Photoacoustic Spectroscopy Gas Detection Device and Method Based on Differential Diffuse Reflection Integrating Sphere

Abstract

The present invention discloses a photoacoustic spectroscopy gas detection device and method based on a differential diffuse reflection integrating sphere. The modulated laser output by a distributed feedback semiconductor laser is incident into the first diffuse reflection sphere of a differential diffuse reflection spherical resonance photoacoustic cell after passing through a laser collimator. After the target gas to be measured absorbs the laser energy, it causes the differential diffuse reflection spherical resonance photoacoustic cell to vibrate, and photoacoustic signals are generated in the first diffuse reflection sphere and the second diffuse reflection sphere. A first microphone and a second microphone are used to respectively detect the two-way photoacoustic signals in the first diffuse reflection sphere and the second diffuse reflection sphere, and the two detected photoacoustic signals are transmitted to a differential amplifier for differential operation. A control and data acquisition system collects the photoacoustic signals after differential operation and processes them by a computer to invert the concentration of the target gas to be measured. The detection device of the present invention has the advantages of large signal intensity, strong noise suppression ability, high system sensitivity, optical path free of adjustment, and small volume.

Description

Technical Field

[0001] The present invention relates to a photoacoustic spectroscopy gas detection device and method, and particularly to a photoacoustic spectroscopy gas detection device and method based on a differential diffuse reflection integrating sphere. Background Art

[0002] With the continuous development of science and technology, absorption spectroscopy has stepped onto the "high-speed road" of development. As an important branch of absorption spectroscopy, photoacoustic spectroscopy technology mainly detects the part of the energy generated by gas absorption of light energy, which is manifested as acoustic pressure in the form of heat energy. It is an ideal technology without background noise signal, with excellent sensitivity and good selectivity, and is an ideal choice for trace gas detection.

[0003] The photoacoustic cell is the core part of the photoacoustic spectroscopy experiment, and its design directly affects the sensitivity of the detection signal. Currently, the photoacoustic cell mainly adopts the resonant type, and the main geometric shapes of the existing resonant photoacoustic cells are cylindrical, spherical and square. Among them, the cylindrical photoacoustic cell has large acoustic losses and a low Q value; the acoustic wave attenuation in the square photoacoustic cell is serious; the spherical photoacoustic cell has received extensive attention from scholars due to its high Q value and low acoustic losses. With the continuous development of the photoacoustic cell, in order to increase the interaction path between light and matter, a diffuse reflection spherical resonant photoacoustic cell has emerged. At the same time, in recent years, the differential characteristics that can suppress incoherent noise have also been widely studied by scholars.

[0004] The photoacoustic spectroscopy technology based on the diffuse reflection spherical resonant photoacoustic cell is a high-sensitivity trace gas detection technology. First, a tunable distributed feedback semiconductor laser is selected as the excitation light source of the photoacoustic signal, and a modulated laser beam is sent into the interior of the diffuse reflection spherical resonant photoacoustic cell filled with the sample to be measured through a collimator to excite the sample to be measured. When the laser beam enters the interior of the sphere, with the accumulation of time, the beam is uniformly reflected inside the sphere, and finally the beam inside the sphere becomes a quite uniform diffuse beam. The sample absorbs the light energy and de-excites by releasing heat energy. The released heat energy causes the sample and the surrounding medium to be periodically heated at the modulation frequency of the light, thereby causing the medium to generate periodic pressure fluctuations. The generated periodic pressure fluctuations are detected by a sensitive microphone and amplified by a preamplifier to obtain a photoacoustic signal. Finally, the generated photoacoustic signal is demodulated using the method of correlation demodulation, and then the concentration of the sample to be measured is inversely calculated.

[0005] In order to expand the detection range of the photoacoustic spectroscopy gas technology, it is necessary to improve its sensitivity. However, when the sensitivity of the current photoacoustic spectroscopy system based on the diffuse reflection spherical resonant photoacoustic cell is further improved, there are two major problems:

[0006] 1. The resonance frequency of the existing photoacoustic cell is generally above 1 kHz. Therefore, as the resonance frequency of the photoacoustic cell increases, the low-frequency noise of the system will be significantly reduced. In order to make the resonance frequency higher than 1 kHz, the diameter of the sphere of the diffuse reflection spherical resonance photoacoustic cell cannot be too large. And this constraint limits the increase of the equivalent optical path of the diffuse reflection spherical resonance photoacoustic cell, and its equivalent optical path length L ep is expressed as follows:

[0007]

[0008] where D is the sphere diameter and ρ is the diffuse reflectivity of the coating inside the sphere.

