CN116660170A - Photoacoustic spectrum gas detection device and method based on differential diffuse reflection integrating sphere - Google Patents
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- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
- G01N2021/1704—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in gases
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Abstract
The invention discloses a photoacoustic spectrum gas detection device and method based on a differential diffuse reflection integrating sphere, wherein modulated laser output by a distributed feedback semiconductor laser enters a first diffuse reflection sphere of a differential diffuse reflection spherical resonance photoacoustic cell after passing through a laser collimator, target gas to be detected absorbs laser energy to cause the differential diffuse reflection spherical resonance photoacoustic cell to vibrate, photoacoustic signals are generated in the first diffuse reflection sphere and a second diffuse reflection sphere, two paths of photoacoustic signals in the first diffuse reflection sphere and the second diffuse reflection sphere are detected by a first microphone and a second microphone respectively, the detected two paths of photoacoustic signals are transmitted to a differential amplifier to carry out differential operation, and a control and data acquisition system acquires the photoacoustic signals after differential operation and processes the photoacoustic signals by a computer to invert the concentration of the target gas to be detected. The detection device has the advantages of high signal intensity, high noise suppression capability, high system sensitivity, no adjustment of the light path, small volume and the like.
Description
Technical Field
The invention relates to a photoacoustic spectrum gas detection device and method, in particular to a photoacoustic spectrum gas detection device and method based on a differential diffuse reflection integrating sphere.
Background
With the continuous development of science and technology, absorption spectroscopy has been developed in the course of "highways". As an important branch of absorption spectroscopy, the photoacoustic spectroscopy technology is mainly used for detecting the part of energy which is shown by the heat energy generated after the gas absorbs the light energy in the form of sound pressure, is an ideal background noise-free signal technology, has excellent sensitivity and good selectivity, and is an ideal choice for trace gas detection.
The photoacoustic cell is the core part of photoacoustic spectroscopy experiments, and the design of the photoacoustic cell directly influences the sensitivity of detection signals. Currently, resonant photoacoustic cells are mainly adopted, and the main geometric shapes of the existing resonant photoacoustic cells are cylindrical, spherical and square. Wherein, the cylindrical photoacoustic Chi Shengxue has larger loss and lower Q value; the sound wave attenuation condition in the square photoacoustic cell is serious; spherical photoacoustic cells receive extensive attention from students with their high Q values and low acoustic losses. With the continuous development of photoacoustic cells, a diffusely reflecting spherical resonant photoacoustic cell has been developed in order to increase the path of light to matter. Meanwhile, in recent years, differential characteristics capable of suppressing incoherent noise have been studied extensively by the scholars.
The photoacoustic spectroscopy technology based on the diffuse reflection spherical resonance photoacoustic cell is a trace gas detection technology with high sensitivity. Firstly, a tunable distributed feedback type semiconductor laser is selected as an excitation light source of a photoacoustic signal, and a modulated laser beam is sent into a diffuse reflection spherical resonance photoacoustic cell filled with a sample to be tested through a collimator to excite the sample to be tested. When the laser beam is incident into the sphere, the beam is uniformly reflected inside the sphere as time passes, and finally the beam in the sphere becomes a quite uniform diffuse beam, the sample absorbs light energy and is de-excited in a manner of releasing heat energy, the released heat energy causes the sample and surrounding medium to generate periodic heating according to the modulation frequency of light, so that the medium generates periodic pressure fluctuation, the generated periodic pressure fluctuation is detected by a sensitive microphone, and a photoacoustic signal is obtained by amplification of a preamplifier. Finally, demodulating the generated photoacoustic signal by using a correlation demodulation method, and inverting the concentration of the sample to be detected.
In order to expand the detection range of the photoacoustic spectroscopy gas technology, the sensitivity of the photoacoustic spectroscopy gas technology needs to be improved, but when the sensitivity of the photoacoustic spectroscopy system based on the diffuse reflection spherical resonance photoacoustic cell is further improved at present, two major problems exist:
1. the resonance frequency of the existing photoacoustic cell is generally above 1kHz, so as the resonance frequency of the photoacoustic cell increases, the low-frequency noise of the system is significantly reduced. In order to make the resonance frequency higher than 1kHz, the sphere diameter of the diffusely reflecting spherical resonant photoacoustic cell cannot be too large. This constraint limits the increase of the equivalent optical path length L of the diffusely reflecting spherical resonant photoacoustic cell ep The expression of (2) is as follows:
wherein D is the diameter of the sphere, and ρ is the diffuse reflectance of the coating inside the sphere.
