A time-frequency conversion method for tunable diode laser absorption signal

Abstract

The present invention discloses a method for time-frequency conversion of a tunable diode laser absorption signal. The method comprises: constructing a wavelength scanning-direct absorption spectroscopy measurement system to collect the transmission signal of a wavelength standard and the measured absorption signal of the measured gas, which has only a single absorption peak; comparing the measured absorption signal with a theoretical spectrum in a database to extract characteristic points of the measured absorption signal and the theoretical spectrum in the database; obtaining a sampling point-wavenumber correspondence diagram based on the extracted characteristic points, and establishing a time-frequency conversion model for the time and frequency domains in TDLAS; and comparing the time-frequency conversion model with the wavenumber calibration results to verify the accuracy of the time-frequency conversion model. The proposed method has the advantages of high precision and simple operation, has great application potential in low-cost TDLAS applications, and plays an important role in gaining a deeper understanding of the dynamic wavelength tuning characteristics of lasers.

Description

Technical Field

[0001] The invention relates to the technical field of laser diagnosis and measurement, and in particular to a time-frequency conversion method for a tunable diode laser absorption signal. Background Art

[0002] Tunable laser absorption spectroscopy (TDLAS) is an optical method that uses the narrow linewidth and fast tuning characteristics of semiconductor lasers to detect an isolated absorption line of an absorbing molecule to achieve rapid gas detection. In practical applications, the horizontal coordinate of the detection signal obtained by the TDLAS system is generally the number of sampling points at equal time intervals, corresponding to a time series. Line area normalization is only valid in the frequency domain, and the spectrum horizontal coordinate needs to be converted into wave numbers (cm -1 ) can be used to solve the integral absorbance value. The accuracy of time-frequency conversion plays an important role in the inversion accuracy of flow field parameters (temperature, concentration, etc.).

[0003] Traditional laser frequency measurement methods are based on the principle of interference, and the typical instrument is a high-precision FP etalon. However, traditional laser frequency measurement methods have the following shortcomings: (1) High-precision etalons are affected by environmental disturbances, system noise, and other factors during the measurement process. The accuracy of the peak-finding algorithm is one of the factors that affect the accuracy of time-frequency conversion; (2) The refractive index of the etalon is different for different laser frequencies. Therefore, the refractive index needs to be recalculated every time the calibration frequency is changed, and different wavelength bands require etalons made of different materials. For laboratories, it is not easy to have etalons suitable for different wavelength bands at the same time.

[0004] Using experiments instead of etalons for time-to-frequency conversion is an economical and efficient method. Existing experimental methods for time-to-frequency conversion often require the presence of multiple absorption peaks within the laser scan range. The time-domain information of these absorption peaks is then mapped to the frequency-domain information in the HITRAN database to establish a functional relationship. However, when only a single molecular absorption peak exists within the laser scan range, these existing experimental methods are not applicable.

[0005] In view of this, and targeting the limitations of the existing technology, the present invention proposes a simple tunable diode laser absorption signal time-frequency conversion method, which can achieve time-frequency conversion under the condition that there is only one molecular absorption peak within the laser scanning range. Summary of the Invention

[0006] In order to overcome the shortcomings and deficiencies of the prior art, the present invention provides a tunable diode laser absorption signal time-frequency conversion method, which is particularly suitable for the case where there is only one absorption peak within the laser scanning range.

[0007] The present invention adopts the following technical solutions:

[0008] A time-frequency conversion method for a tunable diode laser absorption signal, comprising:

[0009] A wavelength scanning-direct absorption spectroscopy measurement system was constructed to collect the transmission signal of the wavelength standard and the measured absorption signal of the measured gas, which has only one absorption peak. The measured absorption signal was compared with the theoretical spectrum in the database, and the characteristic points of the measured absorption signal and the theoretical spectrum in the database were extracted.

[0010] According to the extracted feature points, the corresponding graph is obtained and the time-frequency conversion model between the time domain and the frequency domain in TDLAS is established;

[0011] The time-frequency conversion model is compared with the wavenumber calibration results to verify the accuracy of the time-frequency conversion model.

