CN115902834B - Helium laser radar double-frequency temperature and wind measuring system and method based on Fizeau interferometer - Google Patents
Helium laser radar double-frequency temperature and wind measuring system and method based on Fizeau interferometer Download PDFInfo
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- 239000001307 helium Substances 0.000 title claims abstract description 85
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- 238000001914 filtration Methods 0.000 claims abstract description 12
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/483—Details of pulse systems
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A90/00—Technologies having an indirect contribution to adaptation to climate change
- Y02A90/10—Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation
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- Computer Networks & Wireless Communication (AREA)
- General Physics & Mathematics (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Electromagnetism (AREA)
- Optical Radar Systems And Details Thereof (AREA)
Abstract
The invention provides a helium laser radar double-frequency temperature and wind measuring system based on a Fizeau interferometer, which comprises: a transmission system, the transmission system comprising: at least two lasers, which are suitable for emitting pulse lasers with different frequencies to the region to be measured; a receiving system, the receiving system comprising: a telescope adapted to receive echo signals from the region under test having metastable helium scattering information; the filter is suitable for carrying out first-stage filtering processing on the echo signal with metastable helium scattering information received by the telescope; the Fizeau interferometer is suitable for sequentially carrying out second-stage filtering treatment on the echo signals with metastable helium scattering information after the filter processing; the detector is suitable for receiving the scattered echo signals processed by the Fizeau interferometer; the data processing system is suitable for processing the echo signals received by the detector to obtain the wind speed and the temperature corresponding to the region to be detected. The invention also provides a helium laser radar double-frequency temperature and wind measuring method based on the Fizeau interferometer.
Description
Technical Field
The invention relates to the technical field of laser radars, in particular to a helium laser radar double-frequency temperature and wind measuring system and method based on a Fizeau interferometer.
Background
The thermal layer atmospheric temperature and wind speed are important parameters for studying thermodynamic and kinetic processes associated with thermal layers. However, the density of neutral atmospheric particles in this height range is too low, making it extremely difficult to measure the density, temperature and wind profile of the thermal layer from the foundation. In the past, scientists have mostly used sounding rockets, satellites and other spacecraft to make thermal layer measurements. However, the sounding rocket is high in cost and cannot be continuously sounding for a long time; the satellite has low spatial resolution, and long-term uninterrupted observation of a specific area cannot be obtained.
With the development of laser radar technology, resonant fluorescence laser radar is becoming one of the common means for thermal layer detection. Compared with the traditional spacecraft measurement means, the laser radar can provide continuous data with high space-time resolution of a thermal layer. The traditional three-frequency temperature and wind measuring means are applied to sodium resonance fluorescent laser radar more mature and stable, but compared with sodium laser radar, helium laser radar has high observation height and weak signal, and a method for realizing temperature and wind measuring by alternately transmitting laser with three frequencies by adopting one transmitter needs to rapidly switch and alternately transmit the laser frequencies, which is equivalent to weakening energy by one third in frequency domain and time domain, so that the method is unfavorable for obtaining good signal to noise ratio. Based on the above, developing a more optimized temperature and wind measuring method is a technical problem to be solved at present.
Disclosure of Invention
Aiming at the problems, the application provides a helium laser radar double-frequency temperature and wind measuring system based on a Fizeau interferometer, which can realize double-frequency temperature and wind measuring by introducing at least two lasers into a transmitting system, introducing a novel Fizeau interferometer into a receiving system and collecting metastable helium scattering information of a region to be measured. Not only overcomes the technical defects existing in the prior art, but also provides a new research direction for the follow-up research of temperature measurement and wind measurement.
To achieve the above object, a first aspect of the present invention provides a fizeau interferometer-based helium lidar dual-frequency thermometry and anemometry system, comprising:
a transmitting system, comprising:
At least two lasers, which are suitable for emitting pulse lasers with different frequencies to the region to be measured;
a receiving system, comprising:
a telescope adapted to receive echo signals having metastable helium scattering information from the region to be measured;
the filter is suitable for carrying out first-stage filtering processing on the echo signal with the metastable helium scattering information received by the telescope;
the Fizeau interferometer is suitable for carrying out second-stage filtering processing on the echo signal with metastable helium scattering information processed by the filter;
The detector is suitable for receiving the scattered echo signals processed by the Fizeau interferometer, wherein the echo signals processed by the Fizeau interferometer are represented by a measured spectrum, and the measured spectrum is the convolution of the scattered spectrum and the transmittance function of the Fizeau interferometer;
The data processing system is suitable for processing the echo signals received by the detector to obtain the wind speed and the temperature corresponding to the region to be detected, wherein the linear interpolation comparison result of the echo signal spectrum library is obtained according to the echo signal spectrum and the simulation of the laser, the wind speed and the temperature corresponding to the region to be detected are determined, and the echo signal spectrum of the laser is the deconvolution of the measured spectrum and the transmittance function of the Fizeau interferometer.
According to an embodiment of the present invention, the emission system includes the at least two lasers, including:
A first tunable continuous laser and a second tunable continuous laser adapted to produce a stable and continuous seed laser;
The first pulse laser and the second pulse laser are suitable for carrying out optical amplification on the seed laser and transmitting the seed laser to the region to be detected;
And the first pulse laser and the second pulse laser transmit part of the amplified emission laser to a first wavelength meter and a second wavelength meter to monitor the frequency of the emission laser in real time and feed back the frequency to the first tunable continuous laser and the second tunable continuous laser so as to realize the frequency locking of the emission laser.
According to an embodiment of the present invention, the first tunable continuous laser and the second tunable continuous laser have the same structure, the first pulse laser and the second pulse laser have the same structure, and the first wavelength meter and the second wavelength meter have the same structure.
According to an embodiment of the present invention, the at least two lasers emit the laser light vertically upward.
