Gas detection method and system based on nonlinear resonance photoacoustic cell and optical multi-pass

Abstract

The present invention discloses a gas detection method and system based on a nonlinear resonant photoacoustic cell and an optical multi-pass. The photoacoustic gas detection method adopts a nonlinear resonant photoacoustic cell and a matching optical multi-pass structure; the nonlinear resonant photoacoustic cell contains a nonlinear resonant tube, and the shape of the nonlinear resonant photoacoustic cell is designed to match the shape of the light beam in the optical multi-pass structure to achieve a close combination. The photoacoustic sensing system includes a laser controller, a laser source, a collimator, a nonlinear resonant photoacoustic cell, an optical multi-pass structure, a microphone, a self-made pre-transimpedance amplifier circuit board, a phase-locked amplifier, an acquisition card and a host computer. The nonlinear resonant photoacoustic cell with a high quality factor and optimized shape amplifies the photoacoustic signal, and the optical multi-pass structure is used to improve the effective absorption path, thereby further enhancing the photoacoustic signal. The system has the advantages of low detection limit and high sensitivity.

Description

Technical Field

[0001] The invention belongs to the field of laser technology and relates to a gas detection method and system based on a nonlinear resonance photoacoustic cell and optical multi-pass. Background Art

[0002] Photoacoustic spectroscopy (PAS) has been widely studied due to its advantages such as compact structure, fast response speed, low sample gas consumption, high sensitivity, and continuous real-time and in-situ monitoring. In the fault diagnosis of power transformers, the application of PAS to detect dissolved acetylene gas has received widespread attention.

[0003] The resonant photoacoustic cell greatly improves the detection sensitivity due to its acoustic resonance amplification effect. In addition, since the resonant frequency of the resonant photoacoustic cell is usually designed in the high frequency range (>1 kHz), the environmental noise and 1 / f The effect of noise is greatly reduced. Several variants of resonant photoacoustic cells have been studied in the literature, including cylindrical, T-type, H-type, TT type, and differential Holmholtz type. The above-mentioned resonant photoacoustic cell configurations are limited to cylindrical resonant tubes, which are characterized by a constant cross-section along the tube. However, tubes with different cross-sections will show better acoustic resonance effects. "Nonlinear resonant tubes" refer to axisymmetric closed tubes with a constantly changing cross-section. Compared with cylindrical resonant tubes, nonlinear resonant tubes usually have non-equidistant spectra; their higher resonances are not integer multiples of the fundamental resonance. Gaitan et al. demonstrated that nonlinear resonant tubes reduce the energy transfer from the fundamental to the higher harmonics and significantly increase the pressure amplitude. Lawrenson et al. studied the pressure amplitude in cylindrical resonant tubes and nonlinear resonant tubes (conical, pyramidal hybrid, and spherical). The pressure ratio is defined as the ratio of the maximum absolute pressure to the minimum absolute pressure. The optimal pressure ratio of the pyramidal nonlinear resonant photoacoustic cell exceeds 27, while the pressure ratio of the cylindrical resonant tube is 1.3. Chun et al. numerically investigated conical, ½ cosine, and ¾ cosine nonlinear resonant tubes. The ½ cosine resonant tube had the best performance, with a pressure ratio of 6.5, compared to 1.1 for the cylindrical resonant tube. The ½ cosine suppressed the energy transfer from the fundamental to higher harmonics by more than 22 times compared to the cylindrical resonant tube. In conclusion, nonlinear resonant tubes have better acoustic resonance performance than cylindrical resonant tubes and therefore have great potential for designing into resonant photoacoustic cells.

[0004] Integrating PAS with an optical multipass structure is another strategy to improve detection sensitivity. In a typical optical multipass structure, the laser beam is reflected multiple times between two spherical mirrors to increase the effective absorption path length. Recently, Pan et al. reported an optical multipass structure with 37 reflections and an optical absorption path length of 8.64 m. Zha et al. integrated an optical multipass structure with 30 reflections and an absorption path length of 4.92 m into a differential photoacoustic cell with a volume of 162 mL, and the MDL of methane was 19.9 ppb. Mao et al. proposed an annular dot pattern optical multipass structure with 25 reflections and an absorption path length of 2.5 m. Nevertheless, the above works still have certain limitations. The photoacoustic cell is designed as a larger chamber to load the optical multipass structure inside. The mirror surface is easily contaminated due to direct contact with the gas sample. Summary of the invention