[0012] Furthermore, the characteristic point is specifically a point where the amplitude of the measured absorption signal is consistent with that of the theoretical spectrum in the database.

[0013] Furthermore, the corresponding diagram is specifically a sampling point-wave number correspondence diagram, where the horizontal coordinate in the measured absorption signal is the sampling point, representing the time domain information, and the horizontal coordinate in the theoretical spectrum is the wave number, representing the frequency domain information. The horizontal coordinate of the feature point is extracted to obtain the sampling point-wave number correspondence diagram of the feature point.

[0014] Furthermore, the database theoretical spectrum is specifically a theoretical spectrum obtained by querying the HITRAN database based on actual measurement condition information, and the actual measurement condition information includes temperature, pressure, optical path, gas type and concentration.

[0015] Furthermore, the time-frequency conversion model between the time domain and the frequency domain in TDLAS is established as follows:

[0016] Extract theoretical spectral absorption peak half-maximum full width and integrated absorbance information containing measurement condition information;

[0017] Based on the dynamic wavelength tuning characteristics of the laser, a constrained quadratic polynomial model is used to establish a time-domain-frequency domain model under the constraints of the theoretical spectral absorption peak FWHM and integrated absorbance.

[0018] Furthermore, the time domain-frequency domain conversion model is:

[0019] υ=ax 2 +bx+c

[0020] Where ν is the laser frequency, x is the number of absorption signal sampling points, and a, b, and c are the coefficients of the quadratic polynomial that can achieve the correspondence between the theoretical spectrum in the frequency domain and the actual spectrum in the time domain under restricted conditions.

[0021] Furthermore, the time-frequency conversion model is compared with the wavenumber calibration result to verify the accuracy of the time-frequency conversion model, specifically:

[0022] Compare the correlation coefficient and root mean square error between the time-frequency conversion model and the wavenumber calibration results of the etalon; substitute the quadratic polynomial time-frequency conversion model and the wavenumber calibration results of the etalon as the horizontal coordinates into the absorption spectrum of the same known concentration gas, calculate the gas concentration and compare the relative errors, so as to compare the results of the time-frequency conversion model and the wavenumber calibration and verify the accuracy of the time-frequency conversion model.

[0023] Furthermore, the wavelength scanning-direct absorption spectroscopy measurement system includes a function generator, a near-infrared tunable semiconductor laser, an FP standard, a standard concentration gas cell, and a data acquisition module; the function generator generates a sawtooth current, causing the laser to output continuous laser light centered at a certain wavelength, and the light output by the laser is split. After one beam of laser light passes through the FP standard, a transmission signal with continuous periodic peaks and troughs is generated, which is used to calibrate the laser wave number; the other beam passes through the standard gas cell and is received by a detector to obtain a laser absorption signal.

[0024] Furthermore, the standard concentration gas cell is a quartz glass tube.

[0025] Furthermore, a standard gas to be measured with a concentration of 5%-15% is continuously introduced into the quartz glass tube.

[0026] Beneficial effects of the present invention:

[0027] (1) The present invention adopts an experimental measurement method, which can realize the time-frequency conversion of direct absorption spectrum without using an etalon, thus effectively reducing the cost;

[0028] (2) In view of the limitations of traditional experimental measurement methods, time-frequency conversion is achieved under the condition that there is only one absorption peak within the laser scanning range;

[0029] (3) Since the time-frequency conversion process directly processes the rapidly tuned signal, the response speed is consistent with the laser frequency, and the measurement results are closer to the actual situation of dynamic wavenumber changes.

[0030] (4) The method proposed in the present invention has the characteristics of high precision and simple operation, and has great application potential in low-cost TDLAS applications. It plays an important role in gaining a deeper understanding of the dynamic wavelength tuning characteristics of lasers. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the process of the present invention;

[0032] Figure 2 FIG. 1 is a diagram of the experimental device of the present invention;

[0033] Figure 3 Comparison plot of the constrained quadratic polynomial model and its residuals.

