Dual optical comb absorption spectrum high-precision fast imaging method

Abstract

This invention provides a high-precision and rapid imaging method using dual-comb absorption spectroscopy, mainly comprising: First, describing the gas parameter distribution in the measured area using multiple temperature-concentration pairs, obtaining the dual-comb absorption spectrum projections passing through multiple optical paths in this area; utilizing the high energy concentration characteristic of dual-comb absorption spectra in the time domain, performing an inverse Fourier transform on the spectrum, retaining the energy-concentrated segments, and obtaining the dual-comb time-domain absorption signal projection; Second, substituting the dual-comb time-domain absorption signal projection into a multi-optical-path temperature-concentration histogram time-domain inversion model to reduce the model dimensionality, and solving the model to obtain the temperature-concentration histogram matrix; Finally, substituting the histogram matrix into a temperature-concentration discrete reconstruction model to achieve high-precision imaging of the spatial distribution of temperature and concentration. This invention fully utilizes the rich data of dual-comb absorption spectra to improve reconstruction accuracy, while combining its energy concentration characteristic in the time domain to improve reconstruction speed, thus achieving high-precision and rapid imaging of temperature and concentration.

Description

Technical Field

[0001] This invention relates to a high-precision and rapid imaging method using dual-comb absorption spectroscopy, which can achieve high-precision and rapid reconstruction of combustion field gas temperature and concentration images based on multi-channel dual-comb absorption spectroscopy, and belongs to the field of laser absorption spectroscopy tomography. Background Technology

[0002] Combustion is a core element of energy conversion and utilization, widely existing in aerospace, energy and power, industrial production, and social life. Monitoring the distribution of key combustion parameters, such as temperature and component concentration, provides a basis for optimizing the combustion process, improving combustion efficiency, and reducing combustion pollutant emissions, which is of great significance. Laser absorption spectroscopy (LAS) technology is widely used in combustion monitoring due to its advantages such as fast response speed, high measurement accuracy, simple system structure, and non-invasive measurement that does not affect the flow field. Based on the Beer-Lambert law, LAS calculates the transmittance of the laser after passing through the measured medium based on the incident light intensity and the transmitted light intensity after absorption, thus obtaining a wavelength-related absorption spectrum used to accurately calculate parameters such as temperature, component concentration, and pressure along the absorption path. Among these, tunable diode laser absorption spectroscopy (TDLAS) technology is widely used in combustion diagnostics due to its high reliability, low cost, compact structure, and ease of integration.

[0003] TDLAS technology typically uses distributed feedback (DFB) semiconductor lasers as the laser source. These sources are characterized by narrow laser linewidth, fast tuning speed, and ease of integration. The emitted laser, after collimation, scans the absorption spectrum of the target molecule, enabling high-precision measurement of combustion parameters. TDLAS technology features no pretreatment required, fast response speed, high sensitivity, and simultaneous measurement of multiple parameters. Furthermore, the measurement system does not require direct contact with the analyte gas, making it suitable for measurements in harsh environments such as high temperatures and high corrosiveness. It is currently widely used in the field of combustion diagnostics. By scanning the characteristic spectral lines of two analyte molecules, parameters such as temperature and component concentration can be measured using a two-color thermometry method. In 2016, Zhang et al. published an article entitled "Detection of gas temperature using a distributed feedback laser at O2 absorption wavelength 760 nm" in the Journal of Optical Technology, Volume 83, Issue 11, pp. 673-677. In the article, they measured the temperature of oxygen in a tube furnace based on the TDLAS two-color method, with a measurement range of 300 K to 900 K. The results were compared with those of thermocouple measurements. The maximum temperature measurement error occurred at 900 K, with a maximum error value of 51 K. In 2021, Fang et al. published article number 110411, "Characterizing combustion of a hybrid rocket using laser absorption spectroscopy," in Volume 127 of *Experimental Thermal and Fluid Science*. Using TDLAS, they measured the changes in temperature and water vapor partial pressure at the exhaust nozzle of a hybrid rocket engine throughout the ignition, combustion, and shutdown processes, and evaluated combustion efficiency using computational fluid dynamics simulations. While this research achieved path-averaged results of combustion parameters using TDLAS technology, the information reflected by the average value of parameters along a single path is limited due to the typically non-uniform distribution of combustion parameters, making it difficult to measure the internal structure of the flame.

[0004] By combining TDLAS with a series of tomographic imaging algorithms, the temperature distribution and component concentration distribution of the measured field can be measured. Common tomographic imaging algorithms include analytical reconstruction algorithms represented by Abel transform and Radon transform, and iterative reconstruction algorithms that mainly rely on solving linear equations. Analytical reconstruction algorithms often fail to achieve satisfactory results when the projection data is incomplete or has high noise levels. Therefore, iterative reconstruction algorithms, represented by Algebraic Reconstruction Technique (ART), Simultaneous Algebraic Reconstruction Technique (SART), and the Landweber algorithm, are currently more widely used. These algorithms reconstruct the distribution by establishing and iteratively solving a system of linear equations between the parameters and the projection on each discrete spatial grid. In 2014, Li Fei et al. published an article entitled "TDLAT Tomographic Imaging Technology Based on TDLAS" in the *Acta Mechanica Sinica*, Volume 46, Issue 1, pp. 54-59. In it, they designed a sensor based on parallel beam rotation scanning to measure water vapor at a distance of 7185.6 cm along multiple optical paths. -1 and 7444.3 cm -1Dual-spectral measurements were performed, and preliminary measurements of temperature and water vapor concentration distribution in a CH4 / Air planar flame furnace were achieved using ART. In 2020, Zhao et al. published an article entitled "A WMS Based TDLAS Tomographic System for Distribution Retrievals of Both Gas Concentration and Temperature in Dynamic Flames" in IEEE Sensors Journal, Volume 20, Issue 8, pp. 4179-4188. They used a five-angle fan-beam TDLAS tomographic sensor to acquire absorption spectra on 120 optical paths and combined it with the SART algorithm to measure the temperature and water vapor concentration at the outlet of the planar flame combustion furnace and wind tunnel. In 2015, Liu et al. published an article titled "Development of a fan-beam TDLAS-based tomographic sensor for rapid imaging of temperature and gas concentration" in Optics Express, Volume 23, Issue 17, pp. 22494-22511. In this article, they used a five-angle fan-beam sensor to acquire water vapor at a depth of 7185.6 cm⁻¹ across multiple optical paths. −1 and 7444.36 cm −1The absorption spectral results of the double-line system are used, combined with a modified Landweber algorithm, to reconstruct the flame cross-section temperature and water vapor concentration distribution of a flat-flame combustion furnace. Due to the limited number of measurement projections, the number of variables to be solved during reconstruction is much greater than the number of equations, resulting in strong underdeterminism in the reconstruction problem. To further improve the reconstruction accuracy and alleviate the underdeterminism, prior knowledge of the combustion field parameter distribution is introduced to reduce the number of variables to be solved. In 2023, Gao et al. published a paper titled "Radial Basis Function Coupled SART Method for Dynamic LAS Tomography" in Volume 72, IEEE Transactions on Instrumentation and Measurement (No. 4500110). This method utilizes a five-angle fan-beam TDLAS imaging sensor to acquire absorption spectra along multiple optical paths passing through the measured region. During reconstruction, a linear combination of tightly supported radial basis functions is used to describe the parameter distribution of the measured region, transforming the solution into the coefficients of the basis functions. These coefficients are then solved iteratively using SART to reconstruct the parameter distribution. This method effectively utilizes the prior information of the continuous and smooth integral absorption area distribution of the combustion field, improving the quality and spatial resolution of the reconstructed image. However, this method only utilizes the integral absorption area of ​​the spectral lines during measurement, failing to fully utilize the spectral shape information. Furthermore, the aforementioned method, after reconstructing the integral absorption area distribution, uses colorimetry to calculate the temperature. However, the ratio calculations in colorimetry amplify noise, making the temperature calculation results more susceptible to spectral noise. To address this issue, based on prior knowledge of the correlation between temperature and concentration, the distribution of the measured gas parameters is discretized into multiple temperature-concentration pairs. A linear model of the projected absorptivity and the absorptivity per unit length temperature-concentration pair is constructed at all wavenumber points of the absorption spectrum. Solving this model yields the histogram results of the multi-path temperature-concentration pair, thus reconstructing the temperature and component concentration.In 2023, Qiu et al. published a paper titled "A Binary-Valued Reconstruction Algorithm for Discrete TDLAS Tomography of Dynamic Flames" in IEEE Transactions on Instrumentation and Measurement, Volume 72, No. 9506614. This paper utilizes a five-angle fan-beam TDLAS sensor to obtain absorption spectra across multiple optical paths and combines this with a temperature-concentration discrete imaging method based on histogram information to measure the distribution of combustion field parameters. Compared to traditional methods, this approach offers higher accuracy and noise immunity. However, due to the limited tuning range of DFB lasers, the number of spectral lines that TDLAS technology can simultaneously detect is extremely limited. To increase the number of spectral lines measured to improve measurement accuracy or to achieve multi-component, wide-temperature-range measurements, multiple sets of DFB lasers with different center wavenumbers are required, inevitably increasing the system complexity.

