Femtosecond laser machining apparatus and method based on a radial equal-width rotating slit

Abstract

The application relates to a femtosecond laser processing device and method based on a radial equal-width rotating slit, which comprises a laser emitting unit, a beam adjusting unit, a slit unit and a focusing unit arranged in sequence along an optical path, the slit unit comprises a slit base, a rotating driving mechanism and a plurality of rectangular slits formed on the slit base, each rectangular slit is equal-width and is distributed in a central radial manner relative to a radiation center, the radiation center is collinear with an optical axis, and the rotating driving mechanism is used for driving the slit base to rotate around the optical axis. By arranging the plurality of rectangular slits in a central radial manner on the slit base, not only a smaller three-dimensional spherical focal spot can be obtained than in the prior art, but also the radial equal-width slit provides multi-direction static pre-shaping, so that the initial light field distribution is more balanced in each direction, the tolerance to rotating speed fluctuation is higher, and the processing result is more stable and reliable.

Description

Technical Field

[0001] This invention belongs to the field of laser processing technology, specifically relating to a femtosecond laser processing device and method based on a radially shaped, uniformly wide rotating slit. Background Technology

[0002] Femtosecond laser direct writing technology, with its ultrashort pulse width and extremely high peak power density, has become the mainstream method for high-precision three-dimensional micro-nano fabrication within transparent materials. Compared with traditional ultraviolet laser processing and mechanical processing, femtosecond laser direct writing technology has significant advantages such as true three-dimensional processing capability, small heat-affected zone, and wide applicability to materials, demonstrating enormous application potential in fields such as photonic chips, microfluidic devices, and biomedical engineering.

[0003] However, traditional femtosecond laser direct writing technology faces a fundamental problem when processing the interior of transparent materials: when a femtosecond laser beam is focused into the interior of transparent glass through a high numerical aperture objective lens, the intensity distribution in the focal region exhibits a distinctly ellipsoidal shape due to beam refraction at the air-glass interface and the focusing characteristics of the objective lens itself. Specifically, the focal spot size along the optical axis is typically much larger than the focal spot size in the transverse plane perpendicular to the optical axis, with an aspect ratio generally between 3:1 and 5:1. This ellipsoidal focal field distribution directly results in an elliptical rather than ideal circular cross-section for the laser-processed structure. In optical waveguide applications, the elliptical waveguide cross-section leads to mode field mismatch and increases coupling loss; in microfluidic applications, the elliptical microchannel cross-section affects hydrodynamic characteristics and reduces chip performance. Therefore, how to transform the ellipsoidal focal field into a spherical focal field to achieve three-dimensional isotropic processing resolution has long been a technical challenge that has not been well resolved in this field.

[0004] To overcome the above problems, researchers have developed a variety of beam shaping techniques: (1) Synchronous spatiotemporal focusing technology enables isotropic processing in photosensitive glass and photoresist by synchronously focusing light of different frequency components in time. However, its application in materials such as quartz glass is limited by the non-reciprocal write characteristics caused by the tilt of the pulse leading edge.

[0005] (2) Multiple scanning technology obtains a circular cross section by scanning the same area multiple times in different directions, but the process is complex and the processing efficiency is low, making it difficult to meet the needs of practical applications.

[0006] (3) Fixed slit shaping technology: By inserting a rectangular slit in the optical path, the slit modulates the light beam and can obtain the processing effect of a circular cross section in a specific direction. However, since the slit direction is fixed, this circular cross section can only be obtained on a plane perpendicular to the slit direction. When the processing direction changes, the cross section will become elliptical again, and true three-dimensional isotropy cannot be achieved.

