TDLAS (Tunable Diode Laser Absorption Spectroscopy) gas concentration detection device and method based on digital phase demodulation

Abstract

The invention discloses a TDLAS (Tunable Diode Laser Absorption Spectroscopy) gas concentration detection device and method based on digital phase demodulation, a DFB (Distributed Feedback) laser generates laser, the laser passes through an absorption cell filled with gas to be detected after passing through a collimator and is received by a photoelectric detector, and a data acquisition card converts an analog absorption signal into a digital signal to be detected; a data acquisition card is used for directly acquiring fundamental frequency sine and cosine reference signals which are generated by a signal generator and have the same frequency as a signal to be measured, the fundamental frequency sine and cosine reference signals are respectively multiplied by the signal to be measured and then subjected to low-pass filtering processing, and obtained mutually orthogonal signals are subjected to division operation and arc tangent operation processing to obtain fundamental frequency phase signals. And taking the vertical coordinate phases of the maximum value point and the minimum value point near the absorption peak as a phase difference, wherein the phase difference is used for measuring the gas concentration. The signal containing the concentration information is obtained through a simple method, the signal can be used for replacing a lock-in amplifier, and the cost of a detection device and the system complexity can be effectively reduced.

Description

Technical Field

[0001] The invention relates to the technical field of tunable semiconductor laser absorption spectroscopy, and in particular to a TDLAS gas concentration detection method based on digital phase demodulation. Background Art

[0002] With the development of industry and agriculture, various fossil energy sources used in large quantities in production will produce many toxic, harmful, flammable and explosive gases. These gases will affect atmospheric circulation, cause acid rain, damage the ozone layer, and cause serious environmental problems. Some greenhouse gases such as CO2 will also aggravate the greenhouse effect, lead to global warming, and affect the living environment of human beings. Therefore, it is of great significance to monitor the harmful gas components in the atmosphere and research and develop gas sensors. In the field of industrial safety, gas sensors can be used to monitor the CH4 concentration in mines, which can effectively prevent and avoid safety accidents caused by gas explosions. In the field of electricity, monitoring and analyzing the gas in the transformer insulating oil can complete fault alarm and fault analysis. In the field of medical health, diseases can be diagnosed by analyzing the concentration changes of volatile organic compounds through breath detection.

[0003] Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology is an optical and spectroscopic measurement method that applies laser to absorption spectrum measurement technology. It has the advantages of high measurement accuracy, high sensitivity, strong anti-interference ability and compact measurement device structure, and has been widely used in the field of gas parameter measurement. The traditional gas concentration detection system based on the TDLAS principle includes laser, optical path structure, photodetector and phase-locked amplifier, which increases the cost and system complexity of the detection system. Under some special conditions, high-frequency modulation signals are required, which are easily affected by noise and affect sensitivity. Summary of the invention

[0004] The object of the present invention is to provide a TDLAS gas concentration detection device and method based on digital phase demodulation.

[0005] To this end, the above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0006] The present invention first provides a TDLAS gas concentration detection device based on digital phase demodulation, comprising a signal generator, a temperature controller, a DFB laser, a collimator, a gas absorption cell, a photoelectric detector, a data acquisition card and a signal processor; the output ends of the signal generator and the temperature controller are connected to the DFB laser; the output end of the DFB laser is connected to the collimator; the gas absorption cell is arranged between the collimator and the photoelectric detector; the output end of the photoelectric detector is connected to the data acquisition card, the data acquisition card converts the absorption signal received by the photoelectric detector into a digital signal to be measured, and converts the reference signal generated by the signal generator into a fundamental frequency sine reference signal and a fundamental frequency cosine reference signal in digital form; the signal processor receives the digital signal to be measured output by the data acquisition card, and processes the digital signal to be measured as follows:

[0007] A first orthogonal signal is obtained by multiplying the signal to be tested with a baseband sine reference signal and then processing it through a low-pass filter; a second orthogonal signal is obtained by multiplying the signal to be tested with a baseband cosine reference signal and then processing it through a low-pass filter;

[0008] The first orthogonal signal and the second orthogonal signal are divided to obtain a ratio signal, and the ratio signal is further processed by an inverse tangent operation to obtain a fundamental frequency harmonic phase signal;

[0009] The gas concentration is obtained based on the fundamental frequency harmonic phase signal.

