Femtosecond laser interference direct writing system

Abstract

This application relates to a femtosecond laser interferometric direct-writing system, comprising: a laser generator for generating a femtosecond laser; a frequency doubling module located on the output side of the laser generator for increasing the frequency of the femtosecond laser; a divergence angle adjustment module located on the output side of the frequency doubling module; a beam splitting module located on the output side of the divergence angle adjustment module, comprising a first Fourier lens, a beam splitting grating, and a second Fourier lens arranged sequentially along the optical path, the beam splitting grating being used to split the light into two diffracted beams; a focusing module located on the output side of the second Fourier lens and used to generate interference fringes at the intersection point of the two diffracted beams; and an observation module for acquiring an image of the interference fringes. The beam splitting grating can move along the optical axis of the beam splitting module to adjust the angle between the two diffracted beams at the intersection point, and the divergence angle adjustment module is used to adjust the divergence angle of the femtosecond laser. The above scheme enables adjustable grating period, ensuring that the focal points of the two diffracted beams are located on the interference plane, allowing observation of the interference fringe positions and focal overlap.

Description

Technical Field

[0001] This application relates to the field of laser micro-nano fabrication technology, and in particular to femtosecond laser interferometry direct writing systems. Background Technology

[0002] Structural colors have garnered significant attention in display, anti-counterfeiting, and high-end decorative fields due to their advantages such as permanent colorfastness, environmental friendliness, and the ability to produce iridescent effects. Femtosecond laser dual-beam interferometry direct writing technology can directly fabricate submicron-level grating structures on material surfaces, representing a crucial approach to achieving high-precision, wide-gamut structural colors. However, in related technologies, the grating period in dual-beam interferometry direct writing optical paths is either fixed by the optical element's own structure, allowing only a single or limited number of fixed periods to be output, failing to achieve wide-range continuous adjustment and thus failing to meet the customized requirements of continuously variable periods for wide-gamut structural colors; or the period switching operation is cumbersome and time-consuming. Furthermore, in traditional optical paths, when changing the grating period, the interference plane containing the interference fringes often deviates from the surface being processed, and the focal points of the two diffracted beams also deviate from the interference plane (i.e., the intersection point), a deviation that is imperceptible to the operator. Summary of the Invention

[0003] Therefore, it is necessary to provide a femtosecond laser interferometric direct writing system to achieve a wide range of continuously adjustable grating period, without the need for cumbersome manual calibration when adjusting the period, and to ensure that the focal points of the two diffracted beams are located on the interference surface, and to observe the position of the interference fringes and the state of focal overlap in real time.

[0004] A femtosecond laser interferometry direct writing system, the femtosecond laser interferometry direct writing system comprising:

[0005] Laser generator, used to produce femtosecond lasers;

[0006] A frequency doubling module is disposed on the light-emitting side of the laser generator and is used to increase the frequency of the femtosecond laser;

[0007] A divergence angle adjustment module is disposed on the light-emitting side of the frequency doubling module;

[0008] The beam splitting module is located on the light-emitting side of the divergence angle adjustment module and includes a first Fourier lens, a beam splitting grating and a second Fourier lens arranged sequentially along the optical path. The beam splitting grating is used to split the light output by the first Fourier lens into two diffracted beams.

[0009] A focusing module, disposed on the light-emitting side of the second Fourier lens, is used to cause the two diffracted beams to intersect at the interference surface, thereby generating interference fringes at the intersection point; and

[0010] The observation module is used to acquire images of the interference fringes;

[0011] The beam splitter is configured to move along the optical axis of the beam splitter module to adjust the angle between the two diffracted beams at the intersection point. The divergence angle adjustment module is used to adjust the divergence angle of the femtosecond laser so that the focal points of the two diffracted beams are both located at the intersection point.

[0012] In some embodiments, the femtosecond laser interferometric direct writing system includes a scattering medium and a stage, with the observation module mounted on the stage; the observation module includes an imaging device, and is used to carry the scattering medium and the workpiece arranged at intervals; the stage is configured to be horizontally movable so that the scattering medium or the workpiece is located in the processing area;

[0013] In the calibration state, the scattering medium is located in the processing area, and the imaging device is used to receive the first scattered light generated by the interference fringes on the scattering medium to acquire an image of the interference fringes; in the processing state, the workpiece to be processed is located in the processing area.

[0014] In some embodiments, the observation module includes a light source, a semi-transparent mirror, and a first reflecting mirror; the light emitted by the light source is reflected by the semi-transparent mirror and then irradiates the scattering medium to generate second scattered light; the first scattered light and the second scattered light are transmitted through the semi-transparent mirror and then reflected by the first reflecting mirror to the imaging device.

[0015] In some embodiments, the divergence angle adjustment module includes a first convex lens and a second convex lens arranged sequentially along the optical path and having the same focal length. The first convex lens is configured to move along the optical axis of the divergence angle adjustment module to adjust the divergence angle of the femtosecond laser.

[0016] In some embodiments, the scattering medium and the workpiece to be processed have the same thickness in the vertical direction; in the calibration state, the stage is configured to move in the vertical direction so that the interference surface and the top surface of the scattering medium coincide.

[0017] In some embodiments, a calibration and reproduction module is further included, which is used to establish and store the displacement of the beam splitter grating, the displacement of the first convex lens, and the correspondence between the height of the support stage and the grating period.

