Method for in-situ measurement and processing of curved surface by femtosecond laser and application thereof

Abstract

This invention discloses a method and its application for in-situ measurement and processing of curved surfaces using femtosecond lasers, belonging to the field of laser micro-nano processing technology. The method includes: acquisition of reflected light spot images; three-dimensional image processing and establishment of a three-dimensional coordinate matrix; three-dimensional coordinate matrix interpolation and surface pattern processing. The method uses an image sensor for in-situ measurement of the curved surface during femtosecond laser processing. Then, by performing three-dimensional processing on the light spot image obtained from the image sensor and extracting an ellipse at a fixed height, the distance between the laser focus and the curved surface can be determined. Measurement of the area to be measured can obtain the three-dimensional coordinates of the curved surface, and then a processing pattern is formed on the curved surface by scanning with a femtosecond laser. This method uses the same light source for both in-situ measurement and processing, avoiding errors between the measurement and processing positions. Compared to traditional measurement methods using high-speed galvanometers combined with cameras, it has higher measurement accuracy.

Description

Technical Field

[0001] This invention belongs to the field of laser micro-nano processing technology, specifically relating to a method and its application for in-situ measurement and processing of curved surfaces using femtosecond lasers. Background Technology

[0002] Fabricating micro- and nanostructures on curved surfaces enables the fabrication of many optical devices, such as artificial compound eye structures and hybrid refractive and diffractive optical elements. Laser direct writing technology, a mature and high-precision 3D fabrication technique, has been applied by researchers to the fabrication of curved surfaces. However, because the height variations of curved surfaces are much greater than the laser's depth of focus, it is difficult to achieve uniform fabrication on unmeasured curved surface morphology.

[0003] Currently, most methods for machining curved surfaces use a camera combined with a high-speed galvanometer to complete the measurement and machining, which suffers from insufficient accuracy. Summary of the Invention

[0004] To address the shortcomings of existing technologies that use cameras combined with high-speed galvanometers for measuring and processing curved surfaces, such as insufficient accuracy, this invention provides a method and its application for in-situ measurement and processing of curved surfaces using femtosecond lasers. This method uses an image sensor to perform in-situ measurement of the curved surface during femtosecond laser processing. By performing three-dimensional processing on the spot image obtained from the image sensor and extracting an ellipse at a fixed height, the distance between the laser focus and the surface of the curved surface can be determined. The measurement of the area to be measured can obtain the three-dimensional coordinates of the surface of the curved surface, and then a processing pattern is formed on the surface of the curved surface by scanning with a femtosecond laser.

[0005] This invention is achieved through the following technical solution:

[0006] The method for in-situ measurement and processing of curved surfaces using femtosecond lasers includes the following steps:

[0007] Step 1: Image acquisition of reflected light spots;

[0008] The area to be measured on the curved surface is divided into a grid in the xy plane, and the coordinates of each grid point are (x, y, ...). i y i The laser beam pattern is acquired at each grid point. The x and y axis stages are moved to position the laser focus at the starting grid point. The laser power is adjusted to a certain value and fixed. The z axis stage is moved for measurement. The laser emitted from the femtosecond laser is converted into circularly polarized light by a polarizing beam splitter and a quarter-wave plate. It is then focused onto the sample surface on the sample stage by the objective lens. The laser beam focused on the sample surface is reflected by the sample surface, passes through the objective lens and the quarter-wave plate, and is reflected to the other side by the polarizing beam splitter. It is then focused onto the image sensor by the lens to obtain the spot image of that grid point. The laser focus is moved to the next grid point and the measurement is repeated until spot images of all grid points are obtained.

[0009] Step 2: Image 3D Conversion and 3D Coordinate Matrix Establishment

[0010] The light spot image acquired by the image sensor in step one is processed into a three-dimensional image using a Python program to obtain a three-dimensional light spot image with the corresponding light field distribution; an ellipse is then extracted from this three-dimensional light spot image at a certain height z0, and the major axis σ of the ellipse is calculated. L and minor axis σ s The value is used to obtain the z-axis coordinates of the grid points on the surface corresponding to the light spot image based on the pre-calibrated curve; the above process is repeated for all light spot images to obtain the three-dimensional coordinate lattice (x) of all grid points to be measured. i y i , z i );

[0011] Step 3: 3D coordinate matrix interpolation and surface pattern processing:

[0012] The three-dimensional coordinate lattice (x) along the scanning direction and perpendicular to the scanning direction i y i , z i Interpolation is performed at fixed intervals, that is, M and N points are inserted in the two directions respectively to obtain the three-dimensional coordinate lattice of the corresponding smooth surface. The coordinates of the three-dimensional lattice are imported into the processing program. The sample is placed on the sample stage, the x and y axis displacement stages are moved to make the laser in the starting processing position, and the z axis displacement stage is moved to make the laser focus on the surface of the curved surface to process the curved surface pattern.

[0013] Furthermore, in step one, the mesh is divided according to the size of the area to be measured, where x, y, and z axes represent the movement directions of the displacement stage, respectively. The number of mesh points in the x-axis direction is 5-20, the number of mesh points in the y-axis direction is 5-20, and the total mesh area is 0.01-1 mm. 2 The femtosecond laser power is 2-10mW; the image sensor is placed at the focal length of lens L1; the quarter-wave plate is adjusted so that the polarization direction of the incident light is at an angle of 45° with the two axes of the quarter-wave plate, so that the incident light can be reflected by the polarizing beam splitter after passing through the quarter-wave plate twice.

[0014] Further, in step two, the three-dimensional processing specifically involves normalizing the grayscale value of each pixel in the two-dimensional image according to the maximum grayscale value and using it as the z-axis coordinate for three-dimensional image rendering; the three-dimensional image is a three-dimensional distribution of an approximate Gaussian light field; the height z0 is 0.2-0.4; the z-axis coordinate is the coordinate displayed by a z-axis displacement stage defined when the distance between the laser focus and the curved surface is zero.

[0015] Furthermore, in step two, the pre-calibrated curves include: a set of elliptical major axes σL A curve relating the distance from the laser focus to the curved surface and a set of elliptical minor axes σ s Regarding the curves representing the distance from the laser focus to the curved surface, each set of curves is obtained by changing the angle between the laser and the tangent plane of the curved surface.

[0016] Furthermore, in step three, the interpolation spacing along the scanning direction is 50-500 nm, and the interpolation spacing along the perpendicular scanning direction is 0.5-5 μm.

[0017] Furthermore, in step three, the scanning method of the processing procedure is line-by-line scanning, and the length of the scanned line segment is 50-500nm.

[0018] On the other hand, the present invention also provides the application of the method of in-situ measurement and processing of curved surfaces using femtosecond lasers in the processing of micro and nano structures. Specifically, a grid structure is processed on the curved surface, and after etching, a conical anti-reflection structure can be obtained, which can increase the transmittance of light in a fixed wavelength band.

[0019] Compared with the prior art, the advantages of the present invention are as follows:

[0020] (1) The method of in-situ measurement and processing of curved surfaces using femtosecond lasers in this invention has higher measurement accuracy compared with the traditional measurement method using high-speed galvanometers combined with cameras;

[0021] (2) The in-situ measurement and in-situ processing methods used in this invention use the same light source, which avoids the phenomenon of error between the measurement position and the processing position. Attached Figure Description

[0022] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0023] Figure 1 This is a schematic diagram of the optical path for in-situ measurement and processing of curved surfaces using femtosecond lasers, as described in this invention.

