A temperature measurement method based on low-speed sampling parameter identification of laser absorption spectrum

Abstract

The application provides a temperature measurement method based on low-speed sampling parameter identification of laser absorption spectrum, and belongs to the technical field of tunable semiconductor laser absorption spectrum. The used elements include a tunable semiconductor laser, a beam splitter, a collimating mirror, a standard device, a photoelectric detector, a Butterworth low-pass filter, a data acquisition module and a computer; on the basis of the direct absorption spectrum method, the Butterworth low-pass filter is added, the flatness characteristics of the Butterworth low-pass filter in the passband are utilized, the signal with the absorption spectrum information of the measured gas is filtered to remove high-frequency noise, the complete line type of the absorption spectrum is still retained, low-speed sampling can be carried out without spectrum aliasing, the requirement of the direct absorption spectrum method on the sampling rate is reduced, the rapid extraction of the absorption spectrum parameter is realized under low-speed sampling, the data acquisition, storage and calculation costs are reduced, and the temperature measurement efficiency of the direct absorption spectrum method can be effectively improved.

Description

(I) Technical Field

[0001] This invention proposes a temperature measurement method based on low-speed sampling parameter identification of laser absorption spectroscopy, which belongs to the field of tunable semiconductor laser absorption spectroscopy technology. (II) Background Technology

[0002] Combustion is one of the most direct energy conversion methods in industrial production. Accurate monitoring of the combustion process helps improve energy conversion efficiency, optimize burner design, and reduce pollutant emissions. In combustion diagnostics, temperature parameters indicate combustion efficiency and chemical reaction rates within the flow field, making rapid online measurement of these parameters particularly important. Direct contact measurement of the combustion flame is challenging and disrupts the flame flow field, affecting the combustion process. Therefore, non-contact measurement methods offer more advantages. Non-contact temperature measurement methods are primarily optical, offering advantages such as non-disruptive flow field measurement and high temperature sensitivity, making them suitable for measuring flow field parameters such as temperature and concentration fields. The advent of tunable semiconductor lasers has led to significant advancements in laser absorption spectroscopy. Utilizing the narrow linewidth and wavelength-dependent characteristics of lasers, which change with the injected current, single molecules or multiple difficult-to-distinguish adjacent spectral lines can be measured. Therefore, Tunable Diode Laser Absorption Spectroscopy (TDLAS) has become one of the main optical temperature measurement methods.

[0003] Tunable diode laser absorption spectroscopy (TBLS) is an optical diagnostic technique widely used in engine combustion diagnostics and environmental monitoring. It obtains molecular absorption information by measuring the absorption spectra of absorbing molecules. In 2016, a paper titled "Tunable diode laser absorption spectroscopy: principle and application," published in *Physics of Gases*, Volume 1, Issue 5, pp. 52-63, reviewed the development and application of absorption spectroscopy in measuring temperature and component concentration in high-temperature reaction environments. It detailed different absorption measurement strategies and their corresponding system compositions. Due to its high sensitivity and robustness, it has been widely applied in various studies involving chemical reaction flows. In 2019, the paper "Simultaneous detection of atmospheric CO and CH4 based on TDLAS using a single 2.3μm DFB laser," published in Volume 222 of *Spectrochemical Journal*, demonstrated the simultaneous monitoring of atmospheric CO and CH4 using a single 2.33μm DFB laser and designed a detection system capable of real-time measurement of atmospheric CO and CH4 for 48 hours. In 2022, the paper "Measurement of turbulent supersonic steam jet flow characteristics using TDLAS," published in Volume 87 of *Flow Measurement and Instrumentation*, explored a new application of the TDLAS method in wet steam ejectors, detecting the thermal flux characteristics of the ejectors.

