Laser scanning frequency online calibration method and system based on dual-frequency reference fusion

Abstract

The present application relates to a kind of laser scanning frequency online calibration method and system based on dual-frequency reference fusion, will be based on the second optical zone of optical feedback enhanced absorption spectroscopy or optical cavity ring-down spectroscopy measurement technology is nested in the first optical zone based on tunable laser direct absorption spectroscopy measurement technology, utilize the absolute frequency reference provided by the first optical zone target gas molecule itself absorption transition, and utilize the periodic frequency reference signal with free spectral range as fixed interval output by the second optical zone, obtain the relative frequency variation relationship of laser scanning whole process, fusion absolute frequency reference and relative frequency variation relationship, establish the mapping relationship between laser drive current parameter and actual laser frequency, realize laser scanning frequency axis reconstruction and frequency online calibration.The technical problems, such as that laser scanning frequency calibration in the prior art cannot be calibrated in real time, system and operation process are complex, and cost is high, are solved.

Description

Technical Field

[0001] This invention relates to the field of laser frequency calibration technology, and in particular to an online calibration method and system for laser scanning frequency based on dual-frequency reference fusion. Background Technology

[0002] Tunable semiconductor lasers are widely used in gas detection, environmental monitoring, industrial process control, and precision spectral measurement. Tunable Laser Absorption Spectroscopy (TDLAS) is currently the mainstream quantitative detection technology for laser absorption spectroscopy. Its core principle is to precisely control the laser wavelength by adjusting the injection current and temperature of the semiconductor laser to scan the narrow-band characteristic absorption lines of a specific gas. It is often combined with wavelength modulation and harmonic detection techniques to achieve ultra-low detection limits at the ppb level. Key features include: extremely high selectivity with virtually no gas cross-interference; millisecond-level response speed; and internal mirrors that are less prone to contamination, eliminating the need for frequent optical locking calibration. In TDLAS systems, laser wavelength scanning is typically achieved by changing the laser injection current or operating temperature. However, due to the nonlinearity of current-wavelength tuning, temperature drift, long-term frequency drift, and poor scan repeatability in semiconductor lasers, the relationship between laser current intensity and laser wavelength is not constant during actual scanning. Therefore, laser frequency calibration is necessary.

[0003] Existing laser frequency calibration methods mainly include: real-time laser wavelength measurement using a wavelengthmeter, which has the disadvantages of high integration difficulty, limited response speed, and high cost; the frequency calibration method using the Fabry-Perot etalon, which can provide periodic relative frequency intervals but cannot provide an absolute frequency reference and requires an additional reference source; and the frequency reference method using a standard gas cell, which requires pre-filling with a standard gas of known concentration, making the system complex. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a laser scanning frequency online calibration method and system based on dual-frequency reference fusion, which solves the technical problems of existing laser scanning frequency calibration schemes and systems being complex and difficult to calibrate online in real time and efficiently.

[0005] The technical solution adopted in this invention is as follows: This invention provides an online laser scanning frequency calibration method based on dual-frequency reference fusion, which is based on the coupling of dual optical regions, wherein the dual optical regions include: The first optical region employs a long-path absorption cell or integrating cavity; The second optical region employs a ring-down optical resonator or an optical feedback enhanced resonator, and is a non-spot area nested inside the air cavity of the first optical region. The second optical region has a sealed air cavity independent of the first optical region. The method includes: The target gas is introduced into the first optical zone gas cavity, the second optical zone gas cavity is kept in a vacuum state, and the two optical zone gas cavities are maintained at a preset temperature. Using a tunable laser, the first optical region is scanned according to a first current step to obtain the direct absorption spectrum of the target gas molecules. The laser current value I0 corresponding to the center of the absorption line is extracted, and then the absolute laser frequency corresponding to the center of the absorption spectral line is obtained. n 0, to achieve absolute frequency calibration; Using a tunable laser, the second optical region is tracked and scanned starting from I0 and according to a second current step size to obtain several equally spaced transmission peak electrical signals and corresponding laser current value sequences {I1, I2, ..., I...}. M}, where M is the number of tracking scans in the second optical region; Based on the laser current value sequence and absolute frequency n 0. The free spectral range of the second optical region at the preset temperature. FSR Establish a mapping model ν= of laser current I and frequency ν. f (I,FSR, n 0), of which f (·) represents the fitting function; Based on the mapping model, the absolute frequency axis of the target gas within the gas cavity of the first optical region is reconstructed.