[0007] To address this, researchers have proposed a rotating slit shaping technique. This technique uses a rotating drive unit to rotate a rectangular slit at high speed, causing the elliptical focal field to rapidly superimpose in different directions, ultimately forming a spherical focal field distribution under the time-averaging effect. A rectangular slit of width L is placed within an expanded Gaussian beam. As the beam passes through the slit, its size is compressed in the direction perpendicular to the slit, while its size remains unchanged in the direction parallel to the slit. Therefore, the cross-section of the emitted beam is elliptical. This elliptical beam, after being focused by the objective lens, forms an elliptical focal spot near the focal plane. When the slit rotates around the optical axis at an angular velocity ω, the direction of the elliptical focal spot changes periodically with time; at any time t, the direction angle of the focal spot is θ = ωt. Because the pulse width of a femtosecond laser is extremely short (typically less than 300 fs), while the slit rotation period is relatively long (typically in the range of 0.03 s to 0.1 s), the duration of a single laser pulse is much shorter than the rotation period. Therefore, the slit direction corresponding to each laser pulse can be considered fixed, and different pulses correspond to different slit directions. When a large number of pulses act on the material successively, the total effect on the material is the superposition of elliptical modified regions in various directions. If the number of pulses is sufficient and the rotation speed is fast enough, this superposition, in a statistically average sense, will form a spherical modified region.

[0008] However, in-depth analysis revealed that existing rotary slit shaping techniques still have significant shortcomings: (1) The focal spot size is still too large: Under optimized processing parameters, the focal spot diameter obtained by this technology in various glass materials is generally in the range of 5μm~15μm. This size still has room for improvement for the increasing processing accuracy requirements of current integrated optoelectronic devices.

[0009] (2) The focal field exhibits a significant side lobe structure: Since the light field distribution generated by a single rectangular slit contains a large number of high-frequency diffraction components, although a spherical main focal spot is formed after rotation and superposition, there are still side lobes of considerable intensity around the main focal spot. These side lobes may cause unnecessary modification to the material around the main focal spot during actual processing, affecting the processing quality.

[0010] (3) When a single elliptical spot is rotated and superimposed, it contributes in only one direction at any given moment; to achieve a uniform spherical distribution, it needs to rotate a sufficient number of times, and the material response must be linear. In actual processing, if the direct writing speed is too fast, the slit direction corresponding to adjacent pulses changes significantly, which may lead to uneven modification in local directions, manifesting as a periodic stripe structure. Summary of the Invention

[0011] This invention relates to a femtosecond laser processing apparatus and method based on a radially oriented, uniformly wide rotating slit, which can at least solve some of the defects of the prior art.

[0012] This invention relates to a femtosecond laser processing apparatus based on a radially oriented, equally wide rotating slit, comprising a laser emitting unit, a beam adjustment unit, a slit unit, and a focusing unit arranged sequentially along the optical path. The slit unit includes a slit base, a rotation drive mechanism, and a plurality of rectangular slits formed on the slit base. Each rectangular slit is of equal width and is radially distributed relative to a radiation center, which is collinear with the optical axis. The rotation drive mechanism is used to drive the slit base to rotate around the optical axis.

[0013] As one embodiment, a light-blocking part is provided at the radiation center, and the light-blocking part is connected to the slit base through a connecting bridge.

[0014] As one embodiment, the light-blocking part has a circular structure.

[0015] As one implementation method, both the light-blocking part and the connecting bridge are pre-formed when the plurality of rectangular slits are processed on the slit base.

[0016] As one implementation method, the ratio of the feature size of the light-blocking part to the width of the rectangular slit is controlled within the range of 0.3 to 0.5.

[0017] As one implementation method, the width of the rectangular slit is in the range of 300~600μm.

[0018] As one embodiment, the rotary drive mechanism is used to drive the slit base to rotate at a speed of 10~30 revolutions / s.

[0019] As one implementation method, the femtosecond laser processing device also includes a motion control unit, which is used to carry the sample to be processed and drive the sample to be processed to move along a preset trajectory.

[0020] The present invention also relates to a femtosecond laser processing method based on a radially oriented equal-width rotating slit, implemented using the aforementioned femtosecond laser processing apparatus; The femtosecond laser processing method includes: The laser emitting unit is activated to generate a femtosecond laser. The slit base is driven to rotate by a rotary drive mechanism; The femtosecond laser beam is expanded by the beam adjