CN101477196B - Vibrating Raman lidar scattered light processing system and processing method - Google Patents

Abstract

A vibrating Raman laser radar scattered light processing system and processing method based on a narrow-band filter and a reflective prism. The present invention addresses the two problems of a large dynamic range of laser radar signal strength and a stronger noise signal than the Raman scattering signal. The backscattered light is divided into high and low altitude channels for detection, and the narrow-band filter is fully utilized to improve the temperature measurement accuracy and expand the temperature measurement range. The high-altitude signal noise signal is relatively weak, and its signal is sent to the photoelectric detection system for detection after passing through a narrow-band filter, while the low-altitude noise signal is very strong, using a special optical design to make the atmospheric backscattered light pass through the same filter twice. For the light sheet, the total transmittance spectrum is equivalent to the square of the single transmittance spectrum, which greatly suppresses the optical noise. After the elastic scattering of the emission wavelength and the background light are compressed, under the condition that the multiplier tube is not saturated, the echo intensity of the Raman scattering signal can be increased by increasing the single pulse energy of the emission laser, thereby improving the accuracy of temperature measurement and expanding temperature measurement range purposes.

Description

Vibrational Raman lidar scattered light processing system and processing method

technical field

The present invention designs a vibrating Raman laser radar scattered light processing system and its processing method, through designing the optical path to separately process the back scattered light received by the Raman laser radar system, wherein the high altitude directly processes the signal passing through the narrowband filter For detection, the low-altitude channel makes the atmospheric backscatter signal pass through the narrow-band filter twice to further compress the noise in the Raman lidar system, which can improve the system signal-to-noise ratio. In the process of detecting atmospheric temperature profile information, the invention solves the two problems of large dynamic range of laser radar signals and low-altitude Raman scattering signals being affected by elastic scattering signals.

Background technique

Atmospheric temperature profiles have important applications in many research fields. In terms of climatological research, people pay more and more attention to global warming. Global warming will cause sea level to rise, which will bring disasters to many island countries. At the same time, extreme temperature weather will appear frequently. bring adverse effects. Long-term observations of the temperature profile show that although the temperature at the bottom of the troposphere is decreasing, the temperature at the top of the troposphere and the stratosphere is increasing. Therefore, long-term monitoring of the atmospheric temperature profile is an important aspect of studying climate change.

In terms of meteorological research, atmospheric temperature is an important meteorological parameter in atmospheric physics, weather analysis and forecasting. In the study of atmospheric dynamics, accurate atmospheric temperature information is very important for the study of atmospheric stability and dynamics. At the same time, accurate atmospheric temperature information can help explain natural weather phenomena and improve the accuracy of numerical weather prediction. In stratospheric atmospheric research, temperature has a great relationship with ozone absorbing solar radiation heating, and temperature changes in this region indirectly reflect changes in ozone. At the same time, high-altitude atmospheric temperature is related to high-altitude gravity waves and atmospheric circulation structure. In addition to geophysical studies, atmospheric temperature profiles are an input parameter for other remote sensing techniques, such as scattering and extinction measurements using Raman lidar. In summary, information on the atmospheric temperature profile from the boundary layer to the bottom of the mesosphere is urgently needed for multiple studies.