[0006] The preferred technical solution is: The frequency step corresponding to the first current step is smaller than the full width at half maximum (FWHM) of the absorption spectrum.

[0007] The frequency step corresponding to the second current step is equal to the free spectral range of the second optical region.

[0008] During the tracking scan, if the deviation between the actual current difference between two consecutive transmission peak electrical signals and the second current step exceeds the set standard, the second current step will be automatically adjusted for the next step to track the drift of the free spectral range of the second optical region.

[0009] The tracking of the drift in the free spectral range of the second optical region includes: Real-time monitoring of the temperatures T1 and T2 in the air chambers of the first and second optical zones; When the change in temperature T2 relative to the temperature T0 at the previous calibration step is greater than the set threshold, online correction is performed: Based on the thermal expansion coefficient α of the material in the second optical region, the physical length and free spectral range of the second optical region are corrected in real time. L' = L·[1 + α·(T2- T0)] FSR' = c / (2nL') In the formula, L and L' are the physical lengths before and after correction, respectively; FSR' is the free spectral range after correction; c is the speed of light in vacuum; and n is the refractive index of the medium in the gas cavity of the second optical region. The updated second current step ΔI2' = FSR' / is obtained based on the corrected FSR'. k , k Based on the current value sequence {I1, I2, ..., I M The calibrated current tuning factor.

[0010] The peak center of the transmission peak is located by using the zero-crossing detection of the second derivative, marked as the cavity mode matching point, and the corresponding laser current value is recorded.

[0011] At set intervals, the first optical region is automatically scanned once according to the first current step size to reacquire the current value corresponding to the center of the target gas absorption spectral line and update the absolute frequency to eliminate the influence of the laser's long-term drift.

[0012] The present invention also provides a dual-optical-area coupled gas concentration detection system for performing the calibration method described above, comprising the dual optical areas, a tunable laser, a beam splitter, a detector one, and a detector two; The air cavity of the second optical region is located at the geometric center of the air cavity of the first optical region; The gas source of the target gas is connected to the air inlet of the two optical zone air chambers respectively through an air inlet three-way valve; The air outlets of the two optical zone air chambers are connected to the vacuum pump via three-way air outlet valves; The output end of the tunable laser is connected to the input end of the beam splitter, and the two output ends of the beam splitter emit the split laser signals into the first optical region and the second optical region, respectively. The detector is used to receive the laser signal transmitted through the first optical zone; The second detector is used to receive laser signals transmitted from the second optical zone.

[0013] The present invention also provides a concentration detection method using the aforementioned dual-optical-zone coupled gas concentration detection system, comprising: By maintaining the second optical region in a vacuum state, and fitting the absorption spectrum of the target gas in the first optical region based on the reconstructed absolute frequency axis, the concentration detection value of the target gas is obtained.

[0014] This invention also provides another concentration detection method utilizing the aforementioned dual-optical-zone coupled gas concentration detection system, comprising: Maintain the pressure and temperature of the target gas within the gas cavity of the first optical zone; The target gas is introduced into the gas cavity of the second optical zone, and the measurement begins after the gas pressure and temperature stabilize. The absolute frequency axis of the target gas within the gas cavity of the second optical region is reconstructed according to the calibration method described above. Based on the reconstructed absolute frequency axis, a first concentration detection value is obtained by performing spectral analysis on the transmitted laser signal output from the first optical region; a second concentration detection value is obtained by performing cavity ring-down spectral or cavity enhancement absorption spectral analysis on the laser transmission signal output from the second optical region; the two concentration values ​​are cross-checked or one of them is selected as the final detection value based on the concentration range.

