CN115902834B - Helium laser radar dual-frequency temperature and wind measurement system and method based on Fizeau interferometer - Google Patents

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

Helium laser radar dual-frequency temperature and wind measurement system and method based on Fizeau interferometer

Technical Field

The present invention relates to the field of laser radar technology, and in particular to a helium laser radar dual-frequency temperature and wind measurement system and method based on a Fizeau interferometer.

Background Art

Thermosphere temperature and wind speed are important parameters for studying thermodynamic and kinetic processes related to the thermosphere. However, the density of neutral atmospheric particles in this altitude range is too low, making it extremely difficult to measure the density, temperature and wind field profile of the thermosphere from the ground. In the past, scientists mostly used sounding rockets, satellites and other spacecraft to measure the thermosphere. However, sounding rockets are expensive and cannot conduct long-term continuous sounding; satellites have low spatial resolution and cannot obtain long-term uninterrupted observations of specific areas.

With the development of lidar technology, resonant fluorescence lidar has gradually become one of the common means of thermosphere detection. Compared with traditional spacecraft measurement methods, lidar can provide continuous data of the thermosphere with high temporal and spatial resolution. The traditional three-frequency temperature and wind measurement methods have been relatively mature and stably applied in sodium resonance fluorescence lidar, but compared with sodium lidar, helium lidar has a high observation altitude and weak signal, and uses a transmitter to alternately emit three-frequency lasers to achieve temperature and wind measurement. The method requires rapid switching of the laser frequency and alternate emission, which is equivalent to reducing the energy by one third in both the frequency domain and the time domain, so it is not conducive to obtaining a good signal-to-noise ratio. Based on this, the development of more optimized temperature and wind measurement methods is a technical problem that needs to be solved at present.

Summary of the invention

In view of the above problems, this application provides a dual-frequency temperature and wind measurement system of helium laser radar based on Fizeau interferometer, which can realize dual-frequency temperature and wind measurement by introducing at least two lasers in the transmitting system, introducing a new Fizeau interferometer in the receiving system, and collecting metastable helium scattering information in the area to be measured. It not only overcomes the technical defects existing in the prior art, but also provides a new research direction for subsequent research on temperature and wind measurement.

To achieve the above object, the first aspect of the present invention provides a helium laser radar dual-frequency temperature and wind measurement system based on a Fizeau interferometer, comprising:

Launch system, including:

At least two lasers, adapted to emit pulsed lasers of different frequencies to the area to be measured;

Receiving system, including:

A telescope, adapted to receive an echo signal having metastable helium scattering information from the above-mentioned area to be measured;

A filter, adapted to perform a first-stage filtering process on the echo signal having metastable helium scattering information received by the telescope;

A Fizeau interferometer, adapted to perform a second-stage filtering process on the echo signal having the metastable helium scattering information after the filter has been processed;

A detector adapted to receive a scattered echo signal processed by the Fizeau interferometer, wherein the echo signal processed by the Fizeau interferometer is represented by a measurement spectrum, and the measurement spectrum is a convolution of a scattered spectrum and a transmittance function of the Fizeau interferometer;

The data processing system is suitable for processing the echo signal received by the above-mentioned detector to obtain the wind speed and temperature corresponding to the above-mentioned area to be measured, wherein the wind speed and temperature corresponding to the above-mentioned area to be measured are determined according to the linear interpolation comparison result of the echo signal spectrum library obtained by simulation based on the echo signal spectrum of the above-mentioned laser, and the echo signal spectrum of the above-mentioned laser is the deconvolution of the above-mentioned measurement spectrum and the transmittance function of the above-mentioned Fizeau interferometer.

According to an embodiment of the present invention, the transmitting system comprises the at least two lasers, including:

A first tunable continuous laser and a second tunable continuous laser are adapted to generate stable and continuous seed lasers;

A first pulse laser and a second pulse laser, adapted to amplify the seed laser and emit the amplified seed laser to the region to be measured;

Among them, the above-mentioned first pulse laser and the above-mentioned second pulse laser transmit a part of the emitted laser after light amplification to the first wavelength meter and the second wavelength meter to monitor the frequency of the above-mentioned emitted laser in real time, and feed back to the above-mentioned first tunable continuous laser and the above-mentioned second tunable continuous laser to achieve the above-mentioned emitted laser frequency locking.

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 lasers vertically upwards.