[0024] Figure 2 This is the image after three-dimensional processing of the light spot image acquired by the image sensor in this invention;

[0025] Figure 3 This is a schematic diagram of truncating an ellipse in the xy plane from the processed three-dimensional spot image in this invention;

[0026] Figure 4 For different angles between the laser and the tangent plane of the curved surface in this invention, the major axis σ of the ellipse is... LA curve showing the relationship between the laser focus and the surface distance;

[0027] Figure 5 For different angles between the laser and the tangent plane of the curved surface in this invention, the minor axis σ of the ellipse is... s A curve showing the relationship between the laser focus and the surface distance;

[0028] Figure 6 This is a schematic diagram of the three-dimensional coordinate point matrix obtained by measuring the area to be measured during surface measurement in this invention;

[0029] Figure 7 This is a resolution map obtained by repeatedly measuring at different distances between the laser focus and the surface when the angle between the laser focus and the curved surface is 10° in this invention;

[0030] The horizontal line represents the ellipse length σ obtained by repeated measurements at the same location. L The average value;

[0031] Figure 8 This invention relates to a three-dimensional coordinate lattice (x... i y i , z i A diagram illustrating the interpolation process;

[0032] Among them, hollow circles represent three-dimensional coordinate lattices, and solid circles represent interpolation points;

[0033] Figure 9 This is a microscope photograph of a school emblem pattern prepared using a femtosecond laser to perform in-situ measurement and processing of curved surfaces, as described in Embodiment 1 of the present invention.

[0034] Figure 10 Microscopic photographs showing the comparison of school badges fabricated without using the proposed femtosecond laser in-situ measurement and processing method for curved surfaces in this invention. Detailed Implementation

[0035] To clearly and completely describe the technical solution and its specific working process of the present invention, the specific embodiments of the present invention are as follows, in conjunction with the accompanying drawings:

[0036] Example 1

[0037] The school emblem pattern was prepared on the surface of a fused silica convex lens using a femtosecond laser.

[0038] By using a Python program to analyze and process the light spot image obtained from the image sensor, a three-dimensional coordinate matrix of the area to be processed on the surface of the fused silica convex lens is obtained. Processing data is generated based on the interpolated three-dimensional coordinate matrix. Then, the laser focus is accurately moved to the lens surface by a moving displacement stage, and finally the school emblem pattern is prepared.

[0039] The specific steps of using femtosecond lasers for in-situ measurement and processing of curved surfaces are as follows:

[0040] (1) Image acquisition of reflected light spots:

[0041] The area to be measured on the fused silica convex lens is divided into a 20*20 grid in the xy plane, and the area of ​​the area to be measured is 0.01 mm². 2 The coordinates of each grid point are (x, y, y). i y i The value of i ranges from 0 to 20. A spot image is acquired for each grid point. The x and y axis displacement stages are moved to position the laser focus at the starting grid point. The laser power is adjusted to 2mW, and the z axis displacement stage is moved for measurement. The laser emitted from the femtosecond laser is converted into circularly polarized light by a polarizing beam splitter and a quarter-wave plate, and finally focused onto the sample surface on the sample stage by the objective lens. The laser focused on the sample surface is reflected by the sample surface, passes through the objective lens and the quarter-wave plate, and is reflected to the other side by the polarizing beam splitter. It is then focused onto the image sensor by lens L1 to obtain the spot image of that grid point. The laser focus is moved to the next grid point, and the measurement is repeated until spot images of all grid points are acquired.

[0042] (2) Image 3D processing and 3D coordinate point matrix establishment:

[0043] The image of the light spot acquired by the image sensor at the starting grid point in step (1) is processed into a three-dimensional image using a Python program to obtain a three-dimensional light spot image with the corresponding light field distribution; an ellipse is truncated at a height of 0.2 in this three-dimensional light spot image, and the major axis σ of the ellipse is calculated. L and minor axis σ s The values ​​are 6.12 and 5.49 respectively. These two values ​​are then substituted into the pre-calibrated curves.

