Gas detection method based on quantum cascade laser

Abstract

The invention discloses a gas detection method based on quantum cascade laser, and relates to the technical field of gas sensing. The method comprises the following steps: outputting laser triggered by a composite driving current by using a laser generator; the composite driving current is formed by superposing a high-frequency sinusoidal modulation waveform on a low-frequency scanning waveform; a quantum cascade laser is used for emitting laser, the laser passes through a multi-reflection pool after being collimated, gas to be detected is scanned, and a detector and a data acquisition card are used for synchronously acquiring light intensity scanning signals; in the processor, a time-division multiplexing algorithm and a compensation matrix are used for decoupling the light intensity scanning signal, corresponding gas and gas concentration are inverted and displayed in real time on a human-computer interaction interface, and when any gas concentration exceeds a corresponding preset threshold value, sound-light alarm is triggered, and an alarm signal is output to an administrator port. According to the invention, on the premise of not increasing the complexity of the system, the gas of which the absorption spectral line is very close to or even has cross interference can be accurately detected.

Description

Technical Field

[0001] This invention relates to the field of gas sensing technology, and in particular to a gas detection method based on quantum cascade lasers. Background Technology

[0002] Quantum cascade lasers (QCLs) are semiconductor laser sources that operate in the mid-infrared band (3-25 μm). Their output wavelengths precisely cover the fundamental vibration absorption band of most gas molecules, and their absorption intensity is several orders of magnitude higher than that in the near-infrared band. Therefore, gas detection technology based on QCLs has extremely high sensitivity (up to ppb or even ppt level).

[0003] Existing QCL gas detection systems mostly adopt a one-laser-one-gas approach, meaning that one QCL scans and detects a single absorption peak of a specific gas. To detect multiple gases simultaneously, multiple laser systems, broadband light sources, and spectrometers are typically required. Multiple laser systems suffer from drawbacks such as large system size, complex structure, high cost, and high power consumption. While broadband light sources and spectrometers can detect multiple components, they are usually bulky, and their sensitivity and response speed are often inferior to laser-based detection methods.

[0004] Furthermore, achieving high selectivity detection without increasing system complexity is also a technical challenge for gases with very similar absorption spectra or even cross-interference (such as CO and N2O, NO and SO2). Summary of the Invention

[0005] The purpose of this invention is to provide a gas detection method based on quantum cascade lasers, which aims to solve or improve at least one of the above-mentioned technical problems.

[0006] To achieve the above objectives, the present invention provides the following solution: A gas detection method based on quantum cascade lasers includes: A laser is generated by a laser generator and triggered by a composite driving current; wherein the composite driving current is generated by a high-speed function generator and a current driver and injected into a quantum cascade laser; the composite driving current is composed of a high-frequency sinusoidal modulation waveform superimposed on a low-frequency scanning waveform; The quantum cascade laser emits a laser beam, which is collimated and then passes through multiple reflection cells to scan the gas to be tested. The light intensity scanning signal is simultaneously acquired by the detector and the data acquisition card. In the processor, the light intensity scanning signal is decoupled using a time-decomposition multiplexing algorithm and a compensation matrix, and the corresponding gas and gas concentration are retrieved. The corresponding gas and its concentration are displayed in real time on the human-machine interface. When the concentration of any gas exceeds the corresponding preset threshold, an audible and visual alarm is triggered and an alarm signal is output to the administrator port.

[0007] Optionally, the low-frequency scanning waveform adopts a multi-step sawtooth wave, which is divided into N different current steps or slope segments to correspond to specific absorption peaks of the target gas.

[0008] Optionally, the step of emitting laser light using the quantum cascade laser, collimating it, and then passing it through multiple reflection cells to scan the gas to be tested, with the detector and data acquisition card synchronously acquiring the light intensity scanning signal, specifically includes: The laser emitted by the quantum cascade laser is collimated into parallel light by a collimating lens and then coupled into a multiple reflection cell. After the laser undergoes multiple reflections in the cell, it is focused onto a photodetector by an output concave mirror, outputting a light intensity voltage signal. The gas to be measured passes through the absorption cell at a constant flow rate. The light intensity voltage signal is amplified by a low-noise preamplifier, and the amplified signal is synchronously acquired by the data acquisition card to obtain the light intensity scanning signal.

