Gas component parameter calculation method and system based on single optical frequency comb photo-thermal spectrum
By combining a single optical frequency comb with a Mach-Zehnder interferometer, the downconversion factor is calculated, simplifying the calculation process of gas component parameters. This solves the problem of slow relaxation processes of gas molecules in optical frequency comb photothermal spectroscopy gas sensing schemes, enabling simple and rapid calculation of gas component parameters and promoting the miniaturization and industrialization of the system.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- HEFEI INSTITUTE OF PHYSICAL SCIENCE CHINESE ACADEMY OF SCIENCES
- Filing Date
- 2026-02-10
- Publication Date
- 2026-05-05
AI Technical Summary
In existing optical frequency comb photothermal spectroscopy gas sensing schemes, the relaxation process of gas molecules is slow, requiring two optical frequency combs to work together, which increases the system complexity and cost, making it difficult to achieve miniaturization and industrialization.
By employing a single optical frequency comb combined with a Mach-Zehnder interferometer, the calculation of the downconversion factor is directly utilized to take advantage of the moving speed of the boom mirror assembly, simplifying the calculation process of gas component parameters, avoiding multiple heterodyne interference, and reducing system complexity and cost.
It enables simple and rapid calculation of gas component parameters, reduces system complexity and cost, and promotes miniaturization and industrialization.
Smart Images
Figure CN121978018A_ABST
Abstract
Description
Technical Field
[0001] This application belongs to the field of laser spectroscopy gas sensing technology, and specifically relates to a method and system for calculating gas component parameters based on single optical frequency comb photothermal spectroscopy. Background Technology
[0002] With the continuous development of gas sensing technology, photothermal spectroscopy has gradually become an important means of trace gas sensing due to its advantages such as high sensitivity, zero background, and low gas consumption. However, traditional photothermal spectroscopy usually relies on narrowband laser sources, making it difficult to achieve multi-component, wide-band gas measurement, thus limiting its application range. Since the beginning of this century, the rapid advancement of mode-locked laser technology has promoted the widespread application of optical frequency combs. Optical frequency combs have characteristics such as wide spectral coverage, high frequency accuracy, and excellent stability, providing an ideal light source for high-resolution spectral measurement of multi-component gases. Combining optical frequency combs with photothermal spectroscopy technology, while retaining the original advantages of photothermal spectroscopy, has opened up a new path for broadband, multi-component gas detection, becoming one of the emerging high-sensitivity, zero-background gas detection technologies in recent years.
[0003] However, existing optical frequency comb-based photothermal spectroscopy gas sensing schemes still have limitations, stemming from the relatively slow relaxation process of gas molecules. To match the modulation frequency of the photothermal signal with the molecular relaxation process, the excitation of the photothermal signal by the signal light typically requires multiheterodyne interference between the local oscillator optical comb and the signal optical comb to downconvert the optical frequency to a suitable radio frequency range. During this downconversion process, two optical frequency combs with slightly different repetition frequencies need to work together; the downconversion factor is defined as the ratio of the repetition frequency difference to the repetition frequency. However, achieving a stable downconversion factor requires highly correlated and controlled phase noise of the two optical combs, significantly increasing the difficulty of implementation. Furthermore, the involvement of dual optical combs not only increases the complexity of the sensing system but also significantly raises the system construction cost, posing a clear constraint on the miniaturization and industrialization of the system. Summary of the Invention
[0004] To address the aforementioned issues, this application provides a method and system for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy.
[0005] The first objective of this application is to provide a method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy, including: The characteristic parameters of the Mach-Zehnder interferometer and the frequency domain electrical information of the signal light, which is generated by the Mach-Zehnder interferometer, are obtained. The frequency domain electrical information of the probe light is obtained after the probe light and signal light are periodically modulated by absorption coupling through the gas under test; The downconversion factor is calculated based on the characteristic parameters of the Mach-Zehnder interferometer. Based on the downconversion factor, the frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light are inverted to obtain the gas photothermal spectrum information; Gas composition parameters are obtained based on gas photothermal spectral information.
[0006] In a specific embodiment of this application, the frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light are obtained by Fourier transform of the time domain point information obtained by two detectors of the same model.
[0007] In specific embodiments of this application, the characteristic parameters of the Mach-Zehnder interferometer include the moving speed of the movable arm mirror group in the Mach-Zehnder interferometer and the refractive index of the environment in which the Mach-Zehnder interferometer is located.
