A method for detecting key atmospheric optical parameters based on integrated laser heterodyne detection

Abstract

The present invention discloses a method for detecting key atmospheric optical parameters based on integrated laser heterodyne detection, including: S1, using laser heterodyne detection method to actively detect the airspace to be detected and obtain the detection signal; S2, extracting frequency information from the detection signal to obtain radial wind speed at different distances, based on the detected radial wind speed, using a two-dimensional scanning device combined with a multi-beam method to synthesize a three-dimensional vector wind field, so as to obtain the atmospheric wind field at the corresponding distance; S3, inverting the aerosol particle extinction coefficient from the detection signal, and then performing distance integration on the aerosol particle extinction system number to obtain the atmospheric transmittance; and S4, using the atmospheric wind field, calculating the atmospheric wind field pulsation information, and then calculating the turbulent kinetic energy dissipation rate, and then combining the atmospheric model to obtain the atmospheric turbulence intensity. The present invention provides an effective technical means for the integrated detection of atmospheric turbulence, transmittance and wind field under complex environments.

Description

Technical Field

[0001] The invention relates to the field of atmospheric optical parameter detection, and in particular to a method for detecting key atmospheric optical parameters. Background Art

[0002] When laser is transmitted in the atmosphere, atmospheric turbulence, transmittance and wind field on the transmission path are three key atmospheric optical parameters that restrict the performance of laser system.

[0003] The available space provided by the mobile platform for the atmospheric optical parameter detection system is very limited, and at the same time, the atmospheric optical parameter detection system is required to be able to output high-accuracy, high-reliability and high-fusion detection data.

[0004] In the early stage, two or more separate devices were used to detect atmospheric turbulence, transmittance and wind field. These devices had limitations such as loose structure and time asynchrony. They failed to realize composite detection of atmospheric optical parameters on the laser transmission path, and could not meet the needs of future high-maneuverability targets for atmospheric optical properties within a large-scale changing spatial range. Summary of the invention

[0005] Aiming at the comprehensive perception needs of laser systems for the optical properties of the atmosphere in complex environments, the present invention proposes a method for realizing the integrated detection of atmospheric turbulence, transmittance and wind field common aperture on the laser transmission path based on laser heterodyne detection technology, which provides an effective technical means for real-time acquisition of key atmospheric optical parameters in complex environments.

[0006] To this end, the present invention provides a method for detecting key atmospheric optical parameters based on integrated laser heterodyne detection, comprising: S1, actively detecting a detection airspace to be detected using a laser heterodyne detection method to obtain a detection signal; S2, extracting frequency information from the detection signal to obtain radial wind speeds at different distances, and synthesizing a three-dimensional vector wind field based on the detected radial wind speed using a two-dimensional scanning device combined with a multi-beam method, thereby obtaining an atmospheric wind field at a corresponding distance; S3, inverting an aerosol particle extinction coefficient from the detection signal, and then performing distance integration on the aerosol particle extinction system number to obtain atmospheric transmittance; and S4, using the atmospheric wind field to calculate atmospheric wind field pulsation information, and then calculating the turbulent kinetic energy dissipation rate, and then combining the atmospheric model to obtain the atmospheric turbulence intensity.

[0007] The integrated laser heterodyne detection method of atmospheric turbulence, transmittance and wind field on the laser transmission path formed by the present invention provides an effective technical means for real-time acquisition of key atmospheric optical parameters in complex environments. The atmospheric turbulence, transmittance and wind field common aperture integrated laser active detection system formed based on this technology can provide support for evaluating the efficiency of laser atmospheric transmission and optimizing the performance of laser systems.

[0008] In the prior art, two or more separate laser radars (usually with large aperture and short wavelength) are used to detect the atmospheric wind field, atmospheric transmittance and atmospheric turbulence. Among them, the detection of atmospheric turbulence profile by laser imaging has not made any breakthrough progress so far, and effective detection has not been achieved. The method of the present invention uses an all-fiber laser radar with a compact structure (aperture 100mm) and a working wavelength (1550nm) that is eye-safe, and uses laser heterodyne to achieve common aperture integrated non-imaging active detection of atmospheric wind field, atmospheric transmittance and atmospheric turbulence. This is the first time in the field of composite detection of key atmospheric optical parameters.

