A quartz tuning fork-based differential photoacoustic spectroscopy gas detection device and method

Abstract

The application discloses a quartz tuning fork-based differential photoacoustic spectrum gas detection device and method, and relates to the field of gas detection devices.The device comprises a distributed feedback semiconductor laser, a laser collimator, a focusing lens, a differential photoacoustic cell, a first quartz tuning fork, a second quartz tuning fork, a control and data acquisition system, and a computer.The laser light source output by the distributed feedback semiconductor laser is collimated by the laser collimator and focused by the focusing lens, and then is incident into a first resonant cavity of the differential photoacoustic cell to excite the gas to be detected.After the gas to be detected absorbs the incident laser energy, photoacoustic signals are generated, and the sound waves are transmitted to the surfaces of the first quartz tuning fork and the second quartz tuning fork to cause the tuning forks to produce periodic elastic deformation and further cause the quartz tuning forks to vibrate.The control and data acquisition system collects the sound signals generated by the first quartz tuning fork and the second quartz tuning fork, and the computer processes the sound signals to obtain the concentration of the detected gas.The application has the advantages of low-frequency noise suppression, high sensitivity, low cost, small size, and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to a photoacoustic spectroscopy gas detection device and method, in particular to a photoacoustic spectroscopy detection device and method using a quartz tuning fork instead of a condenser microphone. BACKGROUND

[0002] In recent years, with the continuous development of laser technology, photoacoustic spectroscopy trace gas detection technology has developed rapidly. At present, photoacoustic spectroscopy technology is mainly applied to continuous monitoring of atmospheric environment, medical diagnosis and power equipment fault detection and other fields.

[0003] As a carrier of the photoacoustic effect of gas, the photoacoustic cell is an essential and important part of the photoacoustic spectroscopy system, which directly affects the performance of the photoacoustic spectroscopy system. At present, the existing photoacoustic cell is mainly divided into spherical photoacoustic cell, Helmholtz photoacoustic cell, H-shaped photoacoustic cell, T-shaped photoacoustic cell and differential photoacoustic cell. Among them, the differential photoacoustic cell is widely concerned by scholars because of its advantages of effectively reducing background noise and improving detection signal-to-noise ratio.

[0004] Traditional photoacoustic spectroscopy technology mainly uses a condenser microphone as the acoustic wave detection unit of the system. The photoacoustic spectroscopy technology based on the condenser microphone mainly consists of three parts: an optical system, an electrical system and a photoacoustic cell system. The photoacoustic cell system is the core unit in the photoacoustic spectroscopy technology, and the differential photoacoustic cell mainly consists of two identical acoustic resonant cavities. The two resonant cavities generate acoustic signals with opposite phases and approximately equal amplitudes. The two acoustic signals can be differentially operated by two condenser microphones to ultimately achieve double enhancement of the acoustic signal. At the same time, the differential photoacoustic cell can use a differential amplifier to eliminate airflow, window and electromagnetic noise, and the resonant structure can reduce the influence of 1 / f noise. A semiconductor laser is used as the excitation light source of the photoacoustic signal. The modulated laser is incident into one of the resonant cavities of the differential photoacoustic cell filled with the gas to be measured after passing through the collimator to excite the gas to be measured. The gas molecules absorb the laser energy and jump from the ground state to the excited state. Because the excited state molecules are unstable, the excited state molecules collide with each other and return to the ground state through non-radiative relaxation. In this process, the gas molecules in the excited state convert the absorbed optical energy into kinetic energy and then into thermal energy. Because the volume of the differential photoacoustic cell is constant, as the thermal energy in one of the resonant cavities of the differential photoacoustic cell accumulates, the temperature in the cell also rises. If the modulation frequency of the laser light source used is less than the relaxation frequency of the non-radiative transition of the gas molecules, the temperature of the gas in one of the resonant cavities of the differential photoacoustic cell will periodically change at the same frequency as the modulation frequency, thereby generating an acoustic signal. Finally, the vibration of the acoustic wave in one of the resonant cavities of the differential photoacoustic cell causes the vibration of the other resonant cavity, that is, the two resonant cavities of the differential photoacoustic cell generate acoustic signals with approximately equal amplitudes, but the phases of the two acoustic signals are opposite. The two acoustic signals can be differentially operated by two condenser microphones to ultimately increase the amplitude of the acoustic signal by 1 times. At the same time, the two signals are differentially operated, and the sideband noise cancels each other out, ultimately achieving the effect of signal doubling and noise cancellation.

