Six-aperture dimm turbulent flow dynamic characteristic measuring device and atmospheric turbulent flow characteristic evaluation method

Abstract

This invention discloses a six-aperture DIMM turbulence dynamic characteristic measurement device and an atmospheric turbulence characteristic evaluation method. The method includes: obtaining a spot image with six spots using the six-aperture DIMM turbulence dynamic characteristic measurement device; preprocessing the spot image; traversing the preprocessed spot image to obtain the bounding rectangle of the spot image; processing the ROI region generated by the bounding rectangle using a clustering algorithm; calculating the centroid coordinates of the clustered spots using geometric moments; calculating the relative distance between the spots based on the centroid coordinates; obtaining a jitter variance estimate based on the relative distance; obtaining the atmospheric coherence length based on the jitter variance estimate; and evaluating the turbulence dynamic characteristics based on the atmospheric coherence length. This invention enriches the directional information of atmospheric turbulence statistical characteristics through the six apertures, thereby reducing measurement errors and improving the accuracy of measurement results.

Description

Technical Field

[0001] This invention relates to the field of atmospheric optics technology, specifically to a six-aperture DIMM turbulence dynamic characteristic measurement device and an atmospheric turbulence characteristic evaluation method. Background Technology

[0002] Laser transmission is affected by atmospheric turbulence, leading to phenomena such as laser wavefront distortion, beam drift, beam spread, and flicker, thus impacting communication quality. Therefore, accurate analysis of the impact of atmospheric turbulence on communication systems during laser transmission is crucial. The atmospheric coherence length r0 is generally used to quantitatively describe the atmospheric turbulence medium. Since the atmospheric turbulence effect is influenced by factors such as temperature and wind speed, its value constantly changes. Therefore, rapid measurement of the atmospheric coherence length is of great significance for atmospheric laser communication.

[0003] Atmospheric optical turbulence causes phenomena such as laser scintillation, beam drift, beam spread, and angular arrival fluctuations after laser transmission through the atmosphere. Fried proposed that atmospheric coherence length r0 is a parameter related to the wavefront phase structure function: in ground-based astronomical observations, r0 can characterize the equivalent aperture of a telescope under the influence of diffraction-limited imaging in the entire atmospheric turbulence layer; in laser atmospheric transmission, r0 is usually used to measure the intensity of optical turbulence in the path integral. Therefore, real-time acquisition of atmospheric coherence length is of great significance for studying laser atmospheric transmission effects and ground-based astronomical observations.

[0004] Current methods for measuring atmospheric coherence length include temperature fluctuation method, laser scintillation method, radar measurement method, slope difference method, and differential imaging method. Among these, the differential image motion method is an excellent method for measuring atmospheric coherence length because it effectively eliminates the influence of non-atmospheric factors (such as telescope jitter) on the measurement results. It has become the main means of measuring r0 in modern astronomical site selection and laser atmospheric transmission. However, the traditional differential image motion method uses a two-aperture telescope tube for measurement, which has limited characteristic directional information and low measurement accuracy. Therefore, we propose a method based on the evaluation of turbulent dynamic characteristics of a six-aperture DIMM to solve the above problems. By using six apertures, the directional information of atmospheric turbulence statistical characteristics is enriched, thereby reducing measurement errors and improving the accuracy of the measurement results. Summary of the Invention

[0005] To address the above technical problems, this invention provides a six-aperture DIMM turbulence dynamic characteristic measurement device, comprising: a laser, a small transmitting telescope, a two-dimensional transmitting turntable, a six-aperture lens tube, a small receiving telescope, a two-dimensional receiving turntable, a narrowband filter, and a visible light CCD camera.

[0006] The transmitting end miniature telescope is connected to the laser and the transmitting end two-dimensional turntable respectively; the receiving end miniature telescope is connected to the six-hole mirror tube and the receiving end two-dimensional turntable respectively; the receiving end miniature telescope images the received antenna through a narrowband filter on a visible light CCD camera.

[0007] This invention also provides a method for evaluating atmospheric turbulence characteristics, using the aforementioned six-aperture DIMM turbulence dynamic characteristic measurement device, the method comprising:

[0008] S1. Obtain a spot image with 6 spots using a six-aperture DIMM turbulence dynamic characteristic measurement device, and preprocess the spot image;

[0009] S2. Traverse the preprocessed spot image to obtain the bounding rectangle of the spot image;

[0010] S3. Process the ROI region generated by the bounding rectangle using the K-means clustering algorithm;

[0011] S4. Calculate the centroid coordinates of the clustered light spots using geometric moments;

[0012] S5. Calculate the relative distance between the light spots based on the centroid coordinates of the light spots;

[0013] S6. Obtain a jitter variance estimate based on the relative distance, obtain an atmospheric coherence length based on the jitter variance estimate, and evaluate the dynamic characteristics of turbulence based on the atmospheric coherence length.

