Photoacoustic spectrometry gas detection system and method based on hollow-core optical fiber and quartz tuning fork

Abstract

The invention relates to the technical field of trace gas optical sensing, in particular to a photoacoustic spectrometry gas detection system and method based on a hollow-core optical fiber and a quartz tuning fork, and the system comprises a signal generation module, a laser modulation module, a hollow-core optical fiber enhanced detection module and a signal processing module, the system performs long-range interaction with to-be-measured gas in the hollow-core optical fiber by modulating laser, thermal expansion generated after light energy is absorbed is transmitted to the mechanically coupled quartz tuning fork through the optical fiber wall, the quartz tuning fork converts vibration into an electric signal through a piezoelectric effect, and finally the gas concentration is obtained through treatment. According to the invention, the bottleneck that the optical path is limited in the traditional technology is broken through, the detection sensitivity is greatly improved, and the application potential in trace and complex environment gas detection is expanded.

Description

Technical Field

[0001] This invention relates to the field of trace gas optical sensing technology, specifically to a photoacoustic spectroscopy gas detection system and method based on hollow optical fiber and quartz tuning fork. Background Technology

[0002] Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a highly sensitive optical sensing method for trace gas detection. Its core principle involves modulating a laser to excite the analyte gas, generating a photoacoustic signal, and then using a quartz tuning fork (QTF) to resonate and detect the sound waves at a specific frequency. Due to the high quality factor and stable resonance characteristics of the quartz tuning fork, it can convert weak acoustic vibrations into electrical signals through the piezoelectric effect. These signals are then amplified by lock-in and processed to determine the gas concentration. Currently, QEPAS technology has developed various methods to enhance detection performance, including using internal cavity structures to increase laser power to enhance sound source excitation, and integrating micro-resonant cavities between the tuning fork fingers to amplify sound pressure, thereby achieving higher detection sensitivity.

[0003] However, existing QEPAS technology still faces significant limitations in further improving its performance. Although cavity enhancement and acoustic resonant structures have effectively improved signal intensity, the interaction area between light and gas is always limited to the narrow space near the tuning fork, making it difficult to further extend the effective absorption optical path. This fundamentally restricts further breakthroughs in detection sensitivity and also limits its application potential in low-concentration, multi-component, or complex environments. Summary of the Invention

[0004] To address the above technical problems, this invention provides a technical solution for a photoacoustic spectroscopy gas detection system and method based on hollow optical fiber and quartz tuning fork.

[0005] The technical problem solved by this invention can be achieved by the following technical solutions: A photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork includes: The signal generation module is used to provide drive signals and reference signals; A laser modulation module, connected to the signal generation module, is used to modulate the laser based on the driving signal to obtain an excitation laser; A hollow-core fiber enhanced detection module is connected to the laser modulation module. The hollow-core fiber enhanced detection module includes a hollow-core fiber and a quartz tuning fork. The input end of the hollow optical fiber receives the excitation laser, and the hollow core of the hollow optical fiber contains the gas to be tested, which is used to guide the excitation laser to perform optical path interaction with the gas to be tested, and to transfer the periodic thermal expansion generated by the gas molecules after absorbing light energy to the fiber wall. The fork arm of the quartz tuning fork is mechanically coupled to the outer wall of the hollow optical fiber to receive mechanical vibrations transmitted through the fiber wall and convert the mechanical vibrations into electrical signals based on the piezoelectric effect. The signal processing module, connected to the hollow fiber enhanced detection module and the signal generation module, is used to process the electrical signal based on the reference signal to obtain the concentration value of the gas to be measured.

[0006] Preferably, the laser modulation module includes: A single-frequency laser is used to generate a laser whose wavelength is aligned with the characteristic absorption peak of the gas being measured. A modulator, connected to the single-frequency laser and the drive signal output terminal of the signal generation module, is used to modulate the intensity of the laser based on the drive signal to generate the excitation laser, wherein the modulation frequency of the laser matches the natural resonant frequency of the quartz tuning fork.

[0007] Preferably, the modulator is an electro-optic intensity modulator.

[0008] Preferably, the hollow optical fiber is a hollow anti-resonant optical fiber.

[0009] Preferably, the hollow optical fiber is fixed to the fork arm of the quartz tuning fork by winding.

[0010] Preferably, the hollow optical fiber is wound 2 to 4 times.

[0011] Preferably, the signal processing module includes: A preamplifier, connected to the output of the quartz tuning fork, is used to perform primary amplification of the electrical signal; A lock-in amplifier, connected to the preamplifier and the reference signal output of the signal generation module, is used to perform phase-sensitive demodulation on the amplified signal based on the reference signal; The data processing system is connected to the lock-in amplifier and is used to receive the signal demodulated by the lock-in amplifier, and to calculate and output the concentration value of the gas to be measured.

