the two concave spherical reflectors; a sample to be detected is placed in the quartz cuvette. The device is compact and stable in design structure, laser can excite a high-intensity Raman signal of a detection sample after passing through the multi-pass cell, the device is more suitable for detecting nitrogen fertilizer residues in soil, and the detection precision is high.

Description

Technical Field

[0001] The present invention relates to the field of optical detection technology, and in particular to a multi-pass cell capable of improving Raman detection accuracy, a design method and a detection method. Background Art

[0002] Nitrogen fertilizer is a kind of fertilizer mainly containing nitrogen. It is widely used in agricultural production and can promote plant growth, development and increase crop yield. Farmland operators often blindly use a large amount of nitrogen fertilizer in pursuit of high yield, but ignore the biological characteristics of crops for the absorption capacity of nitrogen fertilizer. According to the current empirical fertilization standards, after deducting crop absorption and fertilizer loss, an average of more than 20% of the fertilizer nitrogen applied in the season remains in the soil. This part of nitrogen is subsequently fixed by microorganisms, incorporated into soil organic matter, or retained as soil mineral nitrogen, becoming a nitrogen source that can be directly absorbed and utilized by subsequent crops. In production, people generally apply nitrogen fertilizer repeatedly every season according to a fixed crop production pattern, but ignore the fertilizer effect of residual nitrogen, resulting in the continuous accumulation of fertilizer nitrogen in the soil, forming an over-fertilization of fertilizer on a time scale. Therefore, in practical applications, testing the residual nitrogen fertilizer concentration in the soil before fertilization can not only increase the yield of crops, but also save the use of nitrogen fertilizer in the next quarter.

[0003] Raman spectroscopy is an analytical method that analyzes the scattering spectrum generated by the inelastic collision of incident light and material molecules, analyzes the scattering spectrum with different frequencies from the incident light to obtain information on molecular vibration and rotation, and is applied to the study of molecular structure. Raman spectroscopy has the characteristics of non-destructive and rapid qualitative / quantitative analysis of gaseous, liquid, and solid samples without pre-processing and sample preparation, and is widely used in physics, chemistry, biology, medicine, mineralogy, food safety testing and other fields. Compared with the common visible light and near-infrared Raman spectra, ultraviolet Raman spectroscopy has the following characteristics: it is roughly separated from the fluorescence spectrum; due to the isolation of ultraviolet light by the ozone layer, the ultraviolet Raman spectrum is less affected by ambient light and has a wider range of application scenarios; the Raman scattering intensity is inversely proportional to the fourth power of the Raman shift. Under the same conditions, ultraviolet Raman spectroscopy has more advantages in detecting weak scattering signals and is more suitable for actual on-site detection. Therefore, ultraviolet Raman spectroscopy is suitable for the detection of nitrogen fertilizer residues in soil.

[0004] The intensity of Raman scattered light accounts for about 10% of the total light scattered light intensity. -6 -10 -10 , which belongs to the detection of weak signals, which places high demands on the performance of the spectrometer. Figure 1In the original 266nm Raman test system, the ultraviolet laser emitted from the 266nm laser 1 passes through the 266nm Raman filter 2 and is focused onto the cuvette by the convex lens 3 with a focal length of 50mm. When the sample is irradiated by the light beam focused by the convex lens 3, the spontaneous Raman scattering signal is excited. The signal collector receives the excited Raman signal and transmits it to the spectrometer 5. The nitrogen fertilizer content is detected by the above conventional Raman spectroscopy experimental device. The Raman spectroscopy signal collected in the experiment is weak and the detection accuracy is low. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a multi-pass cell, a design method and a detection method that can improve the accuracy of Raman detection. After the laser passes through the designed multi-pass cell, it can excite a high-intensity Raman signal of the detection sample, increase the number of times the laser passes through the sample and the integration time, and thus improve the Raman signal detection accuracy.

[0006] In order to achieve the above-mentioned purpose, the present invention provides a multi-pass cell that can improve the accuracy of Raman detection, including two concave spherical reflectors, the two concave spherical reflectors have different focal lengths and different mirror radii, the two concave spherical emitting mirrors are placed confocally and coaxially, and the focal length and mirror radius of the concave spherical reflector that is relatively far away from the first laser incident light of the two concave spherical reflectors are larger, the center distance between the two concave spherical reflectors is the sum of the focal lengths of each concave spherical reflector, a quartz cuvette is placed at the focus of the two concave spherical reflectors, and the sample to be detected is placed in the quartz cuvette.

