Photoacoustic spectrum trace gas detection device and method based on secondary demodulation technology

Abstract

The invention discloses a photoacoustic spectrum trace gas detection device and method based on a secondary demodulation technology. The device comprises a tunable laser, a laser collimation device, a focusing lens, a gas chamber, a quartz tuning fork resonance circuit, a first lock-in amplifier, a trigger capture circuit, a phase modulation circuit, a second lock-in amplifier and a control and signal acquisition device. In the aspect of automatic tracking of the frequency, the quartz tuning fork is integrated into an oscillator circuit, and automatic tracking of the laser modulation frequency is achieved. The oscillator circuit can automatically adjust the output according to the current resonant frequency of the quartz tuning fork, and ensures that the laser is always modulated at the optimal frequency. In the aspect of continuous phase modulation, traditional fixed phase setting is replaced, the phases of electric excitation and photoacoustic excitation are continuously modulated, and pure photoacoustic signals are separated by using two lock-in amplifiers, so that a complex phase calibration process is avoided, and the accuracy and stability of signal detection are improved.

Description

Technical Field

[0001] The present invention relates to a photoacoustic spectroscopy trace gas detection device and method, and particularly to a photoacoustic spectroscopy trace gas detection device and method based on a secondary demodulation technique. Background Technique

[0002] Quartz-enhanced photoacoustic spectroscopy (QEPAS) is an important means for trace gas detection. It uses a quartz tuning fork to replace the microphone as the sensing element, and inversely calculates the gas concentration by detecting the acoustic wave signal generated after the gas absorbs laser energy. With advantages such as fast response speed, good selectivity, and online monitoring, this technique has been widely applied in fields such as environmental monitoring, industrial production, and biomedicine. In a photoacoustic spectroscopy detection device, accurately obtaining gas concentration information depends on a stable and sensitive detection signal. The amplitude of the detection signal of the device is affected by the quality factor and resonance frequency of the quartz tuning fork. Therefore, optimizing the performance of the quartz tuning fork, stabilizing the laser modulation frequency, and accurately extracting the photoacoustic signal are crucial for improving the detection ability of the QEPAS technique.

[0003] The working principle of the traditional quartz-enhanced photoacoustic spectroscopy sensing technique is to place a quartz tuning fork in a gas chamber filled with the gas to be measured. The tunable laser enters the gas chamber after collimation and excites the gas molecules to be measured. After the gas molecules absorb the laser energy, they undergo non-radiative transitions to the ground state, converting the absorbed light energy into heat energy, resulting in local temperature changes in the substance, and then causing pressure changes to generate density waves. When the frequency of the density waves is the same as the laser modulation frequency, an acoustic wave signal is generated. Since the laser is modulated, the generated acoustic wave signal changes periodically, causing the quartz tuning fork to vibrate periodically. When the modulation frequency of the laser is consistent with the resonance frequency of the quartz tuning fork, the quartz tuning fork resonates and amplifies, and the vibration amplitude increases significantly. Utilizing the piezoelectric effect of the quartz tuning fork, its elastic deformation generates an electrical current signal, and by demodulating this electrical current signal, the gas concentration information can be obtained.

[0004] In the traditional quartz-enhanced photoacoustic spectroscopy sensing technique, the following main problems exist:

[0005] (1) Frequency and phase adjustment problems: The resonance frequency of the quartz tuning fork is easily affected by environmental factors and drifts, resulting in a mismatch between the laser modulation frequency and the tuning fork resonance frequency, and frequent recalibration is required. Therefore, for a fixed phase setting, it is only effective within a limited range of environmental parameters. Once the environmental conditions change, the originally optimized phase setting is no longer applicable, and the phase needs to be readjusted, which increases the complexity and uncertainty of the operation. In addition, manual phase adjustment is difficult to achieve real-time and precise control and cannot meet the rapidly changing detection requirements.

[0006] (2) Coordination problem between the tuning fork and the resonator: When a quartz tuning fork is equipped with an acoustic resonator, there is a strong coupling between the tuning fork and the resonator. The resonance frequencies of the two respond differently to environmental changes. Under actual working conditions, a slight change in environmental conditions may disrupt the resonance balance between them, resulting in a decrease in the sensitivity of the device and a deterioration in stability.

