Multi-wavelength lidar system for measuring atmospheric carbon dioxide and water vapor concentrations

Abstract

A multi-wavelength laser radar system for measuring atmospheric carbon dioxide and water vapor concentrations, the system is composed of a multi-wavelength laser transmitting module and a laser radar receiving module. The present invention allows the use of a laser to output three different wavelength pulse lasers in the 1572nm band to independently measure atmospheric carbon dioxide and water vapor concentrations at the same time. The present invention obtains atmospheric carbon dioxide and water vapor concentrations based on a laser integral path differential absorption method.

Description

Technical Field

[0001] The invention relates to a laser radar, in particular to a multi-wavelength laser radar system for measuring atmospheric carbon dioxide and water vapor concentrations. Background Art

[0002] As a greenhouse gas with a high atmospheric content, accurate detection of its concentration, changes, and source-sink distribution in the atmosphere is of great significance for understanding the greenhouse effect and the carbon cycle in the ecosystem. 2 The main methods of concentration measurement include ground-based single-point measurement, airborne track measurement, and satellite global observation. Among them, satellite remote sensing detection technology has realized passive detection of sunlight ground scattering spectrum technology, but passive detection has certain limitations and constraints. It can only be measured during the day and is easily interfered by clouds and aerosols. LiDAR active detection technology uses continuous laser or pulsed laser emitted by itself as a light source. It does not need to rely on sunlight and aims to achieve global, all-day, high-precision monitoring of atmospheric carbon dioxide column concentration. The more mature active detection technologies include tunable semiconductor laser absorption spectroscopy technology and differential absorption lidar technology (DIAL). Among them, integral path differential absorption (IPDA) detection technology is a special application of DIAL technology.

[0003] For the measurement of atmospheric carbon dioxide concentration, most of the current IPDA lidar systems use dual wavelengths to detect it. Based on the weight function and the error term closely related to the radar working wavelength, two suitable wavelengths are selected in the same band as on-line and off-line. Although the selection of the working wavelength has minimized the influence of water vapor, in the actual detection process, H 2 O still affects the final inversion accuracy through molecular interference, dry air number density determination, and carbon dioxide line broadening caused by water. 2 The measurement accuracy of column concentration can reduce the influence of water vapor on carbon dioxide by measuring the atmospheric water vapor concentration. According to the absorption lines of the two gases, it is feasible for lidar to simultaneously measure atmospheric carbon dioxide and water vapor column concentration in the same band. Summary of the invention

[0004] The present invention provides a multi-wavelength laser radar system for measuring atmospheric carbon dioxide and water vapor concentrations, which can use a laser to simultaneously emit multiple wavelengths to simultaneously measure atmospheric CO 2 Column concentration and H 2 O column concentration.

[0005] The technical solution of the present invention is as follows:

[0006] A multi-wavelength laser radar system for measuring atmospheric carbon dioxide and water vapor concentrations, which is characterized by: comprising a multi-wavelength laser transmitting module and an IPDA laser radar receiving module:

[0007] The multi-wavelength laser emission module includes a multi-wavelength seed laser, a reference seed laser, an electro-optical phase modulator, a carbon dioxide gas absorption cell, an absorption line center locking unit, a working wavelength locking unit, a frequency stabilization system, an optical switch, a multi-wavelength pulse laser, a thermal control unit, a first spectroscope and a second spectroscope;

[0008] The IPDA laser radar receiving module includes a first narrowband filter, an integrating sphere, a first collimator, an attenuation plate, a reflector, a visual axis monitoring unit, a receiving telescope, a third spectroscope, a relay optical unit, a beam combiner, a second narrowband filter, a second collimator, a 1572nm photoelectric detector, a data acquisition and processing unit and a computer;

[0009] The multi-wavelength seed laser and the reference seed laser generate continuous, single-frequency and narrow-linewidth lasers. The reference wavelength output by the reference seed laser is the central wavelength of the carbon dioxide R18 absorption line. The phase is modulated by the electro-optic phase modulator. The modulated laser is absorbed by the carbon dioxide gas absorption cell and then fed back to the reference seed laser by the absorption line center locking unit for wavelength locking. A part of the reference wavelength laser output by the reference seed laser is used to compensate and lock the working laser wavelength of the multi-wavelength seed laser through the working wavelength locking unit. The optical switch switches different wavelengths. The frequency stabilization system controls the output wavelength of the pulse laser through a heterodyne beat frequency signal, so that the frequency of the output laser matches the frequency of the multi-wavelength seed laser; the thermal control unit is used to keep the multi-wavelength pulse laser working at a set temperature; the laser emitted by the multi-wavelength pulse laser is divided into transmitted light and reflected light by a first spectroscope, the transmitted light enters the IPDA laser radar receiving module to form an energy monitoring signal, and the reflected light is further divided into reflected light and transmitted light by a second spectroscope: the reflected light enters the line of sight monitoring unit in the IPDA laser radar receiving module, and the transmitted light forms a detection light and is emitted into the atmosphere to be measured;

[0010] Along the direction of the energy monitoring signal are the first narrowband filter, integrating sphere, first collimator, attenuator, reflector and beam combiner. This signal is used as a reference signal for normalizing the atmospheric echo signal and enters the subsequent detection receiving part after passing through the beam combiner.