[0012] Preferably, the system further includes a lens disposed in the optical path between the laser modulation module and the hollow fiber enhancement detection module, for coupling the excitation laser into the hollow fiber.

[0013] Preferably, it further includes a gas communication structure, which is a gas chamber structure or an optical fiber adapter structure, for introducing the gas to be tested into the hollow core of the hollow optical fiber.

[0014] It also includes a photoacoustic spectroscopy gas detection method based on hollow optical fiber and quartz tuning fork, applied to the photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork as described above, including: Step S1: Generate a driving signal and a reference signal through the signal generation module; Step S2: In response to the driving signal, the laser intensity is modulated to generate an excitation laser with a modulation frequency that matches the resonant frequency of the quartz tuning fork. Step S3: Couple the excitation laser to a hollow optical fiber filled with the gas to be tested, guide the excitation laser to interact with the gas to be tested, and transfer the periodic thermal expansion generated by the gas to be tested after absorbing light energy to the fiber wall. Step S4: Receive mechanical vibrations driven by thermal expansion and transmitted through the fiber wall via the quartz tuning fork, and convert the mechanical vibrations into corresponding electrical signals; Step S5: Based on the reference signal, the electrical signal is amplified, demodulated, and analyzed to obtain the concentration value of the gas to be measured.

[0015] Beneficial effects: This invention introduces hollow optical fiber as a medium for gas absorption and sound transmission, enabling the excitation laser to interact with the gas to be measured within the hollow core of the fiber over a long optical path, thereby significantly extending the effective absorption optical path. This overcomes the fundamental bottleneck in traditional QEPAS technology where the photoacoustic interaction area is limited to the narrow space near the tuning fork. Simultaneously, the mechanical coupling between the quartz tuning fork and the outer wall of the optical fiber efficiently captures the acoustic vibrations generated by the photothermal effect and enhances signal detection through piezoelectric conversion. This not only significantly improves the system's detection sensitivity but also expands its application potential for trace gas detection in low-concentration, multi-component, and complex environments. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the system structure of the present invention; Figure 2 This is a schematic diagram of the hollow-core optical fiber enhanced detection module of the present invention; Figure 3 This is a schematic diagram of the photoacoustic spectroscopy detection principle of the hollow fiber enhanced detection module of the present invention; Figure 4 This is a schematic diagram of the air chamber structure in which the hollow optical fiber of the present invention is disposed in the air chamber; Figure 5 This is a schematic diagram of the air chamber structure in which the hollow optical fiber of the present invention is disposed outside the air chamber; Figure 6 This is a flowchart of the method of the present invention.

[0017] Explanation of reference numerals in the attached diagram: 1. Signal generation module; 2. Laser modulation module; 21. Single-frequency laser; 22. Modulator; 3. Hollow-core fiber enhanced detection module; 31. Hollow-core fiber; 32. Quartz tuning fork; 4. Signal processing module; 41. Preamplifier; 42. Lock-in amplifier; 5. Lens; 6. Pressure control system; 7. Input gas chamber; 71. Optical input window; 8. Output gas chamber; 81. Optical output window; 82. Infrared bandpass filter; 83. Detector. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0020] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0021] Reference Figure 1 This invention provides a photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork, comprising: Signal generation module 1 is used to provide drive signals and reference signals; Laser modulation module 2, connected to signal generation module 1, is used to modulate the laser based on the driving signal to obtain excitation laser; A hollow-core fiber enhancement detection module 3 is connected to the laser modulation module 2. The hollow-core fiber enhancement detection module 3 includes a hollow-core fiber 31 and a quartz tuning fork 32. The input end of the hollow fiber 31 receives the excitation laser, and the hollow core of the hollow fiber 31 contains the gas to be tested, which is used to guide the excitation laser to perform optical path interaction with the gas to be tested, and to transfer the periodic thermal expansion generated by the gas molecules after absorbing light energy to the fiber wall. The fork arm of the quartz tuning fork 32 is mechanically coupled to the outer wall of the hollow optical fiber 31 to receive mechanical vibrations transmitted through the optical fiber wall and convert the mechanical vibrations into electrical signals based on the piezoelectric effect. The signal processing module 4 is connected to the hollow fiber enhanced detection module 3 and the signal generation module 1, and is used to process the electrical signal based on the reference signal to obtain the concentration value of the gas to be tested.

[0022] Specifically, in this embodiment of the invention, in order to address the problem that the light-gas interaction area is limited and the effective absorption optical path is difficult to extend in traditional quartz enhanced photoacoustic spectroscopy (QEPAS) technology, thus restricting the further improvement of detection sensitivity, the hollow fiber 31 is used as the core element for long-range absorption and sound transmission, and is externally mechanically coupled with the quartz tuning fork 32. This effectively avoids the narrow working space that traditional designs rely on for the interdigital gap of the tuning fork, and realizes the flexible extension of the effective optical path on the order of centimeters to meters and the efficient transmission and detection of acoustic vibration signals, fundamentally breaking through the bottleneck of optical path limitation.