[0015] The technical solution of the present invention can achieve at least some of the following beneficial effects: This invention utilizes the absorption transitions of target gas molecules to provide an absolute frequency reference, and uses a periodic frequency reference signal with FSR as the fixed interval output by an optical resonant cavity structure to obtain the relative frequency variation relationship, thus obtaining a high-resolution frequency scale. By fusing the absolute frequency reference and the relative frequency variation relationship, a mapping relationship between the laser driving current parameters and the actual laser frequency is established, realizing laser scanning frequency axis reconstruction and online frequency calibration. It can effectively correct nonlinear changes, temperature drift, and long-term frequency drift during the scanning process of tunable lasers, and is suitable for real-time calibration of laser frequencies for gases of unknown concentration.

[0016] This invention eliminates the need for an external wavelength meter and independent frequency reference device, utilizing only the optical structure of the system itself to achieve frequency tracing. It also eliminates the need for pre-installed standard gas, directly using the absorption line of the gas under test as the frequency reference. This significantly reduces the complexity of the calibration system structure and calibration operation, saving costs.

[0017] The concentration detection system of this invention nests a second optical region based on optical feedback enhanced absorption spectroscopy or cavity ring-down spectroscopy measurement technology within a first optical region based on tunable laser direct absorption spectroscopy measurement technology. Both share the same mechanical reference, sealing system, temperature and pressure control system, and are located in the same thermal environment, significantly reducing FSR drift caused by temperature gradients. The gas cavity of the second optical region has an independent sealed shell, and the gas within the gas cavity of the first optical region can serve as a buffer thermal medium for the gas cavity of the second optical region, improving thermal inertia, reducing the impact of ambient temperature fluctuations on the resonant cavity, and ensuring the accuracy of calibration results.

[0018] This invention can achieve the final concentration detection result by cross-verifying concentration values ​​or by selecting a first and a second concentration value based on the concentration range. It can cover an ultra-wide concentration detection range from ppb to percentage level and is suitable for various gas detection conditions from trace to constant levels. Attached Figure Description

[0019] Figure 1This is a schematic diagram of the overall structure of the system in Embodiment 1 of the present invention.

[0020] Figure 2 This is a schematic diagram of the laser optical path arrangement structure of the system in Embodiment 1 of the present invention.

[0021] Figure 3 This is a schematic diagram showing the comparison between the results of polynomial baseline fitting and Voigt line fitting of the direct absorption spectrum obtained by scanning the first optical region based on the absolute frequency axis reconstructed according to Embodiment 2 of the present invention and the experimental measurement values ​​under the same conditions.

[0022] Explanation of reference numerals in the attached diagram: 1. First optical zone; 2. Second optical zone; 3. Tunable laser; 4. Beam splitter; 5. Detector 1; 6. Detector 2; 7. Inlet three-way valve; 8. Outlet three-way valve; 9. Vacuum pump. Detailed Implementation

[0023] The specific embodiments of the present invention are described below with reference to the accompanying drawings.