[0005] In contrast, the output spectrum of a frequency-comb laser can cover widths from several nanometers to hundreds of nanometers, encompassing multiple characteristic spectral lines across different wavelengths and even different molecules. This provides abundant absorption information for combustion parameter measurements, improving measurement accuracy and even enabling simultaneous measurement of multiple components. To reduce the difficulty of acquisition and detection, a dual-comb system is formed by asynchronous sampling or multi-heterodyne interferometry using two frequency-combs with slightly different repetition frequencies, converting absorption information from the optical frequency domain to the radio frequency domain. Absorption spectral measurements based on a dual-comb system can further improve the measurement accuracy of combustion parameters by simultaneously acquiring a large number of characteristic spectral lines, even achieving simultaneous measurement of multiple components. Furthermore, it can distinguish absorption characteristics and perform parameter inversion even when spectral lines aliasing occurs under high pressure, making it suitable for measurements in complex and variable environments. It has significant advantages and enormous application potential in the field of combustion diagnostics.

[0006] Domestic and international researchers have conducted extensive studies on the measurement of absorption spectra, combustion gas temperature, composition, and other parameters based on dual-comb lasers, promoting the application of dual-comb absorption spectroscopy technology in the field of combustion diagnostics. In 2022, Xu et al. published an article entitled "Dual-comb Spectroscopy for Laminar Premixed Flames with a Free-running Fiber Laser" in Volume 194, Issue 12, pp. 2523-2538 of *Combustion Science and Technology*. In this article, they measured the path absorption spectrum of a flat-flame combustion furnace flame using a wavelength-multiplexed single-cavity dual-comb system and compared it with simulated absorption spectra obtained based on CFD and HITEMP. The overall deviations of the peak absorptivity of the absorption spectra under three different operating conditions were 4.6%, 5.0%, and 4.9%, respectively, demonstrating the capability of dual-comb in absorption spectroscopy measurement. In 2020, Yang Kangwen et al. published an article titled "Temperature measurement based on adaptive dual-comb absorption spectral detection" in the journal *Chinese Optics Letters*, Volume 18, Issue 5 (Article No. 051401). The article described how they constructed an adaptive sampling dual-comb system to measure the temperature of water vapor at 7185.6 cm⁻¹ in a tubular furnace. -1 and 7168.4 cm -1Two absorption spectra were measured, and the temperature was calculated using a two-color thermometry method. Within the measurement range of 500 K to 1000 K, the maximum relative deviation between the temperature measurement results and the thermocouple readings was 4.93%. In 2021, Schroeder et al. published an article entitled "Temperature and concentration measurements in a high-pressure gasifier enabled by cepstral analysis of dual frequency comb spectroscopy" in Volume 38, Issue 1, pp. 1561-1569 of the *Proceedings of the Combustion Institute*. Based on a phase-locked dual-comb system, they measured the temperature and concentration of water vapor in a fluidized bed gasifier with temperatures up to 1700 K and pressures up to 15 bar. Cepstral analysis of the laser transmission spectrum enabled the extraction of overlapping spectral features and the calculation of gas parameters under high pressure, achieving accurate parameter measurement under complex and harsh environments such as high temperature and high pressure. In 2024, Pang et al. published an article titled "Precise Gas Temperature Measurement Using a Single Dual-Wavelength Mode-Locked Fiber Laser" in Volume 73, IEEE Transactions on Instrumentation and Measurement (Issue 7005211). In this article, they constructed a wavelength-multiplexed single-cavity dual-comb laser for accurate gas temperature measurement of acetylene at 6492.52 cm⁻¹. -1 up to 6496.4 cm -1 The absorption spectrum within the range was measured, and a baseline immunoassay multispectral fitting parameter extraction method was used. The measured time-domain free induction decay signal was optimized and fitted with the simulation results from the HITRAN database to achieve temperature measurement. Within the measurement range from room temperature to 452 K, the maximum temperature measurement error did not exceed 3.2%. The above studies also focused on the measurement of the average value of single optical path parameters, which cannot provide sufficient information for the analysis of flame structure.

