An automatic calibration device and method for a quantum cascade laser and gas concentration inversion

Abstract

The present invention discloses an automatic calibration and gas concentration inversion device and method for a quantum cascade laser, comprising: a support structure, which includes a support plate, and a plurality of support feet are fixedly connected to the lower end of the support plate; a calibration and inversion structure, which includes a laser fixedly connected to the upper end of the support plate, a reflecting mirror is arranged on the support plate on the front side of the output end of the laser, a vertical circular plate is fixedly connected to the center of the upper end of the support plate, a stepping motor is fixedly connected to the side wall of the circular plate, and a filter wheel is fixedly sleeved on the driving shaft of the stepping motor. A plurality of gas filter cells and a Ge etalon are evenly arranged on the side wall of the filter wheel, and the light beam of the laser passes through the Ge etalon or the gas filter cell on the filter wheel through the reflecting mirror and exits. By adopting a high-bandwidth mid-infrared MCT detector and a high-speed data acquisition device, the present invention can record the spectrum in the time domain, and combined with a high-resolution etalon, information about the spectrum in the frequency domain can be obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor lasers, and in particular, to an automatic calibration and gas concentration inversion device and method for a quantum cascade laser. Background Art

[0002] NO2 is an important air pollutant and a harmful gas released from automobile exhaust, garbage combustion, smoking, etc. After the concentration of NO2 in the atmosphere increases, it can have direct or indirect impacts on the environment in the following aspects: free radical reactions occur in the stratosphere, destroying stratospheric ozone; nitric acid is generated at the bottom of the stratosphere and enters the cloud to form acid precipitation; after being transported through the atmosphere, it enters the troposphere and causes an increase in the concentration of tropospheric ozone through photochemical reactions, and it is a greenhouse gas. Therefore, monitoring the sources of NO2 in the atmosphere for effective control and management is of great significance to the biosphere.

[0003] Currently, the methods for detecting NO2 gas in the atmosphere mainly include chemical methods and optical methods. The chemical methods mainly include chemiluminescence method and electrochemical method. Since chemical methods usually require sample pretreatment, which is complex and time-consuming, and are easily interfered by other substances, and may also cause artificial products, especially not suitable for real-time continuous measurement, the optical method has become a measurement technology that people increasingly favor. In particular, using tunable infrared laser absorption spectroscopy technology to measure NO2 gas in the atmosphere can not only overcome the sampling and pretreatment problems in chemical methods, but also completely avoid the interference of other gases, and achieve highly sensitive detection of the gas to be measured in an ultra-short time.

[0004] Frequency calibration is of great significance in the fields of precision measurement, optical communication, laser technology, etc. The current frequency calibration technology mainly uses an F-P (Fabry-Perot) interferometer. The F-P interferometer has good frequency discrimination ability and high-precision measurement performance, so it is widely used in many precision measurement fields. However, there are still some problems in the existing frequency calibration technology based on the F-P etalon. First, traditional F-P etalons usually require precise manufacturing processes and complex debugging processes, resulting in high costs. Second, the existing F-P etalons are easily affected by factors such as temperature changes and mechanical vibrations after long-term use, resulting in frequency drift and instability. In addition, the F-P etalon frequency calibration method in the existing technology may require complex data processing and calculations, with certain operational complexity and inconvenience. Summary of the Invention

[0005] The purpose of this section is to outline some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of the present application, to avoid obscuring the purpose of this section, the abstract, and the title, and such simplifications or omissions shall not be used to limit the scope of the present invention.

[0006] In view of the problems existing in the above-mentioned prior art quantum cascade laser automatic calibration and gas concentration inversion device and method, the present invention is proposed.

[0007] Therefore, the object of the present invention is to provide a quantum cascade laser automatic calibration and gas concentration inversion device, which can record the spectrum in the time domain by using a high-bandwidth mid-infrared MCT detector and a high-speed data acquisition device, and combine with a high-resolution etalon to obtain information about the spectrum in the frequency domain.

