Trace hydrogen concentration sensing system based on TDLAS absorption spectrum

Abstract

The invention relates to the technical field of hydrogen concentration detection, and particularly discloses a TDLAS (tunable diode laser absorption spectroscopy) absorption spectrum-based trace hydrogen concentration sensing system, which is used for solving the problems of low detection sensitivity, narrow linear range, large system drift and dependence on external calibration in TDLAS hydrogen detection, and comprises a laser emission module and a photoelectric detection module, the laser emission module and the photoelectric detection module are connected through the dual-optical-path absorption module, the photoelectric detection module is connected with a harmonic extraction module, the harmonic extraction module is connected with a normalization processing module, and the normalization processing module is connected with a concentration inversion module; according to the device and the method, a symmetrical double-light-path structure is constructed, a harmonic signal normalization and differential compensation algorithm is adopted, and a high-pressure reference gas pool and a segmented fitting model are combined, so that high-linearity, high-sensitivity and calibration-free concentration detection of hydrogen within 0.01-100% is realized, and the hydrogen trace recognition capability is improved.

Description

Technical Field

[0001] The invention relates to the technical field of hydrogen concentration detection, and in particular to a trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy. Background Art

[0002] As a clean, renewable, high-energy-density new energy carrier, hydrogen is widely used in energy storage, transportation, industrial manufacturing, fuel cells and other fields. However, due to the characteristics of hydrogen being colorless, odorless, extremely easy to diffuse, and flammable and explosive, it is difficult to detect when it leaks, posing a huge threat to environmental safety. Therefore, the development of a hydrogen sensing system with high sensitivity, wide dynamic detection range and rapid response capability is the key support for the safe development of the hydrogen energy utilization industry. The currently used hydrogen sensing technologies mainly include electrochemical, thermal conductivity, capacitance, resistance, photoacoustic and other methods. The Chinese invention patent with publication number CN1187948914 discloses a trace hydrogen concentration detection device By designing a dual-channel photoacoustic cell, trace hydrogen is catalytically oxidized into water vapor, and the trace water vapor is detected using photoacoustic spectroscopy to obtain the corresponding hydrogen concentration. The voltage signal is processed by differential operation to ensure that the output electrical signal and the measured hydrogen concentration show an excellent linear relationship, and the ppb-level trace hydrogen concentration detection is realized. Although this device has certain advantages in low cost and system integration, it has problems such as complex calibration, narrow linear range, response lag, and large drift error in achieving ppm-level detection and a wide-span linear response from 0.01% to 100%. Tunable Diode Laser Absorption Spectroscopy (TDLAS) has gradually become a research hotspot in the field of trace gas detection due to its non-contact, strong selectivity, fast dynamic response, and online monitoring. However, TDLAS detection of hydrogen is limited by its natural physical properties of low infrared absorption coefficient. How to improve detection sensitivity, compress system errors, and achieve accurate quantification without external calibration has become a technical bottleneck that needs to be solved in the current TDLAS detection of hydrogen. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy. By constructing a symmetrical dual optical path structure, adopting harmonic signal normalization and differential compensation algorithm, and combining a high-pressure reference gas cell with a segmented fitting model, high-linearity, high-sensitivity, and calibration-free concentration detection of hydrogen within 0.01% to 100% is achieved, thereby improving the trace hydrogen identification capability.

[0004] To achieve the above object, the present invention provides the following technical solutions: A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy includes a laser emission module and a photoelectric detection module. The laser emission module and the photoelectric detection module are connected by a double-path absorption module. The photoelectric detection module is connected to a harmonic extraction module, the harmonic extraction module is connected to a normalization processing module, and the normalization processing module is connected to a concentration inversion module. The laser emission module includes a distributed feedback tunable diode laser, and its emission wavelength covers the main absorption line of 2121.8 nm in the near-infrared of hydrogen. The double-path absorption module includes a first gas cell and a second gas cell. The first gas cell is filled with the target gas to be measured, and the second gas cell is filled with hydrogen with a known concentration and is in a high-pressure state of greater than or equal to 8 standard atmospheric pressures. The laser emission beam forms a target optical path and a reference optical path through the first gas cell and the second gas cell respectively. The photoelectric detection module includes two photodetectors, which respectively receive the projected laser signals of the target optical path and the reference optical path. The harmonic extraction module uses a lock-in amplifier to extract the first fundamental signal and the first second harmonic signal of the projected laser signal of the target optical path, and extracts the second fundamental signal and the second second harmonic signal of the projected laser signal of the reference optical path. The normalization processing module performs fundamental normalization processing on the first second harmonic signal to form a main signal ratio, and introduces a compensation ratio of the reference optical path to dynamically compensate for the light source power drift and optical path disturbance. The concentration inversion module outputs the trace concentration value of hydrogen in the target gas based on the main signal ratio and the compensation ratio, in combination with a preset fitting model and absorption coefficient parameters.