[0007] In recent years, the measurement of the spatial distribution of combustion parameters based on dual-comb absorption spectroscopy has gradually developed. Utilizing the broad spectral characteristics of dual-comb spectra is expected to further improve the accuracy of combustion parameter distribution measurement. Chinese Patent CN111077110 B, "A Temperature Field and Concentration Field Measurement System and Method Based on Dual-Comb Spectroscopy" (Patent No. 202010045166.0), proposes a combustion field parameter measurement system and method based on dual-comb absorption spectroscopy. This method utilizes a time-division optical switch to achieve multiple angle optical path outputs. The laser light from each angle is spread into a fan-shaped beam and received by a photodetector array, thereby acquiring the laser absorption spectrum on multiple optical paths. The integrated absorbance projections on different spectral lines are calculated, and an iterative reconstruction algorithm is used to obtain the two-dimensional distribution of the integrated absorbance. Finally, a multicolor method is used to obtain the temperature and concentration distributions. In 2023, Yun et al. published an article entitled "Supersonic combustion diagnostics with dual comb spectroscopy" in the Proceedings of the Combustion Institute, Volume 39, Issue 1, pp. 1299-1306. In this article, they measured the combustion field of the isolation section of a dual-mode ramjet engine using a dual comb system, and the measured water vapor absorption spectrum covered 6800 cm⁻¹. -1 ~7200 cm -1 Hundreds of characteristic spectral lines within the range were vertically scanned at different axial positions in the combustion chamber using a mechanical structure to drive the optical emission and detection module. This enabled two-dimensional profile measurements of temperature, water vapor concentration, gas flow velocity, and mass flow rate at different heights and axial positions. The results were consistent with CFD simulation results. In 2023, Zou et al. published an article entitled "Mid-infrared dual-comb spectroscopy for rapid temperature distribution characterization" in Optics Letters, Volume 48, Issue 23, pp. 6336-6339. They constructed a mid-infrared dual-comb system to measure the absorption spectrum and one-dimensional temperature distribution of a high-temperature CO2 gas chamber. Given the composition and pressure of the gas chamber, they discretized the one-dimensional temperature distribution within the chamber into multiple temperature points, solved the path length (i.e., the one-dimensional histogram) occupied by different temperature points, and finally established the temperature distribution along the one-dimensional path. The reconstructed results were highly consistent with those of thermocouples.

[0008] Currently, there are few reports on the spatial distribution measurement of combustion field parameters based on dual-comb absorption spectroscopy, and these methods face problems such as low spectral utilization, large data volume of absorption spectra, and high computational costs. While temperature and concentration discrete imaging methods based on histogram information can fully utilize the shape information of the absorption spectrum, they face problems such as a rapid increase in spectral data volume and excessively long computation time during the extension from TDLAS to dual-comb absorption spectroscopy. Against this background, this invention proposes a high-precision and rapid imaging method using dual-comb absorption spectroscopy. The combustion field parameters are discretized into multiple temperature-concentration pairs. Utilizing the concentrated energy distribution of dual-comb absorption spectra in the time domain, the measured absorption spectrum projection and the simulated absorption spectrum per unit length are inversely Fourier transformed to the time domain. The energy-concentrated segments are then extracted for calculation. A linear model is established between the time-domain signal projections of the absorption spectra along multiple optical paths and the time-domain signal projections of the simulated absorption spectra per unit length. The multi-path temperature-concentration histogram matrix is ​​obtained by solving this model, and a discrete reconstruction model of temperature and concentration is established to obtain the two-dimensional distribution of temperature and concentration. This method effectively utilizes the advantages of the wide absorption spectrum range of the dual-comb system and the ability to obtain a large number of characteristic spectral lines simultaneously, thereby reducing the underdeterminacy of the reconstruction model and improving the reconstruction accuracy. At the same time, it takes advantage of the concentrated energy distribution of the dual-comb absorption spectrum in the time domain to reduce the matrix size of the reconstruction model, thereby reducing the reconstruction time and computational cost, and realizing the measurement of combustion field parameters quickly, accurately, and with strong anti-interference ability. Summary of the Invention

[0009] (a) Technical problems to be solved

[0010] This invention proposes a high-precision, rapid imaging method using dual-comb absorption spectroscopy. The method transforms the dual-comb absorption spectrum to the time domain and extracts regions with concentrated energy for computation. While fully utilizing the absorption spectral information, it significantly reduces the amount of data involved in the calculation. A time-domain inversion model of the temperature-concentration histogram and a discrete reconstruction model of temperature-concentration are established and solved to reconstruct the temperature and component concentration distributions. This method leverages the wide range and high spectral resolution of dual-comb absorption spectra to achieve high-precision reconstruction of temperature and component concentration distributions. Simultaneously, it utilizes the concentrated energy distribution of dual-comb absorption spectra in the time domain to effectively reduce reconstruction time.