[0024] Example 1 See Figure 1 and Figure 2 This embodiment provides a dual-optical-area coupled gas concentration detection system, including the dual optical areas, a tunable laser 3, a beam splitter 4, a detector 1 5, and a detector 2 6; The first optical region 1 preferably employs a long-path absorption cell based on tunable laser absorption spectroscopy (TDLAS) technology or an integrating cavity based on integrating cavity output spectroscopy (ICOS) technology; the second optical region 2 preferably employs an optical feedback enhanced resonant cavity based on optical feedback enhanced absorption spectroscopy (OF-CEAS) technology or a ring-down optical resonant cavity based on cavity ring-down spectroscopy (CRDS) technology; the air cavity of the second optical region 2 is located in the non-spot area inside the air cavity of the first optical region 1, preferably located at the geometric center of the air cavity of the first optical region 1, and the air cavity of the second optical region 2 has an independent shell, that is, the air cavities of the two optical regions are independent and not connected to each other; The gas source of the target gas is connected to the air inlet of the two optical zone air chambers respectively through the three-way air inlet valve 7; The air outlets of the two optical zone air chambers are connected to the vacuum pump 9 through the three-way air outlet valve 8; The output of the tunable laser 3, controlled by the controller, is connected to the beam splitter 4. The two outputs of the beam splitter 4 emit the split laser signals to the first optical region and the second optical region, respectively. Detector 5 is used to receive the laser signal transmitted through the first optical zone 1; Detector 6 is used to receive laser signals transmitted from the second optical zone 2; Specifically, the output signals of the two detectors are sequentially connected to the ADC (analog-to-digital converter) to complete analog-to-digital conversion, then transmitted to the MCU (microcontroller unit) to complete data acquisition and preprocessing, and finally communicate with the controller.

[0025] Specifically, the system in this embodiment also includes a temperature control module and a pressure control module. The signal acquisition devices of the two control modules include a temperature sensor and a pressure sensor, which are used to detect the temperature and pressure of the two air chambers.

[0026] Specifically, taking a long-path absorption cell as an example, the first optical region 1 has two low-reflection mirrors arranged opposite each other, namely a first concave mirror and a second concave mirror; and a first entrance window and a first exit window, respectively set on the first and second concave mirrors, or both set on the first concave mirror; the first entrance window is used to transmit the first laser, which is reflected multiple times between the two oppositely arranged low-reflection mirrors to form the first absorption optical path, and finally emitted from the first exit window and received by detector 5. The reflectivity of the low-reflection mirrors is 99%-99.9%, which is suitable for the design requirements of long-path direct absorption.

[0027] Taking the optical resonant cavity as an example, the second optical region 2 includes two opposing transmission windows: a second entrance window located in the geometric center region of the first concave mirror and a second exit window located in the geometric center region of the second concave mirror, the geometric center region being the non-spot region of the first laser; and two opposing high-reflectivity mirrors: a first high-reflectivity mirror located on the inner wall of the second entrance window and a second high-reflectivity mirror located on the inner wall of the second exit window; an independent outer shell is provided between the first and second high-reflectivity mirrors, forming a gas cavity independent of the first optical region; the second entrance window is used to transmit the second laser, which is reflected multiple times between the first and second high-reflectivity mirrors to form a second absorption optical path, and finally emitted from the second exit window and received by detector 6. The reflectivity of the high-reflectivity mirror is ≥99.99%.

[0028] The aforementioned first entrance window and exit window, and second entrance window and exit window, were not in Figure 1 This is specifically shown in the text. It is understandable that... Figure 1 The blue shell represents the gas cavity of the first optical region, and the green shell represents the gas cavity of the second optical region, which is independent of the gas cavity of the first optical region. The red and green straight lines with arrows represent the first laser optical path and the second laser optical path, respectively. In cross-section, the multiple reflection spot area of ​​the first laser completely surrounds the resonant cavity spot of the second laser, and there is no overlap or interference between the two optical paths.

[0029] Example 2 This embodiment provides a method for online calibration of laser scanning frequency using the system of Embodiment 1, including the following steps: S1. Inject the target gas into the first optical zone gas cavity, keep the second optical zone gas cavity in a vacuum state, and maintain the two optical zone gas cavities at a preset temperature.

[0030] Specifically, in this step, the gas chambers of the two optical zones are evacuated to a base vacuum (<0.01 Torr) using a vacuum pump. The outlet three-way valve is closed, and the target gas, methane (CH4), is introduced into the gas chamber of the first optical zone through the inlet three-way valve, controlling the gas pressure P1 = 100 Torr. The gas chamber of the second optical zone is maintained under vacuum. The temperatures T1 and T2 of the two optical zones are adjusted to the set temperature of 45.0℃ using the temperature control module. Measurements begin after T1 and T2 stabilize (temperature difference <0.1℃).