[0007] (3) Detection limit and continuity problem: For some trace gas detection scenarios, the detection limit of traditional QEPAS technology is difficult to meet the requirements. Moreover, during the measurement process, since it is necessary to interrupt the measurement to check and readjust the modulation frequency, true continuous detection cannot be achieved, which limits its application in the field of real-time monitoring. Summary of the Invention

[0008] To solve the above problems existing in traditional quartz-enhanced photoacoustic spectroscopy technology, the present invention provides a photoacoustic spectroscopy trace gas detection device and method based on a secondary demodulation technique.

[0009] The object of the present invention is achieved through the following technical solutions:

[0010] A photoacoustic spectroscopy trace gas detection device based on a secondary demodulation technique, comprising a tunable laser, a laser collimation device, a focusing lens, a gas cell, a quartz tuning fork resonance circuit, a first lock-in amplifier, a trigger capture circuit, a phase modulation circuit, a second lock-in amplifier, and a control and signal acquisition device, wherein: the quartz tuning fork resonance circuit automatically tracks the resonance frequency of the quartz tuning fork and generates an electrical signal. After receiving the electrical signal transmitted from the quartz tuning fork resonance circuit, the trigger capture circuit generates a trigger pulse. The phase modulation circuit obtains the trigger signal generated by the trigger capture circuit and performs phase modulation on it, so that the signal transmitted to the tunable laser can make the laser modulation phase change continuously, and controls the tunable laser to emit laser with a specific wavelength and frequency; the laser emitted by the tunable laser is collimated by the laser collimation device and focused by the focusing lens and then enters the gas cell, interacts with the gas to be measured therein, the gas molecules absorb the laser energy to generate sound waves, which excite the quartz tuning fork to vibrate, and its vibration is converted into a mixed electrical signal containing photoacoustic and electrical excitation through the piezoelectric effect; the first lock-in amplifier uses the electrical signal provided by the quartz tuning fork resonance circuit as a reference signal, extracts and amplifies a specific frequency signal, and outputs signal A after suppressing noise; the second lock-in amplifier uses the modulation frequency of the phase modulation circuit as a reference, demodulates and amplifies signal A, and separates out a pure photoacoustic signal; the control and signal acquisition device calculates the concentration of the gas to be measured based on the relevant parameters of the pure photoacoustic signal in combination with the calibration relationship between the relevant parameters of the photoacoustic signal and the concentration of the gas to be measured, and realizes the measurement of the gas concentration.

[0011] A method for detecting trace gases in photoacoustic spectroscopy based on the secondary demodulation technique using the above device, comprising the following steps:

[0012] Step 1: The quartz tuning fork resonance circuit starts to work, automatically tracks the resonance frequency of the quartz tuning fork, generates a corresponding electrical signal, and transmits this signal to the trigger capture circuit and the first lock-in amplifier respectively;

[0013] Step 2: After receiving the electrical signal transmitted from the quartz tuning fork resonance circuit, the trigger capture circuit generates a trigger pulse based on this, which is used to control the operation of the tunable laser. At the same time, the phase modulation circuit obtains the trigger signal generated by the trigger capture circuit and performs phase modulation on it, so that the signal transmitted to the tunable laser can make the laser modulation phase change continuously, and then the tunable laser emits laser with a specific wavelength and frequency;

[0014] Step 3: The laser emitted by the tunable laser first undergoes collimation processing through a laser collimation device. The collimated laser beam is then focused by a focusing lens and then enters the gas cell;

[0015] Step 4: The gas cell is filled with the gas to be measured. The laser entering the gas cell interacts with gas molecules. The gas molecules absorb the laser energy, convert the light energy into heat energy through non-radiative transitions, cause local temperature changes, and then generate sound waves. The sound waves excite the quartz tuning fork to vibrate, and the quartz tuning fork converts the vibration into a mixed electrical signal containing photoacoustic and electrical excitation based on the piezoelectric effect;

[0016] Step 5: The first lock-in amplifier receives the mixed electrical signal, uses the electrical signal provided by the quartz tuning fork resonance circuit as a reference signal, extracts the signal component with the same frequency as the reference frequency by using the phase-sensitive detection technology, suppresses other noise and interference signals at the same time, amplifies the extracted signal, and finally outputs signal A;

