Photoacoustic gas sensor and process based on multi-optical-path ellipsoidal cavity and secondary amplification resonance

Abstract

The invention discloses a multi-optical-path ellipsoid cavity and secondary amplification resonance-based photoacoustic gas sensor and process, and relates to a photoacoustic gas sensor in the technical field of photoacoustic spectrum and trace gas detection. The device comprises an optical fiber collimator (1), an air hole (2), a laser incidence hole (3), a multi-optical-path ellipsoid reflection cavity (4), a curved beam waist resonance tube (5) and an electrical microphone (6), the optical fiber collimator (1), the laser incidence hole (3), the multi-optical-path ellipsoid reflection cavity (4), the curved beam waist resonance tube (5) and the electrical microphone (6) are connected in sequence; and the air holes (2) are respectively formed in two sides below the multi-optical-path ellipsoid reflection cavity (4). The invention has the following advantages and positive effects: the photoacoustic sensor has the advantages of high utilization rate, high sensitivity, low noise level and good system stability; the invention provides a new thought for a novel resonant high-sensitivity photoacoustic sensor, and has a huge application prospect in the field of photoacoustic spectrum gas detection.

Description

Technical Field

[0001] The invention relates to a photoacoustic gas sensor in the technical field of photoacoustic spectroscopy and trace gas detection, and in particular to a photoacoustic gas sensor based on a multi-optical path ellipsoidal cavity and secondary amplification resonance and a process. Background Art

[0002] Photoacoustic spectroscopy has attracted widespread attention due to its advantages such as high sensitivity, fast response and non-invasive detection. This technology has been applied to many scenarios: gas breathing detection, harmful gas leakage detection and atmospheric environment monitoring. This technology can design the absorption spectrum characteristics of the target gas, identify the molecular fingerprint and its intensity, and then invert the type and concentration of the monitored gas. This technology involves a variety of physical processes: first, the wavelength of the laser light source must be modulated. When the gas to be measured absorbs the modulated laser energy, it will transition from a low energy level to a high energy level. Then it will return to a low energy level from an unstable high energy level. This process is also called non-radiative thermal relaxation. In the process of non-radiative thermal relaxation, a periodic heat source will be generated, thereby generating a periodic pressure wave, and then an acoustic signal will be generated, that is, a photoacoustic signal. This means that the photoacoustic signal carries the internal information of the detected gas, and the gas concentration and type information can be inverted by detecting the acoustic signal. It is worth noting that the original photoacoustic signal generated in this way is extremely weak. Even if it can be detected by an acoustic detector such as an electrical microphone, an optical microphone, or a quartz tuning fork, the signal-to-noise ratio will not be ideal.

[0003] Therefore, it is necessary to introduce a photoacoustic cell for resonant amplification. The photoacoustic cell is divided into a resonant photoacoustic cell and a non-resonant photoacoustic cell. When the light source modulation angular frequency is much smaller than the lowest order orthogonal angular frequency, the photoacoustic cell works in a non-resonant state. The non-resonant photoacoustic cell has a simple structure and low cost, but its detection sensitivity is low, and the detected signal intensity is weak and the accuracy is low. When the light source modulation angular frequency is equal to the orthogonal angular frequency, the resonant photoacoustic cell works in a resonant state, and a standing wave will be formed inside it, which will amplify the originally tiny photoacoustic signal. Therefore, in order to ensure the detection sensitivity and accuracy of the photoacoustic sensor, more resonant photoacoustic cells will be used as the place where light and matter interact. H-type and T-type resonant photoacoustic cells are common resonant photoacoustic cells. The H-type resonant photoacoustic cell is usually composed of two symmetrical buffer chambers and a resonant cavity, which is easy to process. However, due to the limitations of structural characteristics, the H-type photoacoustic cell has a large volume, a long gas diffusion time, and a slow gas response speed. In contrast, the T-type resonant photoacoustic cell consists of a buffer chamber and a resonant cavity, which is smaller in size and has better performance, making it more suitable for use as a small, highly sensitive sensing unit. However, compared with the high-quality factor quartz tuning fork, traditional photoacoustic cells face the problems of low quality factor and insufficient sensitivity; however, the quartz tuning fork has the defect of a higher operating frequency. Therefore, making a photoacoustic cell with high quality and lower operating frequency is of high value to photoacoustic spectroscopy technology.

[0004] Based on this document, Li Zehao, Ni Wenjun, Yang Chunyong, et al. Design and performance analysis of curved waist photoacoustic cell. Chinese Laser, 2021, 1, 48: 0111002. A photoacoustic sensor based on a multi-path ellipsoidal cavity and secondary amplification resonance is designed. The sensor mainly includes a multi-path ellipsoidal reflection cavity (4) and a curved waist resonance tube (5). The multi-path ellipsoidal reflection cavity uses the properties of the ellipse. When a laser beam passes through one of the foci of the ellipse, it will pass through another focus after being reflected by the inner wall, thereby increasing the path for the interaction between the gas and the laser. Considering the optical power loss during reflection on the inner wall, the optical power loss is reduced by coating the inner wall of the multi-path ellipsoidal reflection cavity (4) with a high reflectivity film. After the laser fully interacts with the material, the photoacoustic signal will be initially amplified. The curved resonance tube (5) collects the acoustic signal by introducing the design of hyperbolic eccentricity, which will perform secondary amplification on the generated photoacoustic signal. Compared with ordinary resonance tubes without the introduction of hyperbolas, the sensor has a higher quality factor and a greater sound pressure response.

[0005] In summary, designing a photoacoustic spectroscopy trace gas sensor with ultra-high quality factor, high sensitivity, low detection limit and good system performance has important application value. Summary of the invention

[0006] The purpose of the present invention is to overcome the weak sensitivity of photoacoustic signals in the photoacoustic cell of the prior art and to provide a photoacoustic gas sensor and process based on a multi-optical path ellipsoidal cavity and secondary amplification resonance.

[0007] The present invention utilizes the geometric advantages of the ellipsoid focus characteristics to improve the utilization rate of the laser; and uses a curved beam waist resonator to perform secondary amplification of the photoacoustic signal, thereby improving the detection sensitivity and detection limit. This provides a new idea for the design and production of resonant photoacoustic sensors, and has great application prospects in the field of photoacoustic spectroscopy trace gas detection.

[0008] To achieve the above object, the technical solution of the present invention is:

[0009] The core parts that affect the sensitivity of the photoacoustic sensor mainly include the light source, the photoacoustic cell and the microphone. The present invention uses a multi-optical path ellipsoidal cavity and a secondary amplification resonant photoacoustic cell as the source of the photoacoustic signal. Whether its design is reasonable directly affects the detection performance of the entire sensor.

[0010] 1. Photoacoustic gas sensor based on multi-optical path ellipsoidal cavity and secondary amplification resonance

[0011] The invention comprises an optical fiber collimator, an air hole, a laser incident hole, a multi-optical path ellipsoid reflection cavity, a curved beam waist resonance tube and an electrical microphone;

[0012] The optical fiber collimator, the laser incident hole, the multi-optical path ellipsoid reflection cavity, the curved beam waist resonator and the electrical microphone are connected in sequence;

[0013] Air holes are respectively arranged on both sides of the bottom of the multi-optical path ellipsoid reflection cavity for air intake and air exhaust.

[0014] 2. Process of photoacoustic gas sensor based on multi-optical path ellipsoidal cavity and secondary amplification resonance

[0015] ① Design a multi-path ellipsoid reflection cavity, abandon the traditional integrating sphere reflection cavity, based on the ellipse focus characteristics, that is, when a laser beam passes through one of the foci of the ellipse, the light path after reflection passes through another focus, and this reciprocating cycle can form a multi-path laser reflection, generating a higher photoacoustic signal, such as Figure 2 As shown;

[0016] ② The design of the curved waist resonance tube can effectively amplify the photoacoustic signal concentrated in the reflection cavity through the resonance tube resonance;

[0017] ③The two combinations form a highly efficient photoacoustic sensor with the advantages of high utilization rate, high sensitivity, low noise level and good system stability.

