CN108931498B - Device and method for simultaneous measurement of gas absorption spectrum and absorption optical path in a multi-pass cell - Google Patents
Device and method for simultaneous measurement of gas absorption spectrum and absorption optical path in a multi-pass cell Download PDFInfo
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Abstract
多通池内气体吸收光谱和吸收光程同步测量的装置和方法，涉及多通池内气体吸收光谱和吸收光程的同步测量技术，为了解决现有的定量气体检测中多通池的光程和气体吸收光谱分开测量且光程的测量精度不高的问题。可调谐光源输出的激光分别入射至辅助干涉仪和主干涉仪；辅助干涉仪用于产生时钟信号，并发送给数据采集卡的时钟端；主干涉仪的光路中包括本征光和测试光，测试光在多通池内传输后与本征光发生拍频相干；数据采集卡用于对主干涉仪产生的拍频相干信号进行采集，并发送给计算模块；计算模块用于根据拍频相干信号计算吸收光程和吸收光谱。本发明适用于同步测量吸收光谱和吸收光程。
A device and method for synchronous measurement of gas absorption spectrum and absorption optical path in a multi-pass cell, involving the simultaneous measurement technology of gas absorption spectrum and absorption optical path in a multi-pass cell. The absorption spectrum is measured separately and the measurement accuracy of the optical path is not high. The laser output from the tunable light source is incident on the auxiliary interferometer and the main interferometer respectively; the auxiliary interferometer is used to generate a clock signal and send it to the clock end of the data acquisition card; the optical path of the main interferometer includes intrinsic light and test light, After the test light is transmitted in the multi-pass cell, the beat frequency coherence occurs with the intrinsic light; the data acquisition card is used to collect the beat frequency coherent signal generated by the main interferometer and send it to the calculation module; the calculation module is used to obtain the beat frequency coherent signal according to the Calculate the absorption path and absorption spectrum. The invention is suitable for simultaneous measurement of absorption spectrum and absorption optical path.
Description
Technical Field
The invention relates to a synchronous measurement technology for gas absorption spectrum and absorption optical path in a multi-pass cell, belonging to the field of laser absorption spectrum and gas sensing.
Background
In laser absorption spectroscopy, multipass cells are widely used to extend the absorption optical path and increase the sensitivity of the measurement, with the most commonly used multipass cells being based on the wytt-type and herriott-type designs. In order to meet the actual different measurement requirements, different types of multipass cells have been developed, such as optical matrix type gas cells, cylindrical mirror combination type gas cells. In order to achieve high compactness of the device, design types such as a chaotic optical multi-pass cavity and a circular multi-reflection air pool have appeared.
For quantitative gas detection, the absorption spectrum and the optical path of the multipass cell must be accurately determined. In the existing detection, the gas absorption spectrum and the absorption optical path are generally measured independently. The absorption path length is calibrated by mechanical measurement, gas absorption spectroscopy, phase shift, or interferometric techniques prior to acquiring the gas absorption spectrum. However, due to structural deformation, especially in long-term measurement under severe environment, the pre-calibration optical path of the multi-pass cell may change, resulting in a decrease in measurement accuracy. On the other hand, in applications requiring different optical depths, although the optical path length of the multipass cell can be changed by adjusting the mirror spacing or adjusting the number of reflections by laser alignment, the process is complex and time consuming.
Disclosure of Invention
The invention aims to solve the problems that the optical path and the gas absorption spectrum of a multi-pass cell in the conventional quantitative gas detection are separately measured and the measurement precision of the optical path is not high, so that the device and the method for synchronously measuring the gas absorption spectrum and the absorption optical path in the multi-pass cell are provided.