[0009] Therefore, in the design process of the diffuse reflection spherical resonance photoacoustic cell, it is necessary to balance the relationship between the equivalent optical path and the resonance frequency of the cell body. Therefore, the performance of the system cannot be maximally improved, and this problem cannot be perfectly solved.

[0010] 2. The inside of the photoacoustic cell generally uses polytetrafluoroethylene (PTFE), commonly known as the "king of plastics", and its ability to reduce the barrier to external noise is limited. Although the outside of the PTFE inner liner is generally wrapped with an aluminum alloy sphere, due to the limited thickness of the aluminum alloy sphere, the overall noise isolation ability of the diffuse reflection spherical resonance photoacoustic cell is average, resulting in generally large noise in the photoacoustic spectroscopy system based on the diffuse reflection spherical resonance photoacoustic cell. Under the same conditions, the greater the noise, the smaller the signal-to-noise ratio, resulting in poorer detection performance of the system. Therefore, the sensitivity of the photoacoustic spectroscopy system based on the diffuse reflection spherical resonance photoacoustic cell cannot be further improved. Summary of the Invention

[0011] Aiming at the problem that the sensitivity of the current photoacoustic spectroscopy system based on the diffuse reflection spherical resonance photoacoustic cell cannot be further improved, the present invention provides a photoacoustic spectroscopy gas detection device and method based on a differential diffuse reflection integrating sphere. The present invention improves the detection sensitivity of the system by enhancing the photoacoustic signal and simultaneously suppressing the low-frequency noise.

[0012] The object of the present invention is achieved by the following technical solutions:

[0013] A photoacoustic spectroscopy gas detection device based on a differential diffuse reflection integrating sphere, comprising a distributed feedback 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:

[0014] The differential diffuse reflection spherical resonance photoacoustic cell includes a first diffuse reflection sphere and a second diffuse reflection sphere. The first diffuse reflection sphere is provided with an air inlet and a light 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 connected through a connecting pipe;

[0015] The first microphone and the second microphone are respectively placed at two ends of a first diffuse reflection sphere and a second diffuse reflection sphere of a differential diffuse reflection spherical resonance photoacoustic cell;

[0016] The distributed feedback semiconductor laser outputs modulated laser light. After being collimated by a laser collimator, the laser light is incident into the first diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell through the light inlet. After the target gas to be measured in the differential diffuse reflection spherical resonance photoacoustic cell absorbs the laser energy, it causes the differential diffuse reflection spherical resonance photoacoustic cell to vibrate, and 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 to respectively detect the two photoacoustic signals in the first diffuse reflection sphere and the second diffuse reflection sphere, and the two detected photoacoustic signals are transmitted to a differential amplifier for differential operation. The control and data acquisition system collects the photoacoustic signals after differential operation and processes them by a computer, and the concentration of the target gas to be measured is inversely calculated.

[0017] A method for photoacoustic spectroscopy gas detection based on a differential diffuse reflection integrating sphere by using the above device, comprising the following steps:

[0018] Step 1: A laser controller controls the output wavelength and the output power of the distributed feedback semiconductor laser; a superimposed signal generated by a low-frequency sawtooth wave and a high-frequency sine wave is used to modulate the laser light source of the distributed feedback semiconductor laser; a control and data acquisition system scans and optimizes the resonance frequency of the differential diffuse reflection spherical resonance photoacoustic cell and the modulation depth of the photoacoustic spectroscopy gas detection device based on the differential diffuse reflection integrating sphere;

[0019] Step 2: The laser output by the distributed feedback semiconductor laser becomes a parallel collimated light beam through a laser collimator, and the collimated light beam is incident into the differential diffuse reflection spherical resonance photoacoustic cell containing the target gas to be measured to excite the target gas to be measured;

