Ultrasonic miniature two-channel differential resonance photoacoustic cell

Abstract

The invention relates to the field of gas sensors, in particular to an ultrasonic miniature two-channel differential resonance photoacoustic cell, which comprises a two-way acoustic resonant cavity, a buffer cavity, an optical window, a gas path interface and a sound-sensitive detection element, and is characterized in that the two-way acoustic resonant cavity comprises two parallel and symmetrical cylindrical resonant cavities; the two cylindrical buffer cavities are respectively arranged on two sides of the double-path acoustic resonant cavity, and the buffer cavities are connected with the circular bottom surface of the resonant cavity; the two optical windows are respectively arranged on the two buffer cavities; the two gas path interfaces are respectively arranged on the two buffer cavities and are used for carrying out gas exchange; the two sound-sensitive detection elements are arranged on the two resonant cavities respectively and located at the positions where sound pressure signals of the two resonant cavities are maximum. The ultrasonic miniature dual-channel differential resonance photoacoustic cell disclosed by the invention has the advantages of small volume, high sensitivity, high response speed, strong anti-interference capability and the like.

Description

Technical Field

[0001] The invention relates to the field of gas sensors, and in particular to an ultrasonic miniature dual-channel differential resonance photoacoustic cell. Background Art

[0002] Photoacoustic spectroscopic gas sensors have huge application demands in the field of gas detection in confined spaces such as commercial airliners and ocean-going ships due to their high sensitivity, high selectivity, and rapid response characteristics, as well as the potential for miniaturization and simultaneous detection of multiple gases. The photoacoustic cell is the core component of the photoacoustic gas sensor. The photoacoustic cell serves as both a sample cavity and a sound resonant cavity, and must satisfy the requirements for dense interaction between laser and gas in a small, limited space, complete the high-efficiency conversion process of light-heat-sound-electricity, realize the resonant amplification of photoacoustic waves, and form a spatially stable distribution of sound waves.

[0003] At present, the mainstream resonant photoacoustic cell structures include Helmholtz resonance structure, one-dimensional resonance cavity, three-dimensional resonance cavity and differential resonance structure. When used in complex detection environments such as commercial airliners and ocean-going ships, they all have limitations such as large size, long response time, large low-frequency noise influence, and poor signal-to-noise ratio, which do not meet the needs of sensor miniaturization and high detection sensitivity in complex environments.

[0004] In the technical solution disclosed in the prior art CN114813574 A, a differential photoacoustic spectroscopy gas concentration detection device based on a dual-channel T-type photoacoustic cell is disclosed, and the technical problem solved is to reduce the resonant frequency of the photoacoustic cell through structural design, so that it can be used with a high-sensitivity acoustic wave detector with an operating frequency in the range of hundreds of hertz. However, due to the structural characteristics, it has serious low-frequency noise interference in complex environments and a large volume, which is subject to certain limitations in terms of sensitivity and volume.

[0005] Therefore, there is an urgent need for a resonant photoacoustic cell with small size, high sensitivity, fast response speed and strong anti-interference ability. Summary of the invention

[0006] In view of this, the present invention discloses an ultrasonic miniature dual-channel differential resonant photoacoustic cell to solve the above problems; comprising: a dual-path acoustic resonant cavity comprising two parallel and symmetrical cylindrical resonant cavities; two cylindrical buffer cavities are respectively arranged on both sides of the dual-path acoustic resonant cavity, and the buffer cavities are connected to the circular bottom surface of the resonant cavity; optical windows are respectively arranged on the circular bottom surfaces of the two buffer cavities that are not connected to the resonant cavity; two gas path interfaces are respectively arranged on the two buffer cavities for gas exchange; and two acoustic sensitive detection elements are respectively arranged on the two resonant cavities.

[0007] The beneficial effects of the present invention include:

[0008] The present invention effectively suppresses atmospheric airflow noise and ensures rapid response by introducing a buffer cavity; by reducing the length of the photoacoustic cell resonance cavity to the millimeter level, which is an order of magnitude smaller than the size of the traditional photoacoustic cell, the volume of the photoacoustic cell is reduced, and the gas circulation renewal time in the photoacoustic cell cavity is accelerated, providing a guarantee for the rapid response of the sensor; by designing a dual-channel balanced acoustic resonance cavity in the structure, environmental noise, gas flow rate noise and electromagnetic field noise are effectively suppressed, ensuring the excellent gas detection performance of the photoacoustic cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is a schematic structural diagram of an ultrasonic miniature dual-channel differential resonance photoacoustic cell of the present invention;

[0010] Figure 2 It is a front view of the ultrasonic miniature dual-channel differential resonance photoacoustic cell of the present invention;

[0011] Figure 3 A top view of the ultrasonic miniature dual-channel differential resonant photoacoustic cell of the present invention;

[0012] Figure 4 It is a side view of the ultrasonic miniature dual-channel differential resonance photoacoustic cell of the present invention;

[0013] Figure 5 It is a simulation diagram of the sound pressure distribution of the ultrasonic miniature dual-channel differential resonance photoacoustic cell of the present invention;

[0014] Figure 6 It is a schematic diagram of the simulation of the relationship between the inner diameter of the resonant cavity and the resonant frequency of the ultrasonic micro dual-channel differential resonant photoacoustic cell of the present invention;

[0015] Figure 7 It is a schematic diagram of the angle between the optical path and the optical window of the ultrasonic miniature dual-channel differential resonance photoacoustic cell of the present invention;

[0016] Figure 8 It is a schematic diagram of the structural connection relationship of the ultrasonic miniature dual-channel differential resonance photoacoustic cell of the present invention when it is working;

[0017] The numbers mean: 1 and 2 are resonant cavities, 3 and 4 are buffer cavities, 5 and 6 are optical windows, 7 and 8 are gas path interfaces, 9 and 10 are acoustic detection elements, 11 is a signal generator, 12 is a laser driving module, 13 is an excitation light source, 14 is a phase-locked amplifier, and 15 is a computer. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solution, characteristics and advantages of the present invention more clearly understood, the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0019] The ultrasonic miniature dual-channel differential resonant photoacoustic cell described in this embodiment is as follows: Figure 1 As shown, it includes: a two-way acoustic resonance cavity includes two parallel and symmetrical cylindrical resonance cavities 1, 2; two cylindrical buffer cavities 3, 4 are respectively arranged on both sides of the two-way acoustic resonance cavity, and the buffer cavities are connected to the circular bottom surface of the resonance cavity; optical windows 5, 6 are respectively arranged on the circular bottom surfaces of the two buffer cavities 3, 4 that are not connected to the resonance cavity; two gas path interfaces 7, 8 are respectively arranged on the two buffer cavities 3, 4 for gas exchange; two acoustic sensitive detection elements 9, 10 are respectively arranged on the two resonance cavities, located at the maximum sound pressure signal of the two resonance cavities. Other views of the ultrasonic miniature dual-channel differential resonance photoacoustic cell are shown in FIG. Figure 2 , Figure 3 , Figure 4 shown.

[0020] Furthermore, when preparing the ultrasonic micro dual-channel differential resonant photoacoustic cell, the process flow is: milling-CNC (Computer Numerical Control)-wire cutting-electric spark-inner and outer circular grinding-inner layer coating-peripheral packaging.

[0021] Furthermore, the geometric shape and relative position accuracy of each device in the photoacoustic cell are at the arc second level; the internal and external circular grinding and polishing accuracy is at the mirror level, and the surface roughness Ra is less than 0.1 μm; the resonant cavity adopts a cylindrical first-order longitudinal mode configuration; and the peripheral packaging material is brass or stainless steel.

[0022] Furthermore, in a complex environment such as an ocean-going ship, the vibration interference sources are mainly low-frequency mechanical vibration and audible sound wave vibration. To avoid interference, the resonant frequency of the photoacoustic pool is preferably 20-22 kHz, which is also the modulation frequency of the excitation light source 13 and the operating frequency of the sensor. According to the relationship between the acoustic cavity parameters and the resonant frequency f0 When the speed of sound is approximately c=343 m / s, the length of the resonant cavity designed in the present invention ranges from 7.8 mm to 8.6 mm.

[0023] Furthermore, regarding the resonant cavity: the resonant cavity is an acoustic cavity, which serves as both a gas sample cavity and a sound resonant cavity, and is the core structure of a photoacoustic spectroscopic gas sensor. The present invention adopts a cylindrical first-order longitudinal mode resonant acoustic cavity structure. When the cross-sectional size of the acoustic cavity is much smaller than the wavelength of sound, the acoustic field excited along the direction of the acoustic cavity can be regarded as a one-dimensional acoustic field of the length of the resonant cavity. Optical windows 5 and 6 are respectively installed at both ends of the acoustic cavity for sound insulation treatment to further reduce environmental noise interference. Figure 6The figure shows the change trend of the acoustic cavity resonance frequency of the resonant photoacoustic pool with a stable sound field distribution under different resonant cavity apertures obtained by simulation research. The results show that as the diameter of the inner hole of the resonant cavity increases, the resonant frequency tends to decrease. However, in addition to the resonant frequency, the inner diameter size also needs to consider the laser spot size: when the acoustic cavity aperture is smaller than the spot of the photoacoustic signal excitation light source 13, the light source will directly irradiate the inner wall of the photoacoustic pool, thereby generating significant thermal noise under the photothermal effect. When the target gas concentration is too low, the noise can even drown out the signal. The inner diameter of the resonant cavity is preferably R = 3mm, which can ensure that the light beam passes through the resonant cavity without collision and avoids the generation of photothermal noise, and at the same time ensure that the resonant frequency of the photoacoustic pool reaches the ultrasonic frequency band to meet the detection needs of complex environments.