[0009] In addition to the above-described purposes, features and advantages, the present invention has other purposes, features and advantages. The present invention will be further described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0011] Figure 1 is a technical principle diagram of a method for detecting key atmospheric optical parameters based on integrated laser heterodyne detection according to the present invention;

[0012] Figure 2 It is a principle diagram of the laser heterodyne detection method on which the method of the present invention is based;

[0013] Figure 3 The two-dimensional scanning device combined with the multi-beam method obtains the schematic diagram of wind speed and direction;

[0014] Figure 4 It is a principle diagram of atmospheric transmittance laser detection in the method of the present invention;

[0015] Figure 5 is a flow chart of inverting atmospheric transmittance in the method of the present invention; and

[0016] Figure 6 It is a schematic diagram of the atmospheric turbulence laser non-imaging active detection method in the method of the present invention. DETAILED DESCRIPTION

[0017] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0018] The present invention focuses on the problem of the comprehensive perception needs of laser systems for the optical properties of the atmosphere in complex environments, and proposes a three-in-one detection method for atmospheric turbulence, transmittance and wind field based on laser heterodyne technology, which realizes the integrated laser active detection of atmospheric turbulence, transmittance and wind field, and provides an effective technical means for real-time acquisition of key atmospheric optical parameters in complex scenarios.

[0019] The present invention relies on atmospheric wind field laser heterodyne detection technology and related research foundations, and further proposes a method for integrated detection of atmospheric wind field, turbulence and transmittance based on laser heterodyne technology. The technical scheme is as follows: Figure 1 shown.

[0020] The laser radar on which the present invention is based is: a laser source wavelength of 1550nm all-fiber laser, an aperture of 100mm, a high atmospheric window transmittance in this band and a working wavelength that is safe for human eyes.

[0021] Technical parameter table of LiDAR

[0022] Laser source 1550nm wavelength pulse laser Measuring range 60～6000m Single pulse energy 110uJ Wind speed measurement range -30~30m / s Pulse Width 300ns Wind speed measurement accuracy <0.5m / s Operating temperature -20-50℃ Spatial resolution 30m power ＜500W Temporal resolution 1s size 750mm×650mm×1500mm Measuring distance blind area ＜60m

[0023] The key atmospheric optical parameters detected by the present invention are described one by one below.

[0024] (1) Laser heterodyne detection of atmospheric wind fields

[0025] The Doppler frequency shift caused by the movement of atmospheric particles is much smaller than the laser frequency, and the frequency shift identified by direct detection technology has low accuracy. Laser heterodyne detection technology can amplify the backscattered signal, suppress background noise and detector noise, and directly extract the frequency of the intermediate frequency signal of the same magnitude, with high sensitivity. The basic principle of laser heterodyne detection, such as Figure 2 shown.

[0026] The backscattered light is mixed with a beam of local oscillator light with highly stable frequency and phase. The field intensity of the mixed signal light and its corresponding photocurrent intensity are the result of the interference of the two beams. The mixed light is then coupled to the target surface of the photodetector, and the optical signal is converted into an electrical signal. The atmospheric wind field is detected by extracting the frequency information of the intermediate frequency electrical signal. The electromagnetic field distribution of the backscattered light and the local oscillator light can be expressed as:

[0027]

[0028] In the formula, A s and A LO represent the amplitude of backscattered light and local oscillator light, respectively, and f s and f LO are the frequencies of backscattered light and local oscillator light, respectively. and are the phases of backscattered light and local oscillator light respectively. Assuming the detector is located at z = 0, the light intensity received by the photodetector can be expressed as:

[0029] I d (x,y,0,t)=I s (x,y,0,t)+I LO (x,y,0,t)+I h (x,y,0,t)------(2)

[0030] In the formula, I s and I LO are the intensity distributions of backscattered light and local oscillator light on the detector, I h is the intensity distribution of the laser heterodyne signal on the detector. The intensity distribution of the laser heterodyne signal can be further expressed as:

[0031]

[0032] In the formula, f d and are the frequency difference and phase difference between the backscattered light and the local oscillator light respectively. The photocurrent obtained by laser heterodyne detection is:

[0033]

[0034] Where η q is the quantum efficiency of the detector, ν is the center frequency of the laser, e is the charge constant, and h is the Planck constant. It can be seen from formula (4) that the information contained in the amplitude, frequency, and phase of the backscattered light signal is all contained in the converted electrical signal. By extracting the frequency information using the propagation time of the laser pulse, the radial wind speed at different distances can be obtained. Furthermore, based on the detection of radial wind speed, the wind speed and wind direction are obtained using a two-dimensional scanning device combined with a multi-beam method, such as Figure 3 As shown. Lidar uses the zenith angle Scanning is performed in four directions (0°, 90°, 180° and 270°, where 0° is due north). The atmospheric wind field consists of horizontal wind speed (u, v) and vertical wind speed (w). A 、V B 、V C 、V D are the radial wind speeds in the four directions of OA, OB, OC, and OD respectively. The relationship between the atmospheric wind field and the radial wind speed at the measurement point is shown in formula (5):

