CN109767420B - Turbulence intensity determination method based on vortex light beam - Google Patents
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Abstract
The invention relates to a turbulence intensity determination method based on vortex beams, which comprises the following steps: s1: interfering the initial vortex light beam with the initial plane light beam to obtain a non-turbulent interference pattern; s2: enabling the initial vortex light beam to enter a turbulent flow environment to obtain a turbulent flow vortex light beam; s3: interfering the turbulent vortex light beam with the initial plane light beam to obtain a turbulent interference pattern carrying turbulent information; s4: and respectively calculating and comparing the turbulence-free interference pattern and the turbulence interference pattern to judge the turbulence intensity of the turbulence environment. The turbulence intensity determination method of the invention adopts vortex light beams to detect turbulence environment, has longer transmission distance, higher transmission efficiency, more stable transmission, more accurate determination of turbulence intensity, easier determination and convenient operation, and is suitable for various turbulence environments.
Description
Technical Field
The invention belongs to the technical field of laser application, and particularly relates to a turbulence intensity determination method based on vortex beams.
Background
The vortex beam is a beam carrying an "optical vortex" phenomenon, and the wave front of the vortex beam is in a spiral vortex shape, which is the biggest difference between the vortex beam and the ordinary beam. In an underwater turbulent environment, a vortex beam has a longer propagation distance, more stable information transmission, higher transmission efficiency, and more data capacity than a normal beam. With the continuous development of ocean resources and the continuous definition of ocean defense consciousness, the detection of dynamic targets in seawater and the trail tracking of ship targets become an important research subject.
At present, the underwater environment detection mostly adopts common light beams, the transmission of the light beams is unstable, and a fine and complex device is needed for judging the turbulence intensity. And the transmission distance of the common light beam in turbid non-static seawater is much closer than that of the vortex light beam, so that the existing turbulence detection method based on the common light beam is not suitable for being used in a long distance.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a turbulence intensity determination method based on vortex beams. The technical problem to be solved by the invention is realized by the following technical scheme:
the invention provides a method for judging turbulence intensity based on vortex beams, which comprises the following steps:
s1: interfering the initial vortex light beam with the initial plane light beam to obtain a non-turbulent interference pattern;
s2: enabling the initial vortex light beam to enter a turbulent flow environment to obtain a turbulent flow vortex light beam;
s3: interfering the turbulent vortex light beam with the initial plane light beam to obtain a turbulent interference pattern carrying turbulent information;
s4: and respectively calculating and comparing the turbulence-free interference pattern and the turbulence interference pattern to judge the turbulence intensity of the turbulence environment.
In an embodiment of the present invention, before the S1, the method further includes:
s0: acquiring the initial vortex beam and the initial plane beam.
In an embodiment of the present invention, the S0 includes: and generating the initial vortex light beam by using a spatial light modulator, and generating a Gaussian light beam by using a laser, wherein the beam waist of the Gaussian light beam can be approximately regarded as the initial plane light beam.
In an embodiment of the present invention, the S4 includes:
s41: respectively carrying out gray level image conversion on the turbulence-free interference pattern and the turbulence interference pattern to obtain a turbulence-free gray level image and a turbulence gray level image;
s42: and respectively carrying out Fourier transform on the turbulence-free gray scale image and the turbulence gray scale image to obtain a turbulence-free frequency spectrum image and a turbulence frequency spectrum image.
S43: respectively carrying out gray level calculation on the turbulence-free frequency spectrogram and the turbulence frequency spectrogram to obtain a turbulence-free gray level average difference value and a turbulence gray level average difference value;
s44: and comparing the turbulence-free gray level average difference value with the turbulence gray level average difference value, and judging the turbulence intensity of the turbulence environment.
In an embodiment of the present invention, the S42 includes:
s421: converting the image data formats of the turbulence-free gray scale map and the turbulence gray scale map into first double-precision floating point type data and second double-precision floating point type data respectively;
s422: respectively performing two-dimensional discrete Fourier transform on the first double-precision floating point type data and the second double-precision floating point type data to obtain first two-dimensional discrete Fourier data and second two-dimensional discrete Fourier data;
s423: respectively performing fast Fourier transform on the first two-dimensional discrete Fourier data and the second two-dimensional discrete Fourier data to obtain first fast Fourier data and second fast Fourier data;
s424: and respectively carrying out spectrum logarithmic transformation on the real parts of the first fast Fourier data and the second fast Fourier data so as to obtain a turbulence-free spectrogram and a turbulence spectrogram.
In an embodiment of the present invention, the step S42 further includes: and respectively carrying out filtering processing on the turbulence-free spectrogram and the turbulence spectrogram.