[0005] As a further solution of the present invention, a beam splitter is provided on one side of the laser. The beam splitter divides the emission beam of the laser into a first beam and a second beam. A first optical component and a second optical component are provided on the other side of the beam splitter. The photoelectric detection module includes a first photodetector and a second photodetector.

[0006] As a further solution of the present invention, the first optical component includes a first off-axis parabolic mirror. A first filter is provided above the first off-axis parabolic mirror. The first filter is located below the first photodetector. A first window is provided on the side of the first off-axis parabolic mirror away from the laser. A second window is provided on the other side of the first window. The first gas cell is located between the first window and the second window. A first full-angle reflector is provided on the other side of the second window.

[0007] As a further solution of the present invention, the optical path of the first beam split by the beam splitter from the laser is as follows: The first beam first passes through the middle opening of the first off-axis parabolic mirror and the first window, enters the first gas cell, and completes the first absorption. Subsequently, the laser beam passes through the second window and then shoots towards the first full-angle reflector. After being reflected back along the original path, it enters the first gas cell again through the second window for the second absorption. The reflected beam passes through the first window and then is incident on the parabolic surface of the first off-axis parabolic mirror. Its reflected light passes through the first filter and is focused on the light-sensitive surface of the first photodetector.

[0008] As a further solution of the present invention, the second optical component includes a second off-axis parabolic mirror. A second filter is provided above the second off-axis parabolic mirror. The second filter is located below the second photodetector. A third window is provided on the side of the second off-axis parabolic mirror away from the laser. A fourth window is provided on the other side of the third window. The second gas cell is located between the third window and the fourth window. A second full-angle reflector is provided on the other side of the fourth window.

[0009] As a further solution of the present invention, the optical path of the second light beam split by the beam splitter from the laser is as follows: The second light beam first passes through the middle opening of the second off-axis parabolic mirror and the third window, enters the second gas cell, and completes the first absorption. Subsequently, the laser beam passes through the fourth window and then shoots towards the second full-angle reflector. After being reflected along the original path, it enters the second gas cell again through the fourth window for the second absorption. The reflected light beam passes through the third window and then is incident on the parabolic surface of the second off-axis parabolic mirror. Its reflected light passes through the second filter and is focused on the photosensitive surface of the second photodetector.

[0010] As a further solution of the present invention, the normalization processing module uses the fundamental wave normalization algorithm to process the first second harmonic signal extracted from the target optical path, obtains the main signal ratio, and introduces the compensation ratio of the reference signal. The specific process includes: dividing the first second harmonic signal by the corresponding first fundamental wave signal to form the main signal ratio, and dividing the second second harmonic signal by the corresponding second fundamental wave signal to form the compensation ratio of the reference signal.

[0011] As a further solution of the present invention, in the concentration inversion module, the preset fitting model is based on the harmonic detection theory of tunable diode laser absorption spectroscopy. A piecewise linear regression function is used to construct independent fitting sub-models for the set concentration interval segments of 0.01% - 1%, 1% - 10%, and 10% - 100%. The fitting function form of the trace hydrogen concentration in each segment is: ; In the formula: is the concentration interval index, is the fitting function value of the trace hydrogen concentration in the th concentration interval, and are both fitting coefficients obtained through experiments, is the first second harmonic signal, is the first fundamental wave signal; ; In the formula: is the fitting function value of the trace hydrogen concentration in the th concentration interval after calibration, and are the second second harmonic signal and the second fundamental wave signal respectively.

[0012] As a further solution of the present invention, the effective absorption optical path of the second gas cell is greater than or equal to 2 meters, and the filled gas is a high-purity gas with a volume fraction greater than 99.99%.

[0013] As a further solution of the present invention, both the first photodetector and the second photodetector are highly sensitive detectors with a response bandwidth higher than 20 kHz and a linear dynamic range greater than or equal to 90 dB. Their photosensitive surfaces are respectively perpendicular to the reflection axes of the target optical path and the reference optical path, and are used to receive the laser signals coupled and focused to the first filter and the second filter after two absorptions and reflections.

[0014] Technical effects of a trace hydrogen concentration sensing system based on TDLAS absorption spectrum according to the present invention: Through a trace hydrogen concentration sensing system based on TDLAS absorption spectrum provided by the present invention, by constructing a double optical path structure to respectively guide the laser beam through the first gas cell filled with the gas to be measured and the second gas cell filled with high-pressure high-purity hydrogen, a symmetric absorption path configuration of the target optical path and the reference optical path is realized. Combining a beam splitter, an off-axis paraboloid mirror, a filter and a full-angle reflector to construct an enhanced absorption optical path with multiple transmissions and reflections, the amplitude of the harmonic signal and the signal-to-noise ratio are improved. The fundamental wave signal and the second harmonic signal in the target optical path and the reference optical path are synchronously extracted, and the main signal ratio and the reference compensation ratio are formed through normalization processing, dynamically eliminating the measurement errors caused by laser power fluctuations, optical path disturbances and device response drifts. Based on this ratio, combined with a preset multi-segment fitting model and absorption coefficient parameters, the hydrogen concentration results covering the range of 0.01% to 100% are output, with high linearity, low detection limit, no need for external calibration, and the ability to adapt to rapid measurement under complex working conditions, improving the quantitative detection accuracy, system stability and industrial adaptability of trace hydrogen. Description of the Drawings

[0015] Figure 1 is the system diagram of the present invention; Figure 2 is the optical path schematic diagram of the present invention; Figure 3 is for the present invention and 、 and The relationship diagram between them, where the green line is and Relationship curve, the blue line is and relationship curve; In the figure: 1. Laser emission module; 2. Photoelectric detection module; 3. Double optical path absorption module; 4. Harmonic extraction module; 5. Normalization processing module; 6. Concentration inversion module; 8. Laser; 9. Phase-locked amplifier; 10. Beam splitter; 11. First optical component; 12. Second optical component; 21. First photodetector; 22. Second photodetector; 31. First gas cell; 32. Second gas cell; 111. First off-axis paraboloid mirror; 112. First filter; 113. First window; 114. Second window; 115. First full-angle reflector; 121. Second off-axis paraboloid mirror; 122. Second filter; 123. Third window; 124. Fourth window; 125. Second full-angle reflector. Specific implementation mode

[0016] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0017] Embodiment 1

[0018] As Figure 1As shown in the figure, a trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy proposed by the present invention includes a laser emission module 1 and a photoelectric detection module 2. The laser emission module 1 and the photoelectric detection module 2 are connected by a double-path absorption module 3. The photoelectric detection module 2 is connected to a harmonic extraction module 4. The harmonic extraction module 4 is connected to a normalization processing module 5. The normalization processing module 5 is connected to a concentration inversion module 6. The laser emission module 1 includes a distributed feedback tunable diode laser 8, and its emission wavelength covers the main absorption line of hydrogen at 2121.8 nm in the near infrared. The double-path absorption module 3 includes a first gas cell 31 and a second gas cell 32. The first gas cell 31 is filled with the target gas to be measured, and the second gas cell 32 is filled with hydrogen with a known concentration and is in a high-pressure state of greater than or equal to 8 standard atmospheres. The laser emission beam forms a target optical path and a reference optical path through the first gas cell 31 and the second gas cell 32 respectively. The photoelectric detection module 2 includes two photodetectors, which respectively receive the projected laser signals of the target optical path and the reference optical path. The harmonic extraction module 4 uses a lock-in amplifier 9 to extract the first fundamental signal and the first second harmonic signal of the projected laser signal of the target optical path, and extracts the second fundamental signal and the second second harmonic signal of the projected laser signal of the reference optical path. The normalization processing module 5 performs fundamental wave normalization processing on the first second harmonic signal to form a main signal ratio, and introduces a compensation ratio of the reference optical path to dynamically compensate for the light source power drift and optical path disturbance. The concentration inversion module 6 outputs the trace concentration value of hydrogen in the target gas based on the main signal ratio and the compensation ratio, in combination with a preset fitting model and absorption coefficient parameters.

[0019] The present invention passes through as Figure 1 shown in the trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy. A double-path absorption module with a symmetric structure is introduced between the laser emission module and the photoelectric detection module. The target optical path and the high-pressure reference optical path are respectively constructed through the first gas cell 31 and the second gas cell 32 to ensure that the laser obtains equivalent optical path absorption and multiple reflection enhanced signals in the two paths. A photodetector with a high response bandwidth is used to collect the double-path transmission signals, and a lock-in amplifier is used to extract the first fundamental signal and the first second harmonic signal of the target optical path, and the second fundamental signal and the second second harmonic signal of the reference optical path. The main signal ratio and the reference compensation ratio are respectively formed through the normalization processing module, which can not only be used to dynamically offset the instability of the laser power, the optical path fluctuation and the system response deviation, but also allow the absorption information such as gas concentration to be extracted from the main signal ratio.