[0011] The detection light is emitted into the atmosphere to be measured, and the laser light returned after being absorbed by the atmosphere is received by the telescope to form an echo signal. The echo signal is divided into two paths by the third spectroscope: one echo signal is emitted into the visual axis monitoring unit, and the other echo signal enters the subsequent detection receiving part after passing through the beam combining mirror of the relay optical unit. The subsequent detection receiving part includes: along the direction of the reference signal and the echo signal, the second narrowband filter, the second collimator and the 1572nm photoelectric detector are arranged in sequence. The output of the 1572nm photoelectric detector enters and is processed by the data acquisition and processing unit. The data acquisition and processing unit is controlled by the computer.

[0012] The first narrowband filter is used to eliminate the influence of background light on the integrating sphere, and the integrating sphere can reduce the influence of pointing jitter on the monitoring error.

[0013] The advantages of the present invention are as follows:

[0014] 1. The present invention allows the use of a multi-wavelength laser to simultaneously and independently measure atmospheric carbon dioxide and water vapor column concentrations, reducing the size, mass and power consumption of the required sensors.

[0015] 2. The present invention is applicable to ground-based, airborne and satellite-borne platforms, and can simultaneously carry out high-precision measurements of atmospheric carbon dioxide and water vapor column concentrations on a global scale.

[0016] 3. The present invention adopts an active detection method, and the three-wavelength laser in the 1572nm band can work both during the day and at night to achieve high-precision measurement throughout the day. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Example diagram of wavelength selection for a three-wavelength lidar.

[0018] Figure 2 This is a structural block diagram of the multi-wavelength laser radar system for measuring atmospheric carbon dioxide and water vapor concentrations according to the present invention.

[0019] In the figure: A-multi-wavelength laser emission module, B-IPDA laser radar receiving module. 1-multi-wavelength seed laser, 2-reference seed laser, 3-electro-optic phase modulator, 4-carbon dioxide gas absorption cell, 5-absorption line center locking unit, 6-working wavelength locking unit, 7-frequency stabilization system, 8-optical switch, 9-multi-wavelength pulse laser, 10-thermal control unit, 11-first beam splitter, 12-first narrow-band filter, 13-integrating sphere, 14-first collimator, 15-attenuation plate, 16-reflector, 17-second beam splitter, 18-line of sight monitoring unit, 19-receiving telescope, 20-third beam splitter, 21-relay optical unit, 22-beam combiner, 23-second narrow-band filter, 24-second collimator, 25-1572nm photoelectric detector, 26-data acquisition and processing unit, 27-computer.

[0020] Figure 3 This is the timing diagram of the three-wavelength optical switch. DETAILED DESCRIPTION

[0021] In order to more clearly illustrate the multi-wavelength lidar system for measuring atmospheric carbon dioxide and water vapor concentration, a three-wavelength example is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0022] Please refer to Figure 2 , Figure 2 This is a structural block diagram of a multi-wavelength laser radar system for measuring atmospheric carbon dioxide and water vapor concentrations according to the present invention. As can be seen from the figure, the multi-wavelength laser radar system for measuring atmospheric carbon dioxide and water vapor concentrations according to the present invention includes a multi-wavelength laser transmitting module A and an IPDA laser radar receiving module B:

[0023] The multi-wavelength laser emission module A comprises a multi-wavelength seed laser 1, a reference seed laser 2, an electro-optical phase modulator 3, a carbon dioxide gas absorption cell 4, an absorption line center locking unit 5, a working wavelength locking unit 6, a frequency stabilization system 7, an optical switch 8, a multi-wavelength pulse laser 9, a thermal control unit 10, a first beam splitter 11 and a second beam splitter 17;

[0024] The IPDA laser radar receiving module B includes a first narrowband filter 12, an integrating sphere 13, a first collimator 14, an attenuation plate 15, a reflector 16, a visual axis monitoring unit 18, a receiving telescope 19, a third beam splitter 20, a relay optical unit 21, a beam combiner 22, a second narrowband filter 23, a second collimator 24, a 1572nm photodetector 25, a data acquisition and processing unit 26 and a computer 27;

