Optical fiber photoacoustic gas sensing system and method suitable for low-pressure environment

Abstract

The invention belongs to the technical field of optical fiber gas sensing, and discloses an optical fiber photoacoustic gas sensing system and method suitable for a low-pressure environment. The system comprises a programmable logic gate array circuit, a distributed feedback laser, a laser transmission optical fiber, an optical fiber photoacoustic sensing probe, a super-radiation light-emitting diode light source, an optical fiber circulator and a spectrograph. The air pressure is monitored according to the linear relation between the resonance frequency of the cantilever beam and the ambient air pressure, the characteristic that the acoustic sensitivity at the resonance position of the cantilever beam is enhanced under the low air pressure is utilized to make up for signal attenuation in the photoacoustic excitation process, and the sensitivity is corrected in combination with the air pressure measured by the cantilever beam. Therefore, high-precision and high-sensitivity detection of trace gas in a low-pressure environment is realized. The system only adopts a single cantilever beam and a single demodulator, has the functions of gas pressure and gas concentration detection, and can realize accurate detection of trace gas in a gas pressure change environment. And an all-fiber scheme is adopted, so that intrinsic safety is realized, and remote measurement and multi-point network group detection are easy.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber gas sensing, and relates to an optical fiber photoacoustic gas sensing system and method suitable for low pressure environment. Background Art

[0002] Photoacoustic spectroscopy provides an indirect method for measuring the light absorption of gas molecules. It has the advantages of high sensitivity, compact structure and flexible system, and plays an important role in homeland security and defense, industrial production process monitoring and environmental pollutant detection. The periodically modulated laser is tuned to the same band as the energy required for the vibrational-rotational energy level transition of the gas molecule to be measured. The gas absorbs light energy and transitions to the excited state, and then returns to the ground state and releases energy through radiationless collision relaxation. In this process, the absorbed light energy is converted into the translational energy of the gas molecules, which is manifested as the periodic expansion and contraction of the local gas on a macroscopic scale. Since the gas photoacoustic signal is closely related to the degree of molecular collision and the physical properties of the gas, the change of air pressure will significantly affect the gas photoacoustic signal. Literature Research on temperature and pressure interference in photoacoustic spectroscopy gas detection. Microwave and Optical Technology Letters, 66.1 (2024): e33971 took the H-type photoacoustic cell as the research object, and studied the relationship between the photoacoustic signal and air pressure. In the air pressure range of 50-100kPa, the signal increases significantly with the increase of air pressure. This is because gas molecules collide more violently in high-pressure environments, and the gas absorption coefficient and the resonant photoacoustic cell constant increase with increasing pressure. Therefore, higher ambient pressure is conducive to gas concentration detection. However, normal pressure and low pressure environments are the main application scenarios for ambient gas detection and diffusion gas detection. Due to changes in terrain and altitude, the ambient pressure is not always close to the standard atmospheric pressure. There are large areas of plateaus and mountainous areas in central and western my country, and the air pressure is significantly lower than the standard atmospheric pressure. For example, the altitude around Mount Everest is higher than 3,000 meters, and the air pressure in Lhasa is only about 64 kPa. Low pressure can seriously affect the detection of photoacoustic signals, resulting in serious degradation of the accuracy and sensitivity of photoacoustic spectroscopy gas detection equipment, and the detection results of the same gas detection device in different regions have large deviations. At the same time, most of the existing photoacoustic gas sensors cannot measure atmospheric pressure at the same time, and lack a method for signal correction based on ambient pressure. Therefore, it is of great application significance to design a fiber optic photoacoustic gas sensing system suitable for high-altitude and low-pressure environments. Summary of the invention

[0003] The purpose of the present invention is to propose a fiber optic photoacoustic gas sensing system and method suitable for low pressure environments, aiming to solve the problems of low sensitivity, low precision and poor stability of current fiber optic photoacoustic gas sensing solutions in low pressure environments due to signal attenuation. At the same time, the system is a diffusion detection method, which has both ambient air pressure detection and signal calibration functions, and provides a new idea for the application of fiber optic photoacoustic gas sensing technology.