[0020] Step 3: The laser excites the target gas to be measured in the first diffuse reflection sphere to cause the generation of a photoacoustic effect. The first microphone and the second microphone are used to detect the photoacoustic signals generated in the first diffuse reflection sphere and the second diffuse reflection sphere, and are transmitted to a differential amplifier for differential operation, and finally transmitted to the control and data acquisition system;

[0021] Step 4: The control and data acquisition system collects the acoustic signals and processes them by a computer, and the concentration of the target gas to be measured is inversely calculated.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] 1. The present invention combines the differential characteristics that can suppress incoherent noise to design a photoacoustic spectroscopy detection system based on a differential diffuse reflection spherical resonance photoacoustic cell. The core of the system is a differential diffuse reflection spherical resonance photoacoustic cell composed of two diffuse reflection spheres and a connecting tube. When the modulated laser beam is incident on the excitation cavity of the differential diffuse reflection spherical resonance photoacoustic cell filled with the sample to be measured, the sample to be measured in the excitation cavity absorbs the incident laser to generate a periodic acoustic signal. The acoustic signal is transmitted through the middle connecting tube to the inside of another diffuse reflection sphere filled with the sample to be measured, thereby causing the other diffuse reflection sphere to vibrate. Two sensitive microphones are used to detect the acoustic signals generated inside the two diffuse reflection spheres respectively and transmit them to the lock-in end for demodulation, and finally the computer terminal is used for data processing. During this process, the laser beam undergoes diffuse reflection inside the diffuse reflection sphere serving as the excitation cavity. Due to the high diffuse reflectivity of PTFE, with the accumulation of time, the light beam inside the sphere finally becomes a quite uniform diffuse beam. According to the Lambert-Beer law, as the interaction path between light and matter continuously increases, that is, the optical path increases, the absorption intensity and the photoacoustic signal also increase. The differential characteristics further achieve the effect of doubling the enhancement of the photoacoustic signal, and at the same time, the differential mode has the characteristics of suppressing incoherent noises such as gas flow noise and optical noise caused by multiple reflections of the excitation beam.

[0024] 2. The detection device of the present invention has the advantages of large signal intensity, strong noise suppression ability, high system sensitivity, no need for optical path adjustment, small volume, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic structural diagram of a photoacoustic spectroscopy gas detection device based on a differential diffuse reflection integrating sphere;

[0026] Figure 2 is a schematic structural diagram of a differential diffuse reflection spherical resonance photoacoustic cell;

[0027] Figure 3 is the experimental result of detecting acetylene gas by using a photoacoustic spectroscopy gas detection device with a differential diffuse reflection spherical resonance photoacoustic cell. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings, but are not limited thereto. Any modification or equivalent replacement of the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention shall be covered by the protection scope of the present invention.

[0029] The present invention provides a photoacoustic spectroscopy gas detection device based on a differential diffuse reflection integrating sphere, as shown in Figure 1As shown in the figure, the device includes 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, where:

[0030] The differential diffuse reflection spherical resonance photoacoustic cell 3 includes a first diffuse reflection sphere and a second diffuse reflection sphere. The first diffuse reflection sphere is provided with an air inlet and a light inlet, and the second diffuse reflection sphere is provided with an air outlet. The first diffuse reflection sphere and the second diffuse reflection sphere are communicated through a connecting pipe;

[0031] The first microphone 4 and the second microphone 5 are respectively placed at both ends of the first diffuse reflection sphere and the second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell;

[0032] The distributed feedback semiconductor laser 1 outputs modulated laser, which is collimated by the laser collimator 2 and then enters the first diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3 through the light inlet. The target gas to be measured in the differential diffuse reflection spherical resonance photoacoustic cell 3 absorbs the laser energy, causing the differential diffuse reflection spherical resonance photoacoustic cell 3 to vibrate, and photoacoustic signals are generated in the first diffuse reflection sphere and the second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3. The first microphone 4 and the second microphone 5 are used to detect the two photoacoustic signals in the first diffuse reflection sphere and the second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3 respectively, and the two detected photoacoustic signals are transmitted to the differential amplifier 6 for differential operation. The control and data acquisition system 7 collects the photoacoustic signals after differential operation and processes them by the computer 8 to invert the concentration of the target gas to be measured. The specific implementation process is as follows:

[0033] Step 1: The laser controller controls the output wavelength and output power of the distributed feedback semiconductor laser 1; a superimposed signal generated by a low-frequency sawtooth wave and a high-frequency sine wave is used to modulate the laser source of the distributed feedback semiconductor laser 1; the control and data acquisition system is used to scan and optimize the resonance frequency of the differential diffuse reflection spherical resonance photoacoustic cell 3 and the modulation depth of the photoacoustic spectroscopy gas detection device based on the differential diffuse reflection integrating sphere.

