A device and method for detecting optical fiber end face dimensions based on Fourier stack imaging

Abstract

The present invention discloses a device and method for detecting the size of an optical fiber end face based on Fourier stack imaging. The device comprises an optical fiber (1), a light source modulator (2), an LED light source array (3), an objective lens (4), a lens (5), a CCD camera (6), and a host computer (7). The host computer is used to execute a process for detecting the size of an optical fiber end face based on Fourier stack imaging: obtaining a large-field-of-view, high-resolution image of an optical fiber digital end face based on Fourier stack imaging; processing the large-field-of-view, high-resolution image of an optical fiber digital end face based on Fourier stack imaging to obtain an image area of ​​the optical fiber end face; and then calculating the actual geometric size of the optical fiber end face based on the image area of ​​the optical fiber end face, thereby obtaining a detection result of the optical fiber end face diameter. Compared with the prior art, the present invention can achieve high-precision measurement of optical fiber end face geometric parameters while taking into account both a large field of view and high resolution without changing the hardware structure of the imaging system.

Description

Technical Field

[0001] The present invention relates to the fields of optical fiber end face digital imaging technology, optical measurement and optical imaging technology, and in particular to a device and method for detecting the geometric dimensions of an optical fiber end face. Background Art

[0002] Optical fiber technology is the core carrier of modern information transmission. Its high bandwidth, low loss, compact size, and mass production make it widely used in communications, sensing, and lasers. Optical fiber geometric parameters include the diameters of the fiber core, cladding, and coating. The core diameter influences the distribution and number of transmission modes, the cladding diameter affects the ability of the transmission modes to be confined within the core, and the coating protects the fiber from bending.

[0003] The fundamental concept behind Fourier stacking microscopy is derived from stacking techniques. It applies the concepts of phase retrieval and synthetic aperture, using computational imaging to address the difficult balance between a large field of view and high resolution. The basic principle is to observe an object with a low-magnification objective lens and then acquire a series of low-resolution images. These low-resolution images represent information in the spatial domain. These low-resolution images are then iterated based on their mapping in the frequency domain, thereby expanding the spatial-bandwidth product of the system and ultimately recovering a high-resolution image. Specifically, while observing a sample with a low-magnification objective lens, a programmable LED array is controlled to illuminate the sample at different angles using a camera. These low-resolution images, in the frequency domain, correspond to sub-spectra of different regions within the reconstructed image. These sub-spectra are then stitched together in the frequency domain to obtain a sample spectrum beyond the objective lens cutoff frequency. By converting the sample spectrum to the spatial domain, a high-resolution image with a large field of view is obtained. The entire system is simple to install, has low hardware cost, and can take into account both a large field of view and high resolution, which makes it have great development prospects. Therefore, this method is now widely used in digital pathology, biological imaging, and phase difference correction of optical imaging systems.

[0004] With the advancement of scientific research, many new types of optical fibers have emerged. For example, antiresonant hollow-core fibers used in fiber-optic communications boast a 47% increase in propagation speed compared to traditional glass-core fibers due to their air-based transmission channel. This is currently the main research focus for hollow-core fibers, but their endface geometries are complex, including single- and double-nested structures. Double-clad ytterbium-doped fibers used in high-energy fiber lasers feature core diameters and claddings that vary uniformly over their length. There are also double-clad fibers with circular, D-shaped, and octagonal inner claddings. Polarization-maintaining fibers with special structures, including panda-shaped, bow-tie, and elliptical cores, are used in fiber gyros. The emergence of various new optical fibers has increased the demand for measuring fiber endface geometry. Existing fiber endface geometry measurement relies on microscopic imaging. Using high-magnification objectives for high-resolution imaging reduces the imaging range, while using low-magnification objectives for wide-field imaging reduces the imaging resolution. It is difficult to achieve both a wide field of view and high resolution when extracting digital images of fiber endface geometry. Therefore, large-field, high-resolution imaging technology based on the Fourier stacking method is of great significance.

