A high-sensitivity multi-gas real-time monitoring device and method

Abstract

The application provides a high-sensitivity multi-gas real-time monitoring device and method, which comprises a controller, wherein the controller is connected with a main control circuit; the main control circuit is connected with an indicating laser and a mid-infrared laser array; the indicating laser and the mid-infrared laser array are connected with a multi-pass gas cell through a fiber coupling structure; a photoelectric detector is arranged on the light outlet side of the multi-pass gas cell; and the photoelectric detector is connected with the main control circuit; the controller generates modulation signals with different frequencies to modulate the mid-infrared laser array; the indicating laser emitted by the indicating laser and the mid-infrared laser emitted by the mid-infrared laser array are coupled into a coaxial light beam through the fiber coupling structure and enter the multi-pass gas cell; the photoelectric detector collects signals with target gas information passing through the multi-pass gas cell and simultaneously analyzes the concentrations of multiple target gases through a signal conditioning algorithm. The application has the characteristics of high sensitivity, low detection limit, real-time monitoring, convenient installation, strong anti-interference ability, maintenance-free and wide application range.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of gas monitoring, and in particular to a high-sensitivity multi-gas real-time monitoring device and method. BACKGROUND

[0002] Qualitative and quantitative detection of multi-component trace gases is increasingly valued in many industries such as environmental pollution monitoring, industrial production and emission measurement, medical diagnosis, agriculture and food safety, etc. Real-time monitoring of key gases in industrial processes can ensure the reliable operation of production and the safety of workers. For example, when a transformer fails, gases such as C2H2, CO and CO2 will be generated, and according to the standard judgment criteria, the safety status and failure cause of the transformer can be determined; when coal is naturally oxidized, it will produce marker gases such as CO, C2H4 and C2H2, and according to the types and concentrations of the gases, the degree of oxidation can be determined to issue an early warning and avoid the occurrence of coal spontaneous combustion. Therefore, it is of great practical significance to study a high-sensitivity multi-component gas simultaneous monitoring technology for safe production and life.

[0003] Traditional multi-component gas simultaneous monitoring technologies are mostly gas chromatography methods, the sampling characteristics of which determine that the gas detection time is relatively long, and continuous online monitoring is not possible. Moreover, the influence of temperature on the separation effect of the chromatographic column and the need to consume carrier gas and periodically replace the chromatographic column also determine that the chromatography technology is not suitable for long-term online monitoring applications in the field. At the same time, in multi-component measurement, due to the limitations of the actual detection principles and manufacturing processes and the similar chemical properties of the target gases to be measured, the selection of target gases by the gas chromatograph cannot achieve the ideal state, and some non-target gases will also produce certain responses during use, leading to gas cross interference. In addition, traditional gas chromatography multi-gas detection devices are bulky and heavy, such as the chromatographic analyzer for special oil in power systems, which has an outer size of 570x510x470 mm and a weight of >55 kg, which is very unfavorable for the handling of the instrument and the use in complex environments.

[0004] Mid-infrared laser gas detection technology is based on the Beer-Lambert law, and utilizes the strong absorption characteristics and fingerprint identification characteristics of gas molecules in the mid-infrared waveband, which has great application potential in the high-precision and specific detection of trace gases.

[0005] The application patent with the application number 202110395933.5 discloses a kind of multi-component trace gas on-line detection device and method under negative pressure, and the measuring device includes laser control center, multiple tunable lasers, polarization maintaining optical fiber, optical switch, optical beam splitter, optical isolator, optical fiber collimator, gas tank, pressure control center, multiple high reflection mirrors, reaction tower, vacuum pump, multiple optical ring-down cavity, photodetector, central data processing and control center, multiple optical switches, laser calibration center and photoelectric transducer.The ring-down cavity of the above-mentioned application passes into the gas to be measured, and the laser control center controls the laser signal of different wavelengths emitted by tunable laser, and the optical switch controls the laser signal of one of the wavelengths to be divided into two laser signals with energy ratio of 99:1 by optical beam splitter, and the laser signal with 99% energy is coupled into optical ring-down cavity after passing through optical isolator, and the laser signal with 1% energy is transmitted to calibration system, and according to one-to-one correspondence of wavelength, photoelectric transducer captures calibration information and transmits to central data processing and control center, photodetector captures the laser signal transmitted from optical ring-down cavity, and transmits the captured information to central data processing and control center to obtain ring-down time, and calculates the concentration of the gas to be measured according to the ring-down time.The above-mentioned application provides a kind of multi-component trace impurity gas on-line detection method with high sensitivity.However, the structure of the above-mentioned application is complex, multiple precise structures such as high reflection mirror and ring-down cavity are used, and the mechanical stability is poor, which is difficult to be applied to relatively complex field environment. SUMMARY

[0006] In view of the technical problems that traditional gas chromatography multi-gas detection device is bulky, has low sensitivity and cannot be monitored in real time, the present application provides a high-sensitivity multi-gas real-time monitoring device and method, which has the characteristics of accurate identification, high sensitivity, real-time monitoring, small size and easy installation.

