Method and device for detecting various gases based on TDLAS (Tunable Diode Laser Absorption Spectroscopy) technology

Abstract

The invention discloses a TDLAS (tunable diode laser absorption spectroscopy)-based multi-gas detection method and device. The device comprises a plurality of paths of lasers with different wavelengths, a laser coupling device, a multi-reflection gas chamber, a detector and a data processing module, the device modulates a plurality of lasers with different wavelengths at different frequencies, then light emitted by the different lasers is coupled and then enters a multi-reflection gas chamber to react with detected gas, the output light passes through a detector, and a signal generated by the detector is demodulated by adopting a demodulation frequency which is the same as the modulation frequency. And the concentrations of different gas components are extracted. According to the device, various gas components can be analyzed under the condition that only one gas chamber and one detector are used, the size and the weight of gas analysis equipment are effectively reduced, and the manufacturing cost is reduced.

Description

Technical Field

[0001] The present invention relates to the field of optical gas analysis, and in particular to a multiple gas detection method based on TDLAS technology. Background Art

[0002] The principle of gas detection technology based on laser absorption spectroscopy is:

[0003] Due to the existence of the absorption spectrum of gas molecules, when light passes through the gas, if the spectrum of the light source overlaps with the absorption spectrum of the gas to be measured, part of the light energy will be absorbed by the gas to be measured and the output light intensity will be weakened. When the gas concentration is low, the absorption of this gas to light energy of a specific wavelength satisfies the Beer-Lambert law.

[0004] According to this law, a specific gas only absorbs a spectrum of a specific wavelength, and the intensity of the absorption is proportional to the gas concentration. By detecting the gas absorption intensity, the concentration of the specific gas can be calculated.

[0005] The existing gas detection methods often integrate multiple single-channel detection devices into one device for detection, such as Figure 1 As shown, this detection method requires multiple detection gas chambers, which is costly and bulky. Summary of the invention

[0006] The present invention provides a multi-gas detection device and method based on TDLAS (Tunable diode laser absorption spectroscopy) technology, which can realize the detection of multiple gas components under the condition of using only one gas chamber and one detector, effectively reducing the size and cost of the equipment.

[0007] Among them, the principle of TDLAS modulation spectroscopy technology is:

[0008] If the wavelength of the light source is directly locked on the gas absorption peak, then when the light source deviates from the gas absorption peak due to the influence of temperature, the absorption coefficient will change, causing the test result to change. In order to avoid this influence and improve the signal-to-noise ratio, the wavelength of the light source can be modulated, and a high-frequency sinusoidal modulation signal can be superimposed on the laser scanning signal, so that the instantaneous frequency and light intensity of the light source can change periodically according to the law of the sinusoidal function and sweep across the gas absorption peak. The light source and the gas absorption peak will generate a harmonic signal after interaction, and the concentration of the gas to be measured can be obtained by analyzing the harmonic signal.

[0009] The present disclosure provides a TDLAS-based multi-gas detection method, which mainly includes the following steps:

[0010] S1: Modulate several lasers with different wavelengths at different frequencies;

[0011] S2: The modulated multi-path laser is coupled and then injected into the multi-reflection air chamber through the collimator, and then emitted after being fully absorbed, and the optical signal is received by the detector;

[0012] S3: Use reference signals with the same frequency as the modulation signal to demodulate the signals generated by the detector and calculate the concentration value of each absorbing gas.

[0013] Furthermore, the modulation method of the laser in step S1 includes:

[0014] The dual-frequency modulation method is adopted, that is, high-frequency sine modulation is superimposed on a low-frequency sawtooth wave. The sawtooth wave controls the speed and range of wavelength scanning to ensure that the absorption line is scanned and separated from other gas absorption lines;

[0015] Among them, lasers of different wavelengths are modulated with high-frequency sine waves of different frequencies to separate their respective absorption spectra for absorbing different gases.

