CN211627368U - A gas concentration remote sensing detection device based on coherent detection method - Google Patents

Abstract

The utility model belongs to the technical field of laser technology application, and provides a gas concentration remote sensing detection device based on a coherent detection method. After the frequency-modulated laser beam emitted by the tunable laser is collimated by the laser collimating device, it is divided into two beams by the first beam splitter. One is the signal light and the other is the local oscillator light. The signal light hits the target wall to detect the gas to be tested. The signal light scattered by the wall passes through a plurality of imaging lenses and the second beam splitter in sequence, and the signal light is beat frequency coherent with the local oscillator light reflected by the second beam splitter. The beat frequency coherent signal is received by the photodetector, and finally collected by the data acquisition card, and the computer module is used to invert the concentration of the gas to be measured. The utility model is suitable for detection of weak light signals, has high sensitivity and signal-to-noise ratio, and can realize simultaneous detection of absorption distance and absorption signal strength.

Description

A gas concentration remote sensing detection device based on coherent detection method

technical field

The utility model belongs to the technical field of laser spectroscopy technology application, in particular to a gas concentration remote sensing detection device based on a coherent detection method.

Background technique

The atmospheric environment is closely related to human survival, and almost every factor in the atmospheric environment affects the survival and development of human beings. Air pollution not only poses a huge threat to human health, but also affects global climate change and causes serious harm to ecosystems, industrial and agricultural production. The use of advanced optical remote sensing detection technology to carry out atmospheric pollution gas concentration measurement is of great importance to atmospheric environment monitoring and governance. significance. The traditional gas concentration remote sensing detection technologies mainly include differential absorption spectroscopy (DOAS) technology, differential absorption laser radar (DIAL) technology, hard target tunable diode laser absorption spectroscopy (TDLAS) and so on.

DOAS technology is a method for detection by using the difference of gas absorption spectrum on the absorption line and outside the line (prior technology [1]: Scante Wallin, "The realization of DOAS method in continuous emission pollution source and on-line monitoring of process gas", Environmental Engineering Technology Journal, 2011). The concentration information of the gas to be measured can be obtained by inversion according to the differential absorption characteristics of the gas to be measured in the corresponding wavelength band and inversion according to Beer-Lambeau's theorem. The DOAS method can measure the concentration of various gas molecules on the laser beam path without affecting the chemical properties of the measured gas, with fast response and no need for preprocessing. When using DOAS technology to carry out remote sensing detection of gas concentration, the emission beam generally has a wide spectrum (such as xenon lamp, etc.), and a reflector needs to be placed at the far end, so as to reflect the light signal back to the system for photoelectric detection (usually using a spectrometer), which can finally be measured. Average gas concentration in the laser path. In practical application, this method is only suitable for remote sensing detection of gas concentration in fixed scenes (or paths), and it is difficult to realize mobile outdoor remote sensing detection.

Differential absorption lidar (DIAL) technology (prior technology [2]: Liang Mei, et al., "Differentialabsorption lidar system employed for background atomic mercury vertical profiling in South China", Optics and Lasers in Engineering, 2014) is an active Optical remote sensing detection technology has the characteristics of high spatial resolution, high detection sensitivity, and large measurement range. The basic principle of DIAL technology is to alternately emit laser pulses with different wavelengths into the atmosphere (one wavelength is located at the absorption peak of the gas to be measured, and the other wavelength is deviated from the absorption peak of the gas to be measured), and at the same time, high-sensitivity detectors (such as photomultiplier tubes, etc.) are used. The atmospheric backscattering signals of two wavelengths are detected, and the concentration of the gas to be measured at different distances is calculated according to the difference (ratio) of the absorption intensity of the atmospheric echo signal of the two wavelengths of the gas to be measured. However, in order to realize the remote sensing detection of gas concentration at different distances, DIAL technology requires the use of nanosecond-scale, tunable, high-energy pulsed laser light sources, which makes the system extremely complex, requires frequent maintenance during measurement, and cannot be continuous for a long time. Stable measurement greatly limits the practical application of this technique.