[0044] In this embodiment, the calibration curve acquisition method is as follows: The planar sample is placed on the sample stage at a 10° angle. The z-axis displacement stage is moved to scan the laser focus from top to bottom. During this time, the maximum grayscale value of the light spot image acquired by the image sensor will first increase and then decrease. The midpoint of the coordinates of the z-axis displacement stage when the maximum grayscale value is 200 during the scanning process is taken. This coordinate is considered the position where the distance between the laser focus and the sample surface is 0. The light spot image at this time is acquired and processed to obtain the corresponding elliptical major axis σ. L and minor axis σ sThe value is obtained by moving the laser beam upwards by 100 nm and repeating the above operation until the distance between the laser focus and the sample surface is 4 μm, thus completing the plotting of two curves. The sample tilt angle is changed, and the above operation is repeated to complete the plotting of the calibration curve for the laser and the tangent plane of the curved surface at 0-10°. The point with the closest value in the calibration curve corresponds to a distance of 0.1 μm between the laser focus and the curved surface, and an angle of 6° between the laser and the tangent plane of the curved surface; at this time, the z-axis displacement stage displays 13.516 mm, and the z-axis coordinate of the grid point on the curved surface corresponding to this spot image is 13.5159 mm. The above processing is repeated for all spot images to obtain the three-dimensional coordinate point matrix (x... i y i , z i );

[0045] (3) Three-dimensional coordinate matrix interpolation and surface pattern processing:

[0046] The three-dimensional coordinate lattice (x) along the scanning direction and perpendicular to the scanning direction i y i , z i Interpolation was performed at intervals of 50 nm and 1 μm, i.e., 100 and 5 points were inserted in the two directions respectively, to obtain the three-dimensional coordinate lattice of the corresponding smooth surface. The coordinates of the three-dimensional lattice were then imported into the processing program. The scanning method of the processing program was line-by-line scanning, and the length of the scanning line segment was 50 nm. The sample was placed on the sample stage, and the x, y, and z axis displacement stages were moved so that the laser was at the starting grid point coordinates (0, 0, 13.5159 mm), and the school emblem pattern on the surface was obtained.

[0047] Depend on Figure 1 As can be seen, the femtosecond laser is reflected by the surface of the convex lens and then reflected by the polarizing beam splitter to the image sensor. The image sensor can be processed to measure the position of points on the curved surface.

[0048] Depend on Figure 2 , 3 It can be seen that after the image obtained by the image sensor in step (1) is processed into three dimensions and normalized, an ellipse is intercepted in the xy plane at a height of 0.2, and the major axis σ of the ellipse is... L The minor axis σ of the ellipse is 6.12. s It is 5.49.

[0049] Depend on Figure 4 , 5 It can be seen that when the values ​​of the major and minor axes of the ellipse in step (2) are simultaneously substituted into the curve, the point with the closest values ​​corresponds to a distance of 0.1 μm between the laser focus and the surface of the curved surface, and the angle between the laser and the tangent plane of the curved surface is 6°. At this time, the z-axis displacement stage displays 13.516 mm, and the z-axis coordinate of the grid point on the curved surface corresponding to this spot image is 13.5159 mm.

[0050] Depend on Figure 6 It can be seen that by performing three-dimensional processing on all light spot images, the three-dimensional coordinate lattice (x) corresponding to all grid points to be measured can be obtained. i y i , z i ).

[0051] Depend on Figure 7 It is known that because of the Gaussian beam diffraction effect in the finite aperture system, the small changes in the distance and angle between the laser focus and the curved surface will cause the spot image collected by the image sensor to undergo corresponding deformation, thereby changing the values ​​of the major axis and minor axis of the ellipse cropped in step (2). Therefore, the repeatability resolution is better than 100nm, which makes the curved surface measurement have high accuracy.

[0052] Depend on Figure 8 It can be seen that in step (3), the three-dimensional coordinate lattice (x) is scanned along the scanning direction. i y i , z i Interpolating at 50nm intervals can make the three-dimensional coordinate lattice more approximate the true coordinates of the curved surface.

[0053] Depend on Figure 9 It is known that the method of in-situ measurement and processing of curved surfaces using femtosecond lasers can completely prepare the school emblem pattern on the surface of a fused silica lens.

[0054] Depend on Figure 10 It is evident that without using the proposed femtosecond laser method for in-situ measurement and processing of curved surfaces, it is impossible to process a complete pattern, demonstrating the effectiveness of the proposed method.

[0055] Example 2

[0056] Anti-reflection structures were fabricated on the surface of a sapphire lens using a femtosecond laser.