[0008] In a specific embodiment of this application, the downconversion factor a The formula for calculating 0 is as follows: a 0=2 nu / c in, u The moving speed of the boom mirror assembly in the Mach-Zehnder interferometer. n It is the refractive index of the environment in which the Mach-Zehnder interferometer is located. c It is the speed of light in a vacuum. u、n These are all characteristic parameters of the Mach-Zehnder interferometer.
[0009] In a specific embodiment of this application, the step of inverting the frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light based on the downconversion factor to obtain the gas photothermal spectrum information includes: The ratio of the frequency domain electrical information of the signal light to the frequency domain electrical information of the probe light is obtained; Multiply the horizontal axis of the obtained ratio information by the reciprocal of the following conversion factor to obtain the gas photothermal spectrum information.
[0010] In a specific embodiment of this application, the expression for the gas photothermal spectral information is as follows:
[0011] in, K p This is the strength ratio correction factor. For normalized absorption linear functions, A The peak absorption coefficient of the gas. For the frequency domain electrical information of the signal light, For the probe optical frequency domain electrical information that is subject to periodic modulation, v p These are the longitudinal mode frequencies of the optical frequency comb.v 0 represents the center frequency of the molecular absorption line.
[0012] In a specific embodiment of this application, the frequency domain electrical information of the signal light The formula for expressing this is as follows:
[0013] in, R s,p The detector for detecting signal light uses the signal light sequence number. p The RF longitudinal mode responsivity, P p For signal light p Root optical frequency longitudinal mode power.
[0014] In a specific embodiment of this application, the periodically modulated probe optical frequency domain electrical information The formula for expressing this is as follows:
[0015] in, R r , p The detector for detecting the probe light has a periodically modulated probe light numbered as follows: p The RF longitudinal mode responsivity, P p For signal light p Root optical frequency longitudinal mode power, For normalized absorption linear functions, A is the peak absorption coefficient of the gas.
[0016] In a specific embodiment of this application, obtaining gas component parameters based on gas photothermal spectral information includes: The gas molecular parameters are calibrated by comparing the gas photothermal spectral information with a gas molecular parameter database. Based on the gas photothermal spectrum information and the calibrated gas molecule parameters, the types and concentrations of gas molecules are inverted.
[0017] In a specific embodiment of this application, when the signal light is generated by the Mach-Zehnder interferometer, the moving distance of the movable arm reflecting lens group in the Mach-Zehnder interferometer within a single pulse acquisition time is... d max The following requirements must be met:
[0018] in, u The moving speed of the boom reflector assembly. n It is the refractive index of the environment in which the Mach-Zehnder interferometer is located.T The time interval between signal light pulses. c It is the speed of light in a vacuum. f rep It is the repetition frequency of the optical frequency comb.
[0019] In a specific embodiment of this application, the effective bandwidth of the two detectors of the same model is... All satisfy the following expression:
[0020] in, This represents the maximum frequency of the radio frequency component of the interference light generated by the Mach-Zehnder interferometer. v c The longitudinal mode frequency closest to the center of the spectrum. v B The bandwidth of the contour function. This indicates the maximum frequency of the longitudinal mode of the optical frequency comb. u The moving speed of the boom reflector assembly. t For the time of travel, n It is the refractive index of the medium surrounding the Mach-Zehnder interferometer. c The speed of light in a vacuum.
[0021] The second objective of this application is to provide a system for calculating gas component parameters based on single-optical-frequency-comb photothermal spectroscopy, comprising: Database module: used to acquire characteristic parameters of the Mach-Zehnder interferometer and frequency domain electrical information of the signal light, which is generated by the Mach-Zehnder interferometer; also used to acquire frequency domain electrical information of the probe light after the probe light and signal light are periodically modulated by absorption coupling through the gas under test; The calculation module is used to calculate the down-conversion factor based on the characteristic parameters of the Mach-Zehnder interferometer; to perform inversion processing on the frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light based on the down-conversion factor to obtain the gas photothermal spectrum information; and to obtain the gas composition parameters based on the gas photothermal spectrum information. The frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light are obtained by two detectors of the same type.
[0022] Compared with the prior art, this application has the following advantages: The method and system for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy in this application are based on a Mach-Zehnder interferometer. It does not require the use of a local oscillator optical comb and a signal optical comb for multi-heterodyne interference. The down-conversion factor can be directly calculated using the moving speed of the movable arm mirror group in the Mach-Zehnder interferometer, making the calculation process of gas component parameters simple and fast.
