Photoacoustic spectrometer and gas detection device

Abstract

The application discloses a photoacoustic spectrum sound detector and a gas detection device, which comprises a first Helmholtz resonant cavity, a quartz tuning fork and a second Helmholtz resonant cavity. The two Helmholtz resonant cavities are coaxially connected by a cylindrical barrel and a thin tube. When laser is incident from the light transmission window of the first Helmholtz cavity, passes through the gap between the vibrating arms of the quartz tuning fork and is emitted from the window of the second Helmholtz cavity, the two Helmholtz resonant cavities resonate and amplify the sound wave signal excited by the laser beam, so that the sound pressure near the vibrating arms of the quartz tuning fork is enhanced, the environmental noise and the window noise are suppressed, and the signal-to-noise ratio of the quartz tuning fork in detecting the sound wave signal near the vibrating arms is improved. In the application, the Helmholtz cavity can resonate and amplify the sound wave, thereby improving the output signal amplitude. In the measurement process, the Helmholtz cavity can suppress the environmental noise and the window noise, further improve the signal-to-noise ratio and improve the gas detection precision.

Description

Technical Field

[0001] This invention belongs to the field of gas sensing, and more specifically, relates to a photoacoustic spectroscopy sound meter and a gas detection device. Background Technology

[0002] Trace gases refer to gases with concentrations below one part per million. Their detection technology has wide applications in industrial control, medical diagnostics, and environmental monitoring. Based on detection principles, gas sensors are mainly divided into two categories: non-optical gas sensors and optical gas sensors. Non-optical gas sensors are more expensive, have complex structures, slow response, and poor stability. With the development of laser technology, optical gas sensors have been widely studied in recent years due to their high sensitivity, strong selectivity, and fast response speed. Optical sensing technologies mainly include direct absorption spectroscopy, wavelength modulation spectroscopy, cavity enhancement spectroscopy, and photoacoustic spectroscopy, among which photoacoustic spectroscopy, which indirectly detects photoacoustic signals, has unique advantages due to its simple structure.

[0003] Photoacoustic spectroscopy is a detection technique based on the photoacoustic effect. When target gas molecules absorb incident light of a specific wavelength, they transition from the ground state to an excited state, then relax and return to the ground state via a non-radiative transition, releasing heat. Local temperature changes within the gas cause pressure changes, generating pressure waves, i.e., sound waves. If the incident light is modulated, sound waves are generated at a corresponding frequency. Using acoustic sensors such as cantilever beams and microphones, the acoustic signal is converted into an electrical signal, allowing the determination of the concentration of the analyte gas. Compared to other optical gas sensors, photoacoustic spectroscopy has the advantage that the strength of the photoacoustic effect does not depend on the length of the optical absorption path. Therefore, detection instruments based on photoacoustic spectroscopy principles can be miniaturized and modularized. Another advantage is that the detection part of photoacoustic spectroscopy detects acoustic signals, not optical signals, thus eliminating the need for photodetectors. These advantages broaden the application range of photoacoustic spectroscopy technology and reduce the cost of instruments based on it.

[0004] To further enhance the signal of photoacoustic spectroscopy, acoustic resonant cavities are typically installed around acoustic sensors to create standing wave resonance, thereby amplifying the photoacoustic signal. Acoustic resonant cavities are generally classified into three types: axial cavities, radial cavities, and Helmholtz cavities.

[0005] In 2002, Dr. Anatoliy Kosterev and Professor Frank Tittel of Rice University invented Quartz Enhanced Photoacoustic Spectroscopy (QEPAS). This technology is based on the principles of photoacoustic spectroscopy and uses a tuning fork-type quartz crystal as the acoustic sensor. The piezoelectric effect of quartz converts the acoustic signal generated by the photoacoustic effect into an electrical signal. Commercially available tuning fork-type quartz crystals are 3×8 mm in size, with a resonant frequency of 32.7 kHz, a bandwidth of approximately 4 Hz, and a quality factor of 10,000 under standard atmospheric pressure. This brings advantages such as small size, low cost, and good noise immunity to QEPAS. Acoustic resonant cavities are also used in QEPAS to improve detection performance. A miniature acoustic resonant cavity made of thin stainless steel tubing is coupled and resonates with the quartz crystal to enhance the signal strength of QEPAS. For axial cavities, there are coaxial, off-axis, and other variations. Radial cavities are larger in size, which improves the detection signal while reducing the difficulty of collimating and focusing the optical path. To date, no Helmholtz cavities have been applied to quartz-enhanced photoacoustic spectroscopy. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this invention is to provide a photoacoustic spectroscopy sound meter and a gas detection device, which aims to solve the problem that the measurement performance of quartz-enhanced photoacoustic spectroscopy cannot be improved because there is no Helmholtz cavity that has been applied to quartz-enhanced photoacoustic spectroscopy technology before.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a photoacoustic spectral sound meter, which, from left to right, comprises: a first Helmholtz resonant cavity, a quartz tuning fork, and a second Helmholtz resonant cavity;

[0008] The first Helmholtz resonant cavity includes: a coaxial first cylindrical tube and a first thin tube; the left end face of the first cylindrical tube is a first light-transmitting window, and the right end face is a non-light-transmitting window with a first small hole in the center, and the first small hole is connected to the left end face of the first thin tube.

[0009] The second Helmholtz resonant cavity includes: a coaxial second cylindrical tube and a second thin tube; the right end face of the second cylindrical tube is a second light-transmitting window, and the left end face is a non-light-transmitting window with a second small hole in the center, and the second small hole is connected to the right end face of the second thin tube;

[0010] The right end face of the first thin tube and the left end face of the second thin tube are open windows, respectively placed on both sides of the gap between the quartz tuning fork arms, and the two Helmholtz resonant cavities are coaxial.

[0011] When the laser beam enters through one of the two light-transmitting windows, passes through the gap between the arms of the quartz tuning fork, and exits through the other window, the two Helmholtz resonant cavities resonate and amplify the acoustic signal excited by the laser beam, thereby increasing the sound pressure near the arms of the quartz tuning fork and suppressing environmental noise and window noise, thus improving the signal-to-noise ratio of the quartz tuning fork in detecting acoustic signals near its arms.

[0012] In one possible embodiment, the laser is coaxial with two Helmholtz resonators, and its beam waist diameter is less than 300 μm.

[0013] In one possible embodiment, the two Helmholtz resonant cavities are of the same size;

[0014] The parameters of the Helmholtz resonant cavity that satisfy the acoustic resonance condition must meet the following equation:

[0015]

[0016] Where R represents the radius of the cylinder, L represents the length of the cylinder, r represents the radius of the capillary tube, l represents the length of the capillary tube, and k represents the wave number.

[0017] In one possible embodiment, the value of R ranges from 2mm to 4mm, the value of L ranges from 3.1mm to 5.1mm, the value of r ranges from 0.5mm to 0.7mm, and the value of l ranges from 3.4mm to 5.4mm.

[0018] In one possible embodiment, the position of the central axis of the two thin tubes relative to the top of the quartz tuning fork arm ranges from 0mm to 3mm.

[0019] In one possible embodiment, the horizontal distance between the two thin tubes and the plane of the quartz tuning fork arm ranges from 0 mm to 0.08 mm.

[0020] In a second aspect, the present invention provides a gas detection device comprising the photoacoustic spectroscopy sound meter provided in the first aspect, wherein the vibrating arm of the quartz tuning fork in the photoacoustic spectroscopy sound meter is placed in the gas to be measured, and the gas detection device further comprises: a laser, an optical fiber collimating lens and a signal demodulation device.

[0021] The laser is used to emit laser light;

[0022] The fiber collimating lens is used to shape the laser and then incident the shaped laser onto the photoacoustic spectroscopy sound meter.