[0057] By analyzing and processing the spot image obtained from the image sensor using a Python program, a three-dimensional coordinate lattice of the area to be processed on the sapphire lens surface is obtained, and processing data is generated based on the three-dimensional coordinate lattice. Then, a moving stage is used to accurately move the laser focus to the lens surface for mesh processing, and finally, a tapered antireflective structure is fabricated after etching.

[0058] The specific steps of using femtosecond lasers for in-situ measurement and processing of curved surfaces are as follows:

[0059] (1) Image acquisition of reflected light spots:

[0060] The area to be measured on the fused silica convex lens is divided into a 20*20 grid in the xy plane, and the area of ​​the area to be measured is 0.04 mm². 2 The coordinates of each grid point are (x, y, y). iy i The value of i ranges from 0 to 20. A spot image is acquired for each grid point. The x and y axis displacement stages are moved to position the laser focus at the starting grid point. The laser power is adjusted to 2mW, and the z axis displacement stage is moved for measurement. The laser emitted from the femtosecond laser is converted into circularly polarized light by a polarizing beam splitter and a quarter-wave plate, and finally focused onto the sample surface on the sample stage by the objective lens. The laser focused on the sample surface is reflected by the sample surface, passes through the objective lens and the quarter-wave plate, and is reflected to the other side by the polarizing beam splitter. It is then focused onto the image sensor by lens L1 to obtain the spot image of that grid point. The laser focus is moved to the next grid point, and the measurement is repeated until spot images of all grid points are acquired.

[0061] (2) Image 3D processing and 3D coordinate point matrix establishment:

[0062] The image of the light spot acquired by the image sensor at the starting grid point in step (1) is processed into a three-dimensional image using a Python program to obtain a three-dimensional light spot image with the corresponding light field distribution; an ellipse is truncated at a height of 0.2 in this three-dimensional light spot image, and the major axis σ of the ellipse is calculated. L and minor axis σ s The values ​​are 6.25 and 5.73, respectively. Substituting these two values ​​into the pre-calibrated curve, the point with the closest value corresponds to a distance of 0 μm between the laser focus and the curved surface, and an angle of 3° between the laser and the tangent plane of the curved surface. At this time, the z-axis displacement stage displays 7.217 mm, and the z-axis coordinate of the grid point on the curved surface corresponding to this spot image is 7.217 mm. The above processing is repeated for all spot images to obtain the three-dimensional coordinate point matrix (x) corresponding to all the grid points to be measured. i y i , z i ).

[0063] (3) Three-dimensional coordinate matrix interpolation and surface pattern processing:

[0064] The three-dimensional coordinate lattice (x) along the scanning direction and perpendicular to the scanning direction i y i , z i Interpolation is performed at intervals of 50 nm and 2 μm, i.e., 200 and 5 points are inserted in the two directions respectively, to obtain the three-dimensional coordinate lattice of the corresponding smooth surface. The coordinates of the three-dimensional lattice are then imported into the processing program. The scanning method of the processing program is line-by-line scanning, and the scanning line segment length is 50 nm. The sample is placed on the sample stage, and the x, y, and z axis displacement stages are moved so that the laser is at the starting grid point coordinates (0, 0, 7.217 mm), and a curved surface grid structure is obtained with a grid period of 2 μm. By etching the grid processed on the sapphire, a tapered anti-reflection structure can be obtained.

