CN114295581B - Method and device for detecting gas concentration insensitive to DFB laser wavelength characteristics - Google Patents
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Abstract
The invention relates to the technical field of gas concentration detection. The invention discloses a gas concentration detection method and a device insensitive to DFB laser wavelength characteristics, the invention utilizes the definite mathematical relationship of the integral of a certain section on the horizontal axis under the Lorentz line type and the integral ratio of the whole section on the horizontal axis, thereby accurately obtaining the gas concentration under the incomplete absorbance. The invention can solve the problem of incomplete absorbance distribution in the prior art, reduce the problem of inaccurate measurement concentration caused by incomplete absorbance distribution, improve the accuracy of gas real-time measurement, reduce the performance requirement on the DFB laser, and effectively expand the application range of the direct absorption spectrum technology.
Description
Technical Field
The invention belongs to the field of gas concentration detection based on a direct absorption spectrum technology, and particularly relates to a gas concentration detection method and device insensitive to DFB laser wavelength characteristics.
Background
Tunable laser diode absorption spectroscopy (TDLAS) is widely used for detection of various trace gases in the energy, petrochemical and mining industries, and in addition, for biomedical diagnosis of human diseases. It evolves into two main approaches: direct Absorption Spectroscopy (DAS) and Wavelength Modulation Spectroscopy (WMS). Compared with WMS, DAS has wide applicability, easy operation, and more intuitive data interpretation, with the status of immobility. DAS has been widely used for measuring gas concentrations in the past few decades. However, DAS is susceptible to particle concentration, laser intensity fluctuations, laser wavelength tuning range, gas absorption line broadening mechanism, and transition overlap, which can lead to incomplete absorbance distribution problems during measurement, thus failing to accurately detect gas concentration.
For the above problems, more solutions are adopted at present to avoid the occurrence of the situations, such as changing the tuning range of the laser wavelength to enable the tuning range to cover the absorption spectrum of the gas, but the tuning range of the output wavelength of the laser is affected by the manufacturing process of the center wavelength of the DFB (distributed feedback) chip, the bearing level and the power range, and the problems cannot be fundamentally solved.
Disclosure of Invention
The invention aims to provide a gas concentration detection method and device insensitive to the wavelength characteristics of DFB laser, which are used for solving the technical problems.
In order to achieve the above purpose, the invention adopts the following technical scheme: a gas concentration detection method insensitive to DFB laser wavelength characteristics comprises the following steps:
S1, controlling the laser wavelength of a DFB laser to scan at the center nu 0 of a gas absorption spectrum line, measuring the background signal light intensity I 0 without gas absorption and the transmission signal light intensity I t with gas absorption, and performing time-frequency conversion on the background signal light intensity I 0 and the transmission signal light intensity I t to obtain a change relation I 0 (v) of the background signal light intensity I 0 along with the laser frequency nu and a change relation I t (v) of the transmission signal light intensity I t along with the laser frequency nu;
s2, calculating a corresponding incomplete gas absorbance curve alpha (v) according to the I 0 (v) and the I t (v);
S3, performing Lorentz linear fitting on the alpha (v) to generate fitting absorbance alpha L (v):
wherein, alpha 0 is the peak value of the fitting absorbance, and gamma is the half-width at half-maximum of absorbance;
S4, calculating a fitting absorbance peak value alpha 0, an incomplete side absorbance endpoint alpha L(ν2), an absorbance integral A 1 of the complete side, and then calculating an absorbance integral A part of the incomplete side:
S5, deducing a complete absorbance integral A 2 on the incomplete side:
S6, the integral A of the absorbance of the gas is the sum of A 1 and A 2, and the concentration of the gas is calculated according to the integral A of the absorbance.
Further, in step S1, the output wavelength of the DFB laser is modulated by the triangular wave signal generated by the function signal generator.
Further, in step S1, the relationship between the frequency v and the time t of the laser is obtained by processing the laser of the DFB laser after passing through the etalon, so as to perform time-frequency conversion on the background signal light intensity I 0 and the transmission signal light intensity I t.
In step S1, the light intensity of the DFB laser is detected after one path of the laser passes through the first absorption cell containing the gas to be detected to obtain the light intensity I t of the transmission signal, and the light intensity of the other path of the laser passes through the second absorption cell not containing the gas to be detected to obtain the light intensity I 0 of the background signal.
Furthermore, the laser is collimated by the first collimating lens and the second collimating lens before entering the first absorbing tank and the second absorbing tank.
