Differential spectroacoustic gas sensing system based on cross-correlation demodulation

Abstract

The invention provides a differential photoacoustic gas sensing system and method based on cross-correlation demodulation, and is suitable for the fields of trace gas monitoring, industrial process fine control and environment detection. The system comprises a gas excitation light source, a symmetrical double-probe gas sensor, an optical fiber circulator, a wide-spectrum light source, a 1 * 2 optical fiber coupler, two pieces of plain glass, a photoelectric detection array and a computer, acoustic wave signals excited by the photoacoustic effect generate difference superposition cavity length changes on the symmetrically-arranged double cantilever beams, and environment noise triggers cavity length responses which are counteracted in the same direction. The system adopts an optical cross-correlation demodulation algorithm to track peak displacement of a cross-correlation function in real time, dynamically demodulates cavity length change information, and inverts the gas concentration according to the information. According to the design scheme, effective photoacoustic signals are remarkably enhanced, meanwhile, effective suppression of environment vibration noise is achieved, and finally the trace gas detection system integrating the advantages of miniaturization, high sensitivity and low cost is constructed.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber photoacoustic gas detection, and is a differential photoacoustic gas sensing system and method based on cross-correlation demodulation, which is applicable to the fields of trace gas monitoring, industrial process refinement control and environmental detection. Background Art

[0002] Photoacoustic spectroscopy (PS), a differential photoacoustic gas sensing system based on cross-correlation demodulation, has shown important application value in environmental monitoring, industrial gas monitoring and other fields due to its advantages such as high sensitivity and good selectivity. This technology achieves concentration measurement by detecting the acoustic pressure signal generated after the gas molecules absorb light energy. As the core detection component of the photoacoustic gas sensing system, the sensitivity of the acoustic sensor directly determines the detection limit of the system. However, while high-sensitivity acoustic sensors improve detection performance, they are often accompanied by the problem of reduced stability. Especially in complex industrial environments, external factors such as strong electromagnetic interference, mechanical vibration and environmental noise will further deteriorate the stability of the system, resulting in a decrease in detection accuracy. This contradictory relationship between sensitivity and stability has become a key bottleneck restricting the practical application of photoacoustic technology. Therefore, under the combined influence of complex environmental interference and system inherent noise, how to improve the signal-to-noise ratio is the core challenge faced in achieving high-precision photoacoustic gas detection.

[0003] In photoacoustic spectroscopy gas detection, effective ways to improve the signal-to-noise ratio can be divided into photoacoustic signal enhancement and noise suppression. In terms of improving the amplitude of the photoacoustic signal, the following optimization measures can be taken: by increasing the incident light power or optimizing the gas absorption line selection to increase the effective absorption power of the excitation light; by adopting a resonant photoacoustic cell structure design and utilizing the acoustic standing wave effect to achieve signal amplification; by selecting a microphone with high signal-to-noise ratio and sensitivity for acoustic wave detection. The coordinated optimization of these methods can significantly improve the system signal-to-noise ratio and detection sensitivity. Literature M.Guo et al., "Differential Photoacoustic Gas Sensing System Based on Cross-Correlation Demodulation" l., "Differential Photoacoustic Gas Sensing System Based on Cross-Correlation Demodulation" l., Optical Spectroscopy-Based Differential Photoacoustic Gas Sensing System Based on Cross-Correlation Demodulation l., Photoacoustic Spectroscopy-Based Differential Photoacoustic Gas Sensing System Based on Cross-Correlation Demodulation" l., ... G. Differential photoacoustic gas sensing system based on cross-correlation demodulation. Differential photoacoustic gas sensing system based on cross-correlation demodulation. Differential photoacoustic gas sensing system based on cross-correlation demodulation. Differential photoacoustic gas sensing system based on cross-correlation demodulation. Lyzer," Differential photoacoustic gas sensing system based on cross-correlation demodulation. L. Chem., vol. 96, no. 37, pp. 14819-14825, Sep. 2024, doi: 10.1021 / cs. Differential photoacoustic gas sensing system based on cross-correlation demodulation. Differential photoacoustic gas sensing system based on cross-correlation demodulation. L. Chem. 4c02440. The scheme of using a resonant photoacoustic cell combined with a high-sensitivity cantilever-enhanced fiber acoustic wave sensor achieves dual enhancement of the photoacoustic signal through the acoustic amplification of the resonant cavity and the high sensitivity of the fiber sensor. At the same time, the introduction of a multi-path structure in the optical path design effectively increases the interaction path between the excitation light and the gas being measured, thereby increasing the amplitude of the photoacoustic signal. However, the inherently large size of the resonant photoacoustic cell not only affects the system's response time, but also limits its application in special and confined environments.In terms of noise suppression, the literature "Differential Cantilever Enhanced Fiber-Optic Photoacoustic Sensor for Diffusion Gas Detection" by C. Li, Y. Zhang, M. Guo, H. Qi, X. Zhou, and K. Chen in Analytical Chemistry, vol. 96, no. 11, pp. 4562-4569, Mar. 2024, doi: 10.1021 / acs.analchem.3c05396 proposed an innovative design of a differential dual photoacoustic cell. This scheme uses two photoacoustic cells with completely matching parameters, one as a detection cell for gas analysis and the other as a reference cell for real-time monitoring and eliminating environmental noise. Through this differential structure and the supporting algorithm processing, the system can effectively suppress the detection noise, and the signal-to-noise ratio is significantly improved. However, the system structure of this scheme is relatively complex, and it uses a dual-channel dynamic demodulation technology based on a micro high-speed spectrometer and a high-performance lock-in amplifier, resulting in a relatively high overall cost.