[0007] The present invention also provides a method for designing a multi-pass cell that can improve the accuracy of Raman detection, comprising the following steps:

[0008] S1: Design the parallel incident point of the laser and generate the optical path diagram of the multi-pass cell;

[0009] The two concave spherical reflectors are named SM1 and SM2 respectively according to the order of the mirrors through which the incident light passes; the focal length corresponding to the concave spherical reflector SM1 is set to f1, the radius of curvature is R1, and the mirror radius is r1; the focal length corresponding to the concave spherical reflector SM2 is set to f2, the radius of curvature is R2, and the mirror radius is r2; and f1>f2, r1>r2; the center distance between the two concave spherical reflectors is L=f1+f2; the two concave spherical reflectors are placed on both sides of the coaxial confocal, and the cylindrical quartz cuvette is placed at the focus;

[0010] Assume that the center of the concave spherical reflector SM1 is the origin, the optical axis is the x-axis, and the plane where the concave spherical reflector is located is the xoy plane. Through geometric algebra, the coordinates of the center point of the concave spherical reflector SM2 are obtained as (L, 0);

[0011] Assume that the incident coordinates of the incident light are (0, y0), and the coordinates after reflection from the focus are (f1+f2, y1). According to the law of concave mirror reflection, when the light passes through the focus and hits the concave reflector, the reflected light should be parallel light with coordinates (0, y1).

[0012] To ensure that the laser parallel light can directly hit the concave spherical reflector SM1 when it is incident for the first time, and is reflected to the concave spherical reflector SM2 through the focus; and to ensure that the first incident light is between the two reflecting surfaces, the range of the incident ordinate y0 of the first incident light is:

[0013] Then, according to the geometric law and the optical path diagram of the concave reflector, the coordinates of all reflection points are calculated; then according to the law of reflection, the vertical coordinate after N reflections can be obtained as:

[0014] S2: Calculate the laser intensity increase multiple to verify the laser intensity enhancement effect;

[0015] The design of the multi-pass cell in step S1 can ensure that when the quartz cuvette is placed at the focal position, the ultraviolet light emitted by the laser can pass through the sample for multiple times. The superposition of the laser in the sample can increase the integration time and excite and amplify the Raman signal. Assuming that the original laser intensity is M when it is incident, it decays by 8% after one reflection. After N reflections, the optical intensity superimposed on the cuvette is

[0016] The present invention also provides a Raman detection method that can improve the accuracy of Raman detection, comprising the following steps: placing a sample to be detected at the common focus of a multi-pass cell, and collecting and amplifying the signal through the multi-pass cell; inputting the collected signal into a spectrometer through an optical fiber, and the system detector of the spectrometer performs Raman signal sampling; processing the collected Raman signal to obtain a Raman spectrum, and achieving accurate collection of the Raman spectrum through multiple collections.

[0017] Beneficial effects of the present invention:

[0018] The invention discloses a multi-pass cell capable of improving the accuracy of Raman detection. The multi-pass cell has a compact and stable design structure. After the laser passes through the multi-pass cell, it can excite a high-intensity Raman signal of a detection sample. The multi-pass cell is more suitable for detecting nitrogen fertilizer residues in soil. The detection accuracy is high and the utilization rate of the laser can be greatly improved. Compared with a traditional 266nm ultraviolet Raman device, the integration time and the laser intensity after superposition are enhanced, and it is easier to excite a high-intensity Raman signal. The amplification effect is achieved while maintaining the same laser power, and the implementation cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of an existing conventional Raman spectroscopy detection system.

[0020] Figure 2 The present invention simulates the light path of 266nm laser in the multi-pass cell.

[0021] Figure 3 It is used in the present invention to reflect the position of the light spot on the reflective mirror surface in the multi-pass cell projected on the xy plane.

[0022] Figure 4 It is a schematic diagram of the multi-pass cell in the present invention applied to a 266 nm Raman detection system.

[0023] In the figure: 1. Laser; 2. Filter; 3. Lens; 4. Dichroic mirror; 5. Spectrometer. DETAILED DESCRIPTION

[0024] The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, which only illustrate the basic structure of the present invention in a schematic manner, and therefore only show the components related to the present invention.

[0025] The invention discloses a multi-pass cell capable of improving Raman detection accuracy and a design method thereof.