Therefore, in the design process of the diffuse reflection spherical resonance photoacoustic cell, the relationship between the equivalent optical path and the resonant frequency of the cell body needs to be balanced, so that the performance of the system cannot be improved to the greatest extent, and the problem cannot be solved perfectly.
2. Polytetrafluoroethylene (PTFE), commonly known as "plastic king", is typically used inside photoacoustic cells, which has limited resistance to external noise reduction. Although the outer part of the PTFE inner container is generally wrapped with an aluminum alloy sphere, the whole noise isolation capability of the diffuse reflection spherical resonance photoacoustic cell is generally high due to the limited thickness of the aluminum alloy sphere, so that the noise of the photoacoustic spectrum system based on the diffuse reflection spherical resonance photoacoustic cell is generally high. Under the same conditions, the larger the noise is, the smaller the signal-to-noise ratio is, and thus the poorer the detection performance of the system is. Therefore, the sensitivity of the photoacoustic spectroscopy system based on the diffuse reflection spherical resonance photoacoustic cell cannot be further improved.
Disclosure of Invention
Aiming at the problem that the sensitivity of the photoacoustic spectrometry system based on the diffuse reflection spherical resonance photoacoustic cell can not be further improved at present, the invention provides a photoacoustic spectrometry gas detection device and method based on a differential diffuse reflection integrating sphere. The invention improves the detection sensitivity of the system by enhancing the photoacoustic signal and simultaneously inhibiting the dual function of low-frequency noise.
The invention aims at realizing the following technical scheme:
the utility model provides a photoacoustic spectrum gas detection device based on difference diffuse reflection integrating sphere, includes distributed feedback semiconductor laser, laser collimator, difference diffuse reflection spherical resonance photoacoustic cell, first microphone, second microphone, differential amplifier, control and data acquisition system and computer, wherein:
the differential diffuse reflection spherical resonance photoacoustic cell comprises a first diffuse reflection sphere and a second diffuse reflection sphere, wherein the first diffuse reflection sphere is provided with an air inlet and an optical inlet, the second diffuse reflection sphere is provided with an air outlet, and the first diffuse reflection sphere and the second diffuse reflection sphere are communicated through a connecting pipe;
the first microphone and the second microphone are respectively arranged at two ends of a first diffuse reflection sphere and a second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell;
the distributed feedback semiconductor laser outputs modulated laser, the modulated laser is collimated by the laser collimator and then enters a first diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell through an optical inlet, the differential diffuse reflection spherical resonance photoacoustic cell is vibrated after the target gas to be detected in the differential diffuse reflection spherical resonance photoacoustic cell absorbs laser energy, photoacoustic signals are generated in the first diffuse reflection sphere and the second diffuse reflection sphere, the first microphone and the second microphone are used for respectively detecting two paths of photoacoustic signals in the first diffuse reflection sphere and the second diffuse reflection sphere, the detected two paths of photoacoustic signals are transmitted to the differential amplifier for differential operation, and the photoacoustic signals after differential operation are collected by the control and data collection system and processed by a computer to invert the concentration of the target gas to be detected.
The method for detecting the photoacoustic spectrum gas based on the differential diffuse reflection integrating sphere by using the device comprises the following steps of:
step one: the laser controller controls the output wavelength of the distributed feedback semiconductor laser and the output power of the laser; modulating a laser light source of the distributed feedback semiconductor laser by using a superposition signal generated by a low-frequency sawtooth wave and a high-frequency sine wave; scanning and optimizing the resonant frequency of the differential diffuse reflection spherical resonant photoacoustic cell and the modulation depth of the photoacoustic spectrum gas detection device based on the differential diffuse reflection integrating sphere by adopting a control and data acquisition system;
step two: the laser output by the distributed feedback semiconductor laser is changed into a parallel collimated beam through a laser collimator, and the collimated beam is incident into a differential diffuse reflection spherical resonance photoacoustic cell containing the target gas to be detected to excite the target gas to be detected;
step three: the laser excites the target gas to be detected in the first diffuse reflection ball to generate a photoacoustic effect, and the photoacoustic signals generated in the first diffuse reflection ball and the second diffuse reflection ball are detected by the first microphone and the second microphone and transmitted to the differential amplifier to perform differential operation, and finally transmitted to the control and data acquisition system;
step four: the control and data acquisition system acquires the acoustic signals and processes the acoustic signals by a computer, and the concentration of the target gas to be detected is inverted.