[0009] Optionally, the data acquisition card and the laser generator use the same clock source.

[0010] Optionally, the step of decoupling the light intensity scanning signal in the processor using a time-decomposition multiplexing algorithm and a compensation matrix, and retrieving the corresponding gas and gas concentration, specifically includes: In the processor, the time-domain signal is divided into time slices using a time decomposition multiplexing algorithm, which are then assigned to the scanning segments of each gas. For a gas with a single absorption line, the second harmonic component of the signal at the modulation frequency is extracted using a lock-in amplifier. Each of the second harmonic components is compared with the calibration curve obtained in advance through standard gas calibration to obtain the corresponding gas and gas concentration. For gases with cross-interference in absorption lines, a compensation matrix is ​​introduced in the time-decomposition multiplexing algorithm to correct the inversion results.

[0011] Optionally, the peak value of the second harmonic component is proportional to the concentration of the corresponding gas.

[0012] Optionally, for a gas with a single absorption line, extracting the second harmonic component of the signal at the modulation frequency for each time slot using a lock-in amplifier specifically includes: Using the second harmonic of the known high-frequency modulation frequency as a reference signal, a pair of orthogonal reference signals are generated; Each time-slice signal is multiplied by the orthogonal reference signal, and the result of the multiplication is low-pass filtered to remove all high-frequency components, resulting in two DC components X and Y; where X represents the real part of the second harmonic component and Y represents the imaginary part of the second harmonic component. The second harmonic component of the signal at the modulation frequency for each time slot is calculated based on the two DC components mentioned above.

[0013] Optionally, for gases with cross-interference in absorption lines, a compensation matrix is ​​introduced into the time-decomposition multiplexing algorithm to correct the inversion results, specifically including: If there is cross interference in the gas absorption lines, the second harmonic component calculated in the first calculation will be biased. In this case, an interference coefficient matrix is ​​introduced and solved to correct the result of the inversion.

[0014] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects: This invention discloses a gas detection method based on quantum cascade lasers. The method includes using a laser generator to output a laser triggered by a composite driving current; the composite driving current is composed of a high-frequency sinusoidal modulated waveform superimposed on a low-frequency scanning waveform; emitting the laser using a quantum cascade laser, which, after collimation, passes through multiple reflection cells to scan the gas to be tested; and simultaneously acquiring the light intensity scanning signal using a detector and a data acquisition card. In a processor, a time-decomposition multiplexing algorithm and a compensation matrix are used to decouple the light intensity scanning signal, retrieve the corresponding gas and its concentration, and display it in real time on a human-machine interface. When the concentration of any gas exceeds a corresponding preset threshold, an audible and visual alarm is triggered, and an alarm signal is output to the administrator port. This invention can accurately detect gases with very similar absorption spectra or even cross-interference without increasing system complexity. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a schematic diagram of the gas detection method based on quantum cascade lasers according to the present invention. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] The purpose of this invention is to provide a gas detection method based on quantum cascade lasers, which aims to solve or improve at least one of the above-mentioned technical problems.

[0019] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] like Figure 1 As shown, this invention provides a gas detection method based on quantum cascade lasers, comprising: Step 100: Output a laser triggered by a composite driving current using a laser generator; wherein the composite driving current is generated by a high-speed function generator and a current driver, and injected into the quantum cascade laser; the composite driving current is composed of a high-frequency sinusoidal modulation waveform superimposed on a low-frequency scanning waveform. The low-frequency scanning waveform employs a multi-step sawtooth wave, which is divided into N different current steps or slope segments to correspond to specific absorption peaks of the target gas.

[0021] Step 200: The laser is emitted using the quantum cascade laser and, after collimation, passes through multiple reflection cells to scan the gas to be tested. The light intensity scanning signal is simultaneously acquired by the detector and the data acquisition card.

[0022] Step 300: In the processor, the light intensity scanning signal is decoupled using a time-decomposition multiplexing algorithm and a compensation matrix, and the corresponding gas and gas concentration are retrieved.

[0023] Step 400: Display the corresponding gas and gas concentration in real time on the human-machine interface, and trigger an audible and visual alarm and output an alarm signal to the administrator port when the concentration of any gas exceeds the corresponding preset threshold.

[0024] As one specific implementation method, step 200 specifically includes: The laser emitted by the quantum cascade laser is collimated into parallel light by a collimating lens and then coupled into a multiple reflection cell. After the laser undergoes multiple reflections in the cell, it is focused onto a photodetector by an output concave mirror, outputting a light intensity voltage signal. The gas to be measured passes through the absorption cell at a constant flow rate.

[0025] The light intensity voltage signal is amplified by a low-noise preamplifier, and the amplified signal is synchronously acquired by the data acquisition card to obtain the light intensity scanning signal. The data acquisition card and the laser generator use the same clock source.

[0026] As one specific implementation method, step 300 specifically includes: In the processor, a time-decomposition multiplexing algorithm is used to divide the acquired time-domain signal into time slices, which are then assigned to the scanning segments of each gas.

[0027] For a gas with a single absorption line, a lock-in amplifier is used to extract the second harmonic component of the signal at the modulation frequency for each time slot. Each second harmonic component is then compared with a calibration curve obtained beforehand using a standard gas to obtain the corresponding gas and its concentration. The peak value of each second harmonic component is proportional to the concentration of the corresponding gas.

[0028] For gases with cross-interference in absorption lines, a compensation matrix is ​​introduced in the time-decomposition multiplexing algorithm to correct the inversion results.

[0029] As a further implementation step, for a gas with a single absorption line, the second harmonic component of the signal at the modulation frequency for each time slot is extracted using a lock-in amplifier, specifically including: Using the second harmonic of the known high-frequency modulation frequency as a reference signal, a pair of orthogonal reference signals are generated; each time-slot signal is multiplied by the orthogonal reference signals respectively, and the result of the multiplication is low-pass filtered to remove all high-frequency components, resulting in two DC components X and Y; where X represents the real part of the second harmonic component and Y represents the imaginary part of the second harmonic component; the second harmonic component of each time-slot signal at the modulation frequency is calculated based on the two DC components.

[0030] As a further implementation step, for gases with cross-interference in absorption lines, a compensation matrix is ​​introduced into the time-decomposition multiplexing algorithm to correct the inversion results, specifically including: If there is cross interference in the gas absorption lines, the second harmonic component calculated in the first calculation will be biased. In this case, an interference coefficient matrix is ​​introduced and solved to correct the result of the inversion.

[0031] Based on the above technical solution, the following embodiments are provided.

[0032] S100: Laser drive and wavelength scanning control. The core of this step is to generate a precisely timed and precisely controlled composite current signal to drive a single tunable QCL, so that its wavelength rapidly and cyclically jumps between the characteristic absorption peaks of multiple target gases.

[0033] Laser selection and operating point setting: Choose a continuous-wave (CW) or pulsed tunable distributed feedback (DFB) quantum cascade laser (QCL). Its tuning range (Δλ) must completely cover the target absorption lines of all the gases to be measured. For example, if it is necessary to simultaneously detect ammonia (NH3 @ 1103.4 cm⁻¹). -1 ) and ethane (C2H6 @ 2986.5 cm) -1 If so, a QCL whose tuning range can cover this range should be selected.

[0034] The QCL's operating temperature is stabilized at an accuracy of ±0.01°C via a temperature control module (TEC controller) to ensure the initial stability of its output wavelength.

[0035] Composite drive current waveform generation: An arbitrary waveform generator (AWG) or a high-precision digital-to-analog converter (DAC) controlled by a microprocessor (such as an FPGA) is used to generate a reference waveform. The reference waveform is digitally superimposed from a low-frequency scanning waveform and a high-frequency sinusoidal modulated waveform.

[0036] Low-frequency scanning waveform: A multi-step sawtooth wave is used. This waveform is no longer a traditional continuous ramp, but consists of N discrete current steps or tiny ramp segments, where N equals the number of types of gas to be measured.

[0037] The start current value (I_start) and stop current value (I_stop) of each segment are precisely pre-calibrated to ensure that the corresponding laser wavelength scanning range can completely cover the single isolated absorption peak of the target gas (typically 1-2 cm⁻¹). -1 (a tiny range), and avoids the strong interference absorption lines of other gases.

[0038] The duration of each segment (T_segment) can be set independently, typically between 1 ms and 100 ms. For gases with weak absorption or requiring higher sensitivity, longer scan times can be allocated.

[0039] All segments are executed sequentially, and after completion, they are quickly reset and the next loop begins, forming a continuous time-division multiplexing scan.