[0014] Optionally, in step S1, the preprocessing of the light spot image includes: grayscale conversion, noise reduction, filtering, and threshold segmentation of the light spot image. Specifically, the threshold segmentation process includes: taking the average light intensity of the four corners of the light spot image as background light noise, taking the background light noise value as the minimum threshold of the big rule method, and using the big rule method to segment the light spot image.

[0015] Optionally, in step S2, traversing the preprocessed spot image to obtain the content of the bounding rectangle of the spot image specifically includes:

[0016] Traverse the light spots in the preprocessed light spot image, find the outline of each light spot, and obtain the bounding rectangle containing all light spots;

[0017] Add pixels to expand the connected region, and use the connected region as the ROI region.

[0018] Optionally, in step S4, the calculation of the centroid coordinates of the clustered light spots using geometric moments specifically includes:

[0019] Geometric moment M ijIt is the weighted sum of the products of the coordinates (x, y) of all pixels in the image and the pixel value I(x, y):

[0020]

[0021] Where i and j are the orders of the moments, and I(x,y) is the pixel value located at (x,y);

[0022] Centroid coordinate calculation:

[0023]

[0024] Optionally, in step S5, calculating the relative distance between light spots based on the centroid coordinates of the light spots specifically includes:

[0025]

[0026] Optionally, in step S6, the formula for calculating the jitter variance is:

[0027]

[0028] Optionally, in step S6, the formula for calculating the atmospheric coherence length is:

[0029]

[0030] Where D is the diameter of the telescope's sub-pupil, d is the sub-pupil spacing, λ is the wavelength selected when calculating r0, and f is the system focal length of the optical antenna.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] The method described in this invention solves the problems of insufficient characteristic direction information and low measurement accuracy in existing differential imaging motion meters. By using a six-aperture system, it enriches the directional information of atmospheric turbulence statistical characteristics, thereby reducing measurement errors and improving the accuracy of measurement results. The method described in this invention obtains the atmospheric coherence length from the light spot data processing results, requiring less data and simplifying calculations. The atmospheric coherence length value is extracted from a single data point, and the atmospheric refractive index structure constant value on the transmission link can be further calculated. The optical antenna used in this invention has a small aperture, small size, and light weight, making it easy to carry. It eliminates the need for a wavefront sensor, effectively overcoming many shortcomings of the Shakhartmann wavefront sensor, such as relatively low resolution, limited dynamic range, and difficulty in precise calibration. Attached Figure Description

[0033] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of the structure of the six-aperture DIMM turbulence dynamic characteristic measurement device of the present invention;

[0035] Figure 2 This is a flowchart of the method for evaluating atmospheric turbulence characteristics according to an embodiment of the present invention;

[0036] Figure 3 The following are three-dimensional views and configuration diagrams of the six apertures at the front of the telescope, from left to right, for this invention.

[0037] Reference numerals: 1. Laser; 2. Transmitter miniature telescope; 3. Transmitter two-dimensional turntable; 4. Six-hole telescope tube; 5. Receiver miniature telescope; 6. Receiver two-dimensional turntable; 7. Narrowband filter; 8. Visible light CCD camera; 9. Computer terminal. Detailed Implementation

[0038] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0039] Example 1

[0040] A six-aperture DIMM turbulence dynamic characteristic measurement device, such as Figure 1 As shown, the device includes: a laser 1; a small transmitting telescope 2; a two-dimensional transmitting turntable 3; a six-aperture lens tube 4; a small receiving telescope 5; a two-dimensional receiving turntable 6; a narrowband filter 7; a visible light CCD camera 8; and a computer processing system 9. The transmitting end emits a 532nm laser beam, which is expanded by an optical antenna and passes through a turbulent channel approximately 864m in length. The beam is then received by a telescope equipped with a six-aperture mask and imaged on the image plane of the visible light CCD camera. Subsequent processing is then performed on the computer. Figure 3 From left to right are the 3D view and configuration diagram of the front-end six-hole sleeve. The transmitting end miniature telescope 2 is connected to the laser 1 and the transmitting end two-dimensional turntable 3 respectively; the receiving end miniature telescope 5 is connected to the six-hole lens tube 4 and the receiving end two-dimensional turntable 6 respectively; the visible light CCD camera 8 is connected to the computer processing system 9; the receiving end miniature telescope 5 images the received antenna through the narrowband filter 7 onto the visible light CCD camera 8. In this embodiment, the structural schematic diagram of the six-hole sleeve is as follows. Figure 3 As shown.

