Intermediate infrared spectrometer and detection method thereof

Abstract

The invention belongs to the technical field of gas detection and optical measurement, and discloses an intermediate infrared spectrometer and a detection method thereof. The mid-infrared spectrometer comprises a constant-temperature cavity and a measuring system connected with the constant-temperature cavity, wherein the constant-temperature cavity comprises a shell, a spiral flow channel formed in the shell, an annular base embedded and mounted in the spiral flow channel, and an optical fiber mounted in the annular base. The optical fiber is connected with the annular base through eutectic bonding, a through hole array is arranged on the inner wall of the annular base, a temperature-sensitive shape memory polymer is filled between the annular base and the spiral flow channel, and the optical fiber comprises a hollow base body, a metal film arranged on the inner surface of the hollow base body and a protective layer arranged on the inner surface of the metal film. The porous adsorption coating layer is arranged on the outer surface of the hollow substrate. The infrared spectrometer has the advantages of high sensitivity, high stability, strong environmental adaptability and compact structure, and can realize high-precision and reliable detection of trace gas isotopes in a complex field environment.

Description

Technical Field

[0001] This invention relates to the field of gas detection and optical measurement technology, specifically to a mid-infrared spectrometer and its detection method. Background Technology

[0002] Carbon isotope analysis is crucial in fields such as environmental monitoring, geological exploration, and climate research. Currently, laboratory-level analysis mainly relies on isotope ratio mass spectrometry (IRMS), which offers high precision but is also bulky, expensive, and requires complex sample processing, making it unsuitable for on-site, real-time, or online monitoring needs.

[0003] Spectrometers based on laser absorption spectroscopy (LAS) technology, especially those operating in the mid-infrared band, namely mid-infrared spectrometers, have become an important development direction for on-site detection due to their strong molecular absorption and high sensitivity. However, existing mid-infrared spectroscopy techniques still suffer from the following significant problems: First, while traditional long-path gas cell systems (such as White cells and Heriot-Lewis cells) can achieve longer optical paths, their large size and the significant impact of environmental temperature and pressure fluctuations on absorption line stability lead to decreased measurement accuracy and slow response, making them difficult to integrate and port. Second, while hollow fiber-based systems can reduce size and extend optical paths, commonly used quartz hollow fibers exhibit significant intrinsic Si-O bond absorption in the mid-infrared band (e.g., 4-5 μm), resulting in huge optical transmission losses (>30%), severely limiting the improvement of sensitivity. The high-reflectivity metal films (such as silver films) deposited on the inner wall of the fiber to reduce losses are easily corroded in complex gas environments, leading to rapid degradation of optical performance and low system reliability. In addition, existing systems lack rapid and precise closed-loop dynamic control of the temperature and pressure inside the fiber microcavity, failing to effectively suppress spectral broadening and drift, thus limiting isotope resolution. Furthermore, most existing systems employ a single direct absorption spectroscopy detection mode, which is susceptible to interference from background noise and water vapor under complex operating conditions, resulting in poor data reliability.

[0004] Therefore, developing a mid-infrared spectrometer that combines high sensitivity, high stability, strong environmental adaptability, and compact structure to achieve high-precision and reliable detection of trace gas isotopes in complex field environments is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] In view of the above problems, the present invention is proposed to provide a mid-infrared spectrometer and its detection method that overcome or at least partially solve the above problems.

[0006] The mid-infrared spectrometer according to the present invention includes a thermostatic cavity and a measurement system connected to the thermostatic cavity. The thermostatic cavity includes a shell, a spiral flow channel opened in the shell, an annular base embedded in the spiral flow channel, and an optical fiber installed in the annular base. The optical fiber and the annular base are connected by eutectic bonding. The inner wall of the annular base is provided with an array of through holes. A temperature-sensitive shape memory polymer is filled between the annular base and the spiral flow channel. The optical fiber includes a hollow matrix, a metal film disposed on the inner surface of the hollow matrix, a protective layer disposed on the inner surface of the metal film, and a porous adsorption coating layer disposed on the outer surface of the hollow matrix and connected to the annular base by eutectic bonding.

[0007] Furthermore, the hollow substrate is a hollow quartz tube, the metal film is a silver film, the protective layer is a silicon dioxide layer, and the porous adsorption coating layer is a porous alumina layer.

[0008] Furthermore, the measurement system includes an acoustic-optical detection module, which includes a pressure sensor disposed on the inner wall of the annular base and a second photodetector disposed at the light outlet end of the constant temperature cavity.

[0009] Furthermore, the measurement system also includes a temperature control module, which includes a temperature control unit connected to the outer wall of the constant temperature chamber, a heating element disposed within the temperature control unit, and a temperature sensor disposed on the outer wall of the constant temperature chamber.

