A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy

Abstract

The present invention relates to the technical field of hydrogen concentration detection, and specifically discloses a trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy, which is used to solve the problems of low detection sensitivity, narrow linear range, large system drift and dependence on external calibration in TDLAS hydrogen detection. The system comprises a laser emission module and a photoelectric detection module, wherein the laser emission module and the photoelectric detection module are connected via a dual-light 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 present invention realizes high linearity, high sensitivity, and calibration-free concentration detection of hydrogen within 0.01% to 100% by constructing a symmetrical dual-light path structure, adopting a harmonic signal normalization and differential compensation algorithm, and combining a high-pressure reference gas cell with a segmented fitting model, thereby improving the trace hydrogen identification capability.

Description

Technical Field

[0001] The present 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 its colorless and odorless nature, easy to diffuse, and flammable and explosive, hydrogen 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 a key support for the safe development of the hydrogen energy utilization industry. Currently used hydrogen sensing technologies mainly include electrochemical, thermal conductivity, capacitance, resistance, photoacoustic and other methods. Chinese invention patent publication number CN1187948914 discloses a trace hydrogen concentration detection device. By designing a dual-channel photoacoustic cell, trace hydrogen is catalytically oxidized to water vapor. This trace water vapor is detected using photoacoustic spectroscopy, and the corresponding hydrogen concentration is derived. This is then processed by differential operation of the voltage signal, ensuring an excellent linear relationship between the output electrical signal and the measured hydrogen concentration, enabling ppb-level trace hydrogen concentration detection. While this device offers advantages in low cost and system integration, achieving ppm-level detection and a wide linear response span from 0.01% to 100% poses challenges such as complex calibration, a narrow linear range, hysteresis, and large drift errors. Tunable diode laser absorption spectroscopy (TDLAS) has become a research hotspot in trace gas detection due to its non-contact, highly selective, fast dynamic response, and online monitoring capabilities. However, TDLAS detection of hydrogen is limited by its naturally low infrared absorption coefficient. Improving detection sensitivity, minimizing system errors, and achieving accurate quantification without external calibration are currently pressing technical bottlenecks in 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, combining a high-pressure reference gas cell and 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:

[0005] 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 through a dual-light 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, whose emission wavelength covers the near-infrared 2121.8nm main absorption line of hydrogen. The dual-light path absorption module includes a first gas pool and a second gas pool. The first gas pool is filled with a target gas to be measured, and the second gas pool is filled with hydrogen of a known concentration and is in a high-pressure state greater than or equal to 8 standard atmospheric pressures. The laser emission beam passes through the first gas pool. The target optical path and the second gas pool are formed respectively. The photoelectric detection module includes two photoelectric detectors, which receive the projected laser signals of the target optical path and the reference optical path respectively. The harmonic extraction module uses a phase-locked amplifier to extract the first fundamental wave signal and the first second harmonic signal of the laser signal projected by the target optical path, and extracts the second fundamental wave signal and the second second harmonic signal of the laser signal projected by the reference optical path. The normalization processing module performs fundamental wave normalization processing on the first and second harmonic signals to form a main signal ratio, and introduces the 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, combined with the preset fitting model and absorption coefficient parameters.

[0006] As a further solution of the present invention, a beam splitter is provided on one side of the laser, which splits 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, and the photoelectric detection module includes a first photodetector and a second photodetector.

[0007] 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, a first gas pool is located between the first window and the second window, and a first full-angle reflector is provided on the other side of the second window.

[0008] As a further solution of the present invention, the optical path of the first light beam split by the beam splitter is:

[0009] The first light beam first passes through the middle opening of the first off-axis parabolic mirror and the first window, enters the first gas pool, and completes the first absorption. Then, the laser beam passes through the second window and is emitted to the first full-angle reflector. After being reflected along the original path, it passes through the second window again and enters the first gas pool for a second absorption. The reflected light beam passes through the first window and is incident on the parabola of the first off-axis parabolic mirror again. Its reflected light passes through the first filter and is focused onto the light-sensitive surface of the first photodetector.

[0010] 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 pool is located between the third window and the fourth window, and a second full-angle reflector is provided on the other side of the fourth window.

[0011] As a further solution of the present invention, the optical path of the second light beam split by the beam splitter is:

[0012] 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 pool, and completes the first absorption. Then the laser beam passes through the fourth window and is emitted to the second full-angle reflector. After reflection along the original path, it passes through the fourth window again and enters the second gas pool for the second absorption. The reflected light beam passes through the third window and is incident on the parabola of the second off-axis parabolic mirror again. Its reflected light passes through the second filter and is focused onto the light-sensitive surface of the second photodetector.