[0005] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a gas detection method and system based on a nonlinear resonant photoacoustic cell and an optical multi-pass. A high-sensitivity photoacoustic sensor for acetylene gas is developed by tightly coupling a high-quality hyperbolic cosine-shaped nonlinear resonant photoacoustic cell with a nearly concentric optical multi-pass structure. The shape of the hyperbolic cosine-shaped nonlinear resonant photoacoustic cell is designed using a hyperbolic cosine function, and the shape parameters are adjustable. A numerical study was conducted on the pressure distribution in the hyperbolic cosine-shaped nonlinear resonant photoacoustic cell to optimize the shape of the hyperbolic cosine-shaped nonlinear resonant photoacoustic cell, thereby producing a high resonance enhancement of the photoacoustic signal. Subsequently, the optimized hyperbolic cosine-shaped nonlinear resonant photoacoustic cell is tightly coupled with the nearly concentric optical multi-pass structure to further enhance the photoacoustic signal. The shape of the nonlinear resonant photoacoustic cell is designed to match the shape of the light beam in the optical multi-pass structure to achieve a tight combination.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A photoacoustic gas detection method based on a nonlinear resonant photoacoustic cell coupled with an optical multi-pass structure, using a nonlinear resonant photoacoustic cell and a matching optical multi-pass structure;

[0008] The nonlinear resonance photoacoustic cell contains a nonlinear resonance tube, and the shape of the nonlinear resonance photoacoustic cell is designed to be closely combined with the shape of the light beam in the optical multi-pass structure.

[0009] The nonlinear resonance tube is an axisymmetric closed tube with a variable cross section, and its shape is described by a nonlinear elementary function or an interpolation spline.

[0010] The nonlinear resonance tube has axial rotational symmetry and is composed of a hyperbolic resonance tube and two buffer cavities; the hyperbolic resonance tube adopts a double horn structure, and the symmetry axis is set along the Z axis. The shape of the hyperbolic resonance tube is given by the upper hyperbolic cosine function, and the δ winding coefficient is used to describe the expansion degree of the resonance tube. There are cylindrical buffer cavities at both ends of the hyperbolic resonance tube to suppress laser window noise.

[0011] The optical multi-pass structure enables multiple reflections of the laser light, using a concave mirror and / or a mirror structure.

[0012] In the optical multi-pass structure, the reflector is placed outside the photoacoustic cell to prevent the humidity of the gas to be measured from affecting the reflectivity of the reflector.

[0013] A photoacoustic sensing system based on a nonlinear resonant photoacoustic cell coupled with an optical multi-pass structure comprises a nonlinear resonant photoacoustic cell and an optical multi-pass structure; the nonlinear resonant photoacoustic cell contains a nonlinear resonant tube, and the shape of the nonlinear resonant photoacoustic cell is closely combined with the shape of a light beam in the optical multi-pass structure.

[0014] The photoacoustic sensing system comprises a laser controller, a laser source, a collimator, a nonlinear resonance photoacoustic cell, an optical multi-pass structure, a microphone, a pre-transimpedance amplifier circuit board, a phase-locked amplifier, an acquisition card and a host computer; the laser controller controls the laser source, the laser source emits laser light to enter the nonlinear resonance photoacoustic cell through the collimator, and is reflected back and forth in the optical multi-pass structure, the microphone is placed in the photoacoustic cell to collect photoacoustic signals, and transmits them to the phase-locked amplifier, the phase-locked amplifier transmits the results to the acquisition card, and then the acquisition card sends them to the computer; the acquisition card is also connected to the laser controller and the phase-locked amplifier, and is responsible for the output of the modulation signal and the reference signal; the photoacoustic sensing system is used to realize laser control, photoacoustic signal excitation and resonance enhancement, effective absorption path enhancement, microphone signal pre-amplification, phase-locked amplifier detection, and display of acetylene concentration inversion results based on LabVIEW program.

[0015] The nonlinear resonant photoacoustic cell is a hyperbolic cosine nonlinear resonant photoacoustic cell, and the resonant tube length, resonant tube radius, δ and buffer cavity length are 40 mm, 3 mm, 120 mm, respectively. -1 and 12 mm, and a resonance frequency of 6871 Hz; the near-concentric optical multi-pass structure consists of two identical concave mirrors with a focal length of 25 mm and a diameter of 25.4 mm, and the two concave mirrors are coaxially positioned and 100 mm apart.

[0016] Beneficial effects of the present invention:

[0017] The present invention provides a photoacoustic gas detection method based on a nonlinear resonant photoacoustic cell coupled with an optical multi-pass structure.