Due to the importance of atmospheric temperature, the measurement of atmospheric temperature has a long history. In addition to the traditional atmospheric temperature measurement method, with the development of optoelectronic technology, atmospheric temperature measurement lidar has gradually developed into a new atmospheric temperature measurement instrument. Atmospheric temperature measurement lidar has the characteristics of high temporal and spatial resolution, generally divided into the following categories: rotational Raman lidar, vibration Raman lidar, Rayleigh lidar, metal ion fluorescence lidar. Lidars of various technologies complement each other and together form a measurement system for the temperature profile from the ground to an altitude of 110Km. Among them, the vibration Raman scattering lidar uses integral technology to invert the atmospheric temperature, and the detection wavelength is different from the emission wavelength, so the high-resolution spectral part is not needed in the receiving system, which simplifies the lidar system. Therefore, when the measurement accuracy permits Instead of pure rotational Raman scattering, vibrational Raman scattering can be utilized. For Rayleigh lidar, the general detection area is the area where the influence of aerosol above 30Km can be ignored. For the area of 5-30Km, it is higher than the detection limit of rotating Raman lidar and lower than the detection limit of Rayleigh lidar. Detection can only be performed using rotational Raman lidar. In 1990, Philippe Keckhut of the French National Research Center reported a lidar system using vibration Raman and Rayleigh technology, and the lowest measurement height of the vibration Raman channel was 12Km. Keith D.Evans of NASA reported in 1997 that the minimum measurement height of the vibration Raman lidar is 5Km, which can coincide with the rotation Raman lidar, but the maximum measurement height is only 10Km, which cannot reach the minimum of Rayleigh temperature measurement. high. Domestically, the vibration Raman lidar measurement results reported by the Anhui Optics and Mechanics Institute of the Chinese Academy of Sciences show that the system has a high accuracy of temperature measurement within the range of 9-15Km. A comprehensive analysis of the detection results of the vibration Raman lidar of many research units shows that there are two main reasons for limiting the increase in the measurement range of the vibration Raman lidar: first, the signal of the rotational Raman lidar itself is relatively weak (compared to The M-Rayleigh signal is 3-4 orders of magnitude smaller), and second, the existence of the distance square factor of the lidar equation makes the dynamic range of the lidar signal vary greatly, and the high-altitude signal is very weak. There is a certain contradiction between the above two reasons. The high-altitude Raman scattering signal is weak, and it is required to enhance the emitted laser to obtain more Raman scattering. However, the increase in laser energy will increase the Mi-Rayleigh signal at the same time, and even make the photoelectric multiplication Tube saturation is not conducive to improving the signal-to-noise ratio of the system.

Contents of the invention

The technical problem solved by the present invention is to solve the problem of the large dynamic range of the laser radar signal by dividing the high-altitude and low-altitude channels for detection, and design an optical path for the low-altitude channel so that the optical signal passes through the filter twice, further compressing the background noise and elasticity Scattering noise improves the signal-to-noise ratio of the system, thereby improving the accuracy of Raman lidar temperature measurement and expanding the temperature measurement range.

Technical solution of the present invention is as follows:

Vibration Raman lidar scattered light processing system, the system includes:

(1) A pulsed laser;

(2) the first reflector and the second reflector for reflecting the pulsed laser;

(3) Telescopes that collect backscattered light signals from the atmosphere;

(4) The pinhole diaphragm and signal light collimating lens that limit the field of view of the receiving system;

(5) The third mirror;

(6) A narrow-band filter for filtering out optical signal noise;

(7) Photomultiplier tubes for detecting optical signals;

(8) Data acquisition and processing system mainly comprises main control computer and photon counting card, and this system links to each other with laser device and photomultiplier tube; It is characterized in that:

(9) the photomultiplier tube includes a first photomultiplier tube and a second photomultiplier tube, and the two photomultiplier tubes are respectively arranged on both sides of the narrowband filter to detect optical signals;

(10) The system also includes a triangular prism, which is arranged between the first photomultiplier tube and the narrow-band filter, and the transmitted part of the light is sent into the first photomultiplier tube, and the reflected part of the light is sent into the second photomultiplier tube after the narrow-band filter. Photomultiplier tube.

