Three-point auxiliary positioning method and device for invisible pulse laser processing focus

Abstract

The invention discloses a three-point auxiliary positioning method and a three-point auxiliary positioning device for an invisible pulse laser processing focus, which realize real-time focus tracking, improved positioning precision and enhanced anti-interference capability by arranging a coaxial pinhole device and combining a plurality of image algorithms and displacement table feedback. Repeated calibration is not needed after first-time calibration, the machining efficiency is improved, single-point errors are restrained through the equilateral triangle structure, and the machining defocusing amount and whether the requirement for vertical machining is met or not can be rapidly determined. The overall cost is reduced, the operation safety is improved, the positioning of the pulse focus is mainly determined by an image acquisition algorithm, the focus calibration is replaced by three visible light indicating lasers, expensive photoelectric detection equipment does not need to be additionally used, and in addition, the whole calibration process is mainly completed by a computer and other controllers. And the harm of invisible pulse to operators is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of precision laser processing, and particularly relates to a three-point auxiliary positioning method and device for the focus of invisible pulsed laser processing. Background Art

[0002] As an important branch in the field of advanced manufacturing, invisible pulsed laser processing technology has been widely applied in scenarios such as precision instrument manufacturing, microelectronic device processing, and micro-nano structure preparation. Among them, femtosecond laser processing technology, due to its unique ultra-short pulse characteristics (typical pulse width of 1 - 1000 fs), exhibits significant advantages of low thermal damage and universal processing capabilities. Its physical mechanism stems from the non-thermodynamic dominant characteristics of the interaction between ultrafast lasers and materials: during the femtosecond pulse action, energy is instantaneously deposited on the material surface through the multi-photon absorption mechanism, while the electron-phonon coupling process undergoes a time delay on the order of several picoseconds to dozens of picoseconds, and the thermal diffusion and material melting processes occur on a time scale of dozens to hundreds of picoseconds, resulting in the completion of the material ablation process before the thermodynamic response occurs. This transient energy deposition characteristic enables femtosecond lasers to break through the traditional heat conduction limit and effectively suppress the expansion of the heat-affected zone, and is defined as a "cold processing" technology, especially suitable for high-precision processing of heat-sensitive materials. Further, femtosecond lasers, with extremely high peak power density (up to 10 12 ~10 15 W / cm 2 ), and non-linear absorption characteristics, can achieve sub-diffraction limit processing accuracy, and its action area is strictly localized within the spot size range. For example, ultraviolet femtosecond lasers can break through the processing limit in the visible light band in the processing of metal and metal oxide films, and obtain nano-scale characteristic structures. The defocus distance between the laser focus and the workpiece surface during the processing is the core parameter determining the processing quality, and it is necessary to accurately calibrate the focus position and optimize the defocus distance to achieve the best processing effect. However, for invisible pulsed lasers (such as ultraviolet or near-infrared bands), due to the lack of intuitive optical signal feedback, their focus positions cannot be directly observed through conventional CCD imaging, and focus detection methods based on optical imaging are difficult to directly apply. Traditional laser focus calibration methods, including numerical control positioning dotting method, burning method, etc., mainly rely on manual methods to determine the focus position by distinguishing the change trend of the small hole diameter or the size of the "spark" formed by laser ablation during the interaction between the laser and the whiteboard. These schemes require debuggers to judge based on experience, are extremely inaccurate, the calibration process cannot quantify data, it is difficult to ensure the repeatability accuracy, and there are safety risks. Therefore, it is necessary to develop a new type of high-precision, non-contact focus calibration technology to adapt to its processing characteristics.

[0003] Patent CN117309328A discloses a method for detecting the reflected light intensity of a laser beam by setting up a photoelectric induction measuring instrument. After converting the photoelectric signal, an intensity curve is obtained, and integral operations are performed on the curve. The integrals of continuous and multiple intensity curves are connected into a curve for trend judgment, thereby determining the focus and the optimal light output position. In this solution, the laser used has a guiding light in the visible light band itself, and the Z-axis origin of the focus is determined by directly observing the point where the guiding light spot is the smallest and brightest. There are problems such as insufficient positioning accuracy and it is not suitable for invisible lasers without guiding light.