[0020] When using TDLAS technology to detect the first fundamental information and the first second harmonic signal, the transmission of the laser beam through the absorbing gas is described by Lambert-Beer's law: ; Among them, 、 are the incident and received light intensities, respectively, is the reception efficiency factor, , , are the absorption coefficient, optical path length, and gas concentration at the absorption center wavelength, respectively; The injection current of the laser 8 is sinusoidally modulated so that the output power of the laser 8 is modulated corresponding to the wavelength of the incident laser. When the modulation center of the laser wavelength is aligned with the gas absorption center, the laser wavelength and intensity are represented by Equation 1 and Equation 2: Equation 1: ; Equation 2: ; Where: is the absorption center wavelength, is the wavelength modulation width, is the modulation frequency, is the time variable, is the laser wavelength at time is the light intensity at time is the intensity modulation index caused by the laser current modulation, can be expanded into a Fourier series containing the fundamental harmonic amplitude and the second harmonic amplitude. Substituting the optical depth of hydrogen, the DC component of the received laser power in Equation 1 is given by: Equation 3: ; Equation 4: ; Equation 5: ; Where, , , are the DC component of the received laser power, fundamental harmonic current, and second harmonic current, respectively, is a constant, is the optical depth of hydrogen, is proportional to the optical depth of hydrogen. By the ratio of the uncertainties of and are eliminated. Considering the interaction between the wavelength modulation width and the intensity modulation index in Equation 1 and Equation 2 due to the characteristics of the laser diode, it is crucial to determine the absorption characteristics of hydrogen to obtain the hydrogen concentration close to the true value. The absorption coefficient of the Lorentz line shape is expanded by Fourier, as Equation 6: , where is the full width at half maximum of the absorption line, Provided by Wahlquist, , , in the expression of is directly applicable to second harmonic detection. To measure trace concentrations (ppm level), when configuring the optimal system parameters, maximize value and adjust the modulation width. The sensitivity will reach the detection limit. After using the above free calibration method to eliminate the light intensity fluctuations caused by other factors except for its gas absorption, use and the relational expression to measure the gas concentration at the ppm level. The relationship between , and is plotted as in Figure 3 . Adjust the value to maximize and . When and are equal to 0, the dimensionless confidence of and reaches the maximum value. At this time, the scale factors and , the dimensionless confidence of is equal to the dimensionless confidence of .

[0021] The emission part uses a distributed feedback laser diode emitting at 2121.83 nm, and the frequency is modulated in the form of a 10 kHz sine wave, with the modulation center locked at the center of the maximum absorption line of hydrogen (2121.83 nm). To lock the laser diode on the maximum absorption line of hydrogen, the injection current and temperature are fixed by monitoring the maximum absorption on the line. The collimated laser beam is split by the beam splitter 10 and transmitted into the first gas cell 31 and the second gas cell 32.

[0022] The concentration inversion module presets a set of fitting functions to achieve linear inversion output of hydrogen concentration in the range of 0.01% - 100%. The minimum detection limit of the system under a 30 - second integration time is better than 0.0055% (55 ppm), and the goodness of linear fit within the full range can reach 0.9995. It has the capabilities of no need for external calibration, high sensitivity, high stability, and on - line real - time detection, and is applicable to industrial scenarios such as hydrogen fuel quality control and storage and transportation leakage warning.

[0023] It should be noted that a beam splitter 10 is provided on one side of the laser 8. The beam splitter 10 divides the emitted beam of the laser 8 into a first beam and a second beam. A first optical component 11 and a second optical component 12 are provided on the other side of the beam splitter 10. The photoelectric detection module 2 includes a first photodetector 21 and a second photodetector 22.

[0024] Specifically, as Figure 2 shown, the first optical component 11 includes a first off-axis parabolic mirror 111. A first filter 112 is provided above the first off-axis parabolic mirror 111. The first filter 112 is located below the first photodetector 21. A first window 113 is provided on the side of the first off-axis parabolic mirror 111 away from the laser 8. A second window 114 is provided on the other side of the first window 113. The first gas cell 31 is located between the first window 113 and the second window 114. A first full-angle reflector 115 is provided on the other side of the second window 114. The optical path of the first beam split by the beam splitter 10 from the laser 8 is as follows: The first beam first passes through the central opening of the first off-axis parabolic mirror 111 and the first window 113, enters the first gas cell 31, and completes the first absorption. Subsequently, the laser beam passes through the second window 114 and then shoots towards the first full-angle reflector 115. After being reflected along the original path, it passes through the second window 114 again and enters the first gas cell 31 for the second absorption. The reflected beam passes through the first window 113 and then is incident on the parabolic surface of the first off-axis parabolic mirror 111. Its reflected light passes through the first filter 112 and is focused on the photosensitive surface of the first photodetector 21.

[0025] In the present invention, after the first beam output by the laser 8 through the beam splitter 10 is changed in direction by a plane mirror, it sequentially passes through the central opening of the first off-axis parabolic mirror 111 and the first window 113 and enters the first gas cell 31 to complete the first gas absorption. Subsequently, it passes through the second window 114 and shoots towards the first full-angle reflector 115 and is reflected along the original path, forming a second gas absorption path. Finally, it is reflected by the first off-axis parabolic mirror 111 and focused by the first filter 112 to the first photodetector 21, realizing double-pass enhanced absorption and efficient signal acquisition. With the collimation and focusing characteristics of the first off-axis parabolic mirror 111, while ensuring the laser coupling efficiency and signal transmission stability, the problem of central occlusion is effectively avoided; the double-window structure and the full-angle reflection path form an equivalent long optical path (2 times the single-pass physical optical path), enhancing the amplitude of the low-concentration hydrogen absorption signal and improving the response sensitivity of the second harmonic; at the same time, the filter is used to isolate background stray light and only allows the target wavelength band to be transmitted, enhancing the signal-to-noise ratio of the detection signal. The system can obtain a minimum detection limit of less than 0.0055% under a 30-second integration time, meeting the high-sensitivity detection requirements for trace hydrogen concentration. The overall design has excellent comprehensive performance in terms of spatial layout, beam quality, and system stability.