[0040] High-frequency sinusoidal modulated waveform: The frequency f is typically in the tens of kHz to several hundred kHz. This signal is superimposed on a low-frequency scanning waveform to achieve wavelength modulation spectrum (WMS) technology. The modulation depth is optimized to produce the strongest second harmonic (2f) signal.

[0041] The composite digital waveform is converted into an analog current signal by a high-bandwidth, low-noise laser current driver and injected into the QCL.

[0042] S200: Signal Detection and Acquisition Optical path design: The laser emitted from the QCL is collimated into parallel light by a collimating lens and then coupled into a multiple reflection cell (such as the Herriott or White type). The optical path length of the reflection cell can be designed according to the detection sensitivity requirements, typically tens of meters or even hundreds of meters.

[0043] After multiple reflections within the cell, the laser light is focused by an output concave mirror onto a high-speed, high-sensitivity photodetector (such as a liquid nitrogen or thermoelectrically cooled mercury cadmium telluride (MCT) detector). The entire optical path is sealed, and the gas to be measured flows through the absorption cell at a constant velocity.

[0044] Synchronous signal acquisition: The light intensity voltage signal (including absorption information) output by the detector is first amplified by a low-noise preamplifier.

[0045] The amplified signal is synchronously acquired by a high-speed data acquisition card (DAQ). The sampling rate of the DAQ card must be much higher than the Nyquist rate of the high-frequency modulation frequency f (typically >10f) to ensure accurate reconstruction of the modulated signal.

[0046] Key point: The DAQ card's acquisition must be strictly synchronized with the drive waveform generated by the AWG. This is achieved through hardware triggering or sharing the same clock source. This ensures that each data point accurately corresponds to an instantaneous value of the drive current, thus corresponding to a precise laser wavelength.

[0047] S300: Signal demodulation and concentration inversion are the core of data processing, which is completed in real time on FPGA or high-speed CPU.

[0048] Data segmentation (time-delay multiplexing): The processor divides the one-dimensional time-series signal V(t) acquired by the DAQ card into N consecutive data segments, namely [V_gas1(t), V_gas2(t), ..., V_gasN(t)], according to the precise time window of each segment (strictly synchronized with the driving waveform). Each data segment corresponds to the scanning process of one gas.

[0049] Harmonic extraction (digital lock-in amplification): For each data segment V_gasi(t) (e.g., corresponding to the i-th gas), a digital lock-in amplifier (DLIA) algorithm is used for processing: Using the second harmonic (2f) of the known high-frequency modulation frequency f as the reference signal, a pair of orthogonal reference signals sin(4πft) and cos(4πft) are generated.

[0050] Multiply V_gasi(t) by the pair of orthogonal reference signals respectively.

[0051] The result of multiplication is low-pass filtered to remove all high-frequency components, resulting in two DC components X and Y (i.e., the real and imaginary parts of the 2f signal).

[0052] The 2f signal amplitude of the gas in this scan was obtained by calculating R = 2 * sqrt(X² + Y²). The factor of 2 is used to correct for the amplitude halving caused by mathematical processing. This process is equivalent to extracting the Fourier component of the signal at the 2f frequency in the digital domain, which has extremely high noise suppression capability.

[0053] Concentration calculation and cross-interference correction: Calibration: Using standard gases of known concentrations, record the 2f signal amplitude R corresponding to each gas at different concentrations, and establish a linear calibration curve (or lookup table) between concentration-C and amplitude-R.

[0054] Inversion: For each gas 2f amplitude R_measured obtained in real time, its initial concentration C_i' can be obtained by querying its corresponding calibration curve.

[0055] Cross-compensation (optional but important): If some gas absorption lines overlap and interfere, the initial values ​​will be biased. In this case, an N x N interference coefficient matrix K needs to be introduced. The final concentration value [C] is obtained by solving the equation system [R_measured] = K * [C]. The matrix K is predetermined by measuring the responses of various pure gases and gas mixtures.

[0056] S400: Results Display and Output: The calculated gas concentration values ​​[C1, C2, ..., CN] are displayed in real time on a human-machine interface (HMI), typically in the form of numbers or trend curves. All data, along with timestamps and environmental parameters (pressure, temperature), are stored in a database.

