CN106772441B - An Ultraviolet Pure Rotational Raman Thermometry Lidar System - Google Patents

Abstract

The invention discloses an ultraviolet pure rotational Raman temperature measuring laser radar system. The lidar system consists of a transmitting unit, an optical receiving unit, a double grating polychromator unit, and a photoelectric conversion and control unit. The emission unit uses a solid-state laser injected with seeds to output a 354.7nm ultraviolet laser with a very narrow linewidth and guides it to the zenith; the optical receiving and double grating polychromator unit is used to collect the backscattered light from atmospheric substances, and it can be used to collect the backscattered light from the atmospheric material, and can be used for 0.2 nm. The spectral resolution extracts the pure rotational Raman spectrum in the scattering spectrum, and uses the variation of the pure rotational Raman spectral line intensity with temperature to obtain the information of atmospheric temperature. The photoelectric conversion and control unit ensures the orderly operation of the entire radar system. The ultraviolet pure rotating Raman laser radar of the present invention makes the radar work in the ultraviolet band, which can obviously reduce the background light of the sun, and can be used for all-day measurement of atmospheric temperature.

Description

An Ultraviolet Pure Rotational Raman Thermometry Lidar System

technical field

The invention relates to an ultraviolet pure rotational Raman laser radar system for detecting atmospheric temperature.

Background technique

Atmospheric temperature is a very important parameter that describes the thermal balance structure of the atmosphere. Temperature is also an important parameter in many atmospheric models and is widely used in the study of atmospheric dynamics, climate change and weather processes. Accurate measurement of atmospheric temperature is of great significance, and structural changes in atmospheric temperature can help us understand phenomena such as climate change.

Lidar is widely used in remote sensing detection of atmosphere, ocean, land and other targets due to its high spatial and temporal resolution, high detection sensitivity and continuous detection, and is especially suitable for the detection of atmospheric parameters. There are many methods for lidar to measure the temperature below the low stratosphere, including: the integration technology of molecular vibrational Raman signals, pure rotational Raman technology, hyperspectral technology, etc. The integration technique must assume that the atmosphere is in a static equilibrium state, first invert the atmospheric molecular number density, and then obtain the temperature. In the troposphere, especially when the turbulence is strong, it takes a long time for the atmosphere to reach a new static equilibrium state, and radar signals need to accumulate for a long time (several hours) to accurately measure the atmospheric temperature. Hyperspectral technology uses the Rayleigh-Brillion spectral half-width as a function of temperature to obtain atmospheric temperature. Due to the extremely narrow half-width of the Rayleigh-Brillion spectrum, some frequencies overlap with the extremely strong Mie scattering spectrum. In a very narrow frequency range, the temperature information is completely free from the interference of strong signals (Mie scattering signals), which requires extremely high performance of the radar system. Pure rotational Raman technology uses the variation of half-width of pure rotational Raman spectrum with temperature to obtain atmospheric temperature, which belongs to spectral thermometry technology. Its temperature measurement principle is very superior. It does not need to assume static equilibrium, but only needs to meet thermal equilibrium. Collisions between air molecules in the lower atmosphere are very frequent and thermal equilibrium is easily satisfied. Pure rotational Raman technology does not have strict requirements on the performance of radar equipment, and it is easier to achieve atmospheric temperature measurement. Considering the temperature measurement principle and technical complexity, pure rotational Raman technology is the most suitable temperature measurement method.

Pure rotational Raman lidar has a variety of spectral extraction methods: interference filter set, double grating polychromator, Fabry-Perot Interferometer plus double grating polychromator, atomic filter plus grating, etc. The interference filter set is to allow multiple interference filters to work in cascade at different angles to extract pure rotational Raman spectrum Stokes or anti-Stokes unilateral spectral lines. The advantages of the interference filter set: simple and stable structure, convenient debugging, very suitable for extracting pure rotational Raman spectrum in severe environment. The double grating polychromator is another main method besides the interference filter set, which can simultaneously extract the spectral lines with the same or similar temperature dependence on both sides of Stokes and anti-Stokes. The dual grating polychromator is also insensitive to ambient temperature, and the center wavelength and receiving bandwidth are very stable. Most pure rotational Raman lidars work in the visible light band (532nm), which is just around the peak of the solar radiation power spectrum, and it is not effective to measure atmospheric temperature during the day. Fabry-Perot Interferometer (FPI) plus double grating polychromator and atomic filter plus grating can also achieve daytime temperature measurement. However, these two pure rotational Raman lidar technologies are complex and the systems are unstable, so they cannot measure the atmospheric temperature reliably for a long time.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose an ultraviolet pure rotational Raman temperature measurement laser radar system, which can perform routine detection of atmospheric temperature throughout the day on the basis of ensuring the temperature measurement principle and technical complexity. The lidar consists of three parts: the transmitting unit, the optical receiving and double grating polychromator unit, and the photoelectric conversion and control unit. The transmitting unit is the part used to generate the 354.7nm wavelength laser. Launched into the air, it interacts with matter in the atmosphere, producing backscattered echoes. The optical receiver and dual grating polychromator unit are used to collect the echo signals and extract the pure rotational Raman spectrum Stokes (J6 and J12) and anti-Stokes (J8 and J14) bilateral spectral lines from atmospheric molecules in the echo signals, and filter out background noise. The photoelectric conversion and control unit mainly realizes the functions of photoelectric conversion, photon counting and data storage.