[0018] The present invention has the following advantages and positive effects:

[0019] ① This photoacoustic sensor, based on the focal characteristics of the ellipse, abandons the traditional integrating sphere reflection cavity, enhances the photoacoustic signal, and solves the problems of low laser utilization and low detection performance of existing resonant photoacoustic sensors;

[0020] ② The design of the curved waist resonator can effectively amplify the sound pressure signal from the photoacoustic signal concentrated in the reflection cavity through the resonance of the resonator; the use of a stable electrical microphone to detect the sound pressure signal solves the problem of low sensor performance and detection limit;

[0021] ③ Compared with the existing photoacoustic sensors based on tube-ball coupling, this photoacoustic sensor has the advantages of high utilization rate, high sensitivity, low noise level and good system stability;

[0022] ④ The present invention provides a new idea for a novel resonant high-sensitivity photoacoustic sensor and has great application prospects in the field of photoacoustic spectroscopy gas detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the results of the photoacoustic sensor, in which:

[0024] 1—Fiber collimator;

[0025] 2—stomata;

[0026] 3—Laser entrance hole;

[0027] 4—Multi-path ellipsoidal reflection cavity;

[0028] 5—curved waist resonant tube;

[0029] 6—Electrical microphone.

[0030] Figure 2 It is the optical path of the laser in the multi-optical path ellipsoidal cavity of the present invention, which is simulated based on the elliptical focus characteristics.

[0031] Figure 3 It is a sound pressure signal curve excited by the multi-path ellipsoid cavity and the secondary amplified resonant photoacoustic cell in the present invention at different frequencies, and a sound pressure comparison diagram at the edge of the end of the curved waist resonance tube and the top of the multi-path ellipsoid reflection cavity. DETAILED DESCRIPTION

[0032] The following is a detailed description with reference to the accompanying drawings and embodiments.

[0033] 1. Photoacoustic sensor based on multi-optical path ellipsoidal cavity and secondary amplification resonance

[0034] 1. Overall

[0035] like Figure 1 The present invention comprises a fiber collimator 1, an air hole 2, a laser incident hole 3, a multi-optical path ellipsoid reflection cavity 4, a curved beam waist resonator 5 and an electrical microphone 6;

[0036] The optical fiber collimator 1, the laser incident hole 3, the optical path ellipsoid reflection cavity 4, the curved beam waist resonator 5 and the electrical microphone 6 are connected in sequence;

[0037] Air holes 2 are respectively arranged on both sides of the bottom of the multi-optical path ellipsoid reflection cavity 4 .

[0038] 2. Functional components

[0039] 1) Fiber collimator 1

[0040] The fiber collimator 1 is a small cylindrical structure integrating an optical fiber and a lens to ensure the correct alignment and focusing of the light beam. It is located at the laser entrance hole 3 on the right side of the lower end of the multi-path ellipsoid reflection cavity 4. It is used to vertically inject the laser beam into the focus of the multi-path ellipsoid reflection cavity 4 to ensure the stability and collimation of the light spot for accurate photoacoustic gas excitation.

[0041] 2) Air hole 2

[0042] The pore 2 plays an important role in the photoacoustic spectroscopy system and is used to control the properties and state of the gas around the sample. The pore 2 can be used to control the pressure, composition, etc. of the gas, thereby affecting the photoacoustic signal response of the sample. The pores are located on both sides below the optical path ellipsoidal reflection cavity 4, with the right side for air intake and the left side for air exhaust, to ensure the accuracy and stability of the measurement. The detection and analysis of the photoacoustic signal can be optimized through the design and adjustment of the pores.

[0043] 3) Laser entrance hole 3

[0044] The laser incident hole 3 can avoid the alignment of the optical system, and can make the laser beam directly vertically incident on the focus of the ellipsoid, and the laser can be reflected in the multi-optical path ellipsoid reflection cavity 4 according to the pre-set route.

