CN109270027A - A kind of gas absorptivity On-line Measuring Method based on the fitting of Sine Modulated time domain - Google Patents

Abstract

A kind of gas absorptivity On-line Measuring Method based on the fitting of Sine Modulated time domain, belongs to tunable diode laser absorption spectroscopy (TDLAS) technical field.This method is modulated semiconductor laser with tunable electric current using sinusoidal signal, while realizing laser wavelength Sine Modulated, it is ensured that incident intensity laser intensity (baseline) has clear expression formula；By converting absorptivity between frequency domain and time domain, in conjunction with specific baseline expression formula, baseline on-line measurement synchronous with gas absorptivity is realized by being fitted in the time domain to transmission laser intensity.This method is easy to operate, it efficiently solves traditional directly absorption process (triangle or serrated signal scanning) baseline fitting and is affected by human factors the problems such as big, Optical thin film precision is low, temporal resolution is low, data processing complex, it is contemplated that measurement means can be provided for gas parameter on-line monitoring in line parameters high-precision calibrating, complex industrial scene.

Description

On-line measurement method of gas absorption rate based on sinusoidal modulation time domain fitting

Technical field

The invention relates to an online measuring method for gas absorption rate, in particular to an absorption rate measuring method based on a tunable diode laser absorption spectrum direct absorption method, belonging to the technical field of laser spectroscopy and gas measurement.

Background technique

Tunable diode laser absorption spectroscopy (TDLAS) has developed into one of the main methods for high-precision gas parameter diagnosis due to its non-contact, high measurement sensitivity and strong anti-interference ability. The method uses a narrow-band tunable semiconductor laser to scan the molecular absorption line, and combines Beer-Lambert's law to calculate the molecular absorption rate, and then determine the gas temperature, concentration, pressure, spectral line physical parameters and other information.

After years of development, TDLAS has formed two main measurement methods: direct absorption and wavelength modulation. The direct absorption method uses low-frequency triangular waves or sawtooth waves to scan molecular absorption lines, and directly calculates the molecular absorption rate by the relationship between incident light and transmitted light in combination with Beer-Lambert's law. The physical concept is clear and the operation is simple. The wavelength modulation method loads high-frequency sinusoidal modulation on the basis of low-frequency scanning, and effectively reduces noise interference such as particulate matter and laser intensity fluctuation in the experiment through harmonic detection, which has high sensitivity and good robustness, and is in a harsh environment such as a complex industrial site. Has a significant advantage. Although the wavelength modulation method has been widely used in gas temperature and concentration measurement by using harmonic detection technology and calibration experiments, the direct absorption method plays an important role in TDLAS technology because it can directly measure the molecular absorption rate and calibration-free.

In the direct absorption method, accurate measurement of the incident laser (baseline) intensity is the key to accurately measure the absorptivity measurement. The traditional direct absorption method generally uses a polynomial to fit the non-absorbed region on both sides of the transmitted laser intensity to obtain a baseline expression, and then according to The Beer-Lambert law and the calibration of the laser wavelength give the absorption rate. The method has the following problems: (1) Since the high frequency components of the triangular wave or the sawtooth wave have high bandwidth requirements on the detection system, the scanning frequency is usually limited to 100 Hz to 10 kHz, which seriously restricts the time resolution of the method; Due to factors such as spectral line broadening, adjacent spectral line interference, and limited laser wavelength scanning range, non-absorbed regions are difficult to obtain in actual measurement. For example, in normal pressure environment, collision broadening is dominant, even at the center of the absorption line. There is also a certain absorption at a wavelength of ten times and a half height, and its absorption intensity is about one percent of the peak value. Therefore, there is a large uncertainty in fitting the baseline through a region with no absorption or absorption. For example, under 1% (peak) weak absorption, a 1% baseline fitting error may result in a 100% measurement error.