[0018] 1) The resonance enhancement of photoacoustic signals was achieved;

[0019] 2) The effective absorption path is enhanced;

[0020] 3) Achieved high sensitivity and extremely low minimum detection limit for acetylene gas.

[0021] The acetylene gas photoacoustic sensing system constructed by the present invention based on a nonlinear resonant photoacoustic cell coupled optical multi-pass structure realizes high sensitivity and extremely low minimum detection limit detection of acetylene. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the structure of the hyperbolic cosine nonlinear resonance photoacoustic cell.

[0023] Figure 2 Schematic diagram of a nearly concentric optical multi-pass structure.

[0024] Figure 3 Schematic diagram of the structure of the acetylene gas photoacoustic sensing system based on the nonlinear resonant photoacoustic cell coupled optical multi-pass structure.

[0025] In the figure, there are a laser controller 1, a laser source 2, a collimator 3, a nonlinear resonant photoacoustic cell 4, an optical multi-pass structure 5, a microphone 6, a homemade preamplifier circuit board 7, a phase-locked amplifier 8, an acquisition card 9 and a host computer 10. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0027] Example 1: Construction of an acetylene gas photoacoustic sensing system based on a nonlinear resonant photoacoustic cell coupled with an optical multi-pass structure

[0028] In order to achieve a low minimum detection limit for acetylene gas, an acetylene gas photoacoustic sensing system based on a nonlinear resonant photoacoustic cell coupled with an optical multi-pass structure was built.

[0029] Step 1: Design a hyperbolic cosine-shaped nonlinear resonant photoacoustic cell with axial rotational symmetry, consisting of a hyperbolic resonant tube and two buffer cavities. Figure 1As shown in the figure, the hyperbolic resonant tube adopts a double horn structure, and the axis of symmetry is set along the Z axis. The shape of the hyperbolic resonant tube can be given by the upper hyperbolic cosine function, and the δ winding coefficient is used to describe the expansion degree of the resonant tube. There are cylindrical buffer cavities at both ends of the hyperbolic resonant tube to suppress the laser window noise and optimize the shape of the hyperbolic resonant tube, including the nonlinear resonant tube length, buffer cavity length, resonant tube radius and winding coefficient. During the optimization, the finite method is used to solve the FLNS equation to simulate the absolute pressure distribution inside the hyperbolic cosine-shaped nonlinear resonant photoacoustic cell. Finally, the resonant tube length, resonant tube radius, winding coefficient and buffer cavity length of the hyperbolic cosine-shaped nonlinear resonant photoacoustic cell are optimized to 40 mm, 3 mm and 120 m, respectively. -1 and 12 mm. Thus, the resonance frequency is 6871 Hz, and a high sound pressure signal and a high quality factor of 90.4 are obtained. The nonlinear resonant photoacoustic cell can be any other shape except a cylinder.

[0030] Step 2: Design an optical multi-pass structure, such as Figure 2 As shown, the near-concentric optical multi-pass structure consists of two identical concave mirrors with a focal length of 25 mm and a diameter of 25.4 mm. The two concave mirrors are coaxially positioned 100 mm apart to meet the near-concentric condition. By adjusting the angle of the incident light and the tilt angle of the mirror, up to 44 reflections can be achieved; the near-concentric optical multi-pass structure can use concave mirrors with different focal lengths, which can be other structures or mirror components.

[0031] Step 3: Build an acetylene gas photoacoustic sensing system based on a nonlinear resonant photoacoustic cell coupled with an optical multi-pass structure. Figure 3 As shown. The system includes a laser controller 1, a laser source 2, a collimator 3, a nonlinear resonance photoacoustic cell 4, an optical multi-pass structure 5, a microphone 6, a pre-transimpedance amplifier circuit board 7, a phase-locked amplifier 8, an acquisition card 9 and a host computer 10. The laser controller 1 controls the laser source 2, and the laser source 2 emits a laser through the collimator 3 to enter the nonlinear resonance photoacoustic cell 4, and reflects back and forth in the optical multi-pass structure 5. The microphone 6 is placed in the photoacoustic cell to collect the photoacoustic signal and transmit it to the phase-locked amplifier 8. The phase-locked amplifier transmits the result to the acquisition card 9, and then the acquisition card sends it to the computer; the acquisition card 9 is also connected to the laser controller 1 and the phase-locked amplifier 2, and is responsible for the output of the modulation signal and the reference signal; the photoacoustic sensing system is used to realize laser control, photoacoustic signal excitation and resonance enhancement, effective absorption path enhancement, microphone signal preamplification, phase-locked amplification detection, and display of acetylene concentration inversion results based on the LabVIEW program.

