CN110471046B - Differential absorption laser radar system constant calibration method - Google Patents

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

A Calibration Method of Differential Absorption LiDAR System Constants

technical field

The invention relates to a method for calibrating differential absorption laser radar system constants, belonging to the technical field of laser radar detection.

Background technique

As an important tool for monitoring environmental pollutants, lidar has the advantages of strong anti-interference ability, high spatial resolution, high detection sensitivity, and long measurement optical path. It is widely used in the detection of aerosols, ozone, and various pollutant gases in the atmosphere. . As a kind of laser radar, differential absorption lidar is a new technology in the field of environmental monitoring in recent years, and has been widely used in the detection of the concentration of pollutant gases. It emits two laser beams with the same power and different wavelengths, one of which is located near the absorption peak of the target gas absorption line, called the on wavelength, and the other beam is located at the bottom of the target gas absorption line, called the off wavelength. The absorption intensity of the target gas to the two laser beams is different, which causes the attenuation of the atmospheric scattering echo signal to be different. By detecting the intensity difference of the two beams of reflected light, the concentration of the measured gas in the atmosphere can be calculated. The light source of the differential absorption lidar system generally adopts the method of one lidar emitting two laser beams alternately or two lasers emitting two laser beams at the same time, and space detection also requires the use of three-dimensional turntables and other equipment, which is more complicated than other systems . Due to the complex optical path characteristics and the different wavelengths of the two laser beams of the differential absorption lidar, the initial energy of the on-wavelength and off-wavelength lasers is often different in actual detection, and the on and off laser beams are in actual detection. Beam combination may not be able to achieve complete coincidence, especially when detecting in invisible bands such as ultraviolet and infrared, because the laser is invisible, it is difficult to guarantee the quality of beam combination, which will make the differential absorption lidar inversion The concentration of the target gas causes a certain error. For those laser radars with a large energy difference between the on wavelength and the off wavelength, it will directly lead to an error in its retrieval.

Contents of the invention

The present invention proposes a method for calibrating the system constants of differential absorption laser radar, which can effectively solve the system error introduced by differential absorption laser radar on-wavelength and off-wavelength laser energy, power jitter, sudden change, and the initial The energy is monitored and the laser state is recorded to improve data reliability.

The present invention adopts following technical scheme for solving its technical problem:

A method for calibrating a differential absorption laser radar system constant, comprising the steps of:

(1) Place the first photodetector, absorption pool and hard target at the exit laser of the system; adjust the position of the absorption pool, and place the first photodetector at the front of the absorption pool as a monitoring signal and initial on and off energy monitoring;

(2) Use a vacuum pump to evacuate the absorption pool, then flush into the target gas with a standard concentration of 20000ppm, and record the pressure in the absorption pool;

(3) Turn on the laser, use the signal acquisition card to record the first photodetector, the on and off signals detected by the second photodetector;

(4) Determine the radar system constants based on the collected data, remove the absorption pool and hard target, and perform normal atmospheric detection, and use the improved inversion formula to perform normal gas concentration detection.

The laser is a mid-infrared laser.

The first photodetector is a mid-infrared VIGO PVI-4TE photodetector.

The second photodetector is a photodetector of the mid-infrared VIGO PVI-4TE model.

The hard target adopts an aluminum hard plate with high reflectivity.

The beneficial effects of the present invention are as follows:

1. Detect the change of the optical signal before and after passing through the absorption cell to calculate the gas concentration in the absorption cell, and compare it with the gas concentration in the absorption cell to determine the correction constant of the entire laser radar system, which is introduced in the actual detection. The initial energy of the laser Do normalization processing to effectively reduce the error caused by the unequal initial energy of the laser, jitter and other issues.

2. Monitor the initial energy of the laser in real time, record the on and off energy of each beam, and achieve the effect of real-time detection.

3. This method is especially suitable for system correction of differential absorption lidar in invisible bands such as ultraviolet and infrared. Using this method can greatly reduce the inversion error caused by the system itself.

4. In this method, an additional initial laser detector, a temporary absorption pool, and a temporary target are installed on the basis of the original differential absorption lidar system to determine the system constants of the entire lidar, and then invert the atmosphere in the atmosphere through the improved inversion formula. concentration of the target gas.

5. The present invention monitors the initial laser energy through the first photodetector, and introduces the initial laser energy into the differential absorption lidar equation, which greatly reduces the inversion caused by problems such as unequal initial laser energy and laser energy jitter error, making the concentration inversion results more accurate.

Description of drawings

Fig. 1 is the structure diagram of the improved system, wherein, 1 is the laser, 2 is the beam combining mirror, 3 is the first photodetector, 4 is the absorption pool, 5 is the first 45° total reflection mirror, 6 is the second 45° ° all-reflective mirror, 7 is the third 45° all-reflective mirror, 8 is the fourth 45° all-reflective mirror, 9 is the fifth 45° all-reflective mirror, 10 is a hard target, 11 is a bovine reverse telescope, and 12 is the first Two photodetectors, 13 is a signal acquisition card, and 14 is an industrial computer.

