CN111208084A - Optical fiber gas concentration remote sensing detection device and method based on coherent detection method - Google Patents
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Abstract
The invention belongs to the field of laser technology application, and provides an optical fiber gas concentration remote sensing detection device and method based on a coherent detection method. A frequency modulation laser beam emitted by the tunable laser passes through the first optical fiber coupler to divide a light beam into two paths, wherein one path is signal light, and the other path is local oscillator light. The signal light is irradiated on the target wall surface and is used for detecting the gas to be detected. The signal light scattered by the wall surface sequentially passes through the plurality of imaging lenses and the optical fiber collimator and is received by the second optical fiber coupler, and the signal light and the local oscillator light accessed to the other end of the second optical fiber coupler are subjected to beat frequency coherence. The beat frequency coherent signal is received by a photoelectric detector and finally collected by a data acquisition card, and the computer module is used for inverting the concentration of the gas to be detected. The invention is suitable for detecting weak light signals, has higher sensitivity and signal-to-noise ratio, and can realize the simultaneous detection of the absorption distance and the absorption signal intensity.
Description
Technical Field
The invention belongs to the field of application of laser spectroscopy technology, and particularly relates to a device and a method for remote sensing detection of optical fiber gas concentration based on a coherent detection method.
Background
The atmospheric environment is closely related to human survival, and almost every factor of the atmospheric environment affects the survival and development of human beings. Atmospheric pollution not only poses great threat to human health, but also affects global climate change and causes serious harm to ecosystem and industrial and agricultural production, and the measurement of atmospheric pollution gas concentration by utilizing an advanced optical remote sensing detection technology has great significance to atmospheric environment monitoring and treatment. The traditional gas concentration remote sensing detection technology mainly comprises a differential absorption spectroscopy (DOAS) technology, a differential absorption lidar (DIAL) technology, a hard target Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology and the like.
The DOAS technique is a method of detecting by using the difference between the absorption spectra of gas on and off the absorption line (prior art [1 ]: ScanteWallin, "implementation of DOAS method in continuous emission of pollution source and on-line monitoring of process gas", proceedings of environmental engineering, 2011). And obtaining the concentration information of the gas to be detected by carrying out inversion according to the Belronbo theorem through the differential absorption characteristics of the gas to be detected on the corresponding wave band. The DOAS method can be used for measuring the concentrations of various gas molecules on a laser beam path, does not influence the chemical characteristics of the measured gas, and has the advantages of quick response and no need of pretreatment. When the DOAS technology is used for carrying out gas concentration remote sensing detection, a light beam is generally wide in spectrum (such as a xenon lamp and the like), a reflector needs to be placed at the far end, and therefore light signals are reflected back to a system for photoelectric detection (a spectrometer is generally used), and finally the average gas concentration on a laser path can be measured. In practical application, the method is only suitable for gas concentration remote sensing detection of a fixed scene (or path), and mobile outdoor remote sensing detection is difficult to realize.
The differential absorption laser radar (DIAL) technology (prior art [2]: Liang Mei, et al., "differential absorption laser system applied for background mechanical calibration in South China", Optics and Lasers in Engineering,2014) is an active optical remote sensing technology, and has the characteristics of high spatial resolution, high detection sensitivity, large measurement range and the like. The DIAL technique is based on the principle that laser pulses with different wavelengths (one wavelength is located at an absorption peak of a gas to be measured, and the other wavelength deviates from the absorption peak of the gas to be measured) are alternately emitted into the atmosphere, simultaneously, atmosphere backscatter signals of the two wavelengths are detected by using a high-sensitivity detector (such as a photomultiplier tube) and the concentration of the gas to be measured at different distances is solved according to the difference (ratio) of the absorption intensity of the atmosphere echo signals of the gas to be measured to the two wavelengths. However, in order to realize remote sensing of gas concentration at different distances, the DIAL technique requires the use of a nanosecond-level tunable high-energy pulsed laser light source, which results in an extremely complex system, requires frequent maintenance during measurement, and cannot continuously and stably measure for a long time, thus greatly limiting the practical application of the technique.
Hard-target tunable semiconductor laser absorption spectroscopy (TDLAS) (prior art [3 ]: Jeremy t. dome, et al, 'demodulation of spatial green house gas mapping using laser absorption spectra on local scales', Journal of Applied Remote Sensing,2017) is based on narrow-linewidth semiconductor laser technology and beer lambert law. Under the conditions of controlling temperature and changing driving current, the wavelength of laser output by the laser is tuned, the laser wavelength is scanned near a gas absorption spectral line, and then the detection of the gas concentration is completed by measuring the intensity of an optical signal of the laser absorbed by the gas. When the TDLAS technique is used to perform remote sensing detection of gas concentration, a narrow-linewidth (10MHz) laser beam is generally subjected to wavelength (or frequency tuning) and then transmitted to a remote hard target, such as a building. The emitted light signal is absorbed by the gas to be detected and reflected back to the detection system by the hard target, and is directly detected by the photoelectric detector, and finally the average concentration of the gas to be detected on the laser path can be obtained according to the intensity and the absorption path of the absorption signal. The distance between the hard target and the system is typically obtained by laser rangefinder measurements. The hard target TDLAS technology is relatively simple, easy to realize and good in stability. However, because the power of the narrow-linewidth semiconductor laser used is generally small, when the laser beam is emitted to a hard target for remote sensing detection, the scattered light signal received by the detector is very weak, so that the system is limited to night short-distance remote sensing detection only. Under the condition of strong background light in the daytime, the system is seriously interfered by background light noise, the detection sensitivity is low, and the remote sensing detection of the atmospheric gas concentration is difficult to realize.
In conclusion, how to realize a gas concentration remote sensing detection technology with convenient outdoor measurement, low cost, good stability and high sensitivity is one of important subjects in the field of atmospheric environment monitoring.
Disclosure of Invention
The invention provides an optical fiber gas concentration remote sensing detection technology based on a coherent detection method, which effectively solves the application bottleneck problems of complex system, poor stability, low sensitivity, poor mobility and the like of the traditional gas concentration remote sensing detection technology.
The technical scheme of the invention is as follows:
a remote sensing detection device of optical fiber gas concentration based on a coherent detection method comprises a signal generator 1, a DFB laser 2, a first optical fiber flange 3, a first optical fiber coupler 4, a first imaging lens 5, a second imaging lens 7, a diaphragm 8, a third imaging lens 9, an optical fiber collimator 10, a second optical fiber coupler 11, a second optical fiber flange 12, a photoelectric detector 13, a data acquisition card 14 and a computer 15;
the sawtooth signal generated by the signal generator 1 is loaded on the DFB laser 2 to modulate the frequency or wavelength of the DFB laser 2; a laser beam generated by the DFB laser 2 enters a first optical fiber coupler 4 through a first optical fiber flange 3 and is divided into two paths of light, wherein one path of light is collimated by an imaging lens 5 and then is emitted onto a hard target 6, the path of light is called signal light, and the other path of light is used as local oscillation light; the signal light is scattered by the hard target 6, and the obtained scattered light signal is changed into parallel light after sequentially passing through a second imaging lens 7, a diaphragm 8 and a third imaging lens 9; the diaphragm 8 is positioned at the back focal length of the second imaging lens 7; the distance between the diaphragm 8 and the third imaging lens 9 is the front focal length of the third imaging lens 9; the collimated parallel light is connected to an input port of a second optical fiber coupler 11 through an optical fiber collimator 10; the local oscillation light is connected to the other input port of the second optical fiber coupler 11 through the second optical fiber flange 12; the parallel light and the local oscillator light interfere in the second optical fiber coupler 11, and the generated coherent light is output to the photoelectric detector 13 from the output end of the second optical fiber coupler 11; the photoelectric detector 13 detects beat frequency signals, and the output end of the photoelectric detector is connected with the signal acquisition input end of the data acquisition card 14; the signals collected by the data acquisition card 14 are transmitted to the computer 15 for analysis and processing, and finally the gas concentration on the measurement path is obtained.
The DFB laser 2 comprises a DFB laser chip, a current drive and temperature control device, wherein the working wavelength of the DFB laser chip is located near the absorption peak of the gas to be detected, and the line width is superior to 10 MHz.
The laser emitted by the DFB laser 2 is a linear frequency modulation laser beam.
The splitting ratio of the first optical fiber coupler 4 is 95:5, wherein the end outputting the signal light is light passing through 95%, and the end outputting the local oscillator light is light passing through 5%.
The splitting ratio of the second optical fiber coupler 11 is 50: 50.
The focal length of the first imaging lens 5 is 15mm, and the caliber of the first imaging lens is 12.7 mm;
the focal length of the second imaging lens 7 is 175mm, and the aperture is 50 mm;
the focal length of the third imaging lens 9 is 15mm, and the aperture is 12.7 mm.
The invention has the beneficial effects that:
the invention relates to an optical fiber gas concentration remote sensing detection device and method based on a coherent detection method, wherein a narrow linewidth semiconductor laser with tuned frequency is used as a light source and is divided into local oscillation light and signal light. The signal light is emitted to the hard target, scattered by the hard target and collected by the lens. The photoelectric detector is used for coherent detection, the generated beat frequency signal can realize the distance detection of a hard target on one hand, and the detection of the absorption signal of the gas concentration to be detected on the other hand can be realized. According to the absorption signal intensity and the absorption distance, the average concentration of the gas to be measured on the measurement path can be obtained. The method has the following advantages:
(1) by means of coherent detection, not only background light can be effectively inhibited, but also optical amplification of signal light can be achieved by using local oscillator light with high intensity, and therefore signal-to-noise ratio and detection sensitivity are greatly improved.
(2) The beat frequency is directly related to the absorption range, and the absorption range can be directly calculated, so that the absorption range does not need to be measured by other technical means, and the outdoor mobile measurement is convenient.
Drawings
FIG. 1 is a schematic diagram of an apparatus of a remote sensing system for detecting gas concentration by using a fiber optic based coherent detection method.
In the figure: 1 a signal generator; a 2DFB laser; 3 a first optical fiber flange; 4 a first fiber coupler; 5 a first imaging lens; 6 hard target; 7 a second imaging lens; 8, a diaphragm; 9 a third imaging lens; 10 a fiber collimator; 11 a second fiber coupler; 12 a second fiber flange; 13 a photodetector; 14, a data acquisition card; 15 computer.
Fig. 2 is a graph of laser output frequency.
Fig. 3 is a beat signal diagram.
Fig. 4 is a diagram of a beat signal spectrum.
Fig. 5 is a graph of absorption signals.
Detailed Description
The following further describes specific embodiments of the present invention with reference to the drawings and technical solutions.
The invention relates to an optical fiber gas concentration remote sensing detection method based on a coherent detection method, which comprises the following steps:
A. the signal generator 1 generates a sawtooth wave, and transmits the sawtooth wave to the drive of the DFB laser 2, so that the DFB laser 2 emits a frequency modulation laser beam, and meanwhile, the data acquisition card 14 receives a trigger signal and prepares for starting data acquisition. The frequency modulation period of the laser is T(s), and the frequency modulation range is delta v (m).
B. The frequency modulated laser beam passes through the first optical fiber flange 3 and enters the first optical fiber coupler 4 according to a 95: the 5 proportion is divided into two paths of light, and the signal light accounts for 95 percent, and the local oscillator light accounts for 5 percent.
C. The signal light is collimated by the first imaging lens 5 and then emitted, and after passing through the target gas, the signal light impinges on the hard target 6. After scattering the signal light, the hard target 6 is received by the second imaging lens 7, the aperture stop 8, and the third imaging lens 9 in sequence, and enters the second fiber coupler 11 after being collimated by the fiber collimator 10.