[0023] Specifically, the excitation laser guided inside the hollow fiber 31 interacts with the gas under test over a long distance with high efficiency. The resulting photoacoustic signal is directly transmitted through the fiber wall to the quartz tuning fork 32 coupled to it. This structure makes full use of the low acoustic loss characteristics of the fiber waveguide and the high quality factor of the tuning fork, and significantly enhances the signal strength while maintaining the system compactness.

[0024] More specifically, in this embodiment of the invention, the signal generation module 1 preferably adopts a signal generator. The signal generator generates a highly stable and precisely adjustable driving signal and a reference signal to ensure that the laser modulation frequency and the tuning fork resonance frequency are precisely matched, thereby optimizing the signal-to-noise ratio and detection lower limit of the system.

[0025] In a preferred embodiment of the present invention, the laser modulation module 2 includes: A single-frequency laser 21 is used to generate a laser whose wavelength is aligned with the characteristic absorption peak of the gas to be measured; Modulator 22, connected to the single-frequency laser 21 and the drive signal output terminal of the signal generation module 1, is used to modulate the intensity of the laser based on the drive signal to generate the excitation laser, wherein the modulation frequency of the laser matches the natural resonant frequency of the quartz tuning fork 32.

[0026] Specifically, in order to achieve highly selective gas excitation and ensure effective detection of subsequent photoacoustic signals, in this embodiment of the invention, reference is made to... Figure 1 Preferably, a single-frequency laser 21 with a precisely tunable output wavelength and extremely narrow linewidth is used. The core wavelength of the single-frequency laser 21 is set to align with a specific absorption peak of the gas molecules to be detected. For example, when detecting acetylene gas, a single-frequency laser with a wavelength of 1530.37 nm is preferred, as this wavelength precisely corresponds to a characteristic absorption peak of acetylene gas in the near-infrared band. Therefore, using a single-frequency laser 21 ensures that the laser energy is highly concentrated on the target gas molecules, greatly reducing cross-interference from other gas components, thereby improving the selectivity and accuracy of the detection.

[0027] More specifically, in this embodiment of the invention, modulator 22 receives a precise drive signal from signal generation module 1 and accordingly performs sinusoidal intensity modulation on the continuous laser output by single-frequency laser 21 to generate periodic excitation laser.

[0028] The modulation frequency of modulator 22 is strictly set to match the resonant frequency of quartz tuning fork 32, for example, 12.4677kHz. This frequency matching design is crucial, as it ensures that the frequency of the sound waves excited by the periodic thermal expansion generated after gas absorption falls precisely within the peak response range of the mechanical resonance of quartz tuning fork 32, thereby maximizing the amplification and extraction of weak photoacoustic signals through its high quality factor.

[0029] It is evident that using an external independent modulator 22 can achieve high-depth, high-linearity intensity modulation, and the modulation frequency can be flexibly set, ensuring optimal coordination between the excitation signal and the detection element.

[0030] In a preferred embodiment of the present invention, the modulator 22 is an electro-optic intensity modulator.

[0031] Specifically, considering that the system requires high modulation depth, excellent linearity, and fast response characteristics to achieve high-fidelity periodic modulation of laser intensity, an electro-optic modulator is used as the core modulation element in this embodiment of the invention. Based on the electro-optic effect, the electro-optic modulator uses an external driving voltage signal to change the refractive index of an optical medium (such as a lithium niobate crystal), thereby achieving high-speed and precise control over the intensity, phase, or frequency of the passing laser.

[0032] In photoacoustic spectroscopy applications, electro-optic modulators are mainly divided into two types: intensity modulators and phase modulators. Intensity modulators directly change the output optical power, and their modulation waveform has a direct amplitude correspondence with the photoacoustic signal generated by gas absorption, making signal demodulation more intuitive and efficient. While phase modulators can indirectly generate intensity changes through structures such as interference and coupling, their system optical path is more complex, they are more sensitive to optical alignment and environmental vibrations, and their long-term stability is relatively poor.

[0033] Therefore, in this embodiment of the invention, an electro-optic intensity modulator is preferably used. This electro-optic intensity modulator can receive the high-precision sinusoidal driving voltage output by the signal generation module 1 and linearly convert it into periodic changes in laser intensity, generating a deep, stable, and pure excitation light. This direct intensity-modulated square wave or sine wave form not only highly matches the demodulation principle of the subsequent lock-in amplifier but also maximizes the excitation of the periodic thermal expansion of the gas synchronized with the modulation frequency, thereby optimizing the generation and detection efficiency of the photoacoustic signal. This is a key choice to ensure that the system has both high sensitivity and high reliability.