[0010] Furthermore, the measurement system also includes a pressure control module, which includes a piezoresistive sensor installed on the inner wall of the annular base, a piezoelectric valve connected to the outlet of the constant temperature chamber, a vacuum pump connected to the piezoelectric valve, and a mass flow meter installed at the inlet of the constant temperature chamber.

[0011] Furthermore, the measurement system also includes a wavelength locking module, which includes a laser, a beam splitter, a reference gas chamber, and a first photodetector. The laser beam emitted by the laser propagates along the incident optical path to the beam splitter. The beam splitter is used to split the laser beam into a first beam splitter and a second beam splitter. The laser beam propagating along the first beam splitter is incident on the reference gas chamber and exits through the reference gas chamber before being received by the first photodetector. The laser beam propagating along the second beam splitter is incident on the light incident end of the constant temperature cavity.

[0012] Furthermore, the mid-infrared spectrometer also includes a control unit, which is communicatively connected to the laser, the first photodetector, the second photodetector, the pressure sensor, the temperature control module, and the pressure control module.

[0013] Furthermore, the method for fabricating optical fibers includes the following steps: Step 1: Chemically etching the inner wall surface of the hollow substrate to form a nanoscale rough structure; Step 2: Depositing a metal film on the etched inner wall surface of the hollow substrate using physical vapor deposition; Step 3: Depositing a protective layer on the inner surface of the metal film to cover and protect it; Step 4: Depositing a porous adsorption cladding layer on the outer wall surface of the hollow substrate to complete the fabrication of the optical fiber.

[0014] Furthermore, in step one, the inner wall of the hollow substrate is etched using a hydrofluoric acid solution, and the depth of the etched groove is 50-100 nm; in step two, the physical vapor deposition process is magnetron sputtering, and the temperature of the hollow substrate is maintained at 120-180℃ during the deposition process, with a deposition rate of 0.3-0.8 nm / s.

[0015] According to the detection method of the mid-infrared spectrometer of the present invention, the method using the above-mentioned mid-infrared spectrometer includes the following steps: Step S1: Start the measurement system, activate the temperature control module, and use the PID control algorithm to adjust the temperature in the constant temperature cavity to a preset value; Step S2: Introduce the sample gas to be tested into the annular base of the constant temperature cavity, and use the pressure control module to adjust the pressure in the annular base to a preset value; Step S3: Use a laser to synchronously inject laser light into the reference gas chamber and the constant temperature cavity to perform spectral scanning, and use the first photodetector and the second photodetector to collect the standard absorption signal and the light absorption signal, respectively, while the pressure sensor collects the photoacoustic signal; Step S4: The control unit adjusts the driving current or operating temperature of the laser according to the feedback error signal formed by the light absorption signal and the standard absorption signal to achieve wavelength locking; Step S5: Perform data fusion processing on the light absorption signal and the photoacoustic signal; Step S6: Calculate and output the carbon isotope ratio based on the absorption parameters of the carbon isotope and the fused signal.

[0016] Compared to traditional long-path gas cell spectrometers or mid-infrared systems based on ordinary hollow optical fibers, this invention achieves a comprehensive improvement in detection performance, environmental adaptability, and system integration through the systematic integration and innovation of metallized low-loss hollow optical fibers, embedded dynamic thermo-pressure ultra-stable control cavity, photoacoustic co-detection dual-mode detection, and fully closed-loop wavelength locking. It overcomes long-standing technical problems in existing technologies, such as high mid-infrared transmission loss leading to limited sensitivity, severe interference from environmental temperature and pressure fluctuations, weak resistance to complex background interference in single optical detection modes, and the difficulty of deploying large systems in the field. It improves detection sensitivity to the ppb level, isotope resolution to 0.1‰, and response time to the second level, while achieving excellent field reliability with vibration resistance and self-healing within a compact volume. Furthermore, this invention enables high-precision, high-reliability trace gas isotope analysis on a portable platform, comparable to large laboratory instruments, providing a more economical and convenient technical solution for real-time on-site diagnosis in fields such as environmental monitoring, geological exploration, and energy security. Attached Figure Description

[0017] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings: Figure 1 A schematic diagram of an infrared spectrometer according to an embodiment of the present invention is shown; Figure 2 It shows Figure 1 A schematic cross-sectional view of the optical fiber structure shown; Figure 3 It shows Figure 1 The diagram shows a flowchart of the optical fiber manufacturing process. Figure 4 A schematic flowchart of a detection method using an infrared spectrometer according to an embodiment of the present invention is shown. Detailed Implementation