The invention relates to a device for synchronously measuring gas absorption spectrum and absorption optical path in a multi-pass cell, which comprises: the system comprises a tunable light source 1, an auxiliary interferometer, a main interferometer, a data acquisition card 10 and a calculation module 11;
laser output by the tunable light source 1 is respectively incident to the auxiliary interferometer and the main interferometer;
the auxiliary interferometer is used for generating a clock signal and sending the clock signal to the clock end of the data acquisition card 10;
the light path of the main interferometer comprises intrinsic light and test light, and the test light is transmitted in the multi-pass cell and then generates beat frequency coherence with the intrinsic light;
the data acquisition card 10 is used for acquiring beat frequency coherent signals generated by the main interferometer and sending the beat frequency coherent signals to the calculation module;
and the calculating module 11 is used for calculating an absorption optical path and an absorption spectrum according to the beat frequency coherent signal.
Preferably, the laser light output by the tunable light source 1 is split into two paths of light by the first coupler 2-1, one path of light is incident to the auxiliary interferometer, and the other path of light is incident to the main interferometer.
Preferably, the auxiliary interferometer comprises a third coupler 2-3, a first Faraday rotator mirror 5-1, a second Faraday rotator mirror 5-2 and an optical fiber 6;
the third coupler 2-3 is a 2 × 2 coupler with a splitting ratio of 50: 50;
one path of light output by the first coupler 2-1 is divided into two paths through the third coupler 2-3, one path of light is reflected by the first Faraday rotating mirror 5-1 and then coupled into the third coupler 2-3, the other path of light is incident to the second Faraday rotating mirror 5-2 through the optical fiber 6 and then coupled into the third coupler 2-3 after being reflected by the second Faraday rotating mirror 5-2, the third coupler 2-3 outputs beat frequency coherent signals, and then square wave signals are generated through the TTL clock generator 9, wherein the square wave signals are clock signals.
Preferably, the main interferometer comprises a second coupler 2-2, a fourth coupler 2-4, a polarization controller 3, a fiber circulator 4 and a balanced detector 8;
the second coupler 2-2 is a 1 x 2 coupler and the fourth coupler 2-4 is a 2 x 2 coupler with a splitting ratio of 50: 50;
the other path of light output by the first coupler 2-1 is divided into two paths of light by the second coupler 2-2, one path of light is incident to an input port of the fourth coupler 2-4 as intrinsic light after passing through the polarization controller 3, the other path of light enters from a port 1 of the optical fiber circulator 4 as test light and is emitted to the multi-pass cell 7 from a port 2 of the optical fiber circulator 4, the reflected light reenters a port 2 of the optical fiber circulator 4 after multi-stage reflection and gas absorption in the multi-pass cell 7 and is coupled into the other input port of the fourth coupler 2-4 from a port 3, the intrinsic light and the test light are subjected to beat frequency coherence and are output to the balance detector 8 from 2 output ends of the fourth coupler 2-4, the output end of the balance detector 8 is connected with the acquisition signal input end of the data acquisition card 10, and the output end of the data acquisition card 10 is connected with the input end of the calculation module 11.
Preferably, the calculating module 11 calculates the absorption optical path and the absorption spectrum according to the beat frequency coherent signal, specifically:
the calculation module 11 performs fourier transform on the received beat frequency coherent signal to realize the transform from a frequency domain spectrum to a time domain spectrum, and obtains an absorption optical path according to an optical path difference between reflection peaks;
and performing inverse Fourier transform on the interested information data segment on the time domain spectrum, squaring the amplitude data to obtain the absorption spectrum intensity information of the gas, and performing normalization processing to obtain a normalized absorption spectrum.