[0013] As a further solution of the present invention, the normalization processing module uses a fundamental normalization algorithm to process the first and second harmonic signals extracted from the target optical path, obtains the main signal ratio, and introduces a compensation ratio of the reference signal. The specific process includes: dividing the first and second harmonic signals by the corresponding first fundamental signal to form a main signal ratio, and dividing the second and second harmonic signals by the corresponding second fundamental signal to form a compensation ratio of the reference signal.

[0014] As a further solution of the present invention, in the concentration inversion module, the preset fitting model is established on the basis of the harmonic detection theory of tunable diode laser absorption spectroscopy, and 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:

[0015] ;

[0016] 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. are the first and second harmonic signals, is the first fundamental signal;

[0017] At the same time, the compensation ratio extracted from the reference optical path is introduced to correct the harmonic response error caused by laser fluctuation and mirror contamination. After correction, the fitting function formula of the trace hydrogen concentration in each concentration range is:

[0018] ;

[0019] 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.

[0020] As a further solution of the present invention, the effective absorption optical path of the second gas pool 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%.

[0021] As a further solution of the present invention, the first photodetector and the second photodetector 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. 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 and the second filter after two absorption and reflection.

[0022] The technical effects of the trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy of the present invention are as follows:

[0023] The present invention provides a trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy. By constructing a dual-optical path structure, the laser beam is guided through a first gas cell filled with the gas to be measured and a second gas cell filled with high-pressure, high-purity hydrogen, respectively, to achieve symmetrical absorption path configuration of the target optical path and the reference optical path. A beam splitter, an off-axis parabolic mirror, a filter, and a full-angle reflector are combined to construct an enhanced absorption optical path with multiple transmission and reflection, thereby improving the harmonic signal amplitude and signal-to-noise ratio. The fundamental signal and second harmonic signal in the target optical path and the reference optical path are synchronously extracted. The main signal ratio and the reference compensation ratio are formed through normalization processing, and the measurement error caused by laser power fluctuation, optical path disturbance, and device response drift is dynamically eliminated. Based on the ratio pair, combined with a preset multi-segment fitting model and absorption coefficient parameters, the hydrogen concentration result covering the range of 0.01% to 100% is output. The system has high linearity, low detection limit, no need for external calibration, and the ability to adapt to rapid measurement under complex working conditions, thereby improving the quantitative detection accuracy of trace hydrogen, system stability, and industrial adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a system diagram of the present invention;

[0025] Figure 2 Schematic diagram of the optical path of the present invention;

[0026] Figure 3 For the present invention and 、 and The relationship diagram between and Relationship curve, the blue line is and Relationship curve;

[0027] In the figure: 1. Laser emission module; 2. Photoelectric detection module; 3. Dual-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 pool; 32. Second gas pool; 111. First off-axis parabolic mirror; 112. First filter; 113. First window; 114. Second window; 115. First full-angle reflector; 121. Second off-axis parabolic mirror; 122. Second filter; 123. Third window; 124. Fourth window; 125. Second full-angle reflector. DETAILED DESCRIPTION

[0028] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0029] Example 1

[0030] like Figure 1 As shown, the present invention proposes a trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy, including a laser emission module 1 and a photoelectric detection module 2, the laser emission module 1 and the photoelectric detection module 2 are connected through a dual-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, the emission wavelength of which covers the near-infrared 2121.8nm main absorption line of hydrogen, the dual-path absorption module 3 includes a first gas pool 31 and a second gas pool 32, the first gas pool 31 is filled with a target gas to be measured, and the second gas pool 32 is filled with hydrogen of known concentration and is in a high-pressure state greater than or equal to 8 standard atmospheric pressures, the laser The emitted light beam passes through the first gas pool 31 and the second gas pool 32 to form a target light path and a reference light path respectively. The photoelectric detection module 2 includes two photoelectric detectors, which receive the projected laser signals of the target light path and the reference light path respectively. The harmonic extraction module 4 uses a phase-locked amplifier 9 to extract the first fundamental wave signal and the first second harmonic signal of the laser signal projected by the target light path, and extracts the second fundamental wave signal and the second second harmonic signal of the laser signal projected by the reference light path. The normalization processing module 5 performs fundamental wave normalization processing on the first and second harmonic signals to form a main signal ratio, and introduces a compensation ratio of the reference light path to dynamically compensate for the light source power drift and light 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, combined with the preset fitting model and absorption coefficient parameters.

