A fiber-optic photoacoustic gas sensing system and method suitable for low-pressure environments

Abstract

The application belongs to the technical field of optical fiber gas sensing, and discloses a kind of optical fiber photoacoustic gas sensing system and method suitable for low pressure environment.System includes programmable logic gate array circuit, distributed feedback laser, laser transmission optical fiber, optical fiber photoacoustic sensing probe, super radiation emitting diode light source, optical fiber circulator and spectrometer.According to the linear relationship between the cantilever beam resonance frequency and the ambient pressure, the gas pressure is monitored, and the acoustic sensitivity at the resonance of the cantilever beam is enhanced at low pressure to compensate for the attenuation of the signal in the photoacoustic excitation process.Combined with the measured pressure of the cantilever beam, the sensitivity is corrected, so as to realize the high precision and high sensitivity detection of trace gas under low pressure environment.The system only uses a single cantilever beam and a single demodulator, and has the functions of pressure and gas concentration detection, and can realize accurate detection of trace gas in a changing pressure environment.The all-fiber scheme has the characteristics of intrinsic safety, and is easy to remote measure and multi-point network detection.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic gas sensing technology, and relates to a fiber optic photoacoustic gas sensing system and method suitable for low-pressure environments. Background Technology

[0002] Photoacoustic spectroscopy provides an indirect method for measuring the light absorption of gas molecules, offering advantages such as high sensitivity, compact structure, and system flexibility. It plays a crucial role in national security and defense, industrial process monitoring, and environmental pollutant detection. A periodically modulated laser is tuned to the same wavelength as the energy required for the vibrational-rotational energy level transition of the gas molecules being measured. The gas absorbs light energy and transitions to an excited state, then returns to the ground state and releases energy through non-radiative collisional relaxation. In this process, the absorbed light energy is converted into the translational kinetic energy of the gas molecules, macroscopically manifested as the periodic expansion and contraction of the local gas. Since the gas photoacoustic signal is closely related to the degree of molecular collisions and the gas's physical properties, changes in gas pressure significantly affect the signal. The literature *Research on temperature and pressure interference in photoacoustic spectroscopy gas detection*, *Microwave and Optical Technology Letters*, 66.1(2024):e33971, using an H-type photoacoustic cell as the research object, investigated the relationship between the photoacoustic signal and gas pressure. Within the pressure range of 50-100 kPa, the signal increases significantly with increasing pressure. This is because gas molecule collisions are more intense under high pressure, and the gas absorption coefficient and the cell constant of the photoacoustic resonant cell increase with increasing pressure. Therefore, higher ambient pressure is beneficial for gas concentration detection. However, atmospheric and low-pressure environments are the main application scenarios for ambient gas detection and diffuse gas detection. Due to variations in terrain and altitude, ambient pressure is not always close to standard atmospheric pressure. Central and western my country have large areas of plateaus and mountains where the air pressure is significantly lower than standard atmospheric pressure. For example, the altitude around Mount Everest is above 3000 meters, while the air pressure in Lhasa is only about 64 kPa. Low air pressure severely affects photoacoustic signal detection, leading to significant degradation in the accuracy and sensitivity of photoacoustic spectroscopy gas detection equipment, and large deviations in detection results from the same gas detection device in different regions. Furthermore, most existing photoacoustic gas sensors cannot simultaneously measure atmospheric pressure and lack methods for signal correction based on ambient pressure. Therefore, designing a fiber optic photoacoustic gas sensing system suitable for high-altitude, low-pressure environments is of great practical significance. Summary of the Invention

[0003] The purpose of this invention is to propose a fiber optic photoacoustic gas sensing system and method suitable for low-pressure environments. It aims to solve the problems of low sensitivity, low accuracy 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 adopts a diffusion detection method and has both environmental pressure detection and signal calibration functions, providing a new approach for the application of fiber optic photoacoustic gas sensing technology.