[0023] Other features and advantages of this application will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the application. The objectives and other advantages of this application may be realized and obtained by means of the structures pointed out in the description, claims and drawings. Attached Figure Description
[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0025] Figure 1 A flowchart of a method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to an embodiment of this application is shown; Figure 2 A framework diagram of a gas composition parameter calculation system based on single-optical frequency comb photothermal spectroscopy according to an embodiment of this application is shown. Figure 3 A schematic diagram of a Mach-Zehnder interferometer according to an embodiment of this application is shown; Figure 4 A spectrum diagram of the signal light according to an embodiment of this application is shown; Figure 5 The spectrum diagrams of the periodically modulated detector photoelectric information and the spectrum diagrams obtained by inversion processing are shown in the embodiments of this application. Figure 5 In Figure 'a', the spectrum of the detector's photoelectric information is subjected to periodic modulation. Figure 5 In the diagram, b represents the normalized spectrum. Figure 5 In the diagram, c represents the spectrum obtained from the inversion (i.e., normalized and multiplied by the above transformation factors); In the diagram: 1. Optical frequency comb; 2. First optical beam splitter; 3. Movable arm reflector group; 4. Beam combiner; 5. Collimating lens group; 51. First collimating lens; 52. Second collimating lens; 6. Fiber collimator; 7. Second optical beam splitter; 8. First photodetector; 9. Wavelength division multiplexer; 10. Hollow-core fiber optic cell; 11. Three-terminal circulator; 111. B port; 112. C port; 113. D port; 12. Second photodetector; 13. First filter; 14. Laser servo control system; 15. Laser; 16. Controller; 17. Second filter; 18. Data acquisition card; 19. Computer; 10. Database module; 20. Calculation module. Detailed Implementation
[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0027] like Figure 1 As shown, a method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to certain embodiments of this application includes: S1. Obtain the characteristic parameters of the Mach-Zehnder interferometer and the frequency domain electrical information of the signal light, wherein the signal light is generated by the Mach-Zehnder interferometer; S2. Obtain the frequency domain electrical information of the probe light after it and the signal light are periodically modulated by the absorption coupling of the gas under test; S3. Calculate the downconversion factor based on the characteristic parameters of the Mach-Zehnder interferometer. S4. Based on the downconversion factor, the frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light are inverted to obtain the gas photothermal spectrum information. S5. Obtain gas component parameters based on gas photothermal spectrum information; In some embodiments of this application, the frequency domain electrical information of the signal light The expression formula is shown in equation (1): (1) In equation (1), R s,p The detector for detecting signal light uses the signal light sequence number. p The RF longitudinal mode responsivity, P p For signal light p Root optical frequency longitudinal mode power.
[0028] In some embodiments of this application, the periodically modulated probe optical frequency domain electrical information The expression formula is shown in equation (2): (2) In equation (2), R r,p The detector for detecting the probe light has a periodically modulated probe light numbered as follows: p The RF longitudinal mode responsivity, P p For signal light p Root optical frequency longitudinal mode power, For normalized absorption linear functions,A The peak absorption coefficient of the gas. v 0 represents the center frequency of the molecular absorption line.
[0029] In some embodiments of this application, in step S3, the downconversion factor a The formula for calculating 0 is shown in equation (3): (The unit is dimensionless) a 0=2 nu / c (3) In equation (3), u The moving speed of the boom mirror assembly in the Mach-Zehnder interferometer. n It is the refractive index of the environment in which the Mach-Zehnder interferometer is located. c It is the speed of light in a vacuum. u、n These are all characteristic parameters of the Mach-Zehnder interferometer.
[0030] In some embodiments of this application, step S4 includes: S4-1. Take the ratio of the frequency domain electrical information of the signal light to the frequency domain electrical information of the periodically modulated probe light, that is, compare equation (1) and equation (2) to eliminate... P p ,eliminate P p The optimal condition is that the frequency domain electrical information of the signal light and the frequency domain electrical information of the periodically modulated probe light are obtained by two detectors of the same type. S4-2. Multiply the obtained ratio information by the reciprocal of the following conversion factor (defined as the upconversion factor) on the horizontal axis to obtain the gas photothermal spectrum information.