[0005] At present, the existing photoacoustic spectroscopy detection technology mainly uses a condenser microphone to detect acoustic signals. The condenser microphone mainly consists of a metal diaphragm and a metal electrode close to each other, which is essentially a flat plate capacitor. The metal diaphragm and the metal electrode form the two plates of the capacitor. The working principle of the condenser microphone can be simply summarized as follows: when the diaphragm is subjected to sound pressure, the diaphragm deforms, the distance between the two plates changes, the capacitance changes, and the voltage of the measurement circuit also changes, thereby realizing the conversion of the sound signal into a voltage signal.

[0006] The capacitance between the two metal plates of the condenser microphone is:

[0007]

[0008] Wherein, epsilon is the dielectric constant of the medium between the two plates; A represents the area of the plate opposite; D is the distance between the two plates.

[0009] The currently used capacitive microphone mainly is a thin film capacitive microphone, which is used for a gas analysis instrument. A gas absorbs light energy to generate a sound wave, which pushes a capacitive moving piece to move relative to a fixed piece, and converts a measured component concentration change into a capacitance change. However, the thin film capacitive microphone has the disadvantages of being afraid of moisture and falling, a fragile sound film, and the like. Meanwhile, the thin film is easily affected by mechanical vibration, causing the receiving gas chamber to leak, the modulation frequency cannot be improved, low-frequency noise is obvious, the amplifier is difficult to manufacture, and the volume is large. SUMMARY

[0010] In view of the deficiencies of the current photoacoustic spectroscopy technology based on the capacitive microphone, the application provides a quartz tuning fork-based differential photoacoustic spectroscopy gas detection device and method.

[0011] The purpose of the application is achieved by the following technical solutions:

[0012] A quartz tuning fork-based differential photoacoustic spectroscopy gas detection device comprises a distributed feedback semiconductor laser, a laser collimator, a focusing lens, a differential photoacoustic cell, a first quartz tuning fork, a second quartz tuning fork, a control and data acquisition system, and a computer, wherein:

[0013] The laser light source output by the distributed feedback semiconductor laser is collimated by the laser collimator and focused by the focusing lens, and then is incident into the first resonant cavity of the differential photoacoustic cell to excite the gas to be measured;

[0014] The first quartz tuning fork is located at the middle position outside the first resonant cavity of the differential photoacoustic cell, and the second quartz tuning fork is located at the middle position outside the second resonant cavity of the differential photoacoustic cell.

[0015] After the gas to be measured in the differential photoacoustic cell absorbs the incident laser energy, a photoacoustic signal is generated, and the sound wave is transmitted to the surfaces of the first quartz tuning fork and the second quartz tuning fork to cause the tuning forks to produce periodic elastic deformation and further cause the quartz tuning forks to vibrate. The control and data acquisition system collects the sound signals detected by the first quartz tuning fork and the second quartz tuning fork, and the computer processes the sound signals to obtain the concentration of the detected gas.

[0016] A method for differential photoacoustic spectroscopy gas detection using the above device, after the modulated laser is incident into the differential photoacoustic cell filled with the gas to be measured, the gas in the cell absorbs the incident laser to generate a photoacoustic signal, the quartz tuning forks are used to detect the sound signal, the control and data acquisition system is used to demodulate the sound signal, and finally the computer is used to process the data to obtain the concentration of the detected gas. The method specifically comprises the following steps:

[0017] Step one: the laser controller controls the output wavelength and output power of the distributed feedback semiconductor laser; the superimposed signal generated by the low-frequency sawtooth wave and the high-frequency sine wave is used to modulate the laser light source of the distributed feedback semiconductor laser; the control and data acquisition system is used to scan and optimize the resonance frequency of the first quartz tuning fork and the second quartz tuning fork and the modulation depth of the photoacoustic spectrum system;

[0018] Step two: the laser light source output by the distributed feedback semiconductor laser first passes through the laser collimator to become a parallel collimated light beam, then the collimated laser light source is focused by the focusing lens, and finally the focused laser light source is input into the differential photoacoustic cell containing the target gas to be detected to excite the target gas to be detected;

[0019] Step three: the laser light source excites the target gas to be detected in the differential photoacoustic cell to cause the generation of photoacoustic effect, and the generated sound wave passes through the sound detection hole of the differential photoacoustic cell to cause the first quartz tuning fork and the second quartz tuning fork to periodically elastically deform and vibrate along with the modulated laser light source;

[0020] Step four: the control and data acquisition system collects the generated sound signals of the first quartz tuning fork and the second quartz tuning fork and processes them by the computer to inversely calculate the concentration of the detected gas.