[0017] Step 6: The second lock-in amplifier demodulates and amplifies the received signal A with the modulation frequency of the phase modulation circuit as a reference, and separates out the pure photoacoustic signal;

[0018] Step 7: The control and signal acquisition device calculates the concentration of the gas to be measured based on the relevant parameters of the pure photoacoustic signal and the calibration relationship between the relevant parameters of the photoacoustic signal and the concentration of the gas to be measured established in advance, so as to realize the measurement of the gas concentration.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] The core of the present invention lies in realizing the automatic tracking of frequency and the continuous modulation of phase. In terms of the automatic tracking of frequency, a quartz tuning fork is integrated into the oscillator circuit to achieve the automatic tracking of the laser modulation frequency. The oscillator circuit can automatically adjust the output according to the current resonance frequency of the quartz tuning fork, ensuring that the laser is always modulated at the optimal frequency. In terms of the continuous modulation of phase, instead of the traditional fixed phase setting, the phases of the electrical excitation and the photoacoustic excitation are continuously modulated, and two lock-in amplifiers are used to separate the pure photoacoustic signal, avoiding the complex phase calibration process and improving the accuracy and stability of signal detection. In addition, each component of the device of the present invention is optimized, including the laser source, the gas chamber, the signal amplification circuit, the data acquisition and processing device, etc., to improve the performance and detection accuracy of the entire device. Description of the Drawings

[0021] Figure 1 It is a schematic structural diagram of a photoacoustic spectroscopy trace gas detection device based on the secondary demodulation technology;

[0022] Figure 2 It is the signal output by the first lock-in amplifier;

[0023] Figure 3 It is the output result of the demodulated signal. Detailed Embodiment

[0024] The technical solution of the present invention will be further described below in conjunction with the drawings, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention shall be covered by the protection scope of the present invention.

[0025] The present invention provides a photoacoustic spectroscopy trace gas detection device based on the secondary demodulation technology, such as Figure 1As shown, the device includes a tunable laser 1, a laser collimation device 2, a focusing lens 3, a gas cell 4, a quartz tuning fork resonance circuit 5, a first lock-in amplifier 6, a trigger capture circuit 7, a phase modulation circuit 8, a second lock-in amplifier 9, and a control and signal acquisition device 10. The quartz tuning fork resonance circuit 5 automatically tracks the resonance frequency of the quartz tuning fork and generates an electrical signal. Based on this, the trigger capture circuit 7 generates a trigger pulse to control the tunable laser 1 to emit laser light of a specific wavelength and frequency. At the same time, the phase modulation circuit 8 performs phase modulation on the trigger signal to make the modulation phase of the laser change continuously. The laser emitted by the tunable laser 1 is collimated by the laser collimation device 2, focused by the focusing lens 3, and then enters the gas cell 4, where it interacts with the gas to be measured. The gas molecules absorb the laser energy and generate sound waves, which excite the vibration of the quartz tuning fork. The vibration is converted into a mixed electrical signal containing photoacoustic and electrical excitation through the piezoelectric effect. The first lock-in amplifier 6 uses the electrical signal provided by the quartz tuning fork resonance circuit 5 as a reference signal, extracts and amplifies the signal of a specific frequency, suppresses the noise, and then outputs signal A. The second lock-in amplifier 9 uses the modulation frequency of the phase modulation circuit 8 as a reference, demodulates and amplifies signal A, separates the pure photoacoustic signal, and finally the control and signal acquisition device 10 calculates the concentration of the gas to be measured based on the relevant parameters of the pure photoacoustic signal combined with the calibration relationship, realizing the measurement of the gas concentration. The specific implementation process is as follows:

[0026] Step 1: The quartz tuning fork resonance circuit 5 starts to work, automatically tracks the resonance frequency of the quartz tuning fork, generates a corresponding electrical signal, and transmits this signal to the trigger capture circuit 7 and the first lock-in amplifier 6 respectively.

[0027] Step 2: After receiving the electrical signal from the quartz tuning fork resonance circuit 5, the trigger capture circuit 7 generates a trigger pulse based on this to control the operation of the tunable laser 1. At the same time, the phase modulation circuit 8 obtains the trigger signal generated by the trigger capture circuit 7 and performs phase modulation on it, so that the signal transmitted to the tunable laser 1 can make the modulation phase of the laser change continuously, and then the tunable laser 1 emits laser light of a specific wavelength and frequency.

