A magneto-optical imaging system based on line-field fourier transform and imaging method thereof

Abstract

This invention discloses a magneto-optical imaging system and method based on line-field Fourier transform, relating to optical instruments. The magneto-optical instrument utilizes a beam shaping module to output a linear flat-top beam with a high polarization ratio and low coherence, which then enters the probe perpendicularly after passing through a depolarizing beam splitter. The probe includes an achromatic lens and an optically active crystal, achieving magnetic detection based on the Faraday effect. Utilizing the multi-layer reflection of the crystal, an optical path difference is formed between the reflected beams. After reflection by the optically active crystal, the linear beam returns along its original optical path, and upon reaching the depolarizing beam splitter, its optical path direction is reversed. Subsequently, a confocal system filters out stray light, and finally, a line-field optical Fourier transform system achieves interference of the linear beam in the spatial frequency domain. A camera then captures the line-field interference fringe image. Ultimately, the magneto-optical instrument and imaging method described in this invention can achieve high-sensitivity magneto-optical imaging with a spatial resolution of m*n within 1 second.

Description

Technical Field

[0001] This invention relates to optical instruments, and more particularly to magneto-optical instruments for measuring magnetic variables. Background Technology

[0002] In industries such as energy and chemical engineering, aerospace, transportation, high-end manufacturing and special equipment, electronics and medical devices, and infrastructure and construction, electromagnetic non-destructive testing (EMD) of metal and composite material components can promptly detect surface and internal damage. EMD of electronic equipment at the circuit board and module levels enables fault analysis and location. Therefore, EMD is of great significance for preventing catastrophic accidents, protecting people's lives and property, and supporting the safe and stable operation of the economy and society. Magneto-optical imaging (MEI), as an electromagnetic field detection instrument in EMD, has the advantages of high spatial resolution and large imaging area. Traditional MEI, based on the Faraday effect, uses a camera to directly capture the light intensity distribution characteristics caused by the magnetic field in the test area, providing rapid imaging. However, this technology has very limited magnetic field sensitivity, which is not conducive to detecting small and deep damage in structural components or monitoring weak currents in electronic equipment. To improve magnetic field sensitivity, many researchers have modified MEI instruments to use a single-point detection method, facilitating the design of differential optical path schemes and the modulation and demodulation of single-point magnetic signals to achieve noise suppression and signal enhancement. However, single-point magneto-optical detection technology is extremely inefficient in achieving magnetic imaging, making it difficult to implement in engineering applications. Firstly, single-point magneto-optical detection requires integration with a mechanical scanning platform, necessitating mechanical scanning of each pixel. Secondly, modulation and demodulation of the single-point magnetic signal require a significant amount of sampling time, and the longer the sampling time, the greater the improvement in magnetic sensitivity. Therefore, electromagnetic nondestructive testing requires a new magneto-optical instrument and imaging method to achieve efficient and highly sensitive magneto-optical imaging, facilitating engineering applications. Summary of the Invention

[0003] To address the issues of high efficiency but low sensitivity and high magnetic sensitivity but low efficiency in existing magneto-optical imaging technologies, this invention designs a magneto-optical instrument and imaging method based on line-field Fourier transform. The system and method utilize a linear light source with low coherence to induce self-coherence at an optically active crystal, outputting a line-field interference fringe image through line-field Fourier transform. One linear dimension of constructive magneto-optical interference signal can be extracted from a single frame image, achieving both high efficiency and high sensitivity. Imaging in another dimension, combined with a scanning platform and continuous camera shooting, ultimately achieves highly efficient and highly sensitive magneto-optical imaging.