[0045] 4) Multi-path ellipsoidal reflection cavity 4

[0046] The multi-path ellipsoid reflection cavity 4 is generally a place where photoacoustic signals are generated. The present invention utilizes the focal characteristics of the ellipse, that is, when a laser beam passes through one of the foci of the ellipse, the light path after reflection passes through another focus, and this reciprocating cycle can form a multi-path laser reflection, such as Figure 2 As shown; it provides a multi-optical path environment where the laser can fully react with the sample gas to accumulate photoacoustic signals.

[0047] 5) Curved waist resonant tube 5

[0048] The curved beam waist resonator 5 is a key component in the photoacoustic spectroscopy system. It transfers the sound pressure signal from the top of the multi-path ellipsoidal reflection cavity to the end of the curved beam waist resonator 5 and resonates and amplifies it. It accumulates light energy in the multi-path ellipsoidal reflection cavity 4 so that the excited photoacoustic signal can be resonated and enhanced in the cavity. The present invention uses the curved beam waist resonator 5 to collect and amplify the photoacoustic signal generated in the multi-path ellipsoidal reflection cavity 4. This structure can improve the signal-to-noise ratio and increase the sensitivity and accuracy of signal detection.

[0049] 6) Electrical microphone 6

[0050] The electrical microphone 6 is usually used as a sensor for detecting photoacoustic signals, which receives sound waves and vibrates the diaphragm, thereby converting them into electrical signals. The movement of the diaphragm causes the change of current in the generator or sensor connected to it, and finally generates an electrical signal corresponding to the frequency and amplitude of the sound wave.

[0051] It can be seen that the above-mentioned parts each play an important role in the photoacoustic spectroscopy system, and working together can achieve high-sensitivity and high-resolution photoacoustic measurement and analysis.

[0052] In the photoacoustic spectroscopy system, the light emitted by the modulated laser through the fiber collimator 1 is incident on the elliptical focus inside the multi-path ellipsoidal reflection cavity 4 through the laser incident hole 3, interacts with the gas to be measured, and generates a photoacoustic signal through the photoacoustic effect; the generated photoacoustic signal is enhanced by the secondary resonance in the curved beam waist resonance tube 5, so that the electrical microphone 6 collects the photoacoustic signal and converts it into an electrical signal.

[0053] In this system, the performance of the multi-path ellipsoidal reflection cavity 4 and the secondary amplification resonant photoacoustic cell 5 determines the system's comprehensive indicators such as the minimum detection limit, sensitivity, and signal-to-noise ratio. The present invention uses a high-performance multi-path ellipsoidal reflection cavity 4 and a curved beam waist resonant tube 5 to perform secondary amplification on the excited sound pressure signal, thereby improving the performance of the system.

[0054] 2. Process

[0055] A. The semi-major axis and semi-minor axis of the multi-path ellipsoidal reflection cavity are 50 mm and 30 mm respectively;

[0056] B. The curved waist resonance tube introduces the hyperbolic eccentricity e as a new optimization dimension into the inner wall of the traditional resonance tube, which greatly improves the quality factor. The eccentricity e is 9, and the cavity length and bottom radius are 20 mm and 2 mm respectively.

[0057] 3. Working Mechanism

[0058] The optical fiber collimator 1 and the end of the ellipsoidal cavity multi-path reflection cavity 4 are connected through a reserved mechanical laser incident hole 3; the end of the curved waist resonance tube 5 is connected to the top of the ellipsoidal cavity multi-path reflection cavity 4, and the end part is the wave node position, so the electrical microphone 6 is placed at the end edge position of the curved waist resonance tube 5 to connect and perform high-sensitivity acquisition of photoacoustic signals; the air hole 2 is used for gas exchange; the laser is transmitted to the photoacoustic cell body composed of the multi-path ellipsoidal cavity reflection cavity 4 and the curved waist resonance tube 5 through the optical fiber collimator 1, and a photoacoustic effect occurs with the gas to generate a photoacoustic signal, and the maximum sound pressure is formed at the end part of the curved waist resonance tube 5, so the electrical microphone 6 collects the maximum sound pressure signal, and transmits it to the phase-locked amplifier for signal processing to invert and obtain the concentration information of the gas to be measured.