In order to achieve the above object, the technical scheme provided by the present invention is:

An ultraviolet pure rotational Raman temperature measurement lidar system, comprising a transmitting unit, an optical receiving and double grating polychromator unit, and a photoelectric conversion and control unit, the transmitting unit includes a seed laser, a Nd:YAG solid-state laser, and a beam expander , catadioptric mirror; the 354.7nm ultraviolet laser generated by the solid-state laser is reflected into the atmosphere by the catadioptric mirror after being expanded by a beam expander by 5 times;

The optical receiving and double grating polychromator unit sequentially includes a telescope, a pinhole diaphragm, a collimating lens, a 45-degree mirror, a bandpass filter, a lens, an optical fiber 1, an optical fiber mode mixer, and an optical fiber 2 according to the light path. , Double grating polychromator;

The backscattered light from the interaction between the laser and the atmosphere is received by the Cassegrain telescope, and then focused on the aperture diaphragm placed on the focal plane of the telescope. The light passing through the aperture diaphragm is converted into quasi-parallel light by the collimating lens. , the quasi-parallel light is reflected by a 45-degree mirror to a band-pass filter with a central wavelength of 354.7nm and an average transmittance of more than 90% at 350-360nm. The light emitted by the band-pass interference filter is coupled into the lens. Fiber 1 with a core diameter of 1000μm and a numerical aperture of 0.22, the outgoing light from fiber 1 is guided into a fiber 2 with a core diameter of 600μm and a numerical aperture of 0.22 through a fiber mode mixer (FMH), and fiber 2 introduces the echo signal into the double grating The colorimeter is used to extract pure rotational Raman signals and measure atmospheric temperature;

The photoelectric conversion and control unit includes a photon counter and a computer; the optical signal output by the double grating polychromator is connected to the photon counter through a photomultiplier tube, and the photon counter is connected to the computer.

The double-grating polychromator includes two-stage single-grating polychromators, each of which has the same structure. The first-stage polychromator includes a grating 1, a collimating-converging lens 1, and an end face 1 of a fiber bundle array; The first-stage single-grating polychromator includes a grating 2, a collimating-converging lens 2, and an end face of the fiber bundle array 2; the two-stage single-grating polychromator is connected through the fiber-optic bundle array 4; the function of the first-stage single-grating polychromator is spectral separation, and its The output elastic signal is exported to the photomultiplier tube 1-PMT1 by fiber 3 for signal collection; the pure rotational Raman signal is introduced by fiber 4 to the second stage of the double grating polychromator, and the output light of the second stage is respectively sent by fiber 5. And the fiber 6 is exported, and the fiber 5 and the fiber 6 are respectively imported into the photomultiplier tube 2-PMT2 and the photomultiplier tube 3-PMT3 for signal acquisition;

Optical fiber 3, optical fiber 5 and optical fiber 6 are respectively connected with photomultiplier tube 1, photomultiplier tube 2 and photomultiplier tube 3, and the photomultiplier tubes PMT1, PMT2 and PMT3 are connected with 3-channel photon counter, and the photomultiplier tube performs photoelectric conversion, and the photon The counter collects the converted electrical signal, and finally outputs it to a computer connected to the photon counter for storage.

The central wavelength of the bandpass filter is 354.7nm, and the average transmittance at 350-360nm is greater than 90%.

The optical fiber 1 and the optical fiber 2 are two optical fibers with different core diameters, which can expand the receiving field of view of the radar, and can make the light intensity distribution converged into the optical fiber (F2) more uniform.