[0010] The present invention also provides a gas concentration detection method, characterized in that the laser light emitted by the laser is collimated and passed through an absorption cell filled with a gas to be detected, and then received by a photoelectric detector, and then a data acquisition card converts the absorption signal into a digital signal to be detected, and converts the reference signal generated by the signal generator into a digital fundamental frequency sine reference signal and fundamental frequency cosine reference signal;

[0011] The signal to be tested is multiplied by the baseband sine reference signal and then processed by a low-pass filter to obtain a mutually orthogonal first orthogonal signal; the signal to be tested is multiplied by the baseband cosine reference signal and then processed by a low-pass filter to obtain a mutually orthogonal second orthogonal signal;

[0012] The first orthogonal signal and the second orthogonal signal are divided to obtain a ratio signal, and the ratio signal is further processed by an inverse tangent operation to obtain a fundamental frequency harmonic phase signal;

[0013] The gas concentration is obtained based on the fundamental frequency harmonic phase signal.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] The detection device and method of the present invention realizes the calibration of gas concentration without using a phase-locked amplifier, so as to reduce the cost of the gas detection system and simplify the complexity of the system.

[0016] Compared with the traditional TDLAS gas concentration detection method, under the condition of high-frequency modulation signal, it can ensure high sensitivity while reducing the impact of noise on concentration detection results. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is an overall schematic diagram of the present invention;

[0018] Figure 2 It is a schematic diagram of the demodulation process of the present invention;

[0019] Figure 3 It is a comparison chart of the experimental results of the present invention;

[0020] Figure 4 It is the linear fitting diagram of the present invention. DETAILED DESCRIPTION

[0021] The specific implementation modes of the present invention are further described in detail below in conjunction with the accompanying drawings. The following embodiments are used to illustrate the present invention, but should not be used to limit the protection scope of the present invention.

[0022] Figure 1 An overall schematic diagram of a TDLAS gas concentration detection device based on digital phase demodulation provided in this embodiment.

[0023] The gas concentration detection device includes:

[0024] Part 1: According to the type of gas to be measured, select a distributed feedback laser (DFB) that meets the gas absorption spectrum; use a signal generator to generate a low-frequency sawtooth scanning wave and a high-frequency sine modulation wave to drive the DFB laser, and modulate the signal frequency and intensity according to the laser parameters;

[0025] In the second part, a semi-open gas absorption cell is designed. The laser emitted by the laser is collimated and passed through the absorption cell filled with the gas to be measured. It is received by the photoelectric detector, and then the data acquisition card converts the absorption signal into a digital signal to be measured; the collected signal is processed by digital phase deconvolution.

[0026] This embodiment takes carbon dioxide gas (CO2) as an example and provides a TDLAS gas concentration detection method based on digital phase demodulation, including:

[0027] The output end of the signal generator 1 and the output end of the temperature controller 2 are commonly connected to the DFB laser 3. The temperature controller 2 is used to adjust the working temperature of the DFB laser 3 to prevent the laser temperature from being too high, which may cause the spectral drift of the emitted light. The output end of the DFB laser 3 is connected to the collimator 4. The output end of the collimator 4 is connected to the gas absorption cell 5. The other end of the gas absorption cell 5 is connected to the photodetector 6. The output end of the photodetector 6 is connected to the data acquisition card 7. The data acquisition card 7 converts the signal into a digital signal to be measured 8, and converts the reference signal generated by the signal generator 1 into a digital fundamental frequency sine reference signal 9 and a fundamental frequency cosine reference signal 10. The signal to be measured 8 is multiplied with the fundamental frequency sine reference signal 9 and the fundamental frequency cosine reference signal 10 respectively, and then processed by a low-pass filter to obtain a mutually orthogonal first orthogonal signal 11 and a second orthogonal signal 12. The first orthogonal signal 11 and the second orthogonal signal 12 are divided to obtain a ratio signal 13. The ratio signal 13 is then processed by an inverse tangent operation to obtain a fundamental frequency harmonic phase signal 14.