[0034] In addition, modulator 22 can also be an acousto-optic modulator (AOM) or a direct current modulated laser. The acousto-optic modulator utilizes the acousto-optic effect to change the refractive index grating of the medium through a radio frequency drive signal to achieve laser modulation. It has a wide modulation frequency range, but usually requires high driving power, and the modulation depth and stability may be limited in some high-frequency bands. On the other hand, the direct current modulated laser modulates its output light intensity by changing the laser injection current. It has the simplest structure, but the modulation linearity is usually poor, and it is easy to introduce additional laser frequency chirp and intensity noise, which is particularly unfavorable for high-precision photoacoustic measurement.

[0035] Therefore, in this embodiment of the invention, an electro-optic intensity modulator is preferred to achieve high-fidelity and high-linearity intensity modulation of the laser, so as to ensure the purity, stability and controllability of the excitation light signal, thereby providing a key excitation source guarantee for the entire system to achieve high-sensitivity and high-reliability gas detection.

[0036] In a preferred embodiment of the present invention, the hollow optical fiber 31 is a hollow anti-resonant optical fiber.

[0037] In order to enable light to travel long distances in a low-loss hollow core and efficiently excite photoacoustic signals, while ensuring that the acoustic energy generated by the excitation can be effectively coupled to the fiber wall and transmitted outward, in this embodiment of the invention, hollow anti-resonant fiber is preferably used as the core waveguide for photoacoustic enhancement and transmission.

[0038] This hollow anti-resonant fiber uses the principle of anti-resonance reflection to confine light waves to the air core for transmission. Its high hollow core ratio and extremely low overlap with the optical mode field of the glass material not only enable ultra-low optical transmission loss and effectively extend the interaction length between light and gas, but also minimize the background thermal noise generated by the absorption of laser energy by the fiber glass material.

[0039] The microcavity structure formed by hollow-core antiresonant optical fibers has the advantages of small gas heat capacity and limited thermal diffusion. Under the same incident laser power, the gas inside the fiber core can generate a local temperature rise that is several times higher than that in an open space, thereby directly enhancing the initial intensity of the acoustic pressure wave. At the same time, the high-frequency collisions between the inner wall of the fiber core and gas molecules effectively accelerate the relaxation process of the gas molecule vibrational energy level to translational thermal energy, causing more absorbed light energy to be released rapidly as heat energy, further improving the photoacoustic conversion efficiency.

[0040] In addition, the unique periodic cladding structure of this hollow anti-resonant fiber maintains excellent light guiding performance while providing an efficient acoustic coupling channel for the propagation of sound waves from the fiber core region to the cladding and outer wall. This ensures that the periodic pressure wave generated after the gas absorbs the laser can be effectively transmitted to the mechanically coupled quartz tuning fork 32, making it an ideal waveguide structure for achieving high sensitivity, low noise, and long-range photoacoustic detection.

[0041] In a preferred embodiment of the present invention, the hollow optical fiber 31 is fixed to the fork arm of the quartz tuning fork 32 by winding.

[0042] Specifically, in order to effectively capture the weak acoustic vibrations generated by the periodic thermal expansion of the gas and efficiently transmit them to the quartz tuning fork 32, in this embodiment of the invention, referring to... Figure 1 and Figure 2 Hollow optical fiber 31 is tightly wound around the fork arm of quartz tuning fork 32.

[0043] Accordingly, considering that the number of winding turns directly affects the balance between the mechanical contact area and the additional mechanical load, too few turns will lead to insufficient coupling area and low signal transmission efficiency; while too many turns will introduce a significant additional mass load to the tuning fork, which may change its vibration mode and reduce its quality factor, thereby weakening the resonance enhancement effect. Based on this, in this embodiment of the invention, the number of winding turns of the hollow optical fiber 31 is preferably 2 to 4. This range can significantly increase the effective contact area and enhance the transmission capability of weak vibrations while maintaining the original high-sensitivity resonance characteristics of the quartz tuning fork 32 to the maximum extent.

[0044] This design, through moderate multi-turn winding, significantly increases the mechanical contact area between the optical fiber and the tuning fork arm, allowing the weak sound pressure signal transmitted through the fiber wall to be more fully collected and coupled to the quartz tuning fork 32. This not only directly improves the effective contact area between the optical fiber and the tuning fork and the sensitivity to the vibration direction, thereby greatly enhancing the sound energy transmission efficiency, but also maximizes the conversion of the signal gain potential brought by "long optical path absorption" into detectable mechanical vibration.

[0045] Specifically, the winding structure improves system performance through two main mechanisms: Firstly, the increased contact area ensures that even the slightest thermal expansion deformation can be effectively transferred to the tuning fork, enhancing the tuning fork's response to weak sound signals. Secondly, the extension of the optical fiber path directly increases the optical path of light-gas interaction, thereby enabling gas molecules to absorb more light energy and generate a stronger thermal expansion effect and initial sound pressure.

[0046] The synergistic effect of these two aspects significantly improves the gas detection sensitivity of the system, which is the key to unleashing the potential of long optical path absorption.