[0031] S2. Using a tunable laser, the first optical region is scanned according to a first current step to obtain the direct absorption spectrum of the target gas molecules. The laser current value I0 corresponding to the center of the absorption line is extracted, and then the absolute laser frequency corresponding to the center of the absorption spectral line is obtained. n 0, to achieve absolute frequency calibration.

[0032] As a preferred method, the controller controls the tunable laser to scan within the target wavelength band with a first current step of ΔI1 = 0.1 mA, and the scanning frequency is 10 Hz. The ADC and MCU monitor the first electrical signal output by the detector to obtain the direct absorption spectrum of CH4 in the gas cavity of the first optical region; wherein, the frequency step corresponding to ΔI1 = 0.1 mA is approximately 100 MHz, which is much smaller than the full width at half maximum (FWHM) of the absorption spectrum, and preferably one-tenth to one-several tenths of the FWHM of the absorption spectrum.

[0033] In this embodiment, polynomial baseline fitting and Voigt line shape fitting are performed on the absorption spectrum to extract the laser current value I0 = 45.3 mA corresponding to the center of the absorption spectral line. The absolute wavenumber corresponding to this absorption spectral line is then queried from the HITRAN database to obtain the absolute laser frequency corresponding to the center of the absorption spectral line. n 0 = 6046.95 cm -1 We take (I0, ν0) as the absolute frequency anchor point.

[0034] Those skilled in the art will understand that this step essentially involves determining the absolute frequency using gas absorption spectroscopy. The calibration principle is as follows: during laser scanning, by changing the laser driving current I, the gas absorption spectrum signal, i.e., the absorption spectrum A, is obtained as the current changes; the corresponding transition frequency can be obtained using spectral databases such as HITRAN. n gas (The absolute frequency of characteristic transitions of gas molecules), determine the measured absorption peak center current I0 and transition frequency ν. gasThe mapping relationship, that is, the mapping between the measured absorption peak center current I0 and the corresponding standard absolute frequency in the database. n gas Binding is equivalent to attaching a precise "absolute scale point" to the laser driving parameter current I. This reference point after binding is the absolute reference for the entire scanning wavelength axis, that is, obtaining the absolute frequency anchor point. Subsequently, based on this anchor point, the "current axis" of the entire scanning process is completely converted into the "absolute frequency axis", correcting the problems of laser wavelength drift and poor scanning repeatability.

[0035] As a preferred method, at a set interval, such as 10 minutes, the first optical region is automatically scanned once according to a first current step size ΔI1 to reacquire the current value I0 corresponding to the center of the target gas absorption spectral line and update the absolute frequency. n 0, to eliminate the effects of long-term laser drift.

[0036] S3. Using a tunable laser, starting from I0 and scanning the second optical region according to the second current step size, obtain several equally spaced transmission peak electrical signals and corresponding laser current value sequences {I1, I2, ..., I...}. M}, where M is the number of tracking scans in the second optical zone.

[0037] Specifically, starting with I0, the controller controls the laser to step in the long-wavelength direction with a second current step size ΔI2 = 0.6 mA. Here, ΔI2 corresponds to a frequency change of approximately 600 MHz, equal to the free spectral range (FSR) of the second optical region, ensuring that the laser frequency falls within the resonance range of the next longitudinal mode after each step, avoiding cross-mode missed detections. After each step, the ADC and MCU monitor the second electrical signal output by detector two. When a peak in transmitted light intensity is detected, indicating that the laser frequency resonates with a certain longitudinal mode of the optical resonant cavity, it is determined to be a cavity mode matching point. The laser current value I1 corresponding to this cavity mode matching point is recorded. The step-detection process is repeated, starting with I1, to sequentially obtain the current value sequence {I1, I2,..., I...}. M The specific value of M is adjusted according to actual needs. Preferably, the zero-crossing detection method of the second derivative is used to accurately locate the peak center position of the transmission peak, suppressing noise interference while achieving peak position marking with sub-sampling accuracy.

