CN110471046B - Differential absorption laser radar system constant calibration method - Google Patents
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- CN110471046B CN110471046B CN201910807136.6A CN201910807136A CN110471046B CN 110471046 B CN110471046 B CN 110471046B CN 201910807136 A CN201910807136 A CN 201910807136A CN 110471046 B CN110471046 B CN 110471046B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/497—Means for monitoring or calibrating
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
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- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
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Abstract
The invention relates to a differential absorption laser radar system constant calibration method, and belongs to the technical field of laser radar detection. The method comprises the following steps: (1) Respectively placing a first photoelectric detector, an absorption cell and a hard target at the system emergent laser; adjusting the position of an absorption tank, and placing a first photoelectric detector at the front end of the absorption tank as a monitoring signal and an initial on/off energy monitor; (2) The absorption tank is vacuumized by a vacuum pump, then target gas with standard concentration of 20000ppm is flushed, and the pressure in the absorption tank is recorded; (3) The method comprises the steps of starting a laser, and recording on and off signals detected by a first photoelectric detector and a second photoelectric detector by using a signal acquisition card; (3) Determining radar system constants from acquired data，And removing the absorption tank and the hard target, performing normal atmospheric detection, and performing normal gas concentration detection by using an improved inversion formula. The method monitors and records the laser state of the initial energy of the laser, and improves the data reliability.
Description
Technical Field
The invention relates to a differential absorption laser radar system constant calibration method, and belongs to the technical field of laser radar detection.
Background
The laser radar is used as an important tool for monitoring environmental pollutants, has the advantages of strong anti-interference capability, high spatial resolution, high detection sensitivity, long measurement optical path length and the like, and is widely applied to detection of various polluted gases in aerosol, ozone and atmosphere. As one of the lidars, the differential absorption lidar is a new technology in the field of environmental monitoring in recent years, and is widely used for concentration detection of a polluted gas. By emitting two laser beams with the same power and different wavelengths, one of the laser beams is positioned near the absorption peak of the absorption spectrum line of the target gas and is called as on wavelength, and the other laser beam is positioned at the valley bottom of the absorption spectrum line of the target gas and is called as off wavelength. The absorption intensity of the target gas on the two laser beams is different, so that the atmospheric scattering echo signals are different in attenuation, and the concentration of the detected gas in the atmosphere can be calculated by detecting the intensity difference of the two reflected light beams. The light source of the differential absorption laser radar system generally adopts a method that one laser radar alternately emits two laser beams or two laser devices emit two laser beams simultaneously, and space detection also needs to use three-dimensional rotary tables and other devices, which is complex compared with other systems. Because of the characteristics of complex light paths and different wavelengths of two laser beams of the differential absorption laser radar, the initial energy of the laser beams with on wavelength and off wavelength is often different in actual detection, and the combined beams of the laser beams with on wavelength and off wavelength can not reach the completely overlapped ground step in actual detection, especially when the detection is carried out in invisible wave bands such as ultraviolet, infrared and the like, the beam quality combined beam effect is difficult to ensure because the laser beams are invisible, so that certain errors are caused for inverting the target gas concentration of the differential absorption laser radar, and inversion errors can be directly caused for the laser radar with larger energy difference between the on wavelength and the off wavelength.
Disclosure of Invention
The invention provides a differential absorption laser radar system constant calibration method, which can effectively solve the problems of system errors caused by unequal laser energy, power jitter and abrupt change of the on wavelength and the off wavelength of a differential absorption laser radar, monitor and record the initial energy of laser, and improve the data reliability.
The invention adopts the following technical scheme for solving the technical problems:
a differential absorption laser radar system constant calibration method comprises the following steps:
(1) Respectively placing a first photoelectric detector, an absorption cell and a hard target at the system emergent laser; adjusting the position of an absorption tank, and placing a first photoelectric detector at the front end of the absorption tank as a monitoring signal and an initial on/off energy monitor;
(2) The absorption tank is vacuumized by a vacuum pump, then target gas with standard concentration of 20000ppm is flushed, and the pressure in the absorption tank is recorded;
(3) The method comprises the steps of starting a laser, and recording on and off signals detected by a first photoelectric detector and a second photoelectric detector by using a signal acquisition card;
(4) And determining radar system constants according to the acquired data, removing the absorption tank and the hard target, performing normal atmospheric detection, and performing normal gas concentration detection by using an improved inversion formula.
The laser adopts a middle infrared laser.
The first photoelectric detector is a photoelectric detector of a medium infrared VIGO PVI-4TE model.
The second photoelectric detector is a photoelectric detector of a medium infrared VIGO PVI-4TE model.
The hard target adopts an aluminum hard plate with high reflectivity.
The beneficial effects of the invention are as follows:
1. detecting the change of the optical signals passing through the front and rear of the absorption tank, calculating the gas concentration in the absorption tank, comparing the gas concentration with the gas concentration in the absorption tank, determining the correction constant of the whole laser radar system, introducing the correction constant in actual detection, carrying out normalization processing on the laser initial energy, and effectively reducing errors caused by the problems of unequal laser initial energy, jitter and the like.