[0023] The photoacoustic spectrometer is used to acquire the electrical signal corresponding to the gas to be tested after the laser acts on it; wherein, the process of generating the electrical signal is as follows: first, the laser excites the gas to be tested to generate an acoustic signal, and then the acoustic signal drives the quartz tuning fork to vibrate to form a corresponding electrical signal; the intensity of the acoustic signal is different depending on the concentration of the gas to be tested.

[0024] The signal demodulation device is used to demodulate the electrical signal to obtain the concentration information of the gas to be measured.

[0025] In one possible embodiment, the signal demodulation device includes: a preamplifier and a lock-in amplifier;

[0026] The preamplifier is used to amplify the electrical signal output by the photoacoustic spectrometer through transimpedance to obtain the corresponding photoacoustic signal.

[0027] The lock-in amplifier is used to demodulate the photoacoustic signal output by the preamplifier and solve for the concentration information of the gas to be measured.

[0028] In one possible embodiment, the emission center wavelength of the laser corresponds to the target absorption line of the gas to be measured.

[0029] In summary, the technical solutions conceived by this invention have the following beneficial effects compared with the prior art:

[0030] This invention provides a photoacoustic spectroscopy sound meter and a gas detection device. It is the first to apply a Helmholtz cavity to quartz-enhanced photoacoustic spectroscopy gas detection technology. Through geometric parameter design, it achieves a low-frequency resonant frequency of 32.7 kHz, suitable for photoacoustic spectroscopy detection. The Helmholtz cavity has a significant cross-sectional area change from a cylinder to a thin tube, resulting in high acoustic impedance, making it difficult for ambient noise from the surrounding environment to enter the resonant cavity. Furthermore, when laser light passes through the window, absorption by the window generates a sound wave with the same modulation frequency as the resonant cavity, known as window noise. By adjusting the length of the cylinder, this sound wave can be destructively interfered with by reflection during propagation within the resonant cavity, suppressing window noise. Therefore, the introduction of a Helmholtz cavity in this invention can resonate and amplify the sound wave, thereby increasing the output signal amplitude. During measurement, the Helmholtz cavity acts as an acoustic silencer, suppressing ambient noise and window noise, further improving the signal-to-noise ratio and enhancing the accuracy of gas detection. Attached Figure Description

[0031] Figure 1 A cross-sectional view of a Helmholtz resonant cavity provided in an embodiment of the present invention.

[0032] Figure 2 This is a perspective view of a photoacoustic spectrometer provided in an embodiment of the present invention, wherein the window component is transparent and not shown.

[0033] Figure 3a The relationship between the sound pressure level and the first type of radius and length of the resonant cavity cylinder in the simulation verification of the photoacoustic spectroscopy sound meter provided in the embodiment of the present invention.

[0034] Figure 3b The relationship between the sound pressure level and the second type of radius and length of the resonant cavity cylinder in the simulation verification of the photoacoustic spectroscopy sound meter provided in the embodiments of the present invention.

[0035] Figure 3c The relationship between the sound pressure level and the third radius and length of the resonant cavity cylinder in the simulation verification of the photoacoustic spectroscopy sound meter provided in the embodiment of the present invention.

[0036] Figure 4 The diagram shows the relationship between the sound pressure level and the length L of the resonant cavity tube in the simulation verification of the photoacoustic spectrometer provided in the embodiment of the present invention.

[0037] Figure 5 A structural block diagram of a gas detection device according to an embodiment of the present invention is provided for reference.

[0038] Figure 6 Resonance curves of bare tuning fork, coaxial and Helmholtz cavity configurations provided in embodiments of the present invention.

[0039] Figure 7 The diagram shows the second harmonic signal of the bare tuning fork, coaxial and Helmholtz cavity configuration provided in the embodiment of the present invention.

