CN101105532A - All-fiber Raman scattering laser radar system based on wavelength-division multiplex technology for diffracting - Google Patents
All-fiber Raman scattering laser radar system based on wavelength-division multiplex technology for diffracting Download PDFInfo
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Abstract
The invention discloses a full optical fiber Roman scattering laser radar system based on WDM technique multiplexer light split which comprises a laser emission system, a receiving system, a light splitting system, an optical electric probing part and a computer system; the invention is characterized of the design of the light splitting system; the light splitting system in the invention splits light by depending on the characters of the WDM technique multiplexer such as sound wave length selectivity, high spectrum resolution and strong out-of-band inhibiting capability; the invention offers five design proposals which splits light by an optical fiber Bragg raster, a cascade connection optical fiber raster coupler, a multi-layer medium thin film filter, the optical fiber raster coupler based on Mach-Zehnder interferometer and a optical fiber Fabry-Perot cavity.
Description
Technical Field
The invention belongs to the technical field of meteorological and environmental observation, relates to a laser radar system for observing meteorological and atmospheric environments, and particularly relates to an all-fiber Raman scattering laser radar system for splitting light by using a Demultiplexer (Demultiplexer) of a Wavelength Division Multiplexing (WDM) technology.
Background
Laser radar as an active remote sensing detection tool is widely applied to the research fields of laser atmospheric transmission, global climate prediction, aerosol radiation effect, atmospheric environment and the like. The laser radar (Light detection and ranging) system mainly comprises a laser emitting system, a receiving system, a Light splitting system, a photoelectric detection component, a computer system and the like. The basic principle is as follows: the pulse laser beam emitted by the laser is emitted to the atmosphere, the scattering signal generated after the action of the pulse laser beam with the substances in the atmosphere is collected by the receiving system, and the scattering signal is converted into an electric signal by the photoelectric detection component after the light splitting treatment of the optical light splitting system and is sent to the computer for data processing.
The basis of laser remote sensing is various physical processes resulting from the interaction between optical radiation and atoms, molecules and aerosol particles in the atmosphere. Mie scattering (Mie scattering) is an elastic scattering whose spectrum width is similar to the spectrum width of an incident laser light, which is caused by aerosol particles having a particle size equal to or larger than the wavelength of the laser light, and whose center wavelength is the same as the wavelength of the incident laser light. Rayleigh scattering (Rayleigh scattering) is also an elastic scattering whose center wavelength is the same as the wavelength of the incident laser, and whose spectral width depends on the change of the atmospheric temperature, and is a scattering phenomenon caused by molecules or atoms whose scatterer particle size is smaller than that of the laser wavelength. Raman scattering (Raman scattering) can be divided into rotational Raman scattering and vibrational Raman scattering, which is inelastic scattering caused by atmospheric molecules or atoms, the scattering spectrum is distributed on both sides of the incident laser line, the scattering cross section is the smaller one of various scattering mechanisms, and the Raman scattering is very suitable for detecting atmospheric temperature and atmospheric components.
The meteorological observation mainly measures the atmospheric temperature, and the atmospheric environment observation mainly measures parameters such as aerosol extinction coefficient and scattering coefficient, aerosol optical thickness and atmospheric visibility. In general, the meteorological observation lidar and the atmospheric environment observation lidar use independent systems, because the atmospheric environment lidar uses a mie-rayleigh scattering signal in an atmospheric echo signal, which is different from a scattering signal used in the meteorological observation lidar. In order to enable the same laser radar system to observe weather parameters and atmospheric environment, the spectrum components of each scattering signal in the atmospheric echo signals must be effectively separated and detected, and the mutual relationship of the spectrum components is effectively utilized to analyze and solve.
However, since the cross section of raman scattering is 3 to 4 orders of magnitude smaller than that of the meter scattering caused by aerosol and the rayleigh scattering caused by atmospheric molecules, it is necessary to suppress the meter and rayleigh scattering signals by more than 7 orders of magnitude, extract a weak raman line from strong meter and rayleigh scattering signals, and further require the out-of-band suppression capability and high spectral resolution capability of the optical splitting system of the laser radar. In addition, during daytime detection, the solar background light needs to be effectively filtered. At present, the light splitting scheme of the rotating raman laser radar for extracting the rotating raman scattering spectrum generally adopts an interference filter, a single grating monochrometer and an atomic resonance absorption filter, or a light splitting method of a double grating monochrometer.
In recent years, with the rapid development of optical fiber communication technology, the widespread adoption of WDM technology in optical fiber communication has matured. WDM technology employs a scheme that transmits optical signals of multiple wavelengths using the same rate in the same optical fiber. The core optics of a WDM system are multiplexers/demultiplexers (multiplexers/demultiplexers) that function to optically separate closely spaced wavelengths (channels) or to enable the addition and subtraction of Optical wavelengths (channels), commonly referred to as Optical Add/Drop multiplexers (Optical Add/Drop multiplexers). I.e. they are means for combining (coupling) and splitting (splitting) optical signals of different wavelengths, respectively, a Multiplexer combines several wavelength channels onto one optical fiber, as opposed to a Demultiplexer, which in practice often is a WDM apparatus that acts as both a Demultiplexer and a Multiplexer. The WDM technology has the advantages of excellent wavelength selectivity, high spectral resolution, small spectral response function sidelobe, small half-peak width, multiple ports, small size, low insertion loss, simple and compact structure, stable performance and easy realization of all-fiber connection.
Therefore, the invention provides an all-fiber Raman laser radar for meteorological observation and atmospheric environment observation based on the WDM technology demultiplexer light splitting technology based on the reference of the WDM technology demultiplexer of optical fiber communication and the strict wavelength selection characteristic, high spectral resolution capability and out-of-band inhibition capability thereof.