The second aspect of the invention provides a fizeau interferometer-based helium lidar dual-frequency temperature and wind measurement method, which is applied to the fizeau interferometer-based helium lidar dual-frequency temperature and wind measurement system in any one of the embodiments, and the method comprises the following steps:
Simulating to obtain an echo signal spectrum database according to preset emission laser information and a plurality of temperature and wind speed information corresponding to a preset certain temperature and wind speed range, wherein the preset emission laser information is frequency information of laser to be emitted corresponding to the at least two lasers;
Transmitting pulse lasers with different frequencies to a region to be measured by using the at least two lasers;
receiving echo signals with metastable helium scattering information from the region to be measured by using the telescope;
Sequentially performing two-stage filtering processing on the echo signals with metastable helium scattering information received by the telescope by using the filter and the Fizeau interferometer, and conveying the processed echo signals to a light path corresponding to a receiving system;
The detector receives an echo signal from a receiving system optical path, wherein the echo signal in the receiving system optical path is represented by a measured spectrum, and the measured spectrum is the convolution of a scattering spectrum and a transmittance function of a Fizeau interferometer;
Performing deconvolution processing on the measured spectrum and the transmittance function of the Fizeau interferometer to obtain an echo signal spectrum of laser;
and performing linear interpolation comparison on the echo signal spectrum of the laser and the echo signal spectrum database to determine a group of data closest to the echo signal spectrum of the laser as the temperature and the wind speed corresponding to the region to be measured.
According to an embodiment of the present invention, the emitting of pulsed laser light with different frequencies to a region to be measured by using the at least two lasers includes:
generating the seed laser light stably and continuously by using the first tunable continuous laser and the second tunable continuous laser;
the first pulse laser and the second pulse laser are utilized to carry out optical amplification on the seed laser and emit the seed laser to the region to be detected;
And after the first pulse laser and the second pulse laser amplify the seed laser, the first wavelength meter and the second wavelength meter are used for monitoring the frequency of a part of the received emitted laser in real time and feeding back the frequency to the first tunable continuous laser and the second tunable continuous laser so as to realize the frequency locking of the emitted laser.
According to the embodiment of the invention, the at least two lasers emit two pulse lasers with different frequencies to the region to be measured, so that four echo signals with metastable helium scattering information with different frequencies can be obtained.
The third aspect of the invention provides a method for determining parameters of a fizeau interferometer, which is applied to the helium laser radar dual-frequency temperature and wind measurement system based on the fizeau interferometer in any one of the above embodiments, and the method comprises the following steps:
Acquiring a plurality of attribute information of an initial Fizeau interferometer, wherein the plurality of attribute information comprises: the method comprises the steps that a set consisting of a plurality of half-height full widths of an initial Fizeau interferometer and a set consisting of a plurality of free spectrum intervals are formed, and an echo signal with metastable helium scattering information processed by the filter sheet reaches an incidence dip angle of the initial Fizeau interferometer;
obtaining an echo signal spectrum database based on the simulation, traversing the set consisting of the plurality of full widths at half maximum and the set consisting of the plurality of free spectrum intervals to determine a target full width at half maximum and a target free spectrum interval;
Determining a target fine factor of the Fizeau interferometer according to the target full width at half maximum and the target free spectrum interval;
Determining a target reflectivity of the Fizeau interferometer according to the target fine factor;
determining the target average cavity length of the Fizeau interferometer according to the wavelength corresponding to the target free spectrum interval;
Determining a target wedge angle of the Fizeau interferometer according to the target average cavity length;
The target wedge angle is an included angle corresponding to two flat plates of the Fizeau interferometer, and is used for regulating and controlling the width of interference fringes of the Fizeau interferometer.
According to the embodiment of the invention, the target fine factor of the Fizeau interferometer and the target reflectivity of the Fizeau interferometer are calculated by a formula (1);
in equation (1), F represents the target fine factor of the fizeau interferometer, FSR represents the target free spectral separation, FWHM represents the target full width at half maximum, and R represents the target reflectivity of the fizeau interferometer.
According to the embodiment of the invention, the target average cavity length and the target wedge angle of the Fizeau interferometer are calculated by a formula (2);
In formula (2), L 0 represents the target average cavity length of the fizeau interferometer, n represents the refractive index of the medium, λ represents the wavelength of the emitted laser light, λ FSR represents the wavelength corresponding to the target free spectrum spacing, and α represents the target wedge angle of the fizeau interferometer.
According to the embodiment of the invention, at least two lasers are introduced into the emission system, so that two pulse lasers with different frequencies are emitted simultaneously, and the defect in the prior art that three-frequency alternate switching is realized by using three lasers and three-frequency lasers are emitted to perform a temperature and wind measuring method is overcome; the dual-frequency transmitting system not only can save the time of alternately transmitting the traditional laser, but also can reduce the complexity and maintenance difficulty of the laser radar.
According to the embodiment of the invention, a part of modules are modified, namely a novel Fizeau interferometer is introduced into a receiving system to replace a traditional F-P standard tool, so that linear stripes which can replace annular stripes can be generated, frequency scanning is not needed, and the position and shape change of the linear stripes can be detected by directly matching with a linear detector; the novel receiving system can save sweep frequency time, continuously accumulate photon signals, and greatly simplify a data processing method.
According to the embodiment of the invention, helium atoms are one of main atmospheric components above 200km, and metastable helium (He (2 3 S)) which is more stable and stronger in fluorescence can be generated through physical processes such as photoelectron excitation and the like; therefore, compared with the temperature and wind measurement by adopting the sodium resonance fluorescence laser radar in the prior art, the method has the advantages that the metastable helium scattering information of the region to be measured is collected to measure the temperature and wind, a new research direction can be provided for the follow-up research of the temperature and wind measurement, and the theoretical blank of the helium laser radar in the aspect of the temperature and wind measurement is also filled. Thus, the thermal layer atmospheric physical process and the multi-turn layer coupling mechanism can be observed and researched more omnidirectionally.
Drawings
The foregoing and other objects, features and advantages of the disclosure will be more apparent from the following description of embodiments of the disclosure with reference to the accompanying drawings, in which:
FIG. 1 schematically illustrates a block diagram of a helium lidar dual-frequency thermometry and anemometry system based on a Fizeau interferometer in accordance with an embodiment of the present invention;
FIG. 2 schematically illustrates a schematic diagram of a helium lidar dual-frequency thermometry and anemometry system based on a Fizeau interferometer in accordance with an embodiment of the present invention;
FIG. 3 schematically shows a plot of backscattering cross-section versus wavelength for three radial lines of metastable helium around 1083nm in accordance with an embodiment of the invention;
FIG. 4 schematically illustrates a flow chart of a helium lidar dual-frequency thermometry and anemometry method based on a Fizeau interferometer in accordance with an embodiment of the present invention;
FIG. 5 schematically shows a graph of the back-scatter cross-section as a function of temperature and wind speed according to an embodiment of the invention;
FIG. 6 schematically illustrates two sets of data example graphs in an echo signal spectrum database according to embodiments of the invention;
FIG. 7 schematically illustrates a back-scattered cross-section of a metastable helium peak and a normalized intensity plot of an echo signal, according to an embodiment of the present invention;
FIG. 8 schematically illustrates a flow chart of a method of determining parameters of a Fizeau interferometer in accordance with an embodiment of the invention;
FIG. 9 schematically illustrates a graph of transmittance curves for different shape factors versus Fizeau interferometers in accordance with embodiments of the invention;
FIG. 10 schematically illustrates a data processing flow diagram according to an embodiment of the invention;
Fig. 11 schematically shows a schematic of the convolution results of the laser line pattern detected by the detector, the total echo signal, and the fizeau interferometer in accordance with an embodiment of the invention.
Reference numerals illustrate:
a first tunable continuous laser;
2, a first pulse laser;
3: a first wavelength meter;
4: a second tunable continuous laser;
5: a second pulsed laser;
6: a second wavelength meter;
7: a telescope;
8: an optical switch;
9: a filter;
10: fizeau interferometers;
11: a detector.
Detailed Description
Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. It should be understood that the description is only exemplary and is not intended to limit the scope of the present disclosure. In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the present disclosure. It may be evident, however, that one or more embodiments may be practiced without these specific details. In addition, in the following description, descriptions of well-known structures and techniques are omitted so as not to unnecessarily obscure the concepts of the present disclosure.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. The terms "comprises," "comprising," and/or the like, as used herein, specify the presence of stated features, steps, operations, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, or components.
All terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art unless otherwise defined. It should be noted that the terms used herein should be construed to have meanings consistent with the context of the present specification and should not be construed in an idealized or overly formal manner.
Where a convention analogous to "at least one of A, B and C, etc." is used, in general such a convention should be interpreted in accordance with the meaning of one of skill in the art having generally understood the convention (e.g., "a system having at least one of A, B and C" would include, but not be limited to, systems having a alone, B alone, C alone, a and B together, a and C together, B and C together, and/or A, B, C together, etc.).
The present invention will be further described in detail below with reference to specific embodiments and with reference to the accompanying drawings, in order to make the objects, technical solutions and advantages of the present invention more apparent.
With the development of laser radar technology, resonant fluorescence laser radar is becoming one of the common means for thermal layer detection. Helium atoms are one of main atmospheric components above 200km, and can generate metastable helium (He (2 3 S)) which is more stable and stronger in fluorescence than the helium atoms through physical processes such as photoelectron excitation and the like, so that the metastable helium can be used as one of the preferred tracers for observing the neutral atmosphere in the height range by the foundation laser radar. The principle of detecting atmospheric parameters by the metastable helium resonance fluorescence laser radar is as follows: and the emitting end of the laser radar vertically emits a narrow linewidth laser beam with the wavelength of 1083nm upwards to the air, and excites helium atoms in the thermal layer to enable out-of-core electrons to transit to a high energy level and to be excited to a lower energy level, namely, resonance fluorescence reaction occurs. On the ground, a receiving end telescope of the laser radar receives echo signals, the photoelectric detector is used for detecting the number of echo photons, and a series of signal processing and data inversion are carried out, so that the metastable helium density of the thermal layer is obtained. Meanwhile, the temperature and wind field of the metastable helium in the detection height range can be further obtained through inversion by enabling the radar laser transmitter to alternately emit laser with three frequencies and through the relative relation between the resonance fluorescence intensities excited by the three frequencies. In combination with the atmospheric mode, and based on some assumptions and boundary conditions, the density, temperature and wind field of the background atmosphere can be deduced.
However, the metastable helium thermometry wind lidar detection technique cannot be well implemented in the related art. The temperature and wind measuring means in the related art are generally limited to three-frequency temperature and wind measuring by using a sodium laser radar. Compared with the helium laser radar, the helium laser radar has higher detection height and weaker signal, and if the helium laser radar has the temperature measurement and wind measurement functions, a more optimized method needs to be developed to realize a higher signal-to-noise ratio so as to obtain more accurate temperature and wind field measurement results.
Based on the two-frequency temperature and wind measuring system of the helium laser radar based on the Fizeau interferometer, the two-frequency temperature and wind measuring system is realized by introducing at least two lasers into a transmitting system, introducing a novel Fizeau interferometer into a receiving system and collecting metastable helium scattering information of a region to be measured. Not only overcomes the technical defects existing in the prior art, but also provides a new research direction for the follow-up research of temperature measurement and wind measurement.
FIG. 1 schematically illustrates a block diagram of a helium lidar dual-frequency thermometry and anemometry system based on a Fizeau interferometer in accordance with an embodiment of the present invention.
As shown in FIG. 1, the helium lidar dual-frequency thermometry and anemometry system based on the Fizeau interferometer comprises a transmitting system, a receiving system and a data processing system.
Fig. 2 schematically illustrates a diagram of a fizeau interferometer-based helium lidar dual-frequency thermometry and anemometry system in accordance with an embodiment of the present invention.