[0011] (II) Technical Solution

[0012] This invention relates to a high-precision, rapid imaging method using dual-comb absorption spectroscopy, which mainly includes the following steps:

[0013] Step 1: Obtaining the projection of the time-domain absorption signal from the dual-comb system: The optical field of the two optical frequency combs in the dual-comb system can be expressed as:

[0014]

[0015] in, , Two optical frequency combs respectively , The amplitude of the root comb teeth, , Two optical frequency combs respectively , The initial phase of the root comb teeth, , Two optical frequency combs respectively , The frequencies of the root comb teeth can be expressed as follows:

[0016]

[0017] in, , These are the repetition frequency and carrier envelope offset frequency of the first optical frequency comb, respectively. , These are the repetition frequency and carrier envelope offset frequency of the second optical frequency comb, respectively.

[0018] The intensity of the interference signal output by the dual optical comb system is the coherent superposition of the optical fields of the two optical frequency combs, expressed as:

[0019]

[0020] The dual-comb interference signal has a bandwidth of less than Optical bandpass filter and bandwidth less than The electrical low-pass filter filters, where, The center wavelength, At the speed of light, The difference in repetition frequency between the two optical frequency combs is the longitudinal mode beat frequency of the two optical frequency combs. The optical frequency comb teeth are mapped to the radio frequency domain; the filtered interference signal is detected by a photodetector, and the resulting voltage signal can be expressed as:

[0021]

[0022] in, The first optical comb after interference is mapped to the radio frequency domain. The amplitude of each comb tooth , ;

[0023] After absorption by characteristic gas molecules, the voltage of the transmitted light intensity obtained by the photodetector can be expressed as:

[0024]

[0025] in, For gas molecules in Absorption rate at the location;

[0026] According to the Beer-Lambert law, the extracted absorption spectrum can be expressed as:

[0027]

[0028] in, , These represent the incident and transmitted light intensities of the laser, respectively. , and Positions Pressure, temperature, and molecular concentration at that location For spectral lines at temperature Spectral line intensity at that location The line shape function of the spectral lines. This is the laser path length. …, The frequency points covered by the extracted absorption spectrum;

[0029] Performing an inverse Fourier transform on the above absorption spectrum yields its time-domain absorption spectrum projection:

[0030]

[0031] in, The absorption spectra obtained by the dual-comb system are characterized by a wide spectral range and high spectral resolution, typically covering multiple characteristic spectral lines, and the number of frequency points covered in its spectral range is [not specified]. The energy distribution of the absorption spectrum is large, but concentrated in the time domain; the concentrated energy region can be extracted. At this point, most of the information in the absorption spectrum is preserved, while the amount of data is greatly reduced, and the time required for data processing is reduced.

[0032] If the total amount obtained If an optical path passes through the region under test, its time-domain absorption signal projection matrix is:

[0033]

[0034] in, For the first After performing an inverse Fourier transform on the absorption spectrum along the optical path, the values ​​at time points... The results of the treatment, among which The length of the retained absorption spectrum time-domain signal is given, and it has... ;

[0035] Based on prior knowledge of the gas parameter distribution in the measured area, the parameter distribution is represented by... Temperature concentration Description, and discretization of the measured region into Calculate the grid number. The laser beam passes through the first Length of each grid Forming a sensitivity matrix .

[0036] Step 2: Rapid extraction of multi-path temperature-concentration histogram: Calculate the frequency corresponding to the measured absorption spectrum projection signal, i.e. …, The laser in the first Temperature concentration The absorption spectrum after passing through the unit absorption length Perform an inverse Fourier transform on it and truncate it. Time-domain signal within , Perform inverse Fourier transform on the unit length absorption spectra for all temperature and concentration pairs and truncate them. The segment forms a time-domain absorption signal basis matrix. ;

[0037] Because the absorption rate has a path accumulation characteristic, the first The absorption spectrum projection along the optical path can be expressed as:

[0038]

[0039] in, For the first The first on the road of light The temperature concentration corresponds to the path length, and the equation is... Perform inverse Fourier transform and truncate. The results are as follows:

[0040]

[0041] in, Rewriting the above equation in matrix form, we have:

[0042]

[0043] in, It is a histogram matrix that represents the absorption length of each temperature concentration pair across all optical paths;

[0044] Based on the length constraints of multiple measurement optical paths, we have:

[0045]

[0046] in, and For a column vector where all elements are 1, the associative formula and have:

[0047]

[0048] Mode This is a time-domain inversion model for multi-path temperature-concentration histograms. The matrix size in this model is much smaller than that used when directly calculating in the frequency domain using absorption spectra, thus reducing the dimensionality of the histogram reconstruction model. Solving this model yields the multi-path histogram matrix. .

