CN104777487A - Atmospheric aerosol optical property measuring method and laser radar system - Google Patents

Abstract

The invention discloses an atmospheric aerosol optical property measuring method and a laser radar system which is high in spectral resolution. By locking the laser emission frequency, the function of the laser radar system which is high in spectral resolution is achieved. According to a Fabry-Perot interference narrowband spectral filter, an aerosol scattering component and a molecular scattering component are separated, the difficulty that in a traditional back scattering laser radar, one radar equation is utilized to invert two unknown quantities, namely a aerosol scattering coefficient and an extinction coefficient is appropriately solved, limitation of the emission laser wavelength do not exist, the measured atmosphere back scattering ratio is high in accuracy, and the relative error is small.

Description

A method for measuring optical characteristics of atmospheric aerosol and a laser radar system

technical field

The invention relates to a method for measuring the optical characteristics of atmospheric aerosols and a high-spectral resolution laser radar system, in particular to a high-resolution laser radar system based on a Fabry-Perot interferometer that ensures the coincidence of the interferometer spectrum and the laser spectrum by locking the laser frequency. Spectral Resolution LiDAR Systems.

Background technique

Atmospheric aerosols refer to solid and liquid particles suspended in the atmosphere with a diameter between 0.001-100 μm, which directly affect the radiation balance of the earth through absorption and scattering, and at the same time change the formation and characteristics of clouds, thereby indirectly affecting radiation transmission. The measurement of aerosol optical properties is very important for atmospheric research and flux transport research. In addition, the aerosols formed by air pollution often contain many harmful substances and even carcinogens, and are a kind of particulate air pollutants that are harmful to the human body. Therefore, atmospheric aerosols are one of the main contents of air pollution monitoring. It can be seen that the physical and optical properties of aerosols will directly or indirectly affect the radiation balance of the climate, and have a very important impact on the quality of the atmospheric environment and human health. Therefore, the in-depth study of aerosol is of great significance.

Generally, the atmospheric backscattering signal of lidar includes both molecular scattering and aerosol backscattering signals. To invert the optical properties of aerosol according to the lidar equation, assumptions must be made, such as horizontal uniformity or aerosol The condition that the ratio of the extinction coefficient to the backscatter is constant with distance leads to uncertainty in the inversion results. The lidar equation is:

  

in, is the received signal in the range of r, is the laser pulse emission signal, η is the quantum efficiency of the detector, A is the area of the receiving telescope system, r is the vertical height, is the geometric overlap factor of the receiving field angle of the laser beam, c is the speed of light, t is the laser pulse period, β and are the total backscattering coefficient and the total extinction coefficient of the atmosphere, respectively, and

where S1 is the aerosol extinction-scattering ratio, and there are two unknowns for a lidar equation ( and ), so the assumption of the ratio of the aerosol extinction coefficient to the backscattering coefficient should be made. It can be seen that there are uncertainties in the current method of using the LiDAR equation to invert the optical properties of aerosols.

Existing methods for extracting atmospheric aerosol parameters from meter-scattering LiDAR, such as the Klett method, need to make an assumption of LiDAR ratio, which affects the detection accuracy of the aerosol extinction coefficient. The HSRL (High Spectral Resolution Lidar) based on the iodine molecular filter uses the high suppression ratio characteristics of the iodine molecular filter to aerosol backscattering to separate the aerosol and molecular scattering, thus obtaining high-precision atmospheric aerosol and Molecular optics parameter profile, but the absorption peak of iodine molecular absorption filter does not exist at many commonly used laser frequencies, which limits its use; HSRL based on Fizeau interference filter separates aerosol scattering and atmospheric molecular scattering, solves A morbid mathematical problem in measuring the optical properties of aerosols by lidar is solved, and there is no need to assume the ratio of lidar, but its light energy collection efficiency is low.

Contents of the invention

The purpose of the present invention is to solve the defects existing in the prior art, provide a method for measuring the optical characteristics of atmospheric aerosols with a relatively small error, and solve the problem of using a radar equation to invert aerosols encountered by traditional backscatter lidar The difficulty of the two unknown quantities of scattering coefficient and extinction coefficient.

In order to achieve the above object, the present invention provides a method for measuring the optical characteristics of atmospheric aerosols, the method is by locking the laser emission frequency at the center position of the transmission spectrum peak of the Fabry-Perot interference filter, and launching the laser into the atmosphere , using the Fabry-Perot interference filter to receive the backscattering signal, and then inverting the atmospheric backscattering ratio and aerosol backscattering coefficient according to the lidar equation.                     