[0004] The main methods for implementing TDLAS technology include wavelength modulation spectroscopy (WMS) and direct absorption spectroscopy (DAS). Wavelength modulation spectroscopy uses a high-frequency sinusoidal modulated laser source with linear wavelength scanning superimposed on the laser. The laser outputs the modulated wavelength and intensity. Absorption information is obtained by analyzing the intensity of the harmonic components of the high-frequency modulation signal. The high-frequency modulated laser intensity can effectively suppress noise interference, and the high-frequency modulated laser wavelength can repeatedly scan the absorption spectrum, thus exhibiting advantages such as strong noise resistance and high measurement sensitivity. In 2003, the paper "Wavelength modulation spectroscopy: combined frequency and intensity laser modulation," published in *Applied Optics*, Volume 42, Issue 33, pp. 6728-6738, introduced a theoretical model of wavelength modulation spectroscopy using a laser diode on a Lorentz absorption line, evaluated the influence of several modulation parameters on the monitoring signal, and provided theoretical and experimental basis for setting modulation parameters in wavelength modulation spectroscopy. In 2006, the paper "Study on Measurement of Methane by Tunable Semiconductor Laser Absorption Spectroscopy" published in the third issue of the Chinese Journal of Quantum Electronics (pages 388-392) used WMS technology to detect the second harmonic signal of methane gas with concentrations ranging from 0.04% to 10% at atmospheric pressure, providing a new detection method for monitoring the concentration of methane gas in industry. In 2013, the paper "Simulation and Analysis of the Second Harmonic Signal of Tunable Diode Laser Absorption Spectrum" published in Spectroscopy and Spectral Analysis, Volume 33, Issue 4, pp. 881-885, proposed a simulation method for the second harmonic signal using wavelength modulation spectroscopy, analyzed the relationship between the harmonic signal and the modulation coefficient, and provided a theoretical basis for the selection of relevant parameters in practical measurements.In 2021, a paper published in *Futures* (Volume 305), titled "High-temperature dual-species (CO / NH3) detection using calibration-free scanned-wavelength-modulation spectroscopy at 2.3 μm," developed a dual-species detector based on wavelength modulation spectroscopy for calibration-free monitoring of CO and NH3 in high-temperature environments. While wavelength modulation spectroscopy offers strong anti-interference capabilities, its data processing is complex, requiring data calibration and demanding high sampling rates, resulting in high data acquisition costs. In contrast, direct absorption spectroscopy is simpler to implement. It utilizes a sawtooth-modulated tunable semiconductor laser to completely scan the absorption spectrum of the analyte gas. After the laser passes through the gas, the absorption spectrum is directly extracted from the transmitted light intensity, providing a direct view of the absorption spectrum. This convenient measurement method is widely used in geological surveying, combustion diagnostics, and atmospheric environmental monitoring. In 2014, he published a paper entitled "Measurement of disubstituted methane isotopes by tunable infrared laser direct absorption spectroscopy" in Analytical Chemistry, Volume 86, Issue 13, pp. 6487-6494. 13 CH3D》(Measurement of a DoublySubstituted Methane Isotopologue, 13 A method for accurately determining the properties of infrared laser direct absorption spectroscopy using tunable infrared lasers was developed in CH3D (by Tunable Infrared Laser Direct Absorption Spectroscopy). 13A novel method for determining the relative abundance of CH3D can be used to differentiate the geological and biological origins of methane in the atmosphere, hydrosphere, and lithosphere. In 2018, a paper titled "Two-dimensional temperature and carbon dioxide concentration profiles in atmospheric laminar diffusion flames measured by mid-infrared direct absorption spectroscopy at 4.2 μm" published in *Applied Physics B-Lasers and Optics*, Volume 124, Issue 4, proposed a method for accurately diagnosing the temperature and concentration distribution of CO2 in ethylene diffusion flames by measuring vibrational fundamentals using direct absorption spectroscopy. In 2015, the paper "Research on Open-Path Detection for Atmospheric Trace Gas CO Based on TDLAS," published in Volume 42, Issue 2 of *Chinese Journal of Lasers*, constructed a long-path open-path atmospheric CO monitoring system based on tunable semiconductor laser absorption spectroscopy. Using direct absorption spectroscopy, it obtained results consistent with those obtained by a CO point analyzer. In 2023, the paper "Preliminary experimental study on combustion characteristics in a solid rocket motor nozzle based on the TDLAS system," published in Volume 268 of *Energy*, conducted a preliminary experimental study on the combustion of solid rocket motor nozzles using direct absorption spectroscopy. It compared the chemical reaction mechanism in the nozzle with numerical simulation results and analyzed the flow characteristics in the nozzle, providing a reference for improving the design level of solid rocket motors. Although the direct absorption spectroscopy method has a simple measurement process, it is susceptible to noise, which can lead to inaccurate extraction of the absorbance parameter. Industrial measurement applications require rapid data acquisition and processing to achieve real-time temperature measurement, thus requiring high parameter extraction speed.