[0005] Currently, several methods have been proposed to achieve optical fiber end-face imaging to measure optical fiber geometric parameters. For example, in the invention patent with publication number CN117754408A, "A Method for Detecting Optical Fiber End Faces," when performing optical fiber end-face detection, several point light sources emit light simultaneously or sequentially, so that the camera can obtain an image of the optical fiber end face when several point light sources emit light. Optical fiber geometric parameters are measured using the video grayscale method and the maximum connected domain algorithm. Another example is the utility model patent with publication number CN220603740U, "A Polarization-Maintaining Optical Fiber End-Face Imaging Device," which uses light sources at different angles and a prismatic reflector to reflect light onto a microscope to obtain a digital image of the polarization-maintaining optical fiber end face, thereby achieving geometric parameter measurement of the polarization-maintaining optical fiber core cladding. The invention patent, "Method for Measuring the Illumination Angle of a Spectral Conjugate Fourier Stacking Microscopy System," with publication number CN118641160A, inserts an additional lens between the tube lens and the camera to ensure the conjugate relationship between the objective lens' back focal plane and the camera's target surface. The illumination angle is measured using the ratio of the LED illumination angle to the objective lens's NA bright field edge angle, resulting in a reconstructed high-resolution image. The invention patent, "Adaptive Aberration Corrected Reflective Fourier Stacking Imaging Method," with publication number CN118502110A, uses a simulated annealing algorithm to correct the subaperture frequency domain position. It then utilizes an adaptive aberration-corrected Fourier stacking imaging reconstruction algorithm to simultaneously reconstruct the object and system pupil functions to achieve aberration correction, addressing issues of poor imaging quality caused by system parameter errors, system instability, and system aberrations. Summary of the Invention

[0006] In response to the defects of the existing technology for detecting the geometric dimensions of optical fiber end faces, the purpose of the present invention is to propose a method and system for detecting the dimensions of optical fiber end faces based on Fourier stack imaging, which utilizes digital imaging of the optical fiber end face to achieve high-precision detection of the geometric dimensions of the optical fiber end face while improving the resolution of the digital image of the optical fiber end face.

[0007] In the first aspect, the present invention provides an optical fiber end face size detection device based on Fourier stack imaging, which includes an optical fiber 1, a light source modulator 2, an LED light source array 3, an objective lens 4, a lens 5, a CCD camera 6, and a host computer 7; the LED light source array 3 further includes an illumination ring 9 and a plurality of monochromatic light sources 8 arranged thereon, and the output signal from the host computer 7 is modulated and processed by the light source modulator 2 to obtain a modulated light signal, and after receiving the modulated light signal, the host computer controls the LED light source array 3 to sequentially light up the monochromatic light sources 8 at different positions on the annular LED light source array 3, forming a structural illumination of the optical fiber 1, so as to change the illumination angle of the optical fiber end face 10, obtain multiple beams of reflected light from the optical fiber end face 10, and the reflected light is amplified by the objective lens 4 and the lens 5, and then The optical fiber end face (10) is captured by the CCD camera (6) under the illumination of light sources at different angles, and the optical fiber end face size detection process of optical fiber digital end face large field of view high resolution imaging based on Fourier stack imaging is executed by the host computer: the optical fiber digital end face large field of view high resolution imaging based on Fourier stack imaging is obtained; the optical fiber end face edge is extracted from the optical fiber digital end face large field of view high resolution imaging based on Fourier stack imaging, the edge profile is fitted, the image is calibrated, the optical fiber is measured, the pixel distance in the optical fiber end face image is converted into the actual physical size, the optical fiber end face image area is obtained, and then the actual geometric size of the optical fiber end face is obtained by calculation according to the optical fiber end face image area, thereby obtaining the detection result of the optical fiber end face diameter.

[0008] In some embodiments, the monochromatic light source 8 is an LED light source.

[0009] In some embodiments, the optical fiber end face size detection device based on Fourier stack imaging according to claim 2 is characterized in that, specifically, 12 monochromatic light sources 8 form a total of 12 illumination points with a wavelength range of 640 to 650 nm.

[0010] In some embodiments, the aperture of the objective lens 4 and the frequency spectrum of the optical fiber end face 10 to be measured are stacked and scanned on the same plane.

[0011] In some embodiments, the LED light source array 3 adopts a reflective annular LED array lighting structure, which is a programmable LED annular lighting array.