Compared with the prior art, the invention has the following advantages:
1. the invention designs a photoacoustic spectrum detection system based on a differential diffuse reflection spherical resonance photoacoustic cell by combining the differential characteristic capable of inhibiting incoherent noise, and the core of the system is that the differential diffuse reflection spherical resonance photoacoustic cell is formed by two diffuse reflection spheres and a connecting pipe. After the modulated laser beam is incident into an excitation cavity of the differential diffuse reflection spherical resonance photoacoustic cell filled with the sample to be detected, the sample to be detected in the excitation cavity absorbs incident laser to generate periodic acoustic signals, the acoustic signals are transmitted into another diffuse reflection sphere filled with the sample to be detected through a connecting pipe in the middle, so that the vibration of the other diffuse reflection sphere is caused, the acoustic signals generated in the two diffuse reflection spheres are detected by using two sensitive microphones respectively and transmitted to a phase-locked end to be demodulated, and finally, data processing is performed by using a computer end. During this process, the laser beam is diffusely reflected inside the diffusely reflecting sphere as an excitation cavity, and eventually the sphere interior beam becomes a fairly uniform diffuse beam due to the high diffuse reflectance of PTFE over time. According to lambert's law, as the action path of light and substances is continuously increased, namely the optical path is increased, the absorption intensity and the photoacoustic signal are also increased, the effect of doubly enhancing the photoacoustic signal is further realized by the differential characteristic, and meanwhile, the differential mode has the characteristic of inhibiting incoherent noise such as gas flow noise, light noise caused by multiple reflections of an excitation light beam and the like.
2. The detection device has the advantages of high signal intensity, high noise suppression capability, high system sensitivity, no adjustment of the light path, small volume and the like.
Drawings
Fig. 1 is a schematic structural diagram of a photoacoustic spectrometry gas detection apparatus based on a differential diffuse reflection integrating sphere;
FIG. 2 is a schematic diagram of the structure of a differential diffuse reflection spherical resonant photoacoustic cell;
fig. 3 is an experimental result of detecting acetylene gas using a differential diffuse reflection spherical resonance photoacoustic Chi Guangsheng spectroscopic gas detection apparatus.
Detailed Description
The following description of the present invention is provided with reference to the accompanying drawings, but is not limited to the following description, and any modifications or equivalent substitutions of the present invention should be included in the scope of the present invention without departing from the spirit and scope of the present invention.
The invention provides a photoacoustic spectrum gas detection device based on a differential diffuse reflection integrating sphere, which is shown in fig. 1, and comprises a distributed feedback semiconductor laser 1, a laser collimator 2, a differential diffuse reflection spherical resonance photoacoustic cell 3, a first microphone 4, a second microphone 5, a differential amplifier 6, a control and data acquisition system 7 and a computer 8, wherein:
the differential diffuse reflection spherical resonance photoacoustic cell 3 comprises a first diffuse reflection sphere and a second diffuse reflection sphere, wherein the first diffuse reflection sphere is provided with an air inlet and an optical inlet, the second diffuse reflection sphere is provided with an air outlet, and the first diffuse reflection sphere and the second diffuse reflection sphere are communicated through a connecting pipe;
the first microphone 4 and the second microphone 5 are respectively arranged at two ends of a first diffuse reflection sphere and a second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell;
the distributed feedback semiconductor laser 1 outputs modulated laser, the laser is collimated by the laser collimator 2 and then enters a first diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3 through a light inlet, the differential diffuse reflection spherical resonance photoacoustic cell 3 is caused to vibrate after absorbing laser energy by target gas to be detected in the differential diffuse reflection spherical resonance photoacoustic cell 3, photoacoustic signals are generated from the first diffuse reflection sphere and a second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3, two paths of photoacoustic signals in the first diffuse reflection sphere and the second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3 are detected by the first microphone 4 and the second microphone 5 respectively, the detected two paths of photoacoustic signals are transmitted to the differential amplifier 6 for differential operation, and the control and data acquisition system 7 acquires the photoacoustic signals subjected to differential operation and processes the photoacoustic signals by the computer 8 to invert the concentration of the target gas to be detected. The specific implementation process is as follows:
step one: the laser controller controls the output wavelength of the distributed feedback semiconductor laser 1 and the output power of the laser; modulating the laser light source of the distributed feedback semiconductor laser 1 with a superimposed signal generated by a sawtooth wave of a low frequency and a sine wave of a high frequency; and scanning and optimizing the resonance frequency of the differential diffuse reflection spherical resonance photoacoustic cell 3 and the modulation depth of the photoacoustic spectrum gas detection device based on the differential diffuse reflection integrating sphere by adopting a control and data acquisition system.
