A photoacoustic gas detection system based on optical path optimization

Abstract

The present invention discloses a photoacoustic gas detection system based on optical path optimization, including a photoacoustic gas probe and a radiation source connected to the photoacoustic gas probe through a lens optical fiber, the photoacoustic gas probe including a substrate and a shell forming a sealed air chamber with the substrate; a light absorption structure is arranged on the inner wall of the shell away from the light transmission hole in the air chamber, and a plurality of arc surface reflection structures are arranged on the inner wall of the shell, and the arc surface reflection structures are distributed at various upper and lower positions of the inner wall of the shell of the air chamber, and the light emitted by the radiation source enters the air chamber through the lens optical fiber, and reaches the light absorption structure after being reflected by the plurality of arc surface reflection structures; an air hole is arranged on the side of the shell of the air chamber, and a waterproof breathable film is covered on the air hole; a microphone is arranged on the substrate in the air chamber. The detection system disclosed by the present invention can utilize lasers with good collimation and high optical power, and can reduce the volume of the photoacoustic gas probe, and at the same time, the optical path optimization in the air chamber can further improve the accuracy, sensitivity and detection limit of the detection system.

Description

Technical Field

[0001] The invention relates to the field of photoacoustic measurement, and in particular to a photoacoustic gas detection system based on optical path optimization. Background Art

[0002] With the continuous development of society, the types of gases people are exposed to in production and life are becoming more and more complex, and gas concentration detection has an increasingly important impact on production and life. Traditional gas detection methods include electrochemical method, electrical method, optical method, etc. Among them, gas sensors based on optical method have many advantages such as high sensitivity and good selectivity.

[0003] The photoacoustic effect refers to the phenomenon that sound pressure is generated after gas absorbs modulated light. Its basic theory is the process from light to heat and then to sound to generate photoacoustic signals. A gas will have a strong absorption peak for light of a certain wavelength, and different types of gases will have strong absorption peaks for light of different wavelengths. After the gas absorbs modulated light, a transition occurs. The molecules that transition to the excited state release this part of energy in some form, and the released energy is converted into heat energy. Gas concentration sensors based on the photoacoustic effect often include four structures: a radiation source, a gas chamber, a closed photoacoustic cell, and a microphone. In a closed photoacoustic cell, the light signal generated by the gas absorbing the modulated light source will generate a heat signal of the same frequency. According to the ideal gas state equation, a pressure signal of the same frequency will be generated accordingly, that is, a photoacoustic signal. The photoacoustic signal will increase with the increase of gas concentration, so the concentration of the gas can be detected according to the size of the photoacoustic signal.

[0004] Traditional photoacoustic gas sensors are often large in size, so that the light has a long enough optical path to be absorbed by gas molecules, so as to ensure its detection accuracy, sensitivity, detection limit and other indicators. At the same time, the volume of the closed photoacoustic cell is often large and the packaging requirements are high, making it difficult to miniaturize the photoacoustic sensor.

[0005] In existing micro-integrated photoacoustic gas sensors, the radiation source is often a MEMS light source, which is convenient for integration, but compared with large solid lasers, semiconductor lasers, etc., the light emitted by MEMS light sources has poor collimation and low optical power. This has brought difficulties to further improve detection indicators such as detection accuracy, detection limit, and detection sensitivity. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a photoacoustic gas detection system based on optical path optimization, so as to utilize lasers with good collimation and high optical power and reduce the volume of the photoacoustic gas probe. At the same time, by optimizing the optical path in the gas chamber, the accuracy, sensitivity, detection limit and other indicators of the detection system can be further improved.