The invention relates to a method for synchronously measuring gas absorption spectrum and absorption optical path in a multi-pass cell, which comprises the following steps:
filling gas to be detected with a certain concentration into a multi-pass tank 7;
starting the linear sweep frequency output of the tunable light source 1, and simultaneously outputting a trigger signal to trigger the data acquisition card 10 to prepare for data acquisition by the tunable light source 1;
thirdly, the auxiliary interferometer generates a clock signal which is transmitted to the data acquisition card 10, so that the data acquisition card 10 performs equal-frequency interval acquisition on the beat frequency signal of the main interferometer;
the test light of the main interferometer is guided into the multi-pass cell 7 from the port 2 of the optical fiber circulator 4, and reflected light enters the optical fiber circulator 4 again after multi-stage reflection and gas absorption in the multi-pass cell 7; the beat-adjusting polarization controller 3 makes the light led out from the port 3 of the optical fiber circulator 4 and the local oscillation light generate beat frequency coherence, and then the beat frequency coherence is transmitted to the data acquisition card 10 by the balance detector 8;
fourthly, the data acquisition card 10 acquires beat frequency signals, and the calculation module 11 stores the beat frequency signals;
fifthly, the calculation module 11 performs Fourier transform on the beat frequency coherent signal to realize the transformation from a frequency domain spectrum to a time domain spectrum, and an absorption optical path is obtained according to the optical path difference between the reflection peaks;
step six, selecting an interested information data segment from the time domain spectrum obtained in the step five, and performing inverse Fourier transform to realize the transformation from the time domain spectrum to the frequency domain spectrum;
step seven, performing square processing on the amplitude data in the frequency domain spectrum to obtain the absorption spectrum intensity information of the gas;
and step eight, carrying out normalization processing on the obtained gas absorption spectrum intensity information to obtain a normalized absorption spectrum.
Preferably, in the fifth step, the computing module 11 performs fourier transform on multiple groups of beat frequency coherent signals to obtain multiple groups of data, and obtains an absorption optical path according to an optical path difference between reflection peaks after averaging the multiple groups of data.
Preferably, the step five is repeated for a plurality of times to obtain a plurality of groups of time domain spectrums, and the step six is performed after the plurality of groups of time domain spectrums are averaged.
In the existing gas sensing, an absorption spectrum and an absorption optical path need to be measured separately and large errors are easy to generate. The device and the method for synchronously measuring the gas absorption spectrum and the absorption optical path in the multi-pass cell can realize high-precision measurement of the absorption optical path and obtain the absorption spectrum at the same time. The invention can obtain the absorption spectra under various optical paths without changing the physical state of the multi-pass cell, and flexibly expand the dynamic range of gas sensing measurement.
Drawings
FIG. 1 is a schematic structural diagram of an apparatus for synchronous measurement of gas absorption spectrum and absorption optical path in a multipass cell according to a first embodiment;
FIG. 2 is a flowchart of a method for synchronous measurement of gas absorption spectrum and absorption optical path in a multi-pass cell according to a second embodiment;
FIG. 3 is a diagram of the acquired beat frequency coherent signal;
FIG. 4 is a time domain spectrum obtained by Fourier transform of 100 sets of beat frequency coherent signals;
cir at the 2 nd port of the optical fiber circulator, Con at the position of the flange between the sealing window I and the optical fiber circulator1And I2The positions of the front end surface and the rear end surface of the inlet sealing window and the position of the O corresponding to the outlet sealing window are respectively corresponding;
FIG. 5 is a graph of absorption spectrum intensity information obtained from the signal at the selected port sealing window O (i.e., within the dashed box);
FIG. 6 is a graph of the corresponding normalized absorption spectra of FIG. 5 compared to simulated spectra of a HITRAN database;
FIG. 7 is a gas absorption spectrum of 1600ppm acetylene at 4 different reflection positions in a multipass cell, corresponding to optical path lengths of 4.8196m, 8.0342m, 16.0713m and 22.4972m, respectively, and the lower 2 plots are magnified images of strong and relatively weak absorption lines, respectively.
Detailed Description
The first embodiment is as follows: specifically, the present embodiment will be described with reference to fig. 1, and the apparatus for synchronously measuring the gas absorption spectrum and the absorption optical length in the multi-pass cell according to the present embodiment includes: the system comprises a tunable light source 1, an auxiliary interferometer, a main interferometer, a data acquisition card 10 and a calculation module 11;
laser output by the tunable light source 1 is respectively incident to the auxiliary interferometer and the main interferometer;
the auxiliary interferometer is used for generating a clock signal and sending the clock signal to the clock end of the data acquisition card 10;
the light path of the main interferometer comprises intrinsic light and test light, and the test light is transmitted in the multi-pass cell and then generates beat frequency coherence with the intrinsic light;
the data acquisition card 10 is used for acquiring beat frequency coherent signals generated by the main interferometer and sending the beat frequency coherent signals to the calculation module;
and the calculating module 11 is used for calculating an absorption optical path and an absorption spectrum according to the beat frequency coherent signal.