[0047] More specifically, considering the stability of mechanical coupling and signal transmission efficiency, the winding position and fixing tension are optimized in this embodiment of the invention.

[0048] The preferred winding position is on the fork arm of the quartz tuning fork 32, near the root but slightly off from its theoretical vibration node. This position has sufficient vibration amplitude to drive the tuning fork to respond effectively, and its proximity to the fixed end provides more stable mechanical characteristics.

[0049] Meanwhile, in terms of fixing method, an elastic bracket or rubber ring is used to apply moderate and uniform radial pressure to the wound optical fiber to ensure a tight and reliable physical contact between the hollow optical fiber 31 and the surface of the fork arm of the quartz tuning fork 32. This moderate tension ensures that the small deformation caused by gas thermal expansion can be transmitted to the quartz tuning fork 32 without loss through the optical fiber wall, while avoiding vibration distortion of the quartz tuning fork 32 or structural damage to the hollow optical fiber 31 due to excessive stress. Thus, while maximizing signal transmission efficiency, it ensures the long-term mechanical stability and reliability of the entire detection module.

[0050] As a preferred embodiment of the present invention, refer to Figure 3 The working principle of the laser modulation module 2 is as follows: First, the high-precision, frequency-stable sinusoidal drive signal generated by the signal generation module 1 is output to the modulator 22, which drives the modulator 22 to perform intensity modulation on the continuous laser emitted by the single-frequency laser 21.

[0051] Next, the modulated laser outputs light intensity that varies periodically according to the frequency of the driving signal. This modulation frequency is precisely set to match the natural resonant frequency of the quartz tuning fork 32.

[0052] Subsequently, this periodically modulated excitation laser is coupled into a hollow anti-resonant optical fiber, where it interacts with the gas to be tested over a long distance. After absorbing the light energy, the gas molecules undergo periodic thermal expansion through a non-radiative relaxation process, thereby exciting sound waves of the same frequency.

[0053] The sound wave then travels through the fiber optic wall and drives the mechanically coupled quartz tuning fork 32 to vibrate under forced vibration.

[0054] Ultimately, the quartz tuning fork 32 utilizes its piezoelectric effect to convert mechanical vibrations into alternating current signals of the same frequency, completing the conversion and detection from optical signals to acoustic signals and then to electrical signals.

[0055] In a preferred embodiment of the present invention, the signal processing module 4 includes: The preamplifier 41 is connected to the output terminal of the quartz tuning fork 32 and is used to perform primary amplification of the electrical signal; The lock-in amplifier 42 is connected to the preamplifier 41 and the reference signal output terminal of the signal generation module 1, and is used to perform phase-sensitive demodulation on the amplified signal with the reference signal as a reference. The data processing system is connected to the lock-in amplifier 42 and is used to receive the demodulated signal from the lock-in amplifier 42, and to calculate and output the concentration value of the gas to be measured.

[0056] Specifically, in order to effectively extract the weak electrical signal generated by the quartz tuning fork 32 and suppress primary noise, in this embodiment of the invention, referring to Figure 1 The current signal output by the quartz tuning fork 32 is amplified and impedance converted by the preamplifier 41.

[0057] The preamplifier 41 is preferably a low-noise transimpedance amplifier and is located close to the output of the quartz tuning fork 32. The low-noise transimpedance amplifier converts the weak current signal output by the quartz tuning fork 32 into a voltage signal and performs preliminary gain amplification. Its low-noise design can minimize the additional electronic noise introduced at the front end of the signal link, thereby maintaining and improving the signal-to-noise ratio of the original signal and laying the foundation for subsequent high-precision demodulation processing.

[0058] Next, in order to selectively extract the photoacoustic response signal that is strictly synchronized with the laser modulation frequency from the amplified signal containing ambient noise and circuit background noise, in this embodiment of the invention, reference is made to... Figure 1 The lock-in amplifier 42 is used as the core demodulation unit. The lock-in amplifier 42 uses the reference signal output by the signal generation module 1, which is in phase and frequency with the laser modulation signal, as a reference to perform phase-sensitive demodulation on the signal from the preamplifier 41.

[0059] The core function of this lock-in amplifier 42 is to accurately measure the signal component that matches the frequency and phase of the reference signal within an extremely narrow equivalent noise bandwidth, thereby greatly suppressing non-co-frequency noise and interference. This processing method can extract the effective photoacoustic signal buried deep in the noise, ultimately leading to a significant order-of-magnitude improvement in the detection sensitivity of the entire photoacoustic spectroscopy detection system.

[0060] Furthermore, in order to accurately and reliably convert the demodulated electrical signal into gas concentration information, the DC level signal output by the lock-in amplifier 42 is calibrated and inverted by a data processing system. Based on a pre-established calibration curve or physical model between photoacoustic signal intensity and gas concentration, this data processing system automatically calculates the concentration value of the gas to be measured and displays or outputs the result in real time, thus completing the entire chain of processing from physical signal to final detection result.