[0004] The principle of the present invention is as follows: The present invention proposes a fiber optic photoacoustic gas sensing system suitable for low pressure environments, in which laser light is incident on a non-sealed non-resonant photoacoustic tube, and environmental pressure monitoring and gas concentration detection are achieved through a micro cantilever beam diaphragm. The photoacoustic pressure wave P excited in the non-sealed non-resonant photoacoustic tube pa The change with frequency f can be expressed as:

[0005]

[0006] Where P0, L and V represent the laser power, the length and volume of the photoacoustic tube, respectively. γ(p) is the heat capacity ratio of the background gas (usually air), which is a function of the air pressure p. β1(p) and β2(p) describe the thermal and acoustic damping in the generation of the photoacoustic signal. When the volume and temperature are fixed, a decrease in air pressure will result in a decrease in gas molecules. The reduced probability of gas molecule collisions causes a decrease in thermal conductivity, gas density and sound wave propagation velocity in the gas medium. In addition, the attenuation degree of constant-pressure heat capacity with air pressure is much higher than the attenuation degree of constant-volume heat capacity, and the heat capacity ratio of the background gas is positively correlated with the ambient air pressure. Therefore, for the air background, the thermal and acoustic-related damping increases with decreasing air pressure, which causes the excited photoacoustic pressure wave to decrease with decreasing air pressure. The frequency response function of the rectangular cantilever beam can be expressed as:

[0007]

[0008] Where F0 represents the amplitude of the cantilever beam driving force. m represents the mass of the cantilever beam. c is the first resonant frequency of the cantilever beam. c (p) is the damping of the cantilever beam when it vibrates. Damping consumes the vibration energy of the cantilever beam and thus weakens the acoustic sensitivity. The reduction in air pressure increases the compressibility of the gas, resulting in a decrease in viscous damping. This increases the acoustic sensitivity of the cantilever. According to the cantilever beam frequency response formula, when the cantilever beam operates at the first-order resonant frequency f c At , the acoustic sensitivity reaches the maximum. In addition, the acoustic sensitivity at the resonant frequency of the cantilever is affected by the resonant frequency. The lower the resonant frequency, the higher the acoustic sensitivity at resonance. Therefore, according to the characteristic that the cantilever response at the resonant frequency increases with the decrease of air pressure, the attenuation of the signal during the photoacoustic excitation process can be compensated, and the detection enhancement of the gas photoacoustic signal in a low-pressure environment can be achieved. The first-order resonant frequency f of the cantilever isc can be written as:

[0009]

[0010] Among them, t, l, w and A c are the geometric parameters of the cantilever beam, which are thickness, length, width and surface area. E is the Young's modulus of the cantilever beam material. V* represents the effective volume of the air cavity on both sides of the cantilever beam. V ad Represents the volume of gas attached to the cantilever beam, and ρ is the gas density. In the air pressure range of 50kPa-120kPa, the resonant frequency of the cantilever beam is linearly related to the air pressure. The decrease in air pressure causes the resonant frequency of the cantilever beam to move to a lower frequency.

[0011] During the detection process, a variable frequency sinusoidal modulated current is first provided to the laser, and the laser generates a solid photoacoustic signal on the wall of the photoacoustic tube. The solid photoacoustic signal is frequency scanned, and the frequency corresponding to the maximum value of the detected signal is the resonant frequency of the cantilever beam. The ambient air pressure can be obtained using the linear relationship between the resonant frequency and the air pressure:

[0012] p=Mfc+N (4)

[0013] Where M and N are the slope and intercept of the linear fit, respectively. M represents the change in gas pressure when the resonant frequency of the cantilever moves 1 Hz toward high frequency. The laser modulation frequency is then locked at half of the resonant frequency of the cantilever, and the second harmonic-wavelength modulation spectroscopy technique is used to detect the gas photoacoustic signal. When the modulated laser irradiates the gas in the photoacoustic tube, the gas absorbs light energy and transitions to a high energy level, releasing heat in the process of returning to a low energy level. The gas expands due to heat, generating periodic photoacoustic pressure waves that are detected by the cantilever. The spectrometer demodulates the vibration amplitude of the cantilever and performs phase-locked amplification processing to obtain the second harmonic amplitude of the photoacoustic signal. Since the air pressure affects both the photoacoustic signal excitation and the cantilever detection, the system's responsiveness to the gas to be measured is a function of the air pressure. The concentration C of the gas to be measured can be calculated based on the measured air pressure and the amplitude of the photoacoustic signal:

[0014]

[0015] Where R(p) represents the responsiveness of the system to the gas to be measured, S is the measured gas photoacoustic signal, and B is the background signal generated by the cell wall absorption in gas detection.

[0016] The technical solution of the present invention:

[0017] A fiber optic photoacoustic gas sensing system suitable for low pressure environment comprises a programmable logic gate array circuit 1, a distributed feedback (DFB) laser 2, a laser transmission optical fiber 3, a fiber optic photoacoustic sensing probe 4, a super luminescent diode (SLED) light source 5, an optical fiber circulator 6 and a spectrometer 7; the programmable logic gate array circuit 1 is respectively connected to the distributed feedback (DFB) laser 2 and the spectrometer 7, the distributed feedback (DFB) laser 2 is further connected to the fiber optic photoacoustic sensing probe 4 through the laser transmission optical fiber 3, and the fiber optic photoacoustic sensing probe 4 and the super luminescent diode (SLED) light source 5 are connected to The optical fiber circulator 6 is connected, and the optical fiber circulator 6 is further connected to the spectrometer 7; the programmable logic gate array circuit 1 issues instructions to control the distributed feedback (DFB) laser 2 to output laser; the laser emitted by the distributed feedback (DFB) laser 2 is incident on the optical fiber photoacoustic sensor probe 4 through the laser transmission optical fiber 3; the wide-spectrum light emitted by the super luminescent diode (SLED) light source 5 is transmitted through the optical fiber circulator 6 into the optical fiber photoacoustic sensor probe 4 and reflected, and the reflected light interferes and is then transmitted to the spectrometer 7 through the optical fiber circulator 6; the programmable logic gate array circuit 1 controls the spectrometer 7 to collect the interference spectrum and demodulate the signal.

[0018] The optical fiber photoacoustic sensor probe 4 comprises a gas diffusion hole 8, a photoacoustic tube 9, a sound guide channel 10, an optical fiber collimator 11, a ceramic pin 12 and a cantilever beam type sound wave sensitive diaphragm 13; the optical fiber photoacoustic sensor probe 4 is provided with a gas diffusion hole 8, the gas diffusion hole 8 connects the photoacoustic tube 9 with the ambient air, and the gas to be measured in the environment enters the photoacoustic tube 9 through the gas diffusion hole 8, and at the same time, the air pressure in the photoacoustic tube 9 is balanced with the ambient air pressure through the gas diffusion hole 8; the optical fiber photoacoustic sensor probe 4 is provided with an optical fiber collimator 11 The optical fiber collimator 11 is aligned with the photoacoustic tube 9 arranged inside the optical fiber photoacoustic sensor probe 4. The laser emitted by the distributed feedback (DFB) laser 2 is incident on the photoacoustic tube 9 through the optical fiber collimator 11 to generate photoacoustic pressure waves. The excited photoacoustic pressure waves include solid photoacoustic pressure waves generated by the absorption of the laser by the inner wall of the photoacoustic tube 9 and gas photoacoustic pressure waves generated by the absorption of the gas to be measured in the photoacoustic tube 9. The frequency of the solid photoacoustic pressure wave is the same as the modulation frequency of the laser, and the frequency of the gas photoacoustic pressure wave is the modulation frequency of the laser. The frequency is twice that of the optical fiber photoacoustic sensor probe 4. Different types of photoacoustic pressure waves are obtained by modifying the harmonic order of the digital phase-locked amplifier in the programmable logic gate array circuit 1. The frequency scanning of the photoacoustic pressure wave is achieved by adjusting the modulation frequency of the distributed feedback (DFB) laser 2 and the spectrum acquisition frequency in the programmable logic gate array circuit 1. The sound-conducting channel 10 in the optical fiber photoacoustic sensor probe 4 is located on the top side wall of the photoacoustic tube 9. The photoacoustic pressure wave is transmitted to the cantilever beam type acoustic wave sensitive diaphragm 13 at the end through the sound-conducting channel 10. The optical fiber photoacoustic sensor probe 4 is provided with a ceramic pin 12. The cantilever beam type acoustic wave sensitive diaphragm 13 and the end face of the ceramic pin 12 constitute a miniature Fabry-Perot interferometer. The photoacoustic pressure wave causes the cantilever beam type acoustic wave sensitive diaphragm 13 to vibrate, thereby causing the cavity length of the Fabry-Perot interferometer to change. The change in air pressure will cause the resonant frequency of the cantilever beam type acoustic wave sensitive diaphragm 13 to change. The solid photoacoustic signal is used to track the resonant frequency to detect the air pressure, and the gas photoacoustic signal is used to detect the concentration of the gas to be measured.