Hard target tunable semiconductor laser absorption spectroscopy (TDLAS) (prior art [3]: Jeremy T. Dobler, et al., "Demonstration of spatial greenhouse gas mapping using laserabsorption sepectrometers on local scales", Journal of Applied Remote Sensing, 2017 ) is based on narrow linewidth semiconductor laser technology and Beer-Lambert's law. Under the conditions of controlling the temperature and changing the driving current, the wavelength of the laser output laser is tuned so that the laser wavelength is scanned near the gas absorption line, and then the gas concentration is detected by measuring the optical signal intensity of the laser absorbed by the gas. When using TDLAS technology to carry out remote sensing detection of gas concentration, a narrow linewidth (10MHz) laser beam is generally wavelength (or frequency tuned) and then emitted to a long-distance hard target, such as a building. The emitted light signal will be absorbed by the gas to be measured, and reflected back to the detection system by the hard target, and directly detected by the photodetector. Finally, according to the intensity of the absorption signal and the absorption range, the average concentration of the gas to be measured on the laser path can be obtained. The distance between the hard target and the system can generally be measured by a laser rangefinder. This hard target TDLAS technology is relatively simple, easy to implement, and has good stability. However, due to the generally low power of the narrow linewidth semiconductor lasers used, 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 the system is limited to short-distance remote sensing detection at night. Under the condition of strong background light during the day, the system is seriously disturbed by background light noise, and the detection sensitivity is low, so it is difficult to realize remote sensing detection of atmospheric gas concentration.

To sum up, how to realize the gas concentration remote sensing detection technology with convenient outdoor measurement, low cost, good stability and high sensitivity is one of the important topics in the field of atmospheric environment monitoring.

Utility model content

The utility model provides a gas concentration remote sensing detection technology based on a coherent detection method, which effectively overcomes the application bottleneck problems of the traditional gas concentration remote sensing detection technology, such as complex system, poor stability, low sensitivity and poor mobility.

The technical scheme of the present utility model:

A gas concentration remote sensing detection device based on a coherent detection method, comprising a signal generator 1, a DFB laser 2, a first imaging lens 3, a first beam splitter 4, a second imaging lens 6, an aperture stop 7, and a third imaging lens 8. The first reflector 9, the second reflector 10, the second beam splitter 11, the photodetector 12, the data acquisition card 13 and the computer 14;

The sawtooth signal generated by the signal generator 1 is loaded on the DFB laser 2 to perform frequency or wavelength modulation on the DFB laser 2 ; the laser beam from the DFB laser 2 is collimated by the first imaging lens 3 . The collimated light is divided into two paths of light through the first beam splitter 4 . One path of light is reflected on the hard target 5 via the first beam splitter 4 as signal light; the other path of light is transmitted through the first beam splitter 4 as local oscillator light. The signal light is scattered by the hard target 5, and the obtained scattered light signal becomes parallel light after passing through the second imaging lens 6, the aperture diaphragm 7, and the third imaging lens 8 in sequence; the aperture diaphragm 7 is located in the second imaging lens 6. at the back focal length of the imaging lens 6; the distance between the diaphragm 7 and the third imaging lens 8 is the front focal length of the third imaging lens 8; the collimated parallel light passes through the second beam splitter 11; the After the reflection of the first reflection mirror 9, the reflection of the second reflection mirror 10, and the reflection of the second beam splitter 11, the local oscillator light meets and interferes with the parallel light; the coherent light is received by the photodetector 12; the coherent light is received by the photodetector 12; The photodetector 12 detects the beat frequency signal, and its output terminal is connected to the signal acquisition input terminal of the data acquisition card 13 .

The DFB laser 2 includes a DFB laser chip, a current drive and a temperature control device. The working wavelength of the DFB laser chip is located near the absorption peak of the gas to be measured, and the line width is better than 10 MHz.