[0035]

[0036] Radial wind speed (V A 、V B 、V C 、VD ), according to formula (5), the expression of atmospheric wind field (u, v, w) is as follows:

[0037]

[0038] (2) Active laser detection of atmospheric transmittance

[0039] When laser interacts with aerosol particles, the backscattering echo signal intensity is related to the backscattering coefficient and extinction coefficient of the aerosol particles, and the relationship between the backscattering coefficient and the extinction coefficient can be approximately described by an empirical model. The present invention proposes to invert the aerosol particle extinction coefficient from the laser radar scattered echo signal, and then obtain the atmospheric transmittance. The principle of atmospheric transmittance laser detection, such as Figure 4 As shown, v0 is the center frequency of the laser radar; v d is the Doppler frequency shift received by the laser radar; V r is the component of wind speed V in the direction of the telescope.

[0040] Generally, the molecular backscattering coefficient and extinction coefficient can be obtained from sounding data or atmospheric models. The Fernald inversion method is used to invert the aerosol extinction coefficient based on the spectrum intensity in the lidar echo signal, and then the aerosol and molecular extinction coefficients are integrated over distance to obtain the atmospheric transmittance. According to the echo signal spectrum intensity, the lidar equation is equivalent to:

[0041]

[0042] Where P(r) is the backscattered echo signal at distance r, A is the proportional relationship between the spectrum intensity and the echo signal intensity, K is the system constant, α(r) and β(r) are the extinction coefficient and backscattering coefficient at distance r, and the subscripts m and a are molecules and aerosol particles, respectively. Generally, the molecular backscattering coefficient β m (r) and extinction coefficient α m (r) can be obtained from sounding data or atmospheric models. The Fernald inversion method is used, combined with formula (7) to obtain the aerosol particle backscattering coefficient β a (r) can be expressed as:

[0043]

[0044] In the formula, is the aerosol extinction backscattering ratio, is the molecular extinction backscattering ratio. S1 is usually taken as 50, and the corresponding S2 is 8π / 3.

[0045] X(r)=P(r)·r 2 is the distance correction factor, r cis the calibration distance. The aerosol backscattering coefficient β at a wavelength of 1550nm a (r c ), by setting the aerosol scattering ratio Therefore, the aerosol extinction coefficient is inverted according to the spectrum intensity in the laser radar echo signal, and then the aerosol and molecular extinction coefficients are integrated over distance to obtain the transmittance. Figure 5 shown.

[0046] (3) Active laser non-imaging detection of atmospheric turbulence

[0047] The present invention proposes high-precision atmospheric wind field information based on laser heterodyne detection, calculates the atmospheric wind field pulsation, and then calculates the turbulent kinetic energy dissipation rate, and further combines the atmospheric model to obtain the atmospheric turbulence intensity. Atmospheric turbulence laser non-imaging active detection technology solution, such as Figure 6 According to the local uniform isotropic turbulence model and the turbulence statistical description theory, the turbulent kinetic energy dissipation rate can be calculated from the high-precision atmospheric wind field information detected by laser heterodyne, and the atmospheric turbulence intensity can be obtained by combining it with the atmospheric model.

[0048] According to the local uniform isotropic turbulence model and the turbulence statistical description theory, the atmospheric turbulence intensity is usually expressed by the refractive index structure constant. In the visible and infrared bands, the influence of water vapor can be ignored, and the atmospheric turbulence intensity can be expressed as:

[0049]

[0050] In the formula, θ is the potential temperature, ε is the turbulent kinetic energy dissipation rate, V is the horizontal wind speed, T(h) is the temperature profile, and P(h) is the pressure profile. The relationship between the turbulent kinetic energy dissipation rate and the atmospheric wind field can be expressed as:

[0051]

[0052] In the formula, is the wind speed shear. Based on the high-precision atmospheric wind field detected by laser heterodyne, the turbulent kinetic energy dissipation rate is obtained by formula (10). It can be seen from formula (9) that the calculation of atmospheric turbulence also requires temperature profile and pressure profile. According to the 1976 US standard atmospheric model, the atmospheric temperature below 11 km can be expressed as:

[0053] T(h)=T(h0)-γh-------------------------------(11)

[0054] Where: T(h) is the temperature at height h, T(h0) is the ground temperature, which is obtained synchronously by the meteorological station equipped with the coherent lidar, and γ is the vertical temperature lapse rate. The atmospheric pressure profile can be expressed by the fluid statics equation combined with equation (11) as follows:

[0055]

[0056] Where P(h) is the pressure at height h, P(h0) is the ground pressure, obtained synchronously from the weather station, and R d is the dry air specific gas constant, and g is the gravitational acceleration.

[0057] Therefore, the turbulent kinetic energy dissipation rate is obtained from equation (10), the atmospheric model temperature and pressure are obtained from equations (11) and (12), and then combined with equation (9), the atmospheric turbulence intensity can be obtained:

[0058] The present invention focuses on the limitations of the previous use of multiple devices to independently detect atmospheric optical parameters such as atmospheric turbulence, transmittance and wind field, such as the loose structure and time asynchrony. Based on the accumulation of atmospheric wind field laser heterodyne detection technology and the relevant theoretical research foundation of atmospheric turbulence and transmittance, a three-in-one laser active non-imaging detection method for atmospheric turbulence, transmittance and wind field is proposed.

[0059] The three-in-one laser radar active non-imaging detection technology of atmospheric turbulence, transmittance and wind field formed according to the method proposed in the present invention can provide support for accurately evaluating the laser atmospheric transmission effect and optimizing the performance of the laser system, filling the gap in my country's common-aperture integrated laser active detection technology for key atmospheric optical parameters such as atmospheric turbulence, transmittance and wind field, and has important prospects in the field of laser system applications.

[0060] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for detecting key atmospheric optical parameters based on laser heterodyne integration, characterized in that: include: S1. Actively detect the airspace to be detected by using laser heterodyne detection method to obtain detection signals; S2. extract frequency information from the detection signal to obtain radial wind speeds at different distances, and synthesize a three-dimensional vector wind field using a two-dimensional scanning device combined with a multi-beam method based on the detected radial wind speed, thereby obtaining the atmospheric wind field at the corresponding distance; S3, invert the aerosol particle extinction coefficient from the detection signal, and then perform distance integration on the aerosol particle extinction system number to obtain the atmospheric transmittance; as well as S4, using the atmospheric wind field, calculating the atmospheric wind field pulsation information, and then calculating the turbulent kinetic energy dissipation rate, and then combining with the atmospheric model to calculate the atmospheric turbulence intensity, Step S4 includes the following steps: S41. Calculate the turbulent kinetic energy dissipation rate (ε) using the atmospheric wind field: In the above formula, is the wind speed shear, which is calculated from the high-precision atmospheric wind field (u, v) detected by laser heterodyne; S42. Calculate the temperature profile T(h) based on the atmospheric model: T(h)=T(h0)-γh-------------------------------(11) In the above formula: T(h) is the temperature at height h, T(h0) is the ground temperature, which can be obtained synchronously by the meteorological station equipped with the coherent lidar, and γ is the vertical lapse rate of temperature; S43. Calculate the pressure profile P(h) according to the atmospheric model: In the above formula, P(h) is the pressure at height h, P(h0) is the ground pressure, which is obtained synchronously from the weather station, and R d is the dry air specific gas constant, g is the gravitational acceleration; S44. Calculate the atmospheric turbulence intensity at height h based on the turbulent kinetic energy dissipation rate ε, temperature profile T(h), and pressure profile P(h) In the above formula, θ is the potential temperature, ε is the turbulent kinetic energy dissipation rate, V is the horizontal wind speed, T(h) is the temperature profile, and P(h) is the pressure profile.

2. The method for detecting key atmospheric optical parameters based on laser heterodyne integration according to claim 1, characterized in that: The frequency information is extracted from the detection signal using the propagation time of the laser pulse to obtain the radial wind speed at different distances.

3. The method for detecting key atmospheric optical parameters based on laser heterodyne integration according to claim 1, characterized in that: The Fernald inversion method is used to invert the aerosol particle extinction coefficient from the detection signal, and then the atmospheric transmittance is obtained by distance integration of the aerosol particle extinction system number.

4. The method for detecting key atmospheric optical parameters based on laser heterodyne integration according to claim 1, characterized in that: The detected atmospheric wind field information is used to calculate the turbulent kinetic energy dissipation rate, and then combined with the atmospheric model to obtain the atmospheric turbulence intensity profile.

5. The method for detecting key atmospheric optical parameters based on laser heterodyne integration according to claim 1, characterized in that: The laser radar used in the detection method is an all-fiber laser radar with a diameter of 100mm and a wavelength of 1550nm.

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