[0005] Against this background, this paper proposes a temperature measurement method based on low-speed sampling parameter identification of laser absorption spectroscopy, which enables rapid extraction of laser absorption spectral parameters under low-speed sampling. By adding a Butterworth low-pass filter to the direct absorption spectroscopy method, utilizing its maximum flatness in the passband, the signal carrying the absorption spectral information of the measured gas is filtered out of high-frequency noise at a suitable cutoff frequency, while still retaining the complete absorption spectral line shape. This simultaneously reduces the sampling rate requirements of the measurement system and the data storage cost. Based on a physical model, the spectral absorbance is identified, improving the speed of absorption information extraction from the laser absorption spectrum, increasing temperature measurement efficiency, and facilitating the miniaturization and integration of the measurement system. (III) Summary of the Invention

[0006] This paper presents a temperature measurement method based on low-speed sampling parameter identification of laser absorption spectroscopy, belonging to the field of tunable semiconductor laser absorption spectroscopy technology. This method introduces a Butterworth low-pass filter on the basis of direct absorption spectroscopy. By filtering the measured light intensity signal, high-frequency components are removed while retaining useful absorption spectral information. The method can meet the measurement requirements with a lower sampling rate and achieve rapid extraction of spectral parameters under low-speed sampling conditions, thereby improving the overall efficiency of temperature measurement.

[0007] The implementation device includes: a tunable semiconductor laser, a beam splitter, a collimating lens, an etalon, a photodetector, a Butterworth low-pass filter, a data acquisition module, and a computer.

[0008] The technical solution adopted in this invention is as follows: Tunable semiconductor lasers 101 and 102 are modulated by sawtooth waves to completely scan different absorption spectral lines of the gas under test 106. The laser output is split into two paths by a beam splitter 103. One laser beam is input to a standard etalon 105 to calibrate the relative wavenumber of the output modulated laser signal. The other beam is collimated by a collimating lens 104 and passes through the gas under test 106 to obtain the spectral information of the gas in the optical path. This information is then detected by a photodetector 107 and converted into a voltage signal output. The voltage signal is then sampled at low speed after passing through a Butterworth low-pass filter 109. The absorption spectral information of the gas under test at two different wavelengths is measured, and the integrated absorption area of ​​the gas under test 106 at the corresponding absorption spectral lines is extracted. The temperature value of the gas under test 106 can then be calculated using a colorimetric method.

[0009] The specific implementation steps are as follows:

[0010] Step 1: Use sawtooth wave modulation to tunable semiconductor lasers 101 and 102 to output laser beams that completely scan different absorption lines of the gas 106 being measured.

[0011] Step 2: After the laser is split by the beam splitter 103, two laser beams are obtained. One laser beam is connected to the etalon 105 and output to the photodetector 108, where it is converted into a voltage signal. The data acquisition module 110 performs high-speed acquisition separately to obtain the relative wavenumber change information of the laser. The other laser beam is connected to the collimating lens 104, collimated, passes through the gas to be measured 106, and is detected by the photodetector 107 to obtain an electrical signal with the absorption information of the gas to be measured 106.

[0012] Step 3: The electrical signal carrying absorption information is passed to a Butterworth low-pass filter 109 with a cutoff frequency 40 times the laser scanning frequency. The output signal is then sampled at low speed by the data acquisition module 110 to obtain the measured light intensity signal I. t ;

[0013] Step 4: Select the light intensity signal I to measure t The original light intensity signals I0 and I0 are obtained by polynomial fitting of the non-absorbing portions at the beginning and end of the middle section. t The relationship with I0 can be expressed as:

[0014] I t (t)=I0(t)exp(-α) (1)

[0015] Where α is the absorption rate of the gas 106 being measured;

[0016] Step 5: Obtain the transfer function of the Butterworth low-pass filter (109) based on its parameters. The general expression is:

[0017]

[0018] Where N is the order of the Butterworth low-pass filter 109, b0, b1…b N-1 The coefficients of different orders in the denominator are a0, a1…a N-1 These are the coefficients for different orders of the molecule;

[0019] Step Six: Obtain the expression for the output signal of the Butterworth low-pass filter 109, which can be represented in the frequency domain as:

[0020]

[0021] Step 7: Obtain the time-domain expression of the output signal of the Butterworth low-pass filter 109, which can be expressed as:

[0022] O(t)=L -1 {O(s)}=L -1 {H(s)·I t (s)}=h(t)*I t (t)=h(t)*[I0(t)exp{-α}] (4)

[0023] Where h(t) is the time-domain expression of the Butterworth low-pass filter 109, which can be obtained by performing an inverse Laplace transform on expression (2);

[0024] Step 8: Calculate the absorptivity parameter α in the low-speed sampling data using expression (4), perform linear fitting on the absorptivity to obtain the integral absorption area, and use colorimetry to obtain the temperature of the gas 106 being measured.