[0026] It should be noted that, as Figure 2 shown, the second optical component 12 includes a second off-axis parabolic mirror 121. Above the second off-axis parabolic mirror 121, there is a second filter 122. The second filter 122 is located below the second photodetector 22. On the side of the second off-axis parabolic mirror 121 away from the laser 8, there is a third window 123. On the other side of the third window 123, there is a fourth window 124. The second gas cell 32 is located between the third window 123 and the fourth window 124. On the other side of the fourth window 124, there is a second full-angle reflector 125. The optical path of the second light beam split by the beam splitter 10 from the laser 8 is as follows: After the direction of the second light beam is changed by the plane mirror, it first passes through the middle opening of the second off-axis parabolic mirror 121 and the third window 123, enters the second gas cell 32, and completes the first absorption. Subsequently, the laser beam passes through the fourth window 124 and then shoots towards the second full-angle reflector 125. After being reflected along the original path, it enters the second gas cell 32 again through the fourth window 124 for the second absorption. The reflected light beam passes through the third window 123 and then is incident on the parabolic surface of the second off-axis parabolic mirror 121. Its reflected light passes through the second filter 122 and is focused on the light-sensitive surface of the second photodetector 22.

[0027] The second optical component 12 in the present invention serves as a reference optical path structure and forms a symmetric layout with the first optical component 11. It uses the second off-axis parabolic mirror 121 to achieve laser collimation and reflection focusing. After the second light beam output by the beam splitter 10 enters through the middle opening, it sequentially passes through the third window 123 and enters the second gas cell 32, first penetrating the known high-concentration hydrogen gas. Subsequently, it is projected onto the second full-angle reflector 125 through the fourth window 124 to form a 180° reflection and return along the original path. After passing through the second gas cell 32 again to form a double-pass absorption path, the laser beam returns to the second off-axis parabolic mirror 121 and is focused through the second filter 122 to the second photodetector 22. This reference optical path uses high-pressure standard hydrogen gas (≥8 atm) as the filling medium to stably output an absorption response signal. By enhancing the double-pass reflection, the amplitude and resolution ability of the compensated harmonic signal are effectively improved. Its structure is symmetric to the target optical path, effectively ensuring the consistency of the main signal and the reference signal in terms of geometric path, mirror group reflection, and optical path conditions, enabling a good alignment relationship between the harmonic normalization ratio and the compensation ratio, which helps to eliminate the systematic errors caused by the fluctuations of the laser 8, optical path drift, and device nonlinearity, thereby enhancing the robustness of the differential fitting in the concentration inversion module 6 and improving the long-term stability and calibration independence of trace hydrogen concentration measurement in complex environments.

[0028] Embodiment 2

[0029] The difference between Embodiment 2 and Embodiment 1 of the present invention lies in that this embodiment introduces the normalization processing module 5, the concentration inversion module, the first gas cell 31, the first photodetector 21, and the second photodetector 22 of a trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy.

[0030] It should be noted that the normalization processing module 5 processes the first second-harmonic signal extracted from the target optical path using the fundamental wave normalization algorithm to obtain the main signal ratio and introduces the compensation ratio of the reference signal. The specific process includes: dividing the first second-harmonic signal by the corresponding first fundamental wave signal to form the main signal ratio, and dividing the second second-harmonic signal by the corresponding second fundamental wave signal to form the compensation ratio of the reference signal.

[0031] The normalization processing module 5 in the present invention processes the first second-harmonic signal extracted from the target optical path through the fundamental wave normalization algorithm, uses the corresponding first fundamental wave signal as the normalization factor to form the main signal ratio, and further introduces the ratio of the second second-harmonic signal to the second fundamental wave signal If 2 in the reference optical path as the compensation ratio. By constructing a double-ratio structure, the system error is canceled. This processing method physically realizes the dynamic elimination of uncertain factors such as the initial power fluctuation of the laser, lens contamination, inconsistent modulation depth, and change in the reflectivity of the mirror, making the main signal ratio only related to the gas concentration, thereby improving the stability and repeatability of the system without external calibration; this algorithm not only improves the normalization accuracy of signal processing, but also realizes the real-time drift self-compensation ability through the introduction of a double-path differential mechanism, effectively reducing the measurement drift and response lag during long-term operation, enabling the entire system to maintain a linear fitting goodness of fit R²≥0.999 in the concentration range of 0.01% to 100%, and having a minimum detection limit better than 55 ppm under a 30-second integration time, meeting the comprehensive requirements of industrial-grade trace hydrogen detection for high sensitivity, anti-interference ability, and long-term maintenance-free operation.