[0004] Therefore, developing a trace gas detection system with the advantages of miniaturization, high sensitivity, and low cost has important engineering application value for meeting the on-site detection requirements in complex industrial environments. Summary of the Invention

[0005] (1) Technical Problems to be Solved

[0006] Aiming at the deficiencies of the prior art, the present invention provides a differential photoacoustic gas sensing system and method based on cross-correlation demodulation. Through a symmetric fiber optic acoustic wave sensor structure and optical demodulation technology, it effectively overcomes the technical defects of existing gas sensors such as insufficient detection limit and weak environmental adaptability. The system has a simple structure and fast response, can significantly suppress the influence of environmental vibration and noise, and improve the detection reliability, providing an innovative solution for the application of fiber optic photoacoustic gas detection technology under complex working conditions.

[0007] (2) Technical Solutions

[0008] To achieve the above objectives, the present invention is implemented through the following technical solutions: a differential photoacoustic gas sensing system and method based on cross-correlation demodulation, including a control circuit gas excitation light source, a symmetrical dual-probe gas sensor, an optical fiber circulator, a broadband light source, a 1×2 optical fiber coupler, flat glass, a photoelectric detection array, and a computer.

[0009] The gas excitation light source is a tunable distributed feedback laser.

[0010] The broad spectrum light source is a superluminescent diode.

[0011] The plane glass has a thickness of 250 μm, is placed vertically to the photoelectric detection array, and is encapsulated as a whole in a light-proof housing to prevent the influence of external stray light on the detection results.

[0012] The symmetrical dual-probe gas sensor mainly includes a photoacoustic cell, a fiber FP acoustic wave sensor, a reflector, a fiber collimator, and a porous breathable membrane. The reflector is provided with a light-through hole structure, which can ensure that the excitation light beam emitted by the fiber collimator is coupled into the interior of the photoacoustic cell and realizes multiple reflections between the reflectors along a preset reflection path, thereby effectively extending the optical path and increasing the excitation efficiency of the photoacoustic signal. The side wall of the photoacoustic cell is provided with a gas exchange hole, which realizes gas diffusion exchange with the external environment through the porous breathable membrane. The porous breathable membrane is made of a polymer material, which can effectively block external noise interference and prevent the leakage of acoustic wave signals generated inside the photoacoustic cell while ensuring the efficient diffusion of target gas molecules, thereby maintaining the acoustic resonance characteristics of the photoacoustic cell unaffected.

[0013] The fiber-optic FP acoustic wave sensor primarily consists of a cantilever beam acoustic-sensitive membrane, a fiber-optic ceramic pin, and a stainless steel housing. The cantilever beam acoustic-sensitive membrane is a rectangular acoustic-wave sensor fabricated using MEMS technology on a circular silicon wafer. Its geometric dimensions are designed using finite element analysis to ensure that the cantilever's resonant frequency matches the photoacoustic signal's frequency band. The inner surface of the cantilever beam is coated with a gold film using magnetron sputtering to enhance its reflectivity. The top of the cantilever beam acoustic-sensitive membrane and the fiber-optic ceramic pin are aligned to form a fiber-optic FP cavity.

[0014] A differential photoacoustic gas sensing system and method based on cross-correlation demodulation. The system adopts a symmetrical dual-sensor structure design and synchronously detects photoacoustic signals through two FP interferometric fiber optic acoustic wave sensors with matching parameters. When the target gas in the photoacoustic cell absorbs the excitation light energy to generate a photoacoustic signal, the effective signals received by the two sensors are in opposite phases; while the environmental noise (including mechanical vibration, electromagnetic interference, etc.) exhibits a co-directional characteristic due to the propagation path. The system processes the dual-path signals through a real-time optical cross-correlation algorithm, breaking through the technical bottleneck of traditional single-sensor systems that are difficult to balance sensitivity and stability in complex environments. The specific steps are as follows:

[0015] First, the gas to be measured diffuses through a porous gas-permeable membrane into the sealed detection chamber of the photoacoustic cell. A control circuit drives the gas excitation light source at a specific frequency that matches the resonant frequency of the cantilever beam's acoustically sensitive membrane. The resulting excitation beam is collimated by a fiber collimator and then incident on the photoacoustic cell at a small angle. When the wavelength of the excitation beam matches the absorption spectrum of the target gas, the photoacoustic effect generates periodic pressure fluctuations, forming a photoacoustic signal with a frequency that matches the modulation frequency.

[0016] The light from the broadband light source is then split into two equal beams by a 1×2 fiber coupler. The beams are then transmitted to two fiber circulators, each ultimately entering two symmetrically placed fiber optic FP acoustic wave sensors. These sensors sense the photoacoustic signal and convert the photoacoustic pressure waves into vibrations of the cantilever beam's acoustically sensitive membrane, which in turn translates into changes in the FP cavity's length.

[0017] Finally, the fiber-optic FP acoustic wave sensor transmits the optical signal carrying the FP cavity length information back to two fiber-optic circulators, where it is incident on two flat glass surfaces at a 45-degree angle. After reflecting off the upper and lower surfaces of the glass, the light beams are received by pixels at different locations on the photodetection array. A computer processes the data, performing a differential operation on the two signals. By calculating the spatial displacement of the two interference fringes on the photodetection array, the change in the dual FP cavity length is demodulated in real time, ultimately outputting gas concentration information that eliminates external noise interference.

[0018] (3) Beneficial effects

[0019] The present invention provides a differential photoacoustic gas sensing system based on cross-correlation demodulation.

[0020] Beneficial effects:

[0021] This differential photoacoustic gas sensing system based on cross-correlation demodulation adopts a symmetrical arrangement design of dual FP sensors combined with a cross-correlation demodulation algorithm to significantly improve the accuracy of gas detection. The device adopts a symmetrical dual-probe differential detection structure, which can effectively suppress interference such as environmental vibration. In conjunction with the optical cross-correlation demodulation algorithm, it not only greatly improves the system's anti-interference ability and signal-to-noise ratio, but also significantly reduces the system complexity and implementation cost. In terms of detection performance, the system adopts cantilever-enhanced collaborative multiple reflection technology to achieve high-sensitivity detection, and significantly enhances the long-term stability and reliability of the system in complex environments. In terms of practicality, the compact modular design greatly reduces the system volume. At the same time, the cross-correlation demodulation module constructed with conventional optical devices effectively controls the overall cost, reduces power consumption, and extends the maintenance cycle. The present invention achieves collaborative optimization in terms of detection accuracy, environmental adaptability and reliability, and proposes a new solution for trace gas monitoring needs in harsh industrial environments such as power equipment and petrochemicals. It solves the technical problem that traditional photoacoustic gas sensors are difficult to balance sensitivity and stability in complex environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Attachment Figure 1 Schematic diagram of a differential photoacoustic gas sensing system and method based on cross-correlation demodulation.

[0023] Attachment Figure 2 Schematic diagram of the symmetrical dual-probe gas sensor structure.

[0024] Attachment Figure 3 It is a schematic diagram of the structure of the optical fiber FP acoustic wave sensor.

[0025] Attachment Figure 4 It is a schematic diagram of the simulated dual-channel optical cross-correlation image.

[0026] In the figure: 1. Control circuit; 2. Gas excitation light source; 3. Symmetrical dual-probe gas sensor; 4. Fiber optic circulator; 5. Fiber optic circulator; 6. Broad-spectrum light source; 7. 1×2 fiber optic coupler; 8. Plane glass; 9. Plane glass; 10. Photoelectric detection array; 11. Computer; 12. Photoacoustic cell; 13. Fiber optic FP acoustic wave sensor; 14. Fiber optic FP acoustic wave sensor; 15. Reflector; 16. Reflector; 17. Fiber optic collimator; 18. Porous breathable membrane; 19. Cantilever beam acoustic sensitive membrane; 20. Fiber optic ceramic pin; 21. Stainless steel housing. DETAILED DESCRIPTION

[0027] The embodiment of the present invention provides a differential photoacoustic gas sensing system and method based on cross-correlation demodulation, such as Figure 1-Figure 3It mainly includes a control circuit 1, a gas excitation light source 2, a symmetrical dual-probe gas sensor 3, a fiber circulator 4, a fiber circulator 5, a broadband light source 6, a 1×2 fiber coupler 7, a plane glass 8, a plane glass 9, a photoelectric detection array 10, a computer 11, a photoacoustic cell 12, a fiber FP acoustic wave sensor 13, a fiber FP acoustic wave sensor 14, a reflector 15, a reflector 16, a fiber collimator 17, a porous gas permeable membrane 18, a cantilever beam acoustic sensitive membrane 19, a fiber optic ceramic pin 20, and a stainless steel housing 21.

[0028] Symmetrical dual-probe gas sensor 3 consists of a photoacoustic cell 12, fiber FP acoustic wave sensors 13 and 14, reflectors 15 and 16, a fiber collimator 17, and a porous gas-permeable membrane 18. Fiber FP acoustic wave sensors 13 and 14 are composed of a cantilever beam acoustic-sensitive membrane 19, a fiber optic ceramic ferrule 20, and a stainless steel housing 21.

[0029] The gas excitation light source 2 is driven by the control circuit 1 to generate a modulated light beam. After being collimated by the fiber collimator 17, this light beam passes through the aperture in the reflector 16 and enters the photoacoustic cell 12. It undergoes multiple reflections between the reflectors 15 and 16, thereby increasing the effective optical power of the excitation light. The porous gas-permeable membrane 18 inside the photoacoustic cell 12 enables gas exchange with the outside world. The target gas absorbs the light energy and generates a photoacoustic signal. This photoacoustic signal is synchronously detected by symmetrically placed fiber optic FP acoustic wave sensors 13 and 14. A 1×2 fiber coupler 7 equally splits the light beam emitted by the broadband light source 6 and transmits it to the fiber optic FP acoustic wave sensors 13 and 14 via fiber circulators 5 and 6. The light beam is then reflected from the two surfaces of the FP cavity formed by the inner surface of the cantilever beam acoustic film 19 and the fiber optic ceramic ferrule 20, forming an interference beam. The interference beam, carrying information about the two cavity lengths, passes through the fiber circulators 5 and 6 again and irradiates the two flat glass panels 8 and 9 at a 45-degree angle. After reflecting off the upper and lower surfaces of glass panels 8 and 9, the light beams arrive at the same point on photodetector array 10, creating an optical path difference. Photodetector array 10 transmits the received optical signals to computer 11, generating an optical cross-correlation pattern. The received optical cross-correlation signals are processed using a pre-calibrated algorithm to analyze the dynamic changes in the two FP cavities, separate the dynamic photoacoustic signal from the noise interference, and ultimately calculate the concentration information of the multi-component gas.

[0030] Working principle: Using Figure 2The symmetrical dual-probe gas sensor 3 shown serves as the site for photoacoustic excitation and sensing. The length and radius of the miniaturized photoacoustic cell 12 are set to 18 mm and 3 mm, respectively. A pair of concave mirrors 16 with equal radii of curvature are symmetrically positioned at either end of the cell. A fiber collimator 17 controls the incident angle of the paraxial light beam, achieving multiple reflections between the two mirrors 16 and extending the effective absorption optical path. To reduce optical loss, the inner wall of the cell 12 is polished.