Disclosure of Invention
The invention aims to provide an all-fiber Raman scattering laser radar system, which utilizes a WDM demultiplexer to split light, and a set of laser radar systems simultaneously observe weather and atmospheric environment to finally realize atmospheric temperature weather parameter measurement and atmospheric environment parameter measurement such as atmospheric aerosol extinction coefficient and scattering coefficient, aerosol optical thickness, atmospheric visibility and the like.
The invention adopts the technical scheme that the WDM technology-based all-fiber Raman scattering laser radar system for demultiplexing by a demultiplexer comprises a laser transmitting system, a receiving system, a light splitting system, a photoelectric detection component and a computer system, wherein the system comprises a laser transmitting system, a receiving system, a light splitting system, a photoelectric detection component and a computer system
YAG pulse laser, beam expander for collimating and expanding pulse laser, multiple reflectors for vertically emitting the collimated and expanded laser to atmosphere,
a receiving system for receiving backward scattered light generated by interaction between laser and molecules and particles in the atmosphere, and coupling the received laser radar atmosphere echo signal to a multimode fiber which sends the atmosphere echo signal into the multimode fiber
The multimode/single-mode optical fiber converter is used for converting the multimode atmospheric echo signal into a single-mode optical fiber signal and sending the single-mode optical fiber signal into the single-mode optical fiber, or directly coupling the laser radar atmospheric echo signal received by the telescope into the single-mode optical fiber and then sending the single-mode optical fiber signal into the telescope
The optical splitting system is used for separating the rotating Raman spectral line and the Mi-Rayleigh scattering spectral line in the echo signal, outputting each separated scattering signal from a corresponding port and sending the signals into
Photoelectric detection component for converting the separated scattered light signals into electric signals and receiving the electric signals
And the computer system is pre-loaded with weather and atmospheric environment parameter inversion algorithm programs and is used for analyzing and processing the received scattering spectral line signals to obtain atmospheric temperature weather parameters, atmospheric aerosol extinction coefficients and scattering coefficients, aerosol optical thickness and atmospheric visibility atmospheric environment parameter values.
The light splitting system performs light splitting by using the characteristics of excellent wavelength selectivity, high spectral resolution and strong out-of-band inhibition capability of a demultiplexer of a WDM (wavelength division multiplexing) technology of optical fiber communication.
Drawings
FIG. 1 is a schematic diagram of a laser radar system according to the present invention, wherein a is a multi-mode fiber, a multi-mode/single-mode converter, and a single-mode fiber for transmitting an echo signal to an optical splitting system, and b is a single-mode fiber for transmitting an echo signal to an optical splitting system;
FIG. 2 is a schematic diagram of the structure of the optical splitting system of the present invention, wherein a is a fiber Bragg grating optical splitting system, b is a fiber grating coupler optical splitting system, c is a multilayer dielectric thin film filter optical splitting system, d is a fiber grating coupler optical splitting system based on a Mach-Zehnder interferometer, and e is a fiber Fabry-Perot cavity optical splitting system;
FIG. 3 shows the Bragg reflection spectrum of the Bragg grating in the optical splitting system and the atmospheric molecule N 2 、O 2 Relation of rotating Raman spectrum ofAn intent;
FIG. 4 is a graph showing the transmittance characteristics of FBG1 and the reflectance characteristics of FBGs 2 and 3 of a fiber Bragg grating in the optical splitting system of the present invention, wherein a is a graph showing the transmittance characteristics of FBG1, and b is a graph showing the reflectance characteristics of FBGs 2 and 3;
FIG. 5 is a diagram of the intensity distribution of the atmospheric echo signal after being filtered by the beam splitter;
FIG. 6 is a high profile of the measured signal-to-noise ratio and temperature measurement error for day and night that is theoretically achievable by the system of the present invention.
In the figure, 1, a laser emission system, 2, a telescope receiving system, 3, a light splitting system and a photoelectric detection component, 4, a computer system, 5, a first optical fiber circulator, 6, a first optical fiber Bragg grating (FBG 1), 7, a second optical fiber circulator, 8, a second optical fiber Bragg grating (FBG 2), 9, a third optical fiber circulator, 10, a third optical fiber Bragg grating (FBG 3), 11, a first photoelectric detection component (PMT 1), 12, a second photoelectric detection component (PMT 1), 13, a third photoelectric detection component (PMT 1), 14, a first optical fiber grating coupler (FBG 1), 15, a second optical fiber grating coupler (FBG 2), 16, a third optical fiber grating coupler (FBG 3), 17, a Multilayer Dielectric Thin Film Filter (MDTFF), 18, a first Lens (Lens), 19, a second Lens, 20, a third Lens, 21, a fourth Lens, 22, a first fiber optic interferometer (M _ ZI 1), 23, a second fiber optic interferometer (M _ ZI 2), 24, a third fiber optic interferometer (M _ ZI 3), 25, a first fiber coupler (FC 1), 26, a second fiber coupler (FC 2), 27, a third fiber coupler (FC 3), 28, a fourth fiber coupler (FC 4), 29, a fifth fiber coupler (FC 5), 30, a sixth fiber coupler (FC 6), 31, a first fiber resonator (F-P cavity 1), 32, a second fiber resonator (F-P cavity 2), 33, and a third fiber resonator (F-P cavity 3).
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.
The invention relates to a Raman laser radar system which can be used for meteorological observation and atmospheric environment observation at the same time, the system of the laser radar is shown in figure 1, wherein, a is to transmit an echo signal to a light splitting system by utilizing a multimode optical fiber, a multimode/single mode converter and a single mode optical fiber, and b is to transmit an echo signal to the light splitting system by utilizing the single mode optical fiber. The laser emitting system 1 comprises an Nd: YAG pulse laser, a collimation beam expander and a plurality of reflecting mirrors.
The laser emission system 1 of the present invention employs an Nd: YAG pulse laser which outputs its second harmonic (lambda) 0 =532.25 nm) as a light source, and optical components and structures emitted.