The vibration Raman lidar scattered light processing method is characterized in that it comprises the following steps:

(1) The main control computer controls the laser to emit pulsed laser, which is totally reflected by two mirrors and enters the atmosphere;

(2) The atmospheric backscatter is collected by the telescope, and the signal light is collimated by the collimating mirror;

(3) The collimated scattered light signal passes through the upper half of the narrow-band filter to filter out background noise and elastic scattering of laser light;

(4) The optical signal passing through the narrow-band filter once enters the prism, and on the first reflective surface, the transmission and reflection ratio of the signal light is 1:1,

(5) The transmitted light part enters the first photomultiplier tube for high-altitude detection;

(6) The optical signal of the reflected part passes through the second reflective surface of the prism and undergoes total reflection, and enters the narrow-band filter for the second time by the lower half of the narrow-band filter (further filtering out background noise and elastic scattering), and is passed by The second photomultiplier tube detects (converts the optical signal into an electrical signal);

(7) The signals detected by the first and second photomultiplier tubes are collected and processed by the data collection and processing system.

Said laser in the present invention is a Nd:YAG laser, the single pulse energy is adjustable, the maximum is 260mJ, the laser works in the external trigger state, and the laser pulse energy can be adjusted by adjusting the pumping current of the laser. The receiving telescope is a Cassegrain telescope with a focal length of 2m and a small hole to limit the receiving angle of the entire lidar. The diameter of the collimating mirror is 1cm, and the spot size of the signal light is 1cm after collimation. The so-called narrow-band filter, whose function is to retain the required Raman optical signal and filter out the optical signal that exists as noise, is the core component of the laser radar system. Its working diameter is 1 inch, and the transmission center wavelength is 607nm. The peak transmittance is greater than 40%, the suppression rate between 200-1200nm is 10 ⁷ , and the theoretical suppression rate at 532nm can reach 10 ¹² . The transmission spectrum of the narrow-band filter is shown in Figure 2. The said reflective prism has a 607nm anti-reflection coating on the incident surface, a 607nm semi-reflective coating on the first reflecting surface, and a 607nm total reflection coating on the second reflecting surface. The first reflective surface of the prism and the first photomultiplier correspond to the upper half of the narrowband filter; the second reflective surface of the prism and the second photomultiplier correspond to the lower half of the narrowband filter.

Said photomultiplier tube has relatively high gain and can realize single photon detection. The said data acquisition and processing system includes a photon counting card and a main control computer. The photon counting card detects the electrical signal output by the photomultiplier tube. The main function of the main control computer is to provide the external trigger pulse signal of the laser, the gate of the two photomultiplier tubes Control signal, trigger signal of photon counting card. The sampling rate of the photon counting card is 200MHz, and the counting threshold voltage is adjustable.

Compared with the prior art, the present invention has the advantages that:

(1) The present invention solves the problem of large dynamic range of laser radar signal strength. The detected laser radar signal is divided into high-altitude segment and low-altitude segment for detection respectively. The high-altitude generally refers to the area of 5-15km, and the low-altitude generally refers to the area of 15-30km. The invention uses two multiplier tubes and photon counting technology to detect separately, and solves the problem that the laser radar signal has a large dynamic range and is difficult to detect.

(2) The present invention further compresses noise. The optical path is designed so that the low-altitude scattered light passes through the narrow-band filter twice to further compress the background noise and elastic scattering noise, improve the signal-to-noise ratio of the system, and extend the detection range to low-altitude.

(3) The method of the present invention can improve the single pulse energy and increase the measurement accuracy of the system. Through laser radar segment detection and twice compressed noise, although the signal received by the system is relatively weak, it is a very pure Raman scattering signal. Therefore, the signal-to-noise ratio of the system can be further improved by increasing the laser radar single pulse energy.

(4) The present invention solves the wavelength matching problem of multiple optical filters. Obviously, if the low-altitude signal passes through two narrow-band filters continuously, the low-altitude signal can be further compressed, but this method will inevitably have a wavelength matching problem (the central wavelengths of the two narrow-band filters are not necessarily the same). In fact, there are technical problems. It is difficult for the two narrow-band filters to match the wavelengths completely, and the wavelength mismatch will reduce the detection efficiency. Even if the two filters can achieve wavelength matching, the central wavelength of the transmittance varies with temperature. That is to say, if the temperature changes slightly, the wavelength will no longer match. However, the signal light proposed by the present invention passes through the same optical filter twice, and because it is the same optical filter, there is no influence of wavelength matching and external temperature, so the wavelength matching problem of the optical filter is solved.

Description of drawings:

Figure 1 is a schematic diagram of the basic structure of a vibration Raman lidar scattered light processing system.

Figure 2 is a schematic diagram of the transmission spectrum of the narrow-band filter.

Fig. 3 is a schematic diagram of the structure of the triangular prism.

In the figure: 1-laser; 2-first mirror; 3-second mirror; 4-telescope; 5-aperture diaphragm; 6-collimating mirror; 7-third total mirror; Light sheet; 81-the transmittance spectrum of the signal light passing through the filter once; 82-the transmittance spectrum of the signal light passing through the filter twice;

9-triangular prism; 91-signal light incident surface (coated with 607nm anti-reflection film); 92-first reflective surface (coated with 607nm semi-reflective film); 93-second reflective surface (coated with 607nm full-reflection film);

10-the first photomultiplier tube; 11-the second photomultiplier tube; 12-data acquisition and processing system.

Detailed ways:

As shown in Figure 1, the present invention is a vibration Raman temperature measurement laser radar based on a novel optical path. It uses a prism to divide the Raman scattered light signal into two channels, high altitude and low altitude. The high altitude optical signal is directly detected, and the low altitude optical signal is passed twice. Narrow-band filter, which can improve the accuracy and temperature measurement range of Raman lidar temperature measurement.

As shown in FIG. 2 , for the transmission spectrum of the narrow-band filter, 81 is the normalized transmittance spectrum of the single-pass filter, and 82 is the normalized transmittance spectrum of the double-pass filter. After passing through the filter twice, the pass rate is the square of the single pass, which has higher noise signal compression capability and narrower passband bandwidth.

As shown in FIG. 3 , the incident surface 91 of the triangular prism is coated with a 607nm anti-reflection coating, the first reflection surface 92 is coated with a 607nm wavelength semi-reflective coating, and the second reflection surface 93 is coated with a 607nm total reflection coating. The direction of the optical signal is shown in the figure, where the arrow indicates the direction of the light, the width of the dotted line box indicates the intensity of the optical signal, and the thickness is as shown in the figure.

The system consists of a laser 1, a first reflector 2, a second reflector 3, a telescope 4, an aperture diaphragm 5, a collimating mirror 6, a third total reflection mirror 7, a narrow band filter 8, a prism 9, the first The photomultiplier tube 10, the second photomultiplier tube 11, and the data acquisition and processing system 12 are composed. The functions of the first and second mirrors guide the laser light of the laser to the atmosphere, and can conveniently adjust the optical axis of the laser to be parallel to the optical axis of the telescope. The telescope 4 is used to collect the backscattered light signal of the atmosphere, and its aperture reflects the ability to receive scattered light. The function of the pinhole diaphragm 5 is to limit the field of view of the telescope. The field of view of the general telescope is much larger than that of the laser radar. Therefore, it is necessary to place a small hole diaphragm near the focal plane of the telescope. Part of the scattered light is lost, so the field of view of the system can be limited. The function of the collimator 6 is to change the backscattered light received by the telescope into parallel light. The function of the narrow-band filter 8 is to filter out noise optical signals, compress background noise and noise caused by elastic scattering noise, and is the core component of Raman lidar. The function of the prism 9 is to send a part of the light to the detector for direct detection, and send the other part of the light to the narrow-band filter to further compress the noise. The function of the first and second photomultiplier tubes is to detect the returned Raman scattering signals at different heights and convert the optical signals into electrical signals. The function of the main control computer of the data acquisition and processing system 12 is to control the working sequence of the whole system and collect the output signals of the two photomultiplier tubes. The high-altitude signal noise signal is relatively weak, and its signal is sent to the photoelectric detection system for detection after passing through the narrow-band filter 8 once. However, the low-altitude noise signal is very strong. Special optical design is used to make the atmospheric backscattered light pass through the same narrow-band filter 8 twice, and the total transmittance spectrum is equivalent to the square of the single transmittance spectrum. suppresses the optical noise. After the elastic scattering of the emission wavelength and the background light are compressed, under the condition that the multiplier tube is not saturated, the echo intensity of the Raman scattering signal can be increased by increasing the single pulse energy of the emission laser, thereby improving the accuracy of temperature measurement and expanding temperature measurement range.