[0008] To solve the above technical problems, the present invention provides the following technical solution: A quantum cascade laser automatic calibration and gas concentration inversion device, comprising:

[0009] A support structure, which includes a support plate, and a plurality of support feet are fixedly connected to the lower end of the support plate;

[0010] A calibration and inversion structure, which includes a laser fixedly connected to the upper end of the support plate. A reflecting mirror is arranged on the support plate in front of the output end of the laser. A vertical circular plate is fixedly connected to the center of the upper end of the support plate. A stepping motor is fixedly connected to the side wall of the circular plate, and a filter wheel is fixedly sleeved on the driving shaft of the stepping motor. A plurality of gas filter cells and a Ge etalon are evenly arranged on the side wall of the filter wheel. The light beam of the laser passes through the reflecting mirror, passes through the Ge etalon or the gas filter cell on the filter wheel and exits. An off-axis parabolic mirror opposite to the reflecting mirror is fixedly connected to the upper end of the support plate. A photodetector is fixedly connected to the support plate on one side of the off-axis parabolic mirror. The light beam is transmitted to the photodetector through the off-axis parabolic mirror.

[0011] As a preferred scheme of the quantum cascade laser automatic calibration and gas concentration inversion device of the present invention, wherein: N2, NO, CO2, NO2, C2H2, and CH4 are respectively filled in the plurality of gas filter cells.

[0012] As a preferred scheme of the quantum cascade laser automatic calibration and gas concentration inversion device of the present invention, wherein: the laser uses a quantum cascade laser, integrates an integrated Ge etalon and a gas filter cell on a filter wheel, and realizes the automatic calibration of the quantum cascade laser through the control of the stepping motor.

[0013] As a preferred embodiment of the quantum cascade laser automatic calibration and gas concentration inversion device of the present invention, the following is provided: the quantum cascade laser is connected to an external host computer, and data acquisition is synchronized with the host computer acquisition card. The photodetector uses a high-bandwidth mid-infrared MCT detector, and the photodetector is connected to an external high-speed data acquisition device to record the spectrum in the time domain. Combining with an etalon, information about the spectrum in the frequency domain can be obtained.

[0014] As a preferred embodiment of the quantum cascade laser automatic calibration and gas concentration inversion device of the present invention, the following is provided: after the emission current pulse, the power of the spectrum in the time domain gradually decreases over time and reaches a minimum value at the end; the heat dissipation of the laser increases the temperature of the laser heat sink, and the laser threshold also increases with the increase of the heat sink temperature, that is, while there is laser negative chirp, the required threshold current is also getting higher and higher.

[0015] A method for automatic calibration of a quantum cascade laser and inversion of gas concentration, wherein: the automatic time-domain to frequency-domain calibration in the inversion method of the quantum cascade laser automatic calibration and gas concentration inversion device includes the following steps:

[0016] Step 1: Control the stepping motor to rotate, place the Ge etalon in the optical path, control the quantum cascade laser to emit light, and synchronize the data acquisition with the host computer acquisition card to obtain a stable Etalon interference peak signal.

[0017] Step 2: Use the acquisition card to collect the interference signal output by the etalon. After multiple averaging, perform automatic peak searching processing by the host computer to achieve relative frequency calibration.

[0018] Step 3: Data processing: By the interference signal peak searching method, obtain the time value corresponding to each interference peak. Take the time point of the interference peak as the abscissa, and the frequency interval between peaks as the free spectral range FSR of the etalon. Fit the above data points with a cubic equation to obtain a fitting equation.

[0019] Step 4: Transfer the gas filter gas cell into the optical path, obtain the time value corresponding to the corresponding gas absorption peak, compare with HITRAN to obtain the theoretical wavenumber position corresponding to the absorption peak, and make a one-to-one correspondence between the two to know the frequency domain position corresponding to the gas absorption peak.

[0020] Step 5: Convert the time axis of the long pulse spectrogram signal into a frequency domain axis represented by wavenumber, time-frequency conversion: convert the data sequence number into time, and then into wavenumber.