[0040] Figure 8 Noise diagram of bare tuning fork, coaxial and Helmholtz cavity configuration provided in an embodiment of the present invention. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0042] In the description of this invention, it should be understood that the orientation or positional relationship indicated by directional terms such as "front, back, up, down, left, right", "horizontal, vertical, horizontal" and "top, bottom" is generally based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing this invention and simplifying the description. Unless otherwise stated, these directional terms do not indicate or imply that the device or element referred to must have a specific orientation or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the scope of protection of this invention. The directional terms "inner" and "outer" refer to the inner and outer contours relative to the outline of each component itself.

[0043] In the description of this invention, "several" means one or more, "multiple" means two or more, and "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, the number of indicated technical features, or the order of the indicated technical features. In the description of this invention, unless otherwise explicitly defined, terms such as "set," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of these terms in this invention based on the specific content of the technical solution.

[0044] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0045] This invention relates to a photoacoustic spectroscopy sound meter, comprising: a pair of identical, coaxially opposite Helmholtz cavities, each consisting of an entrance window, a cylinder, and a thin tube; and a quartz tuning fork fixedly arranged between the Helmholtz cavities such that the central axis of the cavity passes through the gap between the arms of the tuning fork. It also relates to a gas detection device comprising the aforementioned photoacoustic spectroscopy sound meter. The Helmholtz resonant cavity of this invention can resonate and amplify sound waves, and its resonant frequency is the same as that of the quartz tuning fork. It can be applied in quartz-enhanced photoacoustic spectroscopy gas detection technology to increase signal amplitude, suppress noise, and ultimately improve the signal-to-noise ratio.

[0046] This invention provides a Helmholtz cavity photoacoustic spectroscopy sound meter and its gas detection device for photoacoustic spectroscopy detection, aiming to solve at least one of the technical problems existing in the prior art, applying the Helmholtz cavity to photoacoustic spectroscopy gas sensing technology, enhancing the detection signal and suppressing noise.

[0047] The technical solution of this invention is a photoacoustic spectral sound meter, comprising: a pair of miniature Helmholtz resonant cavities of identical size, each resonant cavity consisting of a cylindrical tube and a thin tube. One end of the cylindrical tube is a light-transmitting window, and the other end is coaxially connected to the thin tube. The openings of the thin tubes of the two resonant cavities face each other, maintaining coaxiality, and a quartz tuning fork is placed in the middle, with the central axis of the two resonant cavities perpendicularly passing through the gap between the two arms of the quartz tuning fork.

[0048] Optionally, assume that the radius of the cylindrical tube 50 is R and the length is L; the radius of the thin tube 51 is r and the length is l. The value of R is in the range of 2mm-4mm, the value of L is in the range of 3.1mm-5.1mm, the value of r is in the range of 0.5mm-0.7mm, and the value of l is in the range of 3.4mm-5.4mm.

[0049] Optionally, the position of the central axis of the two thin tubes relative to the top of the quartz tuning fork arm ranges from 0mm to 3mm.

[0050] Optionally, the horizontal distance between the two thin tubes and the plane of the quartz tuning fork arm is in the range of 0mm-0.08mm.

[0051] Furthermore, the light-transmitting window is composed of a thin glass sheet that is highly transparent to near-infrared light.

[0052] Furthermore, the quartz tuning fork is a standard-sized, strongly resonant quartz tuning fork; the cylindrical tube of the resonant cavity has an inner diameter of 3.0 mm and a length of 4.1 mm, while the thin tube has an inner diameter of 0.6 mm and a length of 4.4 mm.

[0053] Furthermore, the resonant cavity is made of stainless steel.