[0004] The technical solution adopted by this invention to solve the technical problem is:

[0005] This invention utilizes a linear light source with low coherence and a high polarization ratio incident on an optically active crystal. After total internal reflection from different reflective layers of the optically active crystal, the linear light source undergoes self-coherence. Following a line-field Fourier transform, it outputs a line-field interference fringe image, containing a one-dimensional linear interference constructive magneto-optical signal. By extracting the constructive magneto-optical signals from multiple line-field interference fringe images, efficient and high-sensitivity magneto-optical imaging can be achieved simultaneously. The specific technical solution is as follows:

[0006] A magneto-optical imaging system based on line field Fourier transform, comprising: a light source, polarization device, achromatic lens, flat-top light shaping module, square pinhole, cylindrical prism, polarizing beam splitter, optical rotation crystal, displacement platform, slit pinhole, mirror, grating, and camera;

[0007] The laser emitted from the light source sequentially passes through a beam shaping module consisting of a first polarizing device, a first achromatic lens, a flat-top beam shaping module, a square aperture, and a cylindrical prism. The light source's wavelength bandwidth ensures that interference fringes appear in the output light even when there is an optical path difference. The polarizing device further improves the polarization ratio of the light source. The achromatic lens collimates the point light source into a circular surface light source. The circular surface light source is modified into a square surface light source by passing through the square aperture, facilitating subsequent focusing by the cylindrical prism into a linear light source with a more uniform power distribution. A flat-top beam shaping module between the achromatic lens and the square aperture converts the Gaussian beam into a flat-top beam, enabling subsequent acquisition of a linear light source with a more uniform power distribution. For example, a Galilean telescope-style beam shaping scheme utilizes the aberrations of a pair of aspherical lenses to alter the energy distribution of the Gaussian beam. Furthermore, the flat-top beam shaping module may also utilize technologies such as flexible fiber bundle arrangement, segmented prisms, and microlens arrays.

[0008] The low-coherence, high-polarization-ratio linear flat-top beam output from the beam shaping module is focused at the center of the polarizing beam splitter. Then, the focusing dimension is changed by the second achromatic lens and focused onto the optical rotator crystal, which has a total reflection layer on its bottom surface. The linear beam is reflected by different reflective layers of the optical rotator crystal and returns along the original path to the center of the polarizing beam splitter prism, where it is reflected to the subsequent optical path. The subsequent optical path passes through the second polarizing device, the third achromatic lens, the slit aperture, and the fourth achromatic lens in sequence. After the angle of the incident grating is adjusted by the mirror, it continues to pass through the fifth achromatic lens, outputting line field interference fringes to the camera.

[0009] The angle between the polarization axes of the first polarization device and the second polarization device constitutes the polarization-detection angle θ.

[0010] The sequential combination of the third achromatic lens, the slit aperture, and the fourth achromatic lens constitutes a confocal system to filter out stray light; at the same time, the confocal system is considered a 4f- system, which also has the functions of image plane scaling and relay.

[0011] The sequential combination of the fourth achromatic lens, the mirror, the grating, and the fifth achromatic lens constitutes a linear field Fourier transform optical system, which can expand the focused linear beam along the spatial frequency domain to form linear field interference fringes. At the same time, the focusing direction of the linear beam at the grating should be opposite to the focusing direction of the linear beam on the optically active crystal and perpendicular to the grating scribe line direction. Furthermore, the linear field Fourier transform optical system is considered a 4f- system, which also has the functions of image plane scaling and relay.

[0012] The calculation method for each interference fringe in the line field interference fringes is as follows:

[0013] I(k) = (1);

[0014] (2);

[0015] Where I(k) is the output light intensity of beams of different wavelengths, S(k) is the light intensity of beams of different wavelengths before they enter the optically rotating crystal, and R... G R is the reflectivity of the surface of an optically active crystal. D It is the reflectivity of the bottom surface of the optically active crystal. The polarization angle is denoted by V, the Wild coefficient of the optically active crystal is V, the magnetic field to be measured is B, the thickness of the optically active crystal is L, and the wavelength of the light beam is k. It is the optical path difference between different reflective layers of the optically active crystal;

[0016] The single-point output response calculation method is as follows:

[0017] I= (3);

[0018] in I is the light intensity before the beam enters the optically rotating crystal, and I is the output light intensity.

[0019] An imaging method for a magneto-optical imaging system based on line-field Fourier transform, such as... Figure 2 As shown, the method includes:

[0020] Step A1: Convert a line field interference fringe image into a two-dimensional matrix, which contains m rows or m columns of interference fringe information; each row or column of interference fringe contains single-point magnetic field information;

[0021] Step A2: Draw m interference lines, with the horizontal axis representing the spatial frequency domain and the vertical axis representing the light intensity grayscale value. Perform preprocessing on each line to suppress noise.