[0031] In some embodiments of this application, the expression for the gas photothermal spectral information is shown in equation (4): (4) In equation (4), K p This is the intensity ratio correction coefficient, which is related to the difference in light intensity loss caused by the different transmission characteristics of the optical devices through which the signal light and the probe light pass, as well as the responsivity ratio between the detector of the signal light and the detector of the probe light. When the two detectors are of the same model, this coefficient can be measured by a nitrogen reference measurement set. For normalized absorption linear functions, A The peak absorption coefficient of the gas. For the frequency domain electrical information of the signal light, For the probe optical frequency domain electrical information that is subject to periodic modulation, v p These are the longitudinal mode frequencies of the optical frequency comb. v0 represents the center frequency of the molecular absorption line.
[0032] In some embodiments of this application, step S5 includes: S5-1. The gas molecular parameters are calibrated by comparing the gas photothermal spectrum information with the gas molecular parameter database, wherein the gas molecular parameter database is a publicly known database in this technical field. S5-2. Based on the gas photothermal spectrum information and the calibrated gas molecule parameters, the inversion of gas molecule types and concentrations is performed. The inversion operation in this step is a well-known inversion operation in this technical field, and will not be described in detail here.
[0033] In some embodiments of this application, when the signal light is generated by the Mach-Zehnder interferometer, the moving distance of the movable arm reflecting lens group in the Mach-Zehnder interferometer during a single pulse acquisition time is... d max (Unit: m) The following requirement (5) must be met to avoid spectral overlap of the optical frequency comb 1 during downconversion in the Mach-Zehnder interferometer, which would affect the quality of the signal light:
[0034] In equation (5), u The moving speed of the boom reflector assembly is in m / s. n It is the refractive index of the environment in which the Mach-Zehnder interferometer is located; it has no dimension. T The time interval between signal light pulses, in seconds. c It is the speed of light in a vacuum, m / s. f rep It is the repetition frequency of the optical frequency comb, in Hz.
[0035] In some embodiments of this application, the effective bandwidth of the two detectors of the same model is... All satisfy the following expression to avoid signal distortion:
[0036] In equation (6), The frequency (Hz) represents the maximum frequency of the radio frequency component of the interference light generated by the Mach-Zehnder interferometer. v c The longitudinal mode frequency closest to the center of the spectrum, in Hz. v B The bandwidth of the contour function is Hz. This indicates the maximum longitudinal mode frequency of the optical frequency comb, in Hz. u The moving speed of the boom reflector assembly. t Let the moving time be s. nIt is the refractive index of the medium surrounding the Mach-Zehnder interferometer, without dimensions. c ρ is the speed of light in a vacuum, in m / s.
[0037] In some embodiments of this application, the Mach-Zehnder interferometer involved in step S1 is as follows: Figure 2 As shown, the optical transmission path includes an optical frequency comb 1 and a first optical beam splitter 2 in sequence; A movable arm reflector group 3 is provided on the optical path of one of the two beams of light split by the first optical beam splitter 2, and the path of beam B is changed by the movable arm reflector group and intersects with the optical path of the other beam C, and a beam combiner 4 is provided at the intersection. The boom reflector group 3 is connected to a drive mechanism, which drives the boom reflector group to perform linear motion. The trajectory of the linear motion is parallel to the line where the light beam B is located. In this embodiment, the light output from the optical frequency comb is split into two paths by the beam splitter 2. One path passes through the motor-driven boom reflector group 3, while the other path propagates through a fixed optical path and is then re-bundled with the output light reflected by the boom reflector group 3 at the beam combiner 4 to form a Mach-Zehnder interference.