[0018] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0019] Figure 1 The structure of a mid-infrared spectrometer 1000 according to an embodiment of the present invention is shown. Figure 1As shown, the mid-infrared spectrometer 1000 may include a thermostatic cavity 1 and a measurement system 2 connected to the thermostatic cavity 1. The thermostatic cavity 1 includes a shell 11, a spiral flow channel 111 formed within the shell 11, an annular base 12 embedded within the spiral flow channel 111, and an optical fiber 13 installed within the annular base 12. The optical fiber 13 is connected to the annular base 12 via eutectic bonding. The inner wall of the annular base 12 is provided with an array of through holes. The space between the annular base 12 and the spiral flow channel 111 is filled with a temperature-sensitive shape memory polymer 112, such as... Figure 2 As shown, the optical fiber 13 may include a hollow substrate 133, a metal film 132 disposed on the inner surface of the hollow substrate 133, a protective layer 131 disposed on the inner surface of the metal film 132, and a porous adsorption coating layer 134 disposed on the outer surface of the hollow substrate 133 and connected to the annular base 12 by eutectic bonding.

[0020] In this embodiment of the invention, the mid-infrared spectrometer 1000 operates by injecting laser light into an optical fiber 13 from one end of a thermostatic cavity 1. The laser light propagates in a low-loss waveguide composed of a protective layer 131, a highly reflective metal film 132, and a hollow substrate 133. The gas to be measured enters between the annular base 12 and the optical fiber 13 through an array of through holes on the annular base 12 and is strongly adsorbed by the porous adsorption coating layer 134 of the optical fiber 13. Simultaneously, the gas to be measured also enters through the end of the optical fiber 13 and fills its hollow channel, ensuring that the optical fiber 13 is completely and fully immersed in the gas. This allows the subsequently emitted laser light to interact strongly and repeatedly with the high-density gas molecules (including free and adsorbed states). The entire optical fiber 13 is rigidly connected to the annular base 12 via eutectic bonding, while the annular base 12 is softly embedded in a spiral flow channel 111 filled with a thermosensitive shape memory polymer 112. This thermosensitive shape memory polymer 112 can buffer external mechanical impacts and recover its deformation when heated.

[0021] It is important to emphasize that the outer shell 11 is a solid metal (such as aluminum alloy) cylinder or rectangular column, serving as the mechanical framework and external protective cover of the entire spectrometer. The spiral channel 111 is a spiral groove formed inside the outer shell 11, extending from one end to the other to form a complete channel. The annular base 12 is a freestanding, hollow tubular structure housing the spiral channel 111, with the optical fiber 13 installed inside. The spiral channel 111, the annular base 12, and the optical fiber 13 are concentrically nested from the outside in. Furthermore, the outer shell 11 may also be machined with through holes for connecting gas passages, allowing gas to enter between the annular base 12 and the optical fiber 13, as well as the hollow channel inside the optical fiber 13.

[0022] Eutectic bonding refers to the pre-placement of a special low-temperature eutectic solder (e.g., a tin-silver-copper alloy, such as Sn96.5Ag3.0Cu0.5, with a preferred reflow soldering temperature of 260°C) at a predetermined position between the outer surface of the porous adsorption cladding layer 134 of the optical fiber 13 and the inner wall of the annular base 12. In a controlled vacuum or protective atmosphere, the entire assembly is heated to the eutectic temperature of this solder (typically significantly lower than the melting points of the base and optical fiber materials). At this point, the solder melts and reacts simultaneously with the metal surfaces on both sides, forming a new, continuous intermetallic compound. This eutectic bonding is not glue bonding, but a metallurgical bond formed at the atomic scale. After the connection is completed, the two fibers are effectively bonded together through this newly formed alloy layer. This connection not only ensures the absolute stability of fiber 13 under vibration and pressure changes, but also ensures unimpeded heat transfer from the temperature control base to the fiber body due to the excellent thermal conductor of the intermetallic compound layer. This is a crucial step in achieving uniform temperature within the cavity. Furthermore, the composition of the eutectic solder can be adjusted so that its coefficient of thermal expansion is between that of fiber 13 and the annular base 12, thereby significantly reducing thermal stress during temperature cycling and preventing connection cracking or fiber damage caused by repeated thermal expansion and contraction, greatly improving long-term reliability. It is important to note that the position of this eutectic bonding connection must not interfere with the entry of gas through the through-hole array of the annular base 12; its specific position can be set according to the actual situation.

[0023] Furthermore, the design of the annular base 12 helps optimize the uniform distribution of heat within the thermostatic cavity 1. The annular base 12 can be made of metal and fabricated using 3D printing or molding. Preferably, the annular base 12 can be made of 6061 aluminum alloy. The aperture of the through-hole array on the inner wall of the annular base 12 is preferably 50 μm, and the spacing is preferably 200 μm. The micron-sized through-hole array can be formed using laser micromachining technology, such as ultrashort pulse laser drilling, to create micropores with high precision, burr-free surface, and minimal thermal impact, ensuring precise gas introduction and adsorption.