[0025] The advantages of this invention are: 1. A direct absorption spectroscopy method with a low-pass filter model is constructed. Before the detection signal is acquired, high-frequency noise is filtered out using a Butterworth low-pass filter, which improves the noise immunity; 2. The Butterworth low-pass filter can prevent spectral aliasing caused by low-speed sampling without destroying spectral information, reducing the sampling rate requirements of the measurement system; 3. The integral absorption area parameter of the absorption spectrum under low data volume is extracted using an actual physical model, which reduces the temperature calculation time and improves the measurement efficiency. (iv) Description of the attached drawings

[0026] Figure 1 This invention relates to a typical structural schematic diagram of a temperature measurement method based on low-speed sampling parameter identification of laser absorption spectrum.

[0027] Attached icon

[0028] 101. Laser; 102. Laser; 103. Beam splitter; 104. Collimating lens; 105. Ereba.

[0029] 106. Measured gas; 107. Photodetector; 108. Photodetector; 109. Butterworth low-pass filter.

[0030] 110. Data acquisition module; 111. Computer

[0031] Figure 2 This is a comparison chart of the temperature calculation results of this patented method and the traditional direct absorption spectroscopy method.

[0032] Figure 3 This is a comparison chart of the temperature calculation time of this patented method and the traditional direct absorption spectroscopy method. (V) Detailed Implementation

[0033] The invention will be further explained below with reference to specific implementation examples.

[0034] The example was conducted under the following conditions: the measured gas 106 was water vapor with a uniform temperature distribution, the pressure was 1 standard atmosphere, the water vapor temperature varied from 308.15 K to 358.15 K in 10 K increments, for a total of 6 temperature points, and two tunable semiconductor lasers 101 and 102 were used at a center wavenumber of 7185.6 cm⁻¹. -1 and 7444.4cm -1 The scan frequency is 1 kHz, which can cover water vapor at a depth of 7185.6 cm. -1 and 7444.4cm -1 For the absorption spectrum line at a given location, the Butterworth low-pass filter 109 is an 8th-order Butterworth low-pass filter with a cutoff frequency 40 times the laser scanning frequency (40kHz). The sampling frequency of the measurement path is 20 times the cutoff frequency of the Butterworth low-pass filter 109 (800kHz). The specific steps include:

[0035] Step 1: Using sawtooth wave modulation, two tunable semiconductor lasers 101 and 102 emit center wavenumbers of 7185.6 cm⁻¹. -1 and 7444.4cm -1 Scanning laser;

[0036] Step 2: After the laser is split by the beam splitter 103, two laser beams are obtained. One laser beam is connected to the etalon 105 and output to the photodetector 108, where it is converted into a voltage signal. The data acquisition module 110 performs high-speed acquisition separately to obtain the relative wavenumber change information of the laser. The other laser beam is connected to the collimating lens 104, collimated, passes through the gas to be measured 106, and is detected by the photodetector 107 to obtain a light intensity signal with the absorption information of the gas to be measured 106.

[0037] Step 3: The light intensity signal with absorption information is connected to the Butterworth low-pass filter 109, and the output signal is sampled at low speed by the data acquisition module 110.

[0038] Step 4: Select the light intensity signal I to measure t The original light intensity signals I0 and I0 are obtained by polynomial fitting of the non-absorbing portions at the beginning and end of the middle section. t The relationship with I0 can be expressed as:

[0039] I t (t)=I0(t)exp(-α) (1)

[0040] Where α is the absorption rate of the gas 106 being measured;

[0041] Step 5: Obtain the transfer function of the Butterworth low-pass filter 109 based on its parameters. The normalized expression for the 8th-order Butterworth filter is:

[0042]

[0043] Step Six: Obtain the expression for the output signal of the Butterworth low-pass filter 109, which can be represented in the frequency domain as:

[0044]

[0045] Step 7: Obtain the time-domain expression of the output signal of the Butterworth low-pass filter 109, which can be expressed as:

[0046] O(t)=L -1 {O(s)}=L -1 {H(s)·I t (s)}=h(t)*I t (t)=h(t)*[I0(t)exp{-α}] (4)

[0047] Where h(t) is the time-domain expression of the Butterworth low-pass filter 109, which can be obtained by performing an inverse Laplace transform on expression (2);

[0048] Step 8: Calculate the absorptivity parameter α in the low-speed sampling data using expression (4), and perform linear fitting on the absorptivity to obtain the absorptivity of the measured gas 106 at 7185.6 cm⁻¹. -1 and 7444.4cm -1 The temperature of the gas 106 was calculated by colorimetry based on the integral absorption area at that point. A comparison of the temperature obtained by the method proposed in this patent with the temperature obtained by the traditional direct absorption spectroscopy method at a sampling frequency of 20 MHz is shown in the attached figure. Figure 2 As shown, the temperature measurement results obtained by the method proposed in this patent are consistent with those obtained by the traditional direct absorption spectroscopy method at a sampling frequency of 20 MHz, and the relative error with the set temperature is within 1%. However, the sampling rate used by the method in this patent is only 1 / 25 of that of the traditional direct absorption spectroscopy method. The method proposed in this patent has a significant advantage in terms of calculation time, as shown in the attached figure. Figure 3 As shown, under the same hardware conditions, 100 sets of samples were calculated respectively. The method proposed in this patent has a faster calculation speed and improves the efficiency of temperature measurement.