[0024] Further, regarding the buffer cavity: although the installation of optical windows at both ends of the acoustic cavity can isolate the external environmental noise, it also blocks the exchange of the gas in the cavity with the environmental gas. Therefore, it is necessary to increase the gas flow rate to ensure a rapid response, and the introduction of airflow will increase the airflow noise. In order to suppress the flow rate noise, thermal noise and environmental noise, the present invention introduces a buffer cavity. The length and inner diameter of the buffer cavity have a significant effect on the photoacoustic signal, noise level and resonant frequency. When its length is one-fourth of the wavelength of the sound wave that forms resonance in the photoacoustic pool, the low-impedance buffer cavity input end can be turned to a high impedance when it is transferred to the resonant cavity, thereby reducing the probability of environmental noise coupling to the resonant cavity, improving the signal-to-noise ratio of the system, and effectively suppressing the window noise and flow rate noise. The inner diameter and length of the buffer cavity are preferably 15mm and 4mm, respectively.

[0025] Further, regarding the dual-path acoustic resonant cavity (structure formed by resonant cavities 1 and 2): since the buffer cavity cannot suppress electromagnetic noise and cannot significantly improve the signal-to-noise ratio of the photoacoustic cell. Therefore, the present invention adopts a dual-path acoustic resonant cavity to further improve the sensor signal-to-noise ratio. By arranging parallel dual-resonant ultrasonic cavities in the same direction, and connecting the two ends of the cavity by two identical buffer cavities 3 and 4, the gas composition in the two acoustic cavities is exactly the same, and the external sound frequency and internal airflow noise are suppressed by differential operation.

[0026] Furthermore, if Figure 8As shown, when working, the outside of the photoacoustic cell also includes a signal generator 11, a laser driving module 12, an excitation light source 13, a phase-locked amplifier 14, and a computer 15; wherein the signal generator 11 generates an initial signal and inputs it into the laser driving module 12 and the phase-locked amplifier 14 respectively; the laser driving module 12 controls the excitation light source 13 to generate a light beam according to the initial signal, and the generated light beam enters the photoacoustic cell, and the photoacoustic cell obtains a current signal that is positively correlated with the gas concentration, and the current signal is input into the phase-locked amplifier 14 after differential amplification, and the phase-locked amplifier 14 demodulates the signal, and inputs the adjusted signal and the initial signal into the computer 15, and the computer 15 analyzes and processes the adjusted signal and the initial signal, and outputs the analysis result.

[0027] Furthermore, one of the acoustic resonant cavities is irradiated by the excitation light beam to generate sound waves, which resonate and propagate and vibrate through two symmetrical dual-channel acoustic resonant cavities. When the light beam enters the photoacoustic pool, only one resonant cavity is activated, and sound waves of equal amplitude and opposite phase are formed in the two resonant cavities. The photoacoustic signal of the target gas can be obtained by differential operation of the output signals of the two cavities. This structure can effectively suppress external sound frequency and internal airflow noise. In addition, this dual-cavity two-end connected structure increases the air pressure in one chamber while reducing the air pressure in the other chamber. The environmental noise, airflow noise, thermal noise generated by the window absorbing the excitation light, and electromagnetic interference are effectively removed by differential means, thereby obtaining twice the photoacoustic signal, further improving the signal-to-noise ratio of the sensor.

[0028] Furthermore, if Figure 7 As shown, a light beam enters from an optical window, passes through the resonant cavity, and exits from another optical window. The angle between the axis of the optical window through which the incident light passes and the optical path is 3°, and the angle between the axis of the optical window through which the outgoing light passes and the optical path is -3°.

[0029] Furthermore, the sound pressure amplitude S in the photoacoustic cell is PA It can be expressed as:

[0030] S PA =C cell αW L

[0031]

[0032] Among them, C cell is the photoacoustic cell constant, which reflects the ability of the photoacoustic system to convert the light energy absorbed by the gas into sound energy and has nothing to do with the gas and the light beam generated by the excitation light source. α is the absorption coefficient which is positively correlated with the gas concentration. W L represents the optical power, r is the radius of the resonant cavity, and l is the length of the resonant cavity.

[0033] Furthermore, regarding the positions of the acoustic detection elements 9 and 10, the sound pressure distribution inside the photoacoustic cell is simulated. The sound pressure distribution simulation result is as follows: Figure 5 As shown, the results show that in the ultrasonic resonant photoacoustic pool, the position with the highest sound field intensity is located exactly at the geometric center of the cylindrical acoustic cavity. Therefore, the sound wave detection position on the two channels, that is, the installation position of the two acoustic sensitive detection elements 9 and 10, can be determined as the geometric center of the acoustic cavity.