Compared with the prior art, the present invention adopts an ultraviolet emitting light source, and utilizes a double grating monochromator to simultaneously extract spectral lines with the same or similar temperature correlations on both sides of Stokes and anti-Stokes; at the same time, elastic scattering and solar background light are suppressed, which can be very efficient. The high-purity rotational Raman spectrum can be obtained well, with long-term stability and reliable measurement of atmospheric temperature. The measurement height ranges from near the ground to about 2km during the day and from near the ground to 10km at night, which is suitable for the promotion of the atmospheric temperature measurement with high temporal and spatial resolution throughout the day.

Description of drawings

FIG. 1 is a working principle diagram of an ultraviolet pure rotational Raman temperature measurement lidar system according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a dual grating polychromator according to an embodiment of the present invention.

Fig. 3 is the optical fiber layout diagram of the fiber bundle array end face 1 and the fiber bundle array end face 2 of the embodiment of the present invention, wherein, F represents the optical fiber, F1 represents the optical fiber 1, and F4 ₁ represents the first optical fiber in the optical fiber 4; analogy.

Detailed ways

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

The key of the present invention is to use the triple frequency laser (355nm) of the ND:YAG solid-state laser implanted with the seed as the emission light, and to use the double-grating polychromator to simultaneously collect the pure rotational Raman spectrum (frequency symmetry) on both sides of Stokes and anti-Stokes. ) for atmospheric temperature measurements. Since the dual grating polychromator is also insensitive to ambient temperature, the center wavelength and receiving bandwidth are very stable, and the laser in the ultraviolet 355nm band is used as the emission light source, which can effectively reduce the influence of the solar background light on the echo signal, and can realize the atmospheric temperature. Observe all day.

The laser radar system of the present invention is composed of three parts, namely a transmitting unit, an optical receiving unit and a double grating polychromator unit, and a photoelectric conversion and sum control unit. As shown in Figure 1.

The emission unit is composed of seed laser, Nd:YAG solid-state laser, beam expander and catadioptric mirror. The 355nm ultraviolet laser generated by the solid-state laser is reflected into the atmosphere by the catadioptric mirror after being expanded by a beam expander by 5 times.

The backscattered light from the interaction between the laser and the atmosphere is received by a Cassegrain-type telescope with an effective aperture of 450 mm, and then focused on a small aperture diaphragm placed in the focal plane of the telescope. The light passing through the aperture diaphragm is converted into quasi-parallel light with a certain divergence angle through the collimating lens. The quasi-parallel light is reflected by a 45-degree mirror to a bandpass filter with a central wavelength of 354.7nm and an average transmittance of more than 90% at 350-360nm. Its function: to limit the light entering the spectroscopic system to a relatively narrow frequency range, to prevent the spectral overlap, and to reduce the bandwidth of the radar signal background light. The light emitted by the bandpass interference filter is coupled by the lens into fiber 1 with a core diameter of 1000 μm and a numerical aperture of 0.22. The light emitted from the fiber 1 is introduced into the fiber 2 with a core diameter of 600 μm and a numerical aperture of 0.22 through a fiber mode mixer (FMH). Fiber 2 introduces the echo signal into the double grating polychromator for extracting the pure rotational Raman signal and measuring the atmospheric temperature. Using a fiber mode mixer (FMH) to connect two fibers with different core diameters will inevitably lose a part of the light intensity. However, using this connection method can bring many advantages: it can expand the receiving field of view of the radar, and can make the light intensity distribution collected into the fiber 2 more uniform.

The double-grating polychromator consists of a two-stage single-grating polychromator. The structure of each level of polychromator is the same. The first-level polychromator includes a grating 1, a collimating-converging lens 1, and an end face of the fiber bundle array 1; the second-level polychromator includes a grating 2, a collimating-converging lens 2, and an optical fiber. The end face of the beam array 2; the two-stage single grating polychromator is connected through the fiber bundle array 4. As shown in Figure 2. The role of the first-stage single-grating polychromator is spectral separation, and the fiber 3 extracts the elastic signal at the first focal plane and exports it to the photomultiplier tube 1 (PMT1) for signal collection. The pure rotational Raman signals are J6 and J12 of Stokes and J8 and J14 of anti-Stokes, respectively, which are extracted by four fibers 4 and introduced into the second-stage single-grating polychromator. The second-stage single-grating polychromator combines the low (J6, J8) and high (J12, J14) spectral lines on both sides of Stokes and anti-Stokes into fiber 5 and fiber 6, and introduces them to photomultiplier tube 2 (PMT2) and photomultiplier tube 3 (PMT3) for signal acquisition. At the same time, the second grating diffraction will separate the elastic stray light carried by the purely rotational Raman signal extracted in the first-stage single-grating polychromator, further improving the out-of-band suppression of the elastic wavelength by the purely rotational Raman channel, achieving The out-of-band suppression of elastic wavelengths by pure rotational Raman channels is better than 8 orders of magnitude.