[0028] Step 3: The laser emitted by the tunable laser 1 first undergoes collimation processing by the laser collimation device 2, converting the divergent laser beam into a parallel beam, reducing energy loss and scattering. Then, the collimated laser beam is focused by the focusing lens 3 to increase the energy density of the laser, and then enters the gas cell 4.

[0029] Step 4: The gas cell 4 is filled with the gas to be measured, and the laser entering the gas cell interacts with the gas molecules. The gas molecules absorb the laser energy, convert the light energy into heat energy through non-radiative transitions, cause local temperature changes, and then generate sound waves. These sound waves excite the vibration of the quartz tuning fork, and the quartz tuning fork converts the vibration into an electrical signal based on the piezoelectric effect. This electrical signal is a mixed electrical signal containing photoacoustic and electrical excitation.

[0030] Step Five: The first lock-in amplifier 6 receives the mixed electrical signal, uses the electrical signal provided by the quartz tuning fork resonance circuit 5 as the reference signal, extracts the signal component with the same frequency as the reference frequency by using the phase-sensitive detection technology, suppresses other noises and interference signals at the same time, amplifies the extracted signal, and finally outputs signal A.

[0031] Step Six: The second lock-in amplifier 9 demodulates and amplifies the received signal A with the modulation frequency of the phase modulation circuit 8 as the reference. After processing, the pure photoacoustic signal is successfully separated.

[0032] Step Seven: The control and signal acquisition device 10 calculates the concentration of the gas to be measured according to the relevant parameters of the pure photoacoustic signal and the pre-established calibration relationship, so as to realize the measurement of the gas concentration.

[0033] In the present invention, the tunable laser 1 is responsible for emitting laser with a specific wavelength, and its wavelength matches the absorption line of the target gas.

[0034] In the present invention, the laser collimation device 2 is responsible for converting the divergent laser beam emitted by the laser into a parallel beam, reducing the loss and scattering of the laser energy during transmission, and ensuring that the laser can accurately and stably enter the subsequent components. In the device, the collimated laser beam can better pass through the gas chamber and act on the quartz tuning fork, improving the generation efficiency of the photoacoustic signal.

[0035] In the present invention, the focusing lens 3 is responsible for focusing the laser beam at a specific position, improving the laser power density. In the experiment, the focusing lens can focus the collimated laser beam between the fingers of the quartz tuning fork or in a specific area in the gas chamber, enhancing the interaction between the laser and gas molecules, and then enhancing the photoacoustic signal and improving the detection sensitivity.

[0036] In the present invention, the gas chamber 4 is responsible for accommodating the gas to be measured and providing a stable environment for the interaction between the laser and the gas. The material of the gas chamber 4 needs to have good optical transparency, and a calcium fluoride window is used to ensure that the laser can smoothly enter the gas chamber 4 and fully interact with the gas molecules. At the same time, the gas chamber 4 should ensure airtightness to prevent gas leakage from affecting the detection result.

[0037] In the present invention, the quartz tuning fork resonance circuit 5 is mainly responsible for making the quartz tuning fork vibrate stably at its own resonance frequency. The quartz tuning fork is placed in this circuit, and the resonance frequency of the quartz tuning fork is automatically tracked and adapted by using the circuit characteristics. When environmental conditions such as gas density, temperature, and pressure change, the resonance frequency of the quartz tuning fork will change, and the resonance circuit can quickly respond and adjust to make the quartz tuning fork always in the best vibration state. This not only provides a stable vibration source for the generation of the photoacoustic signal, but also ensures that the laser modulation frequency can be matched with the resonance frequency of the quartz tuning fork in real time, guaranteeing the stability and accuracy of the device detection.