The photoelectric detection system consists of a first photoelectric detection component 11 (PMT 1), a second photoelectric detection component 12 (PMT 2) and a third photoelectric detection component 13 (PMT 3) and is used for converting echo optical signals into electric signals and receiving the electric signals. The first photoelectric detection component 11 detects a Mi-Rayleigh scattering signal and can be used for measuring the optical characteristics of the atmospheric aerosol; the second photoelectric detection component 12 and the third photoelectric detection component 13 detect 2 rotation Raman scattering signals for temperature measurement.
The optical splitting system has the function of separating a rotating Raman spectral line from a Mi-Rayleigh scattering spectral line and the like in an echo signal, and the interference of Mi scattering, rayleigh scattering signals and solar background light is suppressed to the maximum extent in a temperature detection channel. The main components of the light splitting system are respectively realized by a demultiplexer of WDM technology, and the specific realization scheme is as follows:
The optical Fiber Bragg Grating (FBG) is mainly adopted to realize light splitting.
FIG. 2a shows an embodiment where the system comprises a first fiber circulator 5, a first fiber Bragg grating 6 with a wavelength λ b Is λ b1 =532.25nm, a second optical fiber circulator 7, a second fiber Bragg grating 8, and a wavelength λ thereof b Is λ b2 ＝530.6nmA third optical fiber circulator 9, a third fiber Bragg grating 10 having a wavelength λ b Is λ b3 ＝528.8nm。λ b1 、λ b2 And λ b3 Each FBG is designed to have a high reflectivity and a very low transmissivity to the selected wavelength, corresponding to the mie-rayleigh scattering signal, the low quantum number rotating raman scattering signal and the high quantum number raman scattering signal, respectively.
Atmospheric echo signals containing multiple wavelengths received by a telescope are coupled into a multimode fiber (MMF), converted by a multimode/single-mode fiber converter and then sent into an input port1 of a first fiber circulator 5 through a single-mode fiber (SMF) (or the atmospheric echo signals received by the telescope are directly coupledCombining to a Single Mode Fiber (SMF), then sending into an input port1 of a first optical fiber circulator 5, outputting to an input port2 connected with a first fiber Bragg grating 6 through the first optical fiber circulator 5, wherein the wavelength meeting the Bragg condition of the first fiber Bragg grating 6 is equal to lambda b1 The mie-rayleigh scattering signal is reflected back to the first optical fiber circulator 5, and is output by the output port3 of the first optical fiber circulator 5, and then is received by the first photoelectric detection component 11. This is channel 1. At the channel 1, the parameter of the first fiber Bragg grating 6 is designed, so that the Mi-Rayleigh signal transmitted through the first fiber Bragg grating 6 is very weak, and a high out-of-band rejection rate is achieved.
Because the first fiber bragg grating 6 has no reflection effect on light with other wavelengths, the light with other wavelengths transmits through the output port4 of the first fiber bragg grating 6 and then enters the input port1 of the second fiber circulator 7, and the second fiber bragg grating 8 enables the wavelength to be equal to lambda b2 The low-quantum-number rotational scattered signal is totally reflected back to the second optical fiber circulator 7 and is received by the second photodetection component 12 after being output at its output port 3. This is channel 2.
For the same reason, the other wavelengths are not λ b2 After transmitting through the second fiber bragg grating 8, the optical signal is distributed by a third optical fiber through a third optical fiber circulator 9The reflection of the Lag grating 10 separates the wavelength lambda b3 And received by the third photodetection component 13. This is channel 3.
At the channel 2 and the channel 3, parameters of the second fiber bragg grating 8 and the third fiber bragg grating 10 are designed, so that the residual signals of the meter and the rayleigh scattering are further suppressed respectively.
Therefore, the high-precision extraction of two high-low quantum number rotating Raman scattering spectral lines for temperature measurement is realized while rice and Rayleigh scattering signals are effectively separated and inhibited.
The light splitting is mainly realized by adopting a cascaded Fiber Grating Coupler (FGC).
FIG. 2b shows an embodiment, the system includes a cascade of a first fiber grating coupler 14 and a second fiber grating couplerA fiber grating coupler 15 and a third fiber grating coupler 16, as well as a first photoelectric detection component 11, a second photoelectric detection component 12 and a third photoelectric detection component 13 for receiving reflected signals, and designing the Bragg wavelength lambda of each fiber grating coupler b Setting λ of the first fiber grating coupler 14 b Is λ b1 =532.25nm, and λ of the second fiber grating coupler 15 is set b Is λ b2 =530.6nm, and λ of the third fiber grating coupler 16 is set b Is λ b3 And (3) 528.8nm, namely corresponding to a metric-rayleigh scattering signal, a low-quantum-number rotating raman scattering signal and a high-quantum-number raman scattering signal respectively, and each FBG has a very high reflectivity and a very low transmissivity respectively for the selected wavelength by design.
The atmospheric echo signal received by the telescope and containing a plurality of wavelengths is coupled into a multimode fiber (MMF), is converted by a multimode/single mode fiber converter and then is sent into a first fiber grating coupler 14 through a Single Mode Fiber (SMF), or the atmospheric echo signal received by the telescope is directly coupled into a Single Mode Fiber (SMF) and then is sent into the first fiber grating coupler 14, and the first fiber grating coupler is coupled with a first fiber gratingThe combiner 14 will have a wavelength equal to lambda b1 The mie-rayleigh scattering signal is completely reflected back, and is output from the output port2 and then received by the first photoelectric detection component 11. This is channel 1. At the channel 1, by designing the parameters of the first fiber grating coupler 14, the mie-rayleigh signal transmitted through the first fiber grating coupler 14 is very weak, and a high out-of-band rejection rate is achieved.
Since the first fiber grating coupler 14 does not reflect light with other wavelengths, light with other wavelengths passes through the first fiber grating coupler 14 and enters the second fiber grating coupler 15, and the second fiber grating coupler 15 enables the light with the wavelength equal to λ b2 Is reflected and output at its output port3, and is received by the second photodetection component 12. This is channel 2.