[0034] Step 2: The laser output by the distributed feedback semiconductor laser 1 becomes a parallel collimated beam through the laser collimator 2, and the collimated beam enters the differential diffuse reflection spherical resonance photoacoustic cell 3 containing the target gas to be measured to excite the target gas to be measured.

[0035] Step 3: The laser excites the target gas to be measured in the first diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3, causing the generation of a photoacoustic effect. The first microphone 4 and the second microphone 5 detect the photoacoustic signals generated in the first diffuse reflection sphere and the second diffuse reflection sphere, and transmit them to the differential amplifier 6 for differential operation, and finally transmit them to the control and data acquisition system 7.

[0036] Step 4: The control and data acquisition system 7 collects the acoustic signals and processes them by the computer 8 to invert the concentration of the target gas to be measured.

[0037] In the present invention, the sphere diameters of the first diffuse reflection sphere and the second diffuse reflection sphere need to be < 26 mm. At this time, the resonance frequency of the differential diffuse reflection spherical resonance photoacoustic cell 3 is greater than 1 kHz.

[0038] In the present invention, the length of the connecting pipe between the first diffuse reflection sphere and the second diffuse reflection sphere is 1 - 15 mm, and the diameter is 1 - 10 mm.

[0039] In the present invention, the diameters of the acoustic detection holes of the first diffuse reflection sphere and the second diffuse reflection sphere are 0.2 - 5 mm.

[0040] In the present invention, in order to avoid the interference caused by gas flow noise and achieve high performance of the differential 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 both 1 - 6 mm.

[0041] In the present invention, there are no specific requirements for the position of the light inlet. In actual production and processing design, if the detection port of the first microphone 4 is set as the 0° port, the light inlet is generally located at the 90° port of the first diffuse reflection sphere.

[0042] In the present invention, the air pressure inside the differential diffuse reflection spherical resonance photoacoustic cell 3 is between 50 - 500 Torr, and the specific air pressure value is determined according to the relaxation time of the gas molecules to be measured.

[0043] In the present invention, the differential diffuse reflection spherical resonance photoacoustic cell 3 needs to maintain a constant temperature, and the overall temperature of the differential diffuse reflection spherical resonance photoacoustic cell 3 needs to be maintained between 20 - 35 °C.

[0044] In the present invention, the first microphone 4 and the second microphone 5 need to be placed at both 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 distances 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 need to be < 1 mm.

[0045] In the present invention, the models and performance parameters of the first microphone 4 and the second microphone 5 need to be exactly the same.

[0046] In the present invention, to improve the system signal-to-noise ratio, a high-power laser light source can be used or the power of the laser can be amplified, and the laser power should be > 10 mW.

[0047] 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 (such as various high-diffuse reflection rate coatings such as Spectraflect, Spectralon, Infragold, and BaSO4).

[0048] The acetylene gas is detected by using a differential diffuse reflection spherical resonance photoacoustic cell photoacoustic spectroscopy gas detection device. A 1530 nm laser is used. The diameters of the first diffuse reflection sphere and the second diffuse reflection sphere of the differential diffuse reflection spherical resonance photoacoustic cell 3 are both 24 mm. Compared with a common photoacoustic cell, the obtained photoacoustic signal is increased by 1.84 times, the noise is reduced to 50% of the original, and finally the signal-to-noise ratio is increased by 3.68 times. The relevant experimental results are as Figure 3 shown.