[0054] Reference Figure 1 and Figure 2 In some embodiments, a photoacoustic spectrometer 10 according to the present invention includes: a pair of Helmholtz resonant cavities, each including a cylindrical tube 50, a thin tube 51 connected to the cylindrical tube, and an incident window 53. In these embodiments, the cylindrical tube 50, the thin tube 51, and the laser (LA) are defined to have the same central axis (A1). The quartz tuning fork 52 used in the present invention is a standard-sized, strongly resonant quartz tuning fork (QTF) with a resonant frequency of 32768 Hz. The width, thickness, and gap of the QTF arm are 600 μm, 330 μm, and 300 μm, respectively.

[0055] In a further embodiment, the optimized dimensions of the cylindrical tube 50 and the thin tube 51 of the Helmholtz cavity can be calculated in the following manner. It should be understood that the optimized dimensions in this invention are not simply obtained through a limited number of experiments, but require creative calculations, the reasons and processes of which are explained through the following calculation methods and simulations.

[0056] First, theoretically, acoustic and electrical systems can be compared: sound pressure can be likened to voltage; sound waves to current. Assume the cylindrical tube 50 has a radius of R, a length of L, and a cross-sectional area of ​​A; the thin tube 51 has a radius of r, a length of l, and a cross-sectional area of ​​S. The thin tube can be compared to an electrical conductor with a conductivity of ρlS, where ρ is the air density. (Refer to...) Figure 1 Analyzing the cylindrical tube 50 separately, let the sound pressure equation inside the tube from left to right be P.+ =αe jkx The sound pressure equation from left to right is P - =βe -jkx Where k = ω / c is the wave number, ω is the angular frequency, c is the speed of sound, and α and β are the intensity of the sound pressure. The positive x-axis is the direction from left to right along the axis (A1). The corresponding sound wave equation is I. + =P + / (ρc / S) and I - =-P - / (ρc / S), from which the acoustic impedance at any point inside the resonant cavity can be obtained:

[0057]

[0058] When x = 0, i.e., the position of the resonant cavity window, Z0 is infinite. When x = L, we have:

[0059]

[0060] Meanwhile, the cylindrical tube 50 is connected to the thin tube 51 at x = L, which can be regarded as a section of acoustic impedance formed by an air column, and its value is:

[0061]

[0062] By making equations (2) and (3) equal, we can obtain the equation that satisfies the acoustic resonance condition:

[0063]

[0064] Equation (4) yields the corresponding frequencies of all resonance modes of the Helmholtz cavity. When the resonance frequency is high, this equation becomes inaccurate; finite element simulation using COMSOL software can aid in the analysis. The width, thickness, and gap of the QTF arm are set to 600 μm, 330 μm, and 300 μm, respectively. The radius of the Helmholtz cavity cylinder is set to 3.0 mm, and its length to 4.1 mm. The inner diameter of the thin tube is 0.6 mm, and its length is 4.4 mm. The inner wall of the cavity and the surface of the QTF are set as hard acoustic field boundaries, resulting in total reflection of sound waves. The laser beam is set as a sound source. Keeping the sound source intensity constant, the size of the Helmholtz cavity is adjusted to simulate its response to sound waves of different frequencies. By probing the sound pressure level in the middle of the tuning fork arm, the amplification effect of different sizes on sound waves can be compared, and the optimal size of the Helmholtz cavity can be obtained.

[0065] First, the radius and length of the Helmholtz cavity cylinder were changed, with R taken as 2.9mm, 3.0mm, and 3.1mm, and L taken as 4.0mm, 4.1mm, and 4.2mm. The frequency response of the cavity with different dimensions was simulated as follows: Figure 3a , Figure 3b , Figure 3c As shown, a resonant frequency exists around 32.7 kHz, but the sound pressure level is highest when R = 3.1 mm and L = 4.1 mm, indicating that the Helmholtz cavity of this size amplifies sound waves best. Simulations were also conducted for the case where the length of the thin tube l ranges from 4.1 mm to 4.7 mm, under the optimal cylindrical dimensions. Figure 4 As shown, the optimal length is 4.4 mm. In summary, the optimal dimensions of the Helmholtz cavity in this invention are: a cylindrical radius of 3.0 mm and a length of 4.1 mm, and a thin tube inner diameter of 0.6 mm and a length of 4.4 mm.