[0004] Currently, for the positioning method of the laser processing focus, it is usually based on collecting and detecting the reflected photon signals or information such as plasma generated during the interaction between the laser beam and the processing material, and then judging and processing through a device with a photoelectric detection and conversion function. Finally, the position of the laser focusing spot on the material to be processed is obtained. These solutions rely on the strength of the reflected signal and the sensitivity of the relevant detection equipment, and are easily affected by ambient stray light and laser intensity. Moreover, most of them are observed through the guiding light in the visible light band and are not applicable to invisible lasers without guiding light. Summary of the Invention

[0005] In order to solve the problems existing in the above-mentioned background technology, the present invention provides a three-point auxiliary positioning method and device for the focus of invisible pulsed laser processing, which realizes high-precision positioning of the focus of invisible pulsed laser without guiding light, and does not require repeated calibration after the first calibration. It has the advantages of high precision, high efficiency and low cost, and is applicable to the field of high-end precision manufacturing.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: In the first aspect of the present invention, a three-point auxiliary positioning method for the focus of invisible pulsed laser processing is provided, including the following steps: S1. Horizontal position positioning: Capture the visible processing highlights generated by the interaction between the pulsed laser and the sample surface through a coaxial CCD camera, extract the geometric center of the processing highlight area by using gray processing and binary algorithm, and calculate the relative coordinate values of the laser beam focus in the X and Y directions; S2. Z-axis position positioning: Perform local acquisition on the center of the processing highlight area, and determine the optimal focal plane in combination with the image sharpness algorithm to obtain the spatial coordinates (X, Y, Z) of the laser beam focus; S3. Indicator laser setting: Install three visible light indicating lasers in an equilateral triangle layout. Through observation and adjustment, make the three beams of indicating light emitted by the three visible light indicating lasers overlap and point to the focal position. Use the overlapping and visible indicating light spots emitted by the three visible light indicating lasers to replace the position of the laser beam focus for processing guidance. S4. Defocus amount detection and correction: If the processing surface deviates from the focal plane, the triangle formed by the indicating light spots in the image captured by the CCD camera is deformed compared to the set equilateral triangle. Calculate the distance values between the indicating light spots in the image captured by the CCD camera, and re-adjust the position of the processing surface according to the calculated values so that the pulsed laser is perpendicular to the processing plane, and calculate the defocus amount.

[0007] Specifically, step S4 completes the setting of the indicating light with overlapping light spots at a specific position and the calculation of the laser defocus amount. The visible light indicating laser is in the visible light band. After obtaining the accurate spatial coordinates (X, Y, Z) of the pulsed laser focus, through observation and adjustment, make multiple beams of indicating light overlap and point to the focal position. At this time, the invisible focus is replaced by a visible light spot, and the function of not needing to repeat calibration after the first calibration can be realized during the repeated processing. Based on the good collimation characteristics of the laser, when there is a distance difference between the processing plane and the focal plane, a specific shape determined by the installation position will be projected on the processing plane. By calculating the distance between each indicating laser point, the Z-direction distance difference between the planes, that is, the defocus amount, can be obtained. Moreover, according to the deformation amount of the figure formed by the indicating laser points, the parallelism between the processing surface and the focal plane can also be judged, which can be used to judge whether the pulsed laser is perpendicular to the processing surface when processing complex curved surface materials.

[0008] Further, in step S1, the CCD camera is synchronously triggered to capture an image during the pulsed laser operation and transmit it to the controller. Use the conversion formula to convert the captured color image into a grayscale image, then set the threshold range for region segmentation, and finally calculate the relative coordinates of the geometric center of the processing highlight in the CCD camera field of view to complete the calibration of the focal horizontal position, and map the CCD camera field of view coordinates to the coordinates of the actual processing surface.

[0009] Further, the following formula is used to calculate the relative coordinates of the geometric center of the processing highlight in the CCD camera field of view: ; ; Where, is the geometric center of the pulsed laser beam focus in the image captured by the CCD camera, are the row value and column value of the processed processing highlight in the captured image respectively, represents the gray value of the corresponding coordinate point.

[0010] Preferably, in order to reduce errors, the above dotting process can be repeated for positioning and calibration multiple times.

[0011] Further, in step S2, first, a local image acquisition range of 300×300 is divided with the processed bright spot area as the center. Then, the galvanometer system and the field lens are moved upward until the field of view of the CCD camera becomes blurred, and the distance between the field lens and the processing plane at this time is recorded as f 1. Then, the galvanometer system and the field lens are controlled to move downward at a moving interval of 1μm, denoted as f 1 - n, where n = 1, 2, 3…, and the value of n is determined by the number of movements. Stop moving until the change of blurred - clear - blurred appears in the field of view of the CCD camera. At the same time, save the image information during the movement for subsequent image sharpness evaluation processing, and give the image sharpness evaluation value. Select the f 1 - n value as the Z - axis coordinate value of the laser beam focus.