[0032] It should be noted that in the concentration inversion module 6, the preset fitting model is based on the harmonic detection theory of tunable diode laser absorption spectroscopy and uses a piecewise linear regression function to construct independent fitting sub-models for the set concentration intervals of 0.01% to 1%, 1% to 10%, and 10% to 100%. The fitting function forms of the trace hydrogen concentration in each segment are: ; In the formula: is the concentration interval index, is the fitting function value of the trace hydrogen concentration in the th concentration interval, and are all fitting coefficients obtained through experiments. is the first and second harmonic signals, is the first fundamental wave signal; Meanwhile, introduce the compensation ratio extracted from the reference optical path to correct the harmonic response error caused by the fluctuation of laser 8 and mirror contamination. The fitting function formula of trace hydrogen concentration in each concentration segment after correction is: ; In the formula: is the fitting function value of trace hydrogen concentration in the th concentration interval after correction, , are the second second harmonic signal and the second fundamental wave signal respectively.

[0033] The present invention adopts a piecewise linear regression model established on the basis of the TDLAS harmonic detection theory in the concentration inversion module 6, divides the hydrogen concentration detection range into three concentration intervals of 0.01% - 1%, 1% - 10%, and 10% - 100%, constructs independent fitting functions for each interval to process the non-linear relationship between the main signal ratio and the concentration in different concentration segments, so that the model maintains good local fitting accuracy and inter-segment continuity in the whole detection range; meanwhile, introduce the compensation ratio in the reference optical path as a correction term, and through constructing a differential form, dynamically cancel the systematic errors such as laser power fluctuation, modulation depth inconsistency, mirror contamination, and filter attenuation in mathematics, so that the concentration output is only determined by the absorption spectrum response, and high-precision and maintenance-free hydrogen concentration measurement can be realized without an external calibration curve; the experimental results show that this method can maintain a goodness of fit better than 0.9995 in the concentration range of 0.01% - 100%, the minimum detection limit of the system is better than 0.0055% under the 30-second integration time, significantly improving the sensitivity, linearity and environmental adaptability of trace hydrogen detection, and is applicable to the hydrogen energy industry safety monitoring scenario with high stability requirements.

[0034] It should be noted that the effective absorption optical path of the second gas cell 32 is greater than or equal to 2 meters, and the filled gas is a high-purity gas with a volume fraction greater than 99.99%.

[0035] In the present invention, the effective absorption optical path of the second gas cell 32 is set to be greater than or equal to 2 meters, aiming to increase the total absorption of laser by gas molecules by extending the interaction path between the laser and the target gas, thereby enhancing the amplitude and signal-to-noise ratio of the second harmonic signal. According to the Lambert-Beer law, the gas absorption intensity is proportional to the absorption coefficient, concentration, and optical path. Especially under trace (ppm level) detection conditions, the absorption signal itself is extremely weak. If the optical path is insufficient, the harmonic response amplitude may be overwhelmed by system noise, resulting in an increase in the detection limit; setting an equivalent optical path of ≥2 meters (which can be achieved by multiple-pass reflection) can effectively enhance weak signals while maintaining the compact structure of the system. In addition, the target gas filled in the second gas cell 32 is high-purity hydrogen with a volume fraction of ≥99.99%, aiming to ensure a highly single ratio response and spectral line consistency during concentration inversion, avoiding interference from overlapping absorption peaks of impurity gases such as CH 4 , CO 2 or H 2 O, etc. in the near-infrared band, ensuring the accurate correspondence between the fitting model and the theoretical absorption coefficient, improving the fitting accuracy and repeatability of the system in the trace concentration range of 0.01% - 1%, thereby achieving a minimum detection limit of less than 55 ppm and a linear fitting ability with a goodness of fit greater than or equal to 0.9995 within the full range, meeting the engineering application requirements for high-sensitivity and high-reliability hydrogen sensing in the hydrogen energy field.

[0036] It should be noted that both the first photodetector 21 and the second photodetector 22 are highly sensitive detectors with a response bandwidth higher than 20 kHz and a linear dynamic range greater than or equal to 90 dB. Their photosensitive surfaces are perpendicular to the reflection axes of the target optical path and the reference optical path respectively, and are used to receive the laser signals that are coupled and focused to the first filter 112 and the second filter 122 after two absorptions and reflections.