[0057] The system is equipped with alarm logic. When the concentration of any gas exceeds a preset threshold, it triggers an audible and visual alarm and outputs an alarm signal through the network (Ethernet, 4G / 5G) or digital I / O port.

[0058] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0059] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A gas detection method based on quantum cascade lasers, characterized in that, include: A laser is generated by a laser generator and triggered by a composite driving current; wherein the composite driving current is generated by a high-speed function generator and a current driver and injected into a quantum cascade laser; the composite driving current is composed of a high-frequency sinusoidal modulation waveform superimposed on a low-frequency scanning waveform; The quantum cascade laser emits a laser beam, which is collimated and then passes through multiple reflection cells to scan the gas to be tested. The light intensity scanning signal is simultaneously acquired by the detector and the data acquisition card. In the processor, the light intensity scanning signal is decoupled using a time-decomposition multiplexing algorithm and a compensation matrix, and the corresponding gas and gas concentration are retrieved. The corresponding gas and its concentration are displayed in real time on the human-machine interface. When the concentration of any gas exceeds the corresponding preset threshold, an audible and visual alarm is triggered and an alarm signal is output to the administrator port.

2. The gas detection method based on quantum cascade lasers according to claim 1, characterized in that, The low-frequency scanning waveform uses a multi-step sawtooth wave, which is divided into N different current steps or slope segments to correspond to specific absorption peaks of the target gas.

3. The gas detection method based on quantum cascade lasers according to claim 1, characterized in that, The process involves emitting laser light using the quantum cascade laser, collimating it, and then passing it through multiple reflection cells to scan the gas under test. The intensity scanning signal is simultaneously acquired by a detector and a data acquisition card. Specifically, this includes: The laser emitted by the quantum cascade laser is collimated into parallel light by a collimating lens and then coupled into a multiple reflection cell. After the laser undergoes multiple reflections in the cell, it is focused onto a photodetector by an output concave mirror, outputting a light intensity voltage signal. The gas to be measured passes through the absorption cell at a constant flow rate. The light intensity voltage signal is amplified by a low-noise preamplifier, and the amplified signal is synchronously acquired by the data acquisition card to obtain the light intensity scanning signal.

4. The gas detection method based on quantum cascade laser according to claim 3, characterized in that, The data acquisition card and the laser generator use the same clock source.

5. The gas detection method based on quantum cascade lasers according to claim 1, characterized in that, In the processor, a time-decomposition multiplexing algorithm and a compensation matrix are used to decouple the light intensity scanning signal and retrieve the corresponding gas and gas concentration, specifically including: In the processor, the time-domain signal is divided into time slices using a time decomposition multiplexing algorithm, which are then assigned to the scanning segments of each gas. For a gas with a single absorption line, the second harmonic component of the signal at the modulation frequency is extracted using a lock-in amplifier. Each of the second harmonic components is compared with the calibration curve obtained in advance through standard gas calibration to obtain the corresponding gas and gas concentration. For gases with cross-interference in absorption lines, a compensation matrix is ​​introduced into the time-decomposition multiplexing algorithm to correct the inversion results.

6. The gas detection method based on quantum cascade lasers according to claim 5, characterized in that, The peak value of the second harmonic component is proportional to the concentration of the corresponding gas.

7. The gas detection method based on quantum cascade laser according to claim 5, characterized in that, For a gas with a single absorption line, the second harmonic component of the signal at the modulation frequency for each time slot is extracted using a lock-in amplifier, specifically including: Using the second harmonic of the known high-frequency modulation frequency as a reference signal, a pair of orthogonal reference signals are generated; Each time-slice signal is multiplied by the orthogonal reference signal, and the result of the multiplication is low-pass filtered to remove all high-frequency components, resulting in two DC components X and Y; where X represents the real part of the second harmonic component and Y represents the imaginary part of the second harmonic component. The second harmonic component of the signal at the modulation frequency for each time slot is calculated based on the two DC components mentioned above.

8. The gas detection method based on quantum cascade laser according to claim 5, characterized in that, For gases where absorption lines cross and interfere, a compensation matrix is ​​introduced into the time-decomposition multiplexing algorithm to correct the inversion results, specifically including: If there is cross interference in the gas absorption lines, the second harmonic component calculated in the first calculation will be biased. In this case, an interference coefficient matrix is ​​introduced and solved to correct the result of the inversion.

Images