[0021] As a preferred embodiment of the quantum cascade laser automatic calibration and gas concentration inversion device of the present invention, the following is provided: the fitting equation in the above Step 3 is:

[0022] y = Intercept + B1 * x + B2 * x 2 + B3 * x 3

[0023] Where Intercept is a function, referring to the distance from the intersection point of the function graph and the coordinate axis to the origin;

[0024] B1, B2, and B3 are the coefficients of the cubic function; x and y are the positions of the wave crest and wave trough respectively.

[0025] As a preferred solution of the quantum cascade laser automatic calibration and gas concentration inversion device described in the present invention, wherein: the value of B1 is 0.48327 ± 0.00504; the value of B2 is -0.00137 ± 3.0727E-5; the value of B3 is 1.6479E-6 ± 5.92536E-8.

[0026] As a preferred solution of the quantum cascade laser automatic calibration and gas concentration inversion method described in the present invention, wherein: the following two formulas are used in step five:

[0027] t = x ÷ N

[0028] v = a × t^3 + b × t^2 + c × t + d

[0029] Where x is the data serial number, N is the detector scanning frequency, t is the time, a, b, c, and d are the polynomial coefficients, and v is the wave number.

[0030] As a preferred solution of the quantum cascade laser automatic calibration and gas concentration inversion method described in the present invention, wherein: the specific operation of gas concentration inversion of the quantum cascade laser in the quantum cascade laser automatic calibration and gas concentration inversion method includes the following steps:

[0031] S1. First, take the baseline region of the transmission spectrum, obtain the baseline using a quadratic polynomial, convert the transmission spectrum into an absorbance spectrum, and obtain the absorbance spectrum diagram of the substance;

[0032] S2. Perform multi-group voigt line shape parameter fitting modeling on the absorbance spectrum diagram of the standard substance to obtain a standard substance spectral library. Among them, the intensity and line width will change due to the influence of temperature and pressure, preparing for subsequent concentration inversion;

[0033] S3. Adopt the least squares method, and obtain the concentration of the gas substance in the measured data by comparing the absorbance data of the standard substance voigt model, thereby realizing gas concentration inversion.

[0034] Advantages of the present invention: The present invention uses the method of comparing with a standard spectral database to calibrate the frequency of gas absorption peaks in the time-domain signal, thereby converting the time-domain signal within the entire time range into a frequency-domain signal. The present invention integrates an integrated Ge etalon and a gas filter cell (GFC) on a filter wheel, and through the control of a stepper motor, realizes the automatic calibration of a quantum cascade laser (QCL). BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings. Among them:

[0036] Figure 1 Schematic diagram of the overall Etalon frequency-domain calibration structure of the automatic calibration of the quantum cascade laser and the gas concentration inversion device and method of the present invention;

[0037] Figure 2 Schematic diagram of the side structure of the automatic calibration of the quantum cascade laser and the gas concentration inversion device and method of the present invention;

[0038] Figure 3 Schematic diagram of the excitation pulse spectrum and gas absorption shape of the automatic calibration of the quantum cascade laser and the gas concentration inversion device and method of the present invention;

[0039] Figure 4 Schematic diagram of obtaining a stable Etalon signal and relative frequency calibration of the automatic calibration of the quantum cascade laser and the gas concentration inversion device and method of the present invention;

[0040] Figure 5 Schematic diagram of establishing the functional relationship between the free spectral range and the spectral acquisition time of the automatic calibration of the quantum cascade laser and the gas concentration inversion device and method of the present invention;

[0041] Figure 6 Schematic diagram of the gas absorbance curve in the frequency domain of the laser scanning band of the automatic calibration of the quantum cascade laser and the gas concentration inversion device and method of the present invention;

[0042] Figure 7 Schematic diagram of comparing the peak shapes and determining the center frequency of the characteristic absorption peak of the automatic calibration of the quantum cascade laser and the gas concentration inversion device and method of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the following will make a detailed description of the specific embodiments of the present invention in conjunction with the drawings of the specification.

