Detection method of conjugate superposition vortex beam modal number based on image processing

Abstract

The invention belongs to the technical field of vortex beam modal detection, and particularly relates to a conjugate superposition vortex beam modal number detection method based on image processing. The method comprises the following steps: collecting a disturbed conjugate superposition vortex beam intensity image, firstly carrying out adaptive threshold binarization processing on a disturbed vortex beam intensity distribution diagram, and then replacing and eliminating a large-size background noise interference area by using morphological pixels; eliminating fine connected regions between adjacent modules by adopting a morphological corrosion algorithm of linear structure elements; carrying out median filtering processing on the image after morphological corrosion so as to eliminate residual discrete noise points; a module characteristic boundary is identified based on an edge detection algorithm, and quantitative detection of the modal number of the conjugate superposition vortex beam is realized by delineating an annular path. The conjugate superposition vortex beam modal number detection method provided by the invention has the advantages of atmospheric turbulence interference resistance, high processing efficiency and accurate identification.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of vortex beam mode detection, and particularly relates to a method for detecting the mode number of conjugate superposition vortex beams based on image processing. BACKGROUND

[0002] With the rapid breakthrough of optical communication technology, people's demand for high-speed and large-capacity data transmission is growing exponentially. Traditional optical communication systems mainly rely on limited degrees of freedom such as wavelength, amplitude, phase and time of light for information transmission, but these dimensions have gradually become difficult to cope with the explosive growth of data transmission demand. However, the optical orbital angular momentum (OAM) with a spatial spiral phase can be used as a new degree of freedom. Since different topological charge numbers of OAM beams are mutually orthogonal, the communication capacity can be effectively improved by OAM multiplexing or OAM keying. In addition, when two vortex beams with topological charge numbers of and respectively are coherently superimposed, the superposition state OAM mode formed presents a "petal-shaped" intensity distribution characteristic with rotational symmetry, and the petal number is. Therefore, the mode of the vortex beam after transmission can be detected according to this unique spatial intensity distribution characteristic.

[0003] However, in the vortex optical communication transmission system, atmospheric turbulence will inevitably be encountered, which will significantly affect the transmission performance of the vortex light beam carrying coded information. The turbulence-induced vortex light wavefront distortion not only destroys the inherent intensity distribution characteristics of the OAM mode, but also causes errors in mode detection, thereby significantly reducing the communication transmission performance of the system. Specifically, the conjugate superposition OAM beam sent by the sending end of the vortex optical communication system will be distorted in the transmission process due to turbulence disturbance, resulting in a decrease in beam mode purity and degradation of intensity distribution. These adverse factors seriously affect the accurate detection of the "petal-shaped" characteristics of the conjugate superposition OAM mode at the receiving end, and further cause the mode demodulation to fail and the transmission information to be misjudged.

[0004] Therefore, the present application aims to develop a robust detection method for the number of conjugate superposition OAM modes in an atmospheric turbulence transmission environment, so as to realize accurate identification and demodulation of the vortex beam mode number in a complex transmission channel. SUMMARY

[0005] The present application aims to solve the problem of insufficient detection capability of the number of conjugate superposition OAM modes in an atmospheric turbulence transmission environment, and provides a method for detecting the number of conjugate superposition vortex beam modes based on image processing.

[0006] To achieve the above technical purposes and effects, the present application is implemented by the following technical solutions:

[0007] The application provides a detection method of conjugate superposition vortex beam mode number based on image processing, comprising the following steps:

[0008] S1, using Otsu global threshold method to perform adaptive threshold binaryzation processing on the disturbed conjugate superposition OAM mode intensity distribution diagram, and then using a morphological pixel replacement method based on pixel area threshold to eliminate the background noise interference area; the morphological pixel replacement method calculates the pixel area of the noise interference area, replaces the interference area with an area exceeding a set threshold with a background pixel, so that the large-size noise interference is eliminated;

[0009] S2, using an image erosion algorithm to disconnect the fine connected areas between adjacent modules;

[0010] S3, after the morphological erosion processing, using a median filter of N*N pixel window to further remove the residual discrete noise points;

[0011] S4, using a pixel-by-pixel boundary scanning algorithm to identify the module feature boundary, the pixel-by-pixel boundary scanning algorithm locates the first boundary point with a gray value of 0 in each row / column as a contour feature point by traversing the image pixels row by row, calculates the path radius and the center coordinates of the extracted contour extreme points, and realizes the quantitative detection of the conjugate superposition OAM mode number.

[0012] Further, in step S1, the morphological pixel replacement method used realizes the effect of eliminating the large-size background noise interference area, which is equivalent to processing large-scale noise for unconnected pixel areas; this step is particularly suitable for the case of large noise scale, and for the case that can be processed by linear erosion, the step can be selectively executed.

[0013] Further, in step S2, a detection template is used to scan the image, the detection template is a linear template with a length of N pixels and a direction of θ°; only when the detection template completely covers the target area, the center pixel is retained, the unnecessary small connection is accurately cut off, and the main structure is retained.

[0014] Further, the detection template refers to a pixel area in the conjugate superposition OAM intensity distribution image which can represent the "petal" shaped intensity distribution feature.

[0015] Further, in step S2, the morphological erosion algorithm of the linear structure element eliminates the fine connected areas between adjacent modules, and realizes the effect of disconnecting the connection and retaining the main structure equivalent to the morphological opening operation.

[0016] Further, in step S4, the identification of the module feature boundary based on the edge detection algorithm refers to the first pixel point with a logical value of "1" around the processed superposition OAM mode petal-shaped pattern.

[0017] Further, in step S4, the calculation of the center and radius of the annular detection path according to the annular path is to form a circle with the first pixel point with a logical value of "1" around the image after median filtering, and calculate the radius and center of the circle.

[0018] Further, in step S4, the detection of the mode number is calculated by detecting the extreme pixel points of the annular path intensity distribution curve. Specifically, by counting the number of peaks of the intensity distribution curve, the conjugate superposition OAM mode is detected according to the number of peaks equal to twice the absolute value of the topological charge l of the conjugate superposition OAM mode (i.e. l = 2).

[0019] The beneficial effects of the present application are:

[0020] The present application provides a method for detecting the mode number of conjugate superposition vortex beams based on image processing, which adopts an optimized combination of image processing techniques such as image binarization, pixel replacement, linear erosion and median filtering, and realizes robust detection of the conjugate superposition OAM mode number through the designed annular detection path, which is still effective even in the environment of atmospheric turbulence transmission, providing a new technical solution for reliable detection of optical communication systems, and significantly improving the anti-interference ability of vortex optical communication systems in complex transmission environment. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0022] Figure 1 The flowchart of the detection method of the present application is shown in the figure;

[0023] Figure 2 The intensity diagram of the conjugate superposition vortex beam with topological charge number l = ±2 and ±4 is shown in the figure;

[0024] Figure 3 The working principle flowchart of the interference elimination method is shown in the figure;

[0025] Figure 4 The detection principle flowchart of the pixel area which can represent the "petal" shaped intensity distribution characteristics is shown in the figure;

[0026] Figure 5 The demonstration diagram of the method for detecting the mode number of the conjugate superposition vortex beam is shown in the figure;

[0027] Figure 6The experimental results of mode number identification and detection in the transmission process of the conjugate superimposed OAM mode with a topological charge number of n and m for the method;

[0028] Figure 7 The front interface schematic diagram of the detection system application program developed based on MATLAB App Designer in the application is shown. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the application will be apparently and completely described below with the drawings in the embodiments of the application, obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor belong to the protection scope of the application.

[0030] As Figure 1 shown, the embodiment provides a detection method of conjugate superimposed vortex beam mode number based on image processing, comprising the following steps:

[0031] S1, adaptive threshold binarization processing is performed on the disturbed conjugate superimposed OAM mode intensity distribution diagram, and a morphological pixel replacement method is used to eliminate the large-size background noise interference area;

[0032] Specifically, the effect of eliminating the large-size background noise interference area by the morphological pixel replacement method is equivalent to the effect of processing large-scale noise for unconnected pixel areas. This step is particularly suitable for the case of large noise scale, and can be selectively executed for the case of linear erosion processing.

[0033] S2, a morphological erosion algorithm of a linear structure element is used to eliminate the fine connected areas between adjacent modules;

[0034] Specifically, the module refers to a pixel area in the conjugate superimposed OAM intensity distribution image which can represent the "petal" shaped intensity distribution characteristics.

[0035] Specifically, the morphological erosion algorithm of the linear structure element eliminates the fine connected areas between adjacent modules, and realizes the effect of disconnecting and retaining the main structure equivalent to the morphological opening operation.

[0036] S3, median filtering processing is performed on the image after morphological erosion, so as to eliminate residual discrete noise points;

[0037] S4, an edge detection algorithm is used to identify the module feature boundary, and through circumscribing the annular path, the quantitative detection of the conjugate superimposed OAM mode number is realized.

[0038] Specifically, the feature boundary of the edge detection algorithm recognition module refers to the first pixel point with a logical value of "1" around the petal-shaped pattern of the superimposed OAM mode after the recognition processing.

[0039] Specifically, in step 917, the calculation of the center and radius of the annular detection path according to the annular path refers to a circle formed by the first pixel point with a logical value of "1" around the image after the median filtering (up, down, left and right), and the radius and center of the circle (annular path) are calculated.

[0040] Specifically, the detection of the mode number is calculated by detecting the extreme pixel points of the annular path intensity distribution curve, and specifically, the number of peaks of the intensity distribution curve is counted, and the conjugate superimposed OAM mode is detected according to the fact that the number of peaks is equal to twice the absolute value of the topological charge l of the conjugate superimposed OAM mode (i.e. ±l).

[0041] In combination Figure 2 , the vortex light original intensity pattern provided by the embodiment of the present application is a "petal" pattern with a topological charge number l of ±2 and ±4, and the number of "petal" modules (pixel regions in the image that can represent the "petal" intensity distribution characteristics) of different topological charge numbers is different.

[0042] The feature of the pattern with different topological charge numbers is that the number of "petal" modules increases with the increase of the topological charge number l, and the number of "petal" modules is equal to twice the absolute value of the topological charge l of the conjugate superimposed OAM mode (i.e. ±l).

[0043] As Figure 5 shown, the conjugate superimposed OAM beam emitted by the vortex light communication system sending end will be distorted due to turbulence disturbance during transmission, resulting in a decrease in beam mode purity and degradation of intensity distribution. After the processing of the present method, the turbulence-induced vortex light wavefront distortion is effectively suppressed, and the accurate detection of the conjugate superimposed OAM mode "petal" feature is realized.

[0044] As Figure 6 shown, the conjugate superimposed OAM mode with a topological charge number l of ±2 and ±4 is detected by using the method in the present application.

[0045] Figure 6 The peak characteristics of the intensity distribution curve shown in the figure are that the number of peaks of the intensity distribution curve is counted, and the conjugate superimposed OAM mode is detected according to the fact that the number of peaks is equal to twice the absolute value of the topological charge l of the conjugate superimposed OAM mode (i.e. ±l). The present method provides an intuitive and reliable experimental criterion for OAM mode detection.

[0046] AsFigure 7 As shown, the front interface of the detection system APP made in the method is displayed. When using the APP, each button in the interface is clicked in turn according to the detection sequence (i.e. linear corrosion, median filtering, and circle ring delineation) described above, and the final detection result is displayed in the last dialogue box in the interface.

[0047] The method of the application adopts an optimized combination of image binarization, pixel replacement, linear corrosion, median filtering and other image processing technologies, and realizes robust detection of the number of conjugate superimposed OAM modes through a designed ring-shaped detection path delineation method, which is still effective even in an atmospheric turbulence transmission environment, provides a new technical solution for reliable detection of an optical communication system, and can significantly improve the anti-interference capability of a vortex optical communication system in coping with a complex transmission environment.

[0048] The preferred embodiments of the application disclosed above are only used to help explain the application. The preferred embodiments do not describe all the details and limit the application to the specific embodiments. Obviously, many modifications and variations can be made according to the content of the specification. The specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the application, so that those skilled in the art can well understand and utilize the application. The application is limited only by the claims and their full scope and equivalents.

Claims

1. A method for detecting the mode number of a conjugate superposition vortex beam based on image processing, characterized in that: The following steps are involved: S1. Adaptive threshold binarization is performed on the disturbed conjugate superposition OAM mode intensity distribution map using the Otsu global threshold method, and then a morphological pixel replacement method based on pixel area threshold is used to eliminate the background noise interference area. The morphological pixel replacement method calculates the pixel area of ​​the noise interference area and replaces the interference area with an area exceeding a set threshold with background pixels, thereby eliminating large-scale noise interference. S2, using image erosion algorithm to disconnect the subtle connected areas between adjacent modules; S3, after the morphological corrosion process, the median filter of the N×N pixel window is used to further remove the residual discrete noise points; S4. Use a pixel-by-pixel boundary scanning algorithm to identify the module feature boundary. The pixel-by-pixel boundary scanning algorithm traverses the image pixels row by row, locates the first boundary point with a grayscale value of 0 in each row / column as the contour feature point, and calculates the path radius and circle center coordinates through the extracted contour extreme points to achieve quantitative detection of the conjugate superposition OAM mode number.

2. The method for detecting the mode number of a conjugate superposition vortex beam based on image processing according to claim 1, characterized in that: In step S1, the morphological pixel replacement method used to eliminate large-scale background noise interference areas is equivalent to the effect of processing large-scale noise in disconnected pixel areas; this step is particularly suitable for situations where the noise scale is large, and can be selectively performed for situations where linear corrosion processing is possible.

3. The method for detecting the mode number of a conjugate superposition vortex beam based on image processing according to claim 1, characterized in that: In step S2, the image is scanned using a detection template, which is a linear template with a length of N pixels and a θ° direction. The center pixel is retained only when the detection template completely covers the target area, thereby accurately cutting off unnecessary small connections while retaining the main structure.

4. The method for detecting the mode number of a conjugate superposition vortex beam based on image processing according to claim 3, characterized in that: The detection template refers to the pixel area in the conjugate superposition OAM intensity distribution image that can represent the "petal"-like intensity distribution characteristics.

5. The method for detecting the mode number of a conjugate superposition vortex beam based on image processing according to claim 1, characterized in that: In step S2, the morphological erosion algorithm of the linear structural element eliminates the subtle connected areas between adjacent modules, achieving the effect equivalent to the morphological opening operation of disconnecting and retaining the main structure.

6. The method for detecting the mode number of a conjugate superposition vortex beam based on image processing according to claim 1, characterized in that: In step S4, the module feature boundary is identified based on the edge detection algorithm, which refers to the first pixel point with a logic value of "1" around the petal-shaped pattern of the superimposed OAM mode after the identification process.

7. The method for detecting the mode number of a conjugate superposition vortex beam based on image processing according to claim 1, characterized in that: In step S4, the center and radius of the circular detection path are calculated based on the circled circular path, which refers to the circle formed by the first pixel points with a logic value of "1" around the image after median filtering, and the radius and center of the circle are calculated.

8. The method for detecting the mode number of a conjugate superposition vortex beam based on image processing according to claim 1, characterized in that: In step S4, the mode number is detected by calculating the extreme pixel points of the intensity distribution curve of the circled annular path.

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