[0028] The gas concentration detection method of the present invention comprises the following steps:

[0029] Step 1: According to the type of gas to be tested, select the corresponding absorption spectrum line, and the center frequency of the absorption spectrum is v0;

[0030] Step 2: In order to make the emission spectrum of the DFB laser cover the gas absorption spectrum, a signal generator is needed to generate a low-frequency sawtooth signal (t) to control the scanning of the DFB laser; in order to suppress background noise interference, a high-frequency sine wave signal sin (t) is needed to modulate the output light of the DFB laser; the two signals are superimposed and acted on the DFB laser;

[0031] The frequency of the laser signal generated by the DFB laser in step 2 is:

[0032] v(t)=v0+Esin(2πf m t)

[0033] Where v(t) is the laser output frequency, v0 is the laser center frequency, E is the modulation amplitude, and f m is the modulation frequency, t is the scanning time;

[0034] Step 3: The laser emitted by the laser passes through the collimator and then through the absorption cell filled with the gas to be tested, and is received by the photodetector. Then the data acquisition card converts the analog absorption signal into a digital signal to be tested;

[0035] According to the Beer-Lambert law, the change in light intensity of a laser beam of a specific frequency before and after it enters the gas to be measured can be expressed as:

[0036] I(v)=I0exp[-α(v)CPL]

[0037] Where I0 is the incident light intensity, I(v) is the light intensity at the receiving end after passing through the gas to be measured, α(ν) is the absorption coefficient of the gas to be measured to light at frequency v, L is the optical path length of the laser passing through the gas to be measured, C is the concentration of the gas to be measured, and P is the pressure;

[0038] In step 3, the relationship between the light intensity generated by the light signal absorbed by the gas to be measured and the time variation is as follows:

[0039] I(v(t))=I0exp{-α[v(t)]CPL}=I0exp{-α[v0+Esin(2πf m t)]CPL}

[0040] Step 4: Process the collected digital signal by digital phase deconvolution. The phase deconvolution process is as follows: Figure 2 As shown, D[n] represents the signal to be measured (8), and the expression is as follows:

[0041]

[0042] In the formula, D0 is the DC component in the signal to be measured, A is the amplitude of the signal to be measured, and n is the number of sampling points. is the phase difference between the modulated signal and the reference signal;

[0043] The data acquisition card is used to directly acquire the fundamental frequency sine reference signal S[n] and fundamental frequency cosine reference signal C[n] generated by the signal generator with the same frequency as the signal to be measured. The expression is as follows:

[0044] S[n]=Bsin(2πf m n)

[0045] C[n]=Bcos(2πf m n)

[0046] Where B is the amplitude of the reference signal;

[0047] S[n] and C[n] are multiplied by the measured signal D[n] respectively and then low-pass filtered to remove the high-frequency components and leave the low-frequency DC part containing the amplitude and phase information of the original signal, thus obtaining mutually orthogonal signals Y[n] and X[n];

[0048] Step 5: Divide the mutually orthogonal signals Y[n] and X[n] and then perform an inverse tangent operation to obtain the fundamental frequency harmonic phase signal The expression is as follows:

[0049]

[0050] Step 6: Take the ordinate phase difference between the maximum point and the minimum point at the sampling point near the gas absorption peak. The phase difference has a linear relationship with the gas concentration. As the gas concentration increases, the phase difference also increases.