[0031] In order to reduce the interference of environmental noise, two optical fiber FP acoustic wave sensors 13 / 14 are used to detect the photoacoustic signal. Figure 3 As shown, the three core components include a silicon cantilever acoustically sensitive diaphragm, a fiber optic ceramic ferrule 20, and a metal protective housing. The rectangular silicon cantilever's tip aligns with the center of the fiber optic ceramic ferrule 20, and the two surfaces form a fiber optic FP cavity. The cantilever acts as a highly sensitive mechanical transducer, converting external photoacoustic stimuli into quantifiable displacements through deflection measurement. The wavelength of the excitation light source is modulated sinusoidally, forcing the cantilever to vibrate around its static equilibrium position under the influence of the photoacoustic signal. Two fiber optic FP acoustic wave sensors 13 / 14 are symmetrically positioned on either side of the photoacoustic cell 12, enabling simultaneous capture of the photoacoustic signal generated within the cell and the ambient noise signal. During measurement, the photoacoustic signal generated within the cell 12 excites the two symmetrically arranged cantilever beams to vibrate in opposite phases, while the external ambient noise causes the cantilever beams to vibrate in phase. This unique response characteristic results in differential enhancement of the cavity length change caused by the photoacoustic signal on the two cantilever beams, while the cavity length change caused by the ambient noise cancels out the change in phase. Based on the above characteristics, an optical cross-correlation demodulation algorithm is used to enhance the photoacoustic signal while suppressing the noise interference caused by environmental vibration.

[0032] The optical cross-correlation demodulation module utilizes a vertically orthogonal structure consisting of dual flat glass windows and a highly sensitive photodetector array 10. A fully enclosed black box design effectively suppresses stray light interference and ensures a stable measurement environment. Two interfering beams carrying information about the measured FP cavity length strike two flat glass windows 8 / 9 at a 45-degree angle of incidence. After reflection from the upper and lower surfaces of the glass, they are received by the photodetector array. Each pixel on the array corresponds to a different optical path difference. When the optical path difference at a specific location matches the measured FP cavity length, the cross-correlation spectrum signal at that point reaches its maximum value. The location of the cross-correlation spectrum peak and the corresponding optical path differences at different locations on the photodetector array can be used to demodulate the fiber optic FP sensor cavity length. By optimizing the positions of the flat glass windows 8 / 9 and the photodetector array 10, the optical cross-correlation pattern received by the photodetector array 10 is spatially divided into two regions, corresponding to the two sensing channels. Figure 4This is a two-channel optical cross-correlation image numerically simulated based on the basic theory of optical cross-correlation. When the FP cavity length changes, the peak position of the optical cross-correlation image will periodically oscillate. A peak-finding algorithm can track the peak position of the cross-correlation image in real time to demodulate the cavity length. Ultimately, the target gas concentration information is inverted based on the amplitude of the periodic dynamic changes in the cavity length.

[0033] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A differential photoacoustic gas sensing system based on cross-correlation demodulation, characterized by: It consists of a laser light source module, a wide-spectrum light source module, an optical mutual light demodulation module, a light processing module and a symmetrical dual-probe gas sensor (3); The symmetrical dual-probe gas sensor (3) is composed of a photoacoustic cell (12), optical fiber FP acoustic wave sensors (13, 14), reflectors (15, 16), an optical fiber collimator (17), and a porous gas-permeable membrane (18); the reflector (16) is provided with a light-through hole structure to ensure that the excitation light beam emitted by the optical fiber collimator (17) is coupled into the interior of the photoacoustic cell (12) and realizes multiple reflections between the reflectors (15, 16) along a preset reflection path; the side wall of the photoacoustic cell (12) is provided with a gas exchange hole, and the gas exchange hole realizes gas diffusion exchange with the external environment through the porous gas-permeable membrane (18); The laser light source module comprises a control circuit (1) and a gas excitation light source (2). The gas excitation light source (2) is driven by the control circuit (1) to generate a modulated light beam. After being collimated by a fiber collimator (17), the light beam is incident into a photoacoustic pool (12) through a small hole on a reflector (16), and is reflected multiple times between the reflectors (15, 16), thereby increasing the effective optical power of the excitation light. The broadband light source module comprises a broadband light source (6), a 1×2 optical fiber coupler (7), a first optical fiber circulator (4), and a second optical fiber circulator (5); the 1×2 optical fiber coupler (7) evenly divides the light beam emitted by the broadband light source (6) and transmits the light beam to optical fiber FP acoustic wave sensors (13, 14) through the optical fiber circulators (5, 6); The optical mutual light demodulation module comprises plane glass (8, 9) and a photoelectric detection array (10), wherein the plane glass (8, 9) and the photoelectric detection array (10) are in a vertical orthogonal structure and use a fully enclosed black box design to effectively suppress stray light interference and ensure the stability of the measurement environment; The optical processing module includes a computer (11), which performs data processing, performs differential operation on two signals, calculates the spatial displacement of two interference fringes on the photoelectric detection array (10), demodulates the change in the length of the dual FP cavity in real time, and finally outputs gas concentration information that eliminates external noise interference.