D. The local oscillator light enters the second optical fiber coupler 11 through the second optical fiber flange 12. The signal light and the local oscillator light are coherent in the second fiber coupler 11.
E. The signal output by the second fiber coupler 11 is received by the photodetector 13.
F. The photodetector 13 transmits the received beat frequency signal to the data acquisition card 14 for acquisition.
G. The computer 15 performs fourier transform on the beat frequency signal acquired by the data acquisition card 14. On the frequency domain signal, the size f of the beat frequency signal frequency can be obtainedb. In the case of the known light speed c, the absorption path L of the gas to be measured can be calculated according to the following formula:
H. windowing is performed on the frequency domain signal, and only the frequency spectrum near the beat frequency signal is reserved. The signal is subjected to inverse Fourier transform, and the absorption signal S of the gas to be measured can be obtainedabs. Assuming a known gas concentration of CrefAbsorption range is LrefThe gas absorption signal intensity under the condition is Sref(generally obtained by calibration experiments with known concentrations and absorption paths). The average concentration of the gas in the laser measurement path can be expressed as:
I. the system is aligned to any hard target 6, laser beams are emitted, the intensity of scattered light signals is detected, and according to the method, the average concentration of the gas on the path to be measured can be obtained, so that the mobile (or any path) gas concentration measurement is realized.
The above description is further detailed in connection with the preferred embodiments of the present invention, and it is not intended to limit the practice of the present invention to these descriptions. It will be apparent to those skilled in the art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention.
Claims (9)
1. The remote sensing detection device for the concentration of the gas in the optical fiber based on the coherent detection method is characterized by comprising a signal generator (1), a DFB laser (2), a first optical fiber flange (3), a first optical fiber coupler (4), a first imaging lens (5), a second imaging lens (7), a diaphragm (8), a third imaging lens (9), an optical fiber collimator (10), a second optical fiber coupler (11), a second optical fiber flange (12), a photoelectric detector (13), a data acquisition card (14) and a computer (15);
the sawtooth signal generated by the signal generator (1) is loaded on the DFB laser (2), frequency or wavelength modulation is carried out on the DFB laser (2), and the driving and temperature control of the DFB laser (2) are controlled, so that the wavelength of a laser beam only has one gas absorption peak in a linear output range; laser beams from the DFB laser (2) enter a first optical fiber coupler (4) through a first optical fiber flange plate (3) and are divided into two paths of light, one path of light is collimated through a first imaging lens (5) and is emitted to a hard target (6) to serve as signal light, and the other path of light serves as local oscillation light; the signal light is scattered by a hard target (6), and the obtained scattered light signal is changed into parallel light after sequentially passing through a second imaging lens (7), a diaphragm (8) and a third imaging lens (9); the diaphragm (8) is positioned at the back focal length of the second imaging lens (7); the distance between the diaphragm (8) and the third imaging lens (9) is the front focal length of the third imaging lens (9); the collimated parallel light is connected to an input port of a second optical fiber coupler (11) through an optical fiber collimator (10); the local oscillation light is connected to the other input port of the second optical fiber coupler (11) through a second optical fiber flange plate (12); the parallel light and the local oscillator light interfere in the second optical fiber coupler (11), and the generated coherent light is output to the photoelectric detector (13) from the output end of the second optical fiber coupler (11); the photoelectric detector (13) detects beat frequency signals, and the output end of the photoelectric detector is connected with the signal acquisition input end of the data acquisition card (14).
2. The remote sensing detection device of optical fiber gas concentration based on coherent detection method according to claim 1, characterized in that the laser emitted by the DFB laser (2) is a frequency modulated laser beam, comprising a DFB laser chip, a current driving and temperature control device, the working wavelength of the DFB laser chip is located near the absorption peak of the gas to be detected, and the line width is better than 10 MHz.
3. The remote sensing device for detecting gas concentration by optical fiber based on coherent detection method as claimed in claim 1 or 2, wherein the splitting ratio of the first optical fiber coupler (4) is 95:5, wherein the end outputting signal light is 95% light and the end outputting local oscillator light is 5% light.