The specific implementation process of the Raman lidar system will be described below with reference to the accompanying drawings.

To start up the system, the first step is to set the main control computer, that is, to set the detection range corresponding to the first and second multiplier tubes by setting the delay gate control method. When the laser radar is working, the main control computer gives the light output signal of the laser 1, and the 532nm laser light emitted by the laser 1 enters the atmosphere through the first and second mirrors. The interaction between the laser and the atmosphere produces Rayleigh scattering, Mie scattering, and vibrational Raman scattering. The first two kinds of scattering do not change the wavelength of the laser, which is generally called elastic scattering. Vibrational Raman scattering changes the wavelength, making the laser light into a 607nm optical signal , the energy of the elastically scattered light is much greater than the energy of the Raman scattered light signal. The three kinds of scattered light are all collected by the telescope 4 and enter the laser radar receiving system. The small hole diaphragm 5 located on the focal plane of the telescope 4 limits the field of view of the receiving system, so that the field of view includes the laser beam. Minimize the emission angle as much as possible, which is beneficial to reduce the influence of background light.

In order to facilitate subsequent optical processing, the optical signal collected by the telescope 4 first passes through the aperture diaphragm 5 , is collimated by the collimating mirror 6 , and then is sent to the narrow-band filter 8 after passing through the third total reflection mirror 7 . After the narrow-band filter 8 filters out most of the noise light signals, the signal light is transmitted and reflected at the first reflective surface 92 of the prism 9 at a ratio of 1:1, and the transmitted light is detected by the first photomultiplier tube 10 (that is, the high-altitude detection channel) , by setting the gate of the photomultiplier tube, that is, to make the photomultiplier tube work after a certain period of time after the start pulse of the laser 1 is sent, so that the first photomultiplier tube 10 can only detect high-altitude Raman scattered light. The reflected light at the first reflective surface 92, after being totally reflected by the second reflective surface 93 of the triangular prism 9, is sent into the narrowband optical filter 8 again from the lower half of the narrowband optical filter 8, further compressing the light introduced by elastic scattering noise. It is undeniable that the 607nm optical signal beneficial to the system is also attenuated by passing through the narrow-band filter 8 twice, but the attenuation degree is much smaller than the attenuation to elastic scattering. Therefore, the signal after passing through the narrow-band filter 8 twice is beneficial to improve the overall signal-to-noise ratio of the system. The optical signal after passing through the narrow-band filter 8 twice is detected by the second photomultiplier tube 11. Similarly, by setting the gate control of the photomultiplier tube, the second photomultiplier tube 11 only detects low-altitude Raman optical signals. The gate control of the photomultiplier tube and the detection of the signal after the photomultiplier tube are all monitored by the main control computer of the data acquisition and processing 12, wherein the detection of the photoelectric signal uses a single photon counting card. In this way, every time the laser 1 emits a laser pulse, the main control computer obtains the photon number profiles returned at different heights. Since only one laser pulse is sent, the number of photons collected by the system is very small, and the system must integrate multiple laser pulses. (For example, 10,000 laser pulses), the number of photons obtained can be used to invert the atmospheric temperature profile. Therefore, the lidar system protects the data of the number of photons corresponding to different heights after a period of integration, and updates the The resulting atmospheric temperature profile.

Claims (9)

1. Vibration Raman lidar scattered light processing system, the system includes: (a) a pulsed laser (1); (b) the first reflection mirror (2) and the second reflection mirror (3) reflecting the pulse laser; (c) a telescope (4) for collecting backscattered light signals from the atmosphere; (d) a pinhole diaphragm (5) and a signal light collimating lens (6) that limit the viewing angle of the receiving system; (e) the third mirror (7); (f) a narrow-band filter (8) for filtering out optical signal noise; (g) a photomultiplier tube for detecting light signals; (h) data acquisition and processing system (12), mainly comprising main control computer and photon counting card, this data acquisition and processing system is connected with laser (1) and photomultiplier tube; Its characteristics are: The photomultiplier tube comprises a first photomultiplier tube (10) and a second photomultiplier tube (11), and the two photomultiplier tubes are respectively arranged on both sides of the narrow-band filter (8) to detect optical signals; The vibration Raman laser radar scattered light processing system also includes a prism (9), which is arranged between the first photomultiplier tube (10) and the narrow-band filter (8), and transmits part of the light into the first photomultiplier tube (10), the reflected part of the light is sent into the second photomultiplier tube (11) after passing through the narrow-band filter (8). 2. vibration Raman lidar scattered light processing system according to claim 1, wherein, telescope (4) is a Cassegrain telescope, focal length 2m; The aperture of collimating mirror is 1cm, and signal light spot size after collimating 1cm. 3. The vibration Raman lidar scattered light processing system according to claim 1, wherein the narrowband filter (8) has a central wavelength of 607nm, a diameter of 2.54cm, and a suppression rate of 200-1200nm band of 10 ⁷ . 4. vibration Raman lidar scattered light processing system according to claim 1, wherein, triangular prism (9) is an isosceles rectangular prism, the first reflection surface (92) is coated with semi-reflective film, the second reflection surface (93 ) is coated with a full reflection film. 5. vibration Raman lidar scattered light processing system according to claim 4, wherein, the first reflection surface (92) of described prism (9) and the first photomultiplier tube (10) corresponding narrow-band filter ( 8) the upper half position; the second reflection surface (93) of the prism (9) and the second photomultiplier tube (11) correspond to the lower half position of the narrow band filter (8). 6. The vibration Raman lidar scattered light processing system according to claim 1, wherein the sampling rate of the photon counting card is 200MHz, and the counting threshold voltage is adjustable. 7. The method for processing scattered light of vibration Raman lidar, characterized in that it comprises the following steps: (a) The main control computer controls the laser (1) to emit pulsed laser light, which is totally reflected by two mirrors and enters the atmosphere; (b) The backscattering of the atmosphere is collected by the telescope (4), the viewing angle of the receiving system is limited by the aperture diaphragm (5), and the signal light is collimated by the collimating mirror (6); (c) The collimated scattered light signal passes through the upper half of the narrow-band filter (8), filtering out background noise and elastic scattering of laser light; (d) the optical signal through the narrow-band filter (8) enters the triangular prism (9), and the transmission and reflection ratio of the signal light on the first reflection surface (92) is 1: 1, (e) part of the transmitted light enters the first photomultiplier tube (10) for high-altitude detection; (f) the optical signal of the reflection part is totally reflected through the second reflective surface (93) of the triangular prism (9), and enters the described narrow-band filter (8) for the second time by the lower half of the narrow-band filter (8), And detected by the second photomultiplier tube (11); (g) The signals detected by the first and second photomultiplier tubes are collected and processed by the data collection and processing system (12). 8. vibration Raman lidar scattered light processing method according to claim 7, wherein, triangular prism (9) is an isosceles rectangular prism, and the first reflecting surface (92) is coated with semi-reflective film, and the second reflecting surface (93 ) is coated with a full reflection film. 9. The method for processing scattered light of vibrational Raman lidar according to claim 7, wherein the narrowband filter (8) has a central wavelength of 607nm, a diameter of 2.54cm, and a suppression rate of 200-1200nm band of 10 ⁷ .

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