A second aspect of the present invention provides a dual-frequency temperature and wind measurement method of a helium laser radar based on a Fizeau interferometer, which is applied to a dual-frequency temperature and wind measurement system of a helium laser radar based on a Fizeau interferometer as described in any one of the above embodiments. The method comprises:

According to the preset laser emission information and the corresponding multiple temperature and wind speed information within a preset certain temperature and wind speed range, the echo signal spectrum database is simulated, wherein the preset laser emission information is the frequency information of the lasers to be emitted corresponding to the at least two lasers;

Using the at least two lasers mentioned above to emit pulsed lasers with different frequencies to the area to be tested;

Using the telescope to receive the echo signal with the metastable helium scattering information from the area to be measured;

The echo signal with metastable helium scattering information received by the telescope is subjected to two-stage filtering in sequence by using the filter and the Fizeau interferometer, and the processed echo signal is transmitted to the optical path corresponding to the receiving system;

The detector receives an echo signal from an optical path of a receiving system, wherein the echo signal in the optical path of the receiving system is represented by a measurement spectrum, and the measurement spectrum is a convolution of a scattering spectrum and a transmittance function of a Fizeau interferometer;

Deconvolution is performed on the measurement spectrum and the transmittance function of the Fizeau interferometer to obtain an echo signal spectrum of the laser;

The echo signal spectrum of the laser is linearly interpolated and compared with the echo signal spectrum database to determine that the closest set of data in the echo signal spectrum of the laser is the temperature and wind speed corresponding to the area to be measured.

According to an embodiment of the present invention, using the at least two lasers to emit pulse lasers with different frequencies to the area to be measured includes:

Using the first tunable continuous laser and the second tunable continuous laser to generate the stable and continuous seed laser;

The seed laser is optically amplified by the first pulse laser and the second pulse laser, and emitted to the area to be measured;

Among them, after the above-mentioned first pulse laser and the above-mentioned second pulse laser optically amplify the above-mentioned seed laser, the above-mentioned first wavelength meter and the above-mentioned second wavelength meter are used to monitor the frequency of a part of the above-mentioned emission laser received in real time, and feed back to the above-mentioned first tunable continuous laser and the above-mentioned second tunable continuous laser to achieve the above-mentioned emission laser frequency locking.

According to an embodiment of the present invention, the at least two lasers emit two pulsed lasers with different frequencies to the area to be measured, so that four echo signals with the metastable helium scattering information at different frequencies can be obtained.

A third aspect of the present invention provides a method for determining parameters of a Fizeau interferometer, which is applied to a helium laser radar dual-frequency temperature and wind measurement system based on a Fizeau interferometer as described in any one of the above embodiments, and the method comprises:

Acquire multiple attribute information of the initial Fizeau interferometer, wherein the multiple attribute information includes: a set of multiple half-widths of the initial Fizeau interferometer, a set of multiple free spectrum spacings, and an incident inclination angle of the echo signal with metastable helium scattering information after being processed by the filter to the initial Fizeau interferometer;

Based on the above simulation, an echo signal spectrum database is obtained, and the set consisting of the above multiple full widths at half maximum and the set consisting of the above multiple free spectrum intervals are traversed to determine the target full width at half maximum and the target free spectrum interval;

Determine the target fine factor of the Fizeau interferometer based on the target full width at half maximum and the target free spectrum spacing;

Determining the target reflectivity of the Fizeau interferometer according to the target fine factor;

Determining a target average cavity length of the Fizeau interferometer according to a wavelength corresponding to a target free spectrum spacing;

Determining a target wedge angle of the Fizeau interferometer according to the target average cavity length;

The target wedge angle is the included angle between the two plates of the Fizeau interferometer, and the target wedge angle is used to adjust the width of the interference fringes of the Fizeau interferometer.

According to an embodiment of the present invention, the target fineness factor of the Fizeau interferometer and the target reflectivity of the Fizeau interferometer are calculated by formula (1);

In formula (1), F represents the target fineness factor of the Fizeau interferometer, FSR represents the target free spectrum spacing, FWHM represents the target full width at half maximum, and R represents the target reflectivity of the Fizeau interferometer.

According to an embodiment of the present invention, the target average cavity length and target wedge angle of the above-mentioned Fizeau interferometer are calculated by formula (2);

In formula (2), _L0 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, 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 an embodiment of the present invention, at least two lasers are introduced into the transmitting system to ensure that two pulsed lasers with different frequencies are emitted simultaneously, thereby overcoming the defects introduced by the prior art method of using three lasers to achieve three-frequency switching and emit three-frequency lasers for temperature and wind measurement. The dual-frequency transmission system can not only save the time of traditional laser emission in turn, but also reduce the complexity and maintenance difficulty of the laser radar.

According to an embodiment of the present invention, by modifying some modules, that is, introducing a new Fizeau interferometer in the receiving system to replace the traditional F-P standard, linear fringes that can replace the annular fringes can be generated. There is no need to perform frequency scanning, and the position and shape changes of the linear fringes can be detected directly in conjunction with a linear detector. This new receiving system can not only save scanning time and continuously accumulate photon signals, but also greatly simplify the data processing method.

According to the embodiments of the present invention, helium atoms are one of the main atmospheric components above 200 km. After physical processes such as photoelectron excitation, metastable helium [He(2 ³ S)] which is more stable and has stronger fluorescence can be produced. Therefore, compared with the temperature and wind measurement using sodium resonance fluorescence laser radar in the prior art, the temperature and wind measurement by collecting metastable helium scattering information in the area to be measured can provide a new research direction for subsequent research on temperature and wind measurement, and also fill the theoretical gap in the temperature and wind measurement of helium laser radar. This enables a more comprehensive observation and research on the physical processes of the thermosphere and the multi-sphere coupling mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

The above contents and other purposes, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings, in which:

FIG1 schematically shows a block diagram of a helium laser radar dual-frequency temperature and wind measurement system based on a Fizeau interferometer according to an embodiment of the present invention;

FIG2 schematically shows a schematic diagram of a helium laser radar dual-frequency temperature and wind measurement system based on a Fizeau interferometer according to an embodiment of the present invention;

FIG3 schematically shows a graph showing backscattering cross sections of metastable helium corresponding to three radiation lines around 1083 nm as a function of wavelength according to an embodiment of the present invention;

FIG4 schematically shows a flow chart of a dual-frequency temperature and wind measurement method using a helium laser radar based on a Fizeau interferometer according to an embodiment of the present invention;

FIG5 schematically shows a curve diagram of backscattering cross section changes with temperature and wind speed according to an embodiment of the present invention;

FIG6 schematically shows two sets of data examples in an echo signal spectrum database according to an embodiment of the present invention;

FIG7 schematically shows a schematic diagram of a backscattering cross section of a metastable helium peak and a normalized intensity of an echo signal according to an embodiment of the present invention;

FIG8 schematically shows a flow chart of a method for determining parameters of a Fizeau interferometer according to an embodiment of the present invention;

FIG9 schematically shows a relationship diagram of transmittance curves of a Fizeau interferometer and different shape factors according to an embodiment of the present invention;

FIG10 schematically shows a data processing flow chart according to an embodiment of the present invention;

FIG11 schematically shows a schematic diagram of the laser line shape, the total echo signal and the convolution result of the Fizeau interferometer detected by the detector according to an embodiment of the present invention.

Description of reference numerals:

1: first tunable continuous laser;

2: first pulse laser;

3: First wavelength meter;

4: Second tunable continuous laser;

5: second pulse laser;

6: Second wavelength meter;

7: Telescope;

8: Optical switch;

9: filter;

10: Fizeau interferometer;

11: Detector.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present disclosure. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present disclosure. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present disclosure.

The terms used herein are only for describing specific embodiments and are not intended to limit the present disclosure. The terms "include", "comprising", etc. used herein indicate the existence of the features, steps, operations and/or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

All terms (including technical and scientific terms) used herein have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification, and should not be interpreted in an idealized or overly rigid manner.

When using expressions such as "at least one of A, B, and C", they should generally be interpreted according to the meaning of the expression commonly understood by technical personnel in this field (for example, "a system having at least one of A, B, and C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and/or A, B, C, etc.).

In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

With the development of lidar technology, resonant fluorescence lidar has gradually become one of the common means of thermosphere detection. Helium atoms are one of the main atmospheric components above 200 km. After physical processes such as photoelectron excitation, they can produce metastable helium [He(2 ³ S)] that is more stable and has stronger fluorescence than helium. Therefore, metastable helium can be used as one of the preferred tracers for ground-based lidar to observe the neutral atmosphere in this altitude range. The principle of metastable helium resonance fluorescence lidar to detect atmospheric parameters is as follows: the transmitting end of the lidar emits a narrow linewidth laser beam with a wavelength of 1083nm vertically upward to excite helium atoms in the thermosphere, causing their extranuclear electrons to transition to high energy levels and de-excite to lower energy levels, that is, a resonant fluorescence reaction occurs. On the ground, the receiving end telescope of the lidar receives the echo signal, uses a photodetector to detect the number of echo photons, and performs a series of signal processing and data inversion to obtain the metastable helium density in the thermosphere. At the same time, the temperature and wind field of metastable helium within the detection altitude range can be further inverted by letting the radar laser transmitter emit three frequencies of laser alternately and using the relative relationship between the resonance fluorescence intensities excited by the three frequencies. Combined with the atmospheric model and based on some assumptions and boundary conditions, the density, temperature and wind field of the background atmosphere can be derived.

However, the metastable helium temperature and wind measurement lidar detection technology cannot be well implemented in the related technology. The temperature and wind measurement methods in the related technology are generally limited to three-frequency temperature and wind measurement using sodium lidar. In comparison, the helium lidar detects at a higher altitude and has a weaker signal. If the helium lidar temperature and wind measurement function is to be realized, a more optimized method needs to be developed to achieve a higher signal-to-noise ratio, thereby obtaining more accurate temperature and wind field measurement results.

Based on this, the present application provides a dual-frequency temperature and wind measurement system of helium laser radar based on Fizeau interferometer, which realizes dual-frequency temperature and wind measurement by introducing at least two lasers in the transmitting system, introducing a new Fizeau interferometer in the receiving system, and collecting metastable helium scattering information in the area to be measured. It not only overcomes the technical defects existing in the prior art, but also provides a new research direction for subsequent research on temperature and wind measurement.

FIG1 schematically shows a block diagram of a dual-frequency temperature and wind measurement system of a helium laser radar based on a Fizeau interferometer according to an embodiment of the present invention.

As shown in Figure 1, the helium lidar dual-frequency temperature and wind measurement system based on Fizeau interferometer includes a transmitting system, a receiving system and a data processing system.

FIG2 schematically shows a schematic diagram of a helium laser radar dual-frequency temperature and wind measurement system based on a Fizeau interferometer according to an embodiment of the present invention.

According to an exemplary embodiment of the present invention, the present invention discloses a dual-frequency temperature and wind measurement system of a helium laser radar based on a Fizeau interferometer. As shown in FIG2 , the transmitting system includes: at least two lasers, which are suitable for transmitting pulse lasers with different frequencies to the area to be measured; the receiving system includes: a telescope 7, an optical switch 8, a filter 9, a Fizeau interferometer 10 and a detector 11, wherein the telescope 7 is suitable for receiving an echo signal with metastable helium scattering information from the area to be measured; the filter 9 is suitable for performing a first-stage filtering process on the echo signal with metastable helium scattering information received by the telescope 7; the Fizeau interferometer 10 is suitable for filtering the echo signal with metastable helium scattering information processed by the filter 9. The echo signal of the information is subjected to a second-stage filtering process; the detector 11 is adapted to receive the scattered echo signal processed by the Fizeau interferometer 10, wherein the echo signal processed by the Fizeau interferometer 10 is represented by a measurement spectrum, and the measurement spectrum is the convolution of the scattered spectrum and the transmittance function of the Fizeau interferometer 10; and the data processing system is adapted to process the echo signal received by the detector 11 to obtain the wind speed and temperature corresponding to the area to be measured, wherein the wind speed and temperature corresponding to the area to be measured are determined according to the linear interpolation comparison result of the echo signal spectrum library obtained by simulation and the echo signal spectrum spectrum spectrum of the laser, and the echo signal spectrum of the laser is the deconvolution of the measurement spectrum and the transmittance function of the Fizeau interferometer 10.

According to an embodiment of the present invention, at least two lasers are introduced into the transmitting system to ensure that two pulsed lasers with different frequencies are emitted simultaneously, thereby overcoming the defects introduced by the prior art method of using three lasers to achieve three-frequency switching and emit three-frequency lasers for temperature and wind measurement. The dual-frequency transmission system can not only save the time of traditional laser emission in turn, but also reduce the complexity and maintenance difficulty of the laser radar.

According to an embodiment of the present invention, by modifying some modules, that is, introducing a new Fizeau interferometer 10 in the receiving system to replace the traditional F-P standard, linear stripes that can replace the annular stripes can be generated, and there is no need to perform frequency scanning. The linear stripe position and shape change can be detected directly in conjunction with the linear array detector 11; this new receiving system can not only save scanning time and continuously accumulate photon signals, but also greatly simplify the data processing method.

According to the embodiments of the present invention, helium atoms are one of the main atmospheric components above 200 km. After physical processes such as photoelectron excitation, metastable helium [He(2 ³ S)] which is more stable and has stronger fluorescence can be produced. Therefore, compared with the temperature and wind measurement using sodium resonance fluorescence laser radar in the prior art, the temperature and wind measurement by collecting metastable helium scattering information in the area to be measured can provide a new research direction for subsequent research on temperature and wind measurement, and also fill the theoretical gap in helium laser radar in temperature and wind measurement. This enables a more comprehensive observation and research on the physical processes of the thermosphere and the multi-sphere coupling mechanism.

According to an embodiment of the present invention, the transmitting system includes at least two lasers, including: 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 are adapted to generate a stable and continuous seed laser having a wavelength of about 1083 nm;

The first pulse laser 2 and the second pulse laser 5 are adapted to amplify the seed laser light and emit it to the area to be measured;

Among them, the first pulse laser 2 and the second pulse laser 5 respectively transmit a part of the emitted laser after light amplification to the first wavelength meter 3 and the second wavelength meter 6 to monitor the frequency of the emitted laser in real time, and feed back to the first tunable continuous laser 1 and the second tunable continuous laser 4 to achieve frequency locking of the emitted laser.

According to the embodiment of the present invention, the first tunable continuous laser 1 and the second tunable continuous laser 4 have the same structure, the first pulse laser 2 and the second pulse laser 5 have the same structure, and the first wavelength meter 3 and the second wavelength meter 6 have the same structure.

According to an 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 it vertically upward to the area to be measured.

According to an embodiment of the present invention, for helium atoms, there are two sets of energy levels, singlet and triplet, and there is no transition between them. In the singlet energy level, the first excited state of helium is 2 ¹ S ₀ , with a lifetime of about 19.5 ms; in the triplet energy level, the first excited state of helium is 2 ³ S ₁ , including three radiation lines of 1082.909 nm, 1083.025 nm and 1083.034 nm. Under ideal conditions, the natural lifetime of helium (2 ³ S ₁ ) is about 8000 s, which is suitable as a tracer for detecting the temperature and wind speed of the thermosphere.