[0007] To achieve the above object, the technical solution of the present invention is as follows:

[0008] A photoacoustic gas detection system based on optical path optimization comprises a photoacoustic gas probe and a radiation source connected to the photoacoustic gas probe through a lens optical fiber, the photoacoustic gas probe comprises a substrate and a shell forming a sealed air chamber with the substrate, a light-transmitting hole is provided on one side wall of the shell of the air chamber, one end of the lens optical fiber having a lens is installed in the light-transmitting hole, and the other end is connected to the radiation source; a light-absorbing structure is provided on the inner side wall of the shell in the air chamber away from the light-transmitting hole, a plurality of arc-surface reflection structures are provided on the inner side wall of the shell of the air chamber, and the arc-surface reflection structures are distributed at upper and lower positions of the inner side wall of the shell of the air chamber, light emitted by the radiation source enters the air chamber through the lens optical fiber, and reaches the light-absorbing structure after being reflected by the plurality of arc-surface reflection structures; an air hole is provided on the side of the shell of the air chamber, and the air hole is covered with a waterproof and breathable membrane; a microphone is provided on the substrate in the air chamber.

[0009] In the above solution, the radiation source is a solid laser or a semiconductor laser.

[0010] In a preferred technical solution, the microphone is a MEMS microphone packaged in the form of a bare chip, and an ASIC chip for controlling and preamplifying the microphone signal and a control element for controlling the microphone are also provided on the substrate in the air chamber.

[0011] In a further technical solution, a light-shielding shell is installed outside the microphone.

[0012] In another preferred technical solution, the microphone is a fiber optic microphone, the fiber optic microphone is connected to the radiation source via an input optical fiber, and the fiber optic microphone is connected to an optical receiver outside the air chamber via an output optical fiber.

[0013] In the above solution, the substrate is a PCB board.

[0014] In the above solution, the shell of the air chamber is made of metal or plastic material and the inner wall is coated.

[0015] In the above scheme, the waterproof and breathable membrane is a molecular sieve membrane having the function of screening molecules.

[0016] In the above solution, the air hole and the light absorbing structure are located on the same inner side wall of the shell of the air chamber.

[0017] In a further technical solution, the radiation source is connected to a plurality of optical fibers via an optical splitter, and each optical fiber is connected to a photoacoustic gas probe to form a distributed photoacoustic gas detection system.

[0018] Through the above technical solution, the photoacoustic gas detection system based on optical path optimization provided by the present invention has the following beneficial effects:

[0019] 1. The inner wall of the shell of the air chamber of the present invention is provided with a plurality of arc-surface reflection structures, which are distributed at upper and lower positions of the inner wall of the shell of the air chamber. The light emitted by the radiation source arranged outside the air chamber is reflected by the plurality of arc-surface reflection structures, so that the light has a sufficiently long optical path to be absorbed by the gas molecules, which is beneficial to the detection of the gas to be tested with a very low concentration. While realizing miniaturization and integration, it is ensured to have extremely high detection limit and sensitivity; at the same time, under the same detection index requirements, the volume of the air chamber can be made smaller, which is beneficial to miniaturization.

[0020] 2. The present invention omits the traditional closed photoacoustic cell and combines the air chamber with the closed photoacoustic cell, which greatly reduces the size and avoids the complex packaging of the closed photoacoustic cell.

[0021] 3. The present invention adopts a solid laser or a semiconductor laser as an external radiation source, and transmits the light into the gas chamber through a lens optical fiber, so as to obtain light with good collimation and high optical power, which is beneficial to improve the detection accuracy, detection limit and detection sensitivity.

[0022] 4. The present invention sets the radiation source outside the gas chamber. Compared with the case where the radiation source is made inside the gas chamber, the present invention can obtain a smaller photoacoustic gas probe. At the same time, setting the radiation source outside the gas chamber can shield the influence of the heat generated by the internal radiation source and reduce the noise in the detection system. Moreover, the external radiation source can be reused, which also saves costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0024] Figure 1 A side view of a photoacoustic gas detection system based on optical path optimization disclosed in Example 1 of the present invention;

[0025] Figure 2 A top view of the photoacoustic gas probe disclosed in Embodiment 1 of the present invention;

[0026] Figure 3 A side view of a photoacoustic gas detection system based on optical path optimization disclosed in Embodiment 2 of the present invention;

[0027] Figure 4 This is a side view of a photoacoustic gas distributed detection system based on optical path optimization disclosed in Example 3 of the present invention.