Step two: the laser output by the distributed feedback semiconductor laser 1 is changed into a parallel collimated beam through the laser collimator 2, and the collimated beam is incident into the differential diffuse reflection spherical resonance photoacoustic cell 3 containing the target gas to be detected to excite the target gas to be detected.
Step three: the laser excites the target gas to be detected in the first diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3 to generate a photoacoustic effect, and photoacoustic signals generated in the first diffuse reflection sphere and the second diffuse reflection sphere are detected by the first microphone 4 and the second microphone 5 and transmitted to the differential amplifier 6 to perform differential operation, and finally transmitted to the control and data acquisition system 7.
Step four: the control and data acquisition system 7 acquires the acoustic signals and processes the acoustic signals by the computer 8 to invert the concentration of the target gas to be detected.
In the invention, the diameters of the first diffuse reflection sphere and the second diffuse reflection sphere are required to be less than 26mm, and the resonance frequency of the differential diffuse reflection spherical resonance photoacoustic cell 3 is larger than 1kHz.
In the invention, the length of the connecting pipe between the first diffuse reflection ball and the second diffuse reflection ball is 1-15 mm, and the diameter is 1-10 mm.
In the invention, the diameters of the sound detection holes of the first diffuse reflection ball and the second diffuse reflection ball are 0.2-5 mm.
In the invention, in order to avoid interference caused by gas flow noise and realize high performance of the diffuse reflection spherical resonance photoacoustic cell 3, the diameters of the air inlet and the air outlet of the differential diffuse reflection spherical resonance photoacoustic cell 3 are 1-6 mm.
In the invention, the position of the light inlet is not particularly required. In the actual production design, if the detection port of the first microphone 4 is set to be the 0 ° port, the light entrance port is generally located at the 90 ° port of the first diffuse reflection sphere.
In the invention, the pressure in the differential diffuse reflection spherical resonance photoacoustic cell 3 is between 50 and 500Torr, and the specific pressure value is determined according to the measured relaxation time of gas molecules.
In the invention, the differential diffuse reflection spherical resonance photoacoustic cell 3 needs to be kept at a constant temperature, and the whole temperature of the differential diffuse reflection spherical resonance photoacoustic cell 3 needs to be kept between 20 and 35 ℃.
In the invention, the first microphone 4 and the second microphone 5 are required to be placed at two ends of the first diffuse reflection sphere and the second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3, and the distance between the first microphone 4 and the second microphone 5 and the first diffuse reflection sphere and the second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3 is required to be less than 1mm.
In the present invention, the model and the performance parameters of the first microphone 4 and the second microphone 5 are all required to be completely identical.
In the invention, a high-power laser light source can be used for improving the signal-to-noise ratio of the system or the power of laser is amplified, and the laser power is more than 10mW.
In the present invention, the diffuse reflection coating of the differential diffuse reflection spherical resonance photoacoustic cell 3 includes, but is not limited to, PTFE (e.g., spectraflect, spectralon, infragold and BaSO 4 Etc. various high diffuse reflectance coatings).
The differential diffuse reflection spherical resonance photoacoustic Chi Guangsheng spectrum gas detection device is adopted to detect acetylene gas, 1530nm laser is adopted, the diameters of a first diffuse reflection ball and a second diffuse reflection ball of the differential diffuse reflection spherical resonance photoacoustic cell 3 are 24mm, compared with a common photoacoustic cell, the photoacoustic signal obtained through experiments is improved by 1.84 times, the noise is reduced by 50% of the original value, the final signal to noise ratio is improved by 3.68 times, and the related experimental results are shown in fig. 3.