[0012] Further, an auxiliary illumination light source is arranged around the field lens. The reflected light on the focal plane passes through the focusing lens coaxially, and then enters the CCD camera through the pinhole device. The pinhole device is located at the focus of the focusing lens.

[0013] Further, the methods for image sharpness evaluation include the Laplacian gradient method, the Sobel gradient method, or the method of the sum of absolute values of gray - level variances.

[0014] Further, in step S3, with the laser exit point of the galvanometer system as the center, an equilateral triangle formed by three visible - light indicating lasers is inscribed in a circle with a radius of m. In the initial state, the exit ports of the three visible - light indicating lasers and the exit surface of the field lens are in the same direction and parallel. Electrically - controlled mirrors for adjusting the direction of the indicating light emitted by each of them are respectively installed below each visible - light indicating laser.

[0015] Further, the direction of the indicating light emitted by each visible - light indicating laser is respectively coarsely and finely adjusted through the electrically - controlled mirror. During the coarse - adjustment process, first determine the initial installation deflection angles θ of each electrically - controlled mirror and make corresponding adjustments. Then, respectively adjust the mirror normals of each electrically - controlled mirror around the Z - axis direction of the space to intersect on the laser exit optical axis. During the fine - adjustment process, according to the spatial coordinates (X, Y, Z) of the laser beam focus obtained in steps S1 and S2, finely adjust the deflection and pitch angles of the electrically - controlled mirror to overlap the indicating light spots emitted by the three visible - light indicating lasers at the geometric center of the laser beam focus.

[0016] Further, in step S4, the laser defocus amount d f is calculated by using the following formula: ; wherein, is the angle between the indication light emitted by the visible light indication laser and the spatial Z-axis direction, is the distance between the indication light spots obtained from the images collected by the CCD camera, f is the distance from the exit surface of the field lens to the processing focus, N is the installation distance from the visible light indication laser to the electric mirror, and m is the horizontal distance from the indication light emitted by the visible light indication laser to the laser output optical axis.

[0017] In the second aspect of the present invention, a three-point auxiliary positioning device for the invisible pulsed laser processing focus is provided for implementing the above three-point auxiliary positioning method, including a laser, a beam expander group, an optical path steering module, a galvanometer system, a field lens, a processing platform, a displacement stage, a focusing lens, and a CCD camera; the pulsed laser beam emitted by the laser is collimated by the beam expander group and then enters the galvanometer system through the optical path steering module, and is focused by the field lens on the processing platform. The distance between the exit surface of the field lens and the processing platform is adjusted by the displacement stage so that the reflected light on the focal plane can coaxially pass through the focusing lens and enter the image acquisition field of view of the CCD camera; the distance between the exit surface of the field lens and the processing platform is adjusted by the displacement stage so that the reflected light on the focal plane coaxially passes through the focusing lens and enters the image acquisition field of view of the CCD camera.

[0018] Compared with the prior art, the present invention has the following beneficial effects: (1) Without using guiding light, the present invention utilizes the characteristic that the relative positions of the light spots in the images collected by the coaxial CCD camera remain unchanged, combines image algorithms such as gray processing and binarization, and the local autofocus algorithm to quickly and accurately obtain the spatial position coordinates of the invisible pulsed focus; (2) A coaxial pinhole device is provided between the CCD camera and the focusing lens. By controlling the size of the pinhole aperture, the optical signal at the focus is collected, which plays a role in blocking the reflected light from non-focal planes and reducing the interference of ambient stray light, enhancing the resolution and signal-to-noise ratio of the image, and improving the focus positioning accuracy; (3) According to the fact that three points determine a plane in the case of non-collinearity, visible light indication lasers are arranged around the galvanometer system or the field lens in the structure of an equilateral triangle. The invisible pulsed focus with a determined position is replaced by the visible indication light spots, realizing the processing requirement of no need for repeated calibration after the first calibration. Moreover, by calculating the relative coordinate positions of the indication light spots obtained from the images collected by the CCD camera, when the indication light spots no longer overlap, the defocus amount of the processing surface can be quickly determined by the distance between two points. In addition, if the distances between the indication light spots obtained from the images collected by the CCD camera no longer satisfy the specific relationship of the equilateral triangle, it can be judged that the emitted laser is not perpendicular to the processing at this time, and adjustment and correction need to be carried out by controlling the displacement stage; (4) In the present invention, by setting up a coaxial pinhole device and combining various image algorithms with the displacement stage feedback, real-time focus tracking is achieved, the positioning accuracy is improved, and the anti-interference ability is enhanced. After the first calibration, there is no need for repeated calibration, which improves the processing efficiency. The equilateral triangle structure is used to suppress the single-point error, and it is possible to quickly determine the processing defocus amount and whether the requirements for vertical processing are met. The overall cost is reduced, and the operation safety is improved. The positioning of the pulsed focus mainly relies on the image acquisition algorithm to determine, and its focus calibration is replaced by three visible light indicating lasers, eliminating the need for additional expensive optoelectronic detection equipment. In addition, the entire calibration process is mainly completed by a controller such as a computer, avoiding the harm of invisible pulses to the operator. Description of the Drawings