[0061] As a preferred embodiment of the present invention, a lens 5 is also included, which is disposed in the optical path between the laser modulation module 2 and the hollow fiber enhancement detection module 3, for coupling the excitation laser into the hollow fiber 31.

[0062] Specifically, considering that the hollow-core fiber 31 has a small mode field diameter, in order to efficiently and with low loss couple the modulated excitation laser into its core, thereby maximizing the utilization of optical power and exciting an effective photoacoustic signal, in this embodiment of the invention, referring to... Figure 1 Lens 5 was designed and placed in the optical path.

[0063] The lens 5 is preferably a precision focusing lens, which converges the collimated laser beam from the modulator 22 and precisely adjusts the focal point to coincide with the input end face of the hollow fiber 31 to match the fiber's mode guidance.

[0064] This optimized optical coupling design can significantly reduce the insertion loss of the laser at the fiber optic inlet, ensuring that most of the laser energy enters the hollow core and interacts with the gas. This is an important step in improving the overall signal-to-noise ratio and detection sensitivity of the system.

[0065] As a preferred embodiment of the present invention, it further includes a gas communication structure, which is a gas chamber structure, for introducing the gas to be tested into the hollow core of the hollow optical fiber 31.

[0066] Specifically, since the hollow fiber 31 needs to form a closed or controllable micro-sized gas sample cavity to achieve stable photoacoustic excitation, based on this, in this embodiment of the invention, referring to... Figure 4 An integrated air chamber structure was designed to encapsulate the two ends of the hollow anti-resonant fiber AR-HCF.

[0067] from Figure 4 As can be seen, one end of the gas chamber is connected to the pressure control system 6 via a pipeline to precisely control the gas pressure inside the hollow fiber 31; the other end of the gas chamber is equipped with gas inlet and outlet (Gas in or Gas out) for the injection and discharge of the gas to be measured; the hollow anti-resonant fiber AR-HCF runs through the gas chamber, and its two ends are connected to the gas chamber wall in an airtight manner. This design allows the gas to be measured to fill and flow through the hollow fiber core under controlled pressure, forming a long optical path linear absorption.

[0068] Accordingly, refer to Figure 2 The hollow-core anti-resonant fiber AR-HCF has one end as the gas inlet (Gas in) and the other end as the gas outlet (Gas out). The excitation laser is coupled into the fiber near the gas inlet and interacts with the gas to be measured flowing into the hollow core of the fiber. The gas then flows out from the gas outlet. This linear flow design not only ensures the continuous renewal and uniformity of the gas sample within the hollow core of the fiber, but also constitutes a compact, optically path-controllable, and easily pressure-adjustable miniature gas absorption cell.

[0069] Furthermore, since in some application scenarios it is necessary to spatially separate the excitation region and the detection region of the photoacoustic signal, and in order to facilitate the installation, replacement and winding coupling operation of the optical fiber, in this embodiment of the invention, the hollow optical fiber 31 can also be placed outside the gas chamber. The gas chamber structure is designed as an independent and sealed optical gas chamber, which includes an input gas cell and an output gas cell, and the two are optically connected by a section of hollow optical fiber 31 outside the gas chamber.

[0070] Specifically, refer to Figure 5 Taking a 5-meter-long hollow-core fiber 31 (HCF) and low-pressure hydrogen bromide gas (HBr) as an example, the low-pressure HBr is filled and sealed in the independent input gas cell 7 and output gas cell 8 through the gas inlet and outlet gas, while the HCF runs through and connects the two gas cells, and its main body is exposed outside the gas cells.

[0071] In actual operation, the excitation laser is collimated and focused by a lens 5, and then coupled into the input end of the HCF through the optical input window 71 on the input gas chamber 7. The laser is transmitted in the optical fiber and interacts with the gas to be measured in the gas chambers at both ends. Finally, it is emitted from the output end and passes through the optical output window 81 on the output gas chamber 8. Then, an infrared bandpass filter 82 and a detector 83 are sequentially arranged after the optical output window 81. The infrared bandpass filter 82 is used to filter out stray light and ensure that only signal light of a specific wavelength reaches the detector 83. The detector 83 is used to receive the transmitted light intensity, which can be used by the system to perform reference measurement of the direct absorption spectrum or light intensity monitoring, thereby helping to improve the completeness of the overall system function and the reliability of measurement.

[0072] It is worth mentioning that the gas communication structure can also be a fiber optic adapter structure. This fiber optic adapter structure uses a customized bare wire adapter design to perform gas-tight docking and optical path coupling between single-mode fiber and hollow fiber 31, thereby forming a compact fiber optic microcavity that can be directly connected to an external gas path.