[0066] Figure 5 The diagram shows the structure of a gas detection device according to an embodiment of the present invention. The specific connection structure of the gas detection device is as follows: a function generator 2; an adder 3 connected to the modulation signal output terminal of the function generator 2; a laser driver 4 connected to the signal output terminal of the adder 3; a laser 5 driven by the laser driver 4, the laser 5 emitting at least mid-infrared light; an optical fiber collimating lens 6 disposed in the output optical path of the laser 5; a Helmholtz cavity spectrometer 10 disposed in the output optical path of the lens 6; a preamplifier 8 connected to the quartz crystal oscillator; a lock-in amplifier 9 connected to the output terminal of the preamplifier 8, the lock-in amplifier 9 being connected to the synchronization signal output terminal of the function generator 2; and a computer device 1 with a data acquisition card, the data acquisition card being connected to both the output terminal of the lock-in amplifier 9 and the input terminal of the function generator 2.

[0067] A sine wave with a modulation frequency half that of the quartz tuning fork resonant frequency is sent from function generator 2 to laser driver 4. Laser driver 4 can control the injection current and temperature of laser 5. The emission center wavelength of laser 5 corresponds to the target absorption line of the gas to be measured. The light emitted by laser 5 is optically shaped by lens 6 and then enters spectrophotometer 10. Figure 2 As shown, the collimated beam first enters the first Helmholtz cavity, passes through the entrance window 53 made of BK9 glass, through the cylindrical tube 50 and the thin tube 51, then passes through the gap between the arms of the quartz tuning fork 52, and exits through the second Helmholtz cavity. The excitation light emitted by the laser 5 excites the gas under test to generate sound waves. The sound waves drive the tuning fork to vibrate, thereby generating an electrical signal, which is output to the preamplifier 8 and then sent to the lock-in amplifier 9 for demodulation. The reference signal for demodulation by the lock-in amplifier 9 comes from the synchronization port of the function generator 2. The signal demodulated by the lock-in amplifier is sent to the computer device 1 with a data acquisition card to collect and record data. In addition, the concentration of the gas under test can be measured and displayed on a mobile computer in real time, which has the functions of high precision, high portability, and online monitoring.

[0068] Therefore, when detecting a constant amount of gas, a target detection line close to the center wavelength of the light source is first selected. By controlling the driving current of the laser driver 4, the wavelength of the laser 5 is made to sweep across the target absorption line. Specifically, second harmonic detection technology is used, where the current of the laser 5 is modulated by the f / 2 signal frequency generated by the function generator 1, where f is the resonant frequency of the tuning fork used. After the gas to be measured is excited by the laser, the acoustic signal generated by de-excitation is acquired by the QTF and converted into a corresponding electrical signal. The electrical signal passes through the preamplifier 8 and the lock-in amplifier 9 in sequence, and then enters the computer system through the data acquisition card. After the data is calculated by the software, the gas concentration information is finally displayed on the screen through a human-machine interactive interface. When measuring a gas of unknown concentration, it should be calibrated beforehand using a standard gas of known concentration. The calibrated device can then measure that gas.

[0069] In one embodiment, the gas detection method according to the present invention includes the following steps: A. Triggering a laser to generate an optical path in a resonant cavity, modulating the laser's operating current with the inherent frequency of the quartz tuning fork, so that the wavelength of the laser's output light sweeps across the absorption line of the target gas, and then modulating the laser's operating current until the output photoacoustic signal of the quartz tuning fork reaches a predetermined amplitude; B. When the target gas in the photoacoustic spectrometer is excited by the laser, collecting the electrical signal output by the quartz tuning fork, processing the signal through a preamplifier and a lock-in amplifier, and storing it as a photoacoustic signal count value; C. Calculating the target gas concentration value corresponding to the current photoacoustic signal count value according to a pre-calibrated linear relationship between the photoacoustic signal count value and the target gas concentration.

[0070] In a more specific embodiment, the gas detection device based on the sound meter of the present invention operates as follows.

[0071] First, a 1.39μm near-infrared fiber-coupled distributed feedback semiconductor laser 5 is used as the excitation source. Then, a high-precision semiconductor laser driver board 4 is used to control the temperature and injection current of the semiconductor laser 5. Second harmonic wavelength modulation (HIM) is used to improve the detection sensitivity of the QEPAS. A signal generator produces a triangular wave with a period of 400s and a sine wave with a frequency of f0 / 2 (f0 is the resonant frequency of the spectrometer). HIM is used to reduce the effects of background noise from stray light and crosstalk from other gas absorption lines. The laser beam is focused through the gap between the two arms of the QTF by a self-focusing lens. The focal length of the self-focusing lens is 11mm, and the beam waist diameter of the laser is approximately 100μm. The electrical signal output from the QTF is amplified by a 10MΩ custom transimpedance preamplifier 8. A lock-in amplifier 9 is used to demodulate the second harmonic signal. The entire gas detection system is controlled and the gas concentration is calculated using a LabVIEW program on a computer.

[0072] In a verification example, the resonance characteristics of a bare tuning fork and the Helmholtz cavity of the present invention before and after coupling were compared. Under the aforementioned optimal dimensions, the resonance curves of the Helmholtz cavity-coupled tuning fork and the bare tuning fork are as follows: Figure 6 As shown in the figure, the height of the curve represents the resonance intensity of the tuning fork, reflecting the energy stored inside. The resonant frequency of the quartz tuning fork shifts slightly under coupling, from 32772.8Hz to 32774.3Hz, but coupling is still possible. The quality factor Q decreases from 12527 to 5248. The decrease in Q indicates good coupling between the tuning fork and the Helmholtz cavity, with energy transferred between the tuning fork and the resonant cavity.

[0073] In a verification example, the detection performance of a bare tuning fork before and after coupling with the Helmholtz cavity of this invention was compared. To measure the concentration of water molecules in the air, the laser temperature was set at 18.6°C, and the injection current was varied from 38 mA to 47 mA, corresponding to an emission wavelength from 7194.4 cm⁻¹. -1 Up to 7195.2cm -1 According to the Hitran database, it is located at 7194.8cm. -1 The absorption line intensity is 3.07 × 10⁻⁶. -21 The water molecule absorption line at cm / mol was selected.

[0074] like Figure 7 As shown, the peak value of the second harmonic signal of the bare tuning fork is 0.24 mV, while the peak value of the second harmonic signal of the Helmholtz cavity configuration is 3.32 mV. Furthermore, by adjusting the laser current to move the laser wavelength away from the water molecule absorption line for a period of time, the fluctuation in signal amplitude during this period represents the system noise.

[0075] like Figure 8 As shown, the calculated 1σ noise levels for the bare tuning fork and the Helmholtz cavity configuration are 2.47 μV and 1.75 μV, respectively. The final result shows that the signal-to-noise ratio of the Helmholtz cavity configuration is 20 times higher than that of the bare tuning fork. The noise suppression principle of the Helmholtz cavity mainly involves two aspects. First, the large change in cross-sectional area from the cylinder to the thin tube results in a higher acoustic impedance, making it difficult for ambient noise around the spectrometer to enter the resonant cavity. Second, when the laser passes through the window, the absorption of the laser by the window generates a sound wave with the same modulation frequency as the resonant cavity; this is called window noise. By adjusting the length of the cylinder, this sound wave can be caused to interfere destructively due to reflection as it propagates within the resonant cavity, thus suppressing window noise.