[0019] The gas diffusion hole 8 is located at the side wall of the photoacoustic tube 9 , and its diameter is smaller than the radius of the photoacoustic tube 9 .

[0020] The inner surface of the cantilever beam type acoustic wave sensitive diaphragm 13 is gold-plated, the end surface distance between the cantilever beam type acoustic wave sensitive diaphragm 13 and the ceramic pin 12 is in the range of 150 μm-2 mm, and the resonance frequency is in the range of 100 Hz-7.5 kHz.

[0021] The wavelength range of the spectrometer 7 is 1525nm-1570nm, and the maximum frame rate is higher than 15kHz.

[0022] The wavelength range of the super luminescent diode (SLED) light source 5 covers the wavelength range required by the spectrometer 7 .

[0023] The core of the programmable logic gate array circuit 1 is a white light interferometric Fabry-Perot cavity length demodulator and a digital phase-locked amplifier based on fast Fourier transform (FFT), and has the functions of providing a sinusoidal modulation signal of a distributed feedback (DFB) laser 2, performing signal frequency scanning, peak signal extraction, and controlling sampling of a spectrometer 7. The reference signal of the phase-locked amplifier is provided by the laser modulation frequency, and the fundamental frequency and second harmonic signals can be phase-locked.

[0024] A fiber optic photoacoustic gas sensing method suitable for low pressure environment, wherein a distributed feedback (DFB) laser 2 emits a laser with variable modulation frequency into the side wall of a photoacoustic tube 9 to generate a solid photoacoustic signal, causing a cantilever beam type acoustic wave sensitive diaphragm 13 to vibrate; a super luminescent diode (SLED) light source 5 enters a fiber optic photoacoustic sensing probe 4 through a fiber optic circulator 6 and reflects, and the reflected light contains interference light with the frequency and amplitude of the solid photoacoustic signal and is detected by a spectrometer 7, and a programmable logic gate array circuit 1 collects and demodulates the interference spectrum, and the optimal working frequency and ambient pressure of gas detection are obtained by frequency scanning the solid photoacoustic signal; the laser with optimized modulation frequency irradiates the gas molecules to be detected in the photoacoustic tube 9 to excite the gas photoacoustic signal, and the gas photoacoustic signal is detected and enhanced by the cantilever beam type acoustic wave sensitive diaphragm 13, and the measured pressure and the second harmonic component of the gas photoacoustic signal extracted by the programmable logic gate array circuit 1 are used to calculate the concentration of the gas to be detected, and the specific steps are as follows:

[0025] First, the programmable logic gate array circuit 1 drives the distributed feedback (DFB) laser 2 to emit frequency-adjustable laser light, the wavelength of which is far away from the characteristic absorption line of the gas to be measured, and is incident on the photoacoustic tube 9 through the laser transmission optical fiber 3 and the optical fiber collimator 11; due to the solid photoacoustic effect of the tube wall, a signal with the same frequency as the laser modulation frequency is generated, and the signal is transmitted to the cantilever beam type acoustic wave sensitive diaphragm 13 through the sound guide channel 10 and causes it to vibrate, and the cavity length of the Fabry-Perot cavity formed by the cantilever beam type acoustic wave sensitive diaphragm 13 and the ceramic pin 12 changes; the superluminescent diode (SLED) light source 5 emits The wide-spectrum light emitted is incident on the inner surface of the cantilever beam type acoustic wave sensitive diaphragm 13 through the optical fiber circulator 6 and the ceramic pin 12, and is reflected on the inner end surface of the ceramic pin 12 and the inner surface of the cantilever beam type acoustic wave sensitive diaphragm 13 to form Fabry-Perot interference; the interference light is re-coupled into the optical fiber circulator 6 and received by the spectrometer 7; the programmable logic gate array circuit 1 collects the interference spectrum and demodulates the length change of the interference cavity, and extracts the signal with the same frequency as the laser modulation frequency through the phase-locked amplifier. The frequency corresponding to the maximum value of the signal is the resonance frequency of the cantilever beam type acoustic wave sensitive diaphragm 13, and the air pressure is obtained according to the resonance frequency.