[0031] The present invention is achieved by Figure 1The trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy shown in the figure introduces a structurally symmetrical dual-path absorption module between the laser emission module and the photoelectric detection module, and constructs a target optical path and a high-voltage reference optical path through the first gas pool 31 and the second gas pool 32 respectively, ensuring that the laser obtains equivalent optical path absorption and multiple reflection enhancement signals in the two paths. The dual-path transmission signal is collected in conjunction with a photoelectric detector with a high response bandwidth, and 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 are extracted by a phase-locked amplifier. The main signal ratio and the reference compensation ratio are respectively formed through the normalization processing module. This can not only be used to dynamically offset laser power instability, optical path fluctuation and system response deviation, but also allow absorption information, such as gas concentration, to be extracted from the main signal ratio.

[0032] When using TDLAS technology to detect the first fundamental wave information and the first second harmonic signal, the transmission of the laser beam through the absorbing gas is described by the Lambert-Beer law:

[0033] ;

[0034] in, 、 are the incident and received light intensities, is the receiving efficiency factor, 、 、 are the absorption coefficient, optical path length and gas concentration at the absorption center wavelength respectively;

[0035] The injection current of the laser 8 is sinusoidally modulated so that the output power of the laser 8 is modulated accordingly with 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 expressed by equations 1 and 2:

[0036] Equation 1: ;

[0037] Equation 2: ;

[0038] in: is the absorption center wavelength, is the wavelength modulation width, is the modulation frequency, is the time variable, for The laser wavelength at the moment, for The intensity of the moment, is the intensity modulation index due to laser current modulation, It can be expanded into a Fourier series containing the first 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:

[0039] Equation 3: ;

[0040] Equation 4: ;

[0041] Equation 5: ;

[0042] in, 、 、 are the received laser power DC component, first harmonic current, and second harmonic current respectively. is a constant, is the optical depth of hydrogen, Proportional to the optical depth of hydrogen, The ratio of elimination and The uncertainty is due to the interaction between the wavelength modulation width and the intensity modulation index in equations 1 and 2 due to the characteristics of the laser diode. Determining the absorption characteristics of hydrogen is crucial to obtaining a hydrogen concentration close to the true value. The absorption coefficient of the Lorentzian line is Through Fourier expansion, as shown in Equation 6:

[0043] ,in is the full width at half maximum of the absorption line, Provided by Wahlquist, , , In the expression The relationship can be directly used for second harmonic detection. In order to measure trace concentrations (ppm level), when configuring the optimal system parameters, the maximum value and adjust the modulation width, The sensitivity will reach the detection limit. When using the above-mentioned free calibration method to eliminate the light intensity fluctuation caused by other factors other than gas absorption, use and The relationship expression is used to measure the gas concentration in ppm. and 、 and The relationship between Figure 3 Draw in, adjust To maximize the value of and ,when and When it is equal to 0, and The dimensionless confidence reaches its maximum value, when the scale factor and , The dimensionless confidence level is equal to , The dimensionless confidence level is equal to .

[0044] The emission section uses a distributed feedback laser diode emitting at 2121.83 nm. Its frequency is modulated in a 10 kHz sinusoidal pattern, with the center of the modulation locked to the center of the maximum absorption line of hydrogen (2121.83 nm). To lock the laser diode to the maximum absorption line of hydrogen, the injection current and temperature are fixed by monitoring the maximum absorption at this line. The collimated laser beam is split by a beam splitter 10 and transmitted to the first gas pool 31 and the second gas pool 32.

[0045] The concentration inversion module has a preset fitting function group to achieve linear inversion output of hydrogen concentration in the range of 0.01% to 100%. The system's minimum detection limit at a 30-second integration time is better than 0.0055% (55ppm), and the linear fit goodness of fit can reach 0.9995 within the full range. It does not require external calibration, has high sensitivity, high stability and online real-time detection capabilities, and is suitable for industrial scenarios such as hydrogen fuel quality control and storage and transportation leakage warning.

[0046] It should be noted that a beam splitter 10 is provided on one side of the laser 8, which 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, and the photoelectric detection module 2 includes a first photodetector 21 and a second photodetector 22.