The laser emitted by the DFB laser 2 is a frequency modulated laser beam.

The light splitting ratio of the first beam splitter 4 is 90:10, wherein the end that outputs the signal light is 90% light, and the end that outputs the local oscillator light is 10% light.

The light splitting ratio of the second light splitting plate 11 is 50:50.

The first imaging lens 3 has a focal length of 15mm and a diameter of 12.7mm;

The second imaging lens 6 has a focal length of 175mm and a diameter of 50mm;

The focal length of the third imaging lens 8 is 15mm, and the diameter is 12.7mm.

The beneficial effects of the present utility model:

The gas concentration remote sensing detection device based on the coherent detection method of the utility model adopts a frequency (or wavelength) tuned narrow-linewidth semiconductor laser as a light source, and is divided into local vibration light and signal light. The signal light is emitted to the hard target and collected by the lens after being scattered by the hard target. Using photodetector coherent detection, the generated beat frequency signal can realize the distance detection of hard targets on the one hand, and the detection of the absorption signal of the gas concentration to be measured on the other hand. According to the absorption signal strength and distance, the average concentration of the gas to be measured on the measurement path can be obtained. The main advantages are as follows:

(1) The coherent detection method can not only effectively suppress the background light, but also realize the optical amplification of the signal light by using the local oscillator light with higher intensity, thereby greatly improving the signal-to-noise ratio and detection sensitivity.

(2) The frequency of the beat frequency is directly related to the absorption range, and the absorption range can be directly calculated, so it is not necessary to use other technical means to measure the absorption range, which is convenient for outdoor mobile measurement.

Description of drawings

FIG. 1 is a device diagram of a gas concentration remote sensing detection system based on a coherent detection method.

Figure 2 is a graph of the laser output frequency.

FIG. 3 is a diagram of a beat frequency signal.

FIG. 4 is a spectrogram of the beat signal.

Figure 5 is a graph of the absorption signal.

In the figure: 1 signal generator; 2DFB laser; 3 first imaging lens; 4 first beam splitter; 5 hard target; 6 second imaging lens; 7 diaphragm; 8 third imaging lens; 9 first mirror; 10 11 second beam splitter; 12 photodetector; 13 data acquisition card; 14 computer.

Detailed ways

The specific embodiments of the present utility model are further described below in conjunction with the accompanying drawings and technical solutions.

A method for remote sensing detection of gas concentration based on a coherent detection method described in the utility model, the method is specifically:

A. The signal generator 1 generates a sawtooth wave, which is transmitted to the drive of the DFB laser 2, so that the DFB laser 2 emits a frequency-modulated laser beam. At the same time, the data acquisition card 13 receives the trigger signal and is ready to start data acquisition. The frequency modulation period of the laser is T(s), and the frequency modulation range is Δv(m).

B. The frequency-modulated laser beam is collimated by the first imaging lens 3, and the collimated light is divided into two beams by the ratio of 90:10 through the first beam splitter 4, 90% of which is signal light, and 10% of which is signal light. is the local oscillator. The signal light is reflected by the first beam splitter 4 , passes through the target gas and then reaches the hard target 5 . The other light is transmitted through the first beam splitter as the local oscillator light.

C. After the signal light is scattered by the hard target 5 , it is received by the second imaging lens 6 , the aperture stop 7 , and the third imaging lens 8 and collimated into parallel light, which is transmitted through the second beam splitter 11 .

D. The local oscillator light is coherent with the collimated parallel light after being reflected by the first reflecting mirror 9, the second reflecting mirror 10, and the second beam splitter 11.