Similarly, other wavelengths are not λ b2 The optical signal is further transmitted through the second fiber grating coupler 15 and then reflected by the third fiber grating coupler 16, and the separated wavelengths are respectively lambda b3 High quantum number raman scattering signal. And received by the third photodetection means 13. This is channel 3.
In the channels 2 and 3, parameters of the second fiber grating coupler 15 and the third fiber grating coupler 16 are designed, so that the residual signals of the meter and the rayleigh scattering are further suppressed respectively.
Therefore, the high-precision extraction of two high-low quantum number rotating Raman scattering spectral lines for temperature measurement is realized while rice and Rayleigh scattering signals are effectively separated and inhibited.
Light splitting is mainly realized by a multi-layer dielectric thin film filter (DTMF).
Figure 2c shows an embodiment where the system comprises a multilayer dielectric thin film filter 17 and a first lens 18, a second lens 19, a third lens 20 and a fourth lens 21. The transmission wavelengths in the multilayer dielectric thin film filter 17 are respectively designed to be λ 1 ＝532.25nm、λ 2 =530.6nm and λ 3 And (4) =528.8nm, which respectively corresponds to a meter-rayleigh scattering signal, a low-quantum-number rotating raman scattering signal and a high-quantum-number raman scattering signal, and the DTMF has a very high transmittance and a very low reflectance respectively for the selected wavelength through design.
The atmospheric echo signal received by the telescope and containing a plurality of wavelengths is coupled into a multimode fiber (MMF), is converted by a multimode/single mode fiber converter, is transmitted through a Single Mode Fiber (SMF), is converged by a first lens 18 and then is transmitted into a multilayer dielectric thin film filter 17, or is directly coupled to the Single Mode Fiber (SMF), is transmitted and then is converged by the first lens 18 and then is transmitted into the multilayer dielectric thin film filter 17, the multilayer dielectric thin film filter 17 respectively selects and transmits corresponding wavelengths and then outputs at each output port, and the output optical signals are respectively transmitted through a second lens 19, a third lens 20 and a fourth lens 21 and then are respectively received by a corresponding first photoelectric detection part 11, a corresponding second photoelectric detection part 12 and a corresponding third photoelectric detection part 13. The method realizes the light splitting and detection of the required Mi-Rayleigh scattering signal, low quantum number rotating Raman scattering signal and high quantum number Raman scattering signal.
The method mainly adopts a fiber grating coupler (M-ZI-FGC) based on a Mach-Zehnder interferometer (M-ZI) to realize light splitting.
Fig. 2d shows an embodiment, which is composed of a first fiber coupler 25, a first fiber interferometer 22, a second fiber coupler 26, a third fiber coupler 27, a second fiber interferometer 23, a fourth fiber coupler 28, a fifth fiber coupler 29, a third fiber interferometer 24 and a sixth fiber coupler 30, which are cascaded. Wherein the Bragg wavelength λ of the FBG in each fiber Mach-Zehnder interferometer b Are respectively designed as lambda b1 ＝532.25nm、λ b2 =530.6nm and λ b3 =528.8nm, i.e. for mie-rayleigh scattering signals, low quantum number rotational raman scattering signals and high quantum number raman scattering signals respectively,each fiber interferometer is configured to have a high reflectivity and a very low transmission for the selected wavelength.
The atmospheric echo signal received by the telescope and containing a plurality of wavelengths is coupled into a multimode fiber (MMF), converted by a multimode/single mode fiber converter, and is divided into two beams of light by a first fiber coupler 25 (FC 1) after passing through a Single Mode Fiber (SMF), or the atmospheric echo signal received by the telescope is directly coupled to the Single Mode Fiber (SMF) for transmission, is divided into two beams of light by the first fiber coupler 25, and is sent to a first fiber interferometer 22 and is respectively transmitted at two arms of the first fiber interferometer 22, and the wavelength meeting the grating reflection condition is equal to lambda b1 When the light of the mie-rayleigh scattering signal is reflected from both arms and passes through the first optical fiber coupler 25 again, the two optical signals interfere with each other and are output from the port2 of the output port and received by the first photoelectric detection unit 11. This is channel 1. When the signal light that does not satisfy the grating condition passes through the fiber grating and is transmitted to the second fiber coupler 26, interference occurs and the signal light is output from the output port. This separates the signal light satisfying the grating wavelength condition from the signal light of the remaining wavelengths.
Similarly, the rest of the optical signals passing through the first optical fiber interferometer 22 continue to propagate, and after passing through the second optical fiber interferometer 23 and the third optical fiber interferometer 24, respectively, the interferometers demultiplex corresponding raman scattering signals with low quantum number and high quantum number, and after being output from the output ports port3 and port4 of each interferometer M-ZI, the signals are received by the corresponding second photoelectric detection component 12 (here, channel 2) and third photoelectric detection component 13 (here, channel 3).
Therefore, the high-precision extraction of two high-low quantum number rotating Raman scattering spectral lines for temperature measurement is realized while rice and Rayleigh scattering signals are effectively separated and inhibited.
The optical fiber Fabry-Perot Cavity (F-P Cavity) is mainly adopted to realize light splitting.
FIG. 2e shows an embodiment formed by cascading a first fiber cavity 31 and a second fiber cavityA fiber cavity 32 and a third fiber cavity 33. Wherein the wavelength λ of each fiber cavity Fabry-Perotcavity is respectively designed as λ 1 ＝532.25nm、λ 2 =530.6nm and λ 3 And =528.8nm, i.e. a mie-rayleigh scattering signal, a low quantum number rotating raman scattering signal and a high quantum number raman scattering signal, respectively, each F-PCavity being designed to have a very high reflectivity and a very low transmission for said selected wavelength, respectively.