[0022] Step A3: Take the upper envelope of each of the preprocessed interference fringes, and finally integrate the upper envelope with respect to the spatial frequency domain to obtain m constructive magneto-optical signals in the P dimension. The single-point constructive magneto-optical signal is represented as:

[0023] I= (4);

[0024] Where S is the positive correlation factor.

[0025] Compare formula (4) and formula (3). The value is limited by the exposure level of the single-point magneto-optical signal in the camera, while the S value is the integral value of the exposure levels of multiple constructive magneto-optical signals in the camera; therefore, S > ;at the same time, Therefore, the sensitivity of the magneto-optical instrument of this invention is much greater than that of traditional non-interference magneto-optical imaging technology.

[0026] Furthermore, the optically active crystal is placed on a displacement platform, and the camera continuously captures images while the platform moves rapidly, outputting n line field interference fringe images. The magneto-optical imaging processing method is as follows: Figure 3 As shown:

[0027] Step B1: Perform line field interference fringe image processing on n line field interference fringe images to obtain n line field interference constructive magneto-optical signals;

[0028] Step B2: Take one of the n line field interference constructive magneto-optical signals as the initial signal base value, and subtract the initial signal base value from the n line field interference constructive magneto-optical signals to obtain the initial relative magneto-optical imaging map;

[0029] Step B3: Perform image filtering and enhancement on the initial relative magneto-optical image to obtain the final relative magneto-optical image;

[0030] The imaging time for a relative magneto-optical image depends on the camera's frame rate (FPS), spatial resolution, and the target's magneto-optical imaging spatial resolution. For an imaging spatial resolution of n×m, the required time is n / FPS seconds. For example, with a camera frame rate of 1884 and a spatial resolution of 1216×2043, the magneto-optical instrument and imaging method described in this invention can achieve a relative magneto-optical imaging spatial resolution of 1884×2043 within 1 second, meeting the requirements of high efficiency, high spatial resolution, and high sensitivity in non-destructive testing.

[0031] Furthermore, the preprocessing methods in step A2 include mean filtering, Gaussian filtering, and median filtering.

[0032] Furthermore, the methods for obtaining the upper envelope in step A3 include, but are not limited to, Hilbert transform, extreme point interpolation, sliding window smoothing, and adaptive algorithms.

[0033] Furthermore, the image filtering and enhancement techniques in step B3 include smoothing, noise reduction, sharpening, edge correction, contrast adjustment, complexity enhancement, generation, and repair.

[0034] (I) Beneficial Effects

[0035] The beneficial effects of this invention are that it simultaneously possesses the advantages of high sensitivity and high efficiency in magnetic field imaging. Compared with traditional non-interferometric magneto-optical imaging techniques, this invention, based on the Faraday effect, utilizes the self-coherence phenomenon between different reflective layers of an optically active crystal to extract constructive magneto-optical signals, achieving signal gain without sacrificing sampling time. Simultaneously, this invention filters out stray light from the optical path through a confocal system and suppresses signal noise through image processing, effectively improving the signal-to-noise ratio. Furthermore, this invention obtains a linear light source through a beam-shaping module and establishes a line-field Fourier transform optical system. A single frame image can achieve high spatial resolution imaging of a linear magnetic field, while multiple frames can achieve two-dimensional magnetic field imaging. Attached Figure Description

[0036] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0037] Figure 1 This is a diagram of the magneto-optical imaging instrument based on line field Fourier transform according to the present invention.

[0038] In the diagram, 1. Light source, 2. First polarizing device, 3. First achromatic lens, 4. Flat-top light shaping module, 5. Square pinhole, 6. Cylindrical prism, 7. Polarizing beam splitter, 8. Second achromatic lens, 9. Optical rotator crystal, 10. Displacement platform, 11. Second polarizing device, 12. Third achromatic lens, 13. Slit pinhole, 14. Fourth achromatic lens, 15. Mirror, 16. Grating, 17. Fifth achromatic lens, 18. Camera.