[0018] In some embodiments, the two diffracted beams form a processing spot at the intersection point, the processing spot containing the interference fringes; the support stage is configured to reciprocate along a first direction and step along a second direction, the step size of the step along the second direction being not less than the diameter of the processing spot, wherein the first direction is perpendicular to the second direction, and both are perpendicular to the vertical direction.

[0019] In some embodiments, the system further includes a polarization adjustment module disposed between the frequency doubling module and the divergence angle adjustment module, and includes a half-wave plate and a quarter-wave plate arranged sequentially along the optical path.

[0020] In some embodiments, a second reflector is further included, disposed between the frequency doubling module and the polarization adjustment module. The second reflector is used to reflect the light emitted by the frequency doubling module to the polarization adjustment module and separate the unconverted fundamental frequency light.

[0021] In some embodiments, the beam splitting module further includes a spatial filter disposed between the beam splitting grating and the second Fourier lens, and is used to filter out diffraction light of other orders except for the two first-order diffraction beams.

[0022] In some embodiments, the beam-splitting grating is configured to rotate about the optical axis of the beam-splitting module to adjust the orientation of the interference fringes.

[0023] In some embodiments, the focusing module includes at least one of an aspherical lens, a microscope objective, a telecentric objective, or a combination aberration-correcting focusing lens group.

[0024] In some embodiments, the frequency doubling module includes a beam shrinking mirror, a frequency doubling crystal, and a beam expander arranged sequentially along the optical path.

[0025] In the aforementioned femtosecond laser interferometric direct writing system, the beam-splitting grating in the beam-splitting module can move along the optical axis of the module, thereby adjusting the angle between the two diffracted beams at the intersection point. This angle is related to the grating period, thus enabling a wide range of continuously adjustable grating periods. This adjustment can be achieved simply by moving the beam-splitting grating, making period switching simple and fast, overcoming the shortcomings of fixed periods or cumbersome switching in existing technologies. The divergence angle adjustment module adjusts the divergence angle of the femtosecond laser to ensure that the focal points of the two diffracted beams are both located at the intersection point (i.e., the interference surface), ensuring that the energy density of the processing area reaches the femtosecond laser ablation threshold and avoiding the inability to directly ablate due to focal deviation. The observation module can acquire images of the interference fringes, allowing the operator to observe the position of the interference surface and the focal overlap status in real time, providing a visual basis for optical path calibration and further ensuring processing accuracy and repeatability. In addition, since this system uses femtosecond lasers, its ultrashort pulse and high peak power characteristics enable it to directly perform ablation processing on the surface of non-photosensitive materials such as metals, eliminating the need for post-processing steps such as photosensitive material coating, exposure, and development, thereby simplifying the processing flow and broadening the scope of application. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the optical path of a femtosecond laser interferometry direct writing system in one embodiment of this application.

[0027] Figure 2 This is a schematic diagram of a grating fabricated using a femtosecond laser interferometry direct writing system in one embodiment of this application.

[0028] Figure 3 for Figure 2 A magnified view of a portion of point A in the middle.

[0029] Figure label:

[0030] 100. Laser generator; 200. Frequency doubling module; 210. Beam shrinking mirror; 220. Frequency doubling crystal; 230. Beam expander; 300. Second reflecting mirror; 400. Polarization adjustment module; 410. Half-wave plate; 420. Quarter-wave plate; 500. Divergence angle adjustment module; 510. First convex lens; 520. Second convex lens; 600. Beam splitting module; 610. First Fourier lens; 620. Beam splitting grating; 630. Second Fourier lens; 700. Focusing module; 800. Observation module; 820. Imaging device; 830. Light source; 840. Semi-transparent mirror; 850. First reflecting mirror; 910. Workpiece to be processed; 920. Scattering medium. Detailed Implementation

[0031] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0032] In the description of this application, it should be understood that if terms such as "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" appear, these terms indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0033] Furthermore, where the terms "first" and "second" appear, these terms are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, where the term "multiple" appears, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0034] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0035] In this application, unless otherwise expressly specified and limited, the use of descriptions such as "above" or "below" the second feature indicates that the first and second features are in direct contact or indirect contact via an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. Similarly, "below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0036] It should be noted that if an element is referred to as being "fixed to" or "set on" another element, it can be directly on the other element or there may be an intervening element. If an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be an intervening element. If so, the terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used in this application are for illustrative purposes only and do not represent the only possible implementation.

[0037] See Figures 1 to 3An embodiment of this application provides a femtosecond laser interferometric direct writing system including a laser generator 100, a frequency doubling module 200, a divergence angle adjustment module 500, a beam splitting module 600, a focusing module 700, and an observation module 800. The laser generator 100 generates a femtosecond laser; the frequency doubling module 200 is located on the output side of the laser generator 100 and is used to increase the frequency of the femtosecond laser; the divergence angle adjustment module 500 is located on the output side of the frequency doubling module 200; the beam splitting module 600 is located on the output side of the divergence angle adjustment module 500 and includes a first Fourier lens 610, a beam splitting grating 620, and a second Fourier lens 630 arranged sequentially along the optical path. The beam splitting grating 620 is used to focus the light emitted by the first Fourier lens 610. The output light is split into two diffracted beams; the focusing module 700 is located on the light-emitting side of the second Fourier lens 630 and is used to make the two diffracted beams intersect at the interference surface to generate interference fringes at the intersection point; the observation module 800 is used to acquire an image of the interference fringes; wherein, the beam splitting grating 620 is configured to be able to move along the optical axis of the beam splitting module 600 to adjust the angle between the two diffracted beams at the intersection point, and the divergence angle adjustment module is used to adjust the divergence angle of the femtosecond laser so that the focal points of the two diffracted beams are both located at the intersection point.