[0047] It should be specified that, Figure 2 As shown, the first optical assembly 11 includes a first off-axis parabolic mirror 111, a first filter 112 is provided above the first off-axis parabolic mirror 111, and 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, and a second window 114 is provided on the other side of the first window 113. The 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. The optical path of the first light beam split by the beam splitter 10 from the laser 8 is:

[0048] 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. Then, 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, it passes through the second window 114 again and enters the first gas pool 31 for a second absorption. The reflected light beam passes through the first window 113 and is incident on the parabola of the first off-axis parabolic mirror 111 again. Its reflected light passes through the first filter 112 and is focused onto the light-sensitive surface of the first photodetector 21.

[0049] In the present invention, the first light beam output by the laser 8 through the beam splitter 10 is redirected by the plane mirror, passes through the central opening of the first off-axis parabolic mirror 111, the first window 113, enters the first gas pool 31, and completes the first gas absorption. Then, it passes through the second window 114 and is emitted to the first full-angle reflector 115 and 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 collection. With the collimation and focusing characteristics of the first off-axis parabolic mirror 111, while ensuring the laser While improving coupling efficiency and signal transmission stability, it effectively avoids center occlusion problems; the double-window structure and the full-angle reflection path form an equivalent long optical path (twice the physical optical path of a single path), which increases the amplitude of the low-concentration hydrogen absorption signal and improves the response sensitivity of the second harmonic; at the same time, the filter is used to isolate background stray light and only allow the target band to transmit, thereby improving the signal-to-noise ratio of the detection signal, allowing the system to obtain a minimum detection limit of less than 0.0055% with a 30-second integration time, meeting the needs of high-sensitivity detection of trace hydrogen concentrations. The overall design has excellent comprehensive performance in terms of spatial layout, beam quality and system stability.

[0050] It should be noted that if Figure 2 As shown, the second optical assembly 12 includes a second off-axis parabolic mirror 121, a second filter 122 is provided above the second off-axis parabolic mirror 121, and 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, and a fourth window 124 is provided on the other side of the third window 123. The 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. The optical path of the second light beam split by the beam splitter 10 from the laser 8 is:

[0051] After the second light beam is redirected 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 pool 32, and completes the first absorption. Then, 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, it passes through the fourth window 124 again and enters the second gas pool 32 for a second absorption. The reflected light beam passes through the third window 123 and is incident on the parabola of the second off-axis parabolic mirror 121. Its reflected light passes through the second filter 122 and is focused onto the photosensitive surface of the second photodetector 22.

[0052] The second optical component 12 in the present invention serves as a reference optical path structure and is symmetrically arranged with the first optical component 11. The second off-axis parabolic mirror 121 is used to realize laser collimation and reflective focusing. The second light beam output by the beam splitter 10 is incident through the middle opening, passes through the third window 123 in sequence, enters the second gas pool 32, penetrates the known high-concentration hydrogen for the first time, and then is projected through the fourth window 124 to the second full-angle reflector 125 to form a 180° reflection and return along the original path. After passing through the second gas pool 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 photoelectric detector. Device 22; the reference optical path uses high-pressure standard hydrogen (≥8atm) as the filling medium, stably outputs the absorption response signal, and effectively improves the amplitude and resolution of the compensation harmonic signal through double-pass reflection enhancement. Its structure is symmetrical to the target optical path, effectively ensuring the consistency of the main signal and the reference signal in the geometric path, mirror group reflection and optical path conditions, so that the harmonic normalization ratio and the compensation ratio have a good alignment relationship, which helps to eliminate the systematic errors caused by the fluctuation of the laser 8, the optical path drift and the nonlinearity of the device, thereby enhancing the robustness of the differential fitting in the concentration inversion module 6, and improving the long-term stability and calibration independence of the trace hydrogen concentration measurement in complex environments.

[0053] Example 2

[0054] The difference between Example 2 of the present invention and Example 1 is that this example introduces a normalization processing module 5, a concentration inversion module and a first gas pool 31, a first photodetector 21 and a second photodetector 22 of a trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy.

[0055] It should be noted that the normalization processing module 5 uses a fundamental normalization algorithm to process the first and second harmonic signals 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 signal to form the main signal ratio, and dividing the second second harmonic signal by the corresponding second fundamental signal to form the compensation ratio of the reference signal.