[0016] Furthermore, the modulation method of step S1 specifically includes the following steps:

[0017] Setting the temperature and reference current of the laser to determine the scanning start wavelength;

[0018] The sawtooth wave and the sine wave are superimposed as the final modulation signal to act on the current drive of the laser. The laser is controlled by tuning the current together with the temperature control module to generate the modulated light required by the system.

[0019] Furthermore, in step S3, the step of selecting reference signals with the same frequency as the modulation signal to demodulate the signal generated by the detector includes:

[0020] The modulation frequency of each laser in step S1 is used to generate a reference sine signal and a reference cosine signal of the same frequency, which are respectively multiplied with the signals output by the detector. The multiplication results are respectively filtered by a low-pass filter and then vector synthesized to obtain the detector output signal corresponding to the laser.

[0021] The present disclosure provides a multi-gas detection device based on TDLAS technology using the above method, comprising:

[0022] Several lasers with different wavelengths, laser coupling devices, multiple reflection gas chambers, detectors, and data processing modules; wherein:

[0023] The data processing module is used to modulate the lasers of different wavelengths at different frequencies; and to demodulate the signals generated by the detectors using reference signals with the same frequency as the modulation signal, and calculate the concentration value of each absorbing gas;

[0024] A laser coupling device, used for coupling modulated multi-channel lasers;

[0025] Multiple reflection gas chamber, used to fully absorb the coupled laser;

[0026] A detector is used to detect the light emitted from the multi-reflected gas chamber.

[0027] Furthermore, the laser adopts a DFB distributed feedback laser.

[0028] Furthermore, the data processing module includes: a plurality of modulation submodules and a plurality of demodulation submodules, wherein:

[0029] The modulation submodule is used to modulate the laser; the modulation submodules corresponding to lasers of different wavelengths use different modulation frequencies;

[0030] The demodulation frequency of one demodulation submodule corresponds to the modulation frequency of one modulation submodule, and is used to demodulate the signal generated by the detector with a reference signal having the same frequency as the modulation signal.

[0031] Furthermore, the modulation submodules all use a dual-frequency modulation method in which a high-frequency sine wave is superimposed on a low-frequency sawtooth wave to modulate the laser, wherein the modulation submodules corresponding to lasers of different wavelengths use high-frequency sine waves of different frequencies.

[0032] Further, the modulation submodule includes: an FPGA board and a modulation signal generating component, wherein: the FPGA board is used to control the modulation signal generating component to generate a sawtooth wave and a sine wave signal for modulating the laser, and the two are superimposed as the final modulation signal;

[0033] The laser comprises: a current driving component and a temperature control component, wherein: the current driving component is used to tune the current input to the laser under the action of the final modulation signal; the temperature control component is used to keep the laser at a constant temperature.

[0034] Furthermore, the laser coupling device adopts a fiber coupler.

[0035] Compared with the prior art, the advantages of the present invention are: (1) it is possible to analyze multiple gas components using only one gas chamber and one detector; (2) it effectively reduces the size and weight of the equipment and reduces the manufacturing cost; (3) it adopts a dual-frequency modulation method for the laser, which effectively eliminates the influence of temperature and current jitter on the stability of the system detection signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The above and other objects, features and advantages of the present disclosure will become more apparent through a more detailed description of exemplary embodiments of the present disclosure in conjunction with the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the present disclosure.

[0037] Figure 1 It is a schematic diagram of gas detection in the prior art;

[0038] Figure 2 is a system structure diagram according to an exemplary embodiment of the present disclosure;

[0039] Figure 3 This is the relationship between the central wavelength of the DFB laser and the current and temperature;

[0040] Figure 4 It is the principle block diagram of a single component system;

[0041] Figure 5 Provide a block diagram for implementing dual-frequency modulation of the laser;

[0042] Figure 6 This is the principle block diagram of the demodulation technology. DETAILED DESCRIPTION

[0043] The preferred embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the preferred embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to make the present disclosure more thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art.

[0044] The present disclosure provides a new TDLAS-based multi-gas detection method and device, which can analyze multiple gas components using only one gas chamber and one detector.

[0045] According to the exemplary embodiments of the present disclosure, Figure 2 As shown, the process mainly includes the following steps.

[0046] Step 1: Modulate lasers of different wavelengths at different frequencies;

[0047] Step 2: Couple multiple lasers;

[0048] Step 3: The coupled light is injected into the multi-reflection air chamber through the collimator, and then emitted after being fully absorbed. The detector receives the optical signal;

[0049] Step 4: Select reference signals with the same frequency as the modulation signal to demodulate the signals received by the detector and calculate the concentration values ​​of multiple gases.

[0050] In this embodiment, DFB (Distributed Feedback Laser) is used. The relationship between the central wavelength and the current and temperature is shown in the attached figure. Figure 3 shown.

[0051] In this embodiment, the system principle block diagram for single-component gas is shown in the attached figure. Figure 4 shown.

[0052] 1. Implementation of wavelength modulation

[0053] In this embodiment, the wavelength modulation of the laser adopts a dual-frequency modulation method, and the implementation method is as follows: Figure 5 As shown, high-frequency sinusoidal modulation is superimposed on a low-frequency sawtooth wave, which effectively eliminates the influence of temperature and current jitter on the stability of the system detection signal.

[0054] The starting wavelength of the scan is determined by setting the temperature and reference current of the laser, and the speed and range of the wavelength scan are controlled by the sawtooth wave to ensure that the scan passes through the absorption line and is separated from the absorption lines of other gases. This can solve the problem of background gas interference, and the sawtooth wave scanning can avoid the measurement error caused by the drift of the laser center wavelength to the system. The sawtooth wave and the sine wave are superimposed as the final modulation signal to act on the current drive of the laser, and the laser is controlled by the tuning current and the temperature control module to generate the modulated light required by the system.

[0055] 2. Demodulation

[0056] Since the detector receives modulated signals from multiple lasers, the output signal of the detector is a state of multiple frequency superposition and noise information is added. The signal can be expressed as:

[0057] A1cos(ω1t+α1)+A2cos(ω2t+α2)+A3cos(ω3t+α3)+……+A n cos(ω n t+α n )+n(t)

[0058] Among them, ω1, ω2…ω n represents the modulation frequency of different lasers,

[0059] α1, α2…α n represents the initial phase angle of modulation of different lasers,

[0060] A1, A2…A n represents the modulation amplitude of different lasers,

[0061] n(t) represents the noise signal.

[0062] The superimposed signal cannot directly obtain information, and it is necessary to perform demodulation technology to extract the useful signal. The schematic diagram of the demodulation technology is shown in the attached figure. Figure 6 For example:

[0063] When laser 1 is working, the frequency of the reference signal given by the demodulation signal is ω1. At this time, the light of other frequencies received by the detector will be filtered out by the filter. Similarly, when laser n is working, the reference signal frequency of the demodulation signal is ω n Through a series of demodulation processes, the required signal can be extracted from the output signal of the detector, thereby inferring the concentration information of the gas detected by the laser.

[0064] In this embodiment, wavelengths of multiple lasers are modulated at different frequencies, and the light emitted by different lasers is coupled through a fiber coupler. The coupled laser enters a multiple reflection gas chamber to react with the gas to be measured, and the optical signal is detected by a photoelectric detector. The detector signal enters the data processing module after amplification, filtering, and other processing. The concentration of different gas components is extracted through a specific demodulation algorithm.

[0065] The above technical scheme is only an exemplary embodiment of the present invention. For those skilled in the art, it is easy to make various types of improvements or modifications based on the application methods and principles disclosed in the present invention, and it is not limited to the method described in the above specific embodiment of the present invention. Therefore, the method described above is only preferred and does not have a restrictive meaning.

Claims

1. A method for detecting multiple gases based on TDLAS technology, comprising the following steps: S1: Modulate several lasers with different wavelengths at different frequencies; S2: The modulated multi-path laser is coupled and then injected into the multi-reflection air chamber through the collimator, and then emitted after being fully absorbed, and the optical signal is received by the detector; S3: Use reference signals with the same frequency as the modulation signal to demodulate the signals generated by the detector and calculate the concentration value of each absorbing gas.

2. The method according to claim 1, characterized in that The modulation method of the laser in step S1 includes: The dual-frequency modulation method is adopted, that is, high-frequency sine modulation is superimposed on a low-frequency sawtooth wave. The sawtooth wave controls the speed and range of wavelength scanning to ensure that the absorption line is scanned and separated from other gas absorption lines; Among them, lasers of different wavelengths are modulated with high-frequency sine waves of different frequencies to separate their respective absorption spectra for absorbing different gases.

3. The method according to claim 2, characterized in that The modulation method of step S1 specifically comprises the following steps: Setting the temperature and reference current of the laser to determine the scanning start wavelength; The sawtooth wave and the sine wave are superimposed as the final modulation signal to act on the current drive of the laser. The laser is controlled by tuning the current together with the temperature control module to generate the modulated light required by the system.

4. The method according to any one of claims 1 to 3, characterized in that: In step S3, the reference signals having the same frequency as the modulation signal are respectively selected to demodulate the signals generated by the detectors. The specific method includes: The modulation frequency of each laser in step S1 is used to generate a reference sine signal and a reference cosine signal of the same frequency, which are respectively multiplied with the signal output by the detector. The multiplication results are respectively filtered by a low-pass filter and then vector synthesized to obtain the detector output signal corresponding to the laser.

5. A multi-gas detection device based on TDLAS technology using the method described in any one of claims 1 to 4, characterized in that: include: Several lasers with different wavelengths, laser coupling devices, multiple reflection gas chambers, detectors, and data processing modules; wherein: The data processing module is used to modulate the lasers of different wavelengths at different frequencies; and to demodulate the signals generated by the detectors using reference signals with the same frequency as the modulation signal, and calculate the concentration value of each absorbing gas; A laser coupling device, used for coupling modulated multi-channel lasers; Multiple reflection gas chamber, used to fully absorb the coupled laser; A detector is used to detect the light emitted from the multi-reflected gas chamber.

6. The device according to claim 5, characterized in that The laser adopts DFB distributed feedback laser.

7. The device according to claim 5, characterized in that The data processing module includes: a plurality of modulation submodules and a plurality of demodulation submodules, wherein: The modulation submodule is used to modulate the laser; the modulation submodules corresponding to lasers of different wavelengths use different modulation frequencies; The demodulation frequency of one demodulation submodule corresponds to the modulation frequency of one modulation submodule, and is used to demodulate the signal generated by the detector with a reference signal having the same frequency as the modulation signal.

8. The device according to claim 7, characterized in that The modulation submodules all use a dual-frequency modulation method in which a high-frequency sine wave is superimposed on a low-frequency sawtooth wave to modulate the laser, wherein the modulation submodules corresponding to lasers of different wavelengths use high-frequency sine waves of different frequencies.

9. The device according to claim 7, characterized in that The modulation submodule includes: an FPGA board and a modulation signal generating component, wherein: the FPGA board is used to control the modulation signal generating component to generate a sawtooth wave and a sine wave signal for modulating the laser, and the two are superimposed as the final modulation signal; The laser comprises: a current driving component and a temperature control component, wherein: the current driving component is used to tune the current input to the laser under the action of the final modulation signal; the temperature control component is used to keep the laser at a constant temperature.

10. The device according to any one of claims 5 to 9, characterized in that: The laser coupling device adopts an optical fiber coupler.

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