[0049] Step 3: High-precision reconstruction of temperature and concentration distribution: The gas parameters in each grid of the measured area are determined by... Represented by one of the temperature-concentration pairs, using a 0-1 matrix. This represents the spatial distribution of temperature-concentration pairs, where each row represents a grid and each column represents a temperature-concentration pair. If the element... 1 represents the first The gas parameters on the i-th grid are the i-th For each temperature-concentration pair, if the value is 0, it means that the parameter in this grid is another temperature-concentration pair. Therefore:

[0050]

[0051] Mode For the temperature-concentration discrete reconstruction model, solving this model yields... This allows us to obtain the temperature distribution within the measured area. and component concentration distribution .

[0052] (III) Beneficial Effects

[0053] The beneficial effect of this invention lies in proposing a high-precision and rapid imaging method for dual-comb absorption spectroscopy. Utilizing the characteristic of concentrated energy distribution in the time domain of dual-comb absorption spectra, an inverse Fourier transform is performed on the acquired multi-channel dual-comb absorption spectra, and the energy-concentrated portions are extracted to form a time-domain absorption signal projection matrix. Based on the estimation of the temperature and component concentration range of the measured area, multiple temperature-concentration pairs are used to characterize the gas parameter distribution of the measured area. The time-domain absorption signal basis matrix is ​​calculated, and a temperature-concentration histogram time-domain inversion model is established. The histogram results of each temperature-concentration pair on multiple optical paths are obtained, and a temperature-concentration discrete reconstruction model is established to obtain the spatial distribution of the temperature-concentration pairs. This method fully utilizes the energy concentration characteristic of dual-comb absorption spectra in the time domain. While fully leveraging the characteristics of dual-comb absorption spectra, it reduces the matrix size of the reconstruction model. By extracting regions with highly concentrated time-domain signal energy, the amount of data involved in reconstruction is reduced by approximately two orders of magnitude, and the histogram extraction time is reduced to nearly one-hundredth of the original time, achieving high-precision and rapid reconstruction of temperature and component concentration distributions. Attached Figure Description

[0054] Figure 1 : Detailed implementation diagram of this method

[0055] Figure 2 Original temperature and water vapor concentration distribution maps: (a) Temperature distribution; (b) Concentration distribution.

[0056] Figure 3 Reconstructed temperature and water vapor concentration distribution maps: (a) Temperature distribution; (b) Concentration distribution. Detailed Implementation

[0057] Reference Appendix Figure 1 A detailed implementation diagram of this method is attached. Figure 2 For the original temperature and water vapor concentration distribution, attached Figure 3 To reconstruct the temperature and water vapor concentration distribution using this method, the specific steps are as follows, using an example:

[0058] Step 1: Obtaining the projection of the dual-comb time-domain absorption signal: repetition frequency difference The interference signal output from the dual-comb system is converted into spatial light by a collimating mirror after passing through an optical bandpass filter. This light then passes through the measured region and is received by a photodetector. The collimating mirror and photodetector synchronously perform translational and rotational scanning, forming a total of 150 measurement optical paths. The signal obtained by the photodetector is then phase-corrected and coherently averaged after passing through an electrical low-pass filter, ultimately yielding the absorption spectrum of water vapor along the 150 optical paths, covering a wavenumber range of 5205 cm⁻¹. -1 ~5285 cm -1 The wavenumber resolution is 0.005 cm. -1At this point, the absorption spectrum length of the dual optical comb is 16001. Performing an inverse Fourier transform on the absorption spectra of all optical paths to convert them to the time domain, the first 480 points of the time-domain signal account for 99.9% of the total signal energy, containing most of the signal's information, while the remaining part is almost zero. Therefore, the first 480 points of the time-domain signal are truncated to obtain the time-domain absorption signal projection matrix. Before and after the inverse Fourier transform, the size of the projection matrix of the time-domain absorption signal is reduced from 150×16001 to 150×480, but most of the energy of the absorption spectrum is retained.