[0021] Compared with the prior art, the present application has the following advantages:

[0022] 1. Compared with the capacitive microphone, the quartz tuning fork has the advantages of small size, excellent mechanical performance, high modulation frequency, stable quartz performance, etc.

[0023] 2. In the present application, the sound wave generated in the differential photoacoustic cell causes the vibration of the quartz tuning fork, thereby generating a current signal. At the same time, since the quartz tuning fork is a resonant element, when the frequency of the sound wave is equal to the intrinsic frequency of the quartz tuning fork, the current signal is sharply amplified. Based on the differential principle, the sound waves generated by the two resonant cavities of the differential photoacoustic cell need to be detected by two quartz tuning forks, and the two quartz tuning forks detect two-phase opposite sound waves, so that the signal is sharply enhanced and the noise is obviously canceled.

[0024] 3. The present application realizes further enhancement of the photoacoustic signal through the double resonance of the differential photoacoustic cell and the quartz tuning fork, and proposes a differential photoacoustic spectrum trace gas detection device and method using a quartz tuning fork as a detection element, i.e. the differential photoacoustic cell is used to generate photoacoustic effect, and then the vibration of the quartz tuning fork is caused based on the resonance principle, so as to achieve the effect of signal resonance enhancement. At the same time, based on the differential principle, the noise is suppressed. The core of the method is to use the resonance of the quartz tuning fork to further enhance the vibration of the differential photoacoustic cell, realize the effect of resonance enhancement, and thus improve the amplitude of the photoacoustic signal.

[0025] 4、The application has the advantages of low-frequency noise suppression, high sensitivity, low cost, small size, etc. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is a structure schematic diagram of the differential photoacoustic spectrum gas detection device based on the quartz tuning fork;

[0027] Figure 2 It is a position relationship diagram of the quartz tuning fork and the differential photoacoustic cell;

[0028] Figure 3 It is an acetylene gas detection based on the differential photoacoustic spectrum of the quartz tuning fork;

[0029] In the figure, 1 is a feedback type semiconductor laser, 2 is a laser collimator, 3 is a focusing lens, 4 is a differential photoacoustic cell, 4-1 is an air inlet, 4-2 is an air outlet, 4-3 is a first buffer cell, 4-4 is a second buffer cell, 4-5 is a first resonant cavity, 4-6 is a second resonant cavity, 5 is a first quartz tuning fork, 6 is a second quartz tuning fork, 7 is a control and data acquisition system, and 8 is a computer. DETAILED DESCRIPTION

[0030] The technical solutions of the application are further described below in combination with the drawings, but are not limited thereto, and any modification or equivalent replacement to the technical solutions of the application without departing from the spirit and scope of the technical solutions of the application shall be covered in the protection scope of the application.

[0031] The application provides a differential photoacoustic spectrum gas detection device based on a quartz tuning fork. Figure 1 and Figure 2 As shown in the drawings, the device comprises a distributed feedback type semiconductor laser 1, a laser collimator 2, a focusing lens 3, a differential photoacoustic cell 4, a first quartz tuning fork 5, a second quartz tuning fork 6, a control and data acquisition system 7 and a computer 8, the distributed feedback type semiconductor laser 1 outputs a modulated laser light source, which is collimated by the laser collimator 2 and focused by the focusing lens 3 and then enters the first resonant cavity 4-3 in the differential photoacoustic cell 4, after the gas to be detected in the differential photoacoustic cell 4 absorbs the laser energy, the differential photoacoustic cell 4 generates vibration, thereby generating a photoacoustic signal, the sound wave is transmitted to the surfaces of the first quartz tuning fork 5 and the second quartz tuning fork 6 to cause periodic elastic deformation of the tuning forks and further cause vibration of the quartz tuning forks, and finally the signal is demodulated and subsequently processed by the control and data acquisition system 7 and the computer 8.

[0032] The specific implementation process is as follows:

[0033] Step one: the laser controller controls the output wavelength of the distributed feedback semiconductor laser 1 and the output power of the laser. The superimposed signal generated by the low-frequency sawtooth wave and the high-frequency sine wave is used to modulate the laser light source of the distributed feedback semiconductor laser 1. The control and data acquisition system 7 is used to scan and optimize the resonance frequency of the quartz tuning fork and the modulation depth of the photoacoustic spectrum system.