FIG3 schematically shows a graph showing backscattering cross sections of metastable helium corresponding to three radiation lines around 1083 nm as a function of wavelength according to an embodiment of the present invention.

As shown in Figure 3, curves 1-909, 1-025 and 1-034 represent the backscattering cross sections corresponding to the three radiation lines of metastable helium at 1082.909nm, 1082.025nm and 1082.034nm, respectively. In contrast, the radiation line of metastable helium at 1082.909nm is seriously interfered by hydroxyl groups, so in this application, two radiation lines corresponding to wavelengths of 1083.025nm and 1083.034nm are selected for observation.

FIG4 schematically shows a flow chart of a dual-frequency temperature and wind measurement method using a helium laser radar based on a Fizeau interferometer according to an embodiment of the present invention.

As shown in FIG. 4 , the helium laser radar dual-frequency temperature and wind measurement system method based on the Fizeau interferometer may include operations S410 to S470.

In operation S410, an echo signal spectrum database is simulated based on preset laser emission information and multiple temperature and wind speed information corresponding to a preset temperature and wind speed range, wherein the preset laser emission information is frequency information of lasers 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 distribution law, and the statistical average absorption cross section of each helium atom is calculated by formula (1):

In formula (1), the Doppler absorption cross section v represents the laser frequency, _v0 represents the resonant frequency of each helium atomic transition line, w represents the radial wind speed, λ represents the laser wavelength, i represents the lower energy level, k represents the higher energy level, _gi and _gk represent the degradation factors of the lower and higher energy levels, _Aki represents the transition probability from the k energy level to the i energy level, i.e., the Einstein coefficient (s ^-1 ), _kB represents the Boltzmann constant, T represents the atomic temperature, M represents the atomic mass, and c represents the speed of light. When the radial wind speed is negative, it means that the wind direction is close to the radar; when the radial wind speed is positive, it means that the wind direction is away from the radar.

Assuming that the number of all emitted photons is equal to the number of absorbed photons, the effective cross section is calculated using formula (II):

σ _eff (v, v ₀ ) = σ _abs (v, v ₀ ); (2)

In formula (ii), σ _eff represents the effective cross section of each helium atom, σ _abs represents the statistical average absorption cross section of each helium atom, v ₀ represents the resonant frequency of each helium atom transition line, and v represents the laser frequency.

According to an embodiment of the present invention, under different thermosphere temperatures and wind speeds, the backscattering cross section of metastable helium at the 1083nm peak will change, thereby affecting the relative intensity of the echo signal. By changing the temperature and wind speed, the corresponding effective scattering cross section spectrum can be obtained.

FIG. 5 schematically shows a curve diagram of backscattering cross section changes with temperature and wind speed according to an embodiment of the present invention.

Figure 5 plots the backscattering cross sections at different temperatures and wind speeds. Figure 5 (a) shows the backscattering cross sections at 800K, 1000K and 1200K when the radial wind speed is 0m/s. It can be found that with the increase of temperature, due to Doppler broadening, the full width at half maximum increases, the peak intensity decreases, and the maximum position of the main peak at 1083nm gradually moves toward the shortwave direction. Figure 5 (b) shows the backscattering cross sections corresponding to -200m/s, 0m/s and +200m/s when the temperature is 1000K. If the wind speed is positive, the wind direction is the direction of the atmosphere pointing to the radar, and the spectrum line generally moves toward the shortwave direction; if the wind speed is negative, the wind direction is the direction of the atmosphere away from the radar, and the spectrum line generally moves toward the longwave direction.

In the temperature range of 700K to 2000K and the wind speed range of -200m/s to +200m/s, the temperature and wind speed scales are set to 0.1K and 0.1m/s, and the metastable helium scattering cross sections at different temperatures and wind speeds are obtained. Combined with the frequency information corresponding to the two laser beams to be emitted, each group of scattering cross-section information corresponding to different temperatures and wind speeds is combined into a data set, that is, the echo signal spectrum database corresponding to a specific temperature and wind speed can be simulated.

FIG. 6 schematically shows two sets of data examples in an echo signal spectrum database according to an embodiment of the present invention.

As shown in Figure 6, the dotted line represents the echo signal at (800K, +200m/s), and the solid line represents the echo signal at (1200K, -200m/s). It can be found that under different temperatures and wind speeds, the two groups of echo line types are different, that is, each group of echo line types corresponds to a specific temperature and wind speed.

In operation S420, at least two lasers are used to emit pulse lasers with different frequencies to the area to be measured.

According to an embodiment of the present invention, at least two lasers are used to simultaneously emit pulse lasers with different frequencies to the area to be measured, including:

A first tunable continuous laser 1 and a second tunable continuous laser 4 are used to generate a stable and continuous seed laser with a wavelength of about 1083 nm;

The seed laser is optically amplified by the first pulse laser 2 and the second pulse laser 5, and is emitted vertically upward to the area to be measured;

Among them, 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 emission laser in real time, and feed back to the first tunable continuous laser 1 and the second tunable continuous laser 4 to achieve emission laser frequency locking.

In operation S430, the telescope 7 is used to receive an echo signal having metastable helium scattering information from the region to be measured.

According to an embodiment of the present invention, at least two lasers simultaneously emit two pulse lasers with different frequencies to the area to be measured, and the telescope 7 can receive four echo signals with different frequencies containing metastable helium scattering information.

According to an embodiment of the present invention, assuming that the velocity distribution of atoms obeys Maxwell's distribution law, due to the Doppler effect of electromagnetic waves, a pulsed laser emits laser light to cause resonant fluorescence scattering of helium atoms in the thermosphere atmosphere. Emitting a laser beam can obtain echo signals of two frequencies, and the two echo signals are symmetrical with the frequencies corresponding to 1083.025nm and 1083.034nm in the frequency domain, and the intensity is related to the backscattering cross section of the total peak of metastable helium 1083nm. Under different thermosphere temperatures and wind speeds, the backscattering cross section of metastable helium 1083nm will change, resulting in different intensity and frequency of the echo signal.

It should be noted that when inverting temperature and wind speed based on echo signals, if only one laser beam is emitted, the amount of information contained is too little and the error is too large. At least two laser beams need to be emitted to obtain echo signals at four frequencies.

FIG. 7 schematically shows a schematic diagram of the backscattering cross section of a metastable helium peak and the normalized intensity of an echo signal according to an embodiment of the present invention.

As shown in Fig. 7, the black dotted line and the black dot-dash line represent the wavelengths corresponding to 1083.025nm and 1083.034nm, respectively. Curve A represents the combined peak of metastable helium at 1083nm. Curve 1-1-025 represents the first echo signal corresponding to the laser emitted by the first pulse laser 2. Curve 1-2-034 represents the second echo signal corresponding to the laser emitted by the first pulse laser 2. Curve 2-1-025 represents the first echo signal corresponding to the laser emitted by the second pulse laser 5. Curve 2-2-034 represents the second echo signal corresponding to the laser emitted by the second pulse laser 5. It can be found that the backscattering cross section and intensity corresponding to the echo signal close to the peak of the combined peak of metastable helium at 1083nm are higher, and the backscattering cross section and intensity corresponding to the echo signal far from the peak of the combined peak of metastable helium at 1083nm are lower.

According to an embodiment of the present invention, the laser radar signal received by the telescope 7 can express the effect set of the atmosphere on the scattering, absorption and other effects of the laser beam. Using the echo signal of the thermospheric helium atoms, the connection between the number of photons received by the receiving system and the system parameters can be established to invert the temperature and wind speed corresponding to the thermospheric atmosphere. Let P represent the power of the two pulsed lasers, A represent the aperture of the telescope 7, and η represent the total optical efficiency of the receiving system. Then the resonance fluorescence laser radar equation can be expressed by formula (III):

In formula (III), z represents the measurement altitude, Δz represents the spatial resolution of the measurement, N(z) represents the total number of photons collected between z-Δz/2 and z+Δz/2; τ represents the integration time (s); _h0 represents the Planck constant, h≈6.62607015× ^10-34 J·s; c represents ^the speed of light, c≈299792458m/s; _σeff represents the effective cross section of each helium atom, _λ0 represents the central wavelength of the emitted laser; ρ(z) represents the number density of metastable helium; R represents the branching ratio; T represents the one-way transmittance of the atmosphere; E(z) represents the one-way extinction ratio of the atmosphere; _NB ×σ represents the number of photons caused by background radiation and dark current, which is related to the zenith angle SZA.

Let h represent the height of the bottom metastable helium atom, then the one-way extinction ratio E(z) of the atmosphere can be expressed by formula (IV):

In formula (IV), 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 found that the number of received photons and the number of emitted photons of telescope 7 are proportional to the aperture of telescope 7 and the system efficiency.

According to the embodiment of the present invention, after determining the relationship between the two frequencies of the laser to be emitted and the temperature and wind speed of the thermosphere through formulas (I) and (II), an echo signal spectrum database can be established, and each set of echo line types corresponds to a specific temperature and wind speed. Based on the simulated echo signal spectrum database, multiple parameters of the Fizeau interferometer 10 can be determined.

FIG8 schematically shows a flow chart of a method for determining parameters of a Fizeau interferometer according to an embodiment of the present invention.

As shown in FIG. 8 , the method for determining parameters of a Fizeau interferometer may include operations S810 to S860 .

In operation S810, multiple attribute information of the initial Fizeau interferometer is obtained, wherein the multiple attribute information includes: a set of multiple half-widths at half maximum of the initial Fizeau interferometer, a set of multiple free spectrum spacings, and an incident inclination angle of the echo signal with metastable helium scattering information processed by the filter 9 arriving at the initial Fizeau interferometer.

According to an embodiment of the present invention, the initial Fizeau interferometer is composed of a beam splitter, a collimating objective lens and a standard plane. Assuming that a very narrow segment Δy is selected at the center of the initial Fizeau interferometer, the initial Fizeau interferometer here can be regarded as a traditional F-P standard tool. At the center position, the transmittance function of the initial Fizeau interferometer is expressed by formula (V):

In formula (5), _TP represents the peak transmittance of the Fizeau interferometer; _v0 represents the center frequency of the transmittance of the Fizeau interferometer; v represents the laser frequency; F represents the target fineness factor of the Fizeau interferometer; and FSR represents the target free spectrum spacing.

When the initial Fizeau interferometer is used to obtain the backscattered signal spectrum, the interval of the spectrum frequency range should be within the receiving range of the initial Fizeau interferometer to ensure that the complete spectrum pattern is received by the linear detector 11. In order to reduce the error caused by photon loss, most of the energy of the scattered spectrum corresponding to each echo signal is concentrated in a half-maximum full width (FWHM), and the free spectrum spacing (FSR) needs to be controlled so that the transmittance curve in the scattered spectrum contains all echo signals as much as possible.

In operation S820, an echo signal spectrum database is obtained based on simulation, and a set consisting of a plurality of half-maximum full widths and a set consisting of a plurality of free spectrum intervals are traversed to determine a target half-maximum full width and a target free spectrum interval.

According to an embodiment of the present invention, in combination with the echo signal spectrum database obtained by passive simulation and the scattering information of the echo signal, a subset of the scattering spectrum curve that best matches the echo signal is selected from a set consisting of multiple half-height full widths and a set consisting of multiple free spectrum spacings, that is, the target half-height full width and the target free spectrum spacing. Preferably, the target half-height full width is 0.005nm and the target free spectrum spacing is 0.01nm.

In operation S830, a target fine factor of the Fizeau interferometer 10 is determined based on the target full width at half maximum and the target free spectral spacing.

In operation S840, a target reflectivity of the Fizeau interferometer 10 is determined according to the target fine factor.

According to the embodiment of the present invention, high fineness will improve the resolution of the Fizeau interferometer 10, but it will also bring difficulties in processing. Therefore, choosing a suitable fineness not only needs to consider improving the resolution of the Fizeau interferometer 10, but also facilitate subsequent processing.

The target fineness factor and target reflectivity of the Fizeau interferometer 10 are determined by formula (VI):

In formula (VI), F represents the target fineness factor of the Fizeau interferometer 10 , FSR represents the target free spectrum spacing, FWHM represents the target full width at half maximum, and R represents the target reflectivity of the Fizeau interferometer 10 .

According to the embodiment of the present invention, when the target full width at half maximum and the target free spectrum spacing are known, the target fineness factor F=2 and the target reflectivity R=0.24 of the Fizeau interferometer 10 can be obtained.

In operation S850, a target average cavity length of the Fizeau interferometer 10 is determined according to the wavelength corresponding to the target free spectrum interval.

In operation S860 , a target wedge angle of the Fizeau interferometer 10 is determined according to the target average cavity length; wherein the target wedge angle is the angle between two plates of the Fizeau interferometer 10 , and the target wedge angle is used to adjust the width of the interference fringes of the Fizeau interferometer 10 .

According to an embodiment of the present invention, it is assumed that monochromatic parallel light is incident at an angle θ onto two flat plates with a high reflectivity and an included angle α, a portion of the light directly passes through the two plates, and the other portion of the light is emitted after multiple reflections between the two plates, and the included angle between the two portions of light is 2nα (n is the refractive index of the medium), the two portions of light are coherent and produce fringes of equal thickness, wherein a beam splitter, a collimating objective lens and two flat plates with a high reflectivity constitute an initial Fizeau interferometer.

According to an embodiment of the present invention, the transmittance of the Fizeau interferometer is related to the spatial position of the incident light beam on the wedge of the Fizeau interferometer. Therefore, at different positions of the incident laser, there will be different transmittance curves. Assuming that the telescope 7 receives the light beam and 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 standard tool. The target average cavity length of the Fizeau interferometer 10 can be calculated using the relationship between the free spectral range and the average cavity length of the traditional F-P standard tool. In addition, 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 formula (VII):

In formula (VII), 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, 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 1083 nm, 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 spacing, which is about 0.06 m; and when it is known that the echo signal is vertically incident on the initial Fizeau interferometer, the target wedge angle of the Fizeau interferometer 10 can also be calculated to be 9.2 μrad.

According to the embodiment of the present invention, the shape factor affects the shape of the interference fringes, and different shape factors correspond to different transmittance curves of the Fizeau interferometer 10. Therefore, the rationality of various parameters corresponding to the Fizeau interferometer 10 can be verified by the shape factor.

According to an embodiment of the present invention, the shape factor is determined by the following formula (VIII):

In formula (eight), S represents the shape factor, F represents the target fine factor of the Fizeau interferometer 10, _L0 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 shows a relationship diagram between different shape factors and transmittance curves of the Fizeau interferometer 10 according to an embodiment of the present invention.

As shown in FIG9 , combined with formula (VIII), when the shape factor is less than 0.6, the transmittance curve of the Fizeau interferometer 10 can be approximated as an Airy function, and then the wind speed can be inverted by the Airy function. If S≤0.6 is calculated, the vertical incidence state of the incident angle θ＝0 can be maintained; if S>0.6 is calculated, the interference fringes need to be adjusted by adjusting the incident angle until S≤0.6. Substituting the various parameter values of the Fizeau interferometer 10 into the formula, the calculated S meets the requirements.

According to the embodiment of the present invention, the rationality of various parameters corresponding to the Fizeau interferometer 10 can also be verified by the resolution of the Fizeau interferometer 10 .

Using Taylor's criterion, the resolution of the Fizeau interferometer 10 is determined by formula (IX):

make Formula (IX) can be further expressed as:

In formula (IX), Δv represents the minimum frequency range that the Fizeau interferometer 10 can distinguish, m represents the resolution level, and Δλ represents the minimum wavelength range that the Fizeau interferometer 10 can distinguish. Combining the average cavity length _L0 and the target fine factor F calculated in the previous article, it can be further obtained that the frequency resolution of the Fizeau interferometer 10 is about 1.28 GHz. The minimum frequency interval between the metastable He echo signals is 2.23 GHz, from which it can be known that the various parameter values in the previous article can meet the requirements of distinguishing four echo signals.

In operation S440, the filter 9 and the Fizeau interferometer 10 are used to perform two-stage filtering processing on the echo signal with metastable helium scattering information received by the telescope 7, and the processed echo signal is transmitted to the optical path corresponding to the receiving system.

According to an embodiment of the present invention, the filter 9 performs a first-stage filtering process on the echo signal with metastable helium scattering information received by the telescope 7; the ultra-narrowband Fizeau interferometer 10 performs a second-stage filtering process on the echo signal with metastable helium scattering information processed by the filter 9, wherein the echo signal with metastable helium scattering information processed by the filter 9 undergoes equal-thickness interference in the ultra-narrowband Fizeau interferometer 10, and the parallel light beams of the echo signal after the equal-thickness interference are classified according to different frequencies and transmitted to the optical path corresponding to the receiving system.

FIG. 10 schematically shows a data processing flow chart according to an embodiment of the present invention.

As shown in FIG. 10 , the data processing process includes steps S450 to S470 .

In operation S450 , the detector 11 receives an echo signal from an optical path of the receiving system, wherein the echo signal in the optical path of the receiving system is represented by a measurement spectrum, and the measurement spectrum is a convolution of a scattering spectrum and a transmittance function of the Fizeau interferometer 10 .

The measured spectrum can be determined by formula (10):

In formula (10), S _conv represents the measured spectrum, S _scatter represents the scattered spectrum, and S _Fizeau represents the transmittance function of the Fizeau interferometer.

FIG11 schematically shows a schematic diagram of the laser line shape, the total echo signal and the convolution result of the Fizeau interferometer detected by the detector according to an embodiment of the present invention.

As shown in Figure 11, it is assumed that the laser emitted by the laser radar system is a Gaussian line shape, the line width is 200MHz, the temperature is 850K and the wind speed is 0m/s. When the laser line shape is known, according to the number of photons received by each channel of the detector 11, the convolution result of the scattering spectrum and the transmittance function of the Fizeau interferometer 10 can be obtained.

In operation S460, the measured spectrum and the transmittance function of the Fizeau interferometer 10 are deconvolved to obtain an echo signal spectrum of the laser.

In operation S470, the laser echo signal spectrum is linearly interpolated and compared with the echo signal spectrum database to determine that the closest set of data in the laser echo signal spectrum is the temperature and wind speed corresponding to the area to be measured, wherein the area to be measured corresponds to the thermosphere atmosphere.

According to an embodiment of the present invention, the dual-frequency temperature and wind measurement method of helium laser radar based on Fizeau interferometer, by introducing at least two lasers at the transmitting end and modifying some modules at the receiving end, saves the time of traditional laser emission in turns, and can also reduce the complexity and maintenance difficulty of laser radar; on the other hand, it can also save the frequency sweep time, continuously accumulate photon signals, and greatly simplify the data processing method. Finally, by collecting metastable helium scattering information of the area to be measured for temperature and wind measurement, a new research direction can be provided for subsequent research on temperature and wind measurement, and the theoretical gap in helium laser radar in temperature and wind measurement can be filled, so that more comprehensive observation and research can be carried out on the physical process of the thermosphere atmosphere and the multi-layer coupling mechanism.

The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present 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.