[0038] In the above embodiments, the beam splitter grating 620 in the beam splitter module 600 can move along the optical axis of the beam splitter module 600, thereby adjusting the angle between the two diffracted beams at the intersection point. This angle is related to the grating period, thus enabling a wide range of continuously adjustable grating period. Moreover, this adjustment can be achieved by moving the beam splitter grating 620, making the period switching operation simple and fast, overcoming the defects of fixed period or cumbersome switching in the prior art. The divergence angle adjustment module 500 adjusts the divergence angle of the femtosecond laser so that the focal points of the two diffracted beams are both located at the intersection point (i.e., the interference surface), ensuring that the energy density of the processing area reaches the femtosecond laser ablation threshold and avoiding the inability to directly ablate due to focal deviation. The observation module 800 can acquire images of interference fringes, allowing the operator to observe the position of the interference surface and the focal overlap state in real time, providing a visual basis for optical path calibration and further ensuring processing accuracy and repeatability. In addition, since this system uses femtosecond lasers, its ultrashort pulse and high peak power characteristics enable it to directly perform ablation processing on the surface of non-photosensitive materials such as metals, eliminating the need for post-processing steps such as photosensitive material coating, exposure, and development, thereby simplifying the processing flow and broadening the scope of application.

[0039] Specifically, in related technologies, there are two main types of schemes for direct-write optical paths using two-beam interference. One type uses a beam splitter in conjunction with a delay line, adjusting the optical path difference between the two beams to compensate for the insufficient coherence length of the femtosecond laser by adjusting the delay line; the other type uses diffractive optical elements, utilizing the diffraction and beam splitting characteristics of the elements themselves to directly generate two interference beams.

[0040] However, the first type of solution requires repeated manual calibration of the optical path difference, which is cumbersome and has poor repeatability. In contrast, the beam splitting module 600 in this embodiment employs a symmetrical optical path composed of a first Fourier lens 610, a beam splitting grating 620, and a second Fourier lens 630. A femtosecond laser beam passes through the first Fourier lens 610 and then illuminates the beam splitting grating 620, which splits it into two diffracted beams. These two diffracted beams then exit through the second Fourier lens 630. This symmetrical optical path ensures that the optical path lengths of the two diffracted beams are naturally equal, eliminating the need for delay line compensation and recalibrating the optical path when switching cycles. This solves the problems of cumbersome operation and poor repeatability in the first type of solution.

[0041] For the second type of scheme, the beam splitting period is fixed by the structure of the component itself, and can only output a single or a limited number of fixed periods, which cannot be continuously adjusted. In this embodiment, the beam splitting grating 620 is configured to move along the optical axis of the beam splitting module 600. By continuously changing the angle between the two diffracted beams at the intersection point, a wide range of continuously adjustable grating periods is achieved, overcoming the defect of fixed periods in the second type of scheme.

[0042] Regarding the adjustment of the grating period by changing the angle between the two diffracted beams at the intersection point, the following explanation is provided: d = λ / 2sinθ, where d is the grating period, λ is the wavelength of the two diffracted beams (i.e., the wavelength of the femtosecond laser after frequency doubling), and θ is the angle between the two diffracted beams at the intersection point. As can be seen from the above formula, when θ is adjusted, d will change. For example, increasing θ will decrease d.

[0043] Furthermore, another type of related technology uses continuous laser to perform interferometric imaging exposure on the surface of photosensitive materials. This type of approach requires pre-coating of the photosensitive material, followed by post-processing steps such as development and fixing after exposure. Moreover, the processing targets are limited to photosensitive materials, and it cannot directly form structures on non-photosensitive materials such as metals. In contrast, this application uses a femtosecond laser as the light source. Its ultrashort pulse and high peak power characteristics enable direct ablation and removal of materials. Therefore, it eliminates the need for any photosensitive medium coating and post-processing steps such as exposure and development. This allows for the direct fabrication of submicron-scale grating structures on the surface of non-photosensitive materials such as metals and semiconductors, simplifying the processing flow and expanding the material applicability range of the workpiece 910.

[0044] In this embodiment, the optical axis refers to the central axis of the optical system, that is, the direction of the main optical path for light propagation. "Move along the optical axis" means moving along the direction of light propagation, and "rotate around the optical axis" means rotating about the optical axis.

[0045] See Figures 1 to 3In some embodiments, the beam splitting module 600 further includes a spatial filter, which is disposed between the beam splitting grating 620 and the second Fourier lens 630 and is used to filter out diffraction light of other orders except for the two first-order diffraction beams.