The atmospheric echo signal containing multiple wavelengths received by a telescope is coupled into a multimode fiber (MMF), is converted by a multimode/single mode fiber converter and then is sent into an input port1 of a first fiber circulator 5 through a Single Mode Fiber (SMF), or the atmospheric echo signal received by the telescope is directly coupled to a Single Mode Fiber (SMF), then is sent into an input port1 of the first fiber circulator 5 and is output at an output port2 connected with a first fiber resonant cavity 31 after passing through the first fiber circulator 5, and the wavelength of the atmospheric echo signal is equal to lambda of the first fiber resonant cavity 31 1 The Mi-Rayleigh scattering signal is completely sent to an output port4 to be output and received by a first photoelectric detection component 11; this is channel 1.
The light with other wavelengths is reflected and then continuously transmitted forward, and then enters the input port1 of the second optical fiber circulator 7 after being output from the output port3 of the first optical fiber circulator 5, and the wavelength of the light with the wavelength equal to lambda is transmitted through the second optical fiber resonant cavity 32 2 The low-quantum-number rotation scattering signal is output to the output port4 and then received by the second photoelectric detection component 12. This is channel 2.
Similarly, a high quantum number raman scattered signal having a wavelength λ 3 can be separated and received by the third photodetection component 13. This is channel 3.
Therefore, the high-precision extraction of two high-low quantum number rotating Raman scattering spectral lines for temperature measurement is realized while rice and Rayleigh scattering signals are effectively separated and inhibited.
The computer system comprises a multi-channel synchronous high-speed A/D acquisition card, a computer, a data inversion method for obtaining parameters such as atmospheric temperature, atmospheric aerosol extinction coefficient, aerosol optical thickness and the like, and corresponding application software.
According to Raman scattering signals detected by the photoelectric detection components PMT2 and PMT3, the intensity of the two detected signals and the intensity ratio of the two signals are solved according to a Raman scattering laser radar equation, and the above formula is calibrated and fitted by utilizing radio sounding data to obtain the atmospheric temperature value. And the air temperature detection error of the system is further worked out by utilizing the sensitivity of air temperature detection and the signal-to-noise ratio of the Raman scattering signal obtained by detection.
And the optical characteristic parameters of the atmospheric aerosol can be obtained by inversion according to a Mie-Rayleigh scattering signal detected by the photoelectric detection component PMT1 and a Mie scattering laser radar equation.
YAG pulse laser emits pulse laser beam with wavelength of 532.25nm, after being collimated and expanded by the collimating and expanding system, the pulse laser beam is emitted to atmosphere vertically via several reflectors, the backward scattered light produced by the interaction between laser and the molecules and particles in the atmosphere is received by the telescope receiving system, the received laser radar atmosphere echo signal is coupled to multimode fiber, and after passing through the multimode/single mode fiber converter, the signal is sent to the light splitting system directly or the laser radar atmosphere echo signal received by the telescope is coupled to the single mode fiber and then sent to the light splitting system, the light splitting system splits the atmosphere backward scattered light into various optical signals, which are output from the corresponding port, received by the photoelectric detection part PMT and sent to the computer system for analysis and processing.
FIG. 3 shows the Bragg reflectivity of the fiber Bragg grating and the atmospheric molecules in the optical splitting system of the present inventionN 2 、O 2 Schematic diagram of the relationship of the rotating raman spectra of (a).
When the emitted laser beam propagates in the atmosphere, it interacts with N in the atmosphere 2 And O 2 Interact to produce raman scattering. Due to N 2 And O 2 Of Pure Rotational Raman Spectroscopy (PRRS)The method has dependence on the environment temperature, and meanwhile, the Anti-Stokes branch is selected for detection in consideration of the characteristic that the atmospheric fluorescence interferes with the Stokes branch of the pure rotation Raman spectrum. According to the backscattering section formula of the pure rotation Raman signal, N can be calculated 2 The raman scattering cross-section of the molecule varies at different wavelengths.
The line intensities at temperatures T =200K and T =300K are taken, taking into account that the lower atmospheric temperature varies from 200K to 300K. As can be seen from the figure, N 2 The PRRS intensity distribution of (a) varies with temperature T, with the raman line intensity near the wavelength of the emission laser decreasing with increasing temperature and the raman line intensity further away from the emission line increasing with increasing temperature. That is, the line intensity corresponding to a lower quantum number J decreases with increasing temperature, while the line intensity corresponding to a higher quantum number J increases with increasing temperature. Therefore, two quantum numbers J with the strength varying oppositely with the temperature are selected 1 ＝6，J 2 =14, corresponding wavelengths of 530.6nm and 528.8nm, respectively, i.e. the central wavelengths at which the demultiplexer of the WDM is used to separate the two raman lines. When designing the optical splitting system, in order to ensure that the two detected raman signals have certain intensity, the selected various optical splitting devices need to ensure certain bandwidth,
because the strength of the low quantum number and high quantum number spectral lines of the atmospheric pure rotation Raman spectral line can be respectively reduced and enhanced along with the rise of the temperature, the Raman signals with the 2 central wavelengths are selected and the 2 measurement signals are subjected to differential processing, the temperature measurement sensitivity in the invention becomes the sum of the temperature sensitivities of the 2 channels, and the integral temperature measurement sensitivity characteristic of the system is improved.
Fig. 4 is a schematic diagram of the transmittance characteristic of FBG1 and the reflectance characteristics of FBGs 2 and 3 of the fiber Bragg grating in the optical splitting system of the present invention, and the principle of the optical splitting system is explained in case 1.
Choosing the Bragg wavelength of FBG1 as λ b1 =532.25nm, mainly used for filtering out meter and Rayleigh scatteringA signal. Taking into account Rayleigh scattered signalsThe spectral range is 3GHz, so that FBG1 is chosen to be at λ b1 Has a full width at half maximum (FWHM) of Δ λ FWHM1 =0.018nm, pair λ thereof b1 Has a reflectance of R1 (lambda) b1 ) =0.999, and the transmittance is only T1 (lambda) b1 )＝2.036×10 -4 As shown in a in fig. 4, almost all raman spectrum signals at other wavelengths are transmitted, and of course, some residual signals such as the mie and rayleigh signals are transmitted, but the suppression rate for the mie and rayleigh signals reaches 4 orders of magnitude at this time.