[0056] The normalization processing module 5 of the present invention processes the first and second harmonic signals extracted from the target optical path through the fundamental normalization algorithm, and uses the corresponding first fundamental 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 signal If2 in the reference optical path as the compensation ratio. By constructing a double ratio structure to offset the system error, 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 reflectivity of the reflector, so that the main signal ratio is only related to the gas concentration. The algorithm not only improves the normalization accuracy of signal processing, but also realizes real-time drift self-compensation by introducing a dual-path differential mechanism, effectively reducing measurement drift and response lag during long-term operation. The system maintains a linear fit goodness of fit R² ≥ 0.999 within the concentration range of 0.01% to 100%, and the minimum detection limit is better than 55 ppm at an integration time of 30 seconds, meeting the comprehensive requirements of industrial-grade trace hydrogen detection for high sensitivity, anti-interference and long-term maintenance-free.

[0057] 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. 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:

[0058] ;

[0059] 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. are the first and second harmonic signals, is the first fundamental signal;

[0060] 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:

[0061] ;

[0062] 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.

[0063] The present invention adopts a piecewise linear regression model based on the TDLAS harmonic detection theory in the concentration inversion module 6, and divides the hydrogen concentration detection range into three concentration intervals of 0.01% to 1%, 1% to 10%, and 10% to 100%. An independent fitting function is constructed for each interval to process the nonlinear relationship between the main signal ratio and the concentration in different concentration intervals, so that the model maintains good local fitting accuracy and inter-segment continuity in the entire detection range; at the same time, the compensation ratio in the reference light path is introduced as a correction term, and the laser is mathematically realized by constructing a differential form. The dynamic cancellation of system errors such as instrument power fluctuation, inconsistent modulation depth, mirror contamination, and filter attenuation makes the concentration output determined only by the absorption spectrum response. No external calibration curve is required, and high-precision, maintenance-free hydrogen concentration measurement can be achieved. Experimental results show that this method can maintain a goodness of fit better than 0.9995 in the concentration range of 0.01% to 100%, and the system's minimum detection limit is better than 0.0055% at a 30-second integration time, which significantly improves the sensitivity, linearity and environmental adaptability of trace hydrogen detection, and is suitable for hydrogen energy industry safety monitoring scenarios with high stability requirements.

[0064] It should be noted 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%.

[0065] In the present invention, the effective absorption optical path length of 2 meters or greater is set for the second gas pool 32. This is intended to extend the interaction path between the laser and the target gas, thereby increasing the total absorption of the laser by the gas molecules, 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 length. Especially under trace (ppm level) detection conditions, the absorption signal itself is extremely weak. If the optical path length is insufficient, the harmonic response amplitude may be overwhelmed by the system noise, resulting in an increase in the detection limit. Setting an equivalent optical path length of 2 meters or greater (achievable through multi-pass reflection) can effectively enhance weak signals while maintaining a compact system structure. In addition, the target gas filled in the second gas pool 32 is high-purity hydrogen with a volume fraction of ≥99.99%. The purpose is to ensure that the ratio response during concentration inversion is highly single and spectral consistent, avoid interference from overlapping absorption peaks of impurity gases such as CH4, CO2 or H2O in the near-infrared band, ensure accurate correspondence between the fitting model and the theoretical absorption coefficient, and improve the fitting accuracy and repeatability of the system in the trace concentration range of 0.01% to 1%, thereby achieving a minimum detection limit of less than 55ppm and a linear fitting capability with a goodness of fit greater than or equal to 0.9995 in the full range, meeting the engineering application requirements of the hydrogen energy field for high-sensitivity and high-reliability hydrogen sensing.

[0066] It should be noted 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. Their photosensitive surfaces are perpendicular to the reflection axes of the target light path and the reference light 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 absorption and reflection.

[0067] The first photodetector 21 and the second photodetector 22 used in the present invention are both highly sensitive devices with a signal response bandwidth higher than 20kHz and a linear dynamic range greater than or equal to 90dB. They can respond in real time to the wavelength-modulated laser signal output by the laser at a modulation frequency of 10 to 20kHz, 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 phase-locked amplifier to accurately extract the fundamental and second harmonic signals. At the same time, the linear dynamic range of ≥90dB ensures that under conditions with a very large concentration span (0.01% to 100%), the first and second harmonic components can be completely retained. ) can still accurately respond to laser transmission signals of different intensity levels, 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 light path and the reference light path, respectively, to ensure that the laser signal after two absorption, reflection and focusing by the off-axis parabolic mirror can be vertically incident on the detection surface, maximizing the photoelectric conversion efficiency and reducing the response inconsistency caused by angular deviation, thereby ensuring the response synchronization and comparability of the main signal ratio and the reference ratio during system normalization calculation, and ultimately improving the concentration inversion accuracy and system stability, so that the system can still achieve a minimum detection limit as low as 55ppm and a linear fitting performance better than 0.9995 at trace concentrations.