In an embodiment of the present invention, the S43 includes:
s431: removing low frequency regions in the turbulence free spectrogram and the turbulence spectrogram;
s432: solving a grey level histogram of the turbulence-free spectrogram and the turbulence spectrogram without the low-frequency region to obtain a turbulence-free grey level histogram and a turbulence grey level histogram;
s433: and respectively calculating the gray level average difference of the turbulence-free gray level histogram and the turbulence gray level histogram to obtain a turbulence-free gray level average difference value and a turbulence gray level average difference value.
In an embodiment of the present invention, the S44 includes: when the turbulence gray level average difference value is lower than the turbulence-free gray level average difference value, turbulence exists, and the smaller the difference value between the turbulence-free gray level average difference value and the turbulence gray level average difference value is, the stronger the turbulence intensity based on the vortex light beams is.
Compared with the prior art, the invention has the beneficial effects that:
1. the method for judging the turbulence intensity detects the turbulence environment by adopting the vortex light beam, has longer transmission distance, higher transmission efficiency, more stable transmission and more accurate judgment on the turbulence intensity;
2. the method for judging the turbulence intensity is easier to judge, convenient to operate and suitable for various turbulence environments.
Drawings
FIG. 1 is a schematic flow chart diagram of a vortex beam based turbulence intensity determination method in accordance with an embodiment of the present invention;
FIGS. 2a-2c are non-turbulent interference patterns of interference superposition of initial vortex beams and initial plane beams of different topological charge numbers according to an embodiment of the present invention;
3 a-3 c are weak turbulence interference patterns of different topological charge numbers of a weak turbulence vortex beam interfering with an initial plane beam through a weak turbulence environment according to embodiments of the present invention;
4 a-4 c are highly turbulent interference patterns of different topological charge numbers of highly turbulent vortex beams interfering with an initial planar beam for embodiments of the present invention passing through a highly turbulent environment;
FIGS. 5a-5c are spectrograms of Fourier transforms of interference patterns;
fig. 6a to 6c are spectrograms obtained by performing fourier transform on the interference pattern and then performing filtering processing.
Detailed Description
The present disclosure is further described with reference to specific examples, but the embodiments of the present disclosure are not limited thereto.
Referring to fig. 1, fig. 1 is a schematic flow chart of a turbulent flow strength determination method based on vortex beams in an embodiment of the present invention, and the embodiment provides a method for determining turbulent flow strength based on vortex beams, including:
s1: interfering the initial vortex light beam with the initial plane light beam to obtain a non-turbulent interference pattern;
s2: enabling the initial vortex light beam to enter a turbulent flow environment to obtain a turbulent flow vortex light beam;
s3: interfering the turbulent vortex light beam with the initial plane light beam to obtain a turbulent interference pattern carrying turbulent information;
s4: and respectively calculating and comparing the turbulence-free interference pattern and the turbulence interference pattern to judge the turbulence intensity of the turbulence environment.
Further, before the S1, the method further includes:
s0: acquiring the initial vortex beam and the initial plane beam.
Specifically, the S0 includes: and generating the initial vortex light beam by using a spatial light modulator, and generating a Gaussian light beam by using a laser, wherein the beam waist of the Gaussian light beam can be approximately regarded as the initial plane light beam.
Preferably, the amplitudes of the initial vortex beam and the initial plane beam are consistent, and the interference effect is more obvious when the difference between the amplitudes of the two beams is smaller.
In this embodiment, the initial vortex light beam and the initial plane light beam are obtained by theoretical simulation, and the light intensity distribution of the vortex light beam, the light intensity distribution of the plane light beam and the light intensity distribution after the interference of the two are simulated in MATLAB, wherein the light intensity distribution expression E of the vortex light beam1＝E0exp (il θ), wherein E0Representing amplitude, l represents topological charge number; expression of light intensity distribution of planar light beam
Wherein E is0Representing the amplitude. The expression of the light intensity distribution of the two after the interference in the non-turbulent flow environment is
Referring to fig. 2a-2c, fig. 2a-2c are non-turbulent interference patterns of interference superposition of initial vortex beams with different topological charges and initial plane beams according to an embodiment of the present invention, where the topological charge l of the initial vortex beam of fig. 2a is 1, the topological charge l of the initial vortex beam of fig. 2b is 2, and the topological charge l of the initial vortex beam of fig. 2c is 3, as shown in the figure, the initial vortex beam is no longer a light and dark vertical fringe pattern after interference with the initial plane beam, but changes into an offset fork pattern. The number of the forks in the fork pattern is the sum of the topological charge number and 1, and when the topological charge number is changed, the forks of the fork pattern are also changed. The non-turbulent interference patterns of said fig. 2a-2c serve as reference maps for subsequent judgments.
