A rotating mirror Fourier transform ultraviolet Raman spectrometer for on-site detection of atmospheric CO2

Abstract

The present invention discloses a rotating mirror type Fourier transform ultraviolet Raman spectrometer for realizing on-site detection of atmospheric CO2, which includes an excitation and collection optical path, a Raman spectrometer optical path, a white light reference optical path and a control system. By applying a rotating mirror type Fourier transform spectrometer (Turbo FTS) to the Raman spectrometer and exploring its application in spectral analysis in the ultraviolet band, on-site detection of the weak Raman spectrum of atmospheric CO2 is realized; by designing a set of control systems to precisely control the rotating mirror, PMT and data acquisition card, sampling at equal optical path difference intervals is realized. The present invention can greatly improve the signal-to-noise ratio of detecting atmospheric CO2, effectively improve the accuracy of ultraviolet Raman spectrum restoration and reduce the operation time.

Description

Technical Field

[0001] The invention relates to the field of Fourier transform Raman spectroscopy analysis, and in particular to a rotating mirror Fourier transform ultraviolet Raman spectrometer for realizing on-site detection of atmospheric CO2. Background Art

[0002] CO2 is the most important greenhouse gas causing global warming. Detection of atmospheric CO2 can provide reliable data for the analysis of the sources and sinks of atmospheric CO2. However, the existing mass spectrometry detection method has a complex structure and large volume, and the Fourier transform infrared absorption spectroscopy detection method has strict requirements on the measurement conditions and low accuracy. The application of both methods in field detection is greatly limited. Raman spectroscopy detection method has good fingerprint characteristics (high accuracy) and simple structure, and has broad application prospects. However, there is a key problem with Raman spectroscopy detection method: low signal-to-noise ratio, which makes it difficult to achieve field detection.

[0003] The rotating mirror Fourier transform spectrometer (Turbo FTS) can obtain spectral signals with high signal-to-noise ratio, high resolution and high stability, and is particularly suitable for on-site detection of CO2 in the atmosphere. However, the optical path difference generated by the rotating mirror of the Turbo FTS during rotation is nonlinearly related to the angle of rotation. The uniform time sampling obtains the non-equal optical path difference interval sampling interference signal, and the spectral signal cannot be accurately restored directly through the fast Fourier transform (FFT). To address this problem, the existing solutions are mainly divided into two categories: software methods and hardware methods. Software methods include NUFFT method and secondary sampling method. NUFFT method is not suitable for processing large-angle interference signals, and secondary sampling method is limited by interpolation technology. The software method uses the principle of approximation and has large errors. The hardware method is mainly the reference optical path method, but this method is difficult to meet the working conditions of Raman spectroscopy in the ultraviolet band.

[0004] Therefore, it is necessary to study how to apply Turbo FTS to Raman spectroscopy analysis technology and solve the problem that the optical path difference generated by the rotating mirror of Turbo FTS during rotation is nonlinearly related to the rotation angle, so as to obtain Raman spectra with high signal-to-noise ratio (working in the ultraviolet band), high resolution (large rotation angle range), and high accuracy (equal optical path difference interval sampling) to realize on-site detection of atmospheric CO2. Summary of the invention

[0005] The present invention provides a rotating mirror Fourier transform ultraviolet Raman spectrometer for realizing on-site detection of atmospheric CO2. By designing a high signal-to-noise ratio optical system (including excitation and collection optical paths, Raman spectrometer optical path, and white light reference optical path), Turbo FTS is applied to ultraviolet Raman spectrum analysis. The problem that the optical path difference generated by the rotating mirror of Turbo FTS during rotation is in a nonlinear relationship with the rotation angle is solved by means of hardware improvement and control system design, and an equally spaced sampling interference signal is obtained, thereby realizing accurate restoration of the ultraviolet Raman spectrum of atmospheric CO2, and further realizing on-site detection of atmospheric CO2.

[0006] The specific technical solution adopted by the present invention is as follows:

[0007] A rotating mirror Fourier transform ultraviolet Raman spectrometer for realizing on-site detection of atmospheric CO2, comprising: an excitation and collection optical path, a Raman spectrometer optical path, a white light reference optical path and a control system, characterized in that:

[0008] (1) The excitation and collection optical path includes an ultraviolet laser, a laser beam expander B, a laser beam expander A, a Raman filter, a collimating lens A, an aperture, and a collection lens. According to the direction of the ultraviolet laser, the order of the optical path structure and spatial positioning is as follows: ultraviolet laser → laser beam expander B → laser beam expander A → Raman filter → collimating lens A → aperture → collection lens. After the ultraviolet laser irradiates the atmospheric CO2, Raman scattered light is generated. According to the direction of the Raman scattered light, the order of the optical path structure and spatial positioning is as follows: collection lens → aperture → collimating lens A → Raman filter;

[0009] (2) The optical path of the Raman spectrometer includes a short-wave pass filter, a beam splitter, a reflector A, a reflector B, a rotating mirror, an end mirror A, an end mirror B, a focusing lens, and a PMT. The Raman scattered light continues to enter the optical path of the Raman spectrometer through the Raman filter. According to the direction of the Raman scattered light, the order of the optical path structure composition and spatial positioning is as follows: short-wave pass filter → beam splitter → reflector A / reflector B → rotating mirror → end mirror A / end mirror B → rotating mirror → reflector A / reflector B → beam splitter → focusing lens → PMT;

[0010] (3) The white light reference optical path includes a white light LED light source, a collimating lens B, and a Raman filter. The white light and the Raman scattered light enter the Raman spectrometer optical path coaxially, and the direction is the same as that of the Raman scattered light in feature (2);

[0011] Although the optical path difference generated by the rotating mirror of Turbo FTS when rotating is nonlinearly related to the angle of rotation, this relationship can be expressed by a specific mathematical function. That is to say, if the angle is known, the corresponding optical path difference can be calculated by a mathematical function. To know the angle, a reference point or zero point needs to be determined. We use the position where the normal of the rotating mirror is 45° to the incident light as the reference point, which is also the position of the optical path difference zero point. As long as the time after the rotating mirror reaches this point is known, the angle of the rotating mirror can be calculated according to the rotation speed of the rotating mirror, and the corresponding optical path difference can also be calculated. Therefore, determining this optical path difference zero point is the key, and the role of the white light reference optical path is to determine the optical path difference zero point. According to the characteristics of white light interference fringes, the interference intensity is the largest at the optical path difference zero point, and the intensity decays sharply after deviation. Therefore, the optical path difference zero point is when the PMT detects the brightest light signal.

[0012] (4) The control system includes a coordinated control circuit, a rotating mirror motor mechanism, a rotating mirror motor controller, a rotating mirror zero point sensor, a laser signal generator, and a data acquisition card. The rotating mirror motor mechanism is used to drive the rotating mirror to rotate, and includes a motor, a reducer, and an encoder. The rotating mirror motor controller is used to control the rotation mode of the motor in the rotating mirror motor mechanism and read the feedback signal of the encoder in the rotating mirror motor mechanism. The rotating mirror zero point sensor is used to determine the zero point position of the optical path difference of the rotating mirror. The laser signal generator is used to drive the ultraviolet laser to emit pulsed laser and control the repetition frequency of the pulsed laser. The data acquisition card is used to read the interference signal output by the PMT.

[0013] The traditional Turbo FTS is sampling at equal time intervals (non-equal optical path difference intervals). The present invention realizes sampling at equal optical path difference intervals (unequal time intervals) through the control system, so that the interference signal can be directly restored to an accurate UV Raman spectrum through FFT. The workflow of the control system is as follows:

[0014] 4-1. The cooperative control circuit sends a command to the mirror motor control circuit, so that the motor starts and rotates at a constant speed according to the set speed;

[0015] In order to improve the time resolution of sampling at unequal time intervals, a reducer with a reduction ratio of up to 200:1 is installed at the front end of the rotating mirror motor, so that the rotating mirror only rotates 2-5 times in one minute.

[0016] 4-2. The cooperative control circuit detects the output signal of the rotating mirror zero point sensor, and starts timing or accumulating the number of encoder pulses when the output signal is detected, until the predetermined time is reached or the number of encoder pulses reaches the predetermined value. At this time, the rotating mirror is already at the zero point position of the optical path difference;

[0017] In order to accurately determine the zero point of the optical path difference, a rotating mirror zero point sensor is installed on the rotating mirror bracket. The rotating mirror zero point sensor is installed before the optical path difference zero point position of the rotating mirror (roughly located at the position where the normal line of the rotating mirror is 45° to the incident light) (along the rotation direction). That is to say, it takes a predetermined time (or the encoder sends a predetermined number of pulses) for the rotating mirror to pass through the zero point sensor and then pass through the zero point of the optical path difference. This predetermined time is determined through actual experiments and is related to the rotation speed of the rotating mirror.