[0032] The temperature of the detection laser (1532 nm butterfly distributed feedback tunable laser) is stabilized at 17°C by the laser temperature controller, and the detection laser bias current is 165 mA by the laser current driver, so that its central wavelength is located at the acetylene absorption line. The acquisition card provides modulation and scanning signals. The modulation signal composed of a 0.1 Hz sawtooth wave and a high-frequency sine wave is generated and sent to the laser controller to drive the DFB laser. The temperature controller and current driver are used to stabilize the laser temperature and convert the modulation signal into the laser drive current.

[0033] Step 4: The laser beam is collimated by a fiber collimator with a working distance of 200 mm and then coupled to a near-concentric optical multi-pass structure. Two concave mirrors are coaxially positioned 100 mm apart. Through a high-precision XYR translation stage, the laser beam enters the near-concentric optical multi-pass structure at an oblique angle of 14° to achieve multiple reflections. The hyperbolic cosine-shaped nonlinear resonant photoacoustic cell is tightly coupled with the near-concentric optical multi-pass structure, further enhancing the photoacoustic signal and achieving low detection limit and high sensitivity photoacoustic sensing of acetylene gas.

[0034] Step 4: After acetylene gas is passed into the photoacoustic cell, it absorbs the laser to generate a photoacoustic signal, and a microphone collects the photoacoustic signal;

[0035] Step 5: The signal collected by the microphone is amplified by the preamplifier circuit board and input into the digital lock-in amplifier. The lock-in amplifier demodulates the second harmonic signal according to the reference signal provided by the acquisition card, and collects it to the host computer through the data acquisition card. The LabVIEW program of the host computer realizes the inversion of signal to concentration and the display of results.

[0036] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0037] The above-mentioned implementation case only expresses one embodiment of the present invention, and its description is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be based on the attached claims.

Claims

1. A photoacoustic sensing system based on a nonlinear resonant photoacoustic cell coupled with an optical multi-pass structure, characterized in that: It includes a nonlinear resonance photoacoustic cell and a nearly concentric optical multi-pass structure; the nonlinear resonance photoacoustic cell contains a nonlinear resonance tube, and the shape of the nonlinear resonance photoacoustic cell is closely combined with the shape of the light beam in the nearly concentric optical multi-pass structure; The nonlinear resonance tube has axial rotational symmetry and is composed of a hyperbolic resonance tube and two buffer cavities; the hyperbolic resonance tube adopts a double horn structure, the symmetry axis is arranged along the Z axis, the shape of the hyperbolic resonance tube is given by the upper hyperbolic cosine function, the winding coefficient δ is used to describe the expansion degree of the resonance tube, and both ends of the hyperbolic resonance tube have cylindrical buffer cavities to suppress laser window noise; The nonlinear resonant photoacoustic cell is a hyperbolic cosine nonlinear resonant photoacoustic cell, and the resonant tube length, resonant tube radius, δ and buffer cavity length are 40 mm, 3 mm, 120 mm respectively. -1 and 12 mm, and a resonance frequency of 6871 Hz; the near-concentric optical multi-pass structure consists of two identical concave mirrors with a focal length of 25 mm and a diameter of 25.4 mm, and the two concave mirrors are coaxially positioned and 100 mm apart.

2. The photoacoustic sensing system according to claim 1, characterized in that: It also includes a laser controller, a laser source, a collimator, a microphone, a pre-transimpedance amplifier circuit board, a phase-locked amplifier, an acquisition card and a host computer; the laser controller controls the laser source, the laser source emits laser light through the collimator to enter the nonlinear resonant photoacoustic cell, and reflects back and forth in the optical multi-pass structure, the microphone is placed in the photoacoustic cell to collect photoacoustic signals, and transmits them to the phase-locked amplifier, the phase-locked amplifier transmits the results to the acquisition card, and then the acquisition card sends them to the computer; the acquisition card is also connected to the laser controller and the phase-locked amplifier, and is responsible for the output of the modulation signal and the reference signal; the photoacoustic sensing system is used to realize laser control, excitation and resonance enhancement of photoacoustic signals, improvement of effective absorption paths, microphone signal preamplification, phase-locked amplification detection, and display of acetylene concentration inversion results based on LabVIEW program.

3. A photoacoustic gas detection method, characterized in that: The photoacoustic sensing system according to claim 2 is used for detection.

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