Taking into account the measurement error caused by the uncertainty of baseline fitting, researchers often use the following methods to reduce the baseline error: (1) measure the transmitted laser intensity without absorption and absorption by time-sharing, respectively, without absorption The measured laser intensity is used as a baseline. The method is suitable for environments with closed chambers, such as laboratory line parameter calibration, but is incapable of measuring open space, such as combustion diagnosis, smoke analysis, etc. Eliminate the effects of laser intensity fluctuations, and the light path will also change slightly. (2) Measuring the reference laser intensity by beam splitting and using it as a baseline, the method is suitable for the near-infrared measurement of mature fiber optic technology, and the mid-infrared free-space output laser is complicated by the beam splitting measurement system; in addition, the beam splitting measurement is easy to cause The optical path interferes, and at the same time, the reference light does not strictly represent the baseline due to the difference between the reference optical path and the measuring optical path, thereby causing measurement errors.

Summary of the invention

In order to solve the problem of large uncertainty of baseline fitting of direct absorption method and low time resolution, the present invention provides an on-line measurement method of gas absorption rate based on sinusoidal modulation time domain fitting to further improve gas absorption rate measurement accuracy and simplify measurement. process. The technical solution of the present invention is as follows:

1) determining the spectral center wavelength v _{0 of} the spectral line in the spectral database for the spectral line to be tested;

2) Adjusting the laser controller center current and temperature control tunable semiconductor laser output laser wavelength near v ₀ , generating a sinusoidal signal of frequency ω through the signal generator, inputting to the laser controller for modulating the laser controller Outputting a current, which in turn modulates the wavelength of the output laser of the tunable semiconductor laser;

3) The laser generated by the tunable semiconductor laser is divided into two paths by the fiber splitter, one passes through the collimator and passes through the gas chamber to be tested, and receives the transmitted laser intensity through the first photodetector; the other laser enters the interferometer The second photodetector detects the intensity of the exiting laser of the interferometer, and the first and second photodetectors convert the optical signal into an electrical signal and transmit it to the computer 10;

4) Let the tunable semiconductor laser 3 output the laser instantaneous wavelength as:

The signal collected by the second photodetector 8 is fitted by the Matlab program to obtain the coefficient in the formula (1).

a ₁ , ω, η ₁ , a ₂ , η ₂ ; where t is the detection time, For the laser center wavelength, a ₁ and a ₂ are linear and nonlinear wavelength modulation amplitudes, respectively, and η ₁ and η ₂ are linear and nonlinear wavelength modulation phase angles, respectively;

5) The instantaneous incident laser intensity before entering the gas chamber 5 is:

In the formula: For the average laser intensity, i ₁ and i ₂ are the linear and nonlinear optical emphasis amplitudes, and θ ₁ and θ ₂ are the linear and nonlinear optical emphasis phase angles, respectively;

6) According to Beer-Lambert's law, the instantaneous transmitted laser intensity through the gas chamber 5 is:

Where α(v) is the absorption rate of the line to be measured, and A is the integral area. For the linear function of the line to be measured, it is represented by the Raution function, which is determined by the Gaussian line width γ _{D of} the line to be measured, the Lorentz line width γ _L and the Dick convergence coefficient β;

7) Substituting the formulas (1) and (2) into the formula (3), the relationship between the instantaneous transmitted laser intensity and the detection time t is obtained, by which the second photodetector is applied by the FIT function in the Matlab program. The received instantaneous transmitted laser intensity is fitted in the time domain to obtain the instantaneous incident laser intensity I ₀ , the integral area A, the Gauss line width γ _{D of} the line to be measured, the Lorentz line width γ _L and the Dick convergence coefficient β;

8) Substituting the Gaussian line width γ _D , Lorentz line width γ _L and Dick convergence coefficient β of the line to be measured into the Raution function to calculate the linear function of the line to be measured Combine the integral area A obtained in 7), using the formula Calculate the line absorption rate α(v) of the line to be measured.

The invention has the following advantages and outstanding technical effects: the method of the present invention is directed to the problem of the direct absorption method of the conventional triangular wave or sawtooth modulation, and proposes an absorption rate measurement method based on sinusoidal modulation time domain fitting, which is not only advantageous. The modulation frequency is improved, and the problem that the baseline fitting in the traditional direct absorption method is greatly influenced by human factors, the fitting uncertainty is large, and the absorption rate measurement result is greatly affected, and the data processing repeatability is very high. , can realize automatic processing of software.

DRAWINGS

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a structural schematic view of a gas absorption rate measuring system of the present invention.

2 is a graph showing the instantaneous transmitted laser intensity signal and the interferometer signal obtained by the experimental measurement of the present invention, and the wavelength calibration result obtained by the interferometer signal.