[0012] In some embodiments, the order is clockwise from the inner ring to the outer ring.

[0013] In a second aspect, the present invention implements a method for detecting the size of an optical fiber end face based on Fourier stack imaging, comprising:

[0014] i) obtaining a large field of view and high resolution imaging of the optical fiber digital end face based on Fourier stack imaging; further comprising:

[0015] Light up the monochromatic light sources at different positions in the LED light source array one by one, so that the end face of the optical fiber to be tested is illuminated by light at different angles;

[0016] Use a CCD camera to capture multiple low-resolution images of the fiber end face illuminated by light sources at different positions;

[0017] Find the circular spectrum sub-region p under the condition of the i-th LED illumination mn (x,y), and then perform inverse Fourier transform to get the target image p' mn (x,y);

[0018] The low-resolution image p' measured using this angle mn The amplitude of (x, y) updates the amplitude part of the target image and retains its phase information. The updated image is q mn (x,y), then q mn Perform Fourier transform on (x,y) to get the updated circular spectrum sub-region q' mn (x, y), and the circular spectrum sub-region p under the condition of the i-th LED illumination mn (x,y) is replaced by q' mn (x,y);

[0019] This process is repeated for all illumination angles and iterated until convergence, ultimately obtaining large-field-of-view, high-resolution imaging of the fiber digital end face based on Fourier stack imaging.

[0020] ii) realizing optical fiber end face size detection based on Fourier stack imaging, further comprising: extracting the edge of the optical fiber end face from the optical fiber digital end face large field of view and high resolution imaging based on Fourier stack imaging, performing edge contour fitting, and calibrating the image using a regular dot array of a dot calibration plate; establishing a calibration model, and using the calibration model to realize image alignment and correction of the optical fiber digital end face large field of view and high resolution imaging based on Fourier stack imaging; measuring the optical fiber, converting the pixel distance in the optical fiber end face image into the actual physical size, obtaining the optical fiber end face image area, and then calculating the actual geometric size of the optical fiber end face based on the optical fiber end face image area, thereby obtaining the detection result of the optical fiber end face diameter.

[0021] Compared with the prior art, the beneficial technical effects and technical progress achieved by the present invention are as follows:

[0022] 1) Using multiple low-resolution images obtained by different LED light sources, a large-field-of-view, high-resolution image of the optical fiber end face based on Fourier stacking technology is obtained. By calculating and calibrating the images, the precise measurement of the optical fiber end face dimensions is achieved.

[0023] 2) Without changing the hardware structure of the imaging system, the imaging resolution of the digital image of the optical fiber end face is greatly improved, taking into account both a large field of view and high resolution, and realizing high-precision measurement of the geometric parameters of the optical fiber end face. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a structural diagram of the optical fiber end face size detection device based on Fourier stack imaging of the present invention;

[0025] Figure 2 Schematic diagram of LED light source array;

[0026] Figure 3 Flowchart of the method for large-field-of-view and high-resolution imaging of the optical fiber digital end face based on Fourier stack imaging and the optical fiber end face size detection method based on Fourier stack imaging;

[0027] Figure 4 Schematic diagram of synthetic aperture principle;

[0028] Figure 5 It is a low-resolution image of the fiber end face;

[0029] Figure 6 This is the image of the fiber end face after Fourier stack imaging;

[0030] Figure 7 Schematic diagram of the overall process of the optical fiber end face size detection method based on Fourier stack imaging of the present invention.

[0031] Reference numerals:

[0032] 1. Optical fiber, 2. Light source modulator, 3. LED light source array, 4. Objective lens, 5. Lens, 6. CCD camera, 7. Host computer, 8. Monochromatic light source, 9. Illumination ring, 10. Optical fiber end face. DETAILED DESCRIPTION

[0033] The technical solution of the present invention is described below in conjunction with specific embodiments and drawings.

[0034] The present invention provides an optical fiber end face size detection device and method based on Fourier stack imaging, which realizes large-field-of-view high-resolution imaging and geometric size detection of the optical fiber digital end face through Fourier stack imaging technology.