[0018] 4-3. The collaborative control circuit outputs control signals to the PMT and data acquisition card synchronously to start sampling. During the sampling process, the control signal of the PMT is a stable high level, and the control signal of the data acquisition card is a time series external drive signal calculated according to the equal optical path difference interval, realizing equal optical path difference interval sampling;

[0019] The unequal time intervals (time series) corresponding to the equal optical path difference intervals are determined according to the relationship curve between the optical path difference and the rotation angle generated when the rotating mirror rotates.

[0020] 4-4. The cooperative control circuit detects the output signal of the rotating mirror zero point sensor. When the output signal is detected again, the control signal of the PMT is set to a low level, and the control signal of the acquisition card remains unchanged until the rotating mirror reaches the zero point position of the optical path difference again, so that the acquisition card collects a "zero" signal for a period of time. This "zero" signal is used as the end identification code of this sampling cycle, and the processes (4-3) and (4-4) are repeated to complete the sampling of the next cycle.

[0021] The reason for distinguishing different sampling periods is that not all interference signals of the entire period are valid. When the angle between the incident light and the normal line of the rotating mirror is close to 90°, the incident light is incident from the side of the rotating mirror. At this time and in the vicinity, the interference signal error is large. The present invention intercepts the interference signal when the angle between the incident light and the normal line of the rotating mirror is in the range of [10°-80°].

[0022] Beneficial effects of the present invention:

[0023] The present invention applies the Turbo FTS to the Raman spectrometer and explores its application in ultraviolet spectrum analysis to achieve on-site detection of the weak Raman spectrum of atmospheric CO2. At the same time, the control system is designed to solve the problem that the optical path difference generated by the rotating mirror during rotation is nonlinearly related to the angle of rotation, and to achieve equal optical path difference interval sampling;

[0024] The present invention designs a set of ultraviolet Raman spectrometer optical system based on Turbo FTS, including excitation and collection optical paths, Raman spectrometer optical paths and white light reference optical paths, which can greatly improve the signal-to-noise ratio of atmospheric CO2 detection. Design a set of control systems, through the precise control of the rotating mirror, PMT and data acquisition card to achieve equal optical path difference interval sampling, can effectively improve the accuracy of ultraviolet Raman spectrum restoration and reduce the running time. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of a rotating mirror Fourier transform ultraviolet Raman spectrometer for on-site detection of atmospheric CO2.

[0026] Figure 2 White light interference signal detected by the optical system.

[0027] Figure 3 Design schematics for control systems.

[0028] Figure 4 This is the working flow chart of the control system.

[0029] Figure 5 This is a schematic diagram of the installation position of the zero point sensor.

[0030] Figure 6 This is the relationship curve between the optical path difference and the rotation angle when the rotating mirror rotates. DETAILED DESCRIPTION

[0031] The present invention will be described in detail below with reference to the embodiments and the accompanying drawings, but the present invention is not limited thereto.

[0032] A schematic diagram of a rotating mirror Fourier transform ultraviolet Raman spectrometer for realizing on-site detection of atmospheric CO2 in this embodiment is shown in FIG. Figure 1 As shown, it includes excitation and collection optical paths, Raman spectrometer optical paths, white light reference optical paths and control systems, and is characterized by:

[0033] (1) The excitation and collection optical path includes an ultraviolet laser, a laser beam expander B, a laser beam expander A, a Raman filter, a collimating lens A, an aperture, and a collection lens. According to the direction of the ultraviolet laser, the order of the optical path structure and spatial positioning is as follows: ultraviolet laser → laser beam expander B → laser beam expander A → Raman filter → collimating lens A → aperture → collection lens. After the ultraviolet laser irradiates the atmospheric CO2, Raman scattered light is generated. According to the direction of the Raman scattered light, the order of the optical path structure and spatial positioning is as follows: collection lens → aperture → collimating lens A → Raman filter;

[0034] (2) The optical path of the Raman spectrometer includes a short-wave pass filter, a beam splitter, a reflector A, a reflector B, a rotating mirror, an end mirror A, an end mirror B, a focusing lens, and a PMT. The Raman scattered light continues to enter the optical path of the Raman spectrometer through the Raman filter. According to the direction of the Raman scattered light, the order of the optical path structure composition and spatial positioning is as follows: short-wave pass filter → beam splitter → reflector A / reflector B → rotating mirror → end mirror A / end mirror B → rotating mirror → reflector A / reflector B → beam splitter → focusing lens → PMT;