The end face of the fiber 2 is positioned on the focal plane of the lens 1, and the light emitted by it is collimated by the lens 1 with a diameter of 120 mm and a focal length of 285 mm and then incident on the reflective blazed grating 1. The parameters of the reflective blazed grating 1 are shown in Table 1. shown. The light dispersed by the grating 1 is again condensed to the focal plane through the lens 1, and received by the optical fiber at a specific position on the end face 1 of the optical fiber bundle array placed at the focal plane. Among them, the elastic signal is received by fiber 3 and exported to the first-order monochromatic grating polychromator; the pure rotational Raman spectrum Stokes J6, J12 and anti-Stokes J8 and J14 signals are received by fiber 4 (4 core diameters are 600 μm, fiber with a numerical aperture of 0.22) is received and transmitted into a second-order grating polychromator. The other end of the optical fiber 4 is fixed on the other end face 2 of the fiber bundle array, and is also precisely positioned on the focal plane of the lens 2 with a diameter of 120 mm and a focal length of 285 mm. The light extracted by the first-order grating polychromator is collimated by the lens 2 into parallel light and incident on the reflective blazed grating 2 . The reflective blazed grating 2 has the same parameters and working methods as the grating 1 . The light diffracted by the grating 2 is self-converged on the focal plane through the lens 2 again. Fiber layout of fiber bundle array end face 1 and fiber bundle array end face 2. As shown in Figure 3.

The second-stage single-grating polychromator is the inverse light path of the first-stage, which combines the low (J6, J8) and high (J12, J14) spectral lines on both sides of Stokes and anti-Stokes into fiber 5 and fiber 6, and imported into photomultiplier tube 2 (PMT2) and photomultiplier tube 3 (PMT3) for signal acquisition. At the same time, the second grating diffraction will separate the elastic stray light carried by the purely rotational Raman signal extracted in the first-order single-grating polychromator, further improving the out-of-band suppression of the elastic wavelength by the purely rotational Raman channel. The role of the first-stage single-grating polychromator is spectral separation, and fiber 3 extracts the elastic signal at the first focal plane and exports it to photomultiplier 1 (PMT1) for signal acquisition.

The photoelectric conversion and control unit includes a photomultiplier tube, a multi-channel photon counter and a computer. Mainly realize photoelectric conversion, photon counting, data storage and other functions. Ensure that the entire lidar system works in an orderly manner.

Optical fiber 3, optical fiber 5 and optical fiber 6 are respectively connected with photomultiplier tube 1 (PMT1), photomultiplier tube 2 (PMT2) and photomultiplier tube 3 (PMT3), and the photomultiplier tubes PMT1, PMT2 and PMT3 are connected with a 3-channel photon counter. Photoelectric conversion is performed by a photomultiplier tube, and a 3-channel photon counter collects the converted electrical signal, and finally outputs it to a computer connected to the photon counter for storage.

The elastic and purely rotational Raman signals extracted by the radar are detected by fiber-guided photomultiplier tubes (PMTs). PMT is a photodetection device with extremely high sensitivity and ultra-fast time response, with the advantages of high gain and low dark noise, and is the preferred detection device for lidars operating in the ultraviolet-near-infrared band. Among them, the photomultiplier tube adopts H10721-110 from HAMAMATSU Company of Japan, and the spectral response range is 230-700nm. The output pulse of the photomultiplier tube is sampled and accumulated many times by the signal collector TR20-160 produced by the German Licel company to the rapidly changing echo signal, and finally the original echo data of the lidar can be obtained and uploaded to the computer for calculation, storage.

Table 1 (the following table) is the parameter table of the reflective blazed grating according to the embodiment of the present invention

Claims (1)