[0038] In some embodiments of this application, the device for acquiring optical frequency domain electrical information in step S2 is an integrated photothermal spectral gas sensing device obtained by adding a gas absorption device and a detection device to a Mach-Zehnder interferometer as described in the above embodiments. Specifically, the photothermal spectral gas sensing device includes: a Mach-Zehnder interferometer as described in the above embodiments, and further includes a collimation processing unit and a second optical beam splitter 7 sequentially arranged on the optical path of the beam combined by the beam combiner 4; a first photodetector 8 arranged on the optical path of one beam D of the two beams split by the second optical beam splitter 7; and a wavelength division multiplexer 9 and a hollow fiber gas cell 10 sequentially arranged on the optical path of the other beam E of the two beams split by the second optical beam splitter. The wavelength division multiplexer 9 also receives probe light, and a polarization controller 16 and a three-terminal circulator 11 are provided on the incident optical path of the probe light. One port B111 of the three-terminal circulator 11 is connected to the polarization controller 16, one port C112 of the remaining two ports is connected to the wavelength division multiplexer 9, and the other port D113 of the remaining two ports is connected to the second photodetector 12. The polarization controller 16 is used to adjust the contrast of the interference fringes; The wavelength division multiplexer 9 is used to couple the probe light and the beam E into the hollow fiber gas cell 10, and pick up the secondary backlight carrying phase modulation information that returns through the hollow fiber gas cell 10 (that is, the probe light that is periodically modulated after the probe light and the signal light are absorbed and coupled by the gas to be measured is detected by the second photodetector 12). The second photodetector 12 is the same model as the first photodetector 8. The output signal a of the photodetector 12 is split into two signals. One signal B is connected to the laser 15 of the detection light. In the transmission path between signal B and the laser, a first filter 13 and a laser servo control system 14 are arranged in sequence. The first filter 13 is used to extract the low frequency (<100Hz) component in the signal B. The laser servo control system 14 is used to lock the wavelength of the laser 15 at the orthogonal working point of the interference fringes according to the extracted low frequency component signal. The laser servo control system 14 is a laser servo control system well known in the art, and its specific structure is not specifically limited here. The first photodetector signal 8 is connected to a data acquisition card 18; signal C of the two signals is sent to the data acquisition card 18 to collect the signal detected by the first photodetector 8 and the signal detected by the second photodetector 12. The photothermal spectroscopy gas sensing device also includes a computer 19; The computer 19 is equipped with the calculation method described in the above embodiments, which is used to calculate the component parameters of the gas to be tested.
[0039] In some embodiments of this application, a second filter 17 is provided on the transmission path between the signal C and the data acquisition card 18. The second filter 17 is used to extract the high-frequency (>100Hz) component in the signal A.
[0040] In some embodiments of this application, the collimation processing device includes a collimating lens group 5 and an optical fiber collimator 6, which are arranged sequentially in the optical transmission direction; The collimation processing device ensures that the combined beam (i.e., signal beam) output from the Mach-Zehnder interferometer is shaped so that the beam can be completely incident on the fiber collimator 6 and efficiently coupled into the fiber.
[0041] In some embodiments of this application, the collimating lens group 5 includes a first collimating lens 51 and a second collimating lens 52 for guiding light into the fiber optic collimator.
[0042] In some embodiments of this application, the hollow fiber gas cell 10 contains a Fabry-Perot interference structure, which can form a stable multi-beam interference field on a small scale, serving as a micro gas sample cell and a photothermal effect excitation and amplifier.
[0043] In some embodiments of this application, the fiber collimator 6 is a commercially available fiber collimator commonly used in this technical field, used to receive the light incident from the collimating lens group 5 and couple it into the second beam splitter 7.
[0044] In some embodiments of this application, for example, the second beam splitter 7 is a near-infrared commercial spatial light beam splitter.
[0045] In some embodiments of this application, the laser 15 is, for example, a commercially available narrow-linewidth continuous laser.
[0046] In some embodiments of this application, for example, the data acquisition card 18 is a commercial multi-channel data acquisition card, wherein two channels are used to acquire the output signals of the first photodetector 8 and the second photodetector 12, respectively.
[0047] In some embodiments of this application, for example, the computer 19 is a general commercial computer.
[0048] Frequency domain electrical information involving signal light The reasoning process for expressing the formula is as follows: In this application, the signal light is a beam combined (beam D or beam E) formed by beam combiner 4 in a Mach-Zehnder interferometer. It contains a series of discrete radio frequency modulation components formed by optical frequency longitudinal mode downconversion. Only the positive frequency AC term is analyzed. The intensity of the signal light in the frequency domain is shown in equation (7).
[0049] In equation (7) p This refers to the longitudinal mode number of the optical frequency comb. P p For the first p The signal optical power corresponding to the root longitudinal mode is determined by the spectral profile function of the optical frequency comb. G ( v p -v c )Decide, It is determined by the initial optical path difference between the two interference arms (beam B and beam C). L 0 pairs of serial numbers p The different initial phase differences are caused by different longitudinal modes. p Root RF longitudinal mode frequency f p With corresponding optical frequency longitudinal modev p See equation (8) between them:
[0050] In equation (8), n This represents the refractive index of the medium surrounding the Mach-Zehnder interferometer. u The moving speed of the boom reflector assembly. c At the speed of light, f rep The repetition frequency of the optical frequency comb. v c The longitudinal mode frequency is the closest to the center of the spectrum.
[0051] The beam combining light with intensity modulation information, formed by beam combiner 4, is processed by collimation and then split into two beams by second beam splitter 7, resulting in beam D and beam E. Beam D does not undergo gas absorption, its intensity can be expressed by equation (7), and it is received by first photodetector 8. The signal light time-domain electrical signal is obtained. The expression is:
[0052] In equation (9), R s,p The photodetector's signal light sequence number is p The RF longitudinal mode responsivity, when the detector model is fixed, R s,p And so it became fixed.
[0053] Performing a Fourier transform on the time-domain electrical signal of the signal optical described in equation (9) yields the expression for the frequency-domain electrical signal of the signal optical. And based on the physical meaning, take the positive frequency AC term. The analysis is shown in equation (10).
[0054]
[0055] As can be seen from equation (10), the signal optical frequency domain electrical signal is a series of discrete Dirac functions. Composition, namely discrete radio frequency longitudinal modes, where the frequency of each spectral line is located at the signal light radio frequency longitudinal mode frequency. f p Therefore, equation (10) can be simplified to equation (1).
[0056] Signal optical frequency domain electrical information subject to periodic modulation The reasoning process of the expression formula: Beam D is incident on the hollow fiber optic cell 10, which incorporates a Fabry-Perot interference structure. For the first... pOptical frequency corresponding to the root longitudinal mode v p It is absorbed by the gas to be measured in the hollow fiber cell, and the absorption power is P abs,p This can be expressed using the Beer-Lambert law, as shown in equation (11):
[0057] In equation (11), α ( v p ) is the gas to be tested in v p The absorption coefficient at that location, L This is the optical path length for gas absorption within the hollow fiber. Under weak absorption conditions ( α ( v p ) L <0.05), the absorption condition ensures a linear relationship between the gas absorption power and the absorption coefficient. Gas absorbed light power P abs,p Equation (11) can be approximated by equation (12):
[0058] In equation (12), A The peak absorption coefficient of the gas. v 0 represents the center frequency of the gas absorption line. This is the normalized absorbing linear function. (Normalized absorbing linear function) It can be seen as the "fingerprint" of gas molecules, and is related to the gas peak absorption coefficient. A This combination can be used to identify the type, concentration, temperature, pressure, and other characteristics of gas molecules. For gas molecules (acetylene in this example) with a much lower probability of radiative transitions than collisional relaxation in the near-infrared band, most of the absorbed light energy is converted into heat energy through non-radiative relaxation. P abs,p Power is converted into heat power, forming a periodic heat source in the gas, thereby causing local temperature changes with an amplitude Δ. T p It can be expressed as equation (13):
[0059] In equation (13), For gas at modulation frequency f p The thermal response coefficient is used to characterize the combined effects of factors such as gas diffusion, heat capacity, and relaxation time on the magnitude of temperature rise. Based on the property that the gas refractive index changes with temperature, this temperature change further affects the gas refractive index. np The disturbance can be expressed as equation (14):
[0060] In equation (14), T Δ represents the thermodynamic equilibrium temperature of the gaseous medium. T p For the first p The amplitude of the temperature disturbance caused by radio frequency longitudinal mode modulation. Refractive index disturbance Δ n p Phase modulation is achieved by making the probe light travel back and forth once in a hollow optical fiber containing a Fabry-Perot interference structure. It can be expressed as equation (15):
[0061] In equation (15), L λ is the optical path length for gas absorption within the hollow fiber. pr To detect the wavelength of the light (in this example, the wavelength of the laser), we combine equations (12), (13), (14), and (15) to eliminate the variable Δ. T p . use k * This represents a comprehensive proportionality coefficient related to the probe wavelength, thermal response coefficient, gas thermo-optic coefficient, and fiber geometry. Equation (15) can then be used... k * , No. p Signal optical power corresponding to the root longitudinal mode P p Gas absorption normalized absorption line type function The relationship between them is represented as in equation (16):
[0062] Equation (16) at the phase modulation depth When the value is much less than 1, the above phase modulation In the time domain, it can be expressed as equation (17):
[0063] The hollow fiber gas cell 10 emits primary retroreflected light carrying phase modulation information, including gas absorption characteristics and the amplitude spectrum characteristics of the beam E and the probe light. The probe light in the primary retroreflected light interferes with the incident probe light to obtain secondary retroreflected light carrying periodic intensity modulation information. When the wavelength of the laser 15 is locked at the intersection of the interference fringes, the intensity change of the secondary retroreflected light exhibits a linear response to the round-trip phase change within the cavity, and its intensity change time-domain signal... It can be expressed as equation (18):
[0064] In equation (18), C FP The phase-intensity conversion coefficient of the Fabry-Perot cavity is related to the end-face reflectivity and fineness of the Fabry-Perot interference structure. The secondary return light passes through the wavelength division multiplexer 9, from port C 112 of the three-terminal circulator 11 to port D 113 of the three-terminal circulator, and then from port D 113 to the second photodetector 12, obtaining the periodically modulated temporal electrical information of the probe light. The expression is as shown in equation (19):
[0065] In equation (19), R r,p The photodetector's periodically modulated probe light sequence number is p The RF longitudinal mode responsivity, when the detector model is fixed, R r,p It is also fixed thereafter. In some embodiments of this application, the periodically modulated temporal electrical information of the probe light is processed by the second filter 17 and then received by the data acquisition card.