Figure 3 is a graph showing the instantaneous incident laser intensity I ₀ and the instantaneous transmitted laser intensity I _t and the instantaneous transmitted laser intensity fitting residual obtained by fitting in the time domain by the present invention.

Fig. 4 is a result of optimal fitting of the absorption rate and the Raution function obtained by the present invention.

In the figure: 1-signal generator; 2-laser controller; 3-tunable semiconductor laser; 4-fiber fractional device; 5-air chamber; 6-first photodetector; 7-interferometer; Photodetector; 9-oscilloscope; 10-computer.

Detailed ways

The invention will now be further described with reference to the accompanying drawings.

1 is a schematic structural view of a gas absorption rate measuring system of the present invention, which includes a signal generator 1, a laser controller 2, a tunable semiconductor laser 3, a fiber fractional device 4, a gas chamber 5, and a first photodetector 6 The interferometer 7, the second photodetector 8, the oscilloscope 9, and the computer 10. The signal generator 1 generates a sinusoidal signal of frequency ω, which is input to the laser controller 2 for modulating the output current of the laser controller 2, thereby modulating the wavelength of the laser output from the tunable semiconductor laser 3; and generating the tunable semiconductor laser 3 The laser is split into two paths by the fiber splitter 4, one passes through the collimator and passes through the gas chamber 5, receives the transmitted laser intensity through the first photodetector 6; the other laser enters the interferometer 7), passes through the second photodetection The detector 8 detects the intensity of the exiting laser light of the interferometer 7, and the first and second photodetectors convert the optical signal into an electrical signal which is transmitted to the computer 10 via the oscilloscope 9.

Based on the above measurement system, the present invention provides an on-line measurement method of gas absorption rate based on laser absorption spectrum, which comprises the following steps:

1) determining the spectral center wavelength v _{0 of} the spectral line in the spectral database for the spectral line to be tested;

2) By adjusting the center current and temperature of the laser controller 2, the wavelength of the output laser of the tunable semiconductor laser 3 is near v ₀ , and a sinusoidal signal of frequency ω is generated by the signal generator 1 and input to the laser controller 2 for modulation The output current of the laser controller 2, which in turn modulates the wavelength of the laser output from the tunable semiconductor laser 3;

3) The laser light generated by the tunable semiconductor laser 3 is split into two paths through the fiber splitter 4, one pass through the quasi-straight and then through the gas chamber 5 to be tested, and the first photodetector 6 receives the transmitted laser intensity; the other laser Injection into the interferometer 7, detecting the intensity of the exiting laser light of the interferometer 7 through the second photodetector 8, the first and second detectors converting the optical signal into an electrical signal to the computer 10; the second photodetector 8 detects The signal is shown in Fig. 2 “·”; in Fig. 2, “o” is the wavelength calibration point, and its coordinates are defined as follows: the peak time of “2” in Fig. 2 is recorded as the abscissa of the wavelength calibration point, and the interferometer is free according to the experiment. The FSR value of the spectral region defines the ordinate of each wavelength calibration point, that is, the relative wavelength, as 1×FSR, 2×FSR,...,(n-1)×FSR, n×FSR,(n-1)×FSR. ..., 2 × FSR, 1 × FSR, where n is the number of "·" in a half cycle, and each wavelength point is drawn at time-relative wavelength coordinates, as shown by "o" in Fig. 2;

4) Let the tunable semiconductor laser 3 output the laser instantaneous wavelength as:

Where t is the detection time, For the laser center wavelength, a ₁ , a ₂ are linear and nonlinear wavelength modulation amplitudes, η ₁ , η ₂ are linear and nonlinear wavelength modulation phase angles respectively, for the wavelength calibration point in 3), using Matlab program Get the coefficient in formula (1) a ₁ , ω, η ₁ , a ₂ , η ₂ ;

5) The instantaneous incident laser intensity before entering the gas chamber 5 is:

In the middle For the average laser intensity, i ₁ and i ₂ are the linear and nonlinear optical emphasis amplitudes, and θ ₁ and θ ₂ are the linear and nonlinear optical emphasis phase angles, respectively;

6) According to Beer-Lambert's law, the instantaneous transmitted laser intensity through the gas chamber 5 is:

Where α(v) is the absorption rate of the line to be measured, and A is the integral area. For the line type function to be tested, it is represented by the Raution function, which is defined as:

It is determined by the Gaussian line width γ _{D of} the line to be measured, the Lorentz line width γ _L and the Dick convergence coefficient β; x, y, z are intermediate variables, k is an integral form variable; the line-type function to be tested is divided by Raution In addition, Voigt and Galatry functions can also be used;

7) Substituting the formulas (1) and (2) into the formula (3), the relationship between the instantaneous transmitted laser intensity and the detection time t is obtained, and the instantaneous transmitted laser intensity is composed of I ₀ , γ _D , γ _L , β and A It is decided to use the relationship to fit the instantaneous transmitted laser intensity received by the second photodetector 8 in the time domain by the FIT function in the Matlab program, and obtain the instantaneous incident laser intensity I ₀ , the integral area A, the spectrum to be measured. Line Gaussian line width γ _D , Lorentz line width γ _L and Dick convergence coefficient β;

8) Substituting the Gaussian linewidth γ _D , Lorentz linewidth γ _L and Dick convergence coefficient β of the line to be tested into the formula (4) Raution function, and calculating the linear function of the line to be tested Combine the integral area A obtained in 7), using the formula Calculate the line absorption rate α(v) of the line to be measured.

Example:

1) The example of measuring the R (11) line of CO as an example, measuring its absorption rate, and selecting its center wavelength v ₀ = 4300.699 cm ^-1 from the spectral database;

2) Control the wavelength of the output laser of the tunable semiconductor laser 3 by adjusting the center current of the laser controller 2 at a current of 100 mA and a temperature of 32.07 ° C at a wavelength of v ₀ = 4300.699 cm ^-1 , and generate a frequency ω = 2 × π × 100 by a signal generator. a sinusoidal signal, which is input to the laser controller 2 for modulating the output current of the laser controller 2, thereby modulating the wavelength of the laser output from the tunable semiconductor laser 3;

3) The laser light generated by the tunable semiconductor laser 3 is split into two paths through the fiber splitter 4, one pass through the quasi-straight and then through the gas chamber 5 to be tested, and the first photodetector 6 receives the transmitted laser intensity; the other laser Injecting into the interferometer 7, detecting the intensity of the exiting laser of the interferometer 7 through the second photodetector 8, the first and second detectors converting the optical signal into an electrical signal and transmitting it to the computer 10; the interferometer signal and the transient transmitted laser obtained experimentally The intensity signals are shown as “·” and “▲” in Figure 2 respectively. In Figure 2, “o” is the wavelength calibration point; the peak time of “·” is recorded as the abscissa of the wavelength calibration point, due to the selected interference. The free spectral region (FSR) of the instrument is 0.05 cm ^-1 , and the ordinates of the calibration points of each wavelength are sequentially defined as 0.05, 0.1, 0.15, ..., 1.15, 1.2, 1.15, ... 0.1, 0.05, and are extracted at time-relative wavelength coordinates. Draw the calibration points of each wavelength, as shown by “o” in Figure 2;

4) Let the tunable semiconductor laser 3 output the laser instantaneous wavelength as:

Where t is the detection time, For the laser center wavelength, a ₁ , a ₁ are linear and nonlinear wavelength modulation amplitudes, η ₁ , η ₂ are linear and nonlinear wavelength modulation phase angles, for the wavelength calibration point shown in Figure 2 “o”, using Matlab Program fitting to obtain the coefficients in equation (1) a ₁ =0.598, η ₁ =0.421π, a ₂ =-0.138, η ₂ =0.082π, the fitting result is shown by the solid black line in Fig. 2, and the wavelength fitting residual is shown in the lower part of Fig. 2;

5) The instantaneous incident laser intensity before entering the gas chamber 5 is:

In the middle For the average laser intensity, i ₁ and i ₂ are the linear and nonlinear optical emphasis amplitudes, and θ ₁ and θ ₂ are the linear and nonlinear optical emphasis phase angles, respectively;

6) According to Beer-Lambert's law, the instantaneous transmitted laser intensity through the gas chamber 5 is:

Where α(v) is the absorption rate of the line to be measured, and A is the integral area. For the spectral line type function to be tested, it is represented by the Raution function;