[0038] In the present invention, the first lock-in amplifier 6 is responsible for receiving the mixed electrical signal generated by the quartz tuning fork due to photoacoustic and electrical excitations. This signal is usually relatively weak and contaminated with noise. The first lock-in amplifier 6 uses the resonance frequency signal output by the quartz tuning fork resonance circuit 5 as a reference, and applies the phase-sensitive detection technique to extract the signal component with the same frequency as the reference from the complex mixed electrical signal, amplify it, and effectively suppress the noise and interference signals of other frequencies. After being processed by the first lock-in amplifier 6, the output signal A is enhanced and the signal-to-noise ratio is significantly improved, providing a better input signal for further processing of the photoacoustic signal. In the entire signal processing flow, it is an important link for initially screening and amplifying the photoacoustic signal.

[0039] In the present invention, the trigger capture circuit 7 is responsible for receiving the resonance frequency signal output by the quartz tuning fork resonance circuit 5 and generating trigger pulses based on this. These trigger pulses can accurately control the timing of laser emission and the moment of frequency modulation. In the device, the laser needs to be emitted at a specific moment and the modulation frequency should match the resonance frequency of the quartz tuning fork. The trigger capture circuit 7 plays a crucial bridging role. It ensures the synchronization of the laser with the vibration of the quartz tuning fork, enabling the laser to effectively excite the gas to generate photoacoustic signals. In each experiment, the trigger capture circuit 7 accurately controls the laser emission according to the resonance frequency signal of the quartz tuning fork, ensuring the stable generation of photoacoustic signals and is a key link for realizing the photoacoustic effect.

[0040] In the present invention, the phase modulation circuit 8 is mainly responsible for adjusting the time delay of signal transmission and plays a role in phase adjustment during the detection of photoacoustic signals. By changing the delay time of the signal, it can adjust the phase relationship between the photoacoustic signal and the reference signal. In quartz-enhanced photoacoustic spectroscopy, the phase of the photoacoustic signal is affected by the relaxation dynamics of the gas mixture, and this relaxation dynamics is related to factors such as gas composition, pressure, and temperature. The delay circuit can optimize the phase matching between the photoacoustic signal and other signals according to the actual situation, improving the accuracy of signal detection. In the experiment, the researchers adjusted the delay circuit to synchronize the photoacoustic signal and the reference signal in time for subsequent signal processing and analysis.

[0041] In the present invention, the second lock-in amplifier 9 is responsible for reprocessing the signal A output by the first lock-in amplifier 6 with the phase modulation frequency (f = 0.2 Hz) at a lower frequency as a reference. When there is no target molecule present, the signal A only contains the stable amplitude generated by the oscillator circuit, and the output of the second lock-in amplifier 9 is zero; when there is a target molecule present, the signal A contains the amplitude induced by photoacoustics, and the second lock-in amplifier 9 demodulates and amplifies it, outputting a signal B that is proportional to the pure photoacoustic signal. In this way, the second lock-in amplifier 9 successfully separates the pure photoacoustic signal from the signal containing electrical excitation and noise, avoiding the interference of the electrical excitation signal and improving the accuracy and sensitivity of detection. When measuring the gas concentration, the pure photoacoustic signal output by the second lock-in amplifier 9 provides key data support for accurately retrieving the gas concentration.

[0042] In the present invention, the control and signal acquisition device 10 is mainly responsible for realizing the data acquisition and control of the first lock-in amplifier 6 and the second lock-in amplifier 9, and finally retrieving the gas concentration according to the pure photoacoustic signal output by the second lock-in amplifier 9.

[0043] In the present invention, in order to generate a larger acoustic wave signal and improve the detection sensitivity, the laser power should be > 30 mW.

[0044] In the present invention, the quartz tuning fork resonance circuit 5 includes a quartz tuning fork and a Pierce oscillator circuit. The quartz tuning fork is directly connected to the Pierce oscillator circuit by means of direct insertion, and the function of the resonance circuit can be realized.

[0045] In the present invention, to ensure that the trigger capture circuit 7 can accurately generate trigger pulses and control the tunable laser 1 to emit laser light with a specific wavelength and frequency, the quartz tuning fork resonance circuit 5 needs to stably and accurately automatically track the resonance frequency of the quartz tuning fork and generate an electrical signal that matches it.

[0046] In the present invention, the phase modulation circuit 8 needs to perform phase modulation on the trigger signal generated by the trigger capture circuit 7, so that the laser modulation phase continuously changes between 0 and 360° at a specific frequency (f = 0.2 Hz) less than the resonance frequency of the quartz tuning fork, thereby optimizing the detection effect of the photoacoustic signal.