2. The differential photoacoustic gas sensing system based on cross-correlation demodulation according to claim 1, characterized in that: The optical fiber FP acoustic wave sensor (13, 14) is composed of a cantilever beam acoustic sensitive film (19), an optical fiber ceramic pin (20), and a stainless steel shell (21). The top end of the cantilever beam acoustic sensitive film (19) and the optical fiber ceramic pin (20) are placed correspondingly to form an optical fiber FP cavity.

3. The differential photoacoustic gas sensing system based on cross-correlation demodulation according to claim 1, characterized in that: The gas excitation light source (2) is a tunable distributed feedback laser.

4. The differential photoacoustic gas sensing system based on cross-correlation demodulation according to claim 1, characterized in that: The broadband light source (6) is a superluminescent diode.

5. The differential photoacoustic gas sensing system based on cross-correlation demodulation according to claim 1, characterized in that: The plane glass (8, 9) has a thickness of 250 μm, is placed vertically to the photoelectric detection array (10), and is entirely encapsulated in a light-proof housing to prevent external stray light from influencing the detection results.

6. The differential photoacoustic gas sensing system based on cross-correlation demodulation according to claim 1, characterized in that: The porous breathable membrane (18) is made of a polymer material and can effectively block external noise interference and prevent leakage of acoustic wave signals generated inside the photoacoustic cell (12) while ensuring efficient diffusion of target gas molecules, thereby maintaining the acoustic resonance characteristics of the photoacoustic cell (12) unaffected.

7. A method for using a differential photoacoustic gas sensing system based on cross-correlation demodulation according to any one of claims 1 to 6, characterized in that the steps are as follows: 7.

1. The gas to be measured diffuses through the porous gas-permeable membrane (18) into the sealed detection cavity of the photoacoustic cell (12). The control circuit (1) drives the gas excitation light source (2) at a specific frequency that matches the resonant frequency of the cantilever beam acoustic-sensitive membrane (19). The generated excitation light beam is collimated by the fiber collimator (17) and then incident on the photoacoustic cell (12) at a small angle. When the wavelength of the excitation light beam matches the absorption spectrum of the target gas, periodic pressure fluctuations are generated through the photoacoustic effect, forming a photoacoustic signal whose frequency matches the modulation frequency. 7.

2. The light emitted by the broadband light source (6) is divided into two equal beams by a 1×2 fiber coupler (7) and then transmitted to two fiber circulators (4, 5) respectively. The light is finally incident on two symmetrically placed fiber optic FP acoustic wave sensors (13, 14). The fiber optic FP acoustic wave sensors (13, 14) sense the photoacoustic signal and convert the photoacoustic pressure wave into the vibration of the cantilever beam acoustic sensitive film (19), which then evolves into the change of the cavity length of the FP cavity. 7.

3. The optical fiber FP acoustic wave sensor (13, 14) transmits the optical signal carrying the FP cavity length information to the two optical fiber circulators (4, 5) again, and is incident on two flat glass surfaces (8, 9) at a 45-degree angle. The light beams reflected from the upper and lower surfaces of the flat glass surfaces 8, 9 are received by pixels at different positions on the photoelectric detection array (10). A computer (11) is used to process the data and perform a differential operation on the two signals. By calculating the spatial displacement of the two interference fringes on the photoelectric detection array (10), the change in the dual FP cavity length is demodulated in real time, and finally the gas concentration information is output without external noise interference.

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