[0034] Figure 4 Comparison chart of measurement results of etalon and constrained quadratic polynomial model. DETAILED DESCRIPTION

[0035] The present invention will be further described in detail below with reference to the embodiments and the accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0036] Example

[0037] like Figure 1 As shown, a tunable diode laser absorption signal time-frequency conversion method is applied to a single-line TDLAS measurement spectrum, including:

[0038] S1 constructs a wavelength scanning-direct absorption spectrum measurement system to collect the transmission signal of the wavelength standard and the measured absorption signal of the measured gas. The absorption signal has only one absorption peak.

[0039] Further, if Figure 2 As shown, the wavelength scanning-direct absorption spectrum measurement system includes a function generator, a near-infrared tunable semiconductor laser, an FP etalon, a standard concentration gas cell, and a data acquisition module.

[0040] Specifically, the experiment used methane as the measured gas, and a standard CH4 gas of a certain concentration was continuously introduced into a gas cell made of a quartz glass tube. A function generator was used to generate a sawtooth current. By adjusting the injection current, the laser was tuned to output a continuous laser within a specific wavelength range, approximately 1650-1660 nm. Within this range, methane gas has only a single absorption peak. The DFB laser's light was then split. One laser beam passed through a cell filled with a standard concentration of methane gas and was received by a photodetector. The total optical path of the absorption signal was 20-22 cm, resulting in a direct absorption signal. The other laser beam passed through an etalon to generate an interference signal, which was used to calibrate the laser wavenumber, resulting in the laser time-to-frequency conversion expression ν = f(t).

[0041] The function generator generates a sawtooth wave current, causing the laser to output continuous laser light centered at a certain wavelength. After the laser light is output, it is split into beams. One beam of laser light passes through the FP standard device to generate a transmission signal with continuous periodic peaks and troughs, which is used to calibrate the laser wave number. The other beam passes through a methane gas pool filled with a known standard concentration and is received by the detector to obtain a laser absorption signal. The measured gas has only one absorption peak within the laser scanning range.

[0042] In this embodiment, the measured laser light, after passing through the etalon, produces a transmission spectrum with continuous periodic peaks and troughs. The distance between adjacent peaks is called the free spectral range (FSR), which is a fixed value. A peak-finding algorithm is used to obtain the instantaneous value of the fixed wavelength output in the etalon's output spectrum. Combined with the etalon's free spectral range, the wavelength-time correspondence can be derived, effectively creating a time-frequency conversion model using the etalon.

[0043] S2 compares the measured absorption signal with the theoretical spectrum in the database and extracts the characteristic points of the measured absorption signal and the theoretical spectrum in the database;

[0044] Specifically, the theoretical spectrum is obtained by querying the HITRAN database based on the temperature, pressure, optical path, gas type, concentration and other information of the actual measurement conditions, and the points with consistent amplitudes in the measured signal and the theoretical spectrum are set as feature points; the horizontal coordinates of the feature points in the measured signal are the sampling points, representing the time domain information; the horizontal coordinates of the feature points in the theoretical spectrum are the wavenumbers, representing the frequency domain information; the horizontal coordinates of the feature points are extracted to obtain the sampling point-wavenumber correspondence diagram of the feature point set.

[0045] The actual measurement conditions in this embodiment include a pressure of 1 atm, a temperature of 23°C, a gas cell length of 21.5 cm, a methane standard gas concentration of 5%, and a carrier gas of 95% N2. The line intensity of methane gas near 1654 nm can be found in the HITRAN database to be 7.86×10 -2 cm -2 / atm, and obtain a theoretical spectrum of methane gas within this wavelength range. The ordinates of both the theoretical and measured methane spectra are spectral intensities, but the abscissa of the theoretical spectrum represents wavenumber information, which represents frequency, while the abscissa of the measured signal represents sampling point information, which represents time. Points with identical amplitudes in the measured signal and the theoretical spectrum are designated as feature points, allowing the selection of several feature points within the absorption peak range. A correspondence between the feature points is established based on the corresponding abscissas of the measured signal and the theoretical spectrum.

[0046] S3 obtains the sampling point-wave number correspondence diagram based on the extracted feature points and establishes the time-frequency conversion model between the time domain and the frequency domain in TDLAS.

[0047] Specifically: including:

[0048] S3.1. Extract the full width at half maximum (FWHM) and integrated absorbance information of the theoretical spectral absorption peak, which contain information about the measurement conditions;

[0049] S3.2. Based on the understanding of the dynamic wavelength tuning characteristics of the laser, combined with the constraints of the theoretical spectral absorption peak FWHM and integrated absorbance, a constrained quadratic polynomial model is used to establish the sampling point (time domain)-wavenumber (frequency domain) functional relationship, as follows:

[0050] υ=ax 2 +bx+c

[0051] Where ν is the laser frequency, x is the number of absorption signal sampling points, and a, b, and c are the coefficients of the quadratic polynomial that can achieve the correspondence between the theoretical spectrum's horizontal coordinate frequency domain and the actual spectrum's horizontal coordinate time domain under the restricted conditions. In this embodiment, a = -8.18 × 10 -7 , b = 0.004, c = 6045.546, that is

[0052] ν=-8.180×10 -7 ·x 2 +0.004x+6045.546

[0053] The constraints are that the FWHM and integrated absorbance of the measured methane spectrum are equal to those of the theoretical spectrum, respectively. The FWHM and integrated absorbance of the absorption spectrum contain information about the measured operating conditions, such as gas type and concentration, ambient temperature, pressure, and optical path length. To ensure the accuracy of the time-frequency conversion model, constraints are required when aligning the measured spectrum in the time domain with the theoretical spectrum in the frequency domain. The coefficients of the polynomial are the coefficients that, under these constraints, achieve a correspondence between the theoretical spectrum's abscissa in the frequency domain and the actual spectrum's abscissa in the time domain.

[0054] S4 compares the time-frequency conversion model with the wavenumber calibration results to verify the accuracy of the time-frequency conversion model.

[0055] Specifically: compare the correlation coefficient and root mean square error between the time-frequency conversion model and the wavenumber calibration results of the standard instrument; substitute the quadratic polynomial time-frequency conversion model and the wavenumber calibration results of the standard instrument as the horizontal coordinates into the absorption spectrum of the same known concentration gas, calculate the gas concentration and compare the relative error, so as to compare the results of the time-frequency conversion model and the wavenumber calibration and verify the accuracy of the time-frequency conversion model.

[0056] like Figure 3 As shown in the figure, the correlation coefficient (R 2 ) reaches 0.9949, the root mean square error (RSME) between the two is only 0.0891, and the maximum residual between the two is only 0.185, indicating that the model can better describe the dynamic tuning characteristics of the wavelength within the laser scanning range.

[0057] To verify the accuracy of the model, the constrained quadratic polynomial time-frequency conversion model and the etalon wavenumber measurement results were substituted into the methane absorption spectrum as the abscissa. The concentrations of three different standard gas concentrations were calculated, and the relative errors of the concentration results were compared. The standard methane gas concentrations were 5% ± 500 ppm, 10% ± 1000 ppm, and 15% ± 1500 ppm, respectively.

[0058] The relative error is calculated by the formula:

[0059] σ=AA* / A

[0060] Where A is the concentration value calculated using the etalon, and A* is the concentration value calculated using the constrained quadratic polynomial model.

[0061] Figure 4 The accuracy of the time-to-frequency conversion model was evaluated by comparing the relative errors of the absorption spectrum concentration and the etalon concentration measurement results for the constrained quadratic polynomial model. The relative error between the constrained quadratic polynomial time-to-frequency conversion model and the etalon results when calculating the actual measured gas concentration was less than 0.5%.