[0039] Figure 2 The following is a flowchart of the line field interference fringe image processing method of the present invention: (a) is a line field interference fringe image, (b) is an image of m interference fringes, and (c) is an image of m interference constructive magneto-optical signals.

[0040] Figure 3 The following is a process diagram of the magneto-optical imaging processing method of the present invention: (a) is an image of n line field interference fringes, (b) is an image of n constructive magneto-optical interference signals, (c) is an initial relative magneto-optical imaging image, and (d) is a relative magneto-optical imaging image.

[0041] Figure 4 This is a schematic diagram of the line field interference fringe image processing flow of the present invention.

[0042] Figure 5 This is a schematic diagram of the magneto-optical imaging processing flow for invention. Detailed Implementation

[0043] Figure 1 The diagram shows a magneto-optical imaging instrument based on line field Fourier transform, including a light source (1), a polarizing device (2), an achromatic lens (3), a flat-top light shaping module (4), a square aperture (5), a cylindrical prism (6), a polarizing beam splitter (7), an achromatic lens (8), an optical rotator (9), a displacement platform (10), a polarizing device (11), an achromatic lens (12), a slit aperture (13), an achromatic lens (14), a mirror (15), a grating (16), an achromatic lens (17), and a camera (18).

[0044] The purpose of combining the light source (1), polarizing device (2), achromatic lens (3), a pair of aspherical lenses (4), a small square aperture (5), and cylindrical prism (6) is to form a linear light source with low coherence and a high polarization ratio. Specifically: the light source (1) outputs low-coherence light with a certain bandwidth; the polarizing device (2) further improves the polarization ratio of the low-coherence light; the achromatic lens (3), the small square aperture (5), and the cylindrical prism (6) shape the linear beam. The shaping steps are as follows:

[0045] Step 1: An achromatic lens (3) is used to collimate a point light source into a circular surface light source.

[0046] Step 2: The circular surface light source is modified into a square surface light source by passing through the square hole (5).

[0047] Step 3: The cylindrical prism (6) focuses the square surface light source in a single direction (the same as the side length direction of the square surface light source) and outputs a linear light source at the focal length.

[0048] Furthermore, a flat-top beam shaping module (4) can be added between the achromatic lens (3) and the square aperture (5) to achieve the conversion of the Gaussian beam to a flat-top beam, which facilitates the subsequent output of a linear light source with uniform power distribution. For example, the aberrations of a pair of aspherical lenses can be used to change the energy distribution of the Gaussian beam. In addition, the flat-top beam shaping module also includes, but is not limited to, technologies such as flexible arrangement of fiber bundles, segmented prisms, and microlens arrays.

[0049] The cylindrical prism (6) and the achromatic lens (8) constitute a 4f-system. The selection of the four focal lengths in this 4f-system enables scaling and relaying of the linear light source. The purpose of relaying is to enable the linear beam to be focused successively onto the polarizing beam splitter (7) and the optical rotator (9). The linear light source focused on the optical rotator (9) is the image plane.

[0050] The bottom surface of the optical rotator crystal (9) has a total reflection layer. The image plane is reflected by different reflection layers of the optical rotator crystal (9), and returns along the original path to the center of the polarizing beam splitter (7), and is then reflected to the polarizing device (11). The angle between the polarizing axes of the polarizing device (2) and the polarizing device (11) forms the polarization-analyzing angle θ.

[0051] Subsequently, the achromatic lens (8) and the achromatic lens (12) constitute a 4f-system. The selection of the four focal lengths in this 4f-system enables the scaling and relaying of the image plane. The relaying here is to transfer the image plane to the slit aperture (13). The sequential combination of the achromatic lens (12), the slit aperture (13), and the achromatic lens (14) constitutes a confocal system that enables the filtering of stray light.