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

[0005]

[0006] Where P0, L, and V represent the laser power, the length of the photoacoustic tube, and its volume, respectively. γ(p) is the heat capacity ratio of the background gas (usually air), a function of the gas pressure p. β1(p) and β2(p) describe the thermal and acoustic damping during photoacoustic signal generation. With constant volume and temperature, a decrease in gas pressure leads to a reduction in the number of gas molecules. This reduced probability of gas molecule collisions results in a decrease in thermal conductivity, gas density, and sound wave propagation speed in the gas medium. Furthermore, the attenuation of heat capacity at constant pressure is much greater than that at constant volume, and the heat capacity ratio of the background gas is positively correlated with the ambient gas pressure. Therefore, for an air background, the thermal and acoustic damping increases with decreasing gas pressure, which leads to a decrease in the excited photoacoustic pressure wave with decreasing gas 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's driving force. m represents the mass of the cantilever beam. f c It is the first-order resonant frequency of the cantilever beam. β c (p) represents the damping during cantilever beam vibration. Damping consumes the vibrational energy of the cantilever beam, thus reducing its acoustic sensitivity. The decrease in air pressure increases gas compressibility, leading to a reduction in viscous damping. This improves the acoustic sensitivity of the cantilever. According to the cantilever beam frequency response formula, when the cantilever beam operates at its first resonant frequency f... c At the resonant frequency, the acoustic sensitivity reaches its maximum. Furthermore, the acoustic sensitivity at the cantilever beam's resonant frequency is affected by the resonant frequency; the lower the resonant frequency, the higher the acoustic sensitivity at resonance. Therefore, based on the characteristic that the cantilever beam's response at the resonant frequency increases with decreasing air pressure, the signal attenuation during photoacoustic excitation can be compensated, achieving enhanced detection of gas photoacoustic signals under low-pressure environments. The first-order resonant frequency f of the cantilever beam...c It can be written as:

[0009]

[0010] Where t, l, w, and A c These are the geometric parameters of the cantilever beam: 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 cavities on both sides of the cantilever beam. ad ρ represents the volume of gas attached to the cantilever beam, and ρ is the gas density. Within the gas pressure range of 50 kPa to 120 kPa, the resonant frequency of the cantilever beam has a linear relationship with the gas pressure; a decrease in gas pressure causes the resonant frequency of the cantilever beam to shift to lower frequencies.

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

[0012] p = Mfc + N (4)

[0013] In the formula, 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 beam shifts 1 Hz to a higher frequency. Subsequently, the laser modulation frequency is locked at half the resonant frequency of the cantilever beam, and second harmonic-wavelength modulation spectroscopy is used to detect the gas photoacoustic signal. When the modulated laser irradiates the gas inside the photoacoustic tube, the gas absorbs light energy and transitions to a higher energy level, releasing heat as it returns to a lower energy level. The gas expands due to heat, generating periodic photoacoustic pressure waves that are detected by the cantilever beam. The spectrometer demodulates the vibration amplitude of the cantilever beam and performs phase-locked amplification to obtain the second harmonic amplitude of the photoacoustic signal. Since gas pressure simultaneously affects both the excitation of the photoacoustic signal and the detection by the cantilever beam, the system's responsivity to the gas being tested is a function of the gas pressure. The concentration C of the gas being tested can be calculated based on the measured gas pressure and the photoacoustic signal amplitude.

[0014]

[0015] In the formula, R(p) represents the system's responsivity to the gas being measured, S is the measured photoacoustic signal of the gas, and B is the background signal generated by absorption by the cell wall during gas detection.

[0016] The technical solution of this invention:

[0017] A fiber optic photoacoustic gas sensing system suitable for low-pressure environments includes a programmable gate array (PGA) circuit 1, a distributed feedback (DFB) laser 2, a laser transmission fiber 3, a fiber optic photoacoustic sensing probe 4, a superluminescent diode (SLED) light source 5, a fiber optic circulator 6, and a spectrometer 7. The PGA circuit 1 is connected to the DFB laser 2 and the spectrometer 7. The DFB laser 2 is further connected to the fiber optic photoacoustic sensing probe 4 via the laser transmission fiber 3. The fiber optic photoacoustic sensing probe 4 and the SLED light source 5 are both connected to... Fiber optic circulator 6 is connected, and fiber optic circulator 6 is further connected to spectrometer 7; programmable gate array circuit 1 issues commands to control distributed feedback (DFB) laser 2 to output laser; the laser emitted by distributed feedback (DFB) laser 2 is incident on fiber optic photoacoustic sensor probe 4 through laser transmission fiber optic 3; broadband light emitted by superluminescent diode (SLED) light source 5 is transmitted through fiber optic circulator 6 into fiber optic photoacoustic sensor probe 4 and reflected, the reflected light interferes and is then transmitted through fiber optic circulator 6 to spectrometer 7; programmable gate array circuit 1 controls spectrometer 7 to collect interference spectrum and demodulate signal.

[0018] The fiber optic photoacoustic sensor 4 includes a gas diffuser 8, a photoacoustic tube 9, a sound guiding channel 10, a fiber optic collimator 11, a ceramic ferrule 12, and a cantilever beam-type acoustic wave sensitive diaphragm 13. The fiber optic photoacoustic sensor 4 has a gas diffuser 8 that connects the photoacoustic tube 9 to the ambient air. The gas to be measured in the environment enters the photoacoustic tube 9 through the gas diffuser 8, and the air pressure in the photoacoustic tube 9 is balanced with the ambient air pressure through the gas diffuser 8. The fiber optic photoacoustic sensor 4 is equipped with a fiber optic collimator 11. The fiber collimator 11 is aligned with the photoacoustic tube 9 located inside the fiber optic photoacoustic sensing probe 4. The laser emitted by the distributed feedback (DFB) laser 2 is incident on the photoacoustic tube 9 through the fiber collimator 11, generating a photoacoustic pressure wave. The excited photoacoustic pressure wave includes a solid photoacoustic pressure wave generated by the absorption of the laser on the inner wall of the photoacoustic tube 9 and a gas photoacoustic pressure wave generated by the absorption of the gas to be measured inside 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 same as the modulation frequency of the laser. The frequency is twice that of the photoacoustic pressure waves. Different types of photoacoustic pressure waves are obtained by modifying the harmonic order of the digital lock-in amplifier in the programmable gate array circuit 1. The frequency scanning of the photoacoustic pressure waves is achieved by adjusting the modulation frequency of the distributed feedback (DFB) laser 2 and the spectral acquisition frequency in the programmable gate array circuit 1. The sound guide channel 10 in the fiber optic photoacoustic sensor probe 4 is located on the top side wall of the photoacoustic tube 9. The photoacoustic pressure waves are conducted through the sound guide channel 10 to the cantilever beam type acoustic wave sensitive diaphragm 13 at the end. The fiber optic photoacoustic sensor probe 4 is equipped with a ceramic ferrule 12. The end face of the cantilever beam type acoustic wave sensitive diaphragm 13 and the ceramic ferrule 12 constitute a miniature Fabry-Perot interferometer. The photoacoustic pressure waves cause the cantilever beam type acoustic wave sensitive diaphragm 13 to vibrate, thereby causing a change in the cavity length of the Fabry-Perot interferometer. Since 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 on 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 acoustic wave sensitive diaphragm 13 is plated with gold. The distance between the end face of the cantilever beam acoustic wave sensitive diaphragm 13 and the ceramic pin 12 ranges from 150μm to 2mm, and the resonant frequency ranges from 100Hz to 7.5kHz.

[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 superluminescent diode (SLED) light source 5 covers the wavelength range required in the spectrometer 7.

[0023] The core of the programmable gate array circuit 1 is a white-light interferometric Fabry-Perot cavity length demodulator based on Fast Fourier Transform (FFT) and a digital lock-in amplifier. It also functions to provide the sinusoidal modulation signal from the distributed feedback (DFB) laser 2, perform signal frequency scanning, peak signal extraction, and control the sampling of the spectrometer 7. The reference signal of the lock-in amplifier is provided by the laser modulation frequency, enabling phase-locked processing of the fundamental frequency and second harmonic signals.

[0024] A fiber optic photoacoustic gas sensing method suitable for low-pressure environments is disclosed. A distributed feedback (DFB) laser 2 emits a variable-modulation laser beam that enters the sidewall of a photoacoustic tube 9, generating a solid-state photoacoustic signal that causes vibration of a cantilever beam-type acoustic wave sensitive diaphragm 13. A superluminescent diode (SLED) light source 5 enters the fiber optic photoacoustic sensing probe 4 via a fiber optic circulator 6 and is reflected. The reflected light, containing the frequency and amplitude of the solid-state photoacoustic signal, is detected by a spectrometer 7. A programmable gate array (PGA) circuit 1 collects and demodulates the interference spectrum. By frequency scanning of the solid-state photoacoustic signal, the optimal operating frequency for gas detection and the ambient air pressure are obtained. A laser with optimized modulation frequency irradiates the gas molecules in the photoacoustic tube 9, exciting a gas photoacoustic signal. This signal is detected and amplified 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 PGA circuit 1 are used to calculate the concentration of the gas. The specific steps are as follows:

[0025] First, the programmable gate array circuit 1 drives the distributed feedback (DFB) laser 2 to emit a frequency-tunable laser. The laser wavelength is far from the characteristic absorption line of the gas to be measured. The laser light is incident into the photoacoustic tube 9 via the laser transmission fiber 3 and the fiber collimator 11. Due to the solid-state photoacoustic effect of the tube wall, a signal with the same frequency as the laser modulation is generated. The signal is transmitted through the acoustic guide channel 10 to the cantilever beam acoustic wave sensitive diaphragm 13, causing it to vibrate. The cavity length of the Fabry-Perot cavity formed by the cantilever beam acoustic wave sensitive diaphragm 13 and the ceramic ferrule 12 changes. The superluminescent diode (SLED) light source 5 emits light... The broadband light emitted is incident on the inner surface of the cantilever beam acoustic wave sensitive diaphragm 13 through the fiber optic circulator 6 and ceramic ferrule 12, and is reflected at the inner end face of the ceramic ferrule 12 and the inner surface of the cantilever beam acoustic wave sensitive diaphragm 13 to form a Fabry-Perot interference. The interference light is recoupled into the fiber optic circulator 6 and received by the spectrometer 7. The programmable gate array circuit 1 collects the interference spectrum and demodulates the length change of the interference cavity. The signal with the same frequency as the laser modulation frequency is extracted by the lock-in amplifier. The frequency corresponding to the maximum value of the signal is the resonant frequency of the cantilever beam acoustic wave sensitive diaphragm 13. The air pressure is obtained according to the resonant frequency.

[0026] Then, the programmable gate array circuit 1 drives the distributed feedback (DFB) laser 2 to emit laser light with a center wavelength equal to the characteristic spectral line of the gas to be measured, and controls the laser modulation frequency to be half the resonant frequency of the cantilever beam 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 acoustic wave sensitive diaphragm 13; the spectrometer 7 collects the interference spectrum, which is then demodulated by the programmable gate array circuit 1 to obtain the dynamic cavity length information of the Fabry-Perot cavity, and the amplitude of the second harmonic signal is obtained through lock-in amplification; the concentration of the gas to be measured is calculated by combining the measured gas pressure and the amplitude of the photoacoustic signal.

[0027] The beneficial effects of this invention are as follows: Air pressure is monitored based on the linear relationship between the cantilever beam's resonant frequency and ambient air pressure. The enhanced acoustic sensitivity at the cantilever beam's resonance point under low air pressure compensates for signal attenuation during photoacoustic excitation. Sensitivity is corrected by combining the air pressure measured by the cantilever beam, thus achieving high-precision, high-sensitivity detection of trace gases in low-pressure environments. The system uses only a single cantilever beam and a single demodulator, combining air pressure and gas concentration detection functions, enabling accurate detection of trace gases even under varying air pressure conditions. The all-fiber optic solution offers intrinsic safety and facilitates telemetry and multi-point network detection. The developed high-speed spectral phase demodulation algorithm based on white light interferometry exhibits extremely high detection resolution and stability. This invention provides a highly competitive technical solution for trace gas detection in high-altitude environments. Attached Figure Description

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

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

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

[0031] Figure 4 These are the frequency response curves of a cantilever beam under different air pressures in the simulation.

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

[0033] In the figure: 1 Programmable gate array circuit; 2 DFB laser; 3 Laser transmission fiber; 4 Fiber optic photoacoustic sensor probe; 5 SLED light source; 6 Fiber optic circulator; 7 Spectrometer; 8 Gas diffuser; 9 Photoacoustic tube; 10 Sound guide channel; 11 Fiber optic collimator; 12 Ceramic ferrule; 13 Cantilever beam type acoustic wave sensitive diaphragm. Detailed Implementation

[0034] The specific embodiments of the present invention are described in detail below with reference to the technical solutions and accompanying drawings.