[0047] In the present invention, when the laser emitted by the tunable laser 1 is collimated by the laser collimation device 2, the divergent laser beam needs to be converted into a beam with good parallelism to reduce the energy loss and scattering during laser transmission; after being focused by the focusing lens 3, it needs to accurately enter the gas chamber 4 to ensure that the laser energy acts concentratedly on the gas to be measured in the gas chamber.

[0048] In the present invention, the gas chamber 4 needs to ensure good sealing performance and be filled with the gas to be measured inside, so that the laser entering the gas chamber can fully interact with gas molecules. The gas molecules absorb the laser energy to generate sound waves, and the sound waves should effectively excite the quartz tuning fork to vibrate, and convert the vibration into a mixed electrical signal including photoacoustic and electrical excitation through the piezoelectric effect.

[0049] In the present invention, the first lock-in amplifier 6 needs to use the electrical signal generated by the quartz tuning fork resonance circuit 5 as the reference signal, utilize the phase-sensitive detection technology, extract the signal component with the same reference frequency from the mixed electrical signal, and perform amplification processing, while effectively suppressing other noise and interference signals, and output a signal A with a high signal-to-noise ratio.

[0050] In the present invention, the second lock-in amplifier 9 should use the modulation frequency of the phase modulation circuit 8 as the reference, perform demodulation and amplification operations on the signal A output by the first lock-in amplifier 6, and accurately separate the pure photoacoustic signal.

[0051] In the present invention, it is necessary to pre-establish the calibration relationship between the relevant parameters of the photoacoustic signal and the concentration of the gas to be measured. After the second lock-in amplifier 9 separates the pure photoacoustic signal, based on the relevant parameters such as the amplitude and frequency of the pure photoacoustic signal, combined with the calibration relationship, the concentration of the gas to be measured can be accurately calculated to achieve accurate measurement of the gas concentration. The calibration relationship is the calibration of sensor data, that is, a linear relationship is obtained through a large number of calibration data, and then the calibration relationship is established.

[0052] Embodiment:

[0053] In this embodiment, a distributed feedback (DFB) diode laser with a central wavelength of 1650.96 nm and an output power of 30 mW is selected to detect methane. At the same time, by modulating the laser, its emitted laser intensity or wavelength changes periodically, matching the resonance frequency of the quartz tuning fork to enhance the detection effect of the photoacoustic signal. Implementing the photoacoustic spectroscopy trace gas detection method based on the secondary demodulation technology in the present invention can obtain the signal output by the first lock-in amplifier 6, as Figure 2 shown. In Figure 2 , separate optical excitation, electrical excitation, and optoelectronic hybrid excitation are carried out respectively. It can be found that when optical excitation and electrical excitation are carried out separately, the separate demodulation signal is a stable direct current value, while when optoelectronic hybrid excitation is carried out, a sinusoidal change occurs between the signal and the phase. Therefore, in the secondary demodulation technology, the second lock-in amplifier 9 performs phase modulation with a relatively small frequency. In this embodiment, f = 0.2 Hz is adopted, and the output result of its demodulation signal is as Figure 3 shown. The signal demodulated by the second lock-in amplifier 9 is a stable direct current value and is consistent with the amplitude of the separate optical excitation signal, proving the effectiveness of signal separation.