Among them, the locking of the laser emission frequency is realized by the following method: the backscattering signal after the laser emission enters the atmosphere is divided into two paths, one path enters the reference channel, and the other path enters the Fabry-Perot interference filter to obtain two paths of AD Detection signal; the ratio change of the AD detection signal of the reference channel and the AD detection signal after the Fabry-Perot interference filter is used to feedback and control the output laser frequency; the ratio of the two AD detection signals is locked to make the emission laser frequency The center frequency following the peak of the transmission spectrum of the Fabry-Perot interference filter drifts slowly to lock the laser emission frequency.

The present invention also provides a high spectral resolution laser radar system, the radar system includes a laser emitting system, a laser receiving system, a photoelectric detection system, a data acquisition and analysis system; the laser receiving system includes a telescope, a Fabry-Perot interference filter device, an optical receiving system; the laser emitting system emits laser light, one way enters the atmosphere, the backscattering signal is received by the telescope, and the other way is combined with the backscattering signal received by the telescope, and together they are introduced into the described as the received scattering signal An optical receiving system; after the scattered signal passes through the optical receiving system, it enters the reference channel all the way, and enters the Fabry-Perot interference filter all the way to obtain two echo signals respectively; the two echo signals respectively pass through The corresponding photoelectric detection system collects AD detection signals and atmospheric scattering signals; the data collected by the photoelectric detection system is input into the data acquisition and analysis system, the ratio of the two AD detection signals is locked, and the atmospheric backscattering ratio and the aerosol backscattering ratio are reversed. to the scattering coefficient.

Wherein the optical receiving system includes a lens, an optical filter, and a spectroscope; after the scattered signal passes through the lens and the optical filter in turn, it is divided into two parts by the spectroscope, one part enters the reference channel, and the other part enters the Fab R-Perot interference filter.

The photoelectric detection system includes a photodetector, an AD acquisition card, and a photon counting card; the photodetector receives the corresponding echo signal, converts it into an electrical signal, and collects the photomultiplier signal through the AD acquisition card and the photon counting card respectively.

The laser emitting system includes a seed laser, an oscillator and a beam expander; the seed laser injects laser light into the oscillator, and then outputs the beam after being expanded by a beam expander.

The data acquisition and analysis system includes a control system and a computer; the computer is respectively connected with the photoelectric detection system and the control system; the control system is connected with the laser emission system.

Compared with the prior art, the present invention has the following advantages: 1. Effective separation of atmospheric backscattering signals is realized; 2. Laser frequency is locked to ensure that the interferometer spectrum overlaps with the laser spectrum; 3. The backscattering coefficient for aerosols 4. Solve the difficulty of using one radar equation to invert the two unknown quantities of aerosol scattering coefficient and extinction coefficient encountered by traditional backscatter lidar; 5. The measured atmospheric backscatter High specific precision and small relative error.

Description of drawings

Figure 1 shows the total scattering spectrum of atmospheric molecules and aerosol backscattering;

Figure 2 is a schematic structural diagram of the high spectral resolution laser radar system of the present invention.

In the figure, 1-seed laser, 2-oscillator, 3-beam expander, 4-telescope, 5-reference fiber, 6-optical coupler, 7-signal receiving fiber, 8-lens, 9-filter, 10-beam splitter, 11-photodetector, 12-Fabry-Perot interference filter, 13-AD acquisition card, 14-photon counting card, 15-computer, 16-control system, 17-atmosphere.

Detailed ways

We know that the total scattering signal spectrum received by the telescope includes the Rayleigh scattering signal produced by the scattering of atmospheric molecules and the Mie scattering signal produced by the scattering of aerosol particles. Gaussian linear distributions of varying widths. Among them, due to the rapid thermal movement of atmospheric molecules, the Doppler broadening of the laser is more obvious, so the molecular scattering spectrum is also wider, generally in the order of GHz; the broadening of the laser spectrum by aerosol particles is mainly due to its Brownian motion Due to the slow movement speed, the broadening is not obvious. It is generally believed that the aerosol scattering spectrum has a spectral width equivalent to that of the emitted laser light (about 100MHz). Therefore, the aerosol signal appears as a narrow peak in the center of the total scattering spectrum. As shown in Figure 1.

The HSRL of the present invention mainly utilizes the characteristic of the total scattering spectrum. When the laser frequency is tuned and the transmission spectrum is scanned, the laser frequency is scanned to an appropriate position and the scanning is stopped, and the laser emission frequency is locked according to the ratio change of the transmittance to realize hyperspectral High-resolution lidar system function; through the Fabry-Perot interference narrow-band spectral filter, the aerosol scattering component and the molecular scattering component are separated, which just solves the problem of using a radar equation to invert the gas The two unknown quantities of sol scattering coefficient and extinction coefficient are difficult, and it is not limited by the wavelength of the emitted laser light. The backscattering ratio of the measured aerosol has high precision and small relative error.