According to an exemplary embodiment of the present invention, the present invention discloses a helium lidar dual-frequency thermometry and anemometry system based on a fizeau interferometer, and referring to fig. 2, a transmitting system includes: at least two lasers, which are suitable for emitting pulse lasers with different frequencies to the region to be measured; the receiving system includes: the telescope 7, the optical switch 8, the filter 9, the Fizeau interferometer 10 and the detector 11, wherein the telescope 7 is suitable for receiving echo signals with metastable helium scattering information from a region to be detected; the filter 9 is suitable for performing first-stage filtering processing on the echo signal with metastable helium scattering information received by the telescope 7; the Fizeau interferometer 10 is suitable for carrying out second-stage filtering treatment on the echo signal with metastable helium scattering information processed by the filter 9; the detector 11 is suitable for receiving the scattered echo signal processed by the Fizeau interferometer 10, wherein the echo signal processed by the Fizeau interferometer 10 is represented by a measured spectrum, and the measured spectrum is the convolution of the scattered spectrum and the transmittance function of the Fizeau interferometer 10; and the data processing system is suitable for processing the echo signals received by the detector 11 to obtain the wind speed and the temperature corresponding to the region to be measured, wherein the linear interpolation comparison result of the echo signal spectrum library is obtained according to the echo signal spectrum and simulation of the laser, the wind speed and the temperature corresponding to the region to be measured are determined, and the echo signal spectrum of the laser is the deconvolution of the measured spectrum and the transmittance function of the Fizeau interferometer 10.
According to the embodiment of the invention, at least two lasers are introduced into the emission system, so that two pulse lasers with different frequencies are emitted simultaneously, and the defect in the prior art that three-frequency alternate switching is realized by using three lasers and three-frequency lasers are emitted to perform a temperature and wind measuring method is overcome; the dual-frequency transmitting system not only can save the time of alternately transmitting the traditional laser, but also can reduce the complexity and maintenance difficulty of the laser radar.
According to the embodiment of the invention, by modifying part of the modules, namely introducing a novel Fizeau interferometer 10 to a receiving system to replace a traditional F-P standard tool, linear stripes which can replace annular stripes can be generated, frequency scanning is not needed, and the detection of the position and shape change of the linear stripes can be completed by directly matching with a linear detector 11; the novel receiving system can save sweep frequency time, continuously accumulate photon signals, and greatly simplify a data processing method.
According to the embodiment of the invention, helium atoms are one of main atmospheric components above 200km, and metastable helium (He (2 3 S)) which is more stable and stronger in fluorescence can be generated through physical processes such as photoelectron excitation and the like; therefore, compared with the temperature and wind measurement by adopting the sodium resonance fluorescence laser radar in the prior art, the method has the advantages that the metastable helium scattering information of the region to be measured is collected to measure the temperature and wind, a new research direction can be provided for the follow-up research of the temperature and wind measurement, and the theoretical blank of the helium laser radar in the aspect of the temperature and wind measurement is also filled. Thus, the thermal layer atmospheric physical process and the multi-turn layer coupling mechanism can be observed and researched more omnidirectionally.
According to an embodiment of the invention, the emission system comprises at least two lasers, comprising: a first tunable continuous laser 1, a first pulse laser 2, a second tunable continuous laser 4 and a second pulse laser 5;
a first tunable continuous laser 1 and a second tunable continuous laser 4 adapted to generate a stable and continuous seed laser having a wavelength around 1083 nm;
The first pulse laser 2 and the second pulse laser 5 are suitable for optically amplifying the seed laser and transmitting the seed laser to the region to be measured;
the first pulse laser 2 and the second pulse laser 5 respectively transmit a part of the amplified emission laser to the first wavelength meter 3 and the second wavelength meter 6 to monitor the frequency of the emission laser in real time, and feed back the frequency of the emission laser to the first tunable continuous laser 1 and the second tunable continuous laser 4 to realize frequency locking of the emission laser.
According to an embodiment of the present invention, the first tunable continuous laser 1 and the second tunable continuous laser 4 are identical in structure, the first pulse laser 2 and the second pulse laser 5 are identical in structure, and the first wavemeter 3 and the second wavemeter 6 are identical in structure.
According to the embodiment of the present invention, the first pulse laser 2 and the second pulse laser 5 are solid ytterbium-doped yttrium aluminum garnet crystal (Yb: YAG) pulse lasers, which can output pulse light around 1083nm and emit vertically upward to the region to be measured.
According to embodiments of the present invention, there are two sets of energy levels, singlet and triplet, for helium atoms, without transitions between each other. In the singlet energy level, the first excited state of helium is 2 1S0, with a lifetime of about 19.5ms; in the triplet energy level, the first excited state of helium is 2 3S1, which contains three radiation lines of 1082.909nm, 1083.025nm and 1083.034nm, and in an ideal case helium (2 3S1) has a naturally occurring lifetime of about 8000s and is suitable as a tracer for detecting the temperature and wind speed of the hot layer atmosphere.
FIG. 3 schematically shows a plot of backscattering cross-section versus wavelength for three radial lines of metastable helium around 1083nm, in accordance with an embodiment of the invention.
As shown in FIG. 3, curves 1-909, 1-025, and 1-034 represent the corresponding backscatter cross-sections of three lines of radiation at 1082.909nm, 1082.025nm, and 1082.034nm, respectively, for metastable helium. In contrast, radiation at 1082.909nm of metastable helium is severely disturbed by hydroxyl radicals, so two radiation corresponding to wavelengths 1083.025nm and 1083.034nm were selected for observation in the present application.
FIG. 4 schematically illustrates a flow chart of a helium lidar dual-frequency thermometry and anemometry method based on a Fizeau interferometer in accordance with an embodiment of the present invention.
As shown in fig. 4, the fizeau interferometer-based helium lidar dual-frequency thermometry wind system method may include operations S410-S470.
In operation S410, according to preset emission laser information and a plurality of temperature and wind speed information corresponding to a preset certain temperature and wind speed range, an echo signal spectrum database is obtained through simulation, wherein the preset emission laser information is frequency information of laser to be emitted corresponding to at least two lasers.
According to an embodiment of the present invention, the velocity distribution of helium atoms obeys maxwell's law of distribution, and the statistical average absorption cross section of each helium atom is calculated by formula (one):
in equation (one), doppler absorption cross section V denotes a laser frequency, v 0 denotes a resonance frequency of each helium atom transition line, w denotes a radial wind speed, λ denotes a laser wavelength, i denotes a lower energy level, k denotes a higher energy level, g i and g k are degradation factors of a low energy level and a high energy level, respectively, a ki denotes a transition probability from the k energy level to the i energy level, that is, a einstein coefficient (s -1),kB denotes a boltzmann constant, T denotes an atomic temperature, M denotes an atomic mass, c denotes a light speed, when the radial wind speed is negative, a wind direction is a radar approaching direction, and when the radial wind speed is positive, a wind direction is a radar distancing direction.
Assuming that the number of all emitted photons is equal to the number of absorbed photons, the effective cross section is calculated by equation (two):
σ eff(v,v0)＝σabs(v,v0); (II)
In the formula (two), σ eff represents the effective cross section of each helium atom, σ abs represents the statistically average absorption cross section of each helium atom, v 0 represents the resonance frequency of each helium atom transition line, and v represents the laser frequency.
According to the embodiment of the invention, at different thermal layer temperatures and wind speeds, the backscattering cross section of the metastable helium at the 1083nm peak can change, thereby affecting the relative intensity of the echo signal. The corresponding effective scattering section spectral line can be obtained by changing the temperature and the wind speed.
FIG. 5 schematically shows a graph of the back-scatter cross-section as a function of temperature and wind speed according to an embodiment of the invention.
Fig. 5 plots the backscatter cross-section at different temperatures and different wind speeds. The graph (a) in FIG. 5 shows the backscattering cross section at a radial wind speed of 0m/s at temperatures of 800K, 1000K and 1200K, respectively. It was found that as the temperature increases, the full width at half maximum increases due to doppler broadening, the peak intensity decreases, and the maximum position of the main peak at 1083nm gradually moves in the short wave direction. FIG. 5 (b) shows the corresponding backscattering cross-sections at radial wind speeds of-200 m/s, 0m/s and +200m/s, respectively, at a temperature of 1000K. If the wind speed is positive, the wind direction is the direction of the atmosphere pointing to the radar, and the spectral line moves in the short wave direction as a whole; if the wind speed is negative, the wind direction is the direction that the atmosphere is far away from the radar, and the spectral line moves in the long wave direction as a whole.
And taking the scales of the temperature and the wind speed to 0.1K and 0.1m/s within the temperature range of 700K to 2000K and the wind speed of-200 m/s to +200m/s, so as to obtain metastable helium scattering sections with different temperatures and wind speeds. Combining the frequency information corresponding to the two laser beams to be emitted, and combining the scattering section information of each group which is uniquely corresponding to different temperatures and different wind speeds into a data set, namely, simulating to obtain an echo signal spectrum database corresponding to the specific temperature and wind speed.
Fig. 6 schematically shows two sets of data example diagrams in an echo signal spectrum database according to an embodiment of the invention.
As shown in FIG. 6, the dashed line represents the echo signal at (800K, +200 m/s) and the solid line represents the echo signal at (1200K, -200 m/s). It can be found that in case of different temperatures and wind speeds, the two sets of echo patterns are different, i.e. each set of echo patterns corresponds to a specific temperature and wind speed.
In operation S420, pulsed laser light having different frequencies is emitted to the region to be measured using at least two lasers.
According to an embodiment of the present invention, at least two lasers are used to simultaneously emit pulse lasers with different frequencies to a region to be measured, including:
generating stable and continuous seed laser light with a wavelength around 1083nm with the first tunable continuous laser 1 and the second tunable continuous laser 4;
The seed laser is optically amplified by the first pulse laser 2 and the second pulse laser 5 and vertically emitted upwards to the region to be measured;
After the first pulse laser 2 and the second pulse laser 5 amplify the seed laser, the first wavelength meter 3 and the second wavelength meter 6 are used to monitor the frequency of a part of the received emitted laser in real time, and feed back to the first tunable continuous laser 1 and the second tunable continuous laser 4, so as to realize the frequency locking of the emitted laser.
In operation S430, echo signals with metastable helium scattering information from the region under test are received with the telescope 7.
According to the embodiment of the invention, at least two lasers simultaneously emit two pulse lasers with different frequencies to the region to be measured, and the telescope 7 can receive four echo signals with metastable helium scattering information with different frequencies.
According to the embodiment of the invention, assuming that the velocity distribution of atoms obeys Maxwell's law of distribution, the pulse laser emits laser light due to the Doppler effect of electromagnetic waves, so that helium atoms in the atmosphere of the thermal layer are subjected to resonance fluorescence scattering. Two echo signals with two frequencies can be obtained by emitting one laser beam, and the two echo signals are symmetrical with the frequencies corresponding to 1083.025nm and 1083.034nm respectively in the frequency domain, and the intensity is related to the backward scattering cross section of the total peak of metastable helium 1083 nm. At different thermal layer temperatures and wind speeds, the backscatter cross-section of metastable helium 1083nm will change, thus making the echo signal different in intensity and frequency.
When the temperature and wind speed are inverted according to the echo signals, if only one laser beam is emitted, the information content is too small, the error is too large, and at least two laser beams are emitted to obtain the echo signals on four frequencies.
FIG. 7 schematically illustrates a back-scattered cross-section of a metastable helium peak and a normalized intensity plot of an echo signal according to an embodiment of the present invention.
As shown in fig. 7, the black dotted line and the black dashed line represent wavelengths corresponding to 1083.025nm and 1083.034nm, respectively, curve a represents a peak of metastable helium at 1083nm, curve 1-1-025 represents a first echo signal corresponding to laser light emitted by the first pulse laser 2, curve 1-2-034 represents a second echo signal corresponding to laser light emitted by the first pulse laser 2, curve 2-1-025 represents a first echo signal corresponding to laser light emitted by the second pulse laser 5, and curve 2-2-034 represents a second echo signal corresponding to laser light emitted by the second pulse laser 5. It can be seen that the echo signals near the peak of the peak at 1083nm of metastable helium correspond to higher backscatter cross-sections and intensities, and the echo signals far from the peak of the peak at 1083nm of metastable helium correspond to lower backscatter cross-sections and intensities.