[0007] To achieve the above-mentioned purpose, the technical scheme of the present application is as follows: a high-sensitivity multi-gas real-time monitoring device, comprising a controller, wherein the controller is connected with a main control circuit, the main control circuit is connected with an indicating laser and a mid-infrared laser array respectively, the indicating laser and the mid-infrared laser array are connected with a multi-pass gas cell through an optical fiber coupling structure, a photodetector is arranged on the light outlet side of the multi-pass gas cell, and the photodetector is connected with the main control circuit; the controller generates modulation signals with different frequencies to modulate the mid-infrared laser array, so that mid-infrared lasers with different wavelengths work in parallel; the indicating laser emitted by the indicating laser and the mid-infrared laser emitted by the mid-infrared laser array are coupled into a coaxial light through the optical fiber coupling structure and then enter the multi-pass gas cell; and the controller collects signals with target gas information passing through the multi-pass gas cell through the photodetector, and simultaneously analyzes the concentrations of multiple target gases through a signal conditioning algorithm.

[0008] Preferably, the controller is an MCU, the MCU is connected with a master control circuit; the master control circuit is connected with a power supply, the power supply provides appropriate working voltage for the controller, the indicating laser, the mid-infrared laser array and the photodetector through the master control circuit; the MCU receives the filtered and amplified gas absorption signal of the master control circuit, digitally demodulates the first harmonic signal 1 f and the second harmonic signal 2 f , carries out noise reduction on the harmonic signals through an adaptive wavelet threshold filtering algorithm, and inversely calculates the target gas concentration from the 2 f / 1 f .

[0009] Preferably, the master control circuit comprises a power management module, a laser driving module, a laser temperature control module, a detector temperature control module and a signal processing module, the power supply provides appropriate working voltage for the controller and the remaining modules through the power management module, the laser driving module and the laser temperature control module are connected with the mid-infrared laser array, the laser driving module provides stable driving current for the normal operation of the mid-infrared laser array and the indicating laser, and the laser temperature control module controls the working temperature of the mid-infrared laser; the detector temperature control module and the signal processing module are connected with the photodetector, the detector temperature control module is used for controlling the low-temperature operation of the photodetector to ensure the high response rate of the photodetector, and the signal processing module is used for filtering and amplifying the signals collected by the photodetector and transmitting the signals to the controller.

[0010] Preferably, the optical fiber coupling structure comprises a mid-infrared spatial light fiber coupler and a mid-infrared fiber combiner, the input end of the mid-infrared spatial light fiber coupler is connected with each mid-infrared laser of the mid-infrared laser array, the output end of the mid-infrared spatial light fiber coupler is connected with the mid-infrared fiber combiner, and the mid-infrared fiber combiner is connected with the multi-pass gas cell; the infrared fiber combiner couples the different wavelength lasers emitted by the mid-infrared laser array and the indicating laser emitted by the indicating laser into a coaxial light which is injected into the multi-pass gas cell.

[0011] Preferably, one end of the mid-infrared spatial light fiber coupler is provided with an aspheric lens, the aspheric lens is used to focus and couple the spatial light emitted by the mid-infrared laser into an optical fiber by using the focusing characteristics of short focal length and large numerical aperture, the other end of the mid-infrared spatial light fiber coupler is provided with a fiber interface adapter for connecting the mid-infrared fiber combiner; a focusing lens is arranged on the light outlet of the multi-pass gas cell, the center of the focusing lens is on the same optical axis as the light outlet; and the focusing lens is arranged on the side of the incident window of the photodetector.

[0012] Preferably, the multi-pass gas cell is a long optical path multi-pass gas cell, one side of which is provided with a light inlet and a light outlet; the laser light after beam combination enters the long optical path multi-pass gas cell through the light inlet, is reflected multiple times in the long optical path multi-pass gas cell, and is output from the light outlet and then focused by a focusing lens to reach a photodetector; the long optical path multi-pass gas cell is a pump suction type gas cell, which actively sucks the gas in the detection area.

[0013] Preferably, the mid-infrared laser array includes a plurality of quantum cascade lasers or interband cascade lasers working in parallel, the wavelength of the visible light emitted by the indicator laser ranges from 390 nm to 780 nm, and the wavelength of the mid-infrared laser is determined according to the target gas infrared spectrum obtained from the H IT RAN database and the target gas absorption spectrum measured by a Fourier spectrometer; the photodetector is a mercury cadmium telluride photodetector, and the response range of the photodetector is 2-12 μm.

[0014] A monitoring method of a high-sensitivity multi-gas real-time monitoring device, which comprises the following steps:

[0015] Step one: apply modulation signals of different frequencies to the mid-infrared lasers working in parallel, and couple the mid-infrared lasers and the indicator laser of different modulation frequencies into a coaxial laser through a fiber coupling structure to be injected into a long optical path multi-pass gas cell;

[0016] Step two: the coaxial laser is reflected multiple times in the long optical path multi-pass gas cell filled with target gas, is output from the light outlet, passes through a focusing lens to reach a photodetector, the photodetector converts the light signal carrying the target gas information into an electric signal, and the electric signal is processed by pre-amplification, filtering and noise reduction, and then enters a digital lock-in amplifier for demodulation;

[0017] Step three: the demodulated signal is simultaneously analyzed to obtain the concentrations of multiple target gases through a signal conditioning algorithm.

[0018] Preferably, the modulation signals of different frequencies are sawtooth waves superimposed with sine waves, the modulation frequencies of the sine wave signals are different, the modulation signals of different frequencies are applied to the corresponding mid-infrared lasers through different constant current source circuits, and the sine wave signal modulation frequencies of different mid-infrared lasers are not in an integer multiple relationship.

[0019] The method for demodulation by the digital lock-in amplifier is as follows: a frequency w 1 is used to replace the frequency of the sine wave modulation signal of the mid-infrared laser I, a synchronous reference signal with the same frequency as the frequency of the sine wave modulation signal of the mid-infrared laser I and the signal after amplification are multiplied by a multiplier to obtain a first harmonic direct current component and other mixed signals of the sine wave modulation signal of the mid-infrared laser I, and then the signals of other frequencies are filtered out through a low-pass filter to obtain the first harmonic signal 1 f 1 of the mid-infrared laser I; then, the frequency of the synchronous reference signal is changed to 2w 1. Obtain the second harmonic signal of mid-infrared laser I 2 f 1; Similarly, the first harmonic signal and the second harmonic signal corresponding to each laser in the mid-infrared laser array are obtained through digital demodulation.

[0020] Preferably, the signal conditioning algorithm uses an adaptive wavelet threshold filtering algorithm to further eliminate noise from the harmonic signal output by the digital lock-in amplifier;

[0021] The implementation method of the adaptive wavelet threshold filtering algorithm is as follows: performing wavelet transform on the noisy signal, extracting the low-frequency coefficients and high-frequency coefficients of each layer after wavelet decomposition, estimating the mean square error of the noise from the high-frequency coefficients of the first layer; then performing nonlinear threshold processing on the wavelet coefficients; and finally performing inverse wavelet transform to obtain pure first harmonic signals and second harmonic signals.

[0022] The nonlinear threshold processing method is as follows: the initial value of the threshold is determined by using the universal threshold method proposed by Donoho: the standard deviation of the noise signal is σ, the signal length is N , then the initial threshold ; Compare the wavelet coefficients of the noisy signal with the selected threshold. The wavelet coefficients greater than or equal to the threshold remain unchanged, and the wavelet coefficients less than the threshold are set to zero. If the mean square error of the noise after denoising is greater than the preset value and the number of cycles has not reached the maximum limit, the threshold is adjusted and the denoising process continues; otherwise, the denoising process ends.

[0023] After the adaptive wavelet threshold filtering algorithm is used to eliminate noise, the pure first harmonic signal of the target gas is obtained. f and the second harmonic signal 2 f , using the first harmonic signal 1 f For the second harmonic signal 2 f Normalize and finally calculate the value based on 2 f / 1 f The linear correspondence between the ratio and the concentration of the target gas can be used to invert the concentration of the target gas.

[0024] Compared with the existing technology, the present invention has the following advantages: by combining mid-infrared laser detection technology with tunable diode laser absorption spectroscopy (TDLAS), it can achieve highly sensitive detection of multiple target gases.

[0025] (1) Easy to install: The entire device has a compact structure, small size, light weight, and is easy to move and install.

[0026] (2) Good mechanical stability: the optical fiber coupling structure is used to replace the complex optical element beam combining optical path structure, and the impact resistance of the device is improved, and the device can be used in complex field environments.