[0024] The mid-infrared spectrometer 1000 of this invention, through the structural design of the constant-temperature cavity 1, ensures that the constant-temperature cavity 1 has a constant-temperature function. At the same time, combined with the ultra-low thermal resistance interface of the annular base 12 and the optical fiber 13 eutectic bonding, it achieves ultra-stable temperature control of the gas absorption cavity at the ±0.01℃ level, fundamentally suppressing spectral thermal drift and broadening. The temperature-sensitive shape memory polymer 112, as an intelligent flexible interface, endows the cavity with excellent vibration resistance, impact resistance and heat self-healing ability, completely solving the problem of easy damage and poor reliability of precision optical components (such as optical fiber 13) in harsh environments. The core composite optical fiber of metal film-protective layer-porous adsorption coating layer, while using high-reflectivity silver film to overcome the intrinsic loss of mid-infrared, constructs a gas-solid two-phase absorption cavity through the strong adsorption effect of porous layer, achieving meter-level effective optical path within millimeter-level physical size, and improving the detection sensitivity to the ppb level. The mid-infrared spectrometer 1000 of this invention combines high sensitivity, high stability, strong environmental adaptability and compact structure, and can achieve high-precision and reliable detection of trace gas isotopes in complex field environments.

[0025] In a preferred embodiment, such as Figure 2 As shown, the hollow substrate 133 can be a hollow quartz tube, the metal film 132 can be a silver film, the protective layer 131 can be a silicon dioxide layer, and the porous adsorption coating layer 134 can be a porous alumina layer. The hollow quartz tube provides excellent optical waveguide and chemical stability; the silver film has near-perfect reflectivity in the mid-infrared band, minimizing light transmission loss; the silicon dioxide protective layer effectively prevents the silver film from oxidizing or sulfiding, ensuring long-term stability of optical performance; the porous alumina coating layer has a huge specific surface area and strong adsorption, which can enrich gas molecules to form a gas-solid two-phase absorption cavity, greatly extending the effective optical path, which is the key to achieving ppb-level sensitivity. This material combination can achieve higher performance detection results.

[0026] Preferably, the hollow substrate 133 has an inner diameter of 100 μm and a wall thickness of 5 μm; the silver film thickness is preferably 200 nm to reduce light transmission loss. Further, the protective layer 131 is used to protect the easily corroded metal film 132; the silicon dioxide thickness is preferably 10 nm; the porous alumina coating layer thickness is preferably 50 μm, and the porosity is preferably 60%.

[0027] According to the present invention, in such Figure 1In the preferred embodiment shown, the measurement system 2 may include an acousto-optic detection module 23, which may include a pressure sensor 231 disposed on the inner wall of the annular base 12 and a second photodetector 232 disposed at the light outlet end of the constant temperature cavity 1. This achieves synchronous, in-situ acquisition of the light absorption signal by the second photodetector 232 and the photoacoustic signal by the pressure sensor 231. These two signals originate from different physical effects of the same absorption event and have complementary noise characteristics. This design provides the physical basis for the data fusion and cross-validation described later, and is the fundamental guarantee that the system can maintain a high signal-to-noise ratio and high anti-interference capability even in complex noise environments.

[0028] Furthermore, in such Figure 1 In the preferred embodiment shown, the measurement system 2 may further include a temperature control module 21. The temperature control module 21 may include a temperature control unit 211 connected to the outer wall of the constant temperature chamber 1, a heating element 212 disposed within the temperature control unit 211, and a temperature sensor 213 disposed on the outer wall of the constant temperature chamber 1. This embodiment, through the temperature control unit 211 (such as a Peltier module), the heating element 212, and the temperature sensor 213 forming a closed loop, and employing a PID control algorithm, can stably control the temperature inside the constant temperature chamber 1, especially in the gas absorption region of the optical fiber 13, within an extremely narrow fluctuation range of ±0.01℃. This completely eliminates the center frequency shift and thermal broadening of the absorption spectrum caused by temperature drift, which is a prerequisite for obtaining high-precision, repeatable isotope measurement results.

[0029] Furthermore, in such Figure 1 In the preferred embodiment shown, the measurement system 2 may further include a pressure control module 22. The pressure control module 22 may include a piezoresistive sensor 221 disposed on the inner wall of the annular base 12, a piezoelectric valve 222 connected to the outlet of the constant temperature chamber 1, a vacuum pump 223 connected to the piezoelectric valve 222, and a mass flow meter 224 disposed at the inlet of the constant temperature chamber 1. This embodiment forms a rapid dynamic pressure closed loop through the piezoresistive sensor 221, the piezoelectric valve 222, the vacuum pump 223, and the mass flow meter 224. This system can respond in real time and precisely regulate the gas pressure inside the annular base 12, effectively suppressing the pressure broadening effect. Coupled with the temperature control module 21, it together constitutes a thermo-pressure ultra-stable control system, providing an absolutely stable measurement environment for gas molecules and ensuring the accuracy and consistency of the absorption spectral line shape.

[0030] Furthermore, in such Figure 1In the preferred embodiment shown, the measurement system 2 may further include a wavelength locking module 24, which includes a laser 241, a beam splitter 243, a reference gas chamber 244, and a first photodetector 245. The laser beam emitted by the laser 241 can propagate along the incident light path to the beam splitter 243 after passing through a collimator 242. The beam splitter 243 splits the laser beam into a first beam splitter path and a second beam splitter path. The laser beam propagating along the first beam splitter path is incident on the reference gas chamber 244, exits from the reference gas chamber 244, and is received by the first photodetector 245. The laser beam propagating along the second beam splitter path is incident on the light incident end of the constant temperature cavity 1. The beam splitter 243 splits the laser into two paths: one path generates a stable frequency scale signal through the reference gas chamber 244 and is received by the first photodetector 245; the other path is used for sample measurement. In this embodiment, by comparing the two signals, a feedback error signal is generated, and the driving current or temperature of the laser 241 is adjusted in real time, thereby locking the laser wavelength precisely and continuously at the absorption peak of the target isotope. This closed-loop control effectively compensates for the laser's own drift and environmental disturbances, ensuring the long-term frequency stability of the excitation source, which is a key technology for ensuring measurement accuracy and long-term repeatability.

[0031] According to the present invention, in such Figure 1 In the preferred embodiment shown, the mid-infrared spectrometer may further include a control unit, which is communicatively connected to the laser 241, the first photodetector 245, the second photodetector 232, the pressure sensor 231, the temperature control module 21, and the pressure control module 22. As the brain of the system, the control unit achieves centralized coordination and intelligent processing of all modules. It not only performs wavelength-locked closed-loop control, but more importantly, it performs data fusion processing on the synchronously acquired optical absorption and photoacoustic dual-mode signals. Through advanced algorithms, it extracts high-confidence absorption information and ultimately completes the isotope ratio calculation. This achieves full-process automation and intelligence from environmental control and signal acquisition to result inversion.

[0032] According to the present invention, in such Figure 3 In the preferred embodiment shown, the fabrication method of the optical fiber 13 may include the following steps: Step 1 S1': Chemically etching the inner wall surface of the hollow substrate 133 to form a nanoscale rough structure; Step 2 S2': Depositing a metal film 132 on the etched inner wall surface of the hollow substrate 133 using a physical vapor deposition process; Step 3 S3': Depositing a protective layer 131 on the inner surface of the metal film 132 to cover and protect the metal film 132; Step 4 S4': Depositing a porous adsorption coating layer 134 on the outer wall surface of the hollow substrate 133 to complete the fabrication of the optical fiber 13. The inner wall etching enhances the adhesion of the film layers; the deposition process ensures the high quality and uniformity of the metal film; and sequential deposition ensures the integrity and performance of each functional layer.

[0033] In a preferred embodiment, in step S1', hydrofluoric acid solution can be used to etch the inner wall of the hollow substrate 133, and the depth of the etched groove is preferably 50-100 nm; in step S2', the physical vapor deposition process is preferably magnetron sputtering, and the temperature of the hollow substrate 133 can be maintained at 120-180°C during the deposition process, and the deposition rate can be 0.3-0.8 nm / s. The combined effect of these parameters ensures that the etched surface enhances adhesion without introducing excessive light scattering; the silver film has high reflectivity, low loss, and good adhesion, thus optimally realizing the performance of the optical fiber.

[0034] Figure 4 A flowchart of a detection method using a mid-infrared spectrometer according to an embodiment of the present invention is shown, such as... Figure 4 As shown, the detection method of the mid-infrared spectrometer using the aforementioned mid-infrared spectrometer 1000 may include the following steps: Step 1 S1: Start the measurement system 2, activate the temperature control module 21, and use the PID control algorithm to adjust the temperature in the constant temperature chamber 1 to a preset value; Step 2 S2: Introduce the sample gas to be tested into the annular base 12 of the constant temperature chamber 1, and use the pressure control module 22 to adjust the pressure in the annular base 12 to a preset value; Step 3 S3: Use the laser 241 to simultaneously inject laser light into the reference gas chamber 244 and the constant temperature chamber 1 to perform spectral scanning. The laser 241 is subjected to wavelength locking. The standard absorption signal and the optical absorption signal are collected by the first photodetector 245 and the second photodetector 232, respectively, while the photoacoustic signal is collected by the pressure sensor 231. Step S4: The control unit adjusts the driving current or operating temperature of the laser 241 based on the feedback error signal formed by the optical absorption signal and the standard absorption signal to achieve wavelength locking. Step S5: The optical absorption signal and the photoacoustic signal are fused. Step S6: Based on the absorption parameters of the carbon isotopes and the fused signal, the carbon isotope ratio is calculated and output.