According to the embodiment of the invention, the laser radar signal received through the telescope 7 can express the action set of the scattering, absorption and other effects of the atmosphere on the laser beam. By utilizing the echo signals of the helium atoms of the thermal layer, the relation between the number of photons received by the receiving system and system parameters can be established, and inversion of the temperature and wind speed corresponding to the atmosphere of the thermal layer can be performed. Let P denote the power of two pulse lasers, A denote the caliber of the telescope 7, eta the total optical efficiency of the receiving system, then the resonant fluorescence laser radar equation can be expressed by the formula (III):
In equation (three), z represents the measured height, Δz represents the measured spatial resolution, and N (z) represents the total number of photons collected between z- Δz/2 and z+Δz/2; τ is the integration time(s); h 0 is the Planck constant, h≡ 6.62607015 × - 34 J·s; c is the speed of light, c is approximately 299792458m/s; σ eff denotes an effective cross section of each helium atom, and λ 0 is a center wavelength of the emitted laser light; ρ (z) is the number density of metastable helium; r is a branching ratio; t is the single pass transmission of the atmosphere; e (z) is the single pass extinction ratio of the atmosphere; n B x σ represents the number of photons due to background radiation light and dark current, and is related to zenith angle SZA.
Let h denote the height of the underlying metastable helium atom, then the single pass extinction ratio of the atmosphere, E (z), can be expressed by the formula (four):
In the formula (four), z represents the measurement height; ρ (z) is the number density of metastable helium; σ eff represents the effective cross section of each helium atom. It can be seen that the number of photons received by telescope 7 is proportional to the number of photons emitted, the caliber of telescope 7 and the efficiency of the system.
According to the embodiment of the invention, after determining the relation between the two frequencies of the laser to be emitted and the temperature and the wind speed of the thermal layer through formulas (I) and (II), an echo signal spectrum database can be built, and each group of echo line types corresponds to a specific temperature and a specific wind speed. From the simulated echo signal spectrum database, a plurality of parameters of the fizeau interferometer 10 can be determined.
Fig. 8 schematically shows a flow chart of a method of determining parameters of a fizeau interferometer in accordance with an embodiment of the invention.
As shown in fig. 8, the method of determining parameters of a fizeau interferometer may include operations S810-S860.
In operation S810, a plurality of attribute information for an initial fizeau interferometer is acquired, wherein the plurality of attribute information includes: the echo signal with metastable helium scattering information processed by the filter 9 reaches the incidence dip angle of the initial fizeau interferometer.
According to an embodiment of the invention, the initial fizeau interferometer consists of a beam splitter, a collimator objective and a standard plane, assuming a very narrow segment Δy is chosen in the center of the initial fizeau interferometer, where the initial fizeau interferometer can be considered a conventional F-P etalon. At the center position, the initial fizeau interferometer transmittance function is represented by equation (five):
In equation (five), T P represents the peak transmittance of the fizeau interferometer; v 0 denotes the center frequency of fizeau interferometer transmittance; v represents the laser frequency; f represents the target fine factor of the fizeau interferometer; FSR represents the target free spectral separation.
When the original fizeau interferometer is used to obtain the back-scattered signal spectrum, the intervals of the spectral frequency ranges should be within the reception range of the original fizeau interferometer to ensure that the complete spectral pattern is received by the line detector 11. In order to reduce errors caused by photon losses, most of the energy of the scatter spectrum corresponding to each echo signal is concentrated in one Full Width Half Maximum (FWHM), and it is necessary to control the Free Spectral Range (FSR) such that the transmittance curve in the scatter spectrum contains as much as possible all echo signals.
In operation S820, an echo signal spectrum database is obtained based on the simulation, and a set of multiple full width at half maximum compositions and a set of multiple free spectrum spacing compositions are traversed to determine a target full width at half maximum and a target free spectrum spacing.
According to the embodiment of the invention, the echo signal spectrum database obtained by passive simulation and the scattering information of the echo signals are combined, and one subset of the scattering spectrum curves which most accords with the echo signals is selected from a set consisting of a plurality of full widths at half maximum and a set consisting of a plurality of free spectrum intervals, namely the target full widths at half maximum and the target free spectrum intervals. Preferably, the target full width at half maximum is 0.005nm and the target free spectral separation is 0.01nm.
In operation S830, a target fine factor of the fizeau interferometer 10 is determined from the target full width at half maximum and the target free spectral separation.
In operation S840, a target reflectivity of the fizeau interferometer 10 is determined from the target fine factor.
According to embodiments of the present invention, high finesse increases the resolution of the Fizeau interferometer 10, but at the same time presents processing difficulties. The selection of the appropriate finesse therefore requires consideration not only to increase the resolution of the fizeau interferometer 10, but also to facilitate subsequent processing.
The target fine factor and target reflectivity of the fizeau interferometer 10 are determined by equation (six):
In equation (six), F represents the target fine factor of the fizeau interferometer 10, FSR represents the target free spectral separation, FWHM represents the target full width at half maximum, and R represents the target reflectivity of the fizeau interferometer 10.
According to an embodiment of the present invention, the target fine factor f=2, the target reflectivity r=0.24 of the fizeau interferometer 10 can be obtained with the target full width at half maximum and the target free spectral spacing known.
In operation S850, a target average cavity length of the fizeau interferometer 10 is determined from the wavelengths corresponding to the target free spectral separation.
In operation S860, a target wedge angle of the fizeau interferometer 10 is determined from the target average cavity length; the target wedge angle is an included angle corresponding to two flat plates of the fizeau interferometer 10, and is used for regulating and controlling the width of interference fringes of the fizeau interferometer 10.