[0028] In the figure, 1. photoacoustic gas probe; 2. lens optical fiber; 3. radiation source; 4. substrate; 5. gas chamber; 6. shell; 7. light absorption structure; 8. arc reflection structure; 9. air hole; 10. waterproof breathable membrane; 11. MEMS microphone; 12. ASIC chip; 13. control element; 14. optical splitter; 15. optical fiber microphone; 16. input optical fiber; 17. output optical fiber; 18. optical receiver; 19. light transmission hole; 20. light shielding shell; 21. collimated light. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0030] Example 1

[0031] The present invention provides a photoacoustic gas detection system based on optical path optimization, such as Figure 1 As shown, it includes a photoacoustic gas probe 1 and a radiation source 3 connected to the photoacoustic gas probe 1 through a lens optical fiber 2. In this embodiment, the radiation source 3 is a solid laser or a semiconductor laser, which can emit collimated light 21 with high optical power, which is conducive to obtaining the ultimate detection index.

[0032] The photoacoustic gas probe 1 includes a substrate 4 and a housing 6 that forms a sealed air chamber 5 with the substrate 4. The substrate 4 serves to carry internal components and provide circuit connections between internal components. The material may be a PCB board, such as FR4, an organic polymer, an aluminum substrate 4, etc. The housing 6 is a rectangular parallelepiped structure, and may also be a cube, a hemisphere, or the like.

[0033] like Figure 2 As shown, a light-transmitting hole 19 is provided on one side wall of the shell 6 of the air chamber 5, one end of the lens optical fiber 2 having a lens is installed in the light-transmitting hole 19, and the other end is connected to the radiation source 3. The collimated light 21 emitted by the radiation source 3 enters the air chamber 5 after passing through the lens optical fiber 2, and a light with good collimation can be obtained. A light-absorbing structure 7 is provided on the inner side wall of the shell 6 in the air chamber 5 away from the light-transmitting hole 19. The light-absorbing structure 7 can be a light-absorbing coating, such as a material with black color and rough surface characteristics such as black glue. A plurality of arc-surface reflection structures 8 are provided on the inner side wall of the shell 6 of the air chamber 5, and the arc-surface reflection structures 8 are distributed at various upper and lower positions of the inner side wall of the shell 6 of the air chamber 5. The light emitted by the radiation source 3 enters the air chamber 5 through the lens optical fiber 2, and reaches the light-absorbing structure 7 after being reflected by the plurality of arc-surface reflection structures 8 according to the designed optical path, and is finally absorbed by the light-absorbing structure 7. The purpose of designing the light-absorbing structure 7 is to prevent the light from being reflected multiple times in the air chamber 5 and not being absorbed in time, thereby affecting the detection after the next light emission. Since the arc surface reflection structures 8 are distributed at upper and lower positions of the inner wall of the shell 6 of the gas chamber 5, the light can have a sufficiently long optical path to be absorbed by the gas molecules, which is conducive to detecting the gas to be tested with very low concentration.

[0034] The air hole 9 is provided on the side of the shell 6 of the air chamber 5, and the air hole 9 is covered with a waterproof breathable membrane 10, and the shell 6 of the air chamber 5 is connected to the substrate 4 by an airtight method such as bonding, so that a sound-tight structure is formed in the air chamber 5. The waterproof breathable membrane 10 can be made of materials such as polymers, and its characteristics are that it allows the gas in the air chamber 5 to exchange with the outside world while providing acoustic sealing and retaining pressure changes.

[0035] In particular, the waterproof breathable membrane 10 can use a molecular sieve membrane with a molecular screening function, such as a molecular sieve membrane made of zeolite, etc., which is characterized in that gases with molecular pores larger than the pores of the membrane material cannot pass through the molecular sieve membrane, which can isolate interfering gases such as water vapor and can communicate with the external gas environment so that the gas to be detected can enter. In addition, by controlling the pore size of the molecular sieve, different types of gases can be selectively entered.

[0036] A microphone is arranged on the substrate 4 in the air chamber 5. In the embodiment 1 of the present invention, the microphone is a MEMS microphone 11, such as a silicon-based condenser microphone, which is packaged in the form of a bare chip, and an ASIC chip 12 for controlling and preamplifying the signal of the MEMS microphone 11 and a control element 13 for controlling the MEMS microphone 11 are also arranged on the substrate 4 in the air chamber 5. A light-shielding housing 20 that affects the coupled sound is installed on the outside of the MEMS microphone 11, which can shield the interference of light on the MEMS microphone 11.