[0019] The present invention will be further described in detail below in conjunction with the drawings and specific embodiments.

[0020] Figure 1 It is a schematic structural diagram of the three-point auxiliary positioning device for the focus of invisible pulsed laser processing in Embodiment 1; Figure 2 It is a flowchart of the three-point auxiliary positioning method for the focus of invisible pulsed laser processing in Embodiment 2; Figure 3 It is a schematic position diagram of the visible light indicating lasers installed in an equilateral triangle structure in Embodiment 2; Figure 4 It is a schematic diagram of adjusting the pointing of the visible light indicating lasers to the focus position at the initial moment in Embodiment 2; Figure 5 It is a schematic diagram of judging the defocus amount of the processing plane and whether it is perpendicular to the processing in Embodiment 2; Among them, the specific reference numerals are: Laser 1, beam expander group 2, first reflector 3, second reflector 4, galvanometer system 5, field lens 6, annular shadowless illumination light source 7, processing platform 8, focusing lens 9, pinhole device 10, CCD camera 11, first visible light indicating laser 12, second visible light indicating laser 13, third visible light indicating laser 14, first electric reflector 15, second electric reflector 16, third electric reflector 17, displacement stage 18, controller 19. Detailed Embodiment

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0022] Embodiment 1 This embodiment discloses a three-point auxiliary positioning device for the focus of invisible pulsed laser processing, as Figure 1 shown, which includes a laser 1 module composed of a laser 1 and a beam expander group 2, an optical path turning module composed of a first reflector 3 and a second reflector 4, a processing module composed of a galvanometer system 5, a field lens 6, and a processing platform 8, an image acquisition module composed of a focusing lens 9, a pinhole device 10, a CCD camera 11, and an annular shadowless illumination light source 7. An auxiliary illumination light source (annular shadowless illumination light source 7) is arranged around the field lens 6, an indicating light module composed of a first visible light indicating laser 12, a second visible light indicating laser 13, a third visible light indicating laser 14, a first electric reflector 15, a second electric reflector 16, and a third electric reflector 17, and a motion control module composed of a displacement stage 18 and a controller 19.

[0023] The laser 1 can be an ultrafast laser 1 such as infrared or ultraviolet. The pulsed laser beam emitted by the laser 1 is collimated by the beam expander group 2 and then enters the galvanometer system 5 through the optical path turning module, and is focused by the field lens 6 on the processing platform 8. By adjusting the distance between the exit surface of the field lens 6 and the processing platform 8 through the displacement stage 18, the reflected light on the focal plane can enter the image acquisition field of view of the CCD camera 11 coaxially through the focusing lens 9. Each optical device is placed coaxially. The optical path turning module is adjusted so that the emitted light of the laser 1 enters the galvanometer system 5 at the geometric center and exits through the field lens 6. According to the known working focal length of the field lens 6, the distance between the exit surface of the field lens 6 and the processing platform 8 is adjusted so that the reflected light on the focal plane can enter the image acquisition field of view of the CCD camera 11 coaxially through the focusing lens 9 and then is controlled by the pinhole device 10 to enter. The pinhole device 10 is located at the focus of the focusing lens 9 and has an optical conjugate relationship with the center of the focal plane of the processing laser. In theory, only the signal light returned from the focus can be converged and all pass through the pinhole and be collected by the photodetector. Therefore, the feedback light signal is the strongest at the focus. By adjusting the aperture size of the pinhole device 10, the amount of light flux and the ability to block stray light off the focal plane are determined, improving the system recognition accuracy and anti-interference ability.