[0044] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced in other ways different from those described herein. Those skilled in the art can make similar generalizations without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0045] Secondly, as used herein, "an embodiment" or "embodiments" refers to specific features, structures, or characteristics that may be included in at least one implementation manner of the present invention. The phrase "in an embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it an individual or alternative embodiment that is mutually exclusive with other embodiments.

[0046] Thirdly, the present invention is described in detail in conjunction with schematic diagrams. When detailing the embodiments of the present invention, for the sake of illustration, the cross-sectional views showing the device structure are enlarged locally in a non-general proportion, and the schematic diagrams are only examples and should not limit the scope of protection of the present invention herein. In addition, in actual production, three-dimensional spatial dimensions including length, width, and depth should be included.

[0047] Referring to Figures 1-7 , a quantum cascade laser automatic calibration and gas concentration inversion device is provided, including:

[0048] A support structure 100 and a calibration and inversion structure 200, the support structure 100 includes a support plate 101, and a plurality of support feet 102 are fixedly connected to the lower end of the support plate 101;

[0049] Among them, the calibration and inversion structure 200 includes a laser 201 fixedly connected to the upper end of the support plate 101. A reflecting mirror 202 is arranged on the support plate 101 on the front side of the output end of the laser 201. A vertical circular plate 203 is fixedly connected to the center of the upper end of the support plate 101. A stepping motor 204 is fixedly connected to the side wall of the circular plate 203, and a filter wheel 205 is fixedly sleeved on the driving shaft of the stepping motor 204. A plurality of gas filter chambers 206 and a Ge etalon 207 are evenly arranged on the side wall of the filter wheel 205. The light beam of the laser 201 passes through the Ge etalon 207 or the gas filter chamber 206 on the filter wheel 205 through the reflecting mirror 202 and exits. An off-axis parabolic mirror 208 opposite to the reflecting mirror 202 is fixedly connected to the upper end of the support plate 101. A photodetector 209 is fixedly connected to the support plate 101 on one side of the off-axis parabolic mirror 208, and the light beam is transmitted into the photodetector 209 through the off-axis parabolic mirror 208.

[0050] Among them, N2, NO, CO2, NO2, C2H2, and CH4 are respectively contained in multiple gas filter cells 206; the laser 201 uses a quantum cascade laser, integrates the integrated Ge etalon 207 and the gas filter cell 206 on a filter wheel 205, and is controlled by a stepper motor 204 to realize the automatic calibration of the quantum cascade laser.

[0051] Among them, the quantum cascade laser is connected to an external host computer, and data is collected synchronously with the host computer acquisition card. The photodetector uses a high-bandwidth mid-infrared MCT detector, and the photodetector is connected to an external high-speed data acquisition device to record the spectrum in the time domain. Combining with the etalon, information about the spectrum in the frequency domain can be obtained.

[0052] Furthermore, Figure 3 The light intensity signal for excitation is the change with time. The successful recording of the QCL light pulse stems from the fast time response of the detector and the high-speed digitization process of the digital acquisition card. The baselines before and after the rising and falling edges of the light pulse are the detector voltage responses without laser. It can be seen from the figure that after the emission current pulse, the continuous power of the spectrum in the time domain gradually decreases and reaches the minimum value at the end. This is because the heat dissipation of the laser increases the temperature of the laser heat sink, and the laser threshold will also increase with the increase of the heat sink temperature, that is, while there is laser negative chirp, the required threshold current is also getting higher and higher. Since the laser current is constant, the corresponding output light energy is getting lower and lower.

[0053] Among them, the automatic time-domain to frequency-domain calibration in the process of the automatic calibration of the quantum cascade laser and the gas concentration inversion method includes the following steps:

[0054] Step 1: Control the stepper motor 204 to rotate, place the Ge etalon 207 in the optical path, control the quantum cascade laser to emit light, and synchronously collect data with the host computer acquisition card to obtain a stable Etalon interference peak signal;

[0055] Step 2: Use the acquisition card to collect the interference signal output by the etalon. After multiple averaging, perform automatic peak searching processing through the host computer to achieve relative frequency calibration;

[0056] Step 3: Data processing: Through the interference signal peak searching method, obtain the time value corresponding to each interference peak. Use the time point of the interference peak as the abscissa, and the frequency interval between peaks is the free spectral range FSR of the etalon. Fit the above data points with a cubic equation to obtain a fitting equation;

[0057] Step 4: Transfer the gas filter cell 206 into the optical path, obtain the time value corresponding to the corresponding gas absorption peak, compare with HITRAN to obtain the theoretical wave number position corresponding to the absorption peak, and make a one-to-one correspondence between the two to know the frequency domain position corresponding to the gas absorption peak;

[0058] Step 5: Convert the time axis of the long pulse spectrogram signal into a frequency domain axis represented by wave numbers. Time-frequency conversion: Convert the data sequence number into time and then into wave numbers.

[0059] Furthermore, the fitting equation in Step 3 is:

[0060] y = Intercept + B1 * x + B2 * x 2 + B3 * x 3

[0061] where Intercept is a function referring to the distance from the intersection point of the function graph and the coordinate axis to the origin.

[0062] B1, B2, and B3 are the coefficients of the cubic function; x and y are the positions of the wave crest and wave trough respectively.

[0063] Specifically, referring to Figure 4 , the value of B1 is 0.48327 ± 0.00504; the value of B2 is -0.00137 ± 3.0727E-5; the value of B3 is 1.6479E-6 ± 5.92536E-8.

[0064] Among them, the following two formulas are used in Step 5:

[0065] t = x ÷ N

[0066] v = a × t^3 + b × t^2 + c × t + d

[0067] where x is the data sequence number, N is the detector scanning frequency, t is the time, a, b, c, and d are the polynomial coefficients, and v is the wave number.

[0068] Specifically, the specific operation of gas concentration inversion of the quantum cascade laser in the quantum cascade laser automatic calibration and gas concentration inversion method includes the following steps:

[0069] S1. First, take the baseline region of the transmission spectrum, obtain the baseline using a quadratic polynomial, convert the transmission spectrum into an absorbance spectrum, and obtain the absorbance spectrogram of the substance.

[0070] S2. Perform multi-group Voigt line shape parameter fitting modeling on the absorbance spectrogram of the standard substance to obtain a standard substance spectral library. Among them, the intensity and line width will change due to the influence of temperature and pressure, preparing for subsequent concentration inversion.

[0071] S3. Adopt the least squares method, and obtain the concentration of the gas substance in the measured data by comparing the absorbance data of the standard substance Voigt model, thereby realizing gas concentration inversion.

[0072] Among them, the Voigt model is an existing spectral fitting method, which will not be elaborated here.

[0073] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A quantum cascade laser automatic calibration and gas concentration inversion device, characterized in that: include: A support structure (100), comprising a support plate (101), wherein a plurality of support legs (102) are fixedly connected to the lower end of the support plate (101); The calibration inversion structure (200) comprises a laser (201) fixedly connected to the upper end of a support plate (101), a reflector (202) being arranged on the support plate (101) in front of the output end of the laser (201), a vertical circular plate (203) being fixedly connected to the center of the upper end of the support plate (101), a stepping motor (204) being fixedly connected to the side wall of the circular plate (203), a filter wheel (205) being fixedly sleeved on the driving shaft of the stepping motor (204), and a plurality of gas filter wheels (205) being evenly arranged on the side wall of the filter wheel (205). A gas chamber (206) and a Ge etalon (207), the light beam of the laser (201) passes through the Ge etalon (207) or the gas filter gas chamber (206) on the filter wheel (205) through the reflector (202) and then passes out, the upper end of the support plate (101) is fixedly connected to an off-axis parabolic mirror (208) opposite to the reflector (202), a photoelectric detector (209) is fixedly connected to the support plate (101) on one side of the off-axis parabolic mirror (208), and the light beam is transmitted to the photoelectric detector (209) through the off-axis parabolic mirror (208); The laser (201) adopts a quantum cascade laser, and an integrated Ge etalon (207) and a gas filter chamber (206) are integrated on a filter wheel (205), and controlled by a stepper motor (204) to achieve automatic calibration of the quantum cascade laser; the quantum cascade laser is connected to an external host computer and synchronized with a host computer acquisition card to perform data acquisition; the photoelectric detector adopts a high-bandwidth mid-infrared MCT detector, and the photoelectric detector is connected to an external high-speed data acquisition device to record the spectrum in the time domain, and combined with the etalon, information about the spectrum in the frequency domain can be obtained.

2. The quantum cascade laser automatic calibration and gas concentration inversion device according to claim 1, characterized in that: The plurality of gas filter chambers (206) are respectively equipped with , , , , , .

3. The quantum cascade laser automatic calibration and gas concentration inversion device according to claim 1, characterized in that: After the emission current pulse, the continuous power of the spectrum in the time domain gradually decreases with time and reaches a minimum value at the end; the heat dissipation of the laser (201) increases the heat sink temperature of the laser, and the laser threshold will also increase with the increase of the heat sink temperature, that is, while the laser negative chirps, the required threshold current also becomes higher and higher.

4. A method for automatic calibration of a quantum cascade laser and inversion of gas concentration uses the device for automatic calibration of a quantum cascade laser and inversion of gas concentration as claimed in claim 1, characterized in that: The automatic calibration of cascade lasers and the automatic time-domain to frequency-domain calibration in gas concentration inversion include the following steps: Step 1: Control the stepper motor (204) to rotate, place the Ge etalon (207) in the optical path, control the quantum cascade laser to emit light, and synchronize the host computer acquisition card to perform data acquisition to obtain a stable Etalon interference peak signal; Step 2: Use the acquisition card to collect the interference signal output by the standard tool, average it multiple times, and then use the host computer to automatically find the peak to achieve relative frequency calibration; Step 3: Data processing: The time value corresponding to each interference peak is obtained by the interference signal peak finding method. The time point of the interference peak is used as the horizontal coordinate, and the frequency interval between peaks is the free spectrum range (FSR) of the etalon. The above data points are fitted to the third power to obtain the fitting equation. Step 4: Turn the gas filter chamber (206) into the optical path, obtain the time value corresponding to the corresponding gas absorption peak, compare it with the theoretical wave number position corresponding to the absorption peak obtained by HITRAN, and make a one-to-one correspondence between the two to know the frequency domain position corresponding to the gas absorption peak; Step 5: Change the time axis of the long pulse spectrum signal to the frequency domain axis expressed in wave numbers, time-frequency conversion: convert the data sequence number into time, and then into wave numbers; The following two formulas are used in step 5: Among them, x is the data sequence number, N is the detector scanning frequency, t is the time, a, b, c, d are the polynomial coefficients, and v is the wave number; The specific operation of the quantum cascade laser gas concentration inversion in the quantum cascade laser automatic calibration and gas concentration inversion method includes the following steps: S1. First, take the baseline area of ​​the transmission spectrum, use a quadratic polynomial to obtain the baseline, convert the transmission spectrum into an absorbance spectrum, and obtain the absorbance spectrum of the substance; S2. Perform multiple sets of Voigt line parameter fitting modeling on the absorbance spectra of standard substances to obtain a standard substance spectrum library, in which the intensity and line width will change due to the influence of temperature and pressure, in preparation for subsequent concentration inversion; S3. Using the least squares method, by comparing the absorbance data of the standard substance Voigt model, the concentration of the gas substance in the measured data is obtained, thereby realizing gas concentration inversion.

5. The method for automatic calibration of quantum cascade laser and inversion of gas concentration according to claim 4, characterized in that: The fitting equation in step 3 is: in, is a function, which refers to the distance from the intersection of the function graph and the coordinate axis to the origin; , , are the cubic function coefficients; x and y are the positions of the peak and trough, respectively.

6. The method for automatic calibration of quantum cascade laser and inversion of gas concentration according to claim 5, characterized in that: Said The value of ; The value of ; The value of .

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