[0037] The shell 6 can be made of metal, and machining can obtain an air chamber 5 with a curved reflective structure 8 on the inner wall, and then polishing can obtain a smooth reflective surface. For metal materials with low reflectivity, the reflectivity can be further increased by coating (gold, copper, aluminum, etc.) on the surface; or the shell 6 can be made of plastic, and the injection molding process is used to form an air chamber 5 with an inner wall having a curved reflective structure 8. The inner wall after molding can also obtain a smooth reflective surface. At this time, it is necessary to coat the inner wall with a film (gold, copper, aluminum, etc.) to improve the reflectivity.

[0038] In this embodiment, the air hole 9 and the light absorbing structure 7 are located on the same inner wall of the shell 6 of the air chamber 5, which can reduce the influence on light reflection.

[0039] Example 2

[0040] like Figure 3As shown, in this embodiment, the microphone is a fiber optic microphone 15, which is connected to the radiation source 3 through the input optical fiber 16 and the optical splitter 14, and is connected to the optical receiver 18 outside the air chamber 5 through the output optical fiber 17. At this time, the ASIC chip 12 and the control element 13 do not need to be arranged in the air chamber 5. Compared with the commonly used MEMS microphone 11, the fiber optic microphone 15 outputs an optical signal to represent the sound signal. The light emitted by the external radiation source 3 generates an acoustic signal in the air chamber 5 after the optical path is optimized, and the acoustic signal is received by the fiber optic microphone 15, so that the light input to the fiber optic microphone 15 is modulated by the acoustic signal and then reflected, and is received and demodulated by the optical receiver 18, reflecting the sound pressure signal. Compared with Example 1, the probe part in the embodiment has only an optical signal but no electrical signal, the system has good intrinsic safety characteristics, and is safer to use.

[0041] Example 3

[0042] like Figure 4 As shown, the radiation source 3 is connected to multiple lens optical fibers 2 through an optical splitter 14, and each lens optical fiber 2 is connected to a photoacoustic gas probe 1 to form a distributed photoacoustic gas detection system. Distributed detection systems can play their advantages in various scenarios such as smart city construction. At the same time, since the laser can perform frequency sweeping, multiple photoacoustic gas probes 1 can be used for the detection of multiple gases. Since the laser emitted by the laser has high power characteristics, even if it is divided into multiple paths, it does not affect the detection indicators of each path.

[0043] The detection principle of the photoacoustic gas detection system of the present invention is as follows:

[0044] When there is no detection gas in the air chamber 5, the optical signal generated by the collimated light 21 will not be absorbed in the air chamber 5, so no corresponding heat signal will be generated, and no sound pressure signal will be generated. When there is detection gas in the air chamber 5, the detection gas will absorb part of the light, so a corresponding sound pressure signal will be generated in the air chamber 5, which will be detected and output by the microphone. Obviously, according to the photoacoustic theory, in this embodiment, as the concentration of the detection gas in the external environment increases, the photoacoustic signal increases; the concentration of the detection gas can be inferred from the size of the output photoacoustic signal. The beneficial effect of multiple reflections in the optical path optimization design is that the detection gas and light can be fully contacted and absorbed due to the multiple reflections of the light, so the final sound signal will be more obvious. At the same time, since the photoacoustic gas probe 1 designed by the present invention can use a laser emitted by an external radiation source 3 with good collimation and high optical power, combined with the optical path optimization, it is more conducive to achieving the ultimate in indicators such as detection limit, detection accuracy, and sensitivity.