1. a kind of ultraviolet pure rotation Raman temperature measuring laser radar system, comprises launch unit, optical reception and double grating polychromator unit and photoelectric conversion and control unit, it is characterized in that: described launch unit comprises seed laser, Nd:YAG Solid-state laser, beam expander, and catadioptric mirror; the 354.7nm ultraviolet laser generated by the solid-state laser is reflected into the atmosphere by the catadioptric mirror after 5 times of beam expansion through the beam expander; The optical receiving and double grating polychromator unit sequentially includes a telescope, a pinhole diaphragm, a collimating lens, a 45-degree mirror, a bandpass filter, a lens, an optical fiber 1, an optical fiber mode mixer, and an optical fiber 2 according to the light path. , Double grating polychromator; The backscattered light from the interaction between the laser and the atmosphere is received by the Cassegrain telescope, and then focused on the aperture diaphragm placed on the focal plane of the telescope. The light passing through the aperture diaphragm is converted into quasi-parallel light by the collimating lens. , the quasi-parallel light is reflected by a 45-degree mirror to a bandpass filter with a central wavelength of 354.7nm and an average transmittance at 350-360nm greater than 90%. The light emitted by the bandpass filter is coupled into the core by the lens Fiber 1 with a diameter of 1000 μm and a numerical aperture of 0.22, the outgoing light from fiber 1 is guided into fiber 2 with a core diameter of 600 μm and a numerical aperture of 0.22 through the fiber mode mixer. Extract pure rotational Raman signal and measure atmospheric temperature; The photoelectric conversion and control unit includes a 3-channel photon counter and a computer; the optical signal output by the double grating polychromator is connected to the 3-channel photon counter through a photomultiplier tube, and the 3-channel photon counter is connected to the computer; the photomultiplier tube includes a photomultiplier tube 1. Photomultiplier tube 2, photomultiplier tube 3; The double-grating polychromator includes two-stage single-grating polychromators, each of which has the same structure. The first-stage polychromator includes a grating 1, a collimating-converging lens 1, and an end face 1 of a fiber bundle array; The first-stage polychromator includes a grating 2, a collimating-converging lens 2, and an end face of the fiber bundle array 2; the two-stage single-grating polychromator is connected through the fiber bundle array 4; the function of the first-stage polychromator is spectral separation, and its output The elastic signal is exported by the fiber 3 to the photomultiplier tube 1 for signal acquisition; the pure rotational Raman signal is introduced by the fiber bundle array 4 to the second stage of the double grating polychromator, and the output light of the second stage is respectively transmitted by the fiber 5 and the fiber 6 Export, the optical fiber 5 and the optical fiber 6 are respectively imported into the photomultiplier tube 2 and the photomultiplier tube 3 for signal acquisition; Optical fiber 3, optical fiber 5 and optical fiber 6 are respectively connected with photomultiplier tube 1, photomultiplier tube 2 and photomultiplier tube 3. Photomultiplier tube 1, photomultiplier tube 2 and photomultiplier tube 3 are connected with a 3-channel photon counter, and are connected by photomultiplier tube. The tube performs photoelectric conversion, and the 3-channel photon counter collects the converted electrical signal, and finally outputs it to the computer connected to the 3-channel photon counter for storage; The end face of the optical fiber 2 is positioned on the focal plane of the collimating-converging lens 1, and the light emitted by it is collimated by the collimating-converging lens 1 with a diameter of 120 mm and a focal length of 285 mm and then incident on the grating 1, and is dispersed by the grating 1. The light is again focused on the focal plane through the collimating-converging lens 1, and is received by the fiber at a specific position on the end face 1 of the fiber bundle array placed at the focal plane, wherein the elastic signal is received by the fiber 3 and exported to the first-stage polychromator ; Pure rotational Raman spectrum Stokes J6, J12 and anti-Stokes J8 and J14 signals are received by the fiber bundle array 4 and transmitted to the second-stage polychromator. It is composed of an optical fiber with an aperture of 0.22; the other end of the fiber bundle array 4 is fixed on the end face 2 of the fiber bundle array, and is also precisely positioned on the focal plane of the collimating-converging lens 2 with a diameter of 120mm and a focal length of 285mm; After the light extracted by the polychromator emerges, it is collimated by the collimating-converging lens 2 into parallel light incident on the grating 2. The grating 2 has the same parameters and working mode as the grating 1; the light diffracted by the grating 2 is collimated again - Converging lens 2 self-converges on the focal plane; the second-stage polychromator is the inverse light path of the first stage, which merges Stokes J6 and anti-Stokes J8 spectral lines into fiber 5, Stokes J12 and anti-Stokes The J14 spectral line is merged into optical fiber 6 and introduced into photomultiplier tube 2 and photomultiplier tube 3 for signal acquisition; at the same time, the elastic stray light carried by the pure rotational Raman signal extracted in the first-stage polychromator is separated , further improving the out-of-band suppression of the elastic wavelength by the pure rotational Raman channel; the optical fiber 3 extracts the elastic signal on the focal plane of the collimating-converging lens 1 and exports it to the photomultiplier tube 1 for signal collection.

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