[0051] In order to illustrate the feasibility of the method of the present invention, actual experimental data are used for illustration, as follows:

[0052] The gas to be measured is carbon dioxide (CO2), a DFB laser with a central wavelength of 1580nm and an initial light intensity of 10mW is selected, the modulation signal amplitude is selected to be 0.05V, the modulation signal frequency is selected to be 10kHz, a gas absorption cell with an optical path of 2m is selected, and carbon dioxide gas in the range of 20000ppm to 30000ppm is filled under normal pressure. The digital phase demodulation is performed using the above method of the present invention. With the sampling point as the horizontal coordinate, the fundamental frequency harmonic phase Draw a two-dimensional curve image for the vertical axis, such as Figure 3 As shown; then take the ordinate phase value of the maximum point at the sampling point on the left side of the gas absorption peak and the minimum point on the right side as the phase difference, and then use the phase difference of the fundamental frequency harmonic as the abscissa and the actual gas concentration as the ordinate to draw a two-dimensional curve image, as shown Figure 4 As shown in the figure, it can be observed that as the phase difference of the harmonic signal at the absorption line increases, the gas concentration also increases. It can be seen that the phase difference of the fundamental frequency harmonic at the absorption peak does have a linear relationship with the gas concentration. Finally, the gas concentration value can be determined by observing the phase difference of the fundamental frequency harmonic at the absorption peak.

[0053] This embodiment provides a TDLAS gas concentration detection device based on digital phase demodulation, which can obtain the phase signal of the fundamental frequency harmonic containing the concentration information by performing a digital orthogonal phase solution method on the signal to be measured containing the concentration information. The present invention obtains the signal containing the concentration information by a relatively simple method, replaces the role of the phase-locked amplifier, can effectively reduce the cost and system complexity of the detection device, and can ensure high sensitivity and reduce the influence of noise on the concentration detection result in the case of high-frequency modulation signals.

[0054] The embodiments described above are merely descriptions of preferred implementation modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements to the technical solutions of the present invention made by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A TDLAS gas concentration detection device based on digital phase demodulation, characterized in that: The invention comprises a signal generator (1), a temperature controller (2), a DFB laser (3), a collimator (4), a gas absorption cell (5), a photoelectric detector (6), a data acquisition card (7) and a signal processor; the output ends of the signal generator (1) and the temperature controller (2) are connected to the DFB laser (3); the output end of the DFB laser (3) is connected to the collimator (4); the gas absorption cell (5) is arranged between the collimator (4) and the photoelectric detector (6); the output end of the photoelectric detector (6) is connected to the data acquisition card (7); the data acquisition card (7) converts the absorption signal received by the photoelectric detector into a digital signal to be measured (8), and converts the reference signal generated by the signal generator (1) into a digital fundamental frequency sine reference signal (9) and fundamental frequency cosine reference signal (10); the signal processor receives the digital signal to be measured (8) output by the data acquisition card (7), and processes the digital signal to be measured (8) as follows: A first orthogonal signal (11) is obtained by multiplying the signal to be tested (8) with a baseband sine reference signal (9) and then processing it through a low-pass filter; a second orthogonal signal (12) is obtained by multiplying the signal to be tested (8) with a baseband cosine reference signal (10) and then processing it through a low-pass filter; A ratio signal (13) is obtained by dividing the first orthogonal signal (11) and the second orthogonal signal (12), and an inverse tangent operation is performed on the ratio signal (13) to obtain a fundamental frequency harmonic phase signal (14); The gas concentration is obtained according to the fundamental frequency harmonic phase signal (14).

2. The TDLAS gas concentration detection device based on digital phase demodulation according to claim 1 is characterized in that: The signal generator (1) generates a low-frequency sawtooth scanning wave and a high-frequency sinusoidal modulation wave for driving the DFB laser (3) to emit a detection laser.

3. The TDLAS gas concentration detection device based on digital phase demodulation according to claim 1 is characterized in that: The laser light emitted by the laser (3) passes through the collimator (4) and then through the absorption cell (5) filled with the gas to be measured, and is received by the photodetector (6). Then the data acquisition card (7) converts the analog absorption signal into a digital signal to be measured (8). The frequency and intensity of the signal to be measured (8) are expressed as follows: v(t)=v0+Esin(2πf m t) I(v(t))=I0exp{-α[v(t)]CPL} Where v(t) is the laser output frequency, v0 is the laser center frequency, E is the modulation amplitude, and f m is the modulation frequency, t is the scanning time, I0 is the incident light intensity, I(v(t)) is the light intensity at the receiving end after passing through the gas to be measured, α[ν(t)] is the absorption coefficient of the gas to be measured to light at the frequency v(t), L is the optical path length of the laser passing through the gas to be measured, C is the concentration of the gas to be measured, and P is the pressure.