[0035] (3) The white light reference optical path includes a white light LED light source, a collimating lens B, and a Raman filter. The white light and the Raman scattered light enter the Raman spectrometer optical path coaxially, and the direction is the same as that of the Raman scattered light in feature (2);

[0036] The purpose of the white light reference optical path is to determine the zero point of the optical path difference. Figure 2 The white light interference signal detected by the optical system when a narrow-band filter with a central wavelength of 560 nm and a bandwidth of 10 nm is placed in the emission direction of the white light LED light source is given, and the position with the highest interference intensity is the zero point of the optical path difference.

[0037] (4) The control system includes a coordinated control circuit, a rotating mirror motor mechanism, a rotating mirror motor controller, a rotating mirror zero point sensor, a laser signal generator, and a data acquisition card. Figure 3 As shown. The mirror motor mechanism is used to drive the mirror to rotate, including a motor, a reducer, and an encoder. The mirror motor controller is used to control the rotation mode of the motor in the mirror motor mechanism and read the feedback signal of the encoder in the mirror motor mechanism. The mirror zero point sensor is used to determine the zero point position of the optical path difference of the mirror. The laser signal generator is used to drive the ultraviolet laser to emit pulsed laser and control the repetition frequency of the pulsed laser. The data acquisition card is used to read the interference signal output by the PMT. The workflow of the control system is as follows: Figure 4 As shown, the details are as follows:

[0038] 4-1. The cooperative control circuit sends a command to the mirror motor control circuit, so that the motor starts and rotates at a constant speed according to the set speed;

[0039] 4-2. The cooperative control circuit detects the output signal of the rotating mirror zero point sensor, and starts timing or accumulating the number of encoder pulses when the output signal is detected, until the predetermined time is reached or the number of encoder pulses reaches the predetermined value. At this time, the rotating mirror is already at the zero point position of the optical path difference;

[0040] In order to accurately determine the zero point of the optical path difference, a rotating mirror zero point sensor is installed on the rotating mirror bracket, such as Figure 5 As shown, the rotating mirror zero point sensor is installed before (along the rotation direction) the zero point position of the rotating mirror optical path difference (approximately located at a position where the normal line of the rotating mirror and the incident light form an angle of 45°).

[0041] 4-3. The collaborative control circuit outputs control signals to the PMT and data acquisition card synchronously to start sampling. During the sampling process, the control signal of the PMT is a stable high level, and the control signal of the data acquisition card is a time series external drive signal calculated according to the equal optical path difference interval, realizing equal optical path difference interval sampling;

[0042] The unequal time intervals (time series) corresponding to the equal optical path difference intervals are determined according to the relationship curve between the optical path difference and the rotation angle generated when the rotating mirror rotates, such as Figure 6 The refractive index of the rotating mirror is set to 1.5, the thickness of the rotating mirror is set to 4 mm, the optical path difference interval is 0.3252 mm, 10 sample points are taken, and the corresponding rotating mirror angles are marked in the figure by vertical lines.

[0043] 4-4. The cooperative control circuit detects the output signal of the rotating mirror zero point sensor. When the output signal is detected again, the control signal of the PMT is set to a low level, and the control signal of the acquisition card remains unchanged until the rotating mirror reaches the zero point position of the optical path difference again, so that the acquisition card collects a "zero" signal for a period of time. This "zero" signal is used as the end identification code of this sampling cycle, and the processes (4-3) and (4-4) are repeated to complete the sampling of the next cycle.