As shown in Figure 2, the high spectral resolution lidar system of the present invention uses a narrow-band Nd:YAG laser injected with seed laser as the emitting light source. On the one hand, the detection of atmospheric scattering signals at different heights is completed, and on the other hand, the laser emission frequency is locked at the center of the peak of the transmission spectrum of the Fabry-Perot interference filter. Using the Fabry-Perot interference filter as a high spectral resolution filter device, the aerosol signal passes through the Fabry-Perot interference filter, while all the molecular signals are reflected, realizing the backscattering signal of atmospheric molecules and the air The aerosol backscattering signal is separated, and the aerosol backscattering coefficient is obtained by reversing the obtained signal intensity. The high spectral resolution laser radar system of the present invention is specifically composed of a laser emitting system, a laser receiving system, a photoelectric detection system, and a data acquisition and analysis system. The laser emitting system includes a seed laser 1, an oscillator 2 and a beam expander 3; the laser receiving system includes a telescope 4, an optical coupler 6, a lens 8, a filter 9, a beam splitter 10 and a Fabry-Perot interference filter device 12; the optical detection system includes an optical detector 11, an AD acquisition card 13 and a photon counting card 14; the data acquisition and analysis system includes a computer 15 and a control system 16. The output laser from the seed laser 1 passes through the oscillator 2 and the beam expander 3 in sequence, and is divided into two parts by the beam splitter, one part is directly introduced into the laser receiving system through the reference fiber 5, and the other part enters the atmosphere; the backscattered signal of the laser entering the atmosphere is The telescope 4 receives, passes through the optical coupler 6, combines the laser energy directly introduced through the signal receiving fiber 7 and the reference fiber 5, and jointly serves as the received scattered signal. The received scattered signal is collimated and filtered through the lens 8 and filter 9 in sequence, and passes through the spectroscope 10 , part of the energy enters the reference channel as reference energy, and the other part enters the Fabry-Perot interference filter 12 . The energy entering the reference channel passes through the photomultiplier tube (photodetector 11 ), converts the optical signal into an electrical signal, and then uses the AD acquisition card 13 and the photon counting card 14 to collect the photomultiplied signal. The computer 15 receives the collected signal, and uses the control system 16 to lock the ratio of the two AD detection signals (the control system uses the 16-bit D/A signal of the 16-bit PXI6259 data acquisition system of NI Company to scan, and the laser frequency can be set at a certain The longitudinal mode reaches a continuous tuning range of 30GHz, and its temperature tuning rate is -3.1GHz/°C. The temperature adjustment of the laser crystal is realized by applying an external voltage. The corresponding relationship between temperature and voltage is 1°C/V. Laser plus voltage can achieve high tuning accuracy), so that the emitted laser frequency slowly drifts with the center frequency of the peak of the transmission spectrum of the Fabry-Perot interference filter, thereby locking the laser emission frequency. At the same time, the computer 15 performs real-time analysis according to the collected data, and inverts the atmospheric backscatter ratio and the aerosol backscatter coefficient according to the measured data.

The specific steps for measuring the optical characteristics of atmospheric aerosols using the high spectral resolution laser radar system of the present invention are as follows:

1) The system uses a semiconductor-pumped, narrow-linewidth, continuous Nd:YAG laser as a seed laser and injects it into the high-energy pulse laser oscillator 2 to obtain a high-power, narrow-linewidth, 355nm output laser with a pulse energy of 20mJ. The repetition rate is 100Hz.

2) After the 355nm laser is expanded by the beam expander 3, most of the emitted laser energy enters the atmosphere 17, and a small part of the laser is directly introduced into the optical receiving system through the reference optical fiber 5.

3) The laser energy encounters the target object (atmosphere), and interacts with the target object to produce scattering in different directions, in which the backscattered signal is received by the telescope 4 .

4) The scattered signal received by the telescope 4 is optically coupled into the signal receiving optical fiber 7 through the optical coupler 6, and then combined with the laser energy directly introduced by the reference optical fiber 5 to jointly serve as the received scattered signal.

5) The received scattered signal light is collimated into parallel light through the lens 8, and the background light is compressed by the narrow-band filter 9 with a center wavelength of 355nm and a bandwidth of 0.35nm.