[0049] 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 temperature measurement method based on low-speed sampling parameter identification of laser absorption spectrum, comprising a tunable semiconductor laser, a beam splitter, a collimating lens, an etalon, a photodetector, a Butterworth low-pass filter, a data acquisition module, and a computer. The method is characterized in that the laser emitted by sawtooth-wave modulated tunable semiconductor laser one (101) and tunable semiconductor laser two (102) is split into two paths after passing through a beam splitter (103). One path of the laser is directly received by a photodetector (108) after passing through an etalon (105) and converted into a voltage signal. The data acquisition module (110) performs high-speed sampling on this signal separately to accurately record the relative wavenumber change information of the laser. The other path of the laser... After collimation by the collimating lens (104), the lens passes through one end of the gas to be measured (106). The photodetector (107) detects the gas to be measured (106) at the other end and converts it into a voltage signal. The voltage signal is connected to a Butterworth low-pass filter (109). The data acquisition module (110) performs low-speed sampling on the output signal of the Butterworth low-pass filter to obtain low-speed sampling data containing the absorption spectrum. The acquired data is uploaded to the computer (111). Based on the physical model of the Butterworth low-pass filter (109), the absorption rate parameter is identified, the integral absorption area of ​​the gas to be measured (106) at a specific absorption line is extracted, and the temperature of the gas to be measured (106) is obtained by colorimetry.

2. The temperature measurement method based on low-speed sampling parameter identification of laser absorption spectrum according to claim 1, characterized in that... By adding a Butterworth low-pass filter (109), and utilizing its flatness characteristics within the passband, the signal carrying the absorption spectrum information of the gas under test (106) is filtered out of high-frequency noise, while still retaining the complete absorption spectrum line shape. This allows for low-speed sampling without spectral aliasing. The process includes the following steps: Step 1: Using sawtooth wave modulation, tunable semiconductor laser one (101) and tunable semiconductor laser two (102) are used to output laser beams that completely scan the different absorption lines of the gas under test (106). Step 2: After the laser is split by the beam splitter (103), two laser beams are obtained. One laser beam is connected to the etalon (105) and output to the photodetector (108) to be converted into a voltage signal. The data acquisition module (110) is used to collect the relative wavenumber change information of the laser beam at high speed. The other laser beam is connected to the collimating lens (104) for collimation and passes through the gas to be measured (106). It is then detected by the photodetector (107) to obtain an electrical signal with the absorption information of the gas to be measured (106). Step 3: The electrical signal with absorption information is connected to a Butterworth low-pass filter (109) with a cutoff frequency 40 times that of the laser scanning frequency to ensure the integrity of the absorption spectrum. The output signal is sampled at low speed through the data acquisition module (110). Step 4: Combine the relative wavenumber change information, identify the absorptivity parameters based on the physical model, extract the integral absorption area of ​​the gas (106) at a specific absorption line, and obtain the temperature of the gas (106) using colorimetry.

3. The temperature measurement method based on low-speed sampling parameter identification of laser absorption spectrum according to claim 1, characterized in that... The integrated absorption area of ​​the gas (10⁶) under low-speed sampling was extracted according to the following steps: Step 1: Select the light intensity signal to be measured The original light intensity signal was obtained by polynomial fitting of the non-absorption portions at the beginning and end of the middle section. , and The relationship can be represented as: in, The absorbance of the gas being measured (106) is denoted as . Step 2: Obtain the transfer function of the Butterworth low-pass filter (109) based on its parameters. The general expression is: in, Let be the order of the Butterworth low-pass filter (109). , … The coefficients of different orders in the denominator are... , … These are the coefficients for different orders of the molecule; Step 3: Obtain the expression for the output signal of the Butterworth low-pass filter (109), which can be expressed in the frequency domain as: Step 4: Obtain the time-domain expression of the output signal of the Butterworth low-pass filter (109), which can be expressed as: in, The time-domain expression of the Butterworth low-pass filter (109) can be obtained by performing an inverse Laplace transform on expression (2); Step 5: Calculate the absorption rate parameter in the low-speed sampling data using expression (4). The integral absorption area is obtained by linear fitting of the absorption rate.

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