[0046] Specifically, after the femtosecond laser passes through the first Fourier lens 610, a spatial spectrum distribution is formed on its back focal plane (i.e., the Fourier plane), which reflects the angular information of the incident light. Subsequently, the light on this Fourier plane illuminates the beam splitter 620, which generates multi-order diffraction light, including 0th, ±1st, and ±2nd orders. Only the ±1st order diffracted light consists of two symmetrical beams, which are the effective beams for generating interference fringes; other orders (such as 0th and ±2nd orders) interfere with the fringe quality. A spatial filter is positioned between the beam splitter 620 and the second Fourier lens 630 to filter out diffracted light of all orders except the two first-order diffracted beams, allowing only the ±1st order light to pass through. After the two first-order diffracted beams enter the second Fourier lens 630, they exit as two nearly parallel diffracted beams. Finally, the focusing module 700 causes these two diffracted beams to converge, generating interference fringes at the convergence point.

[0047] In the above embodiments, the spatial filter removes diffracted light of all orders except the two first-order diffracted beams, ensuring that only ±1st-order light enters the second Fourier lens 630. This avoids interference from stray light of the 0th and ±2nd orders, ensuring that the subsequently generated interference fringes have high contrast and good uniformity. High-contrast interference fringes can form a grating structure with clear edges and neat grooves on the surface to be processed, thereby improving the saturation and color consistency of the prepared structural colors, while ensuring the repeatability of processing quality.

[0048] See Figures 1 to 3 In some embodiments, the beam splitter 620 is configured to rotate about the optical axis of the beam splitter module 600 to adjust the orientation of the interference fringes.

[0049] Specifically, when the beam-splitter grating 620 rotates around the optical axis, the arrangement direction of its surface scribe lines changes accordingly. This causes the exit direction of the two first-order diffracted beams to deflect, and the deflection of the exit direction of the diffracted beams directly changes the orientation of the interference fringes on the surface to be processed. In other words, by rotating the beam-splitter grating 620, the orientation of the interference fringes can be continuously adjusted, thereby controlling the direction of the grating grooves ultimately formed on the surface of the workpiece 910.

[0050] In the above embodiments, the rotation of the beam-splitter 620 around the optical axis allows for continuous adjustment of the interference fringe orientation, thereby enabling flexible control of the grating groove direction as needed. This facilitates multi-directional grating splicing, optical anti-counterfeiting pattern design, and anisotropic device fabrication, eliminating the need to replace optical components or readjust the optical path; it can be achieved simply by rotating the beam-splitter 620, simplifying operation and enhancing the system's processing flexibility and applicability.

[0051] Furthermore, in some embodiments, the beam-splitting grating 620 is mounted on a displacement stage, which drives the beam-splitting grating 620 to reciprocate along the optical axis of the beam-splitting module 600, thereby changing the angle between the two diffracted beams and adjusting the period of the interference fringe grating. Simultaneously, the displacement stage is mounted on a rotating stage, which drives both the displacement stage and the beam-splitting grating 620 to rotate around the optical axis, thereby changing the orientation of the interference fringes. Alternatively, the beam-splitting grating 620 can be mounted on a rotating stage, and the rotating stage can be mounted on the displacement stage.

[0052] See Figures 1 to 3 In some embodiments, the femtosecond laser interferometric direct writing system includes a scattering medium 920 and a stage, with an observation module 800 mounted on the stage. The observation module 800 includes an imaging device 820, which carries the spaced scattering medium 920 and the workpiece 910 to be processed. The stage is configured to move horizontally so that the scattering medium 920 or the workpiece 910 is located in the processing area. In the calibration state, the scattering medium 920 is located in the processing area, and the imaging device 820 is used to receive the first scattered light generated by the interference fringes on the scattering medium 920 to acquire an image of the interference fringes. In the processing state, the workpiece 910 is located in the processing area.

[0053] Specifically, the optical elements of the observation module 800 (including components such as the imaging device 820) are encapsulated in a box placed on top of the support platform. A small hole is provided on the top of the box at the location where the scattering medium 920 is placed, but no hole is provided at the location where the workpiece 910 is placed. The scattering medium 920 and the workpiece 910 are respectively positioned at corresponding locations on the top of the box, and are arranged at intervals. The scattering medium 920 and the workpiece 910 have the same thickness in the vertical direction, therefore their top surfaces are flush.

[0054] When the system requires calibration, the stage moves horizontally, causing the observation module 800 to move synchronously, bringing the scattering medium 920 to the processing area. At this time, interference fringes illuminate the scattering medium 920. Due to the microscopic roughness of the surface of the scattering medium 920 (such as frosted glass), the interference fringes generate first scattered light propagating in all directions. This scattered light carries information about the brightness distribution of the interference fringes. The imaging device 820 (such as a CCD or CMOS camera) is positioned on the transmission side of the scattering medium 920, receiving the first scattered light transmitted to this side through a small hole at the top of the housing, forming an image of the interference fringes. This image allows the operator to determine the position of the interference surface and whether the focal points of the two diffracted beams are at their intersection. After calibration, the stage moves in the reverse direction, moving the workpiece 910 to the processing area for processing.

[0055] In the above embodiments, the horizontal movement of the support stage enables the switching between the scattering medium 920 and the workpiece 910 in the processing area, ensuring that calibration and processing do not interfere with each other. When the scattering medium 920 is located in the processing area, the imaging device 820 receives and images the first scattered light generated by the interference fringes, converting the invisible interference fringes into a visible image. This solves the problem that operators cannot detect deviations in the interference surface or focus in traditional optical paths. Therefore, it provides a direct reference for adjusting the height of the support stage and the divergence angle adjustment module (which will be further described later), improving calibration efficiency and processing consistency.

[0056] During calibration, the operator can determine the position of the interference surface and whether the focal point is located at the intersection point by using the images acquired by the imaging device 820. Specifically, when the scattering medium 920 is located in the processing area, the operator observes the light spots formed by the two diffracted beams in the image. If the two light spots completely overlap to form a single processing light spot, it indicates that the interference surface is exactly located on the top surface of the scattering medium 920. If the two light spots are offset from each other or partially separated, it indicates that the interference surface deviates from the top surface of the scattering medium 920. In this case, it is necessary to adjust the support stage to move vertically until the two light spots completely overlap, so that the top surface of the scattering medium 920 coincides with the interference surface. Since the scattering medium 920 is flush with the top surface of the workpiece 910, when the interference surface coincides with the top surface of the scattering medium 920, the interference surface also coincides with the top surface of the workpiece 910 (the surface to be processed). Thus, when the support stage is subsequently moved horizontally to bring the workpiece 910 to the processing area, the interference surface is exactly located on the surface to be processed.

[0057] After confirming that the interference surface coincides with the top surface of the scattering medium 920, the operator further observes the size of the processed spot in the image. If the diameter of the processed spot is the smallest, it indicates that the focal points of the two diffracted beams are both located at the intersection point (i.e., the interference surface), at which point the energy density is the highest, meeting the processing threshold for femtosecond laser direct writing. If the diameter of the processed spot is large or the edges are blurred, it indicates that the focal point has deviated from the intersection point. In this case, the divergence angle adjustment module needs to be adjusted until the diameter of the processed spot is the smallest.

[0058] See Figures 1 to 3 In some embodiments, the observation module 800 includes a light source 830, a semi-transparent mirror 840, and a first reflector 850; the light emitted by the light source 830 is reflected by the semi-transparent mirror 840 and then irradiates the scattering medium 920 to generate second scattered light; the first scattered light and the second scattered light are transmitted through the semi-transparent mirror 840 and then reflected by the first reflector 850 to the imaging device 820.

[0059] Specifically, the optical elements of the observation module 800 (including the light source 830, the semi-transparent mirror 840, the first reflecting mirror 850, and the imaging device 820) are encapsulated within the aforementioned enclosure. From the perspective of the attached diagram, the illumination light emitted by the light source 830 (e.g., an LED) is reflected by the semi-transparent mirror 840 and then passes upwards through a small hole at the top of the enclosure to illuminate the scattering medium 920, generating second scattered light propagating in all directions. The downward-propagating portion illuminates the entire field of view, improving image clarity. Simultaneously, interference fringes illuminate the scattering medium 920, generating first scattered light propagating in all directions, with the downward-propagating portion carrying fringe information. These two downward-propagating scattered light portions enter the enclosure through the small hole, then pass through the semi-transparent mirror 840, and continue propagating downwards to the first reflecting mirror 850. After being reflected by the first reflecting mirror 850, they enter the imaging device 820, forming a clear interference fringe image.

[0060] In the above embodiment, by adding a light source 830 to generate second scattered light, the scattering medium 920 is actively illuminated, enhancing the brightness and contrast of the image acquired by the imaging device 820 and making the light and dark boundaries of the interference fringes clearer. The downward propagation portions of the first scattered light (interference fringe information) and the second scattered light (background illumination) enter the imaging device 820 through the same optical path (transmission by the semi-transparent mirror 840 and reflection by the first reflecting mirror 850), eliminating the need for additional coupling elements and resulting in a compact structure. This solution significantly improves the imaging quality of the observation module 800 while maintaining a simple optical path and reduces the calibration difficulty for the operator.

[0061] See Figures 1 to 3In some embodiments, the divergence angle adjustment module 500 includes a first convex lens 510 and a second convex lens 520 arranged sequentially along the optical path and having the same focal length. The first convex lens 510 is configured to move along the optical axis of the divergence angle adjustment module 500 to adjust the divergence angle of the femtosecond laser.

[0062] Specifically, the first convex lens 510 is mounted on an adjustment mechanism that can move along the optical axis. When the first convex lens 510 moves along the optical axis, the distance between it and the second convex lens 520 changes, thereby altering the divergence angle of the emitted beam (i.e., the degree of convergence or divergence of the beam). This divergence angle adjustment function is used to adjust the focal points of the two diffracted beams to the intersection point (i.e., the interference plane), ensuring that the energy density reaches the femtosecond laser ablation threshold. For example, when the beam splitter grating 620 moves, the focal points of the two diffracted beams may deviate from the intersection point, resulting in a larger processing spot diameter and a decrease in energy density. At this time, by finely adjusting the position of the first convex lens 510, the divergence angle of the femtosecond laser can be changed until the processing spot diameter is minimized, indicating that the focal points of the two diffracted beams have returned to the intersection point.

[0063] In the above embodiments, by moving the first convex lens 510 to change its distance from the second convex lens 520, the divergence angle of the femtosecond laser can be continuously adjusted, thereby adjusting the focus of the two diffracted beams to the intersection point, minimizing the processing spot diameter and maximizing the energy density, thus meeting the requirements of femtosecond laser direct writing. Furthermore, compared to moving the second convex lens 520, moving the first convex lens 510 does not change the axial distance between the second convex lens 520 and the first Fourier lens 610, thus avoiding the introduction of additional beam focusing characteristics due to this distance change, and eliminating the need for additional adjustment steps to compensate for these additional variables. This focus compensation mechanism works in conjunction with grating period adjustment and can be achieved through only a single-axis movement, significantly simplifying the adjustment operation and improving the system's stability and adjustment efficiency.