In this embodiment, the turbulent environment is obtained by theoretical simulation of a random phase screen method, and includes the following steps:
(1) generating a complex Gaussian random number matrix in a simulation mode;
(2) filtering the complex Gaussian random number matrix by adopting a turbulent phase disturbance power spectrum function;
wherein the turbulent phase perturbation power spectrum function is:
wherein the expression inside the integral is:
(3) carrying out inverse Fourier transform on the filtered complex Gaussian random number matrix to obtain a layer of phase screen;
(4) and (4) repeating the steps (1) to (3) to obtain random and different phase screens, and simulating the characteristics of the turbulent flow environment by using the multiple layers of phase screens.
In this embodiment, the turbulent power spectrum function may be atmospheric turbulence or ocean turbulence, the number of layers of the phase screen is 10 to 30, the phase screen simulates a weak turbulent environment and a strong turbulent environment respectively, and the order of magnitude of the phase screen is 10 when the phase screen simulates the weak turbulent environment-7Of the order of 10 when simulating highly turbulent environments-1. Respectively enabling the simulated initial vortex light beams with topological charge numbers l 1, l 2 and l 3 to pass through the simulated weak turbulence environment and the simulated strong turbulence environment, and then performing combination with the simulated weak turbulence environment and the simulated strong turbulence environmentThe initial planar beams of light interfere. The pattern of the initial vortex beam after interference with the initial plane beam after passing through a turbulent environment will carry turbulent information. Referring to fig. 3 a-3 c and fig. 4 a-4 c, fig. 3 a-3 c are weak turbulence interference patterns of interference of weak turbulence vortex beams with different topological charge numbers and initial plane beams of an embodiment of the present invention passing through a weak turbulence environment, fig. 4 a-4 c are strong turbulence interference patterns of interference of strong turbulence vortex beams with different topological charge numbers and initial plane beams of an embodiment of the present invention passing through a strong turbulence environment, wherein the topological charge number l of the turbulence vortex beams of fig. 3a and 4a is 1, the topological charge number l of the turbulence vortex beams of fig. 3b and 4b is 2, and the topological charge number l of the turbulence vortex beams of fig. 3c and 4c is 3. As shown, the weak turbulent interference patterns of fig. 3 a-3 c have granular feelings and are more fuzzy and unstable than the non-turbulent interference patterns of fig. 2a-2c, respectively, and these granular feelings represent noise information of the turbulent flow, and the more granular feelings indicate the stronger turbulent flow. Through the change of the turbulent refractive index, the optical path difference of the two light beams is changed, so that the width of the stripe is slightly changed. The strongly turbulent interference patterns of fig. 4 a-4 c are more grainy, the interference patterns are more blurred, and the illustrative turbulence is also stronger, compared to the less turbulent interference patterns of fig. 3 a-3 c, respectively.
Further, the S4 includes:
s41: respectively carrying out gray level image conversion on the turbulence-free interference pattern and the turbulence interference pattern to obtain a turbulence-free gray level image and a turbulence gray level image;
in the embodiment of the invention, the image obtained through simulation is a gray image, but the interference pattern obtained in an actual experiment is not a gray image, and the interference pattern needs to be converted into a gray image, because the interference pattern is subjected to fourier transform to obtain a spectrogram, the frequency of the spectrogram represents the intensity of gray change of the image, and the fourier transform is performed after the interference pattern is converted into the gray image, so that the gray change of the image can be observed more intuitively and effectively.
S42: respectively carrying out Fourier transform on the turbulence-free gray scale image and the turbulence gray scale image to obtain a turbulence-free frequency spectrum image and a turbulence frequency spectrum image;
s43: respectively carrying out gray level calculation on the turbulence-free frequency spectrogram and the turbulence frequency spectrogram to obtain a turbulence-free gray level average difference value and a turbulence gray level average difference value;
s44: and comparing the turbulence-free gray level average difference value with the turbulence gray level average difference value, and judging the turbulence intensity of the turbulence environment.
Further, the S42 includes:
s421: converting the image data formats of the turbulence-free gray scale map and the turbulence gray scale map into first double-precision floating point type data and second double-precision floating point type data respectively;
specifically, the image data of the turbulence-free gray scale map and the turbulence gray scale map are respectively normalized to be between [0,1 ].