[0037] In the present invention, both the first photodetector 21 and the second photodetector 22 adopted are highly sensitive devices, with a signal response bandwidth higher than 20 kHz and a linear dynamic range greater than or equal to 90 dB. They can respond in real time to the wavelength-modulated laser signal output by the laser at a modulation frequency of 10 - 20 kHz, ensuring that the harmonic components are not filtered out or distorted during the signal acquisition process. This high-bandwidth characteristic can completely retain the first and second harmonic components contained in the laser signal during the modulation process, providing a stable input source for the subsequent lock-in amplifier to accurately extract the fundamental wave and second harmonic signals. At the same time, the characteristic of a linear dynamic range ≥90 dB ensures that laser transmission signals of different intensity levels can still be accurately responded to under conditions of a very large concentration span (0.01% - 100%), avoiding signal saturation at high concentrations or signal distortion at low concentrations. The photosensitive surfaces of the two detectors are arranged perpendicular to the reflection axes of the target optical path and the reference optical path respectively, ensuring that the laser signal after two absorptions, reflections, and focusing by the off-axis parabolic mirror can be perpendicularly incident on the detection surface, maximizing the photoelectric conversion efficiency and reducing the response non-uniformity caused by angular deviation. Thus, during the system normalization calculation, the response synchronization and comparability of the main signal ratio and the reference ratio are ensured, ultimately improving the concentration inversion accuracy and system stability, enabling the system to achieve a minimum detection limit as low as 55 ppm and a linear fitting performance better than 0.9995 even at trace-level concentrations.

[0038] In summary, as described in combination with Embodiment 1 and Embodiment 2, the system proposed by the present invention realizes a symmetric absorption path configuration for the target optical path and the reference optical path by constructing a double optical path structure to respectively guide the laser beam through the first gas cell 31 filled with the gas to be measured and the second gas cell 32 filled with high-pressure high-purity hydrogen. By combining a beam splitter, an off-axis parabolic mirror, a filter, and a corner reflector to construct an enhanced absorption optical path with multiple transmissions and reflections, the harmonic signal amplitude and signal-to-noise ratio are improved. The fundamental wave signal and the second harmonic signal in the target optical path and the reference optical path are synchronously extracted, and the main signal ratio and the reference compensation ratio are formed through normalization processing to dynamically eliminate the measurement errors caused by laser power fluctuations, optical path disturbances, and device response drifts. Based on this ratio pair, combined with a preset multi-segment fitting model and absorption coefficient parameters, the hydrogen concentration results in the range of 0.01% - 100% are output, with high linearity, low detection limit, no need for external calibration, and the ability to adapt to rapid measurements under complex working conditions, improving the quantitative detection accuracy, system stability, and industrial adaptability of trace hydrogen.

[0039] In addition, the distributed feedback type tunable diode laser can be replaced with a narrow linewidth laser covering other near-infrared or infrared bands according to different target absorption line spectra to adapt to the detection requirements of other trace gases; the optical path lengths and pressure parameters of the first gas cell 31 and the second gas cell 32 can be enhanced by selecting a multi-channel mirror group or a ring multi-pass gas chamber to achieve a higher equivalent absorption length or gas compression ratio and adapt to the application scenarios with lower detection limits; the first off-axis parabolic mirror 111 and the second off-axis parabolic mirror 121 in the first optical component 11 and the second optical component 12 can be replaced with an ellipsoidal mirror or a spherical lens + mirror combination to adapt to different optical path layouts and equipment volume requirements while maintaining the laser collimation and focusing functions; the harmonic extraction module can be replaced with high-order harmonic extraction, third-harmonic analysis or digital lock-in amplification technology according to the accuracy requirements and signal frequency characteristics of the detection system to further improve the selectivity and noise immunity of the system; the normalization processing and concentration inversion module can be implemented by an FPGA, a DSP or an embedded AI chip for high-speed calculation and processing, and can establish a connection with the cloud server through a wireless communication module to achieve remote calibration, real-time concentration upload and intelligent concentration warning functions. Therefore, all forms of replacement, equivalent solutions or technical combinations falling within the scope of the claims of the present invention shall be regarded as the protected content of the present invention.

Claims

1. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy, comprising a laser emission module (1) and a photoelectric detection module (2), characterized in that: The laser emission module (1) and the photoelectric detection module (2) are connected via a dual-light path absorption module (3); the photoelectric detection module (2) is connected to a harmonic extraction module (4); the harmonic extraction module (4) is connected to a normalization processing module (5); the normalization processing module (5) is connected to a concentration inversion module (6); the laser emission module (1) comprises a laser (8); the dual-light path absorption module (3) comprises a first gas pool (31) and a second gas pool (32), which are respectively filled with a target gas to be measured and hydrogen gas with a known concentration and at a pressure greater than or equal to 8 standard atmospheres; the laser emission beam passes through and forms a target gas and a second gas pool (32). The target and reference optical paths are provided, the photoelectric detection module (2) comprises two photoelectric detectors, which respectively receive the laser signals of the target optical path and the reference optical path, the harmonic extraction module (4) respectively extracts the first fundamental wave signal and the first second harmonic signal of the target optical path and the second fundamental wave signal and the second second harmonic signal of the reference optical path, the normalization processing module (5) performs fundamental wave normalization on the first and second harmonic signals to form a main signal ratio, and at the same time introduces a compensation ratio to dynamically compensate for the light source power drift and optical path disturbance, and the concentration inversion module (6) outputs the hydrogen trace concentration value based on the main signal ratio and the compensation ratio in combination with the fitting model.

2. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 1, characterized in that: A beam splitter (10) is provided on one side of the laser (8), and the beam splitter (10) splits the emission light beam of the laser (8) into a first light beam and a second light beam. A first optical component (11) and a second optical component (12) are provided on the other side of the beam splitter (10). The photoelectric detection module (2) comprises a first photoelectric detector (21) and a second photoelectric detector (22).

3. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 2, characterized in that: The first optical component (11) comprises a first off-axis parabolic mirror (111), a first filter (112) is provided above the first off-axis parabolic mirror (111), the first filter (112) is located below the first photodetector (21), a first window (113) is provided on the side of the first off-axis parabolic mirror (111) away from the laser (8), a second window (114) is provided on the other side of the first window (113), a first gas pool (31) is located between the first window (113) and the second window (114), and a first full-angle reflector (115) is provided on the other side of the second window (114).

4. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 3, characterized in that: The optical path of the first light beam split by the beam splitter (10) from the laser (8) is: The first light beam first passes through the middle opening of the first off-axis parabolic mirror (111) and the first window (113), enters the first gas pool (31), and completes the first absorption. Subsequently, the laser beam passes through the second window (114) and is emitted to the first full-angle reflector (115). After being reflected along the original path, the laser beam passes through the second window (114) again and enters the first gas pool (31), and is absorbed for the second time. The reflected light beam passes through the first window (113) and is incident on the parabola of the first off-axis parabolic mirror (111). The reflected light passes through the first filter (112) and is focused onto the light-sensitive surface of the first photodetector (21).

5. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 2, characterized in that: The second optical component (12) comprises a second off-axis parabolic mirror (121), a second filter (122) is provided above the second off-axis parabolic mirror (121), the second filter (122) is located below the second photodetector (22), a third window (123) is provided on the side of the second off-axis parabolic mirror (121) away from the laser (8), a fourth window (124) is provided on the other side of the third window (123), a second gas pool (32) is located between the third window (123) and the fourth window (124), and a second full-angle reflector (125) is provided on the other side of the fourth window (124).

6. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 5, characterized in that: The optical path of the second light beam split by the beam splitter (10) from the laser (8) is: The second light beam first passes through the middle opening of the second off-axis parabolic mirror (121) and the third window (123), enters the second gas pool (32), and completes the first absorption. Subsequently, the laser beam passes through the fourth window (124) and is emitted to the second full-angle reflector (125). After being reflected along the original path, the laser beam passes through the fourth window (124) again and enters the second gas pool (32), and is absorbed for the second time. The reflected light beam passes through the third window (123) and is incident on the parabola of the second off-axis parabolic mirror (121). The reflected light passes through the second filter (122) and is focused onto the light-sensitive surface of the second photodetector (22).

7. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 1, characterized in that: The normalization processing module (5) uses a fundamental wave normalization algorithm to process the first and second harmonic signals extracted from the target optical path, obtain a main signal ratio, and introduce a compensation ratio of a reference signal. The specific process includes: dividing the first and second harmonic signals by the first fundamental wave signal corresponding thereto to form a main signal ratio, and dividing the second second harmonic signal by the second fundamental wave signal corresponding thereto to form a compensation ratio of the reference signal.

8. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 7, characterized in that: In the concentration inversion module (6), the preset fitting model is based on the harmonic detection theory of tunable diode laser absorption spectroscopy. A piecewise linear regression function is used to construct independent fitting sub-models for the set concentration intervals of 0.01% to 1%, 1% to 10%, and 10% to 100%. The fitting function form of the trace hydrogen concentration in each segment is: ; Where: is the concentration interval index, For the The trace hydrogen concentration fitting function value of the concentration interval is , are fitting coefficients obtained through experiments. is the first and second harmonic signal, is the first fundamental signal; At the same time, the compensation ratio extracted from the reference optical path is introduced to correct the harmonic response error caused by laser (8) fluctuation and mirror contamination. After correction, the fitting function formula of the trace hydrogen concentration in each concentration range is: ; Where: After correction The trace hydrogen concentration fitting function value of the concentration interval is , They are the second second harmonic signal and the second fundamental signal respectively.

9. The trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 1, characterized in that: The effective absorption optical path of the second gas pool (32) is greater than or equal to 2 meters, and the gas filled therein is a high-purity gas with a volume fraction greater than 99.99%.

10. The trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 1, characterized in that: The first photodetector (21) and the second photodetector (22) are both high-sensitivity detectors with a response bandwidth higher than 20 kHz and a linear dynamic range greater than or equal to 90 dB, and their photosensitive surfaces are respectively perpendicular to the reflection axes of the target optical path and the reference optical path, and are used to receive laser signals that are coupled and focused to the first filter (112) and the second filter (122) after two absorptions and reflections.

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