The linearly tuned narrow linewidth laser output by the tunable light source 1 is divided into two paths of light by the first coupler 2-1, wherein 1% of one path of light is incident to the auxiliary interferometer, and 99% of one path of light is incident to the main interferometer.
The auxiliary interferometer adopts a Michelson interferometer structure and comprises a third coupler 2-3, a first Faraday rotator mirror 5-1, a second Faraday rotator mirror 5-2 and an optical fiber 6;
the third coupler 2-3 is a 2 × 2 coupler with a splitting ratio of 50: 50;
one path of light output by the first coupler 2-1 is divided into two paths through the third coupler 2-3, one path of light is reflected by the first Faraday rotating mirror 5-1 and then coupled into the third coupler 2-3, the other path of light is incident to the second Faraday rotating mirror 5-2 through the optical fiber 6 and then coupled into the third coupler 2-3 after being reflected by the second Faraday rotating mirror 5-2, the third coupler 2-3 outputs beat frequency coherent signals, and then square wave signals are generated through the TTL clock generator 9, wherein the square wave signals are clock signals.
The optical fiber 6 is used for time delay and has a length of 10-3000 m.
The main interferometer comprises a second coupler 2-2, a fourth coupler 2-4, a polarization controller 3, a fiber circulator 4 and a balanced detector 8;
the second coupler 2-2 is a 1 x 2 coupler and the fourth coupler 2-4 is a 2 x 2 coupler with a splitting ratio of 50: 50;
the other path of light output by the first coupler 2-1 is divided into two paths of light by a second coupler 2-2 in a ratio of 1:99, wherein 1% of one path of light is used as intrinsic light, the polarization state of the intrinsic light is adjusted by a polarization controller 3 and then enters an input port of a fourth coupler 2-4, 99% of one path of light is used as test light and enters a port 1 of an optical fiber circulator 4, the test light is emitted to a multi-pass cell 7 from a port 2 of the optical fiber circulator 4, reflected light enters a port 2 of the optical fiber circulator 4 again after multi-stage reflection of a concave mirror I7-1, a concave mirror II 7-2 and a concave mirror III 7-3 and absorption of gas in the multi-pass cell 7 and is coupled into the other input port of the fourth coupler 2-4 from the port 3, beat frequency coherence is generated between the intrinsic light and the test light, a beat frequency coherent signal is detected by a balance detector 8 and converted into a voltage signal, the analog voltage signal is input to the data acquisition card 10, and the data acquisition card 10 converts the analog voltage signal into a digital voltage signal. The auxiliary interferometer is used for acquiring beat frequency coherent signals detected by the balanced detector 8 by the data acquisition card 10.
The multi-pass cell 7 is filled with gas to be measured, the test light enters the multi-pass cell 7, and the test light is subjected to multi-stage reflection inside the multi-pass cell due to the reflection system formed by the three concave mirrors, so that the absorption optical path of the test light is greatly increased. The reflected optical signal contains the absorption intensity information of the gas, the absorption optical path of the gas can be obtained after Fourier transformation by a mathematical method of signal processing, the absorption spectrum of the gas is inverted after inverse Fourier transformation, the absorption optical path and the absorption spectrum information of the gas are obtained, and the concentration of the gas is demodulated.