The Bragg wavelengths of the FBGs 2 and 3 are respectively selected to be lambda b2 =530.6nm and λ b3 =528.8nm, and is mainly used for extracting quantum number J 1 =6 and J 2 Raman lines with a full width at half height of =14, in each case Δ λ FWHM2 ＝0.288nm，Δλ FWHM3 =0.508nm and the reflectance is R2 (lambda) respectively b2 )＝0.908，R3(λ b3 ) =0.945, thus at λ b2 And λ b3 Respectively, raman lines can be efficiently extracted as shown in b of fig. 4. In addition, λ b2 、λ b3 At λ b1 The reflectivity of each of them is R2 (lambda) b1 )＝4.059×10 -4 ，R3(λ b1 )＝6.887×10 -4 So they are paired from λ b1 The transmitted residual signals of the meter and the Rayleigh signals can be further inhibited respectively, and the inhibition rate reaches 3 orders of magnitude. Therefore, the WDM demultiplexer of the system has the suppression rate of the signals of the Mi and Rayleigh numbers of more than 7 orders of magnitude, and ensures the high-precision extraction of the Raman spectral lines with high and low quantum numbers.
Fig. 5 is an intensity distribution diagram of the atmospheric echo signal after being filtered by the optical splitter, where the intensity distribution of the atmospheric echo signal after being split is calculated in schemes 1 and 2.
The sampling period is 300ns during calculation, and the corresponding detection height resolution is 45m. Suppose a wavelength λ in the sky b2 The radiant energy density of the nearby solar background light is 3X 108Wm -2 sr -1 nm -1 Depending on the system parameters of the lidarTo estimate at the wavelength lambda b2 And λ b3 The intensity of the solar background light detected by the system is 3.251 multiplied by 10 near the spectral line -11 W is added. According to an atmosphere Mie scattering signal model and system parameters of the radar, and considering that the light splitting system has 7 orders of magnitude of filtering rate of Mie-Rayleigh scattering signals, each scattering signal entering the radar system and the solar background light can be calculated through a laser radar equationThe intensity was distributed with the detected height, and the result is shown in fig. 5.
As can be seen from FIG. 5, the filtering rate of the light splitting system to the Mie-Rayleigh scattering signals reaches 7 magnitude orders, the signal-to-noise ratio of the system required by temperature measurement is effectively ensured, and the detected Raman signals are stronger than the solar background light below the height of 2.7km, so that the low-altitude atmospheric temperature detection in the daytime can be realized.
Calculating the total signal-to-noise ratio SNR of the system according to the number of Raman scattering signal photons respectively received in the channel 1 and the channel 2, the number of Mie-Rayleigh signal photons remained in the channel, the number of photons of solar background light and the dark noise of a detector total (z) is shown in FIG. 6. The measurement time was taken for 10 minutes.
In addition, FIG. 6 also shows the possible detection temperature error versus height curves for day and night observations. Under the influence of the solar background light in the daytime, when the detection temperature error is required to be less than 1K, the atmospheric temperature distribution below 3.3km can be detected, and the detection height at night reaches 5km. Data inversion of measured parameters
The signals detected by each photoelectric detection component (PMT 1-PMT 3) are sent to a computer for recording and analysis processing after being converted by an A/D acquisition card. For the measurement signal obtained by PMT1, the computer can calculate the optical parameters of aerosol extinction coefficient, backscattering coefficient and the like by inverting the Mie scattering lidar equation. For the measurement signals obtained by the PMT2 and PMT3, the computer can solve the height distribution of the atmospheric temperature by solving the rotational raman scattering lidar equation.
Specifically, the vertical distribution of the atmospheric temperature and the vertical distribution of the extinction coefficient of the atmospheric aerosol and the optical thickness of the atmospheric aerosol can be obtained by the following inversion.
We can calculate the power P (z) of each scattered signal entering the radar system according to the following lidar equation:
wherein K is the optical system efficiency, E0 is the laser pulse energy, tau is the laser pulse interval time, ar is the telescope light receiving area, Y (z) is the optical path overlapping coefficient of the transmitting and receiving devices, z is the detection height, beta (z) is the backscattering coefficient (the function of the atmospheric number density N (z) and the scattering cross section intensity sigma) at the height z, and alpha (z) is the atmospheric extinction coefficient at the height z. In the equation, parameters other than the atmospheric extinction coefficient α (z) and the backscattering coefficient β (z) are known quantities provided by the system.
a) Inversion of atmospheric temperature
The 2 raman scattering signal intensities detected by channel 2 and channel 3 are represented by the following 2 lidar equations:
wherein T is the atmospheric temperature, J 1 And J 2 The number of photons of rotation, sigma, of the Raman scattering signal detected by the two channels, respectively b_1 (J 1 T) and σ b_2 (J 2 T) is the number of rotational quanta J at temperature T 1 Scattering cross-sectional intensity and rotational quantum number J of the rotational Raman scattering signal 2 And J of the rotational Raman scattering signal, and 1 and J 2 Are known.
Then, the ratio of the scattered signal intensities H (T, z) of channel 1 and channel 2 is obtained from the ratio of equation (2) to equation (3):
is directed to theoretical sigma b_1 (J 1 T) and σ b_2 (J 2 And T) solving and performing curve fitting to obtain the following relation:
wherein z is the detection height, and A, B and C are constants. The constants A, B and C can be fitted by calibrating the radar system with the radiosonde data. Thus, the atmospheric temperature T (z) at the height z is
b) Optical characteristics of aerosol
Mi-Rayleigh scattering signal intensity P detected from channel 1 1 (z), expressed by the lidar equation:
wherein β (z) = β m (z)+β a (z)，α(z)＝α m (z)+α a (z)，
And beta is m (z) and beta a (z) backscattering coefficients, α, for atmospheric molecules and aerosol particles, respectively m (z) and alpha a (z) the extinction coefficients of the atmospheric molecules and the aerosol particles, respectively.