[0025] The multi-wavelength seed laser 1 and the reference seed laser 2 generate continuous, single-frequency and narrow-linewidth lasers. The reference wavelength output by the reference seed laser 2 is the central wavelength of the carbon dioxide R18 absorption line, which is phase modulated by the electro-optical phase modulator 3. The modulated laser is absorbed by the carbon dioxide gas absorption cell 4 and then fed back to the reference seed laser 2 by the absorption line center locking unit 5 for wavelength locking. A part of the reference wavelength laser output by the reference seed laser 2 is used to compensate and lock the working laser wavelength of the multi-wavelength seed laser 1 through the working wavelength locking unit 6, and the optical switch 8 switches between different wavelengths. The frequency stabilization system 7 controls the output wavelength of the pulse laser 9 through a heterodyne beat frequency signal, so that the frequency of the output laser matches the frequency of the multi-wavelength seed laser 1; the thermal control unit 10 is used to keep the multi-wavelength pulse laser 9 working at a set temperature; the laser emitted by the multi-wavelength pulse laser 9 is divided into transmitted light and reflected light by a first spectroscope 11, and the transmitted light enters the IPDA laser radar receiving module B to form an energy monitoring signal, and the reflected light is further divided into reflected light and transmitted light by a second spectroscope 17: the reflected light enters the line of sight monitoring unit 18 in the IPDA laser radar receiving module B, and the transmitted light forms detection light and is emitted into the atmosphere to be measured;

[0026] Along the direction of the energy monitoring signal are the first narrowband filter 12, the integrating sphere 13, the first collimator 14, the attenuator 15, the reflector 16 and the beam combiner 22; this signal is used as a reference signal for normalizing the atmospheric echo signal, and enters the subsequent detection receiving part after passing through the beam combiner 22;

[0027] The detection light is emitted into the atmosphere to be measured, and the laser light returned after being absorbed by the atmosphere is received by the telescope 19 to form an echo signal. The echo signal is divided into two paths by the third spectroscope 20: one path is emitted into the visual axis monitoring unit 18, and the other path passes through the relay optical unit 21 and the beam combiner 22 and enters the subsequent detection receiving part. The subsequent detection receiving part is: along the direction of the reference signal and the echo signal are the second narrowband filter 23, the second collimator 24 and the 1572nm photodetector 25 in sequence. The output of the 1572nm photodetector 25 enters and is processed by the data acquisition and processing unit 26 for data acquisition and data processing. The data acquisition and processing unit 26 is controlled by the computer 27.

[0028] The first narrowband filter 12 is used to eliminate the influence of background light on the integrating sphere 13, and the integrating sphere 13 can reduce the influence of pointing jitter on the monitoring error.

[0029] Example

[0030] Figure 1 An example of wavelength selection for a three-wavelength lidar is given, where λ 1 As CO 2 on-line; 2 As CO 2 The off-line is also H 2 O's off-line; λ 3 As H 2 O's on-line.

[0031] The specific devices used in this embodiment are: the multi-wavelength seed laser 1 and the reference seed laser 2 are both DFB laser diodes; the absorption line center locking unit 5 and the working wavelength locking unit 6 include an optical isolator, a fiber beam splitter, a photodiode, a low-pass filter, an amplifier, a servo system, etc. The frequency stabilization system 7 includes an AOM, a signal generator, a fiber coupler, an analog-to-digital / digital-to-analog converter, an amplifier, etc. The receiving telescope 19 is a receiving telescope with parallel light axes; the visual axis monitoring unit 18 is composed of a spectroscopic module and a CCD image sensor; the 1572nm photodetector 25 is an APD detector; the data acquisition and processing unit 26 includes an acquisition card and a data preprocessing module.

[0032] The specific process of measuring atmospheric carbon dioxide and water vapor concentration by the multi-wavelength laser radar implemented in the present invention is as follows:

[0033] 1) The three wavelengths λ detected by the 1572nm photodetector 25 1 , 2 , 3 The energy of the monitoring signal is E 01 、E 02 、E 03 The energy of the echo signals is E 1 , E 2 , E 3 ;

[0034] 2) Calculate the weight function IWF of carbon dioxide gas and water vapor for each pair of wavelengths H2O (λ 1 ,λ 2 ), IWF CO2 (λ 1 ,λ 2 ), IWF H2O (λ 3 ,λ 2 ), IWF CO2 (λ 3 ,λ 2), the weight function is a weight function defined based on the distance range, which is the product of the gas differential absorption cross section and the number density of dry air molecules;

[0035] 3) Calculate the differential optical thickness:

[0036]

[0037]

[0038] The differential optical thickness needs to be calculated by accumulating and averaging the number of times determined by the horizontal resolution and the pulse repetition frequency, so as to meet the signal-to-noise ratio requirement;

[0039] 4) Construct the observation matrix as follows:

[0040]

[0041] 5) Calculate the atmospheric carbon dioxide and water vapor column concentration X H2O , X CO2 .