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

[0036] Programmable gate array circuit 1 drives DFB laser 2 to emit a frequency-tunable laser with a wavelength far from the characteristic absorption spectral line of the gas to be measured. The laser is incident on fiber optic photoacoustic sensor probe 4 via laser transmission fiber 3. Utilizing the solid-state photoacoustic effect on the inner wall of the probe, a signal with the same frequency as the laser modulation is generated. Broadband light emitted by SLED light source 5 is incident on fiber optic photoacoustic sensor probe 4 via fiber optic circulator 6 and reflected. The reflected light contains the amplitude and frequency information of the solid-state photoacoustic signal and is received by spectrometer 7. Programmable gate array circuit 1 demodulates the signal and calculates the gas pressure. The optimal operating frequency of the laser is determined, and the center wavelength of the laser emitted by DFB laser 2 is matched with the characteristic absorption spectral line of the gas to be measured. The gas molecules in the fiber optic photoacoustic sensor probe 4 absorb the light energy and generate a gas photoacoustic signal. The photoacoustic signal causes a change in the interference spectrum received by spectrometer 7. Programmable gate array circuit 1 acquires the interference spectrum and demodulates the amplitude of the gas photoacoustic signal. Based on the relationship between gas pressure, photoacoustic signal, and gas concentration, the concentration of the gas to be measured is further obtained.

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

[0038] The fiber optic photoacoustic sensing probe 4 includes a gas diffuser 8, a photoacoustic tube 9, a sound guiding channel 10, a fiber optic collimator 11, a ceramic ferrule 12, and a cantilever beam-type acoustic wave sensitive diaphragm 13. The gas diffuser 8 has a diameter of 0.8 mm, and the photoacoustic tube 9 has a diameter of 4 mm and a length of 15.5 mm. The cantilever beam-type acoustic wave sensitive diaphragm 13 is an internal structure with a length of 1.6 mm, a width of 0.8 mm, and a thickness of 6 μm. The width of the etched rectangular cantilever beam slit is approximately 6 μm. The ceramic ferrule 12 is the 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 merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A fiber-optic photoacoustic gas sensing system suitable for low-pressure environments, characterized by, The optical fiber photoacoustic gas sensing system comprises a programmable logic gate array circuit (1), a distributed feedback laser (2), a laser transmission optical fiber (3), an optical fiber photoacoustic sensing probe (4), an super radiation light emitting diode light source (5), an optical fiber circulator (6) and a spectrometer (7); the programmable logic gate array circuit (1) is connected with the distributed feedback laser (2) and the spectrometer (7) respectively, the distributed feedback laser (2) is further connected with the optical fiber photoacoustic sensing probe (4) through the laser transmission optical fiber (3), the optical fiber photoacoustic sensing probe (4) and the super radiation light emitting diode light source (5) are connected with the optical fiber circulator (6), and the optical fiber circulator (6) is further connected with the spectrometer (7); the programmable logic gate array circuit (1) sends a command to control the distributed feedback laser (2) to output laser; the laser emitted by the distributed feedback laser (2) is incident on the optical fiber photoacoustic sensing probe (4) through the laser transmission optical fiber (3); the broadband light emitted by the super radiation light emitting diode light source (5) is transmitted into the optical fiber photoacoustic sensing probe (4) through the optical fiber circulator (6) and is reflected, 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; The optical fiber photoacoustic sensing 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 sound guide channel (10) in the optical fiber photoacoustic sensing probe (4) is located at the top side wall of the photoacoustic tube (9), the photoacoustic pressure wave is conducted to the cantilever beam type sound wave sensitive diaphragm (13) at the end through the sound guide channel (10); the ceramic pin (12) is arranged on the optical fiber photoacoustic sensing probe (4), and the cantilever beam type sound wave sensitive diaphragm (13) and the end face of the ceramic pin (12) constitute a micro Fabry-Perot interferometer.

2. The fiber-optic photoacoustic gas sensing system of claim 1, wherein, The optical fiber photoacoustic sensing probe (4) is provided with a gas diffusion hole (8), the gas diffusion hole (8) communicates the photoacoustic tube (9) with ambient air, the gas to be measured in the environment enters the photoacoustic tube (9) through the gas diffusion hole (8), and 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 sensing probe (4) is provided with an optical fiber collimator (11), the optical fiber collimator (11) is aligned with the photoacoustic tube (9) arranged in the optical fiber photoacoustic sensing probe (4), and the laser emitted by the distributed feedback laser (2) is incident on the photoacoustic tube (9) through the optical fiber collimator (11) to generate a photoacoustic pressure wave; the excited photoacoustic pressure wave includes a solid photoacoustic pressure wave generated by the absorption of laser by the inner wall of the photoacoustic tube (9) and a gas photoacoustic pressure wave generated by the absorption of laser by the gas to be measured in the photoacoustic tube (9), wherein the frequency of the solid photoacoustic pressure wave is the same as the modulation frequency of the laser, the frequency of the gas photoacoustic pressure wave is twice the modulation frequency of the laser, different types of photoacoustic pressure waves are obtained by modifying the harmonic number of the digital phase-locked amplifier in the programmable logic gate array circuit (1), and the frequency scanning of the photoacoustic pressure wave is realized by adjusting the modulation frequency of the distributed feedback laser (2) and the spectral acquisition frequency in the programmable logic gate array circuit (1); the photoacoustic pressure wave makes the cantilever beam type acoustic sensitive diaphragm (13) vibrate to cause the change of the cavity length of the Fabry-Perot interferometer, and the change of the gas pressure causes the change of the resonance frequency of the cantilever beam type acoustic sensitive diaphragm (13), the solid photoacoustic signal is used for tracking the resonance frequency to detect the gas pressure, and the gas photoacoustic signal is used for detecting the concentration of the gas to be measured.