7) Substituting the formulas (1) and (2) into the formula (3), the relationship between the instantaneous transmitted laser intensity and the detection time t is obtained, and the instantaneous transmitted laser intensity is composed of I ₀ , γ _D , γ _L , β and A It is decided that the instantaneous transmission laser intensity received by the second photodetector 8 is fitted in the time domain by using the FIT function in the Matlab program, and the integral area A=0.0361 is obtained, and the Gaussian line width of the spectrum to be measured is γ. _D = 5.0 × 10 ^-3 cm ^-1 , Lorentz linewidth γ _L = 5.77 × 10 ^-3 cm ^-1 and Dick convergence coefficient β = 0.02, instantaneous incident laser intensity

I ₀ =1.635+0.2798cos(2π×100t+1.379π)-0.00847cos[(4π×100t+0.885π)]; wherein the best fits I ₀ and I _{t are} as shown in Fig. 3, wherein the lower part of Fig. 3 is Instantaneous transmission laser intensity fitting residual;

8) Substituting the Gaussian linewidth γ _D , Lorentz linewidth γ _L and Dick convergence coefficient β of the line to be tested into the formula (4) Raution function, and calculating the linear function of the line to be tested Combine the integral area A obtained in 7), using the formula Calculate the absorbance of the line to be measured α (v), as shown in Figure 4.

Claims (1)

1. a kind of gas absorptivity On-line Measuring Method based on the fitting of Sine Modulated time domain, it is characterized in that this method includes as follows Step:

1) its core wavelength v is determined in spectra database for spectral line to be measured₀；

2) laser is exported by adjusting laser controller (2) central current and temperature control semiconductor laser with tunable (3) Wavelength is in v₀Near, the sinusoidal signal that frequency is ω is generated by signal generator (1), inputs to laser controller (2), to The output electric current of laser controller (2) is modulated, and then modulates the wavelength of semiconductor laser with tunable (3) output laser；

3) laser that semiconductor laser with tunable (3) generate is divided into two-way by fiber optic splitter (4), all the way by collimation Gas chamber to be measured (5) are passed through afterwards, receive transmission laser intensity by the first photodetector (6)；Another way laser injects interferometer (7), by the shoot laser intensity of the second photodetector (8) detection interferometer (7), the first and second photodetectors are by light Signal is converted to electric signal and is passed to computer (10)；

4) enable semiconductor laser with tunable (3) output laser transient wave long are as follows:

To the second photodetector (8) collected signal, it is fitted to obtain the coefficient in formula (1) using Matlab programa₁, ω,η₁,a₂,η₂；Wherein t is detection time,For laser center wavelength, a₁、a₂Respectively linear and nonlinear wavelength modulated amplitude Value, η₁、η₂Respectively linear and nonlinear wavelength phase modulation angle；

5) it enables and is incident on the instantaneous incident laser intensity before gas chamber (5) are as follows:

In formula:For laser intensity average value, i₁、i₂Respectively linear and nonlinear intensity modulation amplitude, θ₁、θ₂It is respectively linear With non-linear intensity modulation phase angle；

6) according to Beer Lambert law, the instantaneous transmission laser intensity of gas chamber (5) is passed through are as follows:

In formula: α (v) is line absorption rate to be measured, and A is integral area,For spectral line linear function to be measured, with Raution function It indicates, by spectral line Gauss line width γ to be measured_D, Lorentz line width γ_LAnd Dick convergence coefficient β is determined；

7) formula (1) and (2) are substituted into formula (3), obtains the relationship between instantaneous transmission laser intensity and detection time t Formula swashs the instantaneous transmission that the second photodetector (8) receives using the FIT function in Matlab program by the relational expression Luminous intensity is fitted in the time domain, obtains instantaneous incident laser intensity I₀, integral area A, spectral line Gauss line width γ to be measured_D, Lip river Human relations hereby line width γ_LAnd Dick convergence coefficient β；

8) by spectral line Gauss line width γ to be measured_D, Lorentz line width γ_LAnd Dick convergence coefficient β is substituted into Raution function, is calculated Spectral line linear function to be measuredIn conjunction with gained integral area A in 7), formula is utilizedSpectral line to be measured is calculated to inhale Yield α (v).