[0026] Then, the programmable logic gate array circuit 1 drives the distributed feedback (DFB) laser 2 to emit a laser with a central wavelength being the characteristic spectrum line of the gas to be measured, and controls the laser modulation frequency to be half of the resonance frequency of the cantilever beam type acoustic wave sensitive diaphragm 13; the gas to be measured in the photoacoustic tube 9 absorbs the laser energy to generate a gas photoacoustic pressure wave, which is detected by the cantilever beam type acoustic wave sensitive diaphragm 13; the spectrometer 7 collects the interference spectrum, which is then demodulated by the programmable logic gate array circuit 1 to the dynamic cavity length information of the Fabry-Perot cavity, and the amplitude of the second harmonic signal is obtained through phase-locked amplification operation; the concentration of the gas to be measured is calculated in combination with the measured gas pressure and the amplitude of the photoacoustic signal.

[0027] The beneficial effects of the present invention are as follows: the air pressure is monitored based on the linear relationship between the resonant frequency of the cantilever beam and the ambient air pressure, the acoustic sensitivity at the resonance of the cantilever beam is enhanced under low pressure to compensate for the attenuation of the signal during the photoacoustic excitation process, and the sensitivity is corrected in combination with the air pressure measured by the cantilever beam, thereby achieving high-precision and high-sensitivity detection of trace gases in a low-pressure environment. The system uses only a single cantilever beam and a single demodulator, and has the functions of air pressure and gas concentration detection, and can achieve accurate detection of trace gases in an environment with changing air pressure. The all-fiber solution is inherently safe and easy to use for remote sensing and multi-point network group detection. The high-speed spectral phase demodulation algorithm developed based on white light interferometry has extremely high detection resolution and stability. The present invention provides a highly competitive technical solution for trace gas detection in high-altitude environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the structure of the system of the present invention.

[0029] Figure 2 It is a schematic diagram of a fiber optic photoacoustic sensor probe.

[0030] Figure 3 is the intensity of photoacoustic pressure waves excited under simulated different air pressures.

[0031] Figure 4 This is the simulated cantilever beam frequency response curve under different air pressures.

[0032] Figure 5 It is the response of acetylene gas of different concentrations under different pressures.

[0033] In the figure: 1 programmable logic gate array circuit; 2 DFB laser; 3 laser transmission optical fiber; 4 optical fiber photoacoustic sensor probe; 5 SLED light source; 6 optical fiber circulator; 7 spectrometer; 8 gas diffusion hole; 9 photoacoustic tube; 10 sound guide channel; 11 optical fiber collimator; 12 ceramic pin; 13 cantilever beam type sound wave sensitive diaphragm. DETAILED DESCRIPTION

[0034] The specific implementation of the present invention is described in detail below in conjunction with the technical scheme and the accompanying drawings.

[0035] A fiber optic photoacoustic gas sensing system suitable for low pressure environment mainly comprises a programmable logic gate array circuit 1, a DFB laser 2, a laser transmission optical fiber 3, a fiber optic photoacoustic sensing probe 4, a SLED light source 5, a fiber optic circulator 6 and a spectrometer 7.

[0036] The programmable logic gate array circuit 1 drives the DFB laser 2 to emit a frequency-adjustable laser, the wavelength of the laser is far away from the characteristic absorption line of the gas to be measured, and is incident on the fiber optic photoacoustic sensor probe 4 through the laser transmission optical fiber 3; the solid photoacoustic effect of the inner wall of the probe is used to generate a signal with the same frequency as the laser modulation frequency; the wide-spectrum light emitted by the SLED light source 5 is incident on the fiber optic photoacoustic sensor probe 4 through the optical fiber circulator 6 and then reflected, the reflected light contains the amplitude-frequency information of the solid photoacoustic signal and is received by the spectrometer 7, the programmable logic gate array circuit 1 demodulates the signal and calculates the gas pressure; the optimal operating frequency of the laser is determined and the DFB laser 2 is made to emit a laser with a central wavelength matching the characteristic absorption line of the gas to be measured, the gas molecules to be measured in the fiber optic photoacoustic sensor probe 4 absorb light energy to generate a gas photoacoustic signal, the photoacoustic signal causes the interference spectrum received by the spectrometer 7 to change, the logic gate array circuit 1 collects the interference spectrum and demodulates the amplitude of the gas photoacoustic signal, and further inverts the relationship between the gas pressure, the photoacoustic signal and the gas concentration to obtain the concentration of the gas to be measured.

[0037] The central wavelength of the DFB laser 2 is 1532.83 nm, the output power is 15 mW, and there is a strong acetylene gas absorption line at this wavelength. The laser transmission fiber 3 is a G652 single-mode quartz fiber. The central wavelength of the SLED light source 5 is 1550 nm, and the bandwidth is 50 nm. The wavelength range of the spectrometer 7 is 1525 nm-1570 nm, and the frame rate is 15 kHz.

[0038] The optical fiber photoacoustic sensor probe 4 includes a gas diffusion hole 8, a photoacoustic tube 9, a sound guide channel 10, a fiber collimator 11, a ceramic pin 12 and a cantilever beam type acoustic wave sensitive diaphragm 13. The diameter of the gas diffusion hole 8 is 0.8 mm, and the diameter and length of the photoacoustic tube 9 are 4 mm and 15.5 mm respectively. The cantilever beam type acoustic wave sensitive diaphragm 13 is a built-in structure with a length of 1.6 mm, a width of 0.8 mm, and a thickness of 6 μm. The width of the gap of the etched rectangular cantilever beam is about 6 μm. The ceramic pin 12 is a UPC end face, and the distance between it and the cantilever beam type acoustic wave sensitive diaphragm 13 is 190 μm.

[0039] The above description is only a preferred embodiment of the present invention and is 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 fiber optic photoacoustic gas sensing system suitable for low pressure environment, characterized in that: The fiber photoacoustic gas sensing system comprises a programmable logic gate array circuit (1), a distributed feedback laser (2), a laser transmission optical fiber (3), a fiber photoacoustic sensing probe (4), a superluminescent diode light source (5), a fiber optic circulator (6) and a spectrometer (7); the programmable logic gate array circuit (1) is connected to the distributed feedback laser (2) and the spectrometer (7) respectively; the distributed feedback laser (2) is further connected to the fiber photoacoustic sensing probe (4) via the laser transmission optical fiber (3); the fiber photoacoustic sensing probe (4) and the superluminescent diode light source (5) are both connected to the fiber optic circulator (6); The optical fiber circulator (6) is further connected to a spectrometer (7); the programmable logic gate array circuit (1) issues instructions to control the distributed feedback laser (2) to output laser light; the laser light emitted by the distributed feedback laser (2) is incident on the optical fiber photoacoustic sensor probe (4) through the laser transmission optical fiber (3); the wide-spectrum light emitted by the superluminescent diode light source (5) is transmitted through the optical fiber circulator (6) into the optical fiber photoacoustic sensor probe (4) and reflected, and the reflected light is interfered and then transmitted to the spectrometer (7) through the optical fiber circulator (6); the programmable logic gate array circuit (1) controls the spectrometer (7) to collect the interference spectrum and demodulate the signal.

2. The optical fiber photoacoustic gas sensing system according to claim 1, characterized in that: The optical fiber photoacoustic sensor probe (4) comprises a gas diffusion hole (8), a photoacoustic tube (9), a sound guide channel (10), an optical fiber collimator (11), a ceramic pin (12) and a cantilever beam type sound wave sensitive diaphragm (13); the optical fiber photoacoustic sensor probe (4) is provided with a gas diffusion hole (8), the gas diffusion hole (8) connects the photoacoustic tube (9) and the ambient air, the gas to be measured in the environment enters the photoacoustic tube (9) through the gas diffusion hole (8), and at the same time, the air pressure in the photoacoustic tube (9) is balanced with the ambient air pressure through the gas diffusion hole (8); the optical fiber photoacoustic sensor probe (4) comprises a gas diffusion hole (8), a photoacoustic tube (9), and a sound guide channel (10), an optical fiber collimator (11), a ceramic pin (12) and a cantilever beam type sound wave sensitive diaphragm (13); the optical fiber photoacoustic sensor probe (4) is provided with a gas diffusion hole (8), the gas diffusion hole (8) connects the photoacoustic tube (9) and the ambient air, and the gas to be measured in the environment enters the photoacoustic tube (9) through the gas diffusion hole (8), and at the same time, the air pressure in the photoacoustic tube (9) is balanced with the ambient air pressure through the gas diffusion hole (8); The probe (4) is provided with an optical fiber collimator (11), which is aligned with a photoacoustic tube (9) arranged inside the optical fiber photoacoustic sensing probe (4). The laser light emitted by the distributed feedback laser (2) is incident on the photoacoustic tube (9) through the optical fiber collimator (11) to generate photoacoustic pressure waves. The excited photoacoustic pressure waves include solid photoacoustic pressure waves generated by the inner wall of the photoacoustic tube (9) absorbing the laser light and gas photoacoustic pressure waves generated by the gas to be measured in the photoacoustic tube (9) absorbing the laser light. The frequency of the solid photoacoustic pressure waves is the same as the modulation frequency of the laser light. The gas photoacoustic pressure waves are The frequency of the force wave is twice the modulation frequency of the laser. Different types of photoacoustic pressure waves are obtained by modifying the harmonic order of the digital phase-locked amplifier in the programmable logic gate array circuit (1). The frequency scanning of the photoacoustic pressure wave is achieved by adjusting the modulation frequency of the distributed feedback laser (2) and the spectrum acquisition frequency in the programmable logic gate array circuit (1). The sound-conducting channel (10) in the optical fiber photoacoustic sensor probe (4) is located on the top side wall of the photoacoustic tube (9). The photoacoustic pressure wave is transmitted to the cantilever beam type acoustic wave at the end through the sound-conducting channel (10). The optical fiber photoacoustic sensor probe (4) is provided with a ceramic pin (12), and the cantilever beam type sound wave sensitive diaphragm (13) and the end surface of the ceramic pin (12) form a micro Fabry-Perot interferometer; the photoacoustic pressure wave causes the cantilever beam type sound wave sensitive diaphragm (13) to vibrate, thereby causing the cavity length of the Fabry-Perot interferometer to change, and the change in air pressure causes the resonant frequency of the cantilever beam type sound wave sensitive diaphragm (13) to change, the solid photoacoustic signal is used to track the resonant frequency to detect the air pressure, and the gas photoacoustic signal is used to detect the concentration of the gas to be measured.

3. The optical fiber photoacoustic gas sensing system according to claim 1, characterized in that: The gas diffusion hole (8) is located on the side wall of the photoacoustic tube (9), and its diameter is smaller than the radius of the photoacoustic tube (9).

4. The fiber optic photoacoustic gas sensing system according to claim 1, characterized in that: The inner surface of the cantilever beam type sound wave sensitive diaphragm (13) is gold plated, the end surface distance between the cantilever beam type sound wave sensitive diaphragm (13) and the ceramic pin (12) is in the range of 150 μm-2 mm, and the resonance frequency is in the range of 100 Hz-7.5 kHz.

5. The fiber optic photoacoustic gas sensing system according to claim 1, characterized in that: The wavelength range of the spectrometer (7) is 1525nm-1570nm, and the maximum frame rate is higher than 15kHz.

6. The fiber optic photoacoustic gas sensing system according to claim 1, characterized in that: The wavelength range of the superluminescent diode light source (5) covers the wavelength range required by the spectrometer (7).

7. The fiber optic photoacoustic gas sensing system according to claim 1, characterized in that: The core of the programmable logic gate array circuit (1) is a white light interferometric Fabry-Perot cavity length demodulator based on fast Fourier transform and a digital phase-locked amplifier, and has the functions of providing a sinusoidal modulation signal of a distributed feedback laser (2), performing signal frequency scanning, peak signal extraction, and controlling the sampling of a spectrometer (7); wherein the reference signal of the phase-locked amplifier is provided by the laser modulation frequency, and phase-locking processing can be performed on the fundamental frequency and the second harmonic signal.

8. A fiber optic photoacoustic gas sensing method suitable for low pressure environment, characterized in that: A distributed feedback laser (2) emits a laser with a variable modulation frequency into the side wall of a photoacoustic tube (9) to generate a solid photoacoustic signal, causing a cantilever beam type acoustic wave sensitive diaphragm (13) to vibrate; a superluminescent diode light source (5) enters an optical fiber photoacoustic sensor probe (4) through an optical fiber circulator (6) and is reflected, and the reflected light contains interference light with the frequency and amplitude of the solid photoacoustic signal and is detected by a spectrometer (7); a programmable logic gate array circuit (1) collects and demodulates the interference spectrum, and the optimal operating frequency and ambient air pressure for gas detection are obtained by frequency scanning the solid photoacoustic signal; the laser with optimized modulation frequency irradiates the gas molecules to be detected in the photoacoustic tube (9) to excite the gas photoacoustic signal, and the gas photoacoustic signal is detected and enhanced by the cantilever beam type acoustic wave sensitive diaphragm (13); the measured air pressure and the second harmonic component of the gas photoacoustic signal extracted by the programmable logic gate array circuit (1) are used to calculate the concentration of the gas to be detected, and the specific steps are as follows: First, a programmable logic gate array circuit (1) drives a distributed feedback laser (2) to emit frequency-adjustable laser light, the wavelength of which is far from the characteristic absorption line of the gas to be measured, and is incident on the photoacoustic tube (9) through a laser transmission optical fiber (3) and an optical fiber collimator (11); due to the solid photoacoustic effect of the tube wall, a signal with the same frequency as the laser modulation frequency is generated, and the signal is transmitted to the cantilever beam type sound wave sensitive diaphragm (13) through the sound guide channel (10) and causes it to vibrate, so that the cavity length of the Fabry-Perot cavity formed by the cantilever beam type sound wave sensitive diaphragm (13) and the ceramic pin (12) changes; the wide spectrum light emitted by the superluminescent diode light source (5) Light is incident on the inner surface of the cantilever beam type acoustic wave sensitive diaphragm (13) through the optical fiber circulator (6) and the ceramic plug (12), and is reflected on the inner end surface of the ceramic plug (12) and the inner surface of the cantilever beam type acoustic wave sensitive diaphragm (13) to form Fabry-Perot interference; the interference light is re-coupled into the optical fiber circulator (6) and received by the spectrometer (7); the programmable logic gate array circuit (1) collects the interference spectrum and demodulates the length change of the interference cavity, and extracts a signal with the same frequency as the laser modulation frequency through a phase-locked amplifier. The frequency corresponding to the maximum value of the signal is the resonance frequency of the cantilever beam type acoustic wave sensitive diaphragm (13), and the air pressure is obtained according to the resonance frequency; Then, the programmable logic gate array circuit (1) drives the distributed feedback laser (2) to emit laser light having a central wavelength that is a characteristic spectrum line of the gas to be measured, and controls the laser modulation frequency to be half of the resonance frequency of the cantilever beam type acoustic wave sensitive diaphragm (13); the gas to be measured in the photoacoustic tube (9) absorbs the laser energy to generate a gas photoacoustic pressure wave, which is detected by the cantilever beam type acoustic wave sensitive diaphragm (13); the spectrometer (7) collects the interference spectrum, which is then demodulated by the programmable logic gate array circuit (1) to obtain the amplitude of the second harmonic signal through a phase-locked amplification operation; and the concentration of the gas to be measured is calculated by combining the measured gas pressure and the amplitude of the photoacoustic signal.

Images