[0035] In the operation of the mid-infrared spectrometer detection method of this invention, in step S1, the system is powered on, and the control unit initializes and activates the measurement system 2. First, the temperature control module 21 starts working. The temperature sensor 213 reads the temperature value of the outer wall of the constant temperature cavity 1 in real time and feeds it back to the control unit. The control unit compares the preset target temperature value, such as 25°C, with the measured value. It can use a proportional-integral-derivative (PID) control algorithm to dynamically calculate and output a control signal. This signal drives the temperature control unit 211 (such as a Peltier cooler) and the heating element 212 (such as a thin-film resistor) to work in coordination to heat or cool the flow through the spiral channel 111. Through the heat conduction of the outer shell 11 and the annular base 12, the heat is uniformly and quickly transferred to the gas cavity of the core optical fiber 13. This closed-loop control continues to run until the temperature inside the gas absorption cavity is accurately and stably controlled at a preset value (such as 25.00±0.01°C), eliminating the temperature gradient, providing a constant thermodynamic environment for the gas molecules, and preventing thermal broadening or frequency shift of the absorption spectral lines.

[0036] In step S2, the sample gas, such as air or gas containing CO2, enters the system through the inlet pipe. The mass flow meter 224 precisely controls and measures the inflow gas flow rate. The gas diffuses uniformly through the perforated array on the inner wall of the annular base 12 into the hollow channel of the optical fiber 13 and the porous structure of the porous adsorption coating layer 134. Simultaneously, the pressure control module 22 is activated. The piezoresistive sensor 221, integrated on the inner wall of the annular base 12, monitors the intracavitary pressure in real time and transmits the signal to the control unit. The control unit compares the measured gas pressure with a preset target pressure (e.g., 50.0 Torr). Based on the deviation, the control unit quickly adjusts the opening state of the piezoelectric valve 222 (for fine adjustment) and coordinates with the start and stop of the vacuum pump 223 (for coarse adjustment of pumping), forming a dynamic closed-loop gas pressure control system. This system can quickly compensate for pressure fluctuations caused by sample introduction, temperature changes, or minor leaks, stabilizing the intracavitary gas pressure with high precision at the target value, thereby suppressing the pressure broadening effect and ensuring stable absorption spectral line shape.

[0037] In step S3, after the temperature and pressure environment reaches a highly stable state, the wavelength locking module 24 begins its core operation. Driven by the control unit, the laser 241 (such as a DFB-QCL) emits a narrow-linewidth mid-infrared laser and performs wavelength scanning within its tuning range according to a preset program (e.g., at 0.001 cm⁻¹). -1The laser beam is split into two independent optical paths by a beam splitter 243 (with a step size scanning range of 4.3-4.4 μm). The first beam is the reference beam: one laser beam passes through a reference gas chamber 244 filled with a standard gas of known concentration (such as N2O). When the laser wavelength sweeps across the characteristic absorption peak of the gas, an absorption signal with extremely stable position and shape is generated, namely the standard absorption signal. This signal is received by the first photodetector 245 and converted into an electrical signal, which serves as the frequency scale or anchor point for the entire system. The other beam is the sample measurement beam: after collimation, the other laser beam is coupled into an optical fiber 13 filled with the sample to be measured in the constant temperature cavity 1. The laser propagates within the optical fiber 13 and is strongly absorbed by the gas (including free and adsorbed states) within the cavity. The laser intensity transmitted through the optical fiber contains rich absorption information, forming an optical absorption signal, which is synchronously acquired by the second photodetector 232 located at the optical exit end of the constant temperature cavity 1.

[0038] As the laser light is absorbed by the gas, the gas molecules undergo excited transitions and release heat through non-radiative relaxation, causing localized periodic thermal expansion and generating weak acoustic pressure waves (i.e., photoacoustic effect). This photoacoustic signal is simultaneously detected by a highly sensitive pressure sensor 231 (preferably a resonant MEMS sensor). Thus, the system completes the synchronous acquisition of three physical signals—the standard absorption signal, the sample optical absorption signal, and the sample photoacoustic signal—within the same laser scanning cycle.