Further, in step S2, the expression of the corresponding incomplete gas absorbance curve α (v) is calculated according to I 0 (v) and I t (v) as follows
Wherein P is gas pressure, T is gas temperature, S (T) is temperature-dependent line intensity, and phi (v) is gas absorption line function.
Further, in step S6, the formula for calculating the gas concentration χ based on the absorbance integral A is
Where L is the optical path length.
The invention also provides a gas concentration detection device insensitive to the DFB laser wavelength characteristic, which comprises a function signal generator, a DFB laser, an optical fiber beam splitter, a first gas absorption tank containing gas to be detected, a second gas absorption tank not containing gas to be detected, a first photoelectric detector, a second photoelectric detector, an etalon and a signal processing module, wherein the function signal generator is used for modulating the output wavelength of the DFB laser, the laser of the DFB laser is divided into three paths through the optical fiber beam splitter, the first path passes through the first absorption tank and then the first photoelectric detector to detect the light intensity of the first path to obtain the light intensity I t of a transmission signal and output the light intensity I t of the transmission signal to the signal processing module, the second path passes through the second photoelectric detector to detect the light intensity I 0 of the background signal and output the light intensity I 0 of the second path to the signal processing module, the third path passes through the etalon and then outputs the signal processing module, and the signal processing module processes the received signal by adopting the gas concentration detection method insensitive to the DFB laser wavelength characteristic and calculates the gas concentration.
Further, the optical fiber optical system further comprises a first collimating lens and a second collimating lens, wherein the first collimating lens is arranged between the optical fiber beam splitter and the first absorption tank, and the second collimating lens is arranged between the optical fiber beam splitter and the second absorption tank.
Further, the device also comprises a signal acquisition module, wherein the signal acquisition module is used for acquiring signals output by the first photoelectric detector, the second photoelectric detector and the etalon and outputting the acquired signals to the signal processing module.
The beneficial technical effects of the invention are as follows:
The invention can solve the problem of incomplete absorbance distribution in the prior art, reduce the problem of inaccurate measurement concentration caused by incomplete absorbance distribution, improve the real-time measurement accuracy of gas, reduce the performance requirement on a DFB laser, is easy to realize, and can effectively expand the application range of the direct absorption spectrum technology.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings that are needed in the description of the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a flow chart of a method according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a gas concentration detecting apparatus according to an embodiment of the present invention;
FIG. 3 is a graph of background signal intensity and transmission signal intensity in the case of incomplete absorbance distribution according to the invention;
FIG. 4 is a graph of the Lorentzian line-type fit and residual error of the original absorbance distribution with incomplete absorbance distribution according to the invention;
FIG. 5 is a graph of background signal intensity and transmission signal intensity in the case of irregular absorbance distribution according to the invention;
FIG. 6 is a graph of the original absorbance distribution and Lorentzian line fit and residual error for the present invention with incomplete absorbance fractions;
FIG. 7 is a graph showing the relationship between the absorbance integral and the gas concentration at the complete absorbance.
Detailed Description
For further illustration of the various embodiments, the invention is provided with the accompanying drawings. The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate embodiments and together with the description, serve to explain the principles of the embodiments. With reference to these matters, one of ordinary skill in the art will understand other possible embodiments and advantages of the present invention. The components in the figures are not drawn to scale and like reference numerals are generally used to designate like components.
The invention will now be further described with reference to the drawings and detailed description.
As shown in fig. 1, a method for detecting gas concentration insensitive to DFB laser wavelength characteristics includes the steps of:
S1, controlling the laser wavelength of a DFB laser to scan at the center nu 0 of a gas absorption spectrum line, measuring the background signal light intensity I 0 without gas absorption and the transmission signal light intensity I t with gas absorption, and performing time-frequency conversion on the background signal light intensity I 0 and the transmission signal light intensity I t to obtain the change relation I 0 (v) of the background signal light intensity I 0 along with the laser frequency nu and the change relation I t (v) of the transmission signal light intensity I t along with the laser frequency nu.
In this embodiment, the output wavelength of the DFB laser is modulated by the triangular wave signal generated by the function signal generator, so that the laser wavelength of the DFB laser scans at the center v 0 of the gas absorption spectrum line, but the invention is not limited thereto. Preferably, the triangular wave signal is a low frequency signal, which is easy to implement.
In this embodiment, one path of laser light of the DFB laser is passed through a first absorption cell containing the gas to be measured and then detected to obtain the light intensity I t of the transmission signal, and the other path of laser light is passed through a second absorption cell not containing the gas to be measured and then detected to obtain the light intensity I 0 of the background signal, so that the gas absorbance curve can be obtained more accurately.