In continuous wave frequency modulation techniques, the reflected signal essentially contains the absorption information of the gas. The invention relates to a data processing process of double Fourier transform. In the first fourier transform, as in the conventional continuous wave frequency modulation, a beat signal recorded in a frequency domain (spectral domain) is converted into time domain (spatial domain) information. In the space domain, the reflection intensity of each concave mirror in the multi-pass cell distributed along the length can be clearly seen, and the optical path length of the multi-pass cell can be determined through the optical path difference of strong reflection peaks corresponding to the sealing windows at the inlet and the outlet of the multi-pass cell. Within a measuring range of tens of meters, the spatial resolution of the measurement can reach sub-millimeter level. In the second inverse fourier transform, a section of the reflection signal of interest in the spatial domain is selected and inverse fourier transformed, and the reflection signal is converted back to the spectral domain again, so as to obtain the absorption spectrum of the target gas in the absorption optical path corresponding to the reflection signal, for example, the reflection signal at the sealed window of the outlet of the multi-pass cell is selected, and the absorption spectrum at the maximum corresponding absorption optical path can be obtained. Since the reflected signal is measured, the optical path of the absorption of the gas is twice the optical path of the transmission of the multipass cell.
In the main interferometer, the light intensity of the test light is modulated by the spectral absorption information in the multi-pass cell and generates beat frequency interference with the local oscillator light, the obtained beat frequency signal is finally acquired by the data acquisition card 10, the intensity is determined by the vector product of the test light amplitude and the intrinsic light amplitude and the response sensitivity of the balance detector 8, because the square relation exists between the light intensity and the amplitude, the absorption information is the light intensity information, and after two times of Fourier transform processing, the amplitude data of the frequency domain spectrum obtained by inversion is squared, so that the intensity information of the test light is obtained. In order to improve the signal-to-noise ratio, the acquired multiple groups of beat frequency coherent signals are averaged after first Fourier transform, then inverse Fourier transform is carried out on the average result, and the obtained inversion result is squared.
The linear tuning range of the tunable light source 1 can be selected according to the absorption band of the gas to be measured. Different tuning ranges of the light source can be set for different gases to accommodate different ranges of gas absorption spectra. The linear tuning rate and acquisition card parameters are adjustable. The output power of the light source is in milliwatt level and can be set according to specific requirements.
The second embodiment is as follows: the present embodiment will be described in detail with reference to fig. 2 to 7, and a method for synchronously measuring a gas absorption spectrum and an absorption optical length of an apparatus for synchronously measuring a gas absorption spectrum and an absorption optical length in a multi-pass cell according to a first embodiment includes:
filling gas to be detected with a certain concentration into a multi-pass tank 7;
starting the linear sweep frequency output of the tunable light source 1, and simultaneously outputting a trigger signal to trigger the data acquisition card 10 to prepare for data acquisition by the tunable light source 1;
thirdly, the auxiliary interferometer generates a clock signal which is transmitted to the data acquisition card 10, so that the data acquisition card 10 performs equal-frequency interval acquisition on the beat frequency signal of the main interferometer;
the test light of the main interferometer is guided into the multi-pass cell 7 from the port 2 of the optical fiber circulator 4, and reflected light enters the optical fiber circulator 4 again after multi-stage reflection and gas absorption in the multi-pass cell 7; the beat-adjusting polarization controller 3 makes the light led out from the port 3 of the optical fiber circulator 4 and the local oscillation light generate beat frequency coherence, and then the beat frequency coherence is detected by a balance detector 8 and transmitted to a data acquisition card 10;
fourthly, the data acquisition card 10 acquires beat frequency signals, and the calculation module 11 stores the beat frequency signals;
fifthly, the calculation module 11 performs Fourier transform on the beat frequency coherent signal to realize the transformation from a frequency domain spectrum to a time domain spectrum, and an absorption optical path is obtained according to the optical path difference between the reflection peaks;
step six, selecting an interested information data segment from the time domain spectrum obtained in the step five, and performing inverse Fourier transform to realize the transformation from the time domain spectrum to the frequency domain spectrum;
step seven, performing square processing on the amplitude data in the frequency domain spectrum to obtain the absorption spectrum intensity information of the gas;
and step eight, carrying out normalization processing on the obtained gas absorption spectrum intensity information to obtain a normalized absorption spectrum.
In the fifth step, the calculation module 11 performs fourier transform on a plurality of groups of beat frequency coherent signals to obtain a plurality of groups of data, and after averaging the plurality of groups of data, an absorption optical path is obtained according to an optical path difference between reflection peaks.
Repeating the step five for multiple times to obtain multiple groups of time domain spectrums, averaging the multiple groups of time domain spectrums, and then performing the step six.
In this embodiment, taking a White-type multipass cell as an example, referring to fig. 1, three concave mirrors are provided inside the multipass cell, and light is reflected and propagated inside the multipass cell for multiple times to form a total absorption optical path. Fig. 2 is a flowchart of the method of the present embodiment. FIG. 3 is a graph of the raw beat frequency coherent signal collected from a multipass cell charged with 100ppm acetylene at standard atmospheric pressure, digitized to produce correlated interference fringes in the frequency domain.
Fig. 4 shows the results of averaging the fourier transform of 100 consecutive measurements. In the spatial domain obtained by the first fourier transform, reflection peaks are obtained which are generated as a result of light forming multiple reflections in the three concave mirrors, the amplitude of the reflection peaks being two orders of magnitude weaker than the amplitude at the exit sealing window.
Fig. 5 shows the result of inverse fourier transform of the signal at the sealing window O of the selected port during inverse fourier transform, and the absorption spectrum corresponding to the maximum absorption optical length is obtained. And after the data is subjected to inverse Fourier transform, squaring the amplitude data of the obtained frequency domain result to obtain corresponding absorption spectrum information. And correcting by means of base line fitting to obtain a normalized absorption spectrum.
The resulting gas absorption spectrum is shown in fig. 6. For comparison, simulation results based on the HITRAN database are referenced. Comparison shows that the residual error level between the experimental data and the simulation result of the present invention is very low, which demonstrates the success of the method of the present embodiment in accurately detecting the gas absorption spectrum. The minimum detectable absorbance, based on the standard deviation of the residual error, was about 0.01, corresponding to a detection limit of 5ppm for acetylene gas measurement.
According to the Beer-Lambert law of gas sensing, the same absorption intensity is generated, and the gas concentration is higher as the absorption optical path is shorter. When the gas concentration is higher, the absorption spectrum obtained from the outlet sealing window can generate an absorption saturation phenomenon, and the gas sensing is not suitable, and at the moment, the reflection signal of the concave mirror in the multi-pass cell is selected to perform inverse Fourier transform, so that the absorption spectrum without saturation is obtained. Inverse Fourier transform is carried out on the concave mirror reflection signals in the multi-pass cell, so that the measurement dynamic range of the method provided by the invention can be greatly improved. FIG. 7 illustrates the simultaneous measurement of gas absorption spectra for reflection locations corresponding to different optical paths in a multipass cell. With the advantage of this method in terms of expanded measurement dynamic range, a relatively high concentration of 1600ppm acetylene gas was measured. Referring to fig. 7, absorption spectra measured at 4 different reflection positions are shown, and the corresponding absorption optical paths are 4.8196m, 8.0342m, 16.0713m and 22.4972m, respectively. Each absorption spectrum was detected according to the procedure described above. It can be clearly seen that at the 22.4972 meter position corresponding to the maximum absorption optical path, the strong absorption is close to saturation and therefore cannot be used for gas sensing. In contrast, for relatively short absorption optical paths, the absorption lines may still operate. For detection of higher concentrations of gas, short absorption paths may be used in combination with weak absorption lines. Obviously, it is very advantageous to expand the effective dynamic range of the measurement by choosing different optical paths.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
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| CN114002185B (en) * | 2021-11-17 | 2023-08-04 | 哈尔滨工业大学 | Multi-point dispersion spectrum measuring device and method based on optical frequency modulation continuous wave |
| CN114384045B (en) * | 2022-01-10 | 2024-01-12 | 北京航空航天大学 | Trace gas concentration and path length real-time detection system and method |
| CN114384035B (en) * | 2022-01-14 | 2024-11-12 | 谱睿量子科技(山东)有限公司 | Gas component absorption spectrum measurement method and system |
| CN114414530B (en) * | 2022-01-28 | 2023-07-28 | 福州大学 | A biomolecular self-reference detection method and device |
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