[0035] Example 1

[0036] like Figure 1As shown, the optical fiber end face size detection device based on Fourier stack imaging of the present invention includes an optical fiber 1, a light source modulator 2, an LED light source array 3, an objective lens 4, a lens 5, a CCD camera 6, and a host computer 7; the LED light source array 3 further includes an illumination ring 9 and a plurality of monochromatic light sources 8 arranged thereon, and the output signal from the host computer 7 is modulated by the light source modulator 2 to obtain a modulated light signal. After receiving the modulated light signal, the host computer controls the LED light source array 3 to sequentially light up the plurality of monochromatic light sources 8 at different positions on the LED light source array 3 to form a light source. The structured illumination of the optical fiber 1 is used to change the illumination angle of the optical fiber end face 10, thereby obtaining multiple beams of reflected light from the optical fiber end face 10. The reflected light is amplified by the objective lens 4 and the lens 5, and then fed into the CCD camera 6. The CCD camera 6 is used to collect images of the optical fiber end face 10 under the illumination of light sources at different angles. The upper computer is used to execute the optical fiber end face size detection process based on Fourier stack imaging: the optical fiber end face edge is extracted from the optical fiber digital end face large field of view and high resolution imaging based on Fourier stack imaging, the edge contour is fitted, and the image is calibrated using the regular dot array of the dot calibration plate.

[0037] Specifically, the CCD camera 6 is a CCD camera with a resolution of 2448×2048 and a pixel size of 3.45 μm×3.45 μm.

[0038] Specifically, the objective lens 4 adopts a numerical aperture NA OBJ =0.25 low magnification microscope objective lens. In addition, the aperture of the objective lens 4 and the spectrum of the optical fiber end face 10 to be measured are stacked and scanned on the same plane.

[0039] Specifically, the LED light source array 3 adopts a reflective annular LED array lighting structure, which is a programmable LED annular lighting array. Figure 2 As shown, the LED light source array 3 includes an illumination ring 9 and a plurality of monochromatic light sources 8 arranged thereon. The 12 monochromatic light sources 1 form a total of 12 illumination points, and the wavelength range of the illumination points is 640-650 nm.

[0040] Example 2

[0041] like Figure 3 As shown in FIG, the optical fiber end face size detection method based on Fourier stack imaging of the present invention specifically includes the following steps:

[0042] Step 1: Build a fiber end face size detection device based on Fourier stack imaging, and use the device to obtain a large-field-of-view and high-resolution imaging of the fiber digital end face based on Fourier stack imaging;

[0043] Step 1.1, light up the monochromatic light sources at different positions in the LED light source array one by one, so that the end face of the optical fiber to be tested is illuminated by light at different angles;

[0044] Step 1.2, using a CCD camera to capture multiple low-resolution images of the fiber end face illuminated by light sources at different positions;

[0045] Step 1.3, find the circular spectrum sub-region p under the condition of the i-th LED illumination mn (x,y), and then perform inverse Fourier transform to get the target image p' mn (x,y);

[0046] Step 1.4, use the low-resolution image p' measured at this angle mn The amplitude of (x, y) updates the amplitude part of the target image and retains its phase information. The updated image is q mn (x,y), then q mn Perform Fourier transform on (x,y) to get the updated circular spectrum sub-region q' mn (x, y), and the circular spectrum sub-region p in the case of the i-th LED illumination in step 1.3 mn (x,y) is replaced by q' mn (x,y);

[0047] In step 1.5, this process is repeated for all illumination angles and iterated until convergence, ultimately obtaining a high-resolution image.

[0048] After the five steps above, the digital image of the fiber end face is processed, significantly improving its image resolution and achieving high-resolution imaging with a large field of view. After edge extraction, edge profile fitting, and model calibration, the high-resolution geometric end face image is used to calculate the fiber end face geometry, achieving accurate measurement of fiber geometric parameters.

[0049] Step 2, realizing optical fiber end face dimension detection based on Fourier stack imaging, includes the following specific steps:

[0050] Step 2.1, extracting the edge of the optical fiber end face from the large-field-of-view, high-resolution imaging of the optical fiber digital end face based on Fourier stack imaging, for example, using an improved morphological gradient operator to perform coarse edge positioning and obtain pixel-level edge point coordinates and gradient direction information;

[0051] Step 2.2: perform edge contour fitting, for example, using the Zernike moment algorithm to accurately locate edge points at the sub-pixel level. This algorithm utilizes the rotational invariance of the Zernike moment to stably extract the edge of the circular structure in the fiber end face.