Furthermore, in this embodiment, before the laser enters the first absorption cell and the second absorption cell, the laser is collimated by the first collimating lens and the second collimating lens, so as to further improve the measurement accuracy.
Preferably, in this embodiment, the relationship between the frequency v and the time t of the laser is obtained by processing the laser of the DFB laser after passing through the etalon, and then according to the relationship between the obtained frequency v and the time t of the laser, the time-frequency conversion is performed on the background signal light intensity I 0 and the transmission signal light intensity I t, so as to obtain the change relationship I 0 (v) of the background signal light intensity I 0 along with the laser frequency v and the change relationship I t (v) of the transmission signal light intensity I t along with the laser frequency v.
The relation between the laser frequency and time measured by the etalon is more visual and accurate, and is favorable for further experimental analysis, but is not limited to the method.
The background signal light intensity I 0 and the transmission signal light intensity I t are converted from the time domain to the frequency domain so as to carry out the measurement of the subsequent second step and later, improve the measurement accuracy and greatly reduce the storage of useless data in the measurement process.
Specifically, the measured background signal light intensity I 0 and the measured transmission signal light intensity I t are time t-varying, that is, are signals in the time domain, and the relationship between the frequency v of the laser and the time t is transformed, so that the relationship between the background signal light intensity I 0 and the transmission signal light intensity I t along with the change of the laser frequency v, that is, the signal in the frequency domain, can be easily realized by those skilled in the art, and the specific transformation process is not described in detail.
S2, calculating a corresponding incomplete gas absorbance curve alpha (v) according to I 0 (v) and I t (v) by adopting a formula (1).
Wherein P is gas pressure, T is gas temperature, S (T) is temperature-dependent line intensity, and phi (v) is gas absorption line function.
S3, performing Lorentz line fitting on the alpha (v) to generate fitting absorbance alpha L (v), wherein the Lorentz line is shown as a formula (2):
Wherein, alpha 0 is the peak value of the fitting absorbance, and gamma is the half-width at half maximum of absorbance.
S4, calculating a fitting absorbance peak value alpha 0, an incomplete side absorbance endpoint alpha L(ν2), an absorbance integral A 1 of the complete side, and then calculating an absorbance integral A part of the incomplete side, as shown in a formula (3)
S5, deducing a complete absorbance integral A 2 of the incomplete side according to the formula (3), as shown in the formula (4)
S6, calculating the gas concentration χ according to the absorbance integral A by adopting the formula 5, wherein the absorbance integral A of the gas is the sum of A 1 and A 2.
Where L is the optical path length.
The formula (5) may be obtained by performing least square fitting on absorbance integrals of different concentrations of the standard gas under the existing experimental conditions, but is not limited thereto.
As shown in fig. 2, the present invention further provides a gas concentration detection device insensitive to the wavelength characteristics of DFB laser, including a function signal generator 1, a DFB laser 2, an optical fiber beam splitter 3, a first gas absorption tank 5 containing a gas to be detected, a second gas absorption tank 8 not containing a gas to be detected, a first photodetector 6, a second photodetector 9, an etalon 10, and a signal processing module 12, where the function signal generator 1 is configured to generate a triangular wave signal to modulate the output wavelength of the DFB laser 2, so that the laser wavelength of the DFB laser scans at a center v 0 of a gas absorption spectrum, the laser of the DFB laser 2 is split into three paths by the optical fiber beam splitter 3, the first path passes through the first absorption tank 5, the first photodetector 6 detects the light intensity thereof to obtain a transmission signal light intensity I t, and outputs the transmission signal light intensity to the signal processing module 12, the second path passes through the second absorption tank 8, the second photodetector 9 detects the light intensity thereof to obtain a background signal I 0, and outputs the background signal to the signal processing module 12, the third path passes through the etalon 10, and outputs the third path to the signal processing module 12, and the light intensity detector 12 receives the light intensity of the laser light, and the laser intensity on the DFB laser receives the signal processing module, and the light intensity by the signal processing module receives the light, and the sensitivity of the gas concentration is insensitive to the wavelength.
Specifically, in this embodiment, the signal processing module 12 stores a signal processing program for acquiring a relationship between the frequency v and the time t of the laser; according to the relation between the frequency v and the time t of the laser, determining a change relation I 0 (v) of the background signal light intensity I 0 along with the frequency v and a change relation I t (v) of the background signal light intensity I t along with the frequency v; realizing Lorentz line fitting; the fitting absorbance peak value alpha 0, the incomplete side absorbance endpoint alpha L(ν2), the complete side absorbance integral A 1, the incomplete side absorbance integral A part and the complete integral absorbance A are automatically calculated, and the gas concentration χ is calculated through a relation between the built-in absorbance integral and the gas concentration.