The range-squared correction function of the laser radar echo signal is set as follows:
X(z)＝P(z)·z 2 (8)
if the total extinction coefficient alpha (z) of the atmosphere at a certain height Zc is known in advance c ) Or backward powderCoefficient of radiation beta (z) c ) Then Z is c Extinction coefficient alpha of aerosol particles at each height below a (z) or backscattering coefficient beta a (z) are respectively:
in the formula, alpha m (z) and beta m (z) extinction coefficient and backscattering coefficient of atmospheric molecule, S is laser radar ratio, S = alpha a /β a Its value is related to the optical characteristics of different aerosols and is a constant.
In the above two formulae, the extinction coefficient α of atmospheric molecules at the height z m (z) or backscattering coefficient beta m (z) can be obtained from American Standard atmosphere model, and if the measured altitude is high, a near-flat clean atmosphere without aerosol particles can be selected, and the laser radar range logarithm is corrected in the altitude rangeAnd performing least square fitting on the echo signals, wherein half of the slope of the regression curve is the extinction coefficient of the atmospheric molecules, and solving an extinction coefficient height distribution model of the atmospheric molecules, wherein the value taking method is more suitable for the atmospheric condition at the moment. When the measurement height is high, the height of the clean atmosphere containing almost no aerosol particles can be selected as Z c Boundary value α (z) c ) Or beta (z) x ) Contains only atmospheric molecular components; similarly, if the measured height is not high enough, a section of more uniform atmosphere can be selected, least square fitting is carried out on the laser radar distance correction logarithmic echo signal in the height range, and half of the slope of the regression curve is the boundary value alpha (z) of the atmospheric extinction coefficient or the backscattering coefficient at the height zc x ) Or beta (z) c )。
Claims (6)
1. The all-fiber Raman scattering laser radar system based on WDM technology demultiplexer light splitting includes laser emitting system, receiving system, light splitting system, photoelectric detection component and computer system, and is characterized by that said system includes
YAG pulse laser, beam expander for collimating and expanding pulse laser, multiple reflectors for vertically emitting the collimated and expanded laser to atmosphere,
a receiving system for receiving backward scattered light generated by interaction between laser and molecules and particles in the atmosphere and coupling the received laser radar atmosphere echo signal to the multimode fiber which sends the atmosphere echo signal into the multimode fiber
The multimode/single-mode optical fiber converter is used for converting the multimode atmospheric echo signal into a single-mode optical fiber signal and sending the single-mode optical fiber signal into the single-mode optical fiber, or directly coupling the laser radar atmospheric echo signal received by the telescope into the single-mode optical fiber and then sending the single-mode optical fiber signal into the telescope
The optical splitting system is used for separating the rotating Raman spectral line and the Mi-Rayleigh scattering spectral line in the echo signal, outputting each separated scattering signal from a corresponding port and sending the signals into
Photoelectric detection component for converting the separated scattered light signals into electric signals and receiving the electric signals
And the computer system is pre-loaded with weather and atmospheric environment parameter inversion algorithm programs and is used for analyzing and processing the received scattering spectral line signals to obtain atmospheric temperature weather parameters, atmospheric aerosol extinction coefficients and scattering coefficients, aerosol optical thickness and atmospheric visibility atmospheric environment parameter values.
2. The lidar system of claim 1, wherein the beam splitting system comprises,
the first optical fiber circulator (5) is used for receiving echo signals with a plurality of wavelengths sent by the single-mode optical fiber, outputting the echo signals to the first fiber Bragg grating, receiving a Mi-Rayleigh scattering signal reflected by the first fiber Bragg grating and outputting the Mi-Rayleigh scattering signal to the first photoelectric detection component (11);
a first fiber Bragg grating (6) with a set wavelength λ b Is λ b1 =532.25nm for equating wavelength to lambda b1 Almost totally reflecting the Mi-Rayleigh scattering signal back to the first optical fiber circulator (5) and allowing optical signals of other wavelengths to transmit through;
the second optical fiber circulator (7) is used for receiving the signal transmitted by the first fiber Bragg grating and outputting the signal to the second fiber Bragg grating, and receiving the low-quantum-number rotation scattering signal reflected by the second fiber Bragg grating and outputting the rotation scattering signal to the second photoelectric detection component (12);
a second fiber Bragg grating (8) with a designed wavelength λ b Is λ b2 =530.6nm for equalizing the wavelength to λ b2 The low quantum number rotation scattering signal is almost completely reflected back to the second optical fiber circulator, and the optical signals with other wavelengths are transmitted and passed;
a third optical fiber circulator (9) for receiving the signal transmitted by the second fiber Bragg grating and outputting the signal to the third fiber Bragg grating, and receiving the high-quantum-number Raman scattering signal reflected by the third fiber Bragg grating and outputting the scattering signal to a third photoelectric detection component (13);
a third fiber Bragg grating (10) with a designed wavelength λ b Is λ b3 =528.8nm for equating wavelength to λ b3 The high quantum number raman scattered signal is almost completely reflected back to the third fiber optic circulator allowing optical signals of other wavelengths to transmit through.
3. The lidar system of claim 1, wherein the beam splitting system comprises,
a first fiber grating coupler (14) for setting the wavelength λ thereof b Is λ b1 =532.25nm for receiving multiple wavelengths of echo signals transmitted by single-mode fiber b1 Of the Mi-Rayleigh scattering signalReflecting and sending the reflected Mi-Rayleigh scattering signal into a first photoelectric detection component (11);
a second fiber grating coupler (15) for setting the wavelength lambda thereof b Is λ b2 =530.6nm, for receiving an optical signal transmitted through a first fiber grating coupler (14) having a wavelength equal to λ b2 The low-quantum-number rotating Raman scattering signal is reflected, and the reflected low-quantum-number rotating Raman scattering signal is sent to a second photoelectric detection component (12);
a third fiber grating coupler (16) for setting the wavelength lambda thereof b Is λ b3 =528.8nm, is used for receiving the optical signal transmitted by the second fiber grating coupler (15) and leads the wavelength to be equal to lambda b3 And sends the reflected high-quantum-number raman scattering signal to the third photodetection means (13).