[0064] See Figures 1 to 3 In some embodiments, the scattering medium 920 and the workpiece 910 have the same thickness in the vertical direction. In the calibration state, the stage is configured to move in the vertical direction so that the interference surface and the top surface of the scattering medium 920 coincide.

[0065] Specifically, when the beam splitter grating 620 moves along the optical axis to adjust the grating period, the axial position of the plane containing the interference fringes (i.e., the interference surface) also changes. Since the scattering medium 920 and the workpiece 910 have the same thickness in the vertical direction, and both are located on the same supporting surface (top surface of the box), their top surfaces are flush. Therefore, as long as the interference surface and the top surface of the scattering medium 920 coincide, it can be ensured that the interference surface is located on the top surface (the surface to be processed) of the workpiece 910 during processing. Therefore, the height of the supporting stage needs to be adjusted. During the calibration process, the operator observes the light spots formed by the two diffracted beams through the image captured by the imaging device 820: when the two light spots completely overlap into one processing light spot, it indicates that the interference surface coincides with the top surface of the scattering medium 920; if the two light spots are offset from each other or partially separated, it indicates that the interference surface deviates from the top surface of the scattering medium 920. At this time, it is necessary to adjust the supporting stage to move in the vertical direction until the two light spots completely overlap.

[0066] In the above embodiments, by setting a support stage that can move vertically and coordinating with the calibration of the observation module 800, the interference surface and the top surface of the scattering medium 920 are made to coincide. Furthermore, the same thickness ensures that the interference surface and the surface to be processed coincide. This avoids the problem of the interference surface deviating due to the adjustment of the grating period, which affects the processing quality, and ensures that the femtosecond laser can accurately act on the surface of the workpiece 910.

[0067] See Figures 1 to 3 In some embodiments, a calibration and reproduction module is also included, which is used to establish and store the displacement of the beam splitter 620, the displacement of the first convex lens 510, and the correspondence between the height of the stage and the grating period.

[0068] Specifically, the calibration and reproduction module can be a controller or storage unit, electrically connected to the displacement stage of the beam splitter 620, the adjustment mechanism of the first convex lens 510, and the support stage. During initial system debugging or parameter changes, the operator moves the beam splitter 620 to a specific position and adjusts the position of the first convex lens 510 and the height of the support stage accordingly, minimizing the diameter of the processed light spot (indicating the focal points of the two diffracted beams are at their intersection) and ensuring complete overlap of the two spots (indicating the interference surface coincides with the surface to be processed). At this point, the current grating period, the displacement of the beam splitter 620, the displacement of the first convex lens 510, and the height of the support stage are recorded. Through multiple calibrations, a correspondence table is established between these four parameters and stored in the calibration and reproduction module. In subsequent processing, when switching to a specific grating period is required, the calibration and reproduction module automatically calls the corresponding displacement based on the stored correspondence, driving the beam splitter 620, the first convex lens 510, and the support stage to the corresponding positions, eliminating the need for repeated manual adjustments.

[0069] In the above embodiments, the calibration and reproduction module establishes and stores the correspondence between the displacement of the beam splitter 620, the displacement of the first convex lens 510, the height of the stage, and the grating period, enabling rapid retrieval and accurate reproduction of processing parameters. When it is necessary to switch the grating period, the system can automatically drive the relevant components to the preset position according to the stored correspondence, eliminating the need for manual recalibration or adjustment each time, shortening the process changeover time, and improving the consistency of batch processing.

[0070] See Figures 1 to 3 In some embodiments, a polarization adjustment module 400 is also included. The polarization adjustment module 400 is disposed between the frequency doubling module 200 and the divergence angle adjustment module 500, and includes a half-wave plate 410 and a quarter-wave plate 420 arranged sequentially along the optical path.

[0071] Specifically, the half-wave plate 410 is used to rotate the linear polarization direction of the femtosecond laser, and the quarter-wave plate 420 is used to change the polarization state of the femtosecond laser. By combining and adjusting the angles of the two wave plates, any desired polarization state (such as linear polarization, circular polarization, or elliptical polarization) can be generated. In femtosecond laser processing, different polarization states affect material absorption, ablation threshold, and the formation of laser-induced periodic surface structures. By setting up the polarization adjustment module 400, the polarization state can be optimized for different materials and processing requirements. For example, using circularly polarized light can suppress laser-induced periodic surface structures, resulting in cleaner and more uniform grating trenches in the final fabrication.

[0072] In the above embodiments, by setting the half-wave plate 410 and the quarter-wave plate 420, the polarization state of the femtosecond laser can be flexibly controlled to adapt to different materials and processing requirements. For example, using circularly polarized light can effectively suppress laser-induced periodic surface structures, avoid generating unwanted textures in the grating grooves, thereby improving the purity and uniformity of the structural color; adjusting the linear polarization direction can also optimize processing efficiency. This module has a simple structure, is easy to adjust, and improves processing quality without increasing the complexity of the optical path.

[0073] See Figures 1 to 3 In some embodiments, a second reflector 300 is also included, which is disposed between the frequency doubling module 200 and the polarization adjustment module 400. The second reflector 300 is used to reflect the light emitted from the frequency doubling module 200 to the polarization adjustment module 400 and separate the unconverted fundamental frequency light.

[0074] Specifically, the second reflector 300 is positioned between the frequency doubling module 200 and the polarization adjustment module 400. Its surface is coated with a special film that provides high reflectivity for the short-wavelength laser light (i.e., the working band) generated after frequency doubling, while exhibiting high transmittance for the unconverted fundamental frequency light (long wavelength). When the light emitted from the frequency doubling module 200 (including the working band and residual fundamental frequency light) illuminates the second reflector 300, the working band light is reflected to the polarization adjustment module 400, while the fundamental frequency light passes through the second reflector 300 and exits the optical path, thus achieving spectral purification.

[0075] In the above embodiment, by setting the second reflector 300, the working wavelength is reflected to the polarization adjustment module 400 and the unconverted fundamental frequency light is separated, thereby improving the purity of the beam entering the subsequent optical path. The removal of the residual fundamental frequency light avoids its introduction of background noise into the interference fringes, thus improving the contrast and processing stability of the interference fringes. At the same time, the second reflector 300 also realizes the reversal of the optical path, making the system layout more compact and avoiding excessive horizontal dimensions, which is beneficial for integration and industrial applications.

[0076] See Figures 1 to 3 In some embodiments, the frequency doubling module 200 includes a beam shrinking mirror 210, a frequency doubling crystal 220, and a beam expander 230 arranged sequentially along the optical path.

[0077] Specifically, the beam shrinker 210 reduces the diameter of the femtosecond laser spot emitted by the laser generator 100 to increase the optical power density and enhance the frequency doubling efficiency. The frequency doubling crystal 220 (such as a BBO crystal) utilizes nonlinear optical effects to convert the incident fundamental frequency femtosecond laser into frequency-doubled light with increased frequency and decreased wavelength. The beam expander 230 expands the frequency-doubled narrow beam to the required diameter to match the entrance pupil size of subsequent optical elements.

[0078] In the above embodiments, the power density is increased by the beam shrinking mirror 210, thereby enhancing the frequency doubling efficiency. The frequency doubling crystal 220 reduces the femtosecond laser wavelength, shortening the minimum period achievable for subsequent interference fringes, thus expanding the color gamut of the structural colors and enabling the presentation of short-wavelength colors such as blue and violet. The beam expander 230 matches the beam to the entrance pupil of the subsequent large numerical aperture focusing element, ensuring focusing quality. This frequency doubling module has a simple structure and high conversion efficiency, maintaining the system's compactness and reliability while improving processing accuracy.

[0079] See Figures 1 to 3 In some embodiments, the focusing module 700 includes at least one of an aspherical lens, a microscope objective, a telecentric objective, or a combination aberration-correcting focusing lens group.

[0080] Specifically, all of these focusing elements have a large entrance pupil diameter, enabling the two diffracted beams to converge and generate interference fringes. When an aspherical lens is used, its numerical aperture is not less than 0.7, and it has a large working distance (e.g., approximately 25 mm), which helps avoid collisions between the lens and the workpiece 910 and allows space for other auxiliary equipment. Furthermore, the aspherical lens can correct spherical aberration, improving the quality of the light spot at the convergence point. In addition, the larger numerical aperture allows the two diffracted beams to converge at a larger angle. According to the grating period formula d = λ / 2sinθ, increasing the angle θ can further reduce the grating period d, thereby fabricating a finer grating structure.

[0081] In the above embodiments, the focusing module 700 employs a high numerical aperture focusing element, enabling the two diffracted beams to converge and generate interference fringes, thereby achieving grating periods at the submicron or even nanometer scale and meeting the requirement of fine periods for blue-violet structural colors. Simultaneously, the aspherical, telecentric, or apomorphic design effectively corrects aberrations such as spherical aberration and field curvature, ensuring precise overlap of the focal points of the two diffracted beams and obtaining high-contrast interference fringes. This is beneficial for fabricating grating structures with clear edges and neat groove shapes, improving the saturation and consistency of the structural colors.

[0082] See Figures 1 to 3 In some embodiments, two diffracted beams form a processing spot at the intersection point, and the processing spot contains the aforementioned interference fringes; the support stage is configured to reciprocate along a first direction and step along a second direction, and the step size W of the stepping movement along the second direction is not less than the diameter D of the processing spot, wherein the first direction is perpendicular to the second direction, and both are perpendicular to the vertical direction.

[0083] Specifically, the horizontal switching motion of the stage and the scanning motion do not interfere with each other, and the direction of the switching motion is parallel to either the first or second direction. The stage is configured to move along the first direction, causing the interference fringes to scan a straight grating region on the surface of the workpiece 910. Within the scanning region of each processing spot diameter D, there are multiple parallel grooves formed by the interference fringes, and the spacing between adjacent grooves is the grating period d. After a straight line scan is completed, the stage steps forward by a step size W along the second direction, and then performs the next reverse scan, forming an S-shaped scanning path. When the step size W is equal to the processing spot diameter D, the edges of the grating regions of two adjacent scans are exactly connected, achieving seamless splicing of a large area; when the step size W is greater than the processing spot diameter D, there is a gap between adjacent scanning regions, which is suitable for occasions requiring intermittent processing.