Claims

1. An optoacoustic spectroscopy trace gas detection device based on a secondary demodulation technique, characterized in that The method includes a tunable laser, a laser collimation device, a focusing lens, a gas cell, a quartz tuning fork resonance circuit, a first lock-in amplifier, a trigger capture circuit, a phase modulation circuit, a second lock-in amplifier, and a control and signal acquisition device. Among them: The quartz tuning fork resonance circuit automatically tracks the resonance frequency of the quartz tuning fork and generates an electrical signal. After receiving the electrical signal transmitted from the quartz tuning fork resonance circuit, the trigger capture circuit generates a trigger pulse. The phase modulation circuit obtains the trigger signal generated by the trigger capture circuit and performs phase modulation on it, so that the signal transmitted to the tunable laser can make the laser modulation phase continuously change, and controls the tunable laser to emit laser with a specific wavelength and frequency; The laser emitted by the tunable laser is collimated by the laser collimation device and focused by the focusing lens, and then enters the gas cell, interacts with the gas to be measured therein. The gas molecules absorb the laser energy to generate sound waves, which excite the quartz tuning fork to vibrate. Its vibration is converted into a mixed electrical signal containing photoacoustic and electrical excitation through the piezoelectric effect; The first lock-in amplifier uses the electrical signal provided by the quartz tuning fork resonance circuit as a reference signal, extracts and amplifies the signal of a specific frequency, and outputs signal A after suppressing noise; The second lock-in amplifier uses the modulation frequency of the phase modulation circuit as a reference, demodulates and amplifies signal A, and separates out the pure photoacoustic signal; The control and signal acquisition device calculates the concentration of the gas to be measured based on the relevant parameters of the pure photoacoustic signal in combination with the calibration relationship between the relevant parameters of the photoacoustic signal and the concentration of the gas to be measured, realizing the measurement of the gas concentration.

2. The photoacoustic spectroscopy trace gas detection device based on the quadratic demodulation technique according to claim 1, characterized in that The laser power > 30mW.

3. The photoacoustic spectroscopy trace gas detection device based on the secondary demodulation technique according to claim 1, characterized in that The phase modulation circuit performs phase modulation on the trigger signal generated by the trigger capture circuit, so that the laser modulation phase continuously changes between 0 and 360° at a specific frequency less than the resonance frequency of the quartz tuning fork, thereby optimizing the detection effect of the photoacoustic signal.

4. The photoacoustic spectroscopy trace gas detection device based on the secondary demodulation technique according to claim 1, characterized in that The specific frequency f emitted by the tunable laser = 0.2Hz.

5. The photoacoustic spectroscopy trace gas detection device based on the secondary demodulation technology according to claim 1, characterized in that The quartz tuning fork resonance circuit includes a quartz tuning fork and a Pierce oscillation circuit, and the quartz tuning fork is directly connected to the Pierce oscillation circuit by means of direct insertion.

6. A method for detecting trace gases by photoacoustic spectroscopy based on the secondary demodulation technique using the device according to any one of claims 1-5, characterized in that The method includes the following steps: Step 1: The quartz tuning fork resonance circuit starts to work, automatically tracks the resonance frequency of the quartz tuning fork, generates a corresponding electrical signal, and transmits this signal to the trigger capture circuit and the first lock-in amplifier respectively; Step 2: After receiving the electrical signal transmitted from the quartz tuning fork resonance circuit, the trigger capture circuit generates a trigger pulse accordingly to control the operation of the tunable laser. At the same time, the phase modulation circuit obtains the trigger signal generated by the trigger capture circuit and performs phase modulation on it, so that the signal transmitted to the tunable laser can make the laser modulation phase continuously change, and then makes the tunable laser emit laser with a specific wavelength and frequency; Step 3: The laser emitted by the tunable laser is first collimated by the laser collimation device, and the collimated laser beam is then focused by the focusing lens and then enters the gas cell; Step Four: The gas chamber is filled with the gas to be measured. The laser entering the gas chamber interacts with gas molecules. The gas molecules absorb the laser energy, convert the light energy into heat energy through non-radiative transitions, causing local temperature changes, and then generating sound waves. The sound waves excite the quartz tuning fork to vibrate, and the quartz tuning fork converts the vibration into a mixed electrical signal containing photoacoustic and electrical excitation based on the piezoelectric effect; Step Five: The first lock-in amplifier receives the mixed electrical signal, uses the electrical signal provided by the quartz tuning fork resonance circuit as the reference signal, extracts the signal component with the same frequency as the reference frequency by using the phase-sensitive detection technology, suppresses other noise and interference signals at the same time, amplifies the extracted signal, and finally outputs signal A; Step Six: The second lock-in amplifier demodulates and amplifies the received signal A with the modulation frequency of the phase modulation circuit as the reference, and separates the pure photoacoustic signal; Step Seven: The control and signal acquisition device calculates the concentration of the gas to be measured according to the relevant parameters of the pure photoacoustic signal and the calibration relationship between the relevant parameters of the photoacoustic signal and the concentration of the gas to be measured established in advance, so as to realize the measurement of the gas concentration.

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