[0073] Specifically, the fiber optic adapter structure is equipped with a single-mode fiber and a hollow fiber 31 at each end, forming a sealed microchannel inside. The gas to be measured can be introduced through the gas inlet and outlet on the side wall of the adapter and fill the fiber core. The laser is directly coupled into the hollow fiber 31 through the single-mode fiber for long-range absorption. This structure eliminates the need for a separate gas chamber, significantly reducing the system volume and dead volume, facilitating integration and rapid gas replacement, and is suitable for applications with higher requirements for response speed and system integration.

[0074] The two gas communication structures mentioned above, namely the integrated gas chamber structure and the fiber optic adapter structure, provide the system with a flexible and reliable solution for introducing and storing the gas to be tested.

[0075] Among them, the integrated gas chamber structure is suitable for laboratory or online monitoring scenarios where precise control of optical path and gas pressure is required and long optical path absorption is needed. Its design ensures the uniformity and stability of gas samples, and facilitates system calibration and long-term continuous measurement.

[0076] The fiber optic adapter, on the other hand, has a more compact structure, focusing on system integration and rapid response. It achieves rapid replacement of gas samples by minimizing dead volume, making it suitable for portable or field testing applications with stringent response time requirements.

[0077] As can be seen, the two structures can be selected or combined according to different detection needs and environments, so that the gas detection system of the present invention has both high sensitivity, high adaptability and good engineering feasibility.

[0078] Reference Figure 6 The present invention also includes a photoacoustic spectroscopy gas detection method based on hollow optical fiber and quartz tuning fork, applied to a photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork as described above, comprising: Step S1: Generate a driving signal and a reference signal through the signal generation module 1; Step S2: In response to the driving signal, the laser intensity is modulated to generate an excitation laser with a modulation frequency that matches the resonant frequency of the quartz tuning fork 32. Step S3: Couple the excitation laser to the hollow optical fiber 31 filled with the gas to be tested, guide the excitation laser to interact with the gas to be tested, and transfer the periodic thermal expansion generated by the gas to be tested after absorbing light energy to the fiber wall. Step S4: The quartz tuning fork 32 receives the mechanical vibration driven by the thermal expansion and transmitted through the fiber wall, and converts the mechanical vibration into a corresponding electrical signal. Step S5: Based on the reference signal, the electrical signal is amplified, demodulated, and analyzed to obtain the concentration value of the gas to be measured.

[0079] Specifically, in this embodiment of the invention, to achieve highly sensitive and selective detection of trace gases, a highly stable sinusoidal driving signal and a reference signal of the same origin are first generated by the signal generation module 1. The driving signal is used to modulate the intensity of the output laser of the single-frequency laser 21, generating a periodic excitation laser that is precisely synchronized with the resonant frequency of the quartz tuning fork 32. Secondly, the modulated laser is efficiently coupled into a hollow anti-resonant optical fiber 31 filled with the gas to be measured, so that it interacts with gas molecules over a long optical path. The resulting periodic thermal expansion drives the quartz tuning fork 32 to vibrate through the fiber wall and is converted into an electrical signal. Finally, the lock-in amplifier 42 is used to perform phase-sensitive demodulation of the electrical signal based on the reference signal, and the gas concentration is retrieved by the data processing system.

[0080] In summary, this invention constructs a novel hollow-core fiber-enhanced photoacoustic spectroscopy gas detection system by employing hollow-core anti-resonant optical fiber as a long-path gas absorption and acoustic wave transmission medium and mechanically coupling it with a quartz tuning fork. This system significantly extends the effective absorption optical path using fiber waveguides and optimizes the coupling efficiency between acoustic energy and the quartz tuning fork through a winding structure, achieving comprehensive enhancement of the "optical energy-acoustic energy-electrical energy" signal link.

[0081] Compared with the prior art, the present invention has the following advantages: Significantly extended optical path and greatly improved sensitivity: By using hollow anti-resonant fiber as a microcavity waveguide, the limitation of the narrow photoacoustic interaction area in traditional QEPAS technology is effectively overcome, extending the interaction distance between light and gas from the millimeter level to the meter level, fundamentally improving the single-path absorption efficiency and releasing the potential for improved detection sensitivity.

[0082] Enhanced sound pressure signal and high signal-to-noise ratio: The fiber microcavity structure has the characteristics of small gas heat capacity and limited heat diffusion, which can generate stronger local temperature rise and sound pressure under the same laser power; at the same time, the mechanical design of the fiber-wound tuning fork increases the effective contact area, and by optimizing the winding position and tension, weak sound vibrations can efficiently drive the tuning fork resonance, significantly improving the sound energy transmission efficiency and signal-to-noise ratio.

[0083] The system has high integration and good stability: the entire detection module (optical excitation, acoustic detection, and electrical conversion) is integrated into a compact fiber-tuning fork coupling structure, which ensures optical path stability and avoids misalignment problems caused by complex spatial optical path alignment and sensitivity to environmental disturbances, thereby improving the long-term reliability and robustness of the system.

[0084] Flexible design and wide applicability: The system can be flexibly configured as an optical fiber-filled gas type or an independent gas chamber connection type, facilitating gas sampling and module replacement in different application scenarios (such as online monitoring and laboratory analysis). Combined with tunable single-frequency laser and lock-in amplification detection technology, it can achieve highly selective and sensitive detection of a variety of trace gases.

[0085] The above description is merely a preferred embodiment of the present invention and does not limit the implementation and protection scope of the present invention. Those skilled in the art should realize that any equivalent substitutions and obvious changes made based on the description and illustrations of the present invention should be included within the protection scope of the present invention.

Claims

1. A photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork, characterized in that, include: The signal generation module (1) is used to provide drive signals and reference signals; A laser modulation module (2) is connected to the signal generation module (1) and is used to modulate the laser based on the driving signal to obtain an excitation laser. A hollow fiber enhanced detection module (3) is connected to the laser modulation module (2). The hollow fiber enhanced detection module (3) includes a hollow fiber (31) and a quartz tuning fork (32). The input end of the hollow fiber (31) receives the excitation laser. The hollow core of the hollow fiber (31) contains the gas to be tested, which is used to guide the excitation laser to interact with the gas to be tested and to transfer the periodic thermal expansion generated by the gas molecules after absorbing light energy to the fiber wall. The fork arm of the quartz tuning fork (32) is mechanically coupled to the outer wall of the hollow optical fiber (31) to receive mechanical vibrations transmitted through the optical fiber wall and convert the mechanical vibrations into electrical signals based on the piezoelectric effect. The signal processing module (4) is connected to the hollow fiber enhanced detection module (3) and the signal generation module (1) and is used to process the electrical signal based on the reference signal to obtain the concentration value of the gas to be tested.

2. The photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork according to claim 1, characterized in that, The laser modulation module (2) includes: A single-frequency laser (21) is used to generate a laser whose wavelength is aligned with the characteristic absorption peak of the gas to be measured; The modulator (22) is connected to the drive signal output terminal of the single-frequency laser (21) and the signal generation module (1), and is used to modulate the intensity of the laser based on the drive signal to generate the excitation laser, wherein the modulation frequency of the laser is matched with the natural resonant frequency of the quartz tuning fork (32).

3. The photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork according to claim 2, characterized in that, The modulator (22) is an electro-optic intensity modulator.

4. The photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork according to claim 1, characterized in that, The hollow fiber (31) is a hollow anti-resonant fiber.

5. The photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork according to claim 1, characterized in that, The hollow optical fiber (31) is fixed to the fork arm of the quartz tuning fork (32) by winding.

6. The photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork according to claim 5, characterized in that, The hollow optical fiber (31) is wound 2 to 4 times.

7. The photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork according to claim 1, characterized in that, The signal processing module (4) includes: A preamplifier (41) is connected to the output of the quartz tuning fork (32) and is used to amplify the electrical signal at the primary level. A lock-in amplifier (42) is connected to the preamplifier (41) and the reference signal output terminal of the signal generation module (1), and is used to perform phase-sensitive demodulation on the amplified signal based on the reference signal. The data processing system is connected to the lock-in amplifier (42) and is used to receive the demodulated signal from the lock-in amplifier (42) and calculate and output the concentration value of the gas to be measured.

8. The photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork according to claim 1, characterized in that, It also includes a lens (5), which is disposed in the optical path between the laser modulation module (2) and the hollow fiber enhancement detection module (3) to couple the excitation laser into the hollow fiber (31).

9. The photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork according to claim 1, characterized in that, It also includes a gas communication structure, which is a gas chamber structure or an optical fiber adapter structure, for introducing the gas to be tested into the hollow core of the hollow optical fiber (31).

10. A photoacoustic spectroscopy gas detection method based on hollow optical fiber and quartz tuning fork, characterized in that, The photoacoustic spectroscopy gas detection system based on hollow optical fiber and quartz tuning fork as described in any one of claims 1-9 includes: Step S1: Generate a driving signal and a reference signal through the signal generation module (1); Step S2, in response to the driving signal, the laser intensity is modulated to generate an excitation laser with a modulation frequency that matches the resonant frequency of the quartz tuning fork (32); Step S3: Couple the excitation laser to the hollow fiber (31) filled with the gas to be tested, guide the excitation laser to interact with the gas to be tested, and transfer the periodic thermal expansion generated by the gas to be tested after absorbing light energy to the fiber wall. Step S4: The mechanical vibration driven by the thermal expansion and transmitted through the fiber wall is received by the quartz tuning fork (32), and the mechanical vibration is converted into a corresponding electrical signal. Step S5: Based on the reference signal, the electrical signal is amplified, demodulated, and analyzed to obtain the concentration value of the gas to be measured.

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