6) After the filtered and collimated signal passes through the spectroscope 10, part of the energy enters the reference channel as reference energy, and most of the energy enters the Fabry-Perot system (Fabry-Perot interference filter 12).

7) The energy entering the reference channel passes through the photomultiplier tube (photodetector 11) in the photodetection system to convert the optical signal into an electrical signal; the electrical signal is divided into two channels, one of which enters the peak hold circuit for transmittance detection The frequency is locked; the other part enters the photon counting card 14 after rapid amplification, and is used to obtain aerosol and molecular scattering signals.

8) After passing through the Fabry-Perot interference filter 12, the echo signal passes through the photomultiplier tube in the photodetection system to convert the optical signal into an electrical signal; then it is divided into two channels, one of which enters the peak hold circuit for transmission The rate detection locks the frequency; the other path enters the photon counting card 14 after being rapidly amplified, and is used to obtain the aerosol scattering signal.

9) The ratio of the AD detection signal of the reference channel to the AD detection signal after passing through the Fabry-Perot interference filter 12 changes, that is, the output laser frequency is feedback controlled by monitoring the change of the transmittance, and the ratio of the AD detection signal is locked , using the control system 16 (scanning and temperature control) to make the emitted laser frequency follow the slow drift of the center frequency of the transmission spectrum peak of the Fabry-Perot interference filter to lock the laser emission frequency.

10) After the laser emission frequency is locked, the collected data is analyzed in real time by the computer 15, and the atmospheric backscattering ratio and aerosol backscattering coefficient are reversed according to the measured data.

Claims (7)

1. A method for measuring the optical properties of atmospheric aerosols is characterized in that, by locking the laser emission frequency at the central position of the transmission spectrum line peak of the Fabry-Perot interference filter, the laser emission is entered into the atmosphere, and the Fabry-Perot interference filter is used to The R-Perot interference filter receives the backscattering signal, and then inverts the atmospheric backscattering ratio and the aerosol backscattering coefficient according to the lidar equation. 2. The measuring method according to claim 1, wherein the locking of the laser emission frequency is realized by the following method: the backscattering signal after the laser emission enters the atmosphere is divided into two paths, one path enters the reference channel, One way enters the Fabry-Perot interference filter to obtain two AD detection signals; the ratio change of the AD detection signal of the reference channel and the AD detection signal after the Fabry-Perot interference filter is used to feedback and control the output Laser frequency: lock the ratio of the two AD detection signals, so that the emitted laser frequency slowly drifts with the center frequency of the peak of the transmission spectrum of the Fabry-Perot interference filter to lock the laser emission frequency. 3. a high spectral resolution laser radar system adopting the described measuring method of claim 1, is characterized in that, described radar system comprises laser emitting system, laser receiving system, photoelectric detection system, data acquisition analysis system; Said laser The receiving system includes a telescope, a Fabry-Perot interference filter and an optical receiving system; the laser emitting system emits laser light, one way enters the atmosphere, the backscattering signal is received by the telescope, and the other way is backscattered with the backscattering signal received by the telescope The signals are combined and introduced into the optical receiving system together as received scattered signals; after the scattered signals pass through the optical receiving system, they enter the reference channel all the way, and enter the Fabry-Perot interference filter all the way to obtain two channels respectively. Echo signal; the two-way echo signals collect AD detection signals and atmospheric scattering signals through corresponding photoelectric detection systems respectively; the data collected by the photoelectric detection system is input into the data acquisition and analysis system, and the two-way AD detection signals are locked ratio, and invert the atmospheric backscatter ratio and aerosol backscatter coefficient. 4. The lidar system according to claim 3, wherein the optical receiving system comprises a lens, an optical filter, and a beam splitter; after the scattered signal passes through the lens and the optical filter in turn, it passes through the beam splitter Divided into two parts, one part enters the reference channel, and the other part enters the Fabry-Perot interference filter. 5. laser radar system according to claim 3, is characterized in that, described photodetection system comprises photodetector, AD acquisition card, photon counting card; Described photodetector receives corresponding echo signal, converts it After the electrical signal is generated, the photomultiplier signal is collected by the AD acquisition card and the photon counting card respectively. 6. The lidar system according to claim 3, wherein the laser emitting system comprises a seed laser, an oscillator and a beam expander; the seed laser injects laser light into the oscillator, and then expands the beam Output after beam expansion by mirror. 7. laser radar system according to claim 3, is characterized in that, described data acquisition analysis system comprises control system and computer; Described computer is connected with described photoelectric detection system, control system respectively; Described control system is connected with described connected to the laser emission system.