[0038] Those skilled in the art will understand that this step essentially utilizes an optical resonant cavity to provide a frequency scale, the principle of which is: the longitudinal mode frequency of the optical resonant cavity... n m = mc / 2 nl ,in m For the longitudinal modulus ordinal (different) m(Values ​​correspond to different longitudinal mode frequencies), c is the speed of light in a vacuum, n It is the refractive index of the resonant cavity medium. l It is the physical length of the optical resonator (the physical distance between the two mirrors). The interval between adjacent longitudinal mode frequencies is the free spectral range (FSR). c / 2 nl During the tracking and scanning process of the optical resonant cavity, a periodic frequency reference signal with a defined frequency interval—multiple cavity transmission peaks—is obtained from the detector output. The laser driving currents corresponding to these peaks are I1, I2, I3…, and the corresponding specific longitudinal mode frequencies are… n 1, n 2, n 3... By mapping the frequency characteristics to quantifiable and recordable current values ​​through this correspondence, a high-resolution frequency scale can be formed to accurately determine the frequency of the laser.

[0039] S4. Based on the laser current value sequence {I1, I2, ..., I...} M}, absolute frequency n 0. The free spectral range of the second optical region at the preset temperature. FSR This allows the establishment of a mapping model ν = laser current I with frequency ν. f (I,FSR, n 0), of which f (·) is the fitting function; based on the mapping model, the absolute frequency axis of the target gas in the gas cavity of the first optical region is reconstructed to achieve laser frequency calibration.

[0040] As a specific method, taking (I0, ν0) determined in step S2 as the zero point, and using... FSR As an equidistant reference, by using {I1, I2, ..., I... M The data, including the current value and the corresponding laser frequency, are processed. Through methods such as polynomial fitting, spline fitting, and machine learning models, a functional expression that can approximately describe the relationship between the laser frequency ν and the laser driving current I is found. f (·). The controller of the tunable laser can control the laser's operating parameters according to the calibrated laser frequency, so that the laser frequency is kept near the absorption transition of the target gas, and the laser scanning frequency can be calibrated in real time.

[0041] As a preferred method, during the tracking scan in step S3, if the deviation between the actual current difference between two consecutive transmission peak electrical signals and the second current step size ΔI2 exceeds a set standard, for example, 5%, the second current step size for the next step is automatically adjusted to track the drift of the free spectral range of the second optical region. The tracking of the drift of the free spectral range of the second optical region preferably includes: Real-time monitoring of the temperatures T1 and T2 in the air chambers of the first and second optical zones; When the detected change in temperature ΔT between T2 and the temperature T0 in the previous calibration step is greater than a set threshold (e.g., ΔT > 0.5℃), online correction is performed. Based on the thermal expansion coefficient α of the material in the second optical region, the physical length and free spectral range of the second optical region are corrected in real time. L' = L·[1 + α·(T2- T0)] FSR' = c / (2nL') In the formula, L and L' are the physical lengths before and after correction (the distance between the mirrors on both sides of the optical resonant cavity), respectively; FSR' is the free spectral range after correction; c is the speed of light in vacuum; n is the refractive index of the medium in the gas cavity of the second optical region, and n=1 under vacuum conditions; The updated second current step ΔI2' = FSR' / is obtained based on the corrected FSR'. k , k Based on the obtained current value sequence {I1, I2, ..., I... M The calibrated current tuning factor.

[0042] Specifically, the obtained current value sequence {I1, I2, ..., I...} is used. M} Calibration k : k= (M) - 1) FSR / (I M - I1) In the formula, the FSR before correction is c / (2nL).

[0043] Based on the reconstructed absolute frequency axis of methane, the target gas in the first optical region cavity, applicable to this embodiment, polynomial baseline fitting and Voigt line shape fitting were performed on the direct absorption spectrum obtained by scanning the first optical region under vacuum with a first current step size ΔI1 = 0.1 mA. The results were compared with experimental measurements under the same conditions. (See attached image for details.) Figure 3 .

[0044] Figure 3 The upper middle image shows methane molecules in v 0 = 6046.95 cm -1 The nearby single absorption line, with the horizontal axis representing relative frequency (units converted to MHz), has had its absolute frequency subtracted. v 0, with a scanning range of approximately 3 GHz; the green data points are experimental measurements, and the blue solid line is the Voigt linear fitting curve, with the two highly overlapping. Figure 3The lower half of the figure shows the fitted residuals, which have been magnified 100 times for easier observation. It can be seen that even after magnification, the residuals still exhibit a random noise-like distribution, with amplitude controlled within ±1 (corresponding to the original residuals being less than ±10). -2 No systemic structures such as tilting, bending, or side lobes were observed.

[0045] The above results demonstrate that the absolute frequency axis reconstructed by the method in this embodiment successfully eliminates the effects of current-wavelength tuning nonlinearity, temperature drift, and long-term frequency drift in the tunable laser, resulting in accurate and reliable frequency calibration. Only when the frequency axis linearity and accuracy are sufficiently high can the physical line shape (Voigt line shape) be reproduced so precisely, and only then will the residuals exhibit purely random measurement noise. Therefore, the online calibration method of this invention achieves effective real-time calibration of the laser scanning frequency, laying a reliable foundation for subsequent high-precision quantitative gas concentration detection.

[0046] Example 3 This embodiment provides a concentration detection method for the dual-optical-zone coupled gas concentration detection system of Embodiment 1, including: S10. Reconstruct the absolute frequency axis suitable for the target gas according to the method of Example 1; S20. Maintain the second optical region in a vacuum state, and fit the absorption spectrum of the target gas in the first optical region based on the reconstructed absolute frequency axis to obtain the concentration detection value of the target gas.

[0047] Example 4 This embodiment provides a concentration detection method for the dual-optical-zone coupled gas concentration detection system of Embodiment 1, including: S100. Maintain the pressure of the target gas in the first optical zone air cavity P1=100 Torr; at the same time, fill the second optical zone air cavity with the target gas, maintain the pressure P2=50 Torr, and start the measurement after the air pressure and temperature stabilize. S200. Following the method of Example 1, the absolute frequency anchor point (I0, ν0) is obtained through the absorption spectrum of the target gas in the first optical region, and the frequency coordinate scale is determined by tracking and scanning the second optical region to establish an absolute frequency axis suitable for the target gas in the gas cavity of the second optical region. S300. Based on the reconstructed absolute frequency axis, the first electrical signal output by detector one is analyzed by absorption measurement unit spectral analysis to obtain the first concentration detection value; the electrical signal output by detector two is analyzed by cavity ring-down spectroscopy (CRDS) or cavity enhanced absorption spectroscopy (CEAS). Due to the extremely high reflectivity of the optical resonant cavity (R>99.99%), the effective optical path can reach tens of kilometers, thus obtaining a second concentration detection value at the ppb or even ppt level; the two concentration values ​​are cross-checked or selected as the final detection value according to the concentration range. Specifically, when the target gas concentration is below 1 ppm, the cavity ring-down detection value of the second optical region, i.e., the second concentration detection value, is preferentially selected as the final result; when the concentration is above 100 ppm, the direct absorption detection value of the first optical region, i.e., the first concentration detection value, is preferentially selected as the final result.

[0048] Among them, Examples 3 and 4 are applicable to conventional low-precision rapid detection scenarios and high-precision dual-mode verification detection scenarios, respectively.