[0026] Reference Figure 2 A multi-pass cell capable of improving the accuracy of Raman detection comprises two concave spherical reflectors, the two concave spherical reflectors have different focal lengths and different mirror radii, the two concave spherical emitting mirrors are arranged confocally and coaxially, and the focal length and mirror radius of the concave spherical reflector relatively far from the first laser incident light of the two concave spherical reflectors are larger, the center distance between the two concave spherical reflectors is the sum of the focal lengths of the concave spherical reflectors, a quartz cuvette is placed at the focal point of the two concave spherical reflectors, and a sample to be detected is placed in the quartz cuvette.

[0027] The invention also discloses a multi-pass cell design method which can improve the Raman detection accuracy.

[0028] Reference Figure 3 and Figure 4 A multi-pass cell design method for improving Raman detection accuracy comprises the following steps:

[0029] S1: Design the position of the parallel incident point of the laser and generate the optical path diagram of the multi-pass cell. Name the two concave spherical reflectors SM1 and SM2 respectively according to the order of the mirrors through which the incident light passes; the focal length corresponding to the concave spherical reflector SM1 is set to f1, the radius of curvature is R1, and the mirror radius is r1; the focal length corresponding to the concave spherical reflector SM2 is set to f2, the radius of curvature is R2, and the mirror radius is r2; and f1>f2, r1>r2. The center distance between the two concave spherical reflectors L=f1+f2; the two concave spherical reflectors are placed on both sides of the coaxial confocal, and the cylindrical quartz cuvette is placed at the focus.

[0030] Assume that the center of the concave spherical reflector SM1 is the origin, the optical axis is the x-axis, and the plane where the concave spherical reflector is located is the xoy plane, where L is the distance between the center points of the two concave spherical reflectors. Through geometric algebra, the coordinates of the center point of the concave spherical reflector SM2 are (L, 0). Assume that the incident coordinates of the incident light are (0, y0), and the coordinates after reflection from the focus are (f1+f2, y1). According to the law of concave mirror reflection, when the light passes through the focus and hits the concave reflector surface, the reflected light should be parallel light with coordinates (0, y1).

[0031] In this embodiment, the concave spherical reflector SM1 is relatively placed on the left side, and the concave spherical reflector SM2 is relatively placed on the right side. The first incident relationship is from right to left. Figure 3 shown.

[0032] If you want the light to be able to reflect back and forth on the two concave spherical reflectors of the multi-pass cell, you need to ensure that the laser parallel light can directly hit the concave spherical reflector SM1 when it first hits, and is reflected from the focus to the concave spherical reflector SM2. The incident ordinate y0 of the incident light needs to satisfy the following formula:

[0033] r 2 ≤y 0 ≤r 1 (1);

[0034] According to the laws of geometry, the relationship between the coordinate position and the focus can be obtained:

[0035]

[0036] To ensure that the first incident light is within the range of the two reflecting surfaces, the following relationship can be obtained:

[0037]

[0038] According to the above formulas (1) and (4), we can get y 0 Specific scope:

[0039]

[0040] Then, the coordinates of all reflection points can be calculated based on the geometric laws and the optical path diagram of the concave reflector:

[0041]

[0042] According to the law of reflection, the ordinate after N reflections can be deduced to be:

[0043]

[0044] S2: Calculate the laser intensity increase multiple to verify the laser intensity enhancement effect. The design of the multi-pass cell can ensure that when the cylindrical quartz cuvette is placed at the focal position, the ultraviolet light emitted by the laser 1 can pass through the sample multiple times, and the laser superposition in the sample can increase the integration time and excite and amplify the Raman signal.

[0045] Assume that the original incident laser intensity is M, and after one reflection, it decays by 8%. After N reflections, the optical intensity superimposed on the cuvette is:

[0046] The intensity of the first shot hitting the cuvette: M 1 =0.92M;

[0047] The intensity of the second shot hitting the cuvette: M 2 =0.92 3 M;

[0048] …

[0049] The intensity of the Nth shot hitting the cuvette: M N =0.92 (2N-1) M;

[0050] After N times of intensity superposition:

[0051] In the original technology, the laser directly hits the cuvette, and the attenuated optical intensity is 0.92M; therefore, the laser intensity after superposition through the multi-pass cell is the intensity of the laser directly incident before. times.

[0052] Taking N as 10 as an example, the intensity of the laser after superposition is 5.28 times that of the previous direct laser incidence.