Claims

1. A photoacoustic spectroscopy gas detection device based on a differential diffuse reflection integrating sphere, characterized in that The device includes a distributed feedback semiconductor laser, a laser collimator, a differential diffuse spherical resonance photoacoustic cell, a first microphone, a second microphone, a differential amplifier, a control and data acquisition system, and a computer, where: The differential diffuse spherical resonance photoacoustic cell includes a first diffuse sphere and a second diffuse sphere. The first diffuse sphere is provided with an air inlet and a light inlet, and the second diffuse sphere is provided with an air outlet. The first diffuse sphere and the second diffuse sphere are communicated through a connecting pipe; The first microphone and the second microphone are respectively placed at both ends of the first diffuse sphere and the second diffuse sphere of the differential diffuse spherical resonance photoacoustic cell; The distributed feedback semiconductor laser outputs modulated laser, which is collimated by the laser collimator and then enters the first diffuse sphere of the differential diffuse spherical resonance photoacoustic cell through the light inlet. The target gas to be measured in the differential diffuse spherical resonance photoacoustic cell absorbs the laser energy, causing the differential diffuse spherical resonance photoacoustic cell to vibrate, and photoacoustic signals are generated in the first diffuse sphere and the second diffuse sphere. The first microphone and the second microphone are used to respectively detect the two photoacoustic signals in the first diffuse sphere and the second diffuse sphere, and the two detected photoacoustic signals are transmitted to the differential amplifier for differential operation. The control and data acquisition system collects the photoacoustic signals after differential operation and processes them by the computer to invert the concentration of the target gas to be measured.

2. The photoacoustic spectroscopy gas detection device based on a differential diffuse reflection integrating sphere according to claim 1, wherein The sphere diameters of the first diffuse sphere and the second diffuse sphere are < 26 mm, and at this time, the resonance frequency of the differential diffuse spherical resonance photoacoustic cell is greater than 1 kHz.

3. The photoacoustic spectroscopy gas detection device based on a 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 spectroscopy gas detection device based on a differential diffuse reflection integrating sphere according to claim 1 or 2, characterized in that The diameters of the sound detection holes of the first diffuse sphere and the second diffuse sphere are 0.2 - 5 mm.

5. The photoacoustic spectroscopy gas detection device based on a differential diffuse reflection integrating sphere according to claim 1, wherein The diameters of both the air inlet and the air outlet are 1 - 6 mm.

6. The photoacoustic spectroscopy gas detection device based on a differential diffuse reflection integrating sphere according to claim 1, characterized in that The air pressure inside the differential diffuse spherical resonance photoacoustic cell is between 50 - 500 Torr, and the overall temperature is maintained between 20 - 35 °C.

7. The photoacoustic spectroscopy gas detection device based on a differential diffuse reflection integrating sphere according to claim 1, wherein The distances between the first microphone and the second microphone and the first diffuse sphere and the second diffuse sphere are < 1 mm.

8. The photoacoustic spectroscopy gas detection device based on a differential diffuse reflection integrating sphere according to claim 1, characterized in that The power of the laser > 10 mW.

9. The photoacoustic spectroscopy gas detection device based on a differential diffuse reflection integrating sphere according to claim 1, wherein The diffuse reflection coating of the differential diffuse spherical resonance photoacoustic cell is one of various high diffuse reflectivity coatings such as PTFE, Spectraflect, Spectralon, Infragold, and BaSO4.

10. A method for photoacoustic spectroscopy gas detection based on a differential diffuse reflection integrating sphere using the device according to any one of claims 1-9, characterized in that The method includes the following steps: Step 1: The laser controller controls the output wavelength and the output power of the distributed feedback semiconductor laser; uses the superimposed signal generated by a low-frequency sawtooth wave and a high-frequency sine wave to modulate the laser source of the distributed feedback semiconductor laser; uses the control and data acquisition system to scan and optimize the resonance frequency of the differential diffuse spherical resonance photoacoustic cell and the modulation depth of the photoacoustic spectroscopy gas detection device based on the differential diffuse integrating sphere; Step 2: The laser output by the distributed feedback semiconductor laser becomes a parallel collimated beam through the laser collimator, and the collimated beam enters the differential diffuse spherical resonance photoacoustic cell containing the target gas to be measured to excite the target gas to be measured; Step 3: The laser excites the target gas to be measured in the first diffuse reflection sphere, causing the generation of the photoacoustic effect. The first microphone and the second microphone are used to detect the photoacoustic signals generated in the first diffuse reflection sphere and the second diffuse reflection sphere, and are transmitted to the differential amplifier for differential operation, and finally transmitted to the control and data acquisition system; Step 4: The control and data acquisition system collects the acoustic signals and processes them by a computer to invert the concentration of the target gas to be measured.

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