The invention divides the laser into three beams, and can more accurately acquire the gas concentration in real time.
Further, in this embodiment, the laser measuring device further includes a first collimating lens 4 and a second collimating lens 7, the first collimating lens 4 is disposed between the optical fiber beam splitter 3 and the first absorption cell 5, and the second collimating lens 7 is disposed between the optical fiber beam splitter 3 and the second absorption cell 8, so as to improve the collimation of the laser, and further improve the measurement accuracy.
Further, in this embodiment, the device further includes a signal acquisition module 11, where the signal acquisition module 11 is configured to acquire signals output by the first photodetector 6, the second photodetector 9, and the etalon 10, and then convert an analog signal into a digital signal, and output the digital signal to the signal processing module 12, where the digital signal has strong anti-interference capability, and is easy for the signal processing module 12 to perform calculation processing.
In this embodiment, the first photodetector 6 and the second photodetector 9 may be implemented by a CCD sensor or a CMOS sensor, and the signal processing module 12 may be implemented by a single chip microcomputer, but is not limited thereto.
Example 1
S1, modulating the output wavelength of the DFB laser 2 through a triangular wave signal (bias is 464mV and the amplitude is 100 mV) generated by the function signal generator 1, so that the laser wavelength scans at the center of a gas absorption spectrum line v 0 (1653.7 nm), measuring the background signal light intensity I 0 without gas absorption and the transmission signal light intensity I t with gas absorption, wherein the concentration of the gas to be detected is 15%, and the relationship between the background signal light intensity I 0 and the transmission signal light intensity I t is shown in FIG. 3, and performing time-frequency conversion to obtain the change relationship I 0 (v) of the background signal light intensity I 0 along with the laser frequency v and the change relationship I t (v) of the transmission signal light intensity I t along with the laser frequency v.
S2, calculating a corresponding incomplete gas absorbance curve alpha (v) according to I 0 (v) and I t (v) by adopting a formula (1).
S3, performing Lorentz line fitting on the alpha (v) to generate fitting absorbance alpha L (v), wherein the original absorbance distribution, the absorbance and residual error after fitting are shown in fig. 4, and the Lorentz line is shown in formula (2):
S4, calculating a fitting absorbance peak value alpha 0, an incomplete side absorbance endpoint alpha L(ν2), an absorbance integral A 1 of the complete side, and then calculating an absorbance integral A part of the incomplete side, as shown in a formula (3)
S5, deducing a complete absorbance integral A 2 of the incomplete side according to the formula (3), as shown in the formula (4)
S6, calculating the gas concentration χ according to the absorbance integral A by adopting the formula 5, wherein the absorbance integral A of the gas is the sum of A 1 and A 2.
Whereas the conventional direct absorption method omits steps S4 and S5 (which correspond to the complete gas absorbance curve calculated in step S2), integrates the absorbance distribution in formula (2) directly and then substitutes the integrated absorbance distribution into formula (5).
To verify the measurement results, the triangular wave signal (offset 480mV, amplitude 100 mV) generated by the function signal generator 1 in step S1 is repeated, and the absorbance distribution generated is complete, and in the case of complete absorbance, the background signal light intensity I 0 and the transmission signal light intensity I t when there is gas absorption are shown in fig. 5, and the original absorbance distribution and the absorbance and residual after fitting are shown in fig. 6. The relationship between the absorbance fraction and the gas concentration under the complete absorbance calculated by measurement under the same condition is shown in fig. 7.
The experimental results obtained by the method of the present invention and the results obtained by the conventional direct absorption method are shown in table one under the condition that the bias of the triangular wave signal generated by the function signal generator 1 is changed from 456mV to 464mV, different absorbance is generated and the distribution is incomplete. It can be seen that under the condition of incomplete absorbance distribution, the relative error of the integral deduced by the method provided by the invention and the absorbance integral obtained under the condition of complete absorbance is within 3%, and the lowest error of the direct absorption method is 5.83%, which proves that the method provided by the invention is feasible and can effectively reduce experimental errors.
Table one: incomplete absorbance distribution measurements at 15.00% concentration to varying degrees
The invention can solve the problem of incomplete absorbance distribution in the prior art, reduce the problem of inaccurate measurement concentration caused by incomplete absorbance distribution, improve the real-time measurement accuracy of gas, reduce the performance requirement on a DFB laser, is easy to realize, and can effectively expand the application range of the direct absorption spectrum technology.