[0041] like Figure 1 As shown, after the laser is collimated and expanded at end A, the laser passes through the atmospheric turbulence channel to the receiving end at end B. After passing through a six-aperture mask device, receiving optical antenna, filter and other devices, it is imaged on a CCD camera. The centroid jitter of the spot caused by the influence of the channel is observed and analyzed by computer, and then the centroid jitter variance is calculated. The value of r0 can be obtained by substituting the centroid jitter variance into the formula.

[0042] Example 2

[0043] A method for evaluating atmospheric turbulence characteristics using a six-aperture DIMM turbulence dynamics measurement device, such as... Figure 2 As shown, the method includes:

[0044] S1. Obtain a spot image with 6 spots using a six-aperture DIMM turbulence dynamic characteristic measurement device, and preprocess the spot image.

[0045] The entire testing setup consists of two parts, located at ends A and B respectively. End A, which functions as a light source, is situated on the 13th floor of the Science and Technology Building on the South Campus of Changchun University of Science and Technology. An optical antenna collimates, focuses, and expands the laser beam. End B is a six-aperture atmospheric coherence length meter, located on the 9th floor of the Second Teaching Building on the East Campus of Changchun University of Science and Technology. A narrowband CCD camera is connected to the tail of the receiving optical antenna to record the light spot data for analysis. Notably, a six-aperture mask is mounted at the front of the receiving optical antenna. The entire communication link has a straight-line distance of 865m, classifying it as an urban link.

[0046] The obtained spot images were preprocessed, including grayscale processing, image filtering, noise removal, and threshold segmentation. The threshold segmentation process used Otsu's method, and a background light intensity extraction step was also performed. This involved taking the average light intensity at the four corners as the background light noise and using this value as the minimum threshold for Otsu's method. This approach helps filter out some noise and more effectively segment the image.

[0047] S2. Traverse the preprocessed spot image to obtain the bounding rectangle of the spot image. Traverse the processed spot image and use the boundingRect function to obtain the minimum bounding rectangle of each spot contour. Add appropriate pixels to the bounding rectangles of all contours to expand the connected regions and use them as the ROI target regions. For multiple frames of images acquired in a short time, only the ROI region needs to be scanned, which can reduce the number of background pixels scanned and improve processing speed.

[0048] S3. Process the ROI region generated by the circumscribed rectangle using the K-means clustering algorithm. The goal of the K-means algorithm is to minimize the sum of the squared distances from data points within a cluster to their cluster centers. Set the number of clusters k to 6, i.e., the number of light spots is 6, and finally use different colors to distinguish the light spots.

[0049] S4. Calculate the centroid coordinates of the clustered light spots using geometric moments. Obtain the centroid of the light spot using geometric moments. Geometric moment M ij It is the weighted sum of the products of the coordinates (x, y) of all pixels in the image and their pixel values ​​I(x, y). The calculation formula is as follows:

[0050]

[0051] Where i and j are the orders of the moments, and I(x,y) is the pixel value located at (x,y). Centroid The coordinates are calculated using the following formula:

[0052]

[0053] S5. Calculate the relative distance between the light spots based on the centroid coordinates of the light spots.

[0054] The six apertures are arranged in a star-shaped pattern, divided into three groups according to the crossing method. The relative distance between any two pairs of light spots is then calculated using the following formula:

[0055]

[0056] S6. Obtain a jitter variance estimate based on the relative distance, obtain an atmospheric coherence length based on the jitter variance estimate, and evaluate the dynamic characteristics of turbulence based on the atmospheric coherence length.

[0057] By counting several relative distances over consecutive time intervals, the jitter variance can be estimated as follows:

[0058]

[0059] Substituting the jitter variances obtained from the six apertures in three directions into the following formula, the average of the variances can be used to obtain r0:

[0060]

[0061] The proposed method for evaluating the dynamic characteristics of turbulence based on a six-aperture DIMM is effective and accurate. It can replace the atmospheric coherence length measurement that was previously only achievable with large telescopes. It does not use the traditional Shaker-Hartmann sensor and avoids calculation errors caused by the defects of the sensor itself.

[0062] In summary, this invention proposes a method for evaluating the dynamic characteristics of turbulence based on a six-aperture DIMM telescope. A simple system using two small telescopes measures light spot data, and the atmospheric coherence length is then derived by processing the data. Specifically, the relative distance between each pair of light spots is first calculated. Then, the centroid jitter variance in each direction is obtained using the relative distance. Finally, the atmospheric coherence length is successfully derived by substituting the centroid jitter variance in each direction into the formula. The telescopes used in this invention have small apertures, small size, and light weight, thus offering significant advantages in portability.

[0063] The method described in this invention solves the problems of insufficient characteristic direction information and low measurement accuracy of differential imaging motion meters in the prior art. By using a six-aperture system, the directional information of atmospheric turbulence statistical characteristics is enriched, thereby reducing measurement errors and improving the accuracy of measurement results.

[0064] The method described in this invention obtains the atmospheric coherence length by inverting the results of light spot data processing. It requires little data and is easy to calculate. The atmospheric coherence length value can be extracted from a single data point. Furthermore, the scintillation index can be calculated from the light spot intensity to deduce the atmospheric refractive index structure constant.

[0065] Example 3

[0066] An electronic device includes a memory and a processor, wherein the memory stores a computer program executed by the processor, the computer program performing the turbulence dynamic characteristics evaluation method when executed by the processor.

[0067] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for evaluating atmospheric turbulence characteristics, characterized in that, The method includes: S1. Obtain a spot image with 6 spots using a six-aperture DIMM turbulence dynamic characteristic measurement device, and preprocess the spot image; The six-aperture DIMM turbulence dynamic characteristic measurement device includes a laser, a small telescope at the transmitting end, a two-dimensional turntable at the transmitting end, a six-aperture lens tube, a small telescope at the receiving end, a two-dimensional turntable at the receiving end, a narrowband filter, and a visible light CCD camera. The transmitting end miniature telescope is connected to the laser and the transmitting end two-dimensional turntable respectively; the receiving end miniature telescope is connected to the six-hole mirror tube and the receiving end two-dimensional turntable respectively; the receiving end miniature telescope images the received antenna through a narrowband filter on a visible light CCD camera. S2. Traverse the preprocessed spot image to obtain the bounding rectangle of the spot image; S3. Process the ROI region generated by the bounding rectangle using the K-means clustering algorithm; S4. Calculate the centroid coordinates of the clustered light spots using geometric moments; Geometric moment M ij It is the weighted sum of the products of the coordinates (x, y) of all pixels in the image and the pixel value I(x, y): Where i and j are the orders of the moments, I(x,y) is the pixel value located at (x,y), and x i y j These are the i-th moment of the I(x,y) pixel value at the x-coordinate and the j-th moment of the I(x,y) pixel value at the y-coordinate, respectively. centroid coordinates calculate: Among them, M 10 for The first moment of the pixel value I(x,y) represents the x-coordinate; M 00 for M represents the cumulative sum of pixel values ​​I(x,y), 01 for The first moment of the pixel value I(x,y) is represented as the y-coordinate; S5. Calculate the relative distance Δρ between the light spots based on the centroid coordinates of the light spots, specifically including: Where x1 and y1, and x2 and y2 are the centroid coordinates (x1, y1) and (x2, y2) of the two light spots, respectively; S6. Obtain a jitter variance estimate based on the relative distance, obtain an atmospheric coherence length based on the jitter variance estimate, and evaluate the dynamic characteristics of turbulence based on the atmospheric coherence length. jitter variance σ 2 The calculation formula is: Where N is the number of spot centroid jitters, that is, the number of acquired spot images is N, Δρ i Let be the relative distance between the centroids of the two i-th light spots; The formula for calculating the atmospheric coherence length r0 is: Where D is the diameter of the telescope's sub-pupil, d is the sub-pupil spacing, λ is the wavelength selected when calculating r0, and f is the system focal length of the optical antenna.

2. The method for evaluating atmospheric turbulence characteristics according to claim 1, characterized in that, In step S1, the preprocessing of the light spot image includes: grayscale conversion, denoising, filtering, and threshold segmentation of the light spot image. Specifically, the threshold segmentation process includes: taking the average light intensity of the four corners of the light spot image as the background light noise, taking the background light noise value as the minimum threshold of the big rule method, and using the big rule method to segment the light spot image.

3. The method for evaluating atmospheric turbulence characteristics according to claim 1, characterized in that, In step S2, traversing the preprocessed spot image to obtain the content of the bounding rectangle of the spot image specifically includes: Traverse the light spots in the preprocessed light spot image, find the outline of each light spot, and obtain the bounding rectangle containing all light spots; Add pixels to expand the connected region, and use the connected region as the ROI region.

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