3. The fiber-optic photoacoustic gas sensing system of claim 1, wherein, The gas diffusion hole (8) is located at the side wall of the photoacoustic tube (9) and has a diameter smaller than the radius of the photoacoustic tube (9).

4. The fiber-optic photoacoustic gas sensing system of claim 1, wherein, The cantilever beam type acoustic sensitive diaphragm (13) is gold-plated on the inner surface, the distance between the cantilever beam type acoustic sensitive diaphragm (13) and the end surface of 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 of claim 1, wherein, The wavelength range of the spectrometer (7) is 1525 nm-1570 nm, and the maximum frame rate is higher than 15 kHz.

6. The fiber-optic photoacoustic gas sensing system of claim 1, wherein, The wavelength range of the super radiation light emitting diode light source (5) covers the required wavelength range of the spectrometer (7).

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

8. A fiber-optic photoacoustic gas sensing method suitable for low-pressure environments, characterized by, The distributed feedback laser (2) emits laser with variable modulation frequency into the sidewall of the photoacoustic tube (9) to generate solid photoacoustic signal, causing the cantilever beam type acoustic sensitive diaphragm (13) to vibrate; the super radiation light emitting diode light source (5) enters the fiber optic circulator (6) into the fiber optic photoacoustic sensing probe (4) and reflects, and the reflected light containing the frequency and amplitude of the solid photoacoustic signal is detected by the spectrometer (7); the programmable logic gate array circuit (1) collects and demodulates the interference spectrum, and obtains the best working frequency and ambient pressure of gas detection by frequency scanning of 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, which is detected and enhanced by the cantilever beam type acoustic sensitive diaphragm (13); the measured gas 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, the programmable logic gate array circuit (1) drives the distributed feedback laser (2) to emit laser with adjustable frequency, and the wavelength of the laser is far away from the characteristic absorption spectrum of the gas to be detected, which is incident into the photoacoustic tube (9) through the laser transmission fiber (3) and the 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, which is transmitted to the cantilever beam type acoustic sensitive diaphragm (13) through the sound conduction channel (10) and causes the cantilever beam type acoustic sensitive diaphragm (13) to vibrate; the cantilever beam type acoustic sensitive diaphragm (13) and the ceramic pin (12) constitute a Fabry-Perot cavity, and the cavity length changes; the broadband light emitted by the super radiation light emitting diode light source (5) enters the inner surface of the cantilever beam type acoustic sensitive diaphragm (13) through the fiber optic 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 sensitive diaphragm (13) to form Fabry-Perot interference; the interference light is recoupled into the fiber optic 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, extracts the signal with the same frequency as the laser modulation frequency through the lock-in amplifier, and the frequency corresponding to the maximum value of the signal is the resonance frequency of the cantilever beam type acoustic sensitive diaphragm (13), and the gas 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 with a center wavelength of the characteristic spectrum of the gas to be detected, and controls the laser modulation frequency to be half of the resonance frequency of the cantilever beam type acoustic sensitive diaphragm (13); the gas to be detected in the photoacoustic tube (9) absorbs the laser energy to generate gas photoacoustic pressure wave, which is detected by the cantilever beam type acoustic 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 dynamic cavity length information of the Fabry-Perot cavity, and the amplitude of the second harmonic signal is obtained through lock-in amplifier operation; the measured gas pressure and the amplitude of the photoacoustic signal are combined to calculate the concentration of the gas to be detected.

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