[0084] In the above embodiments, the stage reciprocates along a first direction and steps along a second direction, with a step size W not less than the processing spot diameter D, achieving controllable coverage of interference fringes on the surface of the workpiece 910. When W equals D, large-area seamless splicing can be obtained; when W is greater than D, interval scanning can be achieved. This motion mode is independent of the grating period adjustment function and can be controlled separately, ensuring both the accuracy of microscopic grating parameters (period, orientation) and efficient coverage of the macroscopic processing area, which is beneficial for large-scale production and the fabrication of large-area structural color devices.

[0085] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0086] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A femtosecond laser interferometry direct writing system, characterized in that, The femtosecond laser interferometry direct writing system includes: Laser generator, used to produce femtosecond lasers; A frequency doubling module is disposed on the light-emitting side of the laser generator and is used to increase the frequency of the femtosecond laser; A divergence angle adjustment module is disposed on the light-emitting side of the frequency doubling module; The beam splitting module is located on the light-emitting side of the divergence angle adjustment module and includes a first Fourier lens, a beam splitting grating and a second Fourier lens arranged sequentially along the optical path. The beam splitting grating is used to split the light output by the first Fourier lens into two diffracted beams. A focusing module, disposed on the light-emitting side of the second Fourier lens, is used to cause the two diffracted beams to intersect at the interference surface, thereby generating interference fringes at the intersection point; and The observation module is used to acquire images of the interference fringes; The beam splitter is configured to move along the optical axis of the beam splitter module to adjust the angle between the two diffracted beams at the intersection point. The divergence angle adjustment module is used to adjust the divergence angle of the femtosecond laser so that the focal points of the two diffracted beams are both located at the intersection point.

2. The femtosecond laser interferometry direct writing system according to claim 1, characterized in that, The femtosecond laser interferometric direct writing system includes a scattering medium and a stage, with the observation module mounted on the stage. The observation module includes an imaging device and is used to carry the scattering medium and the workpiece arranged at intervals. The stage is configured to move horizontally so that the scattering medium or the workpiece is located in the processing area. In the calibration state, the scattering medium is located in the processing area, and the imaging device is used to receive the first scattered light generated by the interference fringes on the scattering medium to acquire an image of the interference fringes; in the processing state, the workpiece to be processed is located in the processing area.

3. The femtosecond laser interferometry direct writing system according to claim 2, characterized in that, The observation module includes a light source, a semi-transparent mirror, and a first reflecting mirror; the light emitted by the light source is reflected by the semi-transparent mirror and then irradiates the scattering medium to generate second scattered light; the first scattered light and the second scattered light are transmitted through the semi-transparent mirror and then reflected by the first reflecting mirror to the imaging device.

4. The femtosecond laser interferometry direct writing system according to claim 2, characterized in that, The divergence angle adjustment module includes a first convex lens and a second convex lens arranged sequentially along the optical path with the same focal length. The first convex lens is configured to move along the optical axis of the divergence angle adjustment module to adjust the divergence angle of the femtosecond laser.

5. The femtosecond laser interferometry direct writing system according to claim 4, characterized in that, The scattering medium and the workpiece to be processed have the same thickness in the vertical direction; in the calibration state, the support stage is configured to move in the vertical direction so that the interference surface and the top surface of the scattering medium coincide.

6. The femtosecond laser interferometry direct writing system according to claim 5, characterized in that, It also includes a calibration and reproduction module, which is used to establish and store the displacement of the beam splitter grating, the displacement of the first convex lens, and the correspondence between the height of the support stage and the grating period.

7. The femtosecond laser interferometry direct writing system according to claim 2, characterized in that, The two diffracted beams form a processing spot at the intersection point, and the processing spot contains the interference fringes; the support stage is configured to reciprocate along a first direction and step along a second direction, and the step size of the step along the second direction is not less than the diameter of the processing spot, wherein the first direction is perpendicular to the second direction, and both are perpendicular to the vertical direction.

8. The femtosecond laser interferometry direct writing system according to any one of claims 1 to 7, characterized in that, It also includes a polarization adjustment module, which is disposed between the frequency doubling module and the divergence angle adjustment module, and includes a half-wave plate and a quarter-wave plate arranged sequentially along the optical path.

9. The femtosecond laser interferometry direct writing system according to claim 8, characterized in that, It also includes a second reflector disposed between the frequency doubling module and the polarization adjustment module. The second reflector is used to reflect the light emitted from the frequency doubling module to the polarization adjustment module and separate the unconverted fundamental frequency light.

10. The femtosecond laser interferometry direct writing system according to any one of claims 1 to 7, characterized in that, The beam splitting module also includes a spatial filter, which is disposed between the beam splitting grating and the second Fourier lens and is used to filter out diffraction light of other orders except for the two first-order diffraction beams.

11. The femtosecond laser interferometry direct writing system according to any one of claims 1 to 7, characterized in that, The beam splitter is configured to rotate about the optical axis of the beam splitter module to adjust the orientation of the interference fringes.

12. The femtosecond laser interferometric direct writing system according to any one of claims 1 to 7, characterized in that, The focusing module includes at least one of an aspherical lens, a microscope objective, a telecentric objective, or a combination aberration-correcting focusing lens group.

13. The femtosecond laser interferometry direct writing system according to any one of claims 1 to 7, characterized in that, The frequency doubling module includes a beam shrinking mirror, a frequency doubling crystal, and a beam expander arranged sequentially along the optical path.

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