[0027] (3) High selectivity and accurate identification of target gas: in a relatively wide mid-infrared spectral range, the overlap of molecular spectral lines is less, and the cross interference is smaller, and the "fingerprint" spectrum of the molecule is scanned and analyzed by using the mid-infrared laser, and high-precision quantitative analysis of the gas is realized.

[0028] (4) High sensitivity: the mid-infrared laser gas detection technology and the TDLAS technology are fused, the function of extracting weak signals is used by using the mid-infrared band gas absorption line strength and harmonic detection technology, the detection lower limit can reach ppb level, and high-sensitivity detection of the gas is realized.

[0029] (5) Real-time monitoring: the mid-infrared laser array is used to run in parallel and calculate the concentration of multiple target gases at the same time, and the entire gas detection process only needs a few ms of time, and real-time online monitoring is realized.

[0030] The application has the characteristics of high sensitivity, low detection lower limit, real-time monitoring, easy installation, strong anti-interference ability, maintenance-free, wide application range and the like. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description. Obviously, the drawings in the following description only some embodiments of the application, and for those skilled in the art, other drawings can be obtained without creative labor on the basis of these drawings.

[0032] Figure 1 It is a structural schematic diagram of the application.

[0033] Figure 2 It is Figure 1 The flow chart of the adaptive wavelet threshold filtering algorithm shown in the figure.

[0034] In the figure, 1 is a power supply, 2 is a controller, 3 is a main control circuit, 4 is an indication laser, 5 is a mid-infrared laser array, 6 is an optical fiber coupling structure, 7 is a multi-pass gas cell, 8 is a focusing lens, and 9 is a photodetector. DETAILED DESCRIPTION

[0035] 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. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without creative work are within the scope of protection of the present invention.

[0036] Example 1

[0037] like Figure 1 As shown, the present invention provides a highly sensitive multi-gas real-time online monitoring device, including a power supply 1, which is connected to a main control circuit 3, which is respectively connected to a controller 2, an indicator laser 4 and a mid-infrared laser array 5, the indicator laser 4 and the mid-infrared laser array 5 are both connected to a multi-pass gas pool 7 through a fiber coupling structure 6, and a focusing lens 8 and a photodetector 9 are provided on the light outlet side of the multi-pass gas pool 7, and the photodetector 9 is connected to the main control circuit 3.

[0038] This invention combines mid-infrared laser gas detection technology with TDLAS technology. It modulates a mid-infrared laser array (5) with sinusoidal waves of varying frequencies, enabling mid-infrared lasers of varying wavelengths to operate in parallel. Using a signal conditioning algorithm, it simultaneously analyzes the concentrations of multiple target gases. This device offers high sensitivity, a low detection limit, real-time monitoring, ease of installation, strong anti-interference capabilities, is maintenance-free, and has a wide range of applications.

[0039] The power supply 1 provides the operating voltage for the entire device. The power supply 1 is connected to the main control circuit 3, and drives the controller 2, the main control circuit 3, the indicator laser 4, the mid-infrared laser array 5, the photoelectric detector 9 and other modules to work normally through the main control circuit 3.

[0040] The controller 2 is an MCU, which is the control core of the entire device and is electrically connected to the main control circuit 3. The MCU has three main functions: 1. Laser signal drive and temperature control function, which is used to control the operating temperature of each laser in the mid-infrared laser array 5 and the parameters of the modulation signal, specifically including the waveform, frequency, and amplitude of the modulation signal. The mid-infrared laser is driven by the main control circuit 3 to emit modulated laser light, while monitoring the working status of the laser and shutting down the laser output in time when the working current is too large or the working temperature is too high; 2. Detector temperature control function, which is used to control the working temperature of the photodetector 9; 3. Gas concentration inversion function, the MCU receives the gas absorption signal after filtering and amplification by the main control circuit 3, and digitally demodulates the first harmonic signal 1 f and 2nd harmonic signal 2 f Then, the harmonic signal is de-noised by adaptive wavelet threshold filtering algorithm, and finally 2 f / 1 f The target gas concentration is inverted.

[0041] The master control circuit 3 includes a power management module, a laser driver module, a laser temperature control module, a detector temperature control module and a signal processing module. The power supply 1 provides appropriate working voltage for the controller 2 and the remaining modules through the power management module. The laser driver module and the laser temperature control module are connected with the mid-infrared laser array 5. The controller 2 provides stable modulation current for the normal operation of the mid-infrared laser through the laser driver module. The laser temperature control module adjusts the working temperature of the laser. The laser driver module and the laser temperature control module jointly act on the mid-infrared laser to make it output mid-infrared modulation laser of specific wavelength. The detector temperature control module and the signal processing module are connected with the photodetector 9. The detector temperature control module is used to control the low-temperature operation of the photodetector 9 to ensure the high response rate of the photodetector. The signal processing module is used to filter and amplify the signal output by the photodetector 9 and transmit it to the controller 2.