[0066] Fourier transform is performed on the time-domain electrical information of the periodically modulated probe light acquired by data acquisition card 18 to obtain the frequency-domain electrical information of the periodically modulated probe light. Based on its actual physical meaning, its positive frequency AC term is taken. Analysis shows that it can be expressed as equation (20):
[0067] As can be seen from equation (20), the frequency domain signal of the periodic intensity modulation information is composed of a series of discrete Dirac functions. Composition, namely discrete radio frequency longitudinal modes, where the frequency of each spectral line is located at the signal light radio frequency longitudinal mode frequency. f p When the hollow fiber optic air cell structure is determined, L , C FP、 k * For a fixed value, equation (20) can be simplified to equation (2), that is, the frequency domain electrical information of the probe light subject to periodic modulation. From equation (2), it can be seen that the frequency domain signal of the obtained periodic modulation information is related to the absorption line shape of gas molecules and the peak absorption coefficient.
[0068] like Figure 3As shown, a system for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to certain embodiments of this application includes: Database module 10: used to acquire characteristic parameters of the Mach-Zehnder interferometer and frequency domain electrical information of the signal light, which is generated by the Mach-Zehnder interferometer; also used to acquire frequency domain electrical information of the probe light after the probe light and signal light are periodically modulated by absorption coupling through the gas under test; Calculation module 20: used to calculate the downconversion factor based on the characteristic parameters of the Mach-Zehnder interferometer; used to perform inversion processing on the frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light based on the downconversion factor to obtain the gas photothermal spectrum information; and also used to obtain the gas composition parameters based on the gas photothermal spectrum information. The frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light are obtained by two detectors of the same type.
[0069] The calculation method described in the above embodiment is applied to the calculation of the parameters of the gas component to be measured. The frequency domain electrical information spectrum of the signal light detected by the first photodetector 8 is shown in the figure below. Figure 4 As shown, the spectrum of the frequency domain electrical information of the detection light obtained by the second photodetector 12 is as follows. Figure 5 As shown in Figure a, Figure 5 In the diagram, b represents the spectrum of the normalized gas photothermal spectrum. Figure 5 In the image, c represents the photothermal absorption spectrum of acetylene gas obtained through inversion.
[0070] from Figure 5 As can be seen, there are obvious differences in the gas identification by directly using the frequency domain electrical information of the periodically modulated probe light. By normalizing the frequency domain electrical information of the signal light and the frequency domain electrical information of the periodically modulated probe light and multiplying the abscissa by the above conversion factor (i.e., inversion processing), the obtained gas photothermal spectrum information can accurately identify the gas. In this embodiment, the gas to be identified is acetylene.
[0071] Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.
Claims
1. A method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy, characterized in that, include: The characteristic parameters of the Mach-Zehnder interferometer and the frequency domain electrical information of the signal light, which is generated by the Mach-Zehnder interferometer, are obtained. The frequency domain electrical information of the probe light is obtained after the probe light and signal light are periodically modulated by absorption coupling through the gas under test; The downconversion factor is calculated based on the characteristic parameters of the Mach-Zehnder interferometer. Based on the downconversion factor, the frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light are inverted to obtain the gas photothermal spectrum information; Gas composition parameters are obtained based on gas photothermal spectral information.