[0034] Step two: the laser light source output by the distributed feedback semiconductor laser 1 first passes through the laser collimator 2 to become a parallel collimated light beam, then passes through the focusing lens 3 to focus the collimated laser light source, and finally enters the differential photoacoustic cell 4 containing the target gas to be measured to excite the target gas to be measured.

[0035] Step three: the laser light source excites the target gas to be measured in the differential photoacoustic cell 4 to produce photoacoustic effect, and the generated sound wave passes through the differential photoacoustic cell sound detection hole to cause the first quartz tuning fork 5 and the second quartz tuning fork 6 to periodically elastically deform and vibrate in response to the modulated laser light source.

[0036] Step four: the control and data acquisition system 7 collects the generated sound signals of the first quartz tuning fork and the second quartz tuning fork and processes them by the computer 8 to inversely calculate the concentration of the detected gas.

[0037] In the present application, the first resonant cavity 4-5 and the second resonant cavity 4-6 of the differential photoacoustic cell 4 have a length of 4-100 mm and a diameter of 500 μm-10 mm.

[0038] In the present application, the first buffer cell 4-3 and the second buffer cell 4-4 of the differential photoacoustic cell 4 have a length of 2-50 mm and a diameter of 1-40 mm.

[0039] In the present application, in order to avoid the interference caused by gas flow noise, the diameter of the gas inlet 4-1 and the gas outlet 4-2 of the differential photoacoustic cell 4 is 2-6 mm.

[0040] In the present application, the gas pressure in the differential photoacoustic cell 4 is between 50-500 Torr, and the specific gas pressure value is determined according to the relaxation time of the measured gas molecules.

[0041] In the present application, the differential photoacoustic cell 4 needs to maintain a constant temperature, and the overall temperature of the differential photoacoustic cell 4 needs to be maintained at 20-35℃.

[0042] In the present application, the first quartz tuning fork 5 is placed at the middle position outside the first resonant cavity 4-5 of the differential photoacoustic cell 4, the second quartz tuning fork 6 is placed at the middle position outside the second resonant cavity 4-6 of the differential photoacoustic cell 4, and the distance between the first quartz tuning fork 5 and the second quartz tuning fork 6 and the differential photoacoustic cell 4 needs to be <1 mm, at this time, the photoacoustic signal can be better detected, and the best resonance enhancement can be achieved.

[0043] In the present application, the resonance frequency of the first quartz tuning fork 5 and the second quartz tuning fork 6 is 2-40 kHz.

[0044] In the present application, the frequency difference between the first quartz tuning fork 5 and the second quartz tuning fork 6 is within 10 Hz.

[0045] In the present application, the middle position of the outside of the first resonant cavity 4-5 and the second resonant cavity 4-6 of the differential photoacoustic cell 4 (i.e. the position close to the first quartz tuning fork 5 and the second quartz tuning fork 6) is provided with a sound detection hole, and the distance between the sound detection hole and the top of the first quartz tuning fork 5 and the second quartz tuning fork 6 is 0-2 mm.

[0046] In the present application, the diameter of the sound detection hole of the differential photoacoustic cell 4 is 0.2-2 mm.

[0047] In the present application, in order to improve the signal-to-noise ratio of the system, a high-power laser source or a laser power amplifier can be used, and the laser power should be >10 mW.

[0048] In the present application, the differential photoacoustic cell 4 includes but is not limited to various types of photoacoustic cells, such as spherical photoacoustic cell, Helmholtz photoacoustic cell, H-type photoacoustic cell and T-type photoacoustic cell.

[0049] Application example:

[0050] The quartz tuning fork differential photoacoustic gas detection device and method designed above are used to detect acetylene gas, and the results are shown in Figure 3 The quartz tuning fork 1 and the quartz tuning fork 2 represent the signal strength generated by two quartz tuning forks respectively, which are 1.58 mV and 1.53 mV respectively, and the differential mode represents the photoacoustic signal obtained after the two signals are processed by the differential amplifier, which is 3.11 mV. It can be seen that the signal strength generated by the differential mode is the sum of the signal strengths obtained by using two quartz tuning forks respectively, that is, the ideal amplification effect is achieved. At the same time, it can be seen that the noise after the differential is only 55 nV, which is significantly lower than the noise of 88.39 nV and 85.46 nV when two quartz tuning forks are used alone.