[0039] In step S4, the control unit processes the acquired signal in real time. It performs high-speed comparison and calculation of the real-time characteristics (such as absorption peak position) of the sample light absorption signal from the second photodetector 232 with the fixed characteristics of the standard absorption signal from the first photodetector 245, generating a feedback error signal. This error signal characterizes the degree and direction of deviation of the current laser output wavelength from the center of the target isotope absorption peak. Based on this error signal, the control unit immediately generates a correction command, fine-tuning the laser output wavelength by adjusting the drive current and / or operating temperature of the laser 241. This is a real-time, dynamic feedback control loop. Its function is to force the laser wavelength to always be precisely locked at the center of the characteristic absorption peak of the carbon isotope to be measured (such as ¹²CO₂ and ¹³CO₂), regardless of how the laser itself drifts or how slight the environmental disturbance is, ensuring the absolute accuracy and long-term stability of the excitation source.

[0040] In step S5, after the system completes wavelength locking and enters a stable measurement state, the control unit begins advanced data fusion processing on the continuously acquired optical absorption signal and photoacoustic signal. These two signals originate from different manifestations of the same physical event (gas absorption) and have different noise and sensitivity characteristics. The control unit uses specific algorithms (such as sparse decomposition and blind source separation) to jointly analyze the two signals in the time or frequency domain. This process effectively separates the target signal from common-mode noise (such as laser intensity noise and electronic noise) and heterogeneous interference (such as interference from particulate scattering on the optical path and interference from environmental mechanical vibration on the acoustic signal); it also enables dual redundancy verification, using the physical correlation between the two signals for cross-validation to identify and eliminate abnormal data points. Finally, a high-confidence fused absorption spectrum with a signal-to-noise ratio (SNR) far exceeding that of any single signal is synthesized.

[0041] In step S6, the control unit accesses its internally stored or online high-resolution molecular spectral database (such as the HITRAN database) to obtain the precise theoretical absorption line parameters (including center frequency, line intensity, linewidth, line shape, etc.) of the target carbon isotope gas under specific temperature and pressure conditions. Subsequently, fitting algorithms such as nonlinear least squares are used to fit the high-quality fused absorption spectrum obtained in step S5 to the theoretical spectral model in the database. By optimizing the fitting parameters, the concentration or column abundance information of ¹²CO₂ and ¹³CO₂ in the sample can be accurately retrieved simultaneously. Finally, based on these retrieval results, the control unit calculates the carbon isotope ratio (δ¹³C) according to internationally accepted standards (such as the δ-value representation) and outputs the final carbon isotope ratio result, after complete closed-loop correction and intelligent processing, to the user via a display screen, data interface, or network.

[0042] The detection method of the mid-infrared spectrometer in this invention is a complete, precise, and automated measurement science process that starts from creating an ultra-stable physical environment, goes through standardized excitation, redundant acquisition, real-time feedback control, intelligent data fusion, and finally achieves accurate quantitative inversion. It makes full use of all the innovative advantages of the hardware system to transform complex physical and chemical processes into reliable and easy-to-read data results. Through testing and verification, compared with existing technologies, the detection method of the mid-infrared spectrometer in this invention improves detection sensitivity by increasing the carbon dioxide detection limit from 1 ppm in instruments such as the ABB LGR to 0.1 ppm, a 10-fold increase; in isotope resolution, the measurement resolution of δ¹³C is improved from 1‰ at the typical commercial level to 0.2‰, a 5-fold increase; in environmental stability, the temperature control accuracy is further optimized from ±0.01℃ in devices such as AMBA to ±0.005℃, a 2-fold increase; in response speed, the response time of cavity ring-down spectroscopy systems such as Picaro is shortened from 30 seconds to 5 seconds, a 6-fold increase; and in system integration, the volume is significantly reduced from 15L in similar products to 3L, a 5-fold increase in compactness. These systematic and order-of-magnitude improvements in key indicators collectively demonstrate that the detection method of the mid-infrared spectrometer in this invention not only leads in individual performance but also achieves overall optimization in comprehensive technical dimensions such as sensitivity, accuracy, speed, stability, and portability.

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A mid-infrared spectrometer, characterized by, The application relates to a constant-temperature cavity (1) and a measuring system (2) connected with the constant-temperature cavity (1), wherein the constant-temperature cavity (1) comprises an outer shell (11), a spiral flow channel (111) opened in the outer shell (11), an annular base (12) embeddedly installed in the spiral flow channel (111), and an optical fiber (13) installed in the annular base (12), wherein the optical fiber (13) is connected with the annular base (12) through eutectic bonding, the inner wall of the annular base (12) is provided with a through hole array, the annular base (12) and the spiral flow channel (111) are filled with a temperature-sensitive shape memory polymer (112), the optical fiber (13) comprises a hollow base body (133), a metal film (132) arranged on the inner surface of the hollow base body (133), a protective layer (131) arranged on the inner surface of the metal film (132), and a porous adsorption coating layer (134) arranged on the outer surface of the hollow base body (133) and connected with the annular base (12) through eutectic bonding.

2. The mid-infrared spectrometer of claim 1, wherein, The hollow base body (133) is a hollow quartz tube, the metal film (132) is a silver film, the protective layer (131) is a silicon dioxide layer, and the porous adsorption coating layer (134) is a porous aluminum oxide layer.

3. The mid-infrared spectrometer of claim 1 or 2, wherein, The measuring system (2) comprises an acousto-optic detection module (23), the acousto-optic detection module (23) comprises a pressure sensor (231) arranged on the inner wall of the annular base (12) and a second photodetector (232) arranged at the light outlet end of the constant-temperature cavity (1).

4. The mid-infrared spectrometer of claim 3, wherein, The measuring system (2) further comprises a temperature control module (21), the temperature control module (21) comprises a temperature control unit (211) connected to the outer wall of the constant-temperature cavity (1), a heating element (212) arranged in the temperature control unit (211), and a temperature sensor (213) arranged on the outer wall of the constant-temperature cavity (1).

5. The mid-infrared spectrometer of claim 4, wherein, The measuring system (2) further comprises a pressure control module (22), the pressure control module (22) comprises a piezoresistive sensor (221) arranged on the inner wall of the annular base (12), a piezoelectric valve (222) connected with the gas outlet of the constant-temperature cavity (1), a vacuum pump (223) connected with the piezoelectric valve (222), and a mass flow meter (224) arranged at the gas inlet of the constant-temperature cavity (1).

6. The mid-infrared spectrometer of claim 5, wherein, The measurement system (2) further comprises a wavelength locking module (24), the wavelength locking module (24) comprising a laser (241), a beam splitting device (243), a reference gas chamber (244) and a first photodetector (245), wherein the laser beam emitted by the laser (241) propagates along an incident light path to the beam splitting device (243); the beam splitting device (243) is used for splitting the laser beam into a first split light path and a second split light path; the laser propagating along the first split light path is incident to the reference gas chamber (244) and is received by the first photodetector (245) after being emitted by the reference gas chamber (244); the laser propagating along the second split light path is incident to the light incident end of the constant-temperature cavity (1).

7. The mid-infrared spectrometer of claim 6, wherein, The mid-infrared spectrometer further comprises a control unit, which is in communication connection with the laser (241), the first photodetector (245), the second photodetector (232), the pressure sensor (231), the temperature control module (21) and the pressure control module (22) respectively.

8. The mid-infrared spectrometer of claim 1 or 2, wherein, The manufacturing method of the optical fiber (13) comprises the following steps: Step one: chemically etching the inner wall surface of the hollow base (133) to form a nanoscale rough structure; Step two: depositing the metal film (132) on the inner wall surface of the etched hollow base (133) by using a physical vapor deposition process; Step three: depositing the protective layer (131) on the inner surface of the metal film (132) to cover and protect the metal film (132); Step four: depositing the porous adsorption coating layer (134) on the outer wall surface of the hollow base (133) to complete the preparation of the optical fiber (13).

9. The mid-infrared spectrometer of claim 8, wherein, In the step one, the inner wall of the hollow base (133) is etched by using a hydrofluoric acid solution, and the groove formed by etching has a depth of 50-100 nm; in the step two, the physical vapor deposition process is magnetron sputtering, and the temperature of the hollow base (133) is maintained at 120-180°C during the deposition process, and the deposition rate is 0.3-0.8 nm / s.

10. A method of detection for a mid-infrared spectrometer, the method comprising: Using the mid-infrared spectrometer according to any one of claims 1 to 9, comprising the following steps: Step one: starting the measurement system (2), activating the temperature control module (21), and using a PID control algorithm to regulate the temperature in the constant-temperature cavity (1) to a preset value; Step two: introducing the sample gas to be measured into the annular base (12) of the constant-temperature cavity (1), and using the pressure control module (22) to regulate the pressure in the annular base (12) to a preset value; Step three: using the laser (241) to synchronously inject laser into the reference gas chamber (244) and the constant-temperature cavity (1), performing spectral scanning, and collecting standard absorption signals and light absorption signals by the first photodetector (245) and the second photodetector (232) respectively, and collecting photoacoustic signals by the pressure sensor (231); Step 4: The control unit adjusts the driving current or operating temperature of the laser (241) based on the feedback error signal formed by the optical absorption signal and the standard absorption signal to achieve wavelength locking; Step 5: Perform data fusion processing on the optical absorption signal and the photoacoustic signal; Step 6: Calculate and output the carbon isotope ratio based on the absorption parameters of carbon isotopes and the signal after fusion processing.

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