While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (10)
1. A method for detecting gas concentration insensitive to DFB laser wavelength characteristics, comprising the steps of:
S1, controlling the laser wavelength of a DFB laser to scan at a center ν 0 of a gas absorption spectrum line, measuring a background signal light intensity I 0 without gas absorption and a transmission signal light intensity I t with gas absorption, and performing time-frequency conversion on the background signal light intensity I 0 and the transmission signal light intensity I t to obtain a change relation I 0 (ν) of the background signal light intensity I 0 along with the laser frequency ν and a change relation I t (ν) of the transmission signal light intensity I t along with the laser frequency ν;
s2, calculating a corresponding incomplete gas absorbance curve alpha (v) according to the I 0 (v) and the I t (v);
S3, performing Lorentz linear fitting on the alpha (v) to generate fitting absorbance alpha L (v):
wherein, alpha 0 is the peak value of the fitting absorbance, and gamma is the half-width at half-maximum of absorbance;
S4, calculating a fitting absorbance peak value alpha 0, an incomplete side absorbance endpoint alpha L(ν2), an absorbance integral A 1 of the complete side, and then calculating an absorbance integral A part of the incomplete side:
S5, deducing a complete absorbance integral A 2 on the incomplete side:
S6, the integral A of the absorbance of the gas is the sum of A 1 and A 2, and the concentration of the gas is calculated according to the integral A of the absorbance.
2. The method for detecting the concentration of gas insensitive to the wavelength characteristics of the DFB laser according to claim 1, wherein in step S1, the output wavelength of the DFB laser is modulated by the triangular wave signal generated by the function signal generator.
3. The method for detecting the concentration of gas insensitive to the wavelength characteristics of DFB laser according to claim 1, wherein in step S1, the relation between the frequency v and the time t of the laser is obtained by processing the laser beam of the DFB laser after passing through an etalon, so as to perform time-frequency conversion on the background signal light intensity I 0 and the transmission signal light intensity I t.
4. The method for detecting the concentration of gas insensitive to the wavelength characteristics of DFB laser according to claim 1, wherein in step S1, the light intensity of the laser beam of the DFB laser is detected after passing through a first absorption cell containing the gas to be detected to obtain the light intensity I t of the transmission signal, and the light intensity of the laser beam of the DFB laser is detected after passing through a second absorption cell containing no gas to be detected to obtain the light intensity I 0 of the background signal.
5. The method of claim 4, wherein the laser light is collimated by a first collimating lens and a second collimating lens before entering the first absorption cell and the second absorption cell, respectively.
6. The method for detecting a gas concentration insensitive to wavelength characteristics of DFB laser according to claim 1, wherein in step S2, the expression of the corresponding incomplete gas absorbance curve α (v) is calculated from I 0 (v) and I t (v) as
Wherein P is gas pressure, T is gas temperature, S (T) is temperature-dependent line intensity, and phi (v) is gas absorption line function.
7. The method for detecting the concentration of gas insensitive to the wavelength characteristics of DFB laser according to claim 1, wherein in step S6, the formula for calculating the concentration χ of the gas based on the absorbance integral a is
Where L is the optical path length.
8. A gas concentration detection apparatus insensitive to DFB laser wavelength characteristics, characterized by: the method comprises the steps of modulating the output wavelength of a DFB laser, dividing the laser of the DFB laser into three paths through the optical fiber beam splitter, detecting the light intensity of the first path through the first absorption tank by the first photoelectric detector to obtain a transmission signal light intensity I t, outputting the transmission signal light intensity I t to the signal processing module, detecting the light intensity of the second path through the second absorption tank by the second photoelectric detector to obtain a background signal light intensity I 0, outputting the background signal light intensity I 0 to the signal processing module, outputting the third path through the etalon to the signal processing module, processing the received signal by the signal concentration detection method insensitive to the wavelength characteristics of the DFB laser, and calculating the gas concentration.
9. The DFB laser wavelength insensitive gas concentration sensing apparatus of claim 8, wherein: the optical fiber optical system further comprises a first collimating lens and a second collimating lens, wherein the first collimating lens is arranged between the optical fiber beam splitter and the first absorption tank, and the second collimating lens is arranged between the optical fiber beam splitter and the second absorption tank.
10. The DFB laser wavelength insensitive gas concentration sensing apparatus of claim 8, wherein: the system also comprises a signal acquisition module, wherein the signal acquisition module is used for acquiring signals output by the first photoelectric detector, the second photoelectric detector and the etalon and outputting the acquired signals to the signal processing module.
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| CN109991189A (en) * | 2019-04-04 | 2019-07-09 | 东南大学 | A kind of fixed point wavelength modulation gas concentration measuring apparatus and its measurement method based on wave number drift correction |
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