[0052] Step 2.3: Establish a calibration model and use the calibration model to achieve image registration and correction for the large-field-of-view, high-resolution imaging of the optical fiber digital end face based on Fourier stack imaging. For example, the regular dot array of the dot calibration plate is used to accurately align and correct the image of the large-field-of-view, high-resolution imaging of the optical fiber digital end face based on Fourier stack imaging to eliminate distortion introduced by imaging equipment or environmental factors. Utilize the proportional relationship between the pixel distance and the actual physical distance of the center coordinates of the dots in the obtained dot calibration plate image, as well as the perspective transformation model, to establish a mapping relationship from the pixel coordinate system to the world coordinate system, establish a calibration model, and verify the accuracy of the calibration model.

[0053] Step 2.4, obtain the optical fiber end face geometric dimension detection result; for example, measure the sample fiber, convert the pixel distance in the optical fiber end face image into the actual physical dimension, obtain the optical fiber end face image area, and then calculate the actual geometric dimension of the optical fiber end face according to the optical fiber end face image area, including the optical fiber end face diameter, as the detection result of the present invention.

[0054] Before the imaging system can work, the fiber end face must be aligned. Using the mechanical motion unit in the measurement system, three stepper motors are used to control the X, Y, and Z directions respectively. The fiber end face is adjusted to the center of the field of view and the quasi-focal position of the optical path to complete the automatic focusing and detection of the equipment.

[0055] like Figure 4 As shown in the figure, the principle of synthetic aperture specifically includes: according to the frequency shift theorem, the light field diffracts when passing through the end face of the optical fiber to be tested, and the corresponding spectrum of the diffracted light field in the Fourier domain will move a corresponding distance. Combining synthetic aperture technology and phase iterative recovery algorithm, the diffraction limit of the microscope objective lens is broken. The multiple images obtained in the previous step based on the numerical aperture NA of the microscope objective lens are combined. OBJ The lower resolution image is repeatedly iterated in the spatial and frequency domains, and the low resolution image and the synthetic spectrum are continuously updated to gradually restore the complex light field information of the fiber end face. Finally, the complex light field is inverse Fourier transformed to obtain the amplitude and phase images of the fiber end face under high numerical aperture NASYN. Figure 5 As shown in the figure, the low-resolution fiber end face image directly captured by the CCD camera is iterated based on the mapping relationship between light intensity and frequency domain by using the pupil function of overlapping illumination lights at different angles at different positions on the spectrum, thus expanding the frequency domain bandwidth and restoring image information beyond the spatial resolution of the objective lens, thereby obtaining the following: Figure 6 The high-resolution fiber end face image shown has a synthesized numerical aperture of NASYN = 0.75, and the image clarity is improved by about 500%. The obtained high-resolution fiber end face image is subjected to image preprocessing, edge extraction, edge profile fitting, and model calibration before geometric parameter calculation.

[0056] Through the above scheme, large-field-of-view and high-resolution imaging of the optical fiber end face can be achieved to solve the problem of extracting microstructure information from digital images of large-size and special-structure optical fiber end faces.

[0057] The above implementation steps are intended only to facilitate understanding of the specific methods and core concepts of the present invention and are not intended to limit the present invention. Any person skilled in the art may readily conceive of variations or substitutions within the technical scope disclosed herein, or any equivalent substitutions, combinations, and modifications made without departing from the principles of the present invention. Such improvements and modifications shall also be deemed to fall within the scope of protection of the present invention.