E. The generated coherent light is received by the photodetector 12 .

F. The photodetector 12 transmits the received beat frequency signal to the data acquisition card 13 for acquisition.

G. The computer 14 performs Fourier transform on the beat frequency signal collected by the data collection card 13 . On the frequency domain signal, the magnitude f _b of the frequency of the beat signal can be obtained. When the speed of light c is known, the absorption range L of the gas to be measured can be calculated according to the following formula:

H. Perform window processing on the frequency domain signal, and only retain the frequency spectrum near the beat frequency signal. By inverse Fourier transform of the signal, the absorption signal S _abs of the gas to be measured can be obtained. Assuming that the known gas concentration is C _ref , the gas absorption signal intensity under the condition that the absorption range is L _ref is S _ref (generally obtained through calibration experiments with known concentrations and absorption ranges). The average concentration of gas in the laser measurement path can be expressed as:

1. Aim the system at any hard target 5, emit a laser beam, detect the intensity of scattered light signals, according to the above method, the average gas concentration on the path to be measured can be obtained, thereby realizing the gas concentration measurement of the mobile (or any path) .

The above content is a further detailed description of the present invention in combination with the preferred technical solutions, and it cannot be considered that the specific implementation of the present invention is limited to these descriptions. For those of ordinary skill in the technical field of the present invention, simple deductions and replacements can be made without departing from the concept of the present invention, which should be regarded as the protection scope of the present invention.

Claims (5)

1. A gas concentration remote sensing detection device based on a coherent detection method is characterized by comprising a signal generator (1), a DFB laser (2), a first imaging lens (3), a first light splitting sheet (4), a second imaging lens (6), an aperture diaphragm (7), a third imaging lens (8), a first reflector (9), a second reflector (10), a second light splitting sheet (11), a photoelectric detector (12), a data acquisition card (13) and a computer (14);

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; the laser beam from the DFB laser (2) is collimated into parallel light by a first imaging lens (3); the parallel light is divided into two paths of light through a first light splitter (4): one path of light is reflected to a hard target (5) through a first light splitting sheet (4) as signal light; the other path of light is transmitted through the first light splitting sheet (4) and is used as local oscillation light; the signal light is scattered by a hard target (5), and the obtained scattered light signal is changed into parallel light after sequentially passing through a second imaging lens (6), an aperture diaphragm (7) and a third imaging lens (8); the aperture diaphragm (7) is positioned at the back focal length of the second imaging lens (6); the distance between the diaphragm (7) and the third imaging lens (8) is the front focal length of the third imaging lens (8); the collimated parallel light is transmitted through a second light-splitting sheet (11); the local oscillation light meets and interferes with the parallel light after being reflected by the first reflecting mirror (9), the second reflecting mirror (10) and the second light splitting sheet (11) in sequence; the generated coherent light is received by a photodetector (12); the photoelectric detector (12) 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 (13).

2. The remote gas concentration sensing device based on the coherent detection method according to claim 1, wherein the laser emitted by the DFB laser (2) is a frequency modulated laser beam, and comprises a DFB laser chip and a current driving and temperature control device, the working wavelength of the DFB laser (2) 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 gas concentration based on coherent detection method according to claim 1 or 2, wherein the splitting ratio of the first light splitter (4) is 90:10, wherein the signal light accounts for 90% and the local oscillator light accounts for 10%.

4. The remote gas concentration sensing device based on the coherent detection method according to claim 1 or 2, wherein:

the focal length of the first imaging lens (3) is 15mm, and the caliber of the first imaging lens is 12.7 mm;

the focal length of the second imaging lens (6) is 175mm, and the aperture is 50 mm;

the focal length of the third imaging lens (8) is 15mm, and the caliber of the third imaging lens is 12.7 mm.

5. The remote gas concentration sensing device based on the coherent detection method according to claim 3, wherein:

the focal length of the first imaging lens (3) is 15mm, and the caliber of the first imaging lens is 12.7 mm;

the focal length of the second imaging lens (6) is 175mm, and the aperture is 50 mm;

the focal length of the third imaging lens (8) is 15mm, and the caliber of the third imaging lens is 12.7 mm.

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