[0024] Specifically, the actual physical aperture value of the coaxially arranged pinhole device 10 needs to match the pixel size of the CCD camera 11 and the system magnification. Due to the diffraction limit and processing limitations, the equivalent aperture value of the pinhole device 10 is usually made approximately equal to 1 to 3 times the focal plane spot diameter (such as the Airy disk diameter in the case of uniform light intensity distribution). At this time, the aperture value of the pinhole device 10 is about several tens of micrometers. However, in the actual industrial processing process, it is necessary to balance the requirements of signal-to-noise ratio and signal light flux. The aperture value of the pinhole device 10 is generally selected in the sub-millimeter range. In addition, the aperture size of the pinhole device 10 is also directly related to the laser wavelength. The longer the wavelength, the larger the focused spot diameter, and the aperture needs to be increased accordingly. For example, the aperture of the pinhole device 10 required for infrared light will be larger than that for ultraviolet light.

[0025] Embodiment 2 This embodiment discloses a three-point auxiliary positioning method for the focus of invisible pulsed laser processing, as Figure 2 shown, including the following steps: S1. Horizontal position positioning: Since the image acquisition module and the laser processing module are placed on the same optical axis, and the effective Z-direction processing distance of the galvanometer system 5 is controlled by the displacement stage 18, when using the coaxial CCD camera 11 to collect images at the processing focal plane, the relative position of the laser beam in the field of view of the CCD camera 11 is ensured to remain unchanged. At the same time, in actual processing, it can be found that although the spot of the invisible pulsed laser cannot be directly observed, when a single pulsed laser acts on the surface of the processing sample, the process of laser ablation of the sample surface is visible. Specifically, an obvious bright spot appears in the acquisition field of view of the CCD camera 11, and this bright spot is the area where the pulsed laser interacts with the sample surface.

[0026] Therefore, by using the coaxial CCD camera 11 to capture the visible processing bright spot generated by the interaction between the pulsed laser and the sample surface, gray processing and binary algorithm are used to extract the geometric center of the processing bright spot area, and the relative coordinate values of the laser beam focus in the X and Y directions are calculated.

[0027] Specifically: Synchronously trigger the CCD camera 11 to capture images during pulsed laser operation and transmit them to the controller 19. The optional image resolution is 1024×1280 pixels, and the gray level range is 0 to 255; then use the conversion formula to convert the captured color image into a gray image to improve the processing speed; then set the threshold range for region segmentation to improve the recognition accuracy of position judgment; finally, calculate the relative coordinates of the geometric center of the processing bright spot in the field of view of the CCD camera 11 according to the formula to complete the calibration of the focus horizontal position, and map the field of view coordinates of the CCD camera 11 to the coordinates of the actual processing surface.

[0028] The formula for calculating the relative coordinates of the geometric center of the machining highlight in the field of view of the CCD camera 11 is as follows: ; ; Among them, is the geometric center of the focus of the laser beam after focusing in the image collected by the CCD camera 11, are respectively the row value and column value of the machining highlight in the collected image after processing, represents the gray value of the corresponding coordinate point.

[0029] In order to simplify subsequent processing and reduce the amount of calculation, a gray-scale processing algorithm is used to convert the color image into a gray-scale image. For each pixel of the image, the RGB value is usually converted into a gray-scale value by using the weighted average method, such as Gray = 0.299R + 0.587G + 0.114B. Then there is binary processing. The purpose is to segment the highlight area through strong contrast so that it can be recognized more clearly. The specific implementation method is to set a judgment threshold, which can be set manually according to experience or an adaptive threshold method can be used, such as the Otsu algorithm, to adapt to different lighting conditions. All pixels with gray-scale values greater than or equal to the threshold are set to white (255), and the rest are black (0). At this time, the highlight area may be several connected white pixel blocks, so it is also necessary to add a connected component labeling algorithm based on scanning to determine which pixels belong to the same area.

[0030] To reduce errors, the above dotting process can be repeated for positioning and calibration multiple times.

[0031] S2. Z-axis position positioning: From step S1, the position of the focus of the laser beam focused by the field lens 6 in the image collected by the CCD camera 11 can be determined, which is equivalent to solving the coordinate positioning problem of the focus in the X and Y directions of the focal plane. However, the spatial Z coordinate of the focus cannot be determined yet, that is, the distance from the light-emitting surface of the field lens 6 to the focus f .