Figure 2 is the flow chart of calibration inversion.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings.

The present invention takes a set of general differential absorption laser radar system as an example, the laser radar system mainly includes a laser radar transmitting system, a laser radar receiving system and a main control system; wherein, the laser radar transmitting system includes a laser 1, a beam combiner 2, a One 45° full mirror 5, the second 45° full mirror 6, the third 45° full mirror 7, the fourth 45° full mirror 8, the fifth 45° full mirror 9 and hard target 10, wherein the first Three 45 ° total reflection mirrors 7, the fourth 45 ° total reflection mirrors 8, and the fifth 45 ° total reflection mirrors 9 form a three-dimensional turntable; the lidar receiving system includes a cow trans telescope 11, the first photodetector 3, the second Photodetector 12 and signal acquisition card 13; Main control system uses industrial computer 14 to be connected with laser device 1, three-dimensional turntable, signal acquisition card 13 respectively, saves signal acquisition card 13 and collects the first photodetector 3, the second photodetector 12 experimental data. As shown in FIG. 1 , the improved system structure is shown. The first photodetector 3 and the second photodetector 12 respectively record the laser initial energy signal and the laser echo signal.

The entire system constant calibration process is as follows: firstly, at the end of the laser beam combining position, an absorption cell 4 with a target gas of known concentration is placed inside, and a hard target 10 is placed outside the blind area of the system, and the laser initial value is recorded by the first photodetector 3. signal, the second photodetector 12 records the echo signal, and then uses the differential absorption laser radar formula to invert the gas concentration and compare the gas concentration in the actual absorption cell to determine the system constant C, and finally remove the absorption cell 4 and the hard target 10, and use the first A photodetector 3, and a second photodetector 12, the system constant C performs normal gas concentration detection.

Inversion method improvements:

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, _Pt is the peak power, c is the speed of light, η is the efficiency of the receiving system, β(R) is the atmospheric backscattering coefficient, α(R) is the atmospheric extinction coefficient, and N(R) is the gas Concentration distribution, _σon,of is the differential absorption cross section of the gas, R is the distance between the target and the detector, and τ is the transmittance of the lidar system.

First calculate the logarithm of the ratio of the on and off echo signal strength, and then calculate the concentration information of the target gas through the path, and finally convert the concentration of the target gas into the international standard concentration unit according to the Avogadro constant and the molecular mass of the gas. Since the wavelengths of the two laser beams are relatively close, other correction items are ignored, and finally the target gas concentration on the entire path can be expressed as:

Where N(R) is the target gas concentration, ΔR is the entire path length of the beam, Δσ is the differential absorption cross section between on and off, P _off1 is the initial energy of the off laser, P _on1 is the initial energy of the on laser, and P _off2 is the off laser The echo signal after irradiating the hard target, P _on2 is the echo signal after the on laser irradiates the hard target. The previous differential absorption lidar only records the signals of P _on2 and P _off2 . It is generally believed that the energy of P _off1 and P _on1 are equal and Not taken into account, it is generally believed that The ratio of is constant at 1, but this approach will introduce a large error in the inversion of the target gas concentration, especially when the initial laser energy of on and off is not equal or the energy instability is too large, this method will effectively solve this problem .

The inversion equation used can be expressed as:

Among them, C is the constant of the whole radar system, which is the inversion result calculated by the hard target absorption experiment. We will determine the system constant C as follows.

Figure 2 shows the steps of the whole calibration scheme.

Step 1: Place the first photodetector 3, the absorption cell 4, and the hard target 10 at the proper positions of the whole system; adjust the position of the absorption cell 4 so that the optical path of the system passes through the interior of the absorption cell 4 completely, hits the hard target, and ensures that it can be placed The position of the optical path before and after the absorption pool 4 does not change. The first detector 3 is placed at the front end of the absorption pool 4 as a monitoring signal and initial on and off energy monitoring.

Step 2: The absorption pool 4 is evacuated by a vacuum pump, and then the target gas with a standard concentration of 20,000 ppm is flushed in, and the pressure in the absorption pool 4 is recorded.

Step 3: Turn on the laser 1, use the signal acquisition card 13 to record the on and off signals detected by the first photodetector 3 and the second photodetector 12.

Step 4: Determine the constant C according to the collected data, first according to the gas state equation

PV＝NRT (4)

Among them, P is the pressure in the absorption cell, V is the volume of the absorption cell, R is the gas constant, T is the ambient temperature, and N is the concentration of the target gas in the absorption cell, which is also a physical quantity that needs to be obtained.

Step 5: N calculated according to formula (4) is brought into formula (3), ΔσΔR is known, P _on1 and P _off1 are the detection data of the first detector 3, P _on2 and P _off2 are the second detection The echo signal detected by the detector 12, at this time, the only unknown quantity in the formula (3) is the system constant C, and the system constant C can be calculated by the formula (3).

Step 6: Remove the absorbing cell 4 and the hard target 10 to perform normal atmospheric detection, and use the improved inversion formula (3) to perform normal concentration inversion.

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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