2. The initial energy condition of the laser is monitored in real time, and the on and off energy of each beam is recorded, so that the effect of real-time detection is achieved.
3. The method is particularly suitable for correcting the system of the invisible wave band differential absorption laser radar such as ultraviolet, infrared and the like, and inversion errors caused by the defects of the system can be greatly reduced by adopting the method.
4. According to the method, an initial laser detector, a temporary absorption tank and a temporary target are additionally arranged on the basis of an original differential absorption laser radar system to determine the system constant of the whole laser radar, and then the concentration of target gas in the atmosphere is inverted through an improved inversion formula.
5. According to the invention, the first photoelectric detector is used for monitoring the initial laser energy, and the initial laser energy is introduced into the differential absorption laser radar equation, so that inversion errors caused by the problems of unequal laser initial energy, laser energy jitter and the like are greatly reduced, and the concentration inversion result is more accurate.
Drawings
Fig. 1 is a system structure diagram after improvement, wherein 1 is a laser, 2 is a beam combining lens, 3 is a first photoelectric detector, 4 is an absorption tank, 5 is a first 45-degree total reflection lens, 6 is a second 45-degree total reflection lens, 7 is a third 45-degree total reflection lens, 8 is a fourth 45-degree total reflection lens, 9 is a fifth 45-degree total reflection lens, 10 is a hard target, 11 is a bovine trans telescope, 12 is a second photoelectric detector, 13 is a signal acquisition card, and 14 is an industrial personal computer.
FIG. 2 is a scaled inversion flow chart.
Detailed Description
The invention will be described in further detail with reference to the accompanying drawings.
The invention takes a set of general differential absorption laser radar system as an example, and the laser radar system mainly comprises a laser radar transmitting system, a laser radar receiving system and a main control system; the laser radar emission system comprises a laser 1, a beam combining mirror 2, a first 45-degree total reflection mirror 5, a second 45-degree total reflection mirror 6, a third 45-degree total reflection mirror 7, a fourth 45-degree total reflection mirror 8, a fifth 45-degree total reflection mirror 9 and a hard target 10, wherein the third 45-degree total reflection mirror 7, the fourth 45-degree total reflection mirror 8 and the fifth 45-degree total reflection mirror 9 form a three-dimensional turntable; the laser radar receiving system comprises a cattle trans-telescope 11, a first photoelectric detector 3, a second photoelectric detector 12 and a signal acquisition card 13; the main control system is respectively connected with the laser 1, the three-dimensional turntable and the signal acquisition card 13 by using the industrial personal computer 14, and experimental data of the first photoelectric detector 3 and the second photoelectric detector 12 are acquired by the signal acquisition card 13. As shown in fig. 1, the improved system structure is shown, and the first photodetector 3 and the second photodetector 12 record the laser initial energy signal and the laser echo signal respectively.
The whole system constant calibration process comprises the steps of firstly placing an absorption tank 4 into which target gas with known concentration is flushed at the tail end of a laser beam combining position, placing a hard target 10 outside a system blind area, recording laser initial signals through a first photoelectric detector 3, recording echo signals through a second photoelectric detector 12, inverting the gas concentration by using a differential absorption laser radar formula to compare with the gas concentration in an actual absorption tank to determine a system constant C, finally removing the absorption tank 4 and the hard target 10, and detecting the normal gas concentration by using the first photoelectric detector 3 and the second photoelectric detector 12.
The inversion method is improved:
according to the differential absorption lidar equation, the single pulse echo power P on,off Can be expressed as:
in the equation, A is the area of the telescope, P t For peak power, c is the speed of light, η is the efficiency of the receiving systemThe ratio, beta (R) is the atmospheric backscattering coefficient, alpha (R) is the atmospheric extinction coefficient, N (R) is the gas concentration profile, sigma on,of The differential absorption section of the gas is represented by R, the distance between a target object and a detector, and tau, the transmittance of the laser radar system.
The method comprises the steps of firstly, logarithm of the intensity ratio of on echo signals to off echo signals, then, obtaining concentration information of target gas through a path, and finally, converting the concentration of the target gas into an international standard concentration unit according to an A Fu Jiade Luo constant and the molecular mass of the gas. Since the wavelengths of the two laser beams are relatively close, other correction terms are ignored, and finally the target gas concentration on the whole path can be expressed as:
where N (R) is the target gas concentration, ΔR is the total path length through which the beam passes, Δσ is the on and off differential absorption cross-section, P off1 For turning off the laser initial energy, P on1 For on laser initial energy, P off2 For echo signals after off laser irradiates a hard target, P on2 For the echo signal after on laser irradiates a hard target, the conventional differential absorption laser radar only records P on2 ，P off2 Is generally considered as P off1 ，P on1 Is not considered, it is generally considered that
The ratio of (2) is constantly 1, but the method can introduce large errors to inversion of the concentration of the target gas, especially when the initial laser energy of on and off is unequal or the energy instability is too high, and the method can effectively solve the problem.