[0034] Finally, it should be noted that the above only describes some embodiments of the present invention. For those skilled in the art, it is conceivable that various changes, modifications, substitutions and deformations may be made to these embodiments without departing from the principles and spirit of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents, and the above-mentioned actions should be covered within the scope of protection of the present invention.

Claims

1. An ultrasonic miniature dual-channel differential resonant photoacoustic cell, characterized in that: include: A dual-path acoustic resonant cavity, a buffer cavity, an optical window, an air path interface, and an acoustically sensitive detection element, wherein the dual-path acoustic resonant cavity comprises two parallel and symmetrical cylindrical resonant cavities; the two cylindrical buffer cavities are respectively arranged on both sides of the dual-path acoustic resonant cavity, and the buffer cavities are connected to the circular bottom surfaces of the resonant cavities; optical windows are respectively arranged on the circular bottom surfaces of the two buffer cavities that are not connected to the resonant cavities; two air path interfaces are respectively arranged on the two buffer cavities for gas exchange; and two acoustically sensitive detection elements are respectively arranged on the two resonant cavities.

2. The ultrasonic miniature dual-channel differential resonant photoacoustic cell according to claim 1, characterized in that: The inner diameter of the buffer cavity is 15 mm and the length is 4 mm; the inner diameter of the resonant cavity is 3 mm and the length ranges from 7.8 mm to 8.6 mm.

3. The ultrasonic miniature dual-channel differential resonant photoacoustic cell according to claim 1, characterized in that: The process flow for preparing the ultrasonic micro dual-channel differential resonant photoacoustic cell is: milling-CNC (Computer Numerical Control)-wire cutting-electric spark-inner and outer circular grinding-inner layer coating-peripheral packaging.

4. The ultrasonic miniature dual-channel differential resonance photoacoustic cell according to claim 3, characterized in that: The geometric shape and relative position accuracy of each device in the photoacoustic cell are at the arc second level; the internal and external circular grinding and polishing accuracy is mirror level, and the surface roughness Ra is less than 0.1μm; the resonant cavity adopts a cylindrical first-order longitudinal mode configuration; the peripheral packaging material is brass or stainless steel.

5. The ultrasonic miniature dual-channel differential resonant photoacoustic cell according to claim 1, characterized in that: When working, the outside of the photoacoustic cell also includes a signal generator, a laser driving module, an excitation light source, a phase-locked amplifier, and a computer; the signal generator generates an initial signal and inputs it into the laser driving module and the phase-locked amplifier respectively; the laser driving module controls the excitation light source to generate a light beam according to the initial signal, and the generated light beam enters the photoacoustic cell. The photoacoustic cell obtains a current signal that is positively correlated with the gas concentration. The current signal is input into the phase-locked amplifier after differential amplification. The phase-locked amplifier demodulates the signal and inputs the adjusted signal and the initial signal into the computer. The computer analyzes and processes the signal and outputs the analysis result.

6. The ultrasonic miniature dual-channel differential resonant photoacoustic cell according to claim 5, characterized in that: When the light beam enters the photoacoustic cell, only one resonant cavity is activated, and there are sound waves of opposite phase and equal amplitude in the two resonant cavities.

7. The ultrasonic miniature dual-channel differential resonant photoacoustic cell according to claim 6, characterized in that: When the resonant cavity is activated, the resonant frequency of the ultrasonic micro dual-channel differential resonant photoacoustic cell is 20-22 kHz, and the wavelength of the sound wave that resonates in the photoacoustic cell is 4 times the length of the buffer cavity.

8. The ultrasonic miniature dual-channel differential resonance photoacoustic cell according to claim 5, characterized in that: When the light beam enters the photoacoustic cell, it enters from an optical window, passes through the resonant cavity, and exits from another optical window. The angle between the axis of the optical window through which the incident light passes and the optical path is 3°, and the angle between the axis of the optical window through which the outgoing light passes and the optical path is -3°.

9. The ultrasonic miniature dual-channel differential resonance photoacoustic cell according to claim 5, characterized in that: When the light beam enters the photoacoustic cell, the sound pressure amplitude S in the photoacoustic cell is PA It can be expressed as: S PA =C cell αW L Among them, C cell is the photoacoustic cell constant, which reflects the ability of the photoacoustic system to convert the light energy absorbed by the gas into sound energy and is independent of the gas and the excitation light source. α represents the absorption coefficient, and W L represents the optical power, r is the radius of the resonant cavity, and l is the length of the resonant cavity.

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