[0045] Obviously, the photoacoustic gas probe 1 without optical path optimization design is limited by the detection accuracy, limit and sensitivity, which makes it difficult to achieve miniaturization and integration; on the contrary, the detection accuracy, sensitivity, limit and other indicators of the small-sized photoacoustic gas probe 1 are not ideal. In the present invention, the multiple reflections of light between the side walls of the air chamber 5 have the effect of optimizing the optical path. In a small size, its average optical path is not inferior to that of a large-sized traditional photoacoustic sensor. While achieving miniaturization and integration, indicators such as detection limit, detection sensitivity, and detection accuracy are taken into account. At the same time, compared with placing the radiation source 3 on the substrate 4 and obtaining the reflective structure between the substrate 4 and the air chamber 5 by electroplating on the substrate 4, the present invention omits the substrate 4 processing step, which greatly saves costs and processing complexity. At the same time, the air chamber 5 is manufactured by a process such as injection molding, which can also reduce costs.

[0046] The performance indicators of existing micro photoacoustic sensors using MEMS light sources are not as ideal as those of photoacoustic sensors using lasers under the same structural design. The present invention uses a laser with good collimation and high optical power emitted by an external radiation source 3 in the photoacoustic gas probe 1, which is further conducive to achieving the ultimate in indicators such as detection accuracy and sensitivity. Compared with common micro photoacoustic sensors, the photoacoustic gas probe 1 of the present invention has a smaller volume because there is no radiation source 3 inside. At the same time, making the radiation source 3 outside the sensor can shield the impact of internal heating and reduce the noise in the detection system. The external radiation source 3 can be reused, which also saves costs.

[0047] Under the same detection index, the size of the photoacoustic gas probe 1 of the present invention is about 10% to 20% of the conventional photoacoustic sensor; under the same size, its detection index can reach several times or even dozens of times that of the conventional micro photoacoustic sensor.

[0048] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A photoacoustic gas detection system based on optical path optimization, characterized in that: It includes a photoacoustic gas probe and a radiation source connected to the photoacoustic gas probe through a lens optical fiber. The photoacoustic gas probe includes a substrate and a shell that forms a sealed air chamber with the substrate. A light-transmitting hole is provided on one side wall of the shell of the air chamber. One end of the lens optical fiber has a lens and is installed in the light-transmitting hole, and the other end is connected to the radiation source. A light-absorbing structure is provided on the inner wall of the shell away from the light-transmitting hole in the air chamber. A plurality of arc-surface reflection structures are provided on the inner wall of the shell of the air chamber. The arc-surface reflection structures are distributed at upper and lower positions of the inner wall of the shell of the air chamber. Light emitted by the radiation source enters the air chamber through the lens optical fiber, and reaches the light-absorbing structure after being reflected by a plurality of arc-surface reflection structures. An air hole is provided on the side of the shell of the air chamber, and the air hole is covered with a waterproof and breathable film. A microphone is provided on the substrate in the air chamber, and the air hole and the light-absorbing structure are located on the same inner wall of the shell of the air chamber.

2. The photoacoustic gas detection system based on optical path optimization according to claim 1, characterized in that: The radiation source is a solid laser or a semiconductor laser.

3. The photoacoustic gas detection system based on optical path optimization according to claim 1, characterized in that: The microphone is a MEMS microphone, which is packaged in the form of a bare chip, and an ASIC chip for controlling and preamplifying the microphone signal and a control element for controlling the microphone are also arranged on the substrate in the air chamber.

4. The photoacoustic gas detection system based on optical path optimization according to claim 3 is characterized in that: A light shielding shell is installed outside the microphone.

5. The photoacoustic gas detection system based on optical path optimization according to claim 1, characterized in that: The microphone is a fiber optic microphone, which is connected to the radiation source via an input optical fiber and connected to an optical receiver outside the air chamber via an output optical fiber.

6. The photoacoustic gas detection system based on optical path optimization according to claim 1, characterized in that: The substrate is a PCB board.

7. The photoacoustic gas detection system based on optical path optimization according to claim 1, characterized in that: The shell of the air chamber is made of metal or plastic material and the inner wall is coated.

8. The photoacoustic gas detection system based on optical path optimization according to claim 1, characterized in that: The waterproof and breathable membrane is a molecular sieve membrane with the function of screening molecules.

9. A photoacoustic gas detection system based on optical path optimization according to any one of claims 1 to 8, characterized in that: The radiation source is connected to a plurality of lens optical fibers through an optical splitter, and each lens optical fiber is connected to a photoacoustic gas probe to form a distributed photoacoustic gas detection system.

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