The inversion equation used can be expressed as:
where C is the overall radar system constant, i.e. we calculate the result of the inversion using hard target absorption experiments. We will determine the system constant C as follows.
The steps of the overall calibration scheme are shown in fig. 2.
The first step: the first photoelectric detector 3, the absorption cell 4 and the hard target 10 are respectively arranged at the proper positions of the whole system; the position of the absorption tank 4 is adjusted, so that a system light path completely passes through the inside of the absorption tank 4 and is hit to a hard target, and the first detector 3 which does not change the position of the light path before and after the absorption tank 4 is taken and placed at the front end of the absorption tank 4 is ensured to be used as a monitoring signal and an initial on and off energy monitor.
And a second step of: the absorption cell 4 was evacuated using a vacuum pump, and then target gas having a standard concentration of 20000ppm was flushed, and the pressure in the absorption cell 4 was recorded.
And a third step of: the laser 1 is turned on, and the on and off signals detected by the first photodetector 3 and the second photodetector 12 are recorded by using the signal acquisition card 13.
Fourth step: determining a constant C according to the acquired data, and firstly, according to a gas state equation
PV＝NRT (4)
Wherein P is the pressure in the absorption tank, V is the volume of the absorption tank, R is the gas constant, T is the ambient temperature, and N is the concentration of the target gas in the absorption tank, which is also the physical quantity to be obtained.
Fifth step: n calculated according to equation (4) is then brought into equation (3), ΔσΔR is known, and P on1 ，P off1 For the first detector 3 to detect data, P on2 ，P off2 For the echo signal detected by the second detector 12, the only unknown of equation (3) is the system constant C, which can be calculated by equation (3).
Sixth step: and (3) removing the absorption tank 4 and the hard target 10, performing normal atmosphere detection, and performing normal concentration inversion by using an improved inversion formula (3).
Claims (5)
1. The differential absorption laser radar system constant calibration method comprises a laser radar transmitting system, a laser radar receiving system and a main control system; the laser radar emission system comprises a laser (1), a beam combining mirror (2), a first 45-degree total reflection mirror (5), a second 45-degree total reflection mirror (6), a third 45-degree total reflection mirror (7), a fourth 45-degree total reflection mirror (8), a fifth 45-degree total reflection mirror (9) and a hard target (10), wherein the third 45-degree total reflection mirror (7), the fourth 45-degree total reflection mirror (8) and the fifth 45-degree total reflection mirror (9) form a three-dimensional turntable; the laser radar receiving system comprises a cattle trans-telescope (11), a first photoelectric detector (3), a second photoelectric detector (12) and a signal acquisition card (13); the main control system is connected with the laser (1), the three-dimensional turntable and the signal acquisition card (13) respectively by using the industrial personal computer (14); the method is characterized by comprising the following steps of:
(1) A first photoelectric detector (3), an absorption cell (4) and a hard target (10) are respectively arranged at the system emergent laser; the position of the absorption tank (4) is adjusted, and the first photoelectric detector (3) is arranged at the front end of the absorption tank (4) and used as a monitoring signal and an initial on and off energy monitor;
(2) The absorption tank (4) is vacuumized by a vacuum pump, then target gas with standard concentration of 20000ppm is flushed, and the pressure in the absorption tank (4) is recorded;
(3) The laser (1) is turned on, a signal acquisition card (13) is used for recording on and off signals detected by the first photoelectric detector (3) and the second photoelectric detector (12);
(4) Determining radar system constants according to the acquired data, removing the absorption tank (4) and the hard target (10), performing normal atmospheric detection, and performing normal gas concentration detection by using an improved inversion formula; the inversion formula is expressed as:
where N (R) is the target gas concentration, ΔR is the total path length of the beam, Δσ is the on and off differential absorption cross-section, P off1 For turning off the laser initial energy, P on1 For on laser initial energy, P off2 For echo signals after off laser irradiates a hard target, P on2 For the echo signal after on laser irradiates a hard target, C is the whole radar system constant, namely the hard target absorption experiment is used for calculatingAnd (5) inverting the result.
2. A method of calibrating a system constant of a differential absorption lidar according to claim 1, characterized in that the laser (1) is a mid-infrared laser.
3. The method for calibrating the system constants of the differential absorption lidar according to claim 1, wherein the first photodetector (3) is a mid-infrared VIGO PVI-4TE type photodetector.
4. The method for calibrating the system constants of the differential absorption lidar according to claim 1, wherein the second photodetector (12) is a mid-infrared VIGO PVI-4TE type photodetector.
5. A method of calibrating a differential absorption lidar system constant according to claim 1, wherein the hard target (10) is a hard plate of high reflectivity aluminum.
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