Claims (10)
1. The photoacoustic spectrum gas detection device based on the differential diffuse reflection integrating sphere is characterized by comprising a distributed feedback type semiconductor laser, a laser collimator, a differential diffuse reflection spherical resonance photoacoustic cell, a first microphone, a second microphone, a differential amplifier, a control and data acquisition system and a computer, wherein:
the differential diffuse reflection spherical resonance photoacoustic cell comprises a first diffuse reflection sphere and a second diffuse reflection sphere, wherein the first diffuse reflection sphere is provided with an air inlet and an optical inlet, the second diffuse reflection sphere is provided with an air outlet, and the first diffuse reflection sphere and the second diffuse reflection sphere are communicated through a connecting pipe;
the first microphone and the second microphone are respectively arranged at two ends of a first diffuse reflection sphere and a second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell;
the distributed feedback semiconductor laser outputs modulated laser, the modulated laser is collimated by the laser collimator and then enters a first diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell through an optical inlet, the differential diffuse reflection spherical resonance photoacoustic cell is vibrated after the target gas to be detected in the differential diffuse reflection spherical resonance photoacoustic cell absorbs laser energy, photoacoustic signals are generated in the first diffuse reflection sphere and the second diffuse reflection sphere, the first microphone and the second microphone are used for respectively detecting two paths of photoacoustic signals in the first diffuse reflection sphere and the second diffuse reflection sphere, the detected two paths of photoacoustic signals are transmitted to the differential amplifier for differential operation, and the photoacoustic signals after differential operation are collected by the control and data collection system and processed by a computer to invert the concentration of the target gas to be detected.
2. The photoacoustic spectrometry gas detection apparatus based on a differential diffuse reflection integrating sphere according to claim 1, wherein the sphere diameter of the first and second diffuse reflection spheres is <26mm, when the resonance frequency of the differential diffuse reflection spherical resonance photoacoustic cell is greater than 1kHz.
3. The photoacoustic spectrometry gas detection apparatus based on the differential diffuse reflection integrating sphere according to claim 1, wherein the length of the connecting pipe is 1-15 mm and the diameter is 1-10 mm.
4. The photoacoustic spectrometry gas detection apparatus based on the differential diffuse reflection integrating sphere according to claim 1 or 2, characterized in that the acoustic detection hole diameter of the first and second diffuse reflection spheres is 0.2 to 5mm.
5. The photoacoustic spectrometry gas detection device based on the differential diffuse reflection integrating sphere according to claim 1, wherein the diameters of the gas inlet and the gas outlet are 1-6 mm.
6. The photoacoustic spectrometry gas detection apparatus based on the differential diffuse reflection integrating sphere according to claim 1, wherein the gas pressure in the differential diffuse reflection spherical resonance photoacoustic cell is between 50 and 500Torr, and the overall temperature is kept between 20 and 35 ℃.
7. The photoacoustic spectrometry gas detection apparatus based on a differential diffuse reflection integrating sphere according to claim 1, characterized in that the distance of the first and second microphones from the first and second diffuse reflection spheres is <1mm.
8. The photoacoustic spectrometry gas detection apparatus based on a differential diffuse reflection integrating sphere according to claim 1, characterized in that the power of the laser is > 10mW.
9. The photoacoustic spectrometry gas detection apparatus based on the differential diffuse reflection integrating sphere according to claim 1, characterized in that the diffuse reflection coating of the differential diffuse reflection spherical resonance photoacoustic cell is PTFE, spectraflect, spectralon, infragold and BaSO 4 One of various high diffuse reflectance coatings.
10. A method of photoacoustic spectroscopy gas detection based on a differential diffuse reflection integrating sphere using the apparatus of any one of claims 1-9, characterized in that the method comprises the steps of:
step one: the laser controller controls the output wavelength of the distributed feedback semiconductor laser and the output power of the laser; modulating a laser light source of the distributed feedback semiconductor laser by using a superposition signal generated by a low-frequency sawtooth wave and a high-frequency sine wave; scanning and optimizing the resonant frequency of the differential diffuse reflection spherical resonant photoacoustic cell and the modulation depth of the photoacoustic spectrum gas detection device based on the differential diffuse reflection integrating sphere by adopting a control and data acquisition system;
step two: the laser output by the distributed feedback semiconductor laser is changed into a parallel collimated beam through a laser collimator, and the collimated beam is incident into a differential diffuse reflection spherical resonance photoacoustic cell containing the target gas to be detected to excite the target gas to be detected;
step three: the laser excites the target gas to be detected in the first diffuse reflection ball to generate a photoacoustic effect, and the photoacoustic signals generated in the first diffuse reflection ball and the second diffuse reflection ball are detected by the first microphone and the second microphone and transmitted to the differential amplifier to perform differential operation, and finally transmitted to the control and data acquisition system;
step four: the control and data acquisition system acquires the acoustic signals and processes the acoustic signals by a computer, and the concentration of the target gas to be detected is inverted.
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