[0076] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A photoacoustic spectral sound meter, characterized in that, From left to right, it includes: the first Helmholtz resonator, the quartz tuning fork, and the second Helmholtz resonator; The first Helmholtz resonant cavity includes: a coaxial first cylindrical tube and a first thin tube; the left end face of the first cylindrical tube is a first light-transmitting window, and the right end face is a non-light-transmitting window with a first small hole in the center, and the first small hole is connected to the left end face of the first thin tube. The second Helmholtz resonant cavity includes: a coaxial second cylindrical tube and a second thin tube; the right end face of the second cylindrical tube is a second light-transmitting window, and the left end face is a non-light-transmitting window with a second small hole in the center, and the second small hole is connected to the right end face of the second thin tube; The right end face of the first thin tube and the left end face of the second thin tube are open windows, respectively placed on both sides of the gap between the quartz tuning fork arms, and the two Helmholtz resonant cavities are coaxial. When the laser beam enters through one of the two light-transmitting windows, passes through the gap between the arms of the quartz tuning fork, and exits through the other window, the two Helmholtz resonant cavities resonate and amplify the acoustic signal excited by the laser beam, thereby increasing the sound pressure near the arms of the quartz tuning fork and suppressing environmental noise and window noise, thus improving the signal-to-noise ratio of the quartz tuning fork in detecting acoustic signals near its arms.

2. The sound meter according to claim 1, characterized in that, The laser is coaxial with two Helmholtz resonant cavities, and its beam waist diameter is less than 300 μm.

3. The sound meter according to claim 1, characterized in that, The two Helmholtz resonant cavities are the same size; The parameters of the Helmholtz resonant cavity that satisfy the acoustic resonance condition must meet the following equation: Where R represents the radius of the cylinder, L represents the length of the cylinder, r represents the radius of the capillary tube, l represents the length of the capillary tube, and k represents the wave number.

4. The sound meter according to claim 3, characterized in that, The value of R ranges from 2mm to 4mm, the value of L ranges from 3.1mm to 5.1mm, the value of r ranges from 0.5mm to 0.7mm, and the value of l ranges from 3.4mm to 5.4mm.

5. The sound meter according to any one of claims 1 to 4, characterized in that, The position of the two thin tubes relative to the top of the quartz tuning fork arm ranges from 0mm to 3mm.

6. The sound meter according to any one of claims 1 to 4, characterized in that, The horizontal distance between the two thin tubes and the plane of the quartz tuning fork arm ranges from 0mm to 0.08mm.

7. A gas detection device comprising the photoacoustic spectrometer according to any one of claims 1 to 6, wherein the vibrating arm of the quartz tuning fork within the photoacoustic spectrometer is placed in the gas to be measured, characterized in that, Also includes: Laser, fiber optic collimating lens and signal demodulation device; The laser is used to emit laser light; The fiber collimating lens is used to shape the laser and then incident the shaped laser onto the photoacoustic spectroscopy sound meter. The photoacoustic spectrometer is used to acquire the electrical signal corresponding to the gas to be tested after the laser acts on it; wherein, the process of generating the electrical signal is as follows: first, the laser excites the gas to be tested to generate an acoustic signal, and then the acoustic signal drives the quartz tuning fork to vibrate to form a corresponding electrical signal; the intensity of the acoustic signal is different depending on the concentration of the gas to be tested. The signal demodulation device is used to demodulate the electrical signal to obtain the concentration information of the gas to be measured.

8. The apparatus according to claim 7, characterized in that, The signal demodulation device includes: a preamplifier and a lock-in amplifier; The preamplifier is used to amplify the electrical signal output by the photoacoustic spectrometer through transimpedance to obtain the corresponding photoacoustic signal. The lock-in amplifier is used to demodulate the photoacoustic signal output by the preamplifier and solve for the concentration information of the gas to be measured.

9. The apparatus according to claim 7 or 8, characterized in that, The emission center wavelength of the laser corresponds to the target absorption line of the gas to be tested.

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