According to the embodiment of the invention, the monochromatic parallel light is assumed to be incident on two flat plates with high reflectivity and an included angle alpha at an angle theta, one part of the light directly penetrates through the two plates, the other part of the light is emergent after being reflected between the two plates for multiple times, the included angle of the two parts of the light is 2nalpha (n is the refractive index of a medium), the two parts of the light are coherent and generate uniform thickness stripes, and the beam splitter and the collimating objective lens are combined with the two flat plates with high reflectivity to form the initial Fizeau interferometer.
According to an embodiment of the invention, the transmittance of the fizeau interferometer is related to the spatial position of the incident beam on the wedge of the fizeau interferometer. Therefore, there are different transmittance curves at different positions of the incident laser light. Assuming that the telescope 7 receives the beam and then enters the fizeau interferometer 10 vertically, when calculating the target average cavity length of the fizeau interferometer 10, the narrow range at the center of the initial fizeau interferometer can be regarded as a traditional F-P etalon, and the target average cavity length of the fizeau interferometer 10 can be calculated by utilizing the relation between the free spectral range and the average cavity length of the traditional F-P etalon. Furthermore, the target wedge angle determines the width of the interference fringes.
The target average cavity length and target wedge angle of the fizeau interferometer 10 are determined by equation (seven):
In equation (seven), L 0 represents the target average cavity length of the fizeau interferometer 10, n represents the refractive index of the medium, λ represents the wavelength of the emitted laser light, λ FSR represents the wavelength corresponding to the target free spectrum spacing, and α represents the target wedge angle of the fizeau interferometer 10.
When the wavelength of the emitted laser is 1083nm, the refractive index of the medium in the wedge-shaped space is 1, and the target average cavity length L 0 can be calculated according to the wavelength corresponding to the target free spectrum interval, which is about 0.06m; and the target wedge angle of the fizeau interferometer 10 can also be calculated to be 9.2 μrad given that the echo signal is perpendicularly incident to the initial fizeau interferometer.
According to embodiments of the present invention, the shape factor affects the shape of the interference fringes, with different shape factors corresponding to different transmission curves of the fizeau interferometer 10. Thus, the rationality of the parameters corresponding to the fizeau interferometer 10 can be verified by the form factor.
According to an embodiment of the present invention, the shape factor is determined by the following formula (eight):
In equation (eight), S represents the shape factor, F represents the target fine factor of the fizeau interferometer 10, L 0 represents the target average cavity length of the fizeau interferometer 10, α represents the target wedge angle of the fizeau interferometer 10, v represents the laser frequency, and c represents the speed of light.
Fig. 9 schematically illustrates a graph of transmittance curves of different form factors versus fizeau interferometer 10 in accordance with an embodiment of the invention.
As shown in fig. 9, when the shape factor is less than 0.6, the transmittance curve of the fizeau interferometer 10 can be approximated as an Airy function in combination with equation (eight), and the wind speed is inverted by the Airy function form. If S is less than or equal to 0.6, the normal incidence state of the incident angle theta=0 can be maintained; if S >0.6 is calculated, the interference fringes need to be adjusted by adjusting the incidence angle until S is less than or equal to 0.6. The values of the various parameters of the fizeau interferometer 10 are brought into a formula, and the calculated S meets the requirements.
According to embodiments of the invention, the rationality of the parameters corresponding to the fizeau interferometer 10 can also be verified by the resolution of the fizeau interferometer 10.
Using the taylor criterion, the resolution of the fizeau interferometer 10 is determined by the formula (nine):
Order the Equation (nine) can further be expressed as: /(I)
In equation (nine), Δv represents the minimum frequency range that can be resolved by the fizeau interferometer 10, m represents the number of resolution steps, and Δλ represents the minimum wavelength range that can be resolved by the fizeau interferometer 10. The frequency resolution of the fizeau interferometer 10 can be further found to be about 1.28GHz, in combination with the average cavity length L 0 and the target fine factor F calculated previously. The frequency interval between the metastable He echo signals before every two is 2.23GHz, so that the parameter values can be known to meet the requirement of distinguishing four echo signals.
In operation S440, the two-stage filtering processing is sequentially performed on the echo signal with metastable helium scattering information received by the telescope 7 by using the filter 9 and the fizeau interferometer 10, and the processed echo signal is transmitted to the optical path corresponding to the receiving system.
According to the embodiment of the invention, the filter 9 carries out first-stage filtering processing on the echo signal with metastable helium scattering information received by the telescope 7; the ultra-narrow band Fizeau interferometer 10 carries out second-stage filtering processing on the echo signals with metastable helium scattering information processed by the filter 9, wherein the echo signals with metastable helium scattering information processed by the filter 9 generate equal-thickness interference in the ultra-narrow band Fizeau interferometer 10, and parallel beams of the echo signals with equal-thickness interference are classified according to different frequencies and are transmitted to corresponding optical paths of a receiving system.
Fig. 10 schematically shows a data processing flow diagram according to an embodiment of the invention.
As shown in fig. 10, the data processing procedure includes steps S450 to S470.
In operation S450, the probe 11 receives an echo signal from the receive system optical path, wherein the echo signal in the receive system optical path is represented by a measured spectrum that is a convolution of the scatter spectrum and the transmittance function of the fizeau interferometer 10.
The measured spectrum can be determined by the formula (ten):
In the formula (ten), S conv represents a measured spectrum, S scatter represents a scattering spectrum, and S Fizeau represents a transmittance function of the fizeau interferometer.
Fig. 11 schematically shows a schematic of the convolution results of the laser line pattern detected by the detector, the total echo signal, and the fizeau interferometer in accordance with an embodiment of the invention.
As shown in fig. 11, it is assumed that the laser light emitted from the lidar system is gaussian in shape, 200MHz in line width, 850K in temperature and 0m/s in wind speed. With the laser linetype known, the convolution of the scatter spectrum and the transmittance function of the fizeau interferometer 10 can be obtained from the number of photons received by each channel of the detector 11.