4. The lidar system of claim 1, wherein the beam splitting system comprises,
a multilayer dielectric thin film filter (17) whose transmission wavelengths are set to λ respectively 1 ＝532.25nm、 λ 2 =530.6nm and λ 3 =528.8nm for receiving echo signals of multiple wavelengths sent by a single-mode fiber, each having a wavelength equal to λ 1 Of the Mi-Rayleigh scattering signal, wavelength equal to λ 2 Low quantum number rotational raman scattering signal and wavelength equal to λ 3 The high quantum number Raman scattering signal is transmitted, and the transmitted signal is respectively sent to a first photoelectric detection component (11), a second photoelectric detection component (12) and a third photoelectric detection component (13);
the front of the multilayer dielectric thin film filter (17) is also provided with a first lens (18), and the rear of the multilayer dielectric thin film filter (17) is provided with a second lens (19), a third lens (20) and a fourth lens (21), so that each reflected optical signal respectively transmits through the three lenses and then enters the photoelectric detection part.
5. The lidar system of claim 1, wherein the beam splitting system,
comprises a first optical fiber coupler (25), a first optical fiber interferometer (22), a second optical fiber coupler (26), a third optical fiber coupler (27), a second optical fiber interferometer (23), a fourth optical fiber coupler (28), a fifth optical fiber coupler (29), a third optical fiber interferometer (24) and a sixth optical fiber coupler (30) which are cascaded,
a first fiber coupler (25) for splitting the single-mode fiber-fed echo signal into two beams, respectively transmitted on both arms of the first fiber interferometer (22), and receiving the reflected light from the first fiber interferometer (22) with a wavelength equal to λ 1 A mie-rayleigh scattering signal of =532.25nm, which is output by interfering with each other and received by a first photodetection means (11);
a first fiber interferometer (22) having a wavelength λ set to λ 1 =532.25nm for adjusting the wavelength to λ 1 The Mi-Rayleigh scattered signal is almost totally reflected back to the fiber coupler (25) and light signals of other wavelengths are transmitted through;
a second fiber coupler (26) for receiving the signal transmitted by the first fiber interferometer (22) and not satisfying the grating reflection condition and interfering to output to a third fiber coupler (27);
a third optical fiber coupler (27) for splitting the echo signal sent by the second optical fiber coupler (26) into two beams, respectively transmitting the two beams at the two arms of the second optical fiber interferometer (23), and receiving the reflected wave with the wavelength equal to lambda from the second optical fiber interferometer (23) 2 Low-quantum-number rotational Raman scattering signals of =530.6nm, which interfere with each other and are output, and are received by a second photodetection unit (12)Harvesting;
a second fiber interferometer (23) having a wavelength λ set to λ 2 =530.6nm for equalizing the wavelength to λ 2 The low-quantum-number rotating raman scattered signal is almost completely reflected back to the fiber coupler (27) while allowing optical signals of other wavelengths to transmit therethrough;
a fourth fiber coupler (28) for receiving the signal transmitted by the second fiber interferometer (23) and not satisfying the grating reflection condition and generating interference from the output to a fifth fiber coupler (29);
a fifth optical fiber coupler (29) for sending the echo signal sent by the fourth optical fiber coupler (28)Split into two beams, transmitted respectively on the two arms of the third fibre-optic interferometer (24), and receive the reflected light from the third fibre-optic interferometer (24) at a wavelength equal to λ 3 A high quantum number rotational Raman scattering signal of not less than 528.8nm is outputted by interfering with each other and received by the third photodetection means (13);
a third fiber interferometer (24) having a wavelength λ set to 2 =528.8nm for equalizing the wavelength to λ 3 Is reflected almost completely back to the fiber coupler (29).
6. The lidar system of claim 1, wherein the beam splitting system comprises,
the first optical fiber circulator (5) is used for receiving echo signals of a plurality of wavelengths sent by a single-mode optical fiber and outputting the echo signals to the first optical fiber resonant cavity (31), and receiving scattering signals reflected by the first optical fiber resonant cavity (31) and outputting the scattering signals to the second optical fiber circulator (7);
a first fiber cavity (31) with a wavelength λ set to λ 1 =532.25nm for adjusting the wavelength to λ 1 The Mi-Rayleigh scattering signal is output to a first photoelectric detection component (11) and optical signals with other wavelengths are transmitted back to a first optical fiber circulator (5);
the second optical fiber circulator (7) is used for receiving the optical signal output by the first optical fiber circulator (5) and outputting the echo signal to the second optical fiber resonant cavity (32), and receiving the scattering signal reflected by the second optical fiber resonant cavity (32) and outputting the scattering signal to the third optical fiber circulator (9);
a second fiber resonator (32) having a wavelength λ set to 2 =530.6nm for equalizing the wavelength to λ 2 The low quantum number rotation Raman scattering signal is output to a second photoelectric detection component (12) to enable optical signals with other wavelengths to be transmitted back to a second optical fiber circulator (7);
the third optical fiber circulator (9) is used for receiving the optical signal output by the second optical fiber circulator (7) and outputting the echo signal to the third optical fiber resonant cavity (33), and receiving the scattering signal reflected by the third optical fiber resonant cavity (33) and outputting the scattering signal;
a third fiber cavity (33) having a wavelength λ set to λ 3 =528.8nm for equalizing the wavelengthλ 3 The high quantum number Raman scattering signal is output to a third photoelectric detection component (13), and optical signals with other wavelengths are emitted back to a third optical fiber circulator (9).
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