2. The method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to claim 1, characterized in that, The frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light are obtained by Fourier transforming the time domain electrical information obtained from two detectors of the same type. And / or, the characteristic parameters of the Mach-Zehnder interferometer include the moving speed of the movable arm mirror group in the Mach-Zehnder interferometer and the refractive index of the environment in which the Mach-Zehnder interferometer is located; And / or, the calculation formula is as follows: a 0=2 nu / c in, a 0 is the downconversion factor. u The moving speed of the boom mirror assembly in the Mach-Zehnder interferometer. n It is the refractive index of the environment in which the Mach-Zehnder interferometer is located. c It is the speed of light in a vacuum.
3. The method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to claim 1, characterized in that, The process involves inverting the frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light based on the downconversion factor to obtain the gas photothermal spectrum information, including: The ratio of the frequency domain electrical information of the signal light to the frequency domain electrical information of the probe light is obtained; Multiply the horizontal axis of the obtained ratio information by the reciprocal of the following conversion factor to obtain the gas photothermal spectrum information.
4. The method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to claim 3, characterized in that, The expression for the gas photothermal spectrum information is as follows: in, K p This is the strength ratio correction factor. For normalized absorption linear functions, A The peak absorption coefficient of the gas. For the frequency domain electrical information of the signal light, For the probe optical frequency domain electrical information that is subject to periodic modulation, v p These are the longitudinal mode frequencies of the optical frequency comb. v 0 represents the center frequency of the molecular absorption line.
5. The method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to claim 1, characterized in that, The frequency domain electrical information of the signal light The formula for expressing this is as follows: in, R s,p The detector for detecting signal light uses the signal light sequence number. p The RF longitudinal mode responsivity, P p For signal light p Root optical frequency longitudinal mode power.
6. The method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to claim 1, characterized in that, The detected optical frequency domain electrical information The formula for expressing this is as follows: in, R r,p The detector for detecting the probe light is subjected to periodic probe light of number 1. p The RF longitudinal mode responsivity, P p For signal light p Root optical frequency longitudinal mode power, For normalized absorption linear functions, A is the peak absorption coefficient of the gas.
7. The method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to claim 1, characterized in that, The step of obtaining gas component parameters based on gas photothermal spectral information includes: The gas molecular parameters are calibrated by comparing the gas photothermal spectral information with a gas molecular parameter database. Based on the gas photothermal spectrum information and the calibrated gas molecule parameters, the types and concentrations of gas molecules are inverted.
8. The method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to claim 1, characterized in that, When the signal light is generated by the Mach-Zehnder interferometer, the distance the moving arm reflecting lens group in the Mach-Zehnder interferometer moves within a single pulse acquisition time is... d max The following requirements must be met: in, u The moving speed of the boom reflector assembly. n It is the refractive index of the environment in which the Mach-Zehnder interferometer is located. T The time interval between signal light pulses. c It is the speed of light in a vacuum. f rep It is the repetition frequency of the optical frequency comb.
9. The method for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy according to claim 1, characterized in that, The effective bandwidth of the two detectors of the same model All satisfy the following expression: in, This represents the maximum frequency of the radio frequency component of the interference light generated by the Mach-Zehnder interferometer. v c The longitudinal mode frequency closest to the center of the spectrum. v B The bandwidth of the contour function. This indicates the maximum frequency of the longitudinal mode of the optical frequency comb. u The moving speed of the boom reflector assembly. t For the time of travel, n It is the refractive index of the medium surrounding the Mach-Zehnder interferometer. c The speed of light in a vacuum.
10. A system for calculating gas component parameters based on single-optical frequency comb photothermal spectroscopy, characterized in that, include: Database module: used to acquire characteristic parameters of the Mach-Zehnder interferometer and frequency domain electrical information of the signal light, which is generated by the Mach-Zehnder interferometer; also used to acquire frequency domain electrical information of the probe light after the probe light and signal light are periodically modulated by absorption coupling through the gas under test; Calculation module: used to calculate the downconversion factor based on the characteristic parameters of the Mach-Zehnder interferometer; It is used to invert the frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light based on the downconversion factor to obtain the gas photothermal spectrum information; it is also used to obtain the gas composition parameters based on the gas photothermal spectrum information. The frequency domain electrical information of the signal light and the frequency domain electrical information of the probe light are obtained by two detectors of the same type.