[0065] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0066] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable way without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0067] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for in-situ measurement and processing of curved surfaces using femtosecond lasers, characterized in that, Specifically, the steps include the following: Step 1: Image acquisition of reflected light spots; The area to be measured on the curved surface is divided into a grid in the xy plane. The coordinates of each grid point are (x, y, ...). i y i The laser spot image is acquired for each grid point; the x and y axis displacement stages are moved to place the laser focus at the starting grid point, the laser power is adjusted to a certain value and fixed, and the z axis displacement stage is moved to perform measurement; the laser emitted from the femtosecond laser is converted into circularly polarized light through a polarizing beam splitter and a quarter-wave plate, and finally focused on the sample surface on the sample stage through the objective lens. The laser focused on the sample surface is reflected by the sample surface, passes through the objective lens and quarter-wave plate in sequence, and is reflected to the other side by the polarizing beam splitter. It is then focused by the lens onto the image sensor to obtain the spot image of that grid point. The laser focus is moved to the next grid point and the measurement is repeated until the spot images of all grid points are obtained. Step 2: Image 3D Conversion and 3D Coordinate Matrix Establishment The light spot image acquired by the image sensor in step one is processed into a three-dimensional image using a Python program to obtain a three-dimensional light spot image with the corresponding light field distribution; an ellipse is then extracted from this three-dimensional light spot image at a certain height z0, and the major axis σ of the ellipse is calculated. L and minor axis σ s The value is used to obtain the z-axis coordinates of the grid points on the surface corresponding to the light spot image based on the pre-calibrated curve; the above process is repeated for all light spot images to obtain the three-dimensional coordinate lattice (x, y, z) of all grid points to be measured. i y i , z i ); Step 3: 3D coordinate matrix interpolation and surface pattern processing: The three-dimensional coordinate lattice (x) along the scanning direction and perpendicular to the scanning direction i y i , z i Interpolation is performed at fixed intervals, that is, M and N points are inserted in the two directions respectively to obtain the three-dimensional coordinate point matrix of the corresponding smooth surface. The three-dimensional coordinate point matrix of the corresponding smooth surface is imported into the processing program. The sample is placed on the sample stage, the x and y axis displacement stages are moved to make the laser in the starting processing position, and the z axis displacement stage is moved to make the laser focus on the surface of the curved surface to process the curved surface pattern.

2. The method for in-situ measurement and processing of curved surfaces using femtosecond lasers as described in claim 1, characterized in that, In step one, the mesh is divided according to the size of the area to be measured. The x, y, and z axes represent the movement directions of the displacement stage, respectively. The number of mesh points along the x-axis and y-axis is 5-20, and the total mesh area is 0.01-1 mm². 2 The femtosecond laser power is 2-10mW; the image sensor is placed at the focal length of the lens; the quarter-wave plate is adjusted so that the polarization direction of the incident light makes an angle of 45° with the two axes of the quarter-wave plate, so that the incident light can be reflected by the polarizing beam splitter after passing through the quarter-wave plate twice.

3. The method for in-situ measurement and processing of curved surfaces using femtosecond lasers as described in claim 1, characterized in that, In step two, the three-dimensional processing specifically involves normalizing the grayscale value of each pixel in the two-dimensional image according to the maximum grayscale value, and then using it as the z-axis coordinate for three-dimensional image rendering; the three-dimensional image is a three-dimensional distribution of an approximate Gaussian light field; the height z0 is 0.2-0.4; the z-axis coordinate is the coordinate displayed by a z-axis displacement stage defined when the distance between the laser focus and the curved surface is zero.

4. The method for in-situ measurement and processing of curved surfaces using femtosecond lasers as described in claim 1, characterized in that, In step two, the pre-calibrated curves include: a set of elliptical major axes σ L A curve relating the distance from the laser focus to the curved surface and a set of elliptical minor axes σ s Regarding the curves representing the distance from the laser focus to the curved surface, each set of curves is obtained by changing the angle between the laser and the tangent plane of the curved surface.

5. The method for in-situ measurement and processing of curved surfaces using femtosecond lasers as described in claim 1, characterized in that, In step three, the interpolation spacing along the scanning direction is 50-500 nm, and the interpolation spacing along the perpendicular scanning direction is 0.5-5 μm.

6. The method for in-situ measurement and processing of curved surfaces using femtosecond lasers as described in claim 1, characterized in that, In step three, the scanning method of the processing procedure is line-by-line scanning, and the length of the scanned line segment is 50-500nm.

7. The application of the method for in-situ measurement and processing of curved surfaces using femtosecond lasers as described in claim 1 in the processing of micro / nano structures, characterized in that... A grid structure is fabricated on a curved surface, and after etching, a tapered anti-reflective structure can be obtained, which can increase the transmittance of light in a fixed wavelength band.

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