In operation S460, the measured spectrum and the transmittance function of the fizeau interferometer 10 are deconvoluted to obtain an echo signal spectrum of the laser.
In operation S470, the echo signal spectrum of the laser is compared with the echo signal spectrum database by linear interpolation to determine a group of data closest to the echo signal spectrum of the laser as the temperature and wind speed corresponding to the region to be measured, where the region to be measured is a thermal layer atmosphere.
According to the embodiment of the invention, the Fizeau interferometer-based helium laser radar dual-frequency temperature and wind measuring method is characterized in that at least two lasers are introduced into a transmitting end, and partial modules of a receiving end are modified, so that on one hand, the time for alternately transmitting the traditional lasers is saved, and the complexity and maintenance difficulty of the laser radar can be reduced; on the other hand, the method can save the sweep frequency time, continuously accumulate photon signals, and greatly simplify the data processing method. And finally, the metastable helium scattering information of the region to be measured is collected to measure the temperature and wind, so that a new research direction can be provided for the subsequent research of the temperature and wind measurement, and the theoretical blank of the helium laser radar in the aspect of the temperature and wind measurement is filled, thereby being capable of carrying out more omnibearing observation and research on the thermal layer atmospheric physical process and the multi-ring layer coupling mechanism.
The foregoing description of the embodiments has been provided for the purpose of illustrating the general principles of the invention, and is not meant to limit the invention thereto, but to limit the invention thereto, and any modifications, equivalents, improvements and equivalents thereof may be made without departing from the spirit and principles of the invention.
Claims (6)
1. The parameter determining method of the Fizeau interferometer is applied to a helium laser radar double-frequency temperature and wind measuring system based on the Fizeau interferometer, and the helium laser radar double-frequency temperature and wind measuring system comprises the following components:
a transmitting system, comprising:
At least two lasers, which are suitable for emitting pulse lasers with different frequencies to the region to be measured;
a receiving system, comprising:
a telescope (7) adapted to receive echo signals from the region under test having metastable helium scattering information;
the filter (9) is suitable for carrying out first-stage filtering processing on the echo signal with metastable helium scattering information received by the telescope (7);
A Fizeau interferometer (10) adapted to perform a second stage filtering process on the echo signal with metastable helium scattering information processed by the filter (9);
-a detector (11) adapted to receive a scattered echo signal from the fizeau interferometer (10), wherein the echo signal from the fizeau interferometer (10) is represented by a measured spectrum, the measured spectrum being a convolution of the scattered spectrum and a transmittance function of the fizeau interferometer (10);
The data processing system is suitable for processing echo signals received by the detector (11) to obtain wind speed and temperature corresponding to the region to be detected, wherein the linear interpolation comparison result of an echo signal spectrum library is obtained according to the echo signal spectrum and simulation of the laser, the wind speed and the temperature corresponding to the region to be detected are determined, and the echo signal spectrum of the laser is the deconvolution of the measurement spectrum and the transmittance function of the Fizeau interferometer (10);
The method comprises the following steps:
Acquiring a plurality of attribute information of an initial fizeau interferometer (10), wherein the plurality of attribute information comprises: the initial Fizeau interferometer (10) comprises a set of multiple full width at half maximum components and a set of multiple free spectrum spacing components, and the echo signal with metastable helium scattering information processed by the filter (9) reaches the incidence dip angle of the initial Fizeau interferometer (10);
traversing the set of multiple full width at half maximum and the set of multiple free spectrum spacing based on the simulation to obtain an echo signal spectrum database so as to determine a target full width at half maximum and a target free spectrum spacing;
Determining a target fine factor of the Fizeau interferometer (10) according to the target full width at half maximum and the target free spectrum spacing;
determining a target reflectivity of the fizeau interferometer (10) from the target fine factor;
Determining a target average cavity length of the Fizeau interferometer (10) according to the wavelength corresponding to the target free spectrum interval;
Determining a target wedge angle of the fizeau interferometer (10) according to the target average cavity length;
the target wedge angle is an included angle corresponding to two flat plates of the Fizeau interferometer (10), and is used for regulating and controlling the width of interference fringes of the Fizeau interferometer (10).
2. The determination method of claim 1, wherein the target fine factor of the fizeau interferometer (10) and the target reflectivity of the fizeau interferometer (10) are calculated by formula (1);
In equation (1), F represents a target fine factor of the fizeau interferometer (10), FSR represents the target free spectral separation, FWHM represents the target full width at half maximum, and R represents a target reflectivity of the fizeau interferometer (10).
3. The determination method of claim 1, wherein the target average cavity length and target wedge angle of the fizeau interferometer (10) are calculated by formula (2);
In the formula (2), L 0 represents a target average cavity length of the fizeau interferometer (10), n represents a refractive index of a medium, λ represents a wavelength of the emitted laser, λ FSR represents a wavelength corresponding to the target free spectrum interval, and α represents a target wedge angle of the fizeau interferometer (10).
4. The determination method according to claim 1, wherein the emission system includes the at least two lasers, including:
A first tunable continuous laser (1) and a second tunable continuous laser (4) adapted to generate a stable and continuous seed laser;
a first pulse laser (2) and a second pulse laser (5) adapted to optically amplify the seed laser and emit it to the area to be measured;
The first pulse laser (2) and the second pulse laser (5) transmit part of the amplified emission laser to a first wavelength meter (3) and a second wavelength meter (6) to monitor the frequency of the emission laser in real time and feed back the frequency to the first tunable continuous laser (1) and the second tunable continuous laser (4) so as to realize the emission laser frequency locking.
5. The determination method according to claim 4, wherein the first tunable continuous laser (1) and the second tunable continuous laser (4) are identical in structure, the first pulse laser (2) and the second pulse laser (5) are identical in structure, and the first wavelength meter (3) and the second wavelength meter (6) are identical in structure.
6. The determination method according to claim 1, wherein the at least two lasers emit the laser light vertically upward.
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| WO2024113472A1 (en) | 2024-06-06 |
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