[0068] In summary, in combination with the description of Examples 1 and 2, the system proposed in the present invention constructs a dual-optical path structure to guide the laser beam through the first gas pool 31 filled with the gas to be measured and the second gas pool 32 filled with high-pressure, high-purity hydrogen, respectively, to achieve a symmetrical absorption path configuration of the target optical path and the reference optical path. A beam splitter, an off-axis parabolic mirror, a filter, and a full-angle reflector are combined to construct an enhanced absorption optical path with multiple transmission and reflection, thereby improving the harmonic signal amplitude and signal-to-noise ratio. The fundamental signal and second harmonic signal in the target optical path and the reference optical path are synchronously extracted. The main signal ratio and the reference compensation ratio are formed through normalization processing, and the measurement errors caused by laser power fluctuations, optical path disturbances, and device response drift are dynamically eliminated. Based on this ratio pair, combined with a preset multi-segment fitting model and absorption coefficient parameters, a hydrogen concentration result covering the range of 0.01% to 100% is output. The system has high linearity, low detection limit, no need for external calibration, and the ability to adapt to rapid measurement under complex working conditions, thereby improving the quantitative detection accuracy of trace hydrogen, system stability, and industrial adaptability.

[0069] In addition, the distributed feedback tunable diode laser can be replaced with a narrow linewidth laser covering other near-infrared or infrared bands according to the target absorption line spectrum to adapt to the detection requirements of other trace gases; the optical path length and gas pressure parameters of the first gas pool 31 and the second gas pool 32 can be enhanced by selecting a multi-channel reflector group or an annular multi-pass gas chamber to achieve a higher equivalent absorption length or gas compression ratio to adapt to 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 reflector Mirror or spherical lens + reflector 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 by high-order harmonic extraction, third harmonic analysis or digital phase-locked amplification technology according to the accuracy requirements and signal frequency characteristics of the detection system, so as to further improve the selectivity and noise resistance of the system; the normalization processing and concentration inversion module can be realized by FPGA, DSP or embedded AI chip to achieve high-speed computing processing, and can establish a connection with the cloud server through a wireless communication module to realize remote calibration, real-time concentration upload and intelligent concentration warning functions. Therefore, all forms of replacement, equivalent schemes or technical combinations that fall within the scope of the claims of the present invention should be regarded as the protection 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 through 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), and the normalization processing module (5) is connected to a concentration inversion module (6). The laser emission module (1) includes a laser (8), the dual-light path absorption module (3) includes a first gas pool (31) and a second gas pool (32), which are respectively filled with a target gas to be measured and hydrogen with a known concentration and at a pressure greater than or equal to 8 standard atmospheres. The laser emission beam passes through and forms symmetrical target and reference light paths respectively. The photoelectric detection module (2) includes two photoelectric detectors, which respectively receive laser signals of the target light path and the reference light path. The harmonic extraction module (4) extracts the target gas and the reference light path respectively. The first fundamental wave signal and the first second harmonic signal of the standard optical path and the second fundamental wave signal and the second second harmonic signal of the reference optical path are provided. The normalization processing module (5) performs fundamental wave normalization on the first second harmonic signal 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. The concentration inversion module (6) outputs a hydrogen trace concentration value based on the main signal ratio and the compensation ratio in combination with the fitting model. A beam splitter (10) is provided on one side of the laser (8). 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) includes a first photoelectric detector (21) and a second photoelectric detector (22). 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. are the first and second harmonic signals, 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.

2. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 1, characterized in that: 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), 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).

3. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 2, 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. Then, 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, it passes through the second window (114) again and enters the first gas pool (31) for the second absorption. 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).

4. The trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 1, characterized in that: The second optical component (12) includes 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).

5. The trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 4, 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. Then, 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, it passes through the fourth window (124) again and enters the second gas pool (32) for the second absorption. 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).

6. The 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 normalization algorithm to process the first and second harmonic signals extracted from the target optical path, obtains a main signal ratio, and introduces a compensation ratio of a reference signal. The specific process includes: dividing the first and second harmonic signals by the first fundamental signal corresponding thereto to form a main signal ratio, and dividing the second and second harmonic signals by the second fundamental signal corresponding thereto to form a compensation ratio of the reference signal.

7. 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%.

8. 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. The photosensitive surfaces thereof are perpendicular to the reflection axes of the target light path and the reference light path, respectively, 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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