[0042] The indicating laser 4 emits visible light with a wavelength range of 390 nm-780 nm. The indicating laser 4 emits indicating laser and the mid-infrared laser emitted by the mid-infrared laser array 5 are coupled into a coaxial light beam through the optical fiber coupling structure 6. The light path of the visible light is used to indicate the light path of the invisible mid-infrared light, which is convenient for adjusting the emission angle of the mid-infrared laser. The mid-infrared laser array 5 includes several quantum cascade lasers or interband cascade lasers working in parallel. The specific wavelength of the mid-infrared laser is determined according to the target gas infrared spectrum queried from the HITRAN database and the target gas absorption spectrum measured by the Fourier spectrometer. The principle is that the target gas has strong absorption line at this wavelength and is not interfered by other gases. For example, when the high-sensitivity multi-gas real-time monitoring device is used for flue gas analysis in thermal power plants, waste incineration plants and steel plants, the mid-infrared laser array 5 is composed of three mid-infrared lasers with central wavelengths of 7.25 μm, 5.26 μm and 6.15 μm, respectively, which are used for high-sensitivity and accurate detection of SO2, NO and NO2.

[0043] The fiber coupling structure 6 includes a mid-infrared spatial light fiber coupler and a mid-infrared fiber combiner. The input end of the mid-infrared spatial light fiber coupler is respectively connected to each mid-infrared laser of the indicator mid-infrared laser array 5, and the output end of the mid-infrared spatial light fiber coupler is connected to the mid-infrared fiber combiner, which is connected to the multi-pass gas pool 7. An aspheric lens is installed at one end of the mid-infrared spatial light fiber coupler. Its focusing characteristics of short focal length and large numerical aperture are used to focus and couple the spatial light emitted by the mid-infrared laser into the optical fiber. The other end of the mid-infrared spatial light fiber coupler is equipped with a fiber interface adapter for connecting to the mid-infrared fiber combiner. The mid-infrared fiber combiner couples the different wavelength lasers emitted by the mid-infrared laser array 5 and the indicator laser emitted by the indicator laser 4 into a coaxial light beam that is emitted into the multi-pass gas pool 7. The fiber coupling structure of the fiber coupling structure 6 replaces the complex optical component beam combining light path structure, enhancing the mechanical stability of the entire device, while facilitating system integration and reducing the device size.

[0044] The multi-pass gas cell 7 is a long-path multi-pass gas cell with a light inlet and a light outlet on one side. The combined laser beam enters the multi-pass gas cell through the light inlet. To extend the optical path within the limited volume of the gas cell and improve gas detection accuracy, the light is reflected multiple times within the gas cell before being output through the light outlet and passing through a focusing lens 8 to a photodetector 9.

[0045] The long optical path multi-pass gas pool 7 is a pump-suction gas pool, which actively absorbs the gas in the area to be inspected, avoiding the defect of slow detection speed brought by the passive free diffusion detection method, and can detect the appearance and concentration change of the target gas more quickly. The gas pool mainly includes a light inlet, a light outlet, an air inlet, an air outlet, a cavity, etc. The air inlet and the air outlet are located on the upper side of the cavity and are both connected to the cavity. The air inlet end of the air inlet is used to connect the gas in the area to be inspected, and a filter and an air pump are set at the air inlet. The setting of the filter can effectively remove dust, water vapor, etc. in the working environment to avoid affecting the test results. It also increases the service life of the detection device and reduces maintenance costs. The air outlet is used to discharge the gas after detection in time to avoid the detected gas that has not been discharged affecting the subsequent test results.

[0046] A focusing lens 8 is installed on the light outlet side of the long optical path multi-pass gas cell, with its center aligned with the light outlet. Positioned in front of a photodetector 9, focusing the laser light, which is reflected multiple times by the long optical path multi-pass gas cell, onto the photodetector 9, enhancing signal strength and improving system sensitivity.

[0047] The photoelectric detector 9 is a mercury cadmium telluride photoelectric detector with a response range of 2-12 μm. It converts the received light signal into an electrical signal, thereby realizing the function of a single detector to simultaneously detect multiple components of target gas.

[0048] Example 2

[0049] like Figure 1 As shown, the present invention provides a highly sensitive multi-gas real-time online monitoring method, the steps of which are as follows:

[0050] Step 1: Modulation signals of different frequencies are applied to mid-infrared lasers working in parallel. Mid-infrared lasers of different modulation frequencies are coupled into a coaxial laser beam through a fiber coupling structure and injected into a long optical path multi-pass gas cell.

[0051] Each mid-infrared laser in the mid-infrared laser array 5 works in parallel. First, the controller 2 generates modulation signals of different frequencies (sawtooth wave superimposed on sine wave, and the sine wave signal modulation frequency is different). Then, different modulation signals are applied to the corresponding mid-infrared lasers through the laser driver module. That is, the modulation signal 1 is applied to the mid-infrared laser 51 through the constant current source circuit 1 in the laser driver module. The sine wave signal modulation frequency is w 1; The modulation signal 2 is applied to the mid-infrared laser 52 through the constant current source circuit 2 in the laser driver module. The sine wave signal modulation frequency is w 2; ...; The modulation signal n is applied to the mid-infrared laser 5n through the constant current source circuit n in the laser driver module. The modulation frequency of the sine wave signal is w n , and the laser's sinusoidal signal modulation frequency w 1. w 2.…… w n The relationship between them is not an integer multiple. Taking the high-sensitivity multi-gas real-time monitoring device for coal spontaneous combustion warning as an example, the mid-infrared laser array 5 is composed of three mid-infrared lasers, namely lasers 51, 52, and 53, with central wavelengths of 4.6 μm, 10.5 μm, and 3.03 μm, respectively, for detecting CO, C2H4, and C2H2, and the sinusoidal wave signal modulation frequencies of the three lasers are w 1. w 2. w 3, among which w 2 is 3.5 times w 1, w 3 is 3.5 times w 2. Other relationships are also possible, as long as they are not integer multiples of each other. Lasers with different modulation frequencies are coupled into a coaxial laser beam through the optical fiber coupling structure 6 and injected into the long optical path multi-pass gas cell 7.

[0052] Step two: the coaxial laser is emitted from the light outlet after multiple reflections in the long-path multi-pass gas cell 7 filled with target gas, and then enters the focusing lens 8 and the photodetector 9. The photodetector 9 converts the light signal carrying the target gas information into an electrical signal, which is processed by pre-amplification, filtering and noise reduction, and then enters the digital lock-in amplifier for demodulation.

[0053] Step three: the concentrations of multiple target gases are simultaneously analyzed by a signal conditioning algorithm.

[0054] The demodulation algorithm is as follows: when the signal is processed at the receiving end, the amplified signal is input into the digital lock-in amplifier, and a frequency w 1 identical to the modulation signal frequency of the mid-infrared laser 51 and the amplified signal are multiplied by the multiplier to obtain the first harmonic direct current component and other mixed signals of the mid-infrared laser 51 modulation signal. After filtering by a low-pass filter, the signals of other frequencies are filtered out to obtain the first harmonic signal 1 of the laser 51. f 1. Then, the frequency of the synchronous reference signal is changed to 2 w 1 to obtain the second harmonic signal 2 of the mid-infrared laser 51 modulation signal. f 1. Similarly, changing the corresponding frequency of the reference signal can obtain the first harmonic signal and the second harmonic signal of C2H4 and C2H2.

[0055] The measured signal output by the digital lock-in amplifier still has a certain degree of noise due to the influence of the modulation frequencies of the lasers in the mid-infrared laser array 5 and the influence of the residual amplitude modulation. The adaptive wavelet threshold filtering algorithm is used to further denoise the harmonic signal output by the lock-in amplifier.

[0056] As shown in Figure 2 , the implementation method of the adaptive wavelet threshold filtering algorithm is as follows: first, the noisy signal is subjected to wavelet transform, and the low-frequency coefficients and high-frequency coefficients of each layer are extracted after wavelet decomposition. Then, the mean square error of the noise is estimated from the high-frequency coefficients of the first layer. Then, the wavelet coefficients are subjected to nonlinear threshold processing. Finally, the pure first harmonic signal and the second harmonic signal are obtained by inverse wavelet transform. The method of nonlinear threshold processing is as follows: the universal threshold method proposed by Donoho is used to determine the initial value of the threshold. Let the standard deviation of the noise signal be σ , and the signal length be N , then the initial threshold size is . The wavelet coefficients of the noisy signal are compared with the selected threshold. The wavelet coefficients greater than or equal to the threshold remain unchanged, and the wavelet coefficients less than the threshold are set to zero. If the mean square error of the noise after denoising is greater than the preset value and the number of cycles does not reach the maximum number of times, the threshold is adjusted and the denoising process is continued; otherwise, the denoising process is ended.

[0057] After the adaptive wavelet threshold filtering algorithm denoising processing, the target gas pure first harmonic signal 1 f and second harmonic signal 2 f , then using the first harmonic signal 1 f The second harmonic signal 2 f is normalized, and finally the MCU2 according to the linear corresponding relationship between the ratio of 2 f / 1 f and the concentration of the target gas, the concentration of the target gas is inverted.

[0058] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A highly sensitive multi-gas real-time monitoring device, characterized in that: The invention comprises a controller (2), wherein the controller (2) is connected to a main control circuit (3), and the main control circuit (3) is respectively connected to an indicator laser (4) and a mid-infrared laser array (5), and the indicator laser (4) and the mid-infrared laser array (5) are both connected to a multi-pass gas pool (7) through an optical fiber coupling structure (6), and a photodetector (9) is provided on the light outlet side of the multi-pass gas pool (7), and the photodetector (9) is connected to the main control circuit (3); the controller (2) generates modulation signals of different frequencies to modulate the mid-infrared laser array (5), so that mid-infrared lasers of different wavelengths work in parallel; the indicator laser emitted by the indicator laser (4) and the mid-infrared laser emitted by the mid-infrared laser array (5) are coupled into a coaxial beam through the optical fiber coupling structure (6) and are emitted into the multi-pass gas pool (7); the controller (2) collects signals with target gas information passing through the multi-pass gas pool according to the photodetector (9), and simultaneously analyzes the concentrations of multiple target gases through a signal conditioning algorithm; The main control circuit (3) includes a power management module, a laser driving module, a laser temperature control module, a detector temperature control module and a signal processing module. The power management module provides a suitable operating voltage to the controller (2) and the remaining modules. The laser driving module and the laser temperature control module are both connected to the mid-infrared laser array (5). The laser driving module provides a stable driving current for the normal operation of the mid-infrared laser array (5) and the indicator laser (4). The laser temperature control module controls the operating temperature of the mid-infrared laser. The detector temperature control module and the signal processing module are both connected to the photodetector (9). The detector temperature control module is used to control the photodetector (9) to work at a low temperature to ensure a high response rate of the photodetector (9). The signal processing module is used to filter and amplify the signal collected by the photodetector (9) and transmit it to the controller (2) for digital demodulation. The signal conditioning algorithm uses an adaptive wavelet threshold filtering algorithm to further eliminate noise on the harmonic signal output by the digital lock-in amplifier.

2. The highly sensitive multi-gas real-time monitoring device according to claim 1, characterized in that: The power supply (1) is connected to the main control circuit (3), and provides a suitable operating voltage to the controller (2), the indicator laser (4), the mid-infrared laser array (5), and the photodetector (9) through the main control circuit (3); the controller (2) is an MCU, which receives the gas absorption signal after filtering and amplification by the main control circuit (3), and digitally demodulates the first harmonic signal 1 f and the second harmonic signal 2 f After the harmonic signal is denoised by the adaptive wavelet threshold filtering algorithm, the 2 f / 1 f The target gas concentration is inverted.

3. The highly sensitive multi-gas real-time monitoring device according to claim 1, characterized in that: The fiber coupling structure (6) includes a mid-infrared spatial light fiber coupler and a mid-infrared fiber combiner. The input end of the mid-infrared spatial light fiber coupler is respectively connected to each mid-infrared laser of the mid-infrared laser array (5), the output end of the mid-infrared spatial light fiber coupler is connected to the mid-infrared fiber combiner, and the mid-infrared fiber combiner is connected to the multi-pass gas pool (7). The infrared fiber combiner couples the laser beams of different wavelengths emitted by the mid-infrared laser array (5) and the indicator laser beam emitted by the indicator laser (4) into a coaxial light and injects it into the multi-pass gas pool (7).

4. The highly sensitive multi-gas real-time monitoring device according to claim 3, characterized in that: An aspheric lens is installed at one end of the mid-infrared spatial light fiber coupler, and the spatial light emitted by the mid-infrared laser is focused and coupled into the optical fiber by utilizing the focusing characteristics of the aspheric lens with a short focal length and a large numerical aperture. The other end of the mid-infrared spatial light fiber coupler is equipped with an optical fiber interface adapter for connecting to a mid-infrared optical fiber combiner; a focusing lens (8) is provided on the light outlet side of the multi-pass gas pool (7), and the center of the focusing lens (8) is on the same optical axis as the light outlet; the focusing lens (8) is arranged on the incident window side of the photodetector (9).

5. The highly sensitive multi-gas real-time monitoring device according to any one of claims 1 to 4, characterized in that: The multi-pass gas pool (7) is a long optical path multi-pass gas pool, and a light inlet and a light outlet are provided on one side of the long optical path multi-pass gas pool; the laser beam after beam combination enters the long optical path multi-pass gas pool through the light inlet, and the light is reflected multiple times in the long optical path multi-pass gas pool and then output from the light outlet and focused by the focusing lens (8) to reach the photoelectric detector (9); the long optical path multi-pass gas pool is a pump-suction gas pool, which actively absorbs the gas in the area to be inspected.

6. The highly sensitive multi-gas real-time monitoring device according to claim 1, characterized in that: The mid-infrared laser array (5) includes a plurality of quantum cascade lasers or interband cascade lasers arranged in parallel. The wavelength of the visible light emitted by the indicator laser (4) is in the range of 390 nm to 780 nm. The wavelength of the mid-infrared laser is determined based on the infrared spectrum of the target gas queried from the HITRAN database and the absorption spectrum of the target gas measured by the Fourier spectrometer. The photodetector (9) is a mercury cadmium telluride photodetector, and the response range of the photodetector (9) is 2-12 μm.

7. The monitoring method of the highly sensitive multi-gas real-time monitoring device according to any one of claims 1 to 4 or 6, characterized in that: The steps are as follows: Step 1: Modulation signals of different frequencies are applied to the mid-infrared lasers working in parallel. The mid-infrared lasers of different modulation frequencies and the indicator laser are coupled into a coaxial laser beam through a fiber coupling structure and injected into a long optical path multi-pass gas cell. Step 2: After multiple reflections in a long optical path multi-pass gas pool (7) filled with target gas, the coaxial laser is emitted from the light outlet through a focusing lens (8) to reach a photodetector (9). The photodetector (9) converts the optical signal carrying target gas information into an electrical signal, which is then pre-amplified, filtered, and subjected to noise reduction processing before entering a digital lock-in amplifier for demodulation. Step 3: The demodulated signal is analyzed through a signal conditioning algorithm to simultaneously determine the concentrations of multiple target gases.

8. The monitoring method according to claim 7, characterized in that: The modulation signals of different frequencies are sawtooth waves superimposed on sine waves, and the modulation frequencies of the sine wave signals are different. The modulation signals of different frequencies are applied to corresponding mid-infrared lasers through different constant current source circuits; the modulation frequencies of the sine wave signals of different mid-infrared lasers are not integer multiples of each other; The demodulation method of the digital lock-in amplifier is: using a frequency that is consistent with the mid-infrared laser (51) sinusoidal wave modulation signal frequency w The same synchronous reference signal and the amplified signal are multiplied by a multiplier to obtain the first harmonic DC component of the mid-infrared laser (51) modulation signal and other mixed signals. After filtering by a low-pass filter, the signals of other frequencies are filtered out to obtain the first harmonic signal of the mid-infrared laser (51). f 1; then, change the synchronization reference signal frequency to 2 w 1. Obtain the second harmonic signal of the mid-infrared laser (51) 2 f 1; Similarly, the first harmonic signal and the second harmonic signal corresponding to each laser in the mid-infrared laser array are obtained through digital demodulation.

9. The monitoring method according to claim 8, characterized in that: The adaptive wavelet threshold filtering algorithm is implemented as follows: performing wavelet transform on the noisy signal, extracting the low-frequency coefficients and high-frequency coefficients of each layer after wavelet decomposition, estimating the mean square error of the noise from the high-frequency coefficients of the first layer; then performing nonlinear threshold processing on the wavelet coefficients; and finally performing inverse wavelet transform to obtain pure first harmonic signals and second harmonic signals; The nonlinear threshold processing method is as follows: the initial value of the threshold is determined by using the universal threshold method proposed by Donoho: the standard deviation of the noise signal is σ, the signal length is N , then the initial threshold ; Compare the wavelet coefficients of the noisy signal with the selected threshold. The wavelet coefficients greater than or equal to the threshold remain unchanged, and the wavelet coefficients less than the threshold are set to zero. If the mean square error of the noise after denoising is greater than the preset value and the number of cycles has not reached the maximum limit, the threshold is adjusted and the denoising process continues; otherwise, the denoising process ends. After the adaptive wavelet threshold filtering algorithm is used to eliminate noise, the pure first harmonic signal of the target gas is obtained. f and the second harmonic signal 2 f , using the first harmonic signal 1 f For the second harmonic signal 2 f Normalize it and finally calculate it based on 2 f / 1 f The linear correspondence between the ratio and the concentration of the target gas can be used to invert the concentration of the target gas.

Images