Claims

1. A device for detecting the end face size of an optical fiber based on Fourier stack imaging, characterized in that: The device comprises an optical fiber (1), a light source modulator (2), an LED light source array (3), an objective lens (4), a lens (5), a CCD camera (6) and a host computer (7); the LED light source array (3) further comprises an illumination ring (9) and a plurality of monochromatic light sources (8) arranged thereon; an output signal from the host computer (7) is modulated by the light source modulator (2) to obtain a modulated light signal; after receiving the modulated light signal, the host computer controls the LED light source array (3) to sequentially illuminate the monochromatic light sources (8) at different positions on the annular LED light source array (3) to form structural illumination of the optical fiber (1), thereby changing the illumination angle of the optical fiber end face (10) and obtaining a plurality of beams of reflected light from the optical fiber end face (10); the reflected light is amplified by the objective lens (4) and the lens (5) , and then imported into a CCD camera (6), and the CCD camera (6) is used to collect images of the optical fiber end face (10) under the illumination of light sources at different angles, and the upper computer is used to execute the optical fiber end face size detection process of optical fiber digital end face large field of view high resolution imaging based on Fourier stack imaging: obtaining optical fiber digital end face large field of view high resolution imaging based on Fourier stack imaging; extracting the edge of the optical fiber end face from the optical fiber digital end face large field of view high resolution imaging based on Fourier stack imaging, performing edge contour fitting, calibrating the image, measuring the optical fiber, converting the pixel distance in the optical fiber end face image into the actual physical size, obtaining the optical fiber end face image area, and then calculating the actual geometric size of the optical fiber end face according to the optical fiber end face image area, thereby obtaining the detection result of the optical fiber end face diameter.

2. The optical fiber end face size detection device based on Fourier stack imaging according to claim 1, characterized in that: The monochromatic light source (8) adopts an LED light source.

3. The optical fiber end face size detection device based on Fourier stack imaging according to claim 2, characterized in that: Specifically, 12 monochromatic light sources (8) form a total of 12 illumination points, with a wavelength range of 640-650nm.

4. The optical fiber end face size detection device based on Fourier stack imaging according to claim 1, characterized in that: The aperture of the objective lens (4) and the frequency spectrum of the optical fiber end face (10) to be measured are stacked and scanned on the same plane.

5. The optical fiber end face size detection device based on Fourier stack imaging according to claim 1, characterized in that: The LED light source array (3) adopts a reflective annular LED array lighting structure and is a programmable LED annular lighting array.

6. The optical fiber end face size detection device based on Fourier stack imaging according to claim 1, characterized in that: In clockwise order from inner ring to outer ring 7. The method for detecting optical fiber end face size based on Fourier stack imaging of the optical fiber end face size detection device based on Fourier stack imaging according to claim 1, characterized in that: include: i) constructing an optical fiber end face dimension detection device based on Fourier stack imaging to obtain optical fiber digital end face large field of view and high resolution imaging based on Fourier stack imaging; further comprising: Light up the monochromatic light sources at different positions in the LED light source array one by one, so that the end face of the optical fiber to be tested is illuminated by light at different angles; Use a CCD camera to capture multiple low-resolution images of the fiber end face illuminated by light sources at different positions; Find the circular spectrum sub-region p under the condition of the i-th LED illumination mn (x,y), and then perform inverse Fourier transform to get the target image p' mn (x,y); The low-resolution image p' measured using this angle mn The amplitude of (x, y) updates the amplitude part of the target image and retains its phase information. The updated image is q mn (x,y), then q mn Perform Fourier transform on (x,y) to get the updated circular spectrum sub-region q' mn (x, y), and the circular spectrum sub-region p under the condition of the i-th LED illumination mn (x,y) is replaced by q' mn (x,y); This process is repeated for all illumination angles and iterated until convergence, ultimately obtaining large-field-of-view, high-resolution imaging of the fiber digital end face based on Fourier stack imaging. ii) realizing optical fiber end face size detection based on Fourier stack imaging, further comprising: extracting the edge of the optical fiber end face from the optical fiber digital end face large field of view and high resolution imaging based on Fourier stack imaging, performing edge contour fitting, and calibrating the image using a regular dot array of a dot calibration plate; establishing a calibration model, and using the calibration model to realize image alignment and correction of the optical fiber digital end face large field of view and high resolution imaging based on Fourier stack imaging; measuring the optical fiber, converting the pixel distance in the optical fiber end face image into the actual physical size, obtaining the optical fiber end face image area, and then calculating the actual geometric size of the optical fiber end face based on the optical fiber end face image area, thereby obtaining the detection result of the optical fiber end face diameter.

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