[0032] The Z-axis focusing accuracy of the pulsed laser determines the quality of subsequent laser processing. Although the spot size is invisible, the image of the sample surface can still be collected by the CCD camera 11 with the help of the auxiliary light source (ring shadowless illumination light source 7). Therefore, the focusing determination can be completed by evaluating the image sharpness. However, different from the common autofocus algorithm that collects images from the entire sample surface, the present invention makes local collection with the center of the machining highlight area obtained by the previous position positioning algorithm, reducing the deviation of the Z-axis position of the focus.

[0033] The specific implementation process is as follows: First, a local image acquisition range of 300×300 is divided with the processed highlight area as the center. Then, the galvanometer system 5 and the field lens 6 are moved upward until the field of view of the CCD camera 11 becomes blurred, and the distance between the field lens 6 and the processing plane at this time is recorded as f 1. Since the Rayleigh length of the pulsed laser is in the micron range, the galvanometer system 5 and the field lens 6 are controlled to move downward at a moving interval of 1μm, denoted as f 1-n, where n = 1, 2, 3…, and the value of n is determined by the number of movements. Stop moving until the change of blurred-clear-blurred appears in the field of view of the CCD camera 11. At the same time, save the image information during the movement for subsequent image sharpness evaluation processing, and give the image sharpness evaluation value. Select the f 1-n value corresponding to the maximum image sharpness evaluation value as the Z-axis coordinate value of the laser beam focus.

[0034] Specifically, the methods for image sharpness evaluation include the Laplacian gradient method, the Sobel gradient method, or the method of the sum of absolute values of gray-scale variances.

[0035] In addition, in order to enhance the recognition and extraction of the surface feature information of the processed image, a pinhole device 10 is added between the coaxial CCD camera 11 and the focusing lens 9 in the present invention. The reflected light signal at the focal plane is extracted according to the pinhole aperture size of the pinhole device 10, and the stray light at the defocus plane is blocked, improving the anti-interference ability. Moreover, the field lens 6 is achromatically corrected, effectively reducing the focusing deviation between the visible light band and the invisible light, and further improving the positioning accuracy. In a specific embodiment, the galvanometer and the field lens 6 of the processing module can be replaced with a processing objective lens.

[0036] After steps S1 and S2, the spatial coordinates (X, Y, Z) of the laser beam focus are obtained.

[0037] S3. Setting of the indicating laser 1: Install a visible light indicating laser 1 at a specific position and superimpose it to point to the laser focus. Using the visible light indicating light with an approximate spot size to replace the invisible laser focus can achieve the function of not requiring repeated calibration after the first calibration during subsequent repeated processing. According to the principle that three non-collinear points determine a plane, a specific structure of an equilateral triangle is selected as the installation position of the visible light indicating laser 1.

[0038] As Figure 3As shown in the figure, with the laser exit point of the galvanometer system 5 as the center, an equilateral triangle formed by three visible light indicating lasers 1 is inscribed in a circle with a radius of m, and they are respectively marked as the first visible light indicating laser 12, the second visible light indicating laser 13, and the third visible light indicating laser 14. At this time, the visible light indicating laser 1 emits light vertically and the exit port is flush with the exit surface of the field lens 6. In order to make each indicating light deflect and point to the pulse focus position, a first electric mirror 15, a second electric mirror 16, and a third electric mirror 17 are respectively installed behind the first visible light indicating laser 12, the second visible light indicating laser 13, and the third visible light indicating laser 14, and rough adjustment and fine adjustment are respectively carried out.

[0039] First, perform rough adjustment. In the front view of the installation of the indicating light as shown in Figure 4 , determine the initial installation deflection angle of each electric mirror according to the following formula : ; ; where is the angle between the indicating light emitted by the visible light indicating laser 1 and the Z-axis direction of space, f is the distance from the exit surface of the field lens 6 to the processing focus, N is the installation distance from the visible light indicating laser 1 to the electric mirror, and m is the horizontal distance from the indicating light emitted by the visible light indicating laser 1 to the laser exit optical axis.

[0040] After obtaining the initial deflection angle of the electric mirror, then adjust the mirror normals of each electric mirror to intersect on the laser exit optical axis around the Z direction respectively.

[0041] Finally, perform fine adjustment. According to the spatial coordinates (X, Y, Z) of the laser beam focus obtained in steps S1 and S2, the controller 19 finely adjusts the deflection and pitch angles of the electric mirror to make the indicating light spot overlap on the geometric center of the laser beam focus. The model of the used electric mirror is Picomotor-8807, and its angular resolution is 0.7µrad, which can well meet the extreme optical alignment.