[0053] The invention also discloses a Raman detection method which can improve the Raman detection accuracy.

[0054] A Raman detection method for improving the accuracy of Raman detection comprises the following steps: placing a sample to be detected at the common focus of a multi-pass cell, collecting and amplifying the signal through the multi-pass cell, and inputting the collected signal into a spectrometer 5 through an optical fiber. The incident slit width of the spectrometer 5 used in this embodiment is 50um, the slit length is 1mm, the detector adopts a Hamamatsu S10240 CCD array, with its non-cooling circuit, using a Littrow structure, the spectral resolution is better than 0.07nm, and the corresponding Raman spectral resolution is better than 10cm -1 The detection spectrum is 269nm~293.5nm, and the corresponding Raman spectrum range is 400cm -1 ~3500cm -1The system detector of the spectrometer 5 samples the Raman signal, processes the collected Raman signal, and obtains a Raman spectrum. After multiple acquisitions, accurate acquisition of the Raman spectrum can be achieved, and Raman signal recognition of trace samples can be achieved.

[0055] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A multi-pass cell capable of improving the accuracy of Raman detection, characterized in that: It comprises two concave spherical reflectors, the focal lengths and mirror radii of the two concave spherical reflectors are different, the two concave spherical emitting mirrors are placed in a confocal and coaxial manner, and the focal length and mirror radius of the concave spherical reflector relatively far from the first laser incident light of the two concave spherical reflectors are larger, the center distance between the two concave spherical reflectors is the sum of the focal lengths of each concave spherical reflector, a quartz cuvette is placed at the focal point of the two concave spherical reflectors, and the sample to be detected is placed in the quartz cuvette.

2. A method for designing a multi-pass cell that can improve the accuracy of Raman detection, characterized in that: The following steps are involved: S1: Design the parallel incident point of the laser and generate the optical path diagram of the multi-pass cell; The two concave spherical reflectors are named SM1 and SM2 respectively according to the order of the mirrors through which the incident light passes; the focal length corresponding to the concave spherical reflector SM1 is set to f1, the radius of curvature is R1, and the mirror radius is r1; the focal length corresponding to the concave spherical reflector SM2 is set to f2, the radius of curvature is R2, and the mirror radius is r2; and f1>f2, r1>r2; the center distance between the two concave spherical reflectors is L=f1+f2; the two concave spherical reflectors are placed on both sides of the coaxial confocal, and the cylindrical quartz cuvette is placed at the focus; Assume that the center of the concave spherical reflector SM1 is the origin, the optical axis is the x-axis, and the plane where the concave spherical reflector is located is the xoy plane. Through geometric algebra, the coordinates of the center point of the concave spherical reflector SM2 are obtained as (L, 0); Assume that the incident coordinates of the incident light are (0, y0), and the coordinates after reflection from the focus are (f1+f2, y1). According to the law of concave mirror reflection, when the light passes through the focus and hits the concave reflector, the reflected light should be parallel light with coordinates (0, y1). To ensure that the laser parallel light can directly hit the concave spherical reflector SM1 when it is incident for the first time, and is reflected to the concave spherical reflector SM2 through the focus; and to ensure that the first incident light is between the two reflecting surfaces, the range of the incident ordinate y0 of the first incident light is: Then, according to the geometric law and the optical path diagram of the concave reflector, the coordinates of all reflection points are calculated; then according to the law of reflection, the vertical coordinate after N reflections can be obtained as: S2: Calculate the laser intensity increase multiple to verify the laser intensity enhancement effect; The design of the multi-pass cell in step S1 can ensure that when the quartz cuvette is placed at the focal position, the ultraviolet light emitted by the laser (1) can pass through the sample for multiple times. The superposition of the laser in the sample can increase the integration time and excite and amplify the Raman signal. Assuming that the original laser intensity at the time of incident is M, it decays by 8% after one reflection. After N reflections, the optical intensity superimposed on the cuvette is 3. A Raman detection method capable of improving the accuracy of Raman detection, characterized in that: The method comprises the following steps: placing a sample to be detected at the common focus of a multi-pass cell, and collecting and amplifying signals through the multi-pass cell; The collected signal is input into a spectrometer (5) through an optical fiber, and a system detector of the spectrometer (5) performs Raman signal sampling; The collected Raman signals are processed to obtain a Raman spectrum, and accurate collection of the Raman spectrum is achieved through multiple acquisitions.

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