[0044] The above description is only an example of a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A rotating mirror Fourier transform ultraviolet Raman spectrometer for realizing on-site detection of atmospheric CO2, comprising: The excitation and collection optical path, the Raman spectrometer optical path, the white light reference optical path and the control system are characterized by: (1) The excitation and collection optical path includes an ultraviolet laser, a laser beam expander B, a laser beam expander A, a Raman filter, a collimating lens A, an aperture, and a collecting lens. According to the direction of the ultraviolet laser, the order of the optical path structure composition and spatial positioning is as follows: ultraviolet laser → laser beam expander B → laser beam expander A → Raman filter → collimating lens A → aperture → collecting lens. After the ultraviolet laser irradiates the atmospheric CO2, Raman scattered light is generated. According to the direction of the Raman scattered light, the order of the optical path structure composition and spatial positioning is as follows: collecting lens → aperture → collimating lens A → Raman filter; (2) The optical path of the Raman spectrometer includes a short-wave pass filter, a beam splitter, a reflector A, a reflector B, a rotating mirror, an end mirror A, an end mirror B, a focusing lens, and a PMT. The Raman scattered light continues to enter the optical path of the Raman spectrometer through the Raman filter. According to the direction of the Raman scattered light, the order of the optical path structure composition and spatial positioning is as follows: short-wave pass filter → beam splitter → reflector A / reflector B → rotating mirror → end mirror A / end mirror B → rotating mirror → reflector A / reflector B → beam splitter → focusing lens → PMT; (3) The white light reference optical path includes a white light LED light source, a collimating lens B, and a Raman filter. The white light and the Raman scattered light enter the Raman spectrometer optical path coaxially, and the direction is also the same as that of the Raman scattered light. (4) The control system includes a coordinated control circuit, a rotating mirror motor mechanism, a rotating mirror motor controller, a rotating mirror zero point sensor, a laser signal generator, and a data acquisition card. The rotating mirror motor mechanism is used to drive the rotating mirror to rotate, and includes a motor, a reducer, and an encoder. The rotating mirror motor controller is used to control the rotation mode of the motor in the rotating mirror motor mechanism and read the feedback signal of the encoder in the rotating mirror motor mechanism. The rotating mirror zero point sensor is used to determine the zero point position of the optical path difference of the rotating mirror. The laser signal generator is used to drive the ultraviolet laser to emit pulsed laser and control the repetition frequency of the pulsed laser. The data acquisition card is used to read the interference signal output by the PMT. The workflow of the control system is as follows: (2.1) The cooperative control circuit sends a command to the mirror motor control circuit, so that the motor starts and rotates at a constant speed according to the set speed; (2.2) The cooperative control circuit detects the output signal of the rotating mirror zero point sensor, and starts timing or accumulating the number of encoder pulses when the output signal is detected, until a predetermined time is reached or the number of encoder pulses reaches a predetermined value, at which time the rotating mirror is already at the optical path difference zero point position; (2.3) The collaborative control circuit synchronously outputs control signals to the PMT and the data acquisition card to start sampling. During the sampling process, the control signal of the PMT is a stable high level, and the control signal of the data acquisition card is a time series external drive signal calculated according to the equal optical path difference interval, thereby realizing equal optical path difference interval sampling; (2.4) The cooperative control circuit detects the output signal of the rotating mirror zero point sensor. When the output signal is detected again, the control signal of the PMT is set to a low level, and the control signal of the acquisition card remains unchanged until the rotating mirror reaches the zero point of the optical path difference again, so that the acquisition card collects the "zero" signal for a period of time. This "zero" signal serves as the end identification code of this sampling cycle, and repeats the processes (2.3) and (2.4) to complete the sampling of the next cycle.

2. The rotating mirror Fourier transform ultraviolet Raman spectrometer for realizing on-site detection of atmospheric CO2 as claimed in claim 1, characterized in that: The reduction ratio of the reducer is 200:1 to obtain high angular resolution.

3. The rotating mirror Fourier transform ultraviolet Raman spectrometer for realizing on-site detection of atmospheric CO2 as claimed in claim 1, characterized in that: The rotating mirror zero point sensor is installed before the rotating mirror optical path difference zero point position, wherein the rotating mirror optical path difference zero point position is approximately located at a position where the rotating mirror normal line and the incident light form a 45° angle.

4. The rotating mirror Fourier transform ultraviolet Raman spectrometer for realizing on-site detection of atmospheric CO2 as claimed in claim 1, characterized in that: The predetermined time or the predetermined value of the number of encoder pulses described in the process (2.2) is related to the installation position of the rotating mirror zero point sensor and the rotation speed of the rotating mirror, and its value is measured based on actual experiments.

5. The rotating mirror Fourier transform ultraviolet Raman spectrometer for realizing on-site detection of atmospheric CO2 as claimed in claim 1, characterized in that: The time series external driving signal calculated according to the equal optical path difference interval described in the process (2.3) is determined according to the relationship curve between the optical path difference generated when the rotating mirror rotates and the rotation angle.

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