[0042] S4. Defocus detection and correction: Judge whether the processing surface and the focal plane are defocused and deflected. During the subsequent processing, use the visible indicating light spot to replace the position of the laser processing focus for processing guidance. Utilizing the good collimation characteristic of the laser beam, when the focal plane where the focus is located is deflected or non-coplanar with the processing surface, the indicating light will project a specific shape determined by the installation position on the processing surface, such as Figure 5As shown. For the case where the processed surface is deflected relative to the focal plane, in the acquisition field of view of the CCD camera 11, it can be observed that the triangle formed by the projected light spot is deformed compared to the set equilateral triangle. The distances between points can be calculated by relative coordinates, and the position of the processed surface is readjusted according to the calculated values so that the pulsed laser is perpendicular to the processing plane.

[0043] For the calculation of the laser defocus amount d f After adjusting the focal plane and the processing plane to be parallel, it is calculated using the following formula: ; Where is the angle between the indicating light emitted by the visible light indicating laser 1 and the spatial Z-axis direction, is the distance between the indicating light spots obtained from the images collected by the CCD camera 11, f is the distance from the exit surface of the field lens 6 to the processing focus, N is the installation distance from the visible light indicating laser 1 to the electric mirror, and m is the horizontal distance from the indicating light emitted by the visible light indicating laser 1 to the laser emission optical axis.

[0044] It can be seen from the above formula that at this time, the laser defocus amount is only related to the side length of the projected triangle, the installation position of the visible light indicating laser 1, and the Z-direction distance of the pulsed laser focus. Compared with the traditional method of using devices such as distance sensors to measure the defocus value, the present invention converts the slightly changing defocus amount into the calculation of relatively large spatial dimension values, making the measurement accuracy higher, the implementation method simpler, and the cost relatively lower. After calibrating the focus of the processing plane, further processing can be started; otherwise, the above debugging steps need to be repeated in sequence to recalibrate the focus.

[0045] In the present invention, by setting the coaxial pinhole device 10 and combining various image algorithms with the feedback of the displacement stage 18, real-time focus tracking is achieved, the positioning accuracy is improved, and the anti-interference ability is enhanced. After the first calibration, there is no need to repeat the calibration, which improves the processing efficiency. The equilateral triangle structure is used to suppress single-point errors, and it is possible to quickly determine the processing defocus amount and whether the requirements for vertical processing are met. The overall cost is reduced, the operation safety is improved, and the positioning of the pulsed focus mainly relies on the image acquisition algorithm to determine. Its focus calibration is replaced by three visible light indicating lasers, and there is no need to additionally use expensive optoelectronic detection equipment. In addition, the entire calibration process is mainly completed by a controller such as a computer, avoiding the harm of invisible pulses to operators.

[0046] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A three - point auxiliary positioning method for the focus of invisible pulsed laser processing, characterized in that, It includes the following steps: S1. Horizontal position positioning: Capture the visible processing highlights generated by the interaction between the pulsed laser and the sample surface through a coaxial CCD camera, extract the geometric center of the processing highlight area by using gray-scale processing and binary algorithm, and calculate the relative coordinate values of the laser beam focus in the X and Y directions; S2. Z-axis position positioning: Perform local acquisition on the center of the processing highlight area, determine the best focal plane in combination with the image sharpness algorithm, and obtain the spatial coordinates (X, Y, Z) of the laser beam focus; S3. Indicator laser setting: Install three visible light indicator lasers in an equilateral triangle layout. Through observation and adjustment, make the three indicator lights emitted by the three visible light indicator lasers overlap and point to the focus position. Use the overlapping and visible indicator light spots emitted by the three visible light indicator lasers to replace the position of the laser beam focus for processing guidance; S4. Defocus amount detection and correction: If the processing surface deviates from the focal plane, the triangle formed by the indicator light spots in the image collected by the CCD camera is deformed compared with the set equilateral triangle. Calculate the distance values between the indicator light spots in the image collected by the CCD camera, and readjust the position of the processing surface according to the calculated values so that the pulsed laser is perpendicular to the processing plane, and calculate the defocus amount.

2. The three - point auxiliary positioning method for the focus of invisible pulsed laser processing according to claim 1, characterized in that, In the step S1, the CCD camera is synchronously triggered to capture an image during the pulsed laser operation and transmit it to the controller. The captured color image is converted into a gray-scale image by using a conversion formula, then a threshold range is set for region segmentation, and finally the relative coordinates of the geometric center of the processing highlight in the CCD camera field of view are calculated to complete the calibration of the focus horizontal position, and the CCD camera field of view coordinates are mapped to the coordinates of the actual processing surface.

3. The three - point auxiliary positioning method for the focus of invisible pulsed laser processing according to claim 2, characterized in that, The following formula is used to calculate the relative coordinates of the geometric center of the processing highlight in the CCD camera field of view: ； ； Wherein, is the geometric center of the focus of the laser beam after focusing in the image collected by the CCD camera, are respectively the row value and column value of the processed machining bright spot in the collected image, represents the gray value of the corresponding coordinate point.

4. The three - point auxiliary positioning method for the focus of invisible pulsed laser processing according to claim 1, characterized in that, In the step S2, first, a local image acquisition range of 300×300 is divided with the processed highlight area as the center. Then, the galvanometer system and the field lens are moved upward until the field of view of the CCD camera becomes blurred, and the distance between the field lens and the processing plane at this time is recorded as f 1. Then, the galvanometer system and the field lens are controlled to move downward at a moving interval of 1μm, denoted as f 1-n, where n = 1, 2, 3…, and the value of n is determined by the number of movements. Stop moving until the change of blurred-clear-blurred appears in the field of view of the CCD camera. At the same time, save the image information during the movement for subsequent image sharpness evaluation processing, and give the image sharpness evaluation value. Select the f 1-n value as the Z-axis coordinate value of the laser beam focus.

5. The three - point auxiliary positioning method for the focus of invisible pulsed laser processing according to claim 4, characterized in that, An auxiliary illumination light source is arranged around the field lens. The reflected light on the focal plane passes through the focusing lens coaxially, and then enters the CCD camera through the control of the pinhole device. The pinhole device is located at the focus of the focusing lens.

6. The three - point auxiliary positioning method for the focus of invisible pulsed laser processing according to claim 4, characterized in that, The methods for evaluating image sharpness include the Laplace gradient method, the Sobel gradient method, or the method of the sum of absolute values of gray-scale variances.

7. The three - point auxiliary positioning method for the focus of invisible pulsed laser processing according to claim 1, characterized in that, In the step S3, with the laser exit point of the galvanometer system as the center, the equilateral triangle formed by the three visible light indicator lasers is inscribed in a circle with a radius of m. In the initial state, the exit ports of the three visible light indicator lasers and the exit surface of the field lens are in the same direction and parallel. Electrically controlled mirrors for adjusting the direction of the indicator light emitted by each of them are respectively installed below each visible light indicator laser.

8. The three - point auxiliary positioning method for the focus of invisible pulsed laser processing according to claim 7, characterized in that, Coarse and fine adjustments are respectively made to the directions of the indication lights emitted by each visible light indication laser through an electric mirror; during the coarse adjustment process, first determine the initial installation deflection angles of each electric mirror θ , and make corresponding adjustments, and then respectively adjust the mirror normals of each electric mirror to intersect on the laser output optical axis in the Z-axis direction of space; during the fine adjustment process, according to the spatial coordinates (X, Y, Z) of the laser beam focus obtained in steps S1 and S2, finely adjust the deflection and pitch angles of the electric mirror, and overlap the indication light spots emitted by the three visible light indication lasers at the geometric center of the laser beam focus.

9. The three - point auxiliary positioning method for the focus of invisible pulsed laser processing according to claim 1, characterized in that, In the step S4, the laser defocus amount d f is calculated by using the following formula: ； Wherein, is the angle between the indication light emitted by the visible light indication laser and the spatial Z-axis direction, is the distance between the respective indication light spots obtained from the image collected by the CCD camera, f is the distance from the exit surface of the field lens to the processing focus, N is the installation distance from the visible light indication laser to the electric mirror, and m is the horizontal distance from the indication light emitted by the visible light indication laser to the laser emission optical axis.

10. A three - point auxiliary positioning device for the focus of invisible pulsed laser processing, used to implement the three - point auxiliary positioning method according to any one of claims 1 to 9, characterized in that, It includes a laser, an expander lens group, an optical path steering module, a galvanometer system, a field lens, a processing platform, a displacement stage, a focusing lens, and a CCD camera; The pulsed laser beam emitted by the laser is collimated by the expander lens group, enters the galvanometer system through the optical path steering module, and is then focused by the field lens on the processing platform. The distance between the exit surface of the field lens and the processing platform is adjusted through the displacement stage so that the reflected light on the focal plane can pass through the focusing lens coaxially and enter the image acquisition field of view of the CCD camera.

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