Claims

1. A quartz tuning fork-based differential photoacoustic spectroscopy gas detection device, characterized by The device comprises a distributed feedback semiconductor laser, a laser collimator, a focusing lens, a differential photoacoustic cell, a first quartz tuning fork, a second quartz tuning fork, a control and data acquisition system and a computer, wherein: The laser light source output by the distributed feedback semiconductor laser is collimated by the laser collimator, focused by the focusing lens, and then incident into the first resonant cavity of the differential photoacoustic cell to excite the gas to be detected; The first quartz tuning fork is located at the middle position outside the first resonant cavity of the differential photoacoustic cell, and the second quartz tuning fork is located at the middle position outside the second resonant cavity of the differential photoacoustic cell; After the gas to be detected in the differential photoacoustic cell absorbs the incident laser energy, an acoustic signal is generated, and the acoustic wave is transmitted to the surface of the first quartz tuning fork and the second quartz tuning fork to cause periodic elastic deformation of the tuning forks and further cause vibration of the quartz tuning forks. The control and data acquisition system collects the acoustic signal generated by the first quartz tuning fork and the second quartz tuning fork, and the computer processes the collected signal to obtain the concentration of the detected gas.

2. The quartz tuning fork-based differential photoacoustic spectroscopy gas detection device according to claim 1, characterized by The length of the first resonant cavity and the second resonant cavity of the differential photoacoustic cell is 4-100 mm, and the diameter is 500 μm-10 mm. The length of the first buffer cell and the second buffer cell is 2-50 mm, and the diameter is 1-40 mm.

3. The quartz tuning fork-based differential photoacoustic spectroscopy gas detection device according to claim 1, characterized by The diameter of the gas inlet and the gas outlet of the differential photoacoustic cell is 2-6 mm.

4. The quartz tuning fork-based differential photoacoustic spectroscopy gas detection device according to claim 1, characterized by The gas pressure in the differential photoacoustic cell is between 50-500 Torr, and the overall temperature of the differential photoacoustic cell is maintained at 20-35℃.

5. The quartz tuning fork-based differential photoacoustic spectroscopy gas detection device according to claim 1, characterized by The distance between the first quartz tuning fork and the second quartz tuning fork and the differential photoacoustic cell is <1 mm.

6. The quartz tuning fork-based differential photoacoustic spectroscopy gas detection device according to claim 1 or 5, characterized by The resonance frequency of the first quartz tuning fork and the second quartz tuning fork is 2-40 kHz, and the frequency difference between the first quartz tuning fork and the second quartz tuning fork is <10 Hz.

7. The quartz tuning fork-based differential photoacoustic spectroscopy gas detection device according to claim 1 or 2, characterized by The middle position outside the first resonant cavity and the second resonant cavity is provided with a sound detection hole, and the diameter of the sound detection hole is 0.2-2 mm. The distance between the sound detection hole and the top of the first quartz tuning fork and the second quartz tuning fork is 0-2 mm.

8. The quartz tuning fork-based differential photoacoustic spectroscopy gas detection device according to claim 1, characterized by The output laser power of the distributed feedback semiconductor laser is >10 mW.

9. The quartz tuning fork based differential photoacoustic spectroscopy gas detection device according to claim 1, 2, 3 or 4, characterized by The differential photoacoustic cell is a spherical photoacoustic cell, a Helmholtz photoacoustic cell, an H-shaped photoacoustic cell and a T-shaped photoacoustic cell.

10. A method for differential photoacoustic spectroscopy gas detection using the apparatus of any one of claims 1-9, characterized in that The method comprises the following steps: Step one: the laser controller controls the output wavelength and output power of the distributed feedback semiconductor laser; the superimposed signal generated by the low-frequency sawtooth wave and the high-frequency sine wave is used to modulate the laser light source of the distributed feedback semiconductor laser; the control and data acquisition system is used to scan and optimize the resonance frequency of the first quartz tuning fork and the second quartz tuning fork and the modulation depth of the photoacoustic spectrum system; Step two: the laser light source output by the distributed feedback semiconductor laser is first changed into a parallel collimated beam by the laser collimator, then the collimated laser light source is focused by the focusing lens, and finally the focused laser light source is incident into the differential photoacoustic cell containing the gas to be detected to excite the target gas to be detected; Step three: the laser light source passes through the differential photoacoustic cell to excite the target gas to cause the generation of photoacoustic effect, and the generated sound wave passes through the sound detection hole of the differential photoacoustic cell to cause the first quartz tuning fork and the second quartz tuning fork to periodically elastically deform along with the modulated laser light source and further generate vibration; Step four: the control and data acquisition system collects the generated sound signals detected by the first quartz tuning fork and the second quartz tuning fork and processes them by the computer to inversely calculate the concentration of the detected gas.

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