[0049] It will be understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A laser scanning frequency online calibration method based on dual-frequency reference fusion, which is based on the coupling of dual optical regions, wherein the dual optical regions include: The first optical region employs a long-path absorption cell or integrating cavity; The second optical region employs a ring-down optical resonator or an optical feedback enhancement resonator, and is a non-spot region nested inside the air cavity of the first optical region. The second optical region is characterized by having a sealed air cavity independent of the first optical region. The method includes: The target gas is introduced into the first optical zone gas cavity, the second optical zone gas cavity is kept in a vacuum state, and the two optical zone gas cavities are maintained at a preset temperature. Using a tunable laser, the first optical region is scanned according to a first current step to obtain the direct absorption spectrum of the target gas molecules. The laser current value I0 corresponding to the center of the absorption line is extracted, and then the absolute laser frequency corresponding to the center of the absorption spectral line is obtained. ν 0, to achieve absolute frequency calibration; Using a tunable laser, the second optical region is tracked and scanned starting from I0 and according to a second current step size to obtain several equally spaced transmission peak electrical signals and corresponding laser current value sequences {I1, I2, ..., I...}. M }, where M is the number of tracking scans in the second optical region; Based on the laser current value sequence and absolute frequency ν 0. The free spectral range of the second optical region at the preset temperature. FSR Establish a mapping model ν= of laser current I and frequency ν. f (I,FSR, ν 0), of which f (·) represents the fitting function; Based on the mapping model, the absolute frequency axis of the target gas in the gas cavity of the first optical region is reconstructed. During the tracking scan, if the deviation between the actual current difference between two consecutive transmission peak electrical signals and the second current step exceeds the set standard, the second current step will be automatically adjusted in the next step to track the drift of the free spectral range of the second optical region. The tracking of the drift in the free spectral range of the second optical region includes: Real-time monitoring of the temperatures T1 and T2 in the air chambers of the first and second optical zones; When the change in temperature T2 relative to the temperature T0 at the previous calibration step is greater than the set threshold, online correction is performed: Based on the thermal expansion coefficient α of the material in the second optical region, the physical length and free spectral range of the second optical region are corrected in real time. L' = L·[1 + α·(T2 - T0)] FSR' = c / (2nL') In the formula, L and L' are the physical lengths before and after correction, respectively; FSR' is the free spectral range after correction; c is the speed of light in vacuum; and n is the refractive index of the medium in the gas cavity of the second optical region. The updated second current step ΔI2' = FSR' / is obtained based on the corrected FSR'. k , k Based on the current value sequence {I1, I2, ..., I M The calibrated current tuning factor.

2. The calibration method according to claim 1, characterized in that, The frequency step corresponding to the first current step is smaller than the full width at half maximum (FWHM) of the absorption spectrum.

3. The calibration method according to claim 1, characterized in that, The frequency step corresponding to the second current step is equal to the free spectral range of the second optical region.

4. The calibration method according to claim 1, characterized in that, The peak center of the transmission peak is located by using the zero-crossing detection of the second derivative, marked as the cavity mode matching point, and the corresponding laser current value is recorded.

5. The calibration method according to claim 1, characterized in that, At set intervals, the first optical region is automatically scanned once according to the first current step size to reacquire the current value corresponding to the center of the target gas absorption spectral line and update the absolute frequency to eliminate the influence of the laser's long-term drift.

6. A dual-optical-zone coupled gas concentration detection system for implementing the calibration method according to any one of claims 1-5, characterized in that, Includes the dual optical regions, tunable laser, beam splitter, detector one, and detector two; The air cavity of the second optical region is located at the geometric center of the air cavity of the first optical region; The gas source of the target gas is connected to the air inlet of the two optical zone air chambers respectively through an air inlet three-way valve; The air outlets of the two optical zone air chambers are connected to the vacuum pump via three-way air outlet valves; The output end of the tunable laser is connected to the input end of the beam splitter, and the two output ends of the beam splitter emit the split laser signals into the first optical region and the second optical region, respectively. The detector is used to receive the laser signal transmitted through the first optical zone; The second detector is used to receive laser signals transmitted from the second optical zone.

7. A concentration detection method using the dual-optical-zone coupled gas concentration detection system of claim 6, characterized in that, include: By maintaining the second optical region in a vacuum state, and fitting the absorption spectrum of the target gas in the first optical region based on the reconstructed absolute frequency axis, the concentration detection value of the target gas is obtained.

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