[0059] Based on prior knowledge of the water vapor temperature and concentration distribution in the measured area, the gas parameter distribution in the measured area is compared with 10 temperature and concentration parameters. , , , , , , , , , Description, forming a gas parameter matrix The measured area is spatially discretized into 40×40 grids, and the sensitivity matrix is ​​calculated. ;

[0060] Step 2: Rapid extraction of multi-path temperature-concentration histogram: Calculate the... Absorption spectrum per unit length at a given temperature concentration The wavenumber range and resolution are consistent with the measured absorption spectrum projection. An inverse Fourier transform is performed on it, and the results within the first 480 points are truncated to form the time-domain absorption signal basis matrix. ;

[0061] Therefore, the time-domain inversion model for the multi-path temperature-concentration histogram is as follows:

[0062]

[0063] in, and Given a column vector with all elements equal to 1, the matrix size in this model is much smaller than the matrix size when directly using absorption spectra for calculations in the frequency domain. The solution formula... Obtain the histogram matrix The extraction time for the histogram matrix is ​​1.2% of that for frequency domain computation.

[0064] Step 3: High-precision reconstruction of temperature and concentration distribution: using a 0-1 matrix This represents the spatial distribution of temperature-concentration pairs, where each row represents a grid and each column represents a temperature-concentration pair. Elements with a value of 1 represent the gas parameters on that grid as expressed by that temperature-concentration pair. A discrete reconstruction model of temperature and concentration is established as follows:

[0065]

[0066] Iterative solution This yields a 0-1 matrix. Temperature distribution within the measured area and component concentration distribution It can be represented as:

[0067]

[0068] The original distribution of temperature and water vapor concentration used is shown in the attached figure. Figure 2 As shown in the attached figure, the reconstructed temperature and water vapor concentration distributions are as follows. Figure 3 As shown, the L2 norm errors of the reconstructed temperature and water vapor concentration images are 6.5% and 9.8%, respectively.

[0069] The above description of the present invention and its embodiments is not limited thereto, and the accompanying drawings are only one embodiment of the present invention. Any structure or embodiment similar to this technical solution designed without departing from the spirit of the present invention shall fall within the protection scope of the present invention.

Claims

1. A high-precision and rapid imaging method using dual-comb absorption spectroscopy, characterized in that: The interference signal output by a dual-comb system with a certain repetition frequency is filtered by an optical bandpass filter and then generated into multiple rays passing through the measured area by beam splitting or scanning. The multiple interference signals after passing through the measured medium are obtained by a photodetector, filtered by an electrical low-pass filter, and uploaded to the signal acquisition and processing module. The dual-comb absorption spectrum projection matrix of the multiple optical paths is obtained. Based on the characteristic of concentrated energy distribution of the dual-comb absorption spectrum signal in the time domain, an inverse Fourier transform is performed on the dual-comb absorption spectrum projection matrix, and the energy-concentrated segment in the time domain signal is extracted to obtain the time-domain absorption signal projection matrix. The length of each optical path passing through each spatial grid is obtained according to the layout of the multiple optical paths, forming a sensitivity matrix. The temperature and gas molecule concentration range of the measured field are estimated in advance. The gas parameters of the measured field are discretized into a finite number of temperature-concentration pairs. The absorbance per unit path of each temperature-concentration pair is calculated, and its wavenumber range is consistent with the resolution and the projection of the measured absorption spectrum. After inverse Fourier transform and truncating the time-domain signal segment with concentrated energy distribution, the time-domain absorption signal basis matrix is ​​obtained. The multi-path temperature-concentration histogram inversion model in the time domain is solved to obtain the absorption length occupied by the temperature-concentration pairs on multiple optical paths, i.e., the temperature-concentration histogram matrix. A discrete reconstruction model of temperature and concentration is established, and the spatial distribution of temperature-concentration pairs is solved to achieve rapid reconstruction of the two-dimensional image of temperature and concentration in the measured area. The specific steps include: Step 1: Obtaining the projection of the dual-comb time-domain absorption signal: [Setup] An optical path is used to pass through the area under test, and the absorption spectra of multiple optical paths are obtained using a dual-comb optical path. Taking advantage of the highly concentrated energy distribution of this absorption spectrum in the time domain, an inverse Fourier transform is performed on it to convert it to the time domain and the energy-concentrated segment is extracted. Time-domain signal within, The projection matrix of the time-domain absorbed signal is determined by simulation. ,in To extract the length of the time-domain signal, Much shorter than the length of the original dual-comb absorption spectrum; Based on prior knowledge of the distribution of the measured gas parameters, the gas parameter distribution of the measured area is used... Temperature concentration The description involves discretizing the measured region into... Calculate the grid number. The laser beam passes through the first Length of each grid Forming a sensitivity matrix ; Step 2: Rapid extraction of multi-path temperature-concentration histogram: The calculated and measured absorption spectrum projection frequencies are consistent, i.e., the frequencies are... …, The laser in the first Temperature concentration The absorption spectrum after passing through the unit absorption length Perform an inverse Fourier transform on it and truncate it. Time-domain signal results , Perform inverse Fourier transform on the unit length absorption spectra for all temperature and concentration pairs and truncate the segments. Forming a time-domain absorption signal basis matrix ; Because the absorption rate has a path accumulation characteristic, the first The absorption spectrum projection along the optical path can be expressed as: in, For the first The first on the road of light The temperature concentration corresponds to the path length, and the equation is... Perform an inverse Fourier transform and extract the time segment. The results are as follows: in, Rewriting the above equation in matrix form, we have: in, It is a histogram matrix that represents the absorption length of each temperature concentration pair across all optical paths; Based on the length constraint of each measurement optical path, we have: in, and For a column vector where all elements are 1, the associative formula and have: Mode This is a time-domain inversion model for multi-path temperature-concentration histograms. The matrix size in this model is much smaller than that calculated directly using absorption spectra in the frequency domain, thus reducing the dimensionality of the histogram reconstruction model. Solving this model yields the multi-path histogram matrix. ; Step 3: High-precision reconstruction of temperature and concentration distribution: The gas parameters in each grid of the measured area are determined by... Represented by one of the temperature-concentration pairs, using a 0-1 matrix. This represents the spatial distribution of temperature-concentration pairs, where each row represents a grid and each column represents a temperature-concentration pair. If the element... 1 represents the first The gas parameters on the i-th grid are the i-th For each temperature-concentration pair, if the value is 0, it means that the parameter in this grid is another temperature-concentration pair. Therefore: Mode For the temperature-concentration discrete reconstruction model, solving this model yields... This allows us to obtain the temperature distribution within the measured area. and component concentration distribution .