[0059] This sensor is designed with a multi-path ellipsoid reflection cavity, abandoning the traditional integrating sphere reflection cavity. It is based on the elliptical focus characteristics, that is, when a laser beam passes through one of the foci of the ellipse, the light path after reflection passes through another focus. Such a reciprocating cycle can form a multi-path laser reflection and generate a higher photoacoustic signal. The design of the curved waist resonator can effectively amplify the sound pressure signal from the photoacoustic signal concentrated in the reflection cavity through the resonance of the resonator. The combination of the two forms an efficient photoacoustic sensor with the advantages of high utilization rate, high sensitivity, low noise level and good system stability.

[0060] The semi-major axis and semi-minor axis of the multi-path ellipsoid reflection cavity are 50 mm and 30 mm respectively;

[0061] The curved waist resonance tube introduces the hyperbolic eccentricity e as a new optimization dimension in the inner wall of the traditional resonance tube, which greatly improves the quality factor. Its eccentricity e is 9, and the cavity length and bottom radius are 20mm and 2mm respectively.

[0062] Figure 2 It is the sound pressure signal curve excited at different frequencies by the ellipsoid cavity multi-optical path and secondary amplification resonant photoacoustic cell in the present invention, and the sound pressure comparison diagram at the end of the curved waist resonance tube and the top of the multi-optical path ellipsoid reflection cavity. Curve A represents the frequency response of the curved waist resonance tube, and curve B represents the frequency response of the top of the multi-optical path ellipsoid reflection cavity. It can be seen that the sound pressure amplitude excited by the resonant frequency of 6978Hz after secondary amplification by the curved waist resonance tube is larger. This feature can further improve the detection limit of the photoacoustic spectroscopy system in trace gas detection.

[0063] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A photoacoustic sensor based on a multi-optical path ellipsoidal cavity and secondary amplification resonance, characterized in that: It comprises an optical fiber collimator (1), an air hole (2), a laser incident hole (3), a multi-optical path ellipsoid reflection cavity (4), a curved beam waist resonator (5) and an electrical microphone (6); The optical fiber collimator (1), the laser incident hole (3), the multi-optical path ellipsoid reflection cavity (4), the curved beam waist resonator (5) and the electrical microphone (6) are connected in sequence; Air holes (2) for air intake and air exhaust are respectively arranged on both sides of the bottom of the multi-light-path ellipsoid reflection cavity (4).

2. The process based on multi-optical path ellipsoidal cavity and secondary amplified resonant photoacoustic sensor according to claim 1 is characterized in that: ① Design a multi-path ellipsoid reflection cavity, abandon the traditional integrating sphere reflection cavity, based on the ellipse focus characteristics, that is, when a laser beam passes through one of the foci of the ellipse, the light path after reflection passes through another focus, and this reciprocating cycle can form a multi-path laser reflection, generating a higher photoacoustic signal, as shown in Figure 2; ② The design of the curved waist resonance tube can effectively amplify the photoacoustic signal concentrated in the reflection cavity through the resonance tube resonance; ③The two combinations form a highly efficient photoacoustic sensor with the advantages of high utilization rate, high sensitivity, low noise level and good system stability.

3. The process based on multi-optical path ellipsoidal cavity and secondary amplified resonant photoacoustic sensor according to claim 2 is characterized in that: A. The semi-major axis and semi-minor axis of the multi-path ellipsoidal reflection cavity are 50 mm and 30 mm respectively; B. The curved waist resonance tube introduces the hyperbolic eccentricity e as a new optimization dimension into the inner wall of the traditional resonance tube, which greatly improves the quality factor. The eccentricity e is 9, and the cavity length and bottom radius are 20 mm and 2 mm respectively.

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