4. The TDLAS gas concentration detection device based on digital phase demodulation according to claim 3 is characterized in that: The collected digital signal is processed by digital phase deconvolution, and D[n] represents the signal to be measured (8), which is expressed as follows: In the formula, D0 is the DC component in the signal to be measured, A is the amplitude of the signal to be measured, and n is the number of sampling points. is the phase difference between the modulated signal and the reference signal.

5. The TDLAS gas concentration detection device based on digital phase demodulation according to claim 3 is characterized in that: The fundamental frequency sine reference signal (9) has the same frequency as the signal to be measured (8), and the fundamental frequency cosine reference signal (10) has the same frequency as the signal to be measured (8); The expressions of the fundamental frequency sine reference signal and the fundamental frequency cosine reference signal are as follows: S[n]=Bsin(2πf m n) C[n]=Bcos(2πf m n) Where S[n] is the fundamental frequency sine reference signal (9), C[n] is the fundamental frequency cosine reference signal (10), and B is the amplitude of the reference signal. S[n] and C[n] are respectively multiplied by the signal D[n] and then low-pass filtered to leave the low-frequency DC part containing the amplitude and phase information of the original signal, thereby obtaining a mutually orthogonal first orthogonal signal (11) and a second orthogonal signal (12).

6. The TDLAS gas concentration detection device based on digital phase demodulation according to claim 5 is characterized in that: A fundamental frequency sine reference signal (9) and a fundamental frequency cosine reference signal (10) are generated by a signal generator (1) and converted into digital reference signals by a data acquisition card (7).

7. The TDLAS gas concentration detection device based on digital phase demodulation according to claim 5 is characterized in that: The first orthogonal signal (11) and the second orthogonal signal (12) are divided and then subjected to an inverse tangent operation to obtain a fundamental frequency harmonic phase signal (14) expressed as follows: In the formula, is the fundamental frequency harmonic phase signal (14), Y[n] is the first orthogonal signal, and X[n] is the second orthogonal signal.

8. The TDLAS gas concentration detection device based on digital phase demodulation according to claim 7 is characterized in that: Taking the sampling point as the horizontal coordinate, the fundamental frequency harmonic phase signal Draw a two-dimensional curve image for the ordinate, and then take the ordinate phase values ​​of the maximum point and the minimum point at the sampling point near the gas absorption peak as the phase difference; The gas concentration is obtained based on the linear relationship between the phase difference and the concentration.

9. The TDLAS gas concentration detection device based on digital phase demodulation according to claim 8, characterized in that: The linear relationship between the phase difference and the concentration is determined according to the following method: the phase difference of the fundamental frequency harmonic is used as the abscissa and the actual gas concentration is used as the ordinate to plot and perform a linear fit.

10. A gas concentration detection method based on the TDLAS gas concentration detection device according to any one of claims 1 to 9, characterized in that: include: The laser light emitted by the laser is collimated and passed through an absorption cell filled with the gas to be measured, and then received by a photoelectric detector. Then the data acquisition card converts the absorption signal into a digital signal to be measured. The signal to be tested (8) and the baseband sine signal (9) are multiplied and then processed by a low-pass filter to obtain a mutually orthogonal first orthogonal signal (11); the signal to be tested (8) and the baseband cosine signal (10) are multiplied and then processed by a low-pass filter to obtain a mutually orthogonal second orthogonal signal (12); A ratio signal (13) is obtained by dividing the first orthogonal signal (11) and the second orthogonal signal (12), and an inverse tangent operation is performed on the ratio signal (13) to obtain a fundamental frequency harmonic phase signal (14); The gas concentration is obtained according to the fundamental frequency harmonic phase signal (14).

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