[0062] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for time-frequency conversion of a tunable diode laser absorption signal, characterized in that: include: A wavelength scanning-direct absorption spectroscopy measurement system is constructed to collect the transmission signal of the wavelength transmission etalon and the measured absorption signal of the measured gas, wherein the measured absorption signal has only one absorption peak; the measured absorption signal is compared with the theoretical spectrum in the database, and characteristic points of the measured absorption signal and the theoretical spectrum in the database are extracted. The characteristic points are specifically points where the amplitude of the measured absorption signal and the theoretical spectrum in the database are consistent. Several characteristic points are selected within the absorption peak range, and a corresponding relationship between the characteristic points is established based on the horizontal coordinates corresponding to each characteristic point in the measured absorption signal and the theoretical spectrum; According to the extracted characteristic points, a corresponding graph is obtained, wherein the corresponding graph is specifically a sampling point-wave number correspondence graph, wherein the abscissa in the measured absorption signal is the sampling point, representing the time domain information, and the abscissa in the theoretical spectrum is the wave number, representing the frequency domain information. The abscissa of the characteristic point is extracted to obtain a sampling point-wave number correspondence graph of the characteristic point, and a time-frequency conversion model between the time domain and the frequency domain in the tunable laser absorption spectroscopy technology is established; include: S1. Extracting the full width at half maximum and integrated absorbance of the theoretical spectrum absorption peak containing the measurement condition information; S2. Based on the understanding of the dynamic wavelength tuning characteristics of the laser, combined with the constraints of the theoretical spectral absorption peak half-width and integrated absorbance, a constrained quadratic polynomial model is used to establish the sampling point-wavenumber function relationship, as follows: υ=ax 2 +bx+c Where, υ is the laser frequency, x is the sampling point of the measured absorption signal, a 、 b 、 c Under the restricted conditions, the quadratic polynomial coefficients corresponding to the theoretical spectrum in the frequency domain and the measured absorption spectrum in the time domain can be realized, where: a= -8.18×10 -7 , b =0.004, c= 6045.546; The constraint is that the full width at half maximum (FWHM) and the integrated absorbance of the measured methane spectrum are equal to those of the theoretical spectrum, respectively. The FWHM and integrated absorbance of the measured absorption spectrum contain information about the gas type, concentration, ambient temperature, pressure, and optical path length. The time-frequency conversion model is compared with the standard instrument wavenumber calibration results to verify the accuracy of the time-frequency conversion model.

2. The method for time-frequency conversion of a tunable diode laser absorption signal according to claim 1, wherein: The database theoretical spectrum is specifically a theoretical spectrum obtained by querying the HITRAN database based on actual measurement condition information, and the actual measurement condition information includes temperature, pressure, optical path, gas type and concentration.

3. The method for time-frequency conversion of a tunable diode laser absorption signal according to claim 1, wherein: The time-frequency conversion model is compared with the standard instrument wave number calibration result to verify the accuracy of the time-frequency conversion model, specifically: Compare the correlation coefficient and root mean square error between the time-frequency conversion model and the wavenumber calibration results of the etalon; substitute the quadratic polynomial time-frequency conversion model and the wavenumber calibration results of the etalon as horizontal coordinates into the absorption spectrum of the same known concentration gas, calculate the gas concentration and compare the relative errors, so as to compare the results of the time-frequency conversion model and the wavenumber calibration and verify the accuracy of the time-frequency conversion model.

4. The method for time-frequency conversion of a tunable diode laser absorption signal according to any one of claims 1 to 3, characterized in that: The wavelength scanning-direct absorption spectroscopy measurement system includes a function generator, a near-infrared tunable semiconductor laser, an etalon, a standard concentration gas cell, and a data acquisition module. The function generator generates a sawtooth current, causing the laser to output continuous laser light centered at a certain wavelength. The light output by the laser is split. After one beam passes through the etalon, it generates a transmission signal in the form of continuous periodic peaks and troughs, which is used to calibrate the laser wave number. The other beam passes through the standard gas cell and is received by a detector to obtain a laser absorption signal.

5. The method for time-frequency conversion of a tunable diode laser absorption signal according to claim 4, characterized in that: The standard concentration gas cell is a quartz glass tube.

6. The method for time-frequency conversion of a tunable diode laser absorption signal according to claim 5, characterized in that: The standard gas to be tested with a concentration of 5%-15% is continuously introduced into the quartz glass tube.

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