S422: respectively performing two-dimensional discrete Fourier transform on the first double-precision floating point type data and the second double-precision floating point type data to obtain first two-dimensional discrete Fourier data and second two-dimensional discrete Fourier data;
s423: respectively performing fast Fourier transform on the first two-dimensional discrete Fourier data and the second two-dimensional discrete Fourier data to obtain first fast Fourier data and second fast Fourier data;
specifically, the fast fourier transform functions to perform quadrant conversion on the obtained data and move low-frequency components in the data to a spectrum data center.
S424: and respectively carrying out spectrum logarithmic transformation on the real parts of the first fast Fourier data and the second fast Fourier data so as to obtain a turbulence-free spectrogram and a turbulence spectrogram.
Further, the step S42 is followed by: and respectively carrying out filtering processing on the turbulence-free spectrogram and the turbulence spectrogram.
The filtering functions to remove some of the interfering noise, preserving the turbulence information.
Referring to fig. 5a-5c, fig. 5a-5c are spectrograms of fourier transforms of interference patterns, wherein fig. 5a-5c are spectrograms of fourier transforms of the turbulence-free interference patterns of fig. 2a-2c, respectively, as shown, fig. 5a has a distinct central principal maximum, two dark fringes, fig. 5b has three dark fringes, and fig. 5c has four dark fringes. The spectrogram reflects the degree of frequent change of the corresponding original image, the low-frequency signal represents the contour signal of the image with frequent change, the number of forks in the fork diagram of fig. 2a-2c is the topological charge number plus 1, therefore, the number of times of change of the spectrogram is the topological charge number plus 1, and the number of stripes in fig. 5a-5c corresponds to the topological charge number of 1, 2, and 3, respectively.
Referring to fig. 6a to 6c, fig. 6a to 6c are frequency spectrums obtained by performing fourier transform on the interference pattern and then performing filtering processing. Wherein fig. 6a is a spectrogram obtained by performing fourier transform re-filtering on the turbulence-free interference pattern of fig. 2c, fig. 6b is a spectrogram obtained by performing fourier transform re-filtering on the weak turbulence interference pattern of fig. 3c, and fig. 6c is a spectrogram obtained by performing fourier transform re-filtering on the strong turbulence interference pattern of fig. 4 c. As shown in the figure, the turbulence intensity changes according to the change of the background gray scale, the brighter the gray scale indicates the stronger the turbulence, and it can be seen from the figure that the gray scale of FIG. 6c is the brightest and the turbulence intensity is the strongest; FIG. 6b is a gray scale image having a lower turbulence intensity than that of FIG. 6 c; the grey scale of fig. 6a is darkest and its turbulence intensity is weakest.
Further, the S43 includes:
s431: removing low frequency regions in the turbulence free spectrogram and the turbulence spectrogram;
s432: solving a grey level histogram of the turbulence-free spectrogram and the turbulence spectrogram without the low-frequency region to obtain a turbulence-free grey level histogram and a turbulence grey level histogram;
s433: and respectively calculating the gray level average difference of the turbulence-free gray level histogram and the turbulence gray level histogram to obtain a turbulence-free gray level average difference value and a turbulence gray level average difference value.
Specifically, the turbulence-free spectrogram and the turbulence spectrogram without low-frequency components can be obtained through screenshot or computer program interception, and then the gray level histograms of the turbulence-free spectrogram and the turbulence-free spectrogram are respectively obtained through computer program interception, and the turbulence-free gray level average difference value and the turbulence gray level average difference value are respectively obtained through calculation.
Further, the S44 includes: when the turbulence gray level average difference value is lower than the turbulence-free gray level average difference value, turbulence exists, and the smaller the difference value between the turbulence-free gray level average difference value and the turbulence gray level average difference value is, the stronger the turbulence intensity based on the vortex light beams is.
In the embodiment of the present invention, gray level calculation is performed on the spectrograms of fig. 6a to 6c, and the gray level average difference values are 225, 223, and 211, respectively, fig. 6a is the spectrogram obtained by performing fourier transform re-filtering processing on the turbulence-free interference pattern of fig. 2c, and the gray level average difference value indicates that the background gray level of the vortex light beam in the still water environment is 225, that is, the turbulence-free gray level average difference value is not present; FIG. 6b is a spectrogram obtained by Fourier transform and re-filtering the weak turbulence interference pattern of FIG. 3c, wherein the average gray scale difference is the average gray scale difference of the weak turbulence; FIG. 6c is a frequency spectrum diagram obtained by Fourier transform and re-filtering the strong turbulence interference pattern of FIG. 4c, wherein the average gray scale difference is the average gray scale difference of the strong turbulence; turbulence is present when the grey value is below 225, the lower the grey value indicating greater turbulence. In a specific experimental process, the method can set a threshold value aiming at the actual environment, and when the gray mean difference value is lower than a certain value, the turbulent flow exists. And (3) performing reference sampling according to different experimental environments with different reference gray scales according to the step S1, taking a non-turbulent environment or an extremely-weak turbulent environment as the reference gray scale, and performing comparative analysis on subsequent results, namely when the gray scale average difference is lower than the reference gray scale average difference, indicating that the turbulent flow exists, and the lower the gray scale average difference is, indicating that the turbulent flow is stronger.
According to the method for judging the turbulence intensity based on the vortex light beams, the vortex light beams are adopted to detect the turbulence environment, the transmission distance is longer, the transmission efficiency is higher, the transmission is more stable, and the judgment on the turbulence intensity is more accurate; the method for judging the turbulence intensity is easy to judge and convenient to operate, and is suitable for various turbulence environments such as a marine turbulence environment or an atmospheric turbulence environment. When the intensity of turbulence is judged, the topological charge number of the vortex light beam can be accurately judged.
The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.
Claims (7)
1. A turbulence intensity determination method based on vortex beams is characterized by comprising the following steps:
s1: interfering the initial vortex light beam with the initial plane light beam to obtain a non-turbulent interference pattern;
s2: enabling the initial vortex light beam to enter a turbulent flow environment to obtain a turbulent flow vortex light beam;
s3: interfering the turbulent vortex light beam with the initial plane light beam to obtain a turbulent interference pattern carrying turbulent information;
s4: respectively calculating and comparing the turbulence-free interference pattern and the turbulence interference pattern to judge the turbulence intensity of the turbulence environment;
the S4 includes:
s41: respectively carrying out gray level image conversion on the turbulence-free interference pattern and the turbulence interference pattern to obtain a turbulence-free gray level image and a turbulence gray level image;
s42: respectively carrying out Fourier transform on the turbulence-free gray scale image and the turbulence gray scale image to obtain a turbulence-free frequency spectrum image and a turbulence frequency spectrum image;
s43: respectively carrying out gray level calculation on the turbulence-free frequency spectrogram and the turbulence frequency spectrogram to obtain a turbulence-free gray level average difference value and a turbulence gray level average difference value;
s44: and comparing the turbulence-free gray level average difference value with the turbulence gray level average difference value, and judging the turbulence intensity of the turbulence environment.
2. The method according to claim 1, further comprising, before the S1:
s0: acquiring the initial vortex beam and the initial plane beam.
3. The method according to claim 2, wherein the S0 includes: and generating the initial vortex light beam by using a spatial light modulator, and generating a Gaussian light beam by using a laser, wherein the beam waist of the Gaussian light beam can be approximately regarded as the initial plane light beam.
4. The method according to claim 1, wherein the S42 includes:
s421: converting the image data formats of the turbulence-free gray scale map and the turbulence gray scale map into first double-precision floating point type data and second double-precision floating point type data respectively;
s422: respectively performing two-dimensional discrete Fourier transform on the first double-precision floating point type data and the second double-precision floating point type data to obtain first two-dimensional discrete Fourier data and second two-dimensional discrete Fourier data;
s423: respectively performing fast Fourier transform on the first two-dimensional discrete Fourier data and the second two-dimensional discrete Fourier data to obtain first fast Fourier data and second fast Fourier data;
s424: and respectively carrying out spectrum logarithmic transformation on the real parts of the first fast Fourier data and the second fast Fourier data so as to obtain a turbulence-free spectrogram and a turbulence spectrogram.
5. The method according to claim 1, wherein the step S42 is followed by further comprising: and respectively carrying out filtering processing on the turbulence-free spectrogram and the turbulence spectrogram.
6. The method according to claim 1, wherein the S43 includes:
s431: removing low frequency regions in the turbulence free spectrogram and the turbulence spectrogram;
s432: solving a grey level histogram of the turbulence-free spectrogram and the turbulence spectrogram without the low-frequency region to obtain a turbulence-free grey level histogram and a turbulence grey level histogram;
s433: and respectively calculating the gray level average difference of the turbulence-free gray level histogram and the turbulence gray level histogram to obtain a turbulence-free gray level average difference value and a turbulence gray level average difference value.
7. The method according to claim 1, wherein the S44 includes:
when the turbulence gray level average difference value is lower than the turbulence-free gray level average difference value, turbulence exists, and the smaller the difference value between the turbulence-free gray level average difference value and the turbulence gray level average difference value is, the stronger the turbulence intensity based on the vortex light beams is.
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