2. The high-precision and rapid imaging method for dual-comb absorption spectroscopy according to claim 1, characterized in that... After obtaining the absorption spectra of multiple optical paths passing through the measured area, the high concentration of energy distribution in the time domain is utilized to perform an inverse Fourier transform on them, and the highly concentrated energy segment is extracted to obtain the projection matrix of the multi-path time-domain absorption signal. The optical field of the two optical frequency combs in a dual-comb system can be expressed as: in, , Two optical frequency combs respectively , The amplitude of the root comb teeth, , Two optical frequency combs respectively , The initial phase of the root comb teeth, , Two optical frequency combs respectively , The frequencies of the root comb teeth can be expressed as follows: in, , These are the repetition frequency and carrier envelope offset frequency of the first optical frequency comb, respectively. , These are the repetition frequency and carrier envelope offset frequency of the second optical frequency comb, respectively. The intensity of the interference signal output by the dual optical comb system is the coherent superposition of the optical fields of the two optical frequency combs, expressed as: The dual-comb interference signal has a bandwidth of less than Optical bandpass filter and bandwidth less than Electrical low-pass filter filtering, where The center wavelength, At the speed of light, The difference in repetition frequency between the two optical frequency combs is the longitudinal mode beat frequency of the two optical frequency combs. The optical frequency comb teeth are mapped to the radio frequency domain; the filtered interference signal is detected by a photodetector, and the resulting voltage signal can be expressed as: in, The first optical comb after interference is mapped to the radio frequency domain. The amplitude of each comb tooth , ; After absorption by characteristic gas molecules, the voltage of the transmitted light intensity obtained by the photodetector can be expressed as: in, For gas molecules in Absorption rate at the location; According to the Beer-Lambert law, the extracted absorption spectrum can be expressed as: in, , These represent the incident and transmitted light intensities of the laser, respectively. , and Positions Pressure, temperature, and molecular concentration at that location For spectral lines at temperature Spectral line intensity at that location The line shape function of the spectral lines. This is the laser path length. …, The frequency points covered by the extracted absorption spectrum; Performing an inverse Fourier transform on the above absorption spectrum yields its time-domain absorption spectrum projection: in, The absorption spectra obtained by the dual-comb system are characterized by a wide spectral range and high spectral resolution, typically covering multiple characteristic spectral lines, and the number of frequency points covered within the spectral range is [not specified]. The energy distribution of the absorption spectrum is very large, and the energy distribution in the time domain is highly concentrated. Therefore, a rectangular time-domain window is used. By extracting regions with highly concentrated energy and removing portions with very small energy percentages, most of the information in the absorption spectrum is preserved, while the amount of data is significantly reduced, and the data processing time is greatly shortened. If the total amount obtained If an optical path passes through the region under test, its time-domain absorption signal projection matrix is: in, For the first After performing an inverse Fourier transform on the absorption spectrum along the optical path, the values ​​at time points... The results of the treatment, among which The length of the retained time-domain signal, and has .

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