[0042] Experiments show that the advantages of the present invention are as follows:

[0043] 1. The present invention allows the use of a multi-wavelength laser to simultaneously and independently measure atmospheric carbon dioxide and water vapor column concentrations, reducing the size, mass and power consumption of the required sensors.

[0044] 2. The present invention is applicable to ground-based, airborne and satellite-borne platforms, and can simultaneously carry out high-precision measurements of atmospheric carbon dioxide and water vapor column concentrations on a global scale.

[0045] 3. The present invention adopts an active detection method, and the three-wavelength laser in the 1572nm band can work both during the day and at night to achieve high-precision measurement throughout the day.

[0046] The above specific implementation methods are used to specifically illustrate the present invention, but they cannot be used to limit the protection scope of the present invention. For those skilled in the art, the present invention can have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A multi-wavelength laser radar system for measuring atmospheric carbon dioxide and water vapor concentrations, characterized in that: Includes multi-wavelength laser transmitting module (A) and IPDA laser radar receiving module (B): The multi-wavelength laser emission module (A) comprises a multi-wavelength seed laser (1), a reference seed laser (2), an electro-optical phase modulator (3), a carbon dioxide gas absorption cell (4), an absorption line center locking unit (5), an operating wavelength locking unit (6), a frequency stabilization system (7), an optical switch (8), a multi-wavelength pulse laser (9), a thermal control unit (10), a first beam splitter (11) and a second beam splitter (17); The IPDA laser radar receiving module (B) comprises a first narrowband filter (12), an integrating sphere (13), a first collimator (14), an attenuation plate (15), a reflector (16), a visual axis monitoring unit (18), a receiving telescope (19), a third beam splitter (20), a relay optical unit (21), a beam combiner (22), a second narrowband filter (23), a second collimator (24), a 1572nm photodetector (25), a data acquisition and processing unit (26) and a computer (27); The multi-wavelength seed laser (1) and the reference seed laser (2) generate continuous, single-frequency and narrow-linewidth lasers. The reference wavelength output by the reference seed laser (2) is the central wavelength of the carbon dioxide R18 absorption line. The reference wavelength is phase modulated by the electro-optical phase modulator (3). The modulated laser is absorbed by the carbon dioxide gas absorption cell (4) and then fed back to the reference seed laser (2) by the absorption line center locking unit (5) for wavelength locking. A portion of the reference wavelength laser output by the reference seed laser (2) is used to compensate and lock the working laser wavelength of the multi-wavelength seed laser (1) through the working wavelength locking unit (6). The optical switch (8) switches between different wavelengths. The frequency stabilization system (7) controls the output wavelength of the pulse laser (9) through a heterodyne beat frequency signal, so that the frequency of the output laser matches the frequency of the multi-wavelength seed laser (1); the thermal control unit (10) is used to keep the multi-wavelength pulse laser (9) working at a set temperature; the laser emitted by the multi-wavelength pulse laser (9) is divided into transmitted light and reflected light by a first spectroscope (11), the transmitted light enters the IPDA laser radar receiving module (B) to form an energy monitoring signal, and the reflected light is further divided into reflected light and transmitted light by a second spectroscope (17): the reflected light enters the line of sight monitoring unit (18) in the IPDA laser radar receiving module (B), and the transmitted light constitutes detection light and is emitted into the atmosphere to be measured; Along the direction of the energy monitoring signal are the first narrowband filter (12), the integrating sphere (13), the first collimator (14), the attenuator (15), the reflector (16) and the beam combiner (22); the signal is used as a reference signal for normalizing the atmospheric echo signal and enters the subsequent detection receiving part after passing through the beam combiner (22); The detection light is injected into the atmosphere to be measured, and the laser light returned after being absorbed by the atmosphere is received by the telescope (19) to form an echo signal. The echo signal is divided into two paths by the third spectroscope (20): one path is injected into the visual axis monitoring unit (18), and the other path passes through the relay optical unit (21) and the beam combiner (22) and enters the subsequent detection receiving part. The subsequent detection receiving part includes: along the direction of the reference signal and the echo signal, the second narrowband filter (23), the second collimator (24) and the 1572nm photodetector (25) are arranged in sequence. The output of the 1572nm photodetector (25) enters and is processed by the data acquisition and processing unit (26). The data acquisition and processing unit (26) is controlled by the computer (27).

2. The multi-wavelength laser radar system for measuring atmospheric carbon dioxide and water vapor concentration according to claim 1, characterized in that: The first narrowband filter (12) is used to eliminate the influence of background light on the integrating sphere (13), and the integrating sphere (13) can reduce the influence of pointing jitter on monitoring errors.

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