4. The remote sensing device for detecting gas concentration based on coherent detection method as claimed in claim 1 or 2, wherein the splitting ratio of the second fiber coupler (11) is 50: 50.
5. The remote sensing device for detecting gas concentration based on coherent detection method as claimed in claim 3, wherein the second fiber coupler (11) has a splitting ratio of 50: 50.
6. The remote sensing device for gas concentration based on coherent detection method according to claim 1, 2 or 5,
the focal length of the first imaging lens (5) is 15mm, and the caliber of the first imaging lens is 12.7 mm;
the focal length of the second imaging lens (7) is 175mm, and the aperture is 50 mm;
the focal length of the third imaging lens (9) is 15mm, and the caliber of the third imaging lens is 12.7 mm.
7. The remote sensing device for gas concentration based on coherent detection method as claimed in claim 3,
the focal length of the first imaging lens (5) is 15mm, and the caliber of the first imaging lens is 12.7 mm;
the focal length of the second imaging lens (7) is 175mm, and the aperture is 50 mm;
the focal length of the third imaging lens (9) is 15mm, and the caliber of the third imaging lens is 12.7 mm.
8. The remote sensing device for gas concentration based on coherent detection method as claimed in claim 4,
the focal length of the first imaging lens (5) is 15mm, and the caliber of the first imaging lens is 12.7 mm;
the focal length of the second imaging lens (7) is 175mm, and the aperture is 50 mm;
the focal length of the third imaging lens (9) is 15mm, and the caliber of the third imaging lens is 12.7 mm.
9. A remote sensing detection method of optical fiber gas concentration based on a coherent detection method is characterized by comprising the following steps:
A. the signal generator (1) generates sawtooth waves which are transmitted to the drive of the DFB laser (2), so that the DFB laser (2) emits frequency-modulated laser beams, and meanwhile, the data acquisition card (14) receives a trigger signal and prepares for starting data acquisition; the frequency modulation period of the DFB laser (2) is T, and the frequency modulation range is delta v;
B. the frequency modulated laser beam enters the first optical fiber coupler (4) through the first optical fiber flange plate (3) according to a 95:5 proportion is divided into two paths of light, signal light accounts for 95 percent, and local oscillator light accounts for 5 percent;
C. the signal light is collimated by the imaging lens (5) and then emitted out, and is projected on a hard target (6) after passing through target gas; after signal light is scattered by the hard target (6), the signal light is received by the second imaging lens (7), the aperture diaphragm (8) and the third imaging lens (9), and enters the second optical fiber coupler (11) after being collimated by the optical fiber collimator (10);
D. the local oscillation light enters a second optical fiber coupler (11) through a second optical fiber flange plate (12); the signal light and the local oscillator light are coherent in the second optical fiber coupler (11);
E. the signal output by the second optical fiber coupler (11) is received by a photoelectric detector (12);
F. the photoelectric detector (12) transmits the received beat frequency signal to a data acquisition card (14) for acquisition;
G. the computer (15) performs Fourier transform on the beat frequency signals acquired by the data acquisition card (14); on the frequency domain signal, the size f of the beat frequency signal frequency is obtainedb(ii) a Under the condition that the speed of light is known, calculating the absorption path L of the gas to be measured according to the following formula:
H. windowing the frequency domain signal, and only reserving frequency spectrums near the beat frequency signal; performing inverse Fourier transform on the signal to obtain an absorption signal S of the gas to be measuredabs(ii) a Assuming a known gas concentration of CrefAbsorption range is LrefThe gas absorption signal intensity under the condition is Sref(ii) a The average concentration of the gas in the laser measurement path is expressed as:
I. the system is aligned to any hard target (6), laser beams are emitted, the intensity of scattered light signals is detected, and the average concentration of the gas on a path to be measured is obtained according to the method, so that the mobile (or any position) gas concentration measurement is realized.
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