[0052] Finally, the sequential combination of the achromatic lens (14), mirror (15), grating (16), and achromatic lens (17) constitutes a linear field Fourier transform optical system, which can expand the focused linear beam into linear field interference fringes in another dimension along the spatial frequency domain. Simultaneously, the focusing direction of the linear beam at the grating (15) should be opposite to the focusing direction of the linear beam on the optically active crystal (9), and perpendicular to the grating (16) scribe line direction. Meanwhile, the achromatic lens (14) and achromatic lens (17) also constitute a 4f-system. This 4f-system scales and relays the image plane after the linear field Fourier transform to the camera (18). The camera (18) outputs a linear field interference fringe image.

[0053] Methods and procedures for processing line field interference fringe images are as follows: Figure 2 As shown.

[0054] Step 1: Convert a line field interference fringe image into a two-dimensional matrix. The two-dimensional matrix contains m rows (or m columns) of interference fringe information. Each row (or each column) of interference fringe contains single-point magnetic field information.

[0055] Step 2: Draw m interference lines, with the horizontal axis representing the spatial frequency domain and the vertical axis representing the light intensity grayscale value. Perform preprocessing on each line to suppress noise. Preprocessing methods include, but are not limited to, mean filtering, Gaussian filtering, and median filtering.

[0056] Step 3: Extract the upper envelope of each of the preprocessed interference fringes. Methods for extracting the upper envelope include, but are not limited to, Hilbert transform, extremum interpolation, sliding window smoothing, and adaptive algorithms. Finally, integrate the upper envelope over the spatial frequency domain to obtain m constructive magneto-optical signals in the P-dimensional region.

[0057] by Figure 2 (a) shows a single line-field interference fringe image as an example. m interference fringes can be drawn along the direction of position P. Then, each interference fringe is preprocessed, its upper envelope is taken, and the upper envelope is integrated, as shown below. Figure 2As shown in (b). Finally, the upper envelope integral values ​​of the m interference fringes are plotted to obtain a linear field interference constructive magneto-optical signal, as shown in [example]. Figure 2 As shown in (c).

[0058] To further enhance magneto-optical imaging, a displacement platform (10) is introduced into the magneto-optical instrument. While the displacement platform (10) moves rapidly, the camera (18) continuously captures images, outputting n line-field interference fringe images. A schematic diagram of the magneto-optical imaging processing method and process of this invention is shown below. Figure 3 As shown:

[0059] Step 1: Perform line field interference fringe image processing on n line field interference fringe images to obtain n line field interference constructive magneto-optical signals.

[0060] Step 2: Take one of the n line-field interference constructive magneto-optical signals as the initial signal base value. Subtract the initial signal base value from the n line-field interference constructive magneto-optical signals to obtain the initial relative magneto-optical image.

[0061] Step 3: Perform image filtering and enhancement on the initial relative magneto-optical image to obtain the final relative magneto-optical image. Image filtering and enhancement techniques include, but are not limited to, smoothing, denoising, sharpening, edge correction, contrast adjustment, complexity enhancement, generation, and repair.

[0062] by Figure 3 (a) shows an example of n line-field interference fringe images. The n line-field interference fringe images correspond to the imaging region position in the F direction. According to the above-mentioned line-field interference fringe image processing method and process, n line-field interference constructive magneto-optical signals can be plotted from the n line-field interference fringe images, such as... Figure 3 As shown in (b). Subtracting the initial signal base value (one of the line field interference constructive magneto-optical signals) from the n line field interference constructive magneto-optical signals yields the initial relative magneto-optical image, as shown in [example image]. Figure 3 As shown in (c), the initial relative magneto-optical image is filtered and enhanced to obtain the final relative magneto-optical image, as shown in (c). Figure 3 As shown in (d).

[0063] The imaging time for a relative magneto-optical image depends on the camera's frame rate (FPS), spatial resolution, and the target's magneto-optical imaging spatial resolution. For an imaging spatial resolution of n×m, the required time is n / FPS seconds. For example, with a camera frame rate of 1884 and a spatial resolution of 1216×2043, the magneto-optical instrument and imaging method described in this invention can achieve a relative magneto-optical imaging spatial resolution of 1884×2043 within 1 second.

Claims

1. A magneto-optical imaging system based on line-field Fourier transform, the system comprising: It consists of a light source, polarizing device, achromatic lens, flat-top light shaping module, square pinhole, cylindrical prism, polarizing beam splitter, optical rotator crystal, displacement platform, slit pinhole, reflector, grating, and camera; The laser emitted by the light source passes sequentially through a beam shaping module consisting of a first polarizing device, a first achromatic lens, a flat-top light shaping module, a square aperture, and a cylindrical prism; the light source has a wide wavelength bandwidth to ensure that interference fringes appear in the output light when there is an optical path difference. The low-coherence, high-polarization-ratio linear flat-top beam output from the beam shaping module is focused at the center of the polarizing beam splitter. Then, the focusing dimension is changed by the second achromatic lens and focused onto the optical rotator crystal, which has a total reflection layer on its bottom surface. The linear beam is reflected by different reflective layers of the optical rotator crystal and returns along the original path to the center of the polarizing beam splitter prism, where it is reflected to the subsequent optical path. The subsequent optical path passes through the second polarizing device, the third achromatic lens, the slit aperture, and the fourth achromatic lens in sequence. After the angle of the incident grating is adjusted by the mirror, it continues to pass through the fifth achromatic lens, outputting line field interference fringes to the camera.

2. The angle between the polarization axes of the first polarization device and the second polarization device constitutes the polarization-detection angle θ; The method for calculating each interference fringe in the line field interference fringes of this system is as follows: I(k)= (1); (2)； in, I(k) is the output light intensity of beams of different wavelengths, S(k) is the light intensity of beams of different wavelengths before they enter the optically rotating crystal, and R G R is the reflectivity of the surface of an optically active crystal. D It is the reflectivity of the bottom surface of the optically active crystal. The polarization angle is denoted by V, the Wild coefficient of the optically active crystal is V, the magnetic field to be measured is B, the thickness of the optically active crystal is L, and the wavelength of the light beam is k. It is the optical path difference between different reflective layers of the optically active crystal; The single-point output response calculation method is as follows: I= (3)； in I is the light intensity before the beam enters the optically rotating crystal, and I is the output light intensity.

3. An imaging method using the magneto-optical imaging system based on line field Fourier transform as described in claim 1, the method comprising: Step A1: Convert a line field interference fringe image into a two-dimensional matrix, which contains m rows or m columns of interference fringe information; each row or column of interference fringe contains single-point magnetic field information; Step A2: Draw m interference lines, with the horizontal axis representing the spatial frequency domain and the vertical axis representing the light intensity grayscale value. Perform preprocessing on each line to suppress noise. Step A3: Take the upper envelope of each of the preprocessed interference fringes, and finally integrate the upper envelope with respect to the spatial frequency domain to obtain m constructive magneto-optical signals in the P dimension. The single-point constructive magneto-optical signal is represented as: I= (4)； Where S is the positive correlation factor.

4. An imaging method using the magneto-optical imaging system based on line field Fourier transform as described in claim 1, wherein the method first places the optically rotating crystal on a displacement platform, and while the displacement platform moves rapidly, the camera continuously takes pictures, outputting n line field interference fringe images. The magneto-optical imaging processing method is as follows: Step B1: Perform line field interference fringe image processing on n line field interference fringe images to obtain n line field interference constructive magneto-optical signals; Step B2: Take one of the n line field interference constructive magneto-optical signals as the initial signal base value, and subtract the initial signal base value from the n line field interference constructive magneto-optical signals to obtain the initial relative magneto-optical imaging map; Step B3: Perform image filtering and enhancement on the initial relative magneto-optical image to obtain the final relative magneto-optical image.

5. The imaging method as described in claim 2, characterized in that, The preprocessing methods in step A2 include mean filtering, Gaussian filtering, and median filtering.

6. The imaging method as described in claim 2, characterized in that, The methods for obtaining the upper envelope in step A3 include, but are not limited to, Hilbert transform, extreme point interpolation, sliding window smoothing, and adaptive algorithms.

7. The imaging method as described in claim 3, characterized in that, The image filtering and enhancement techniques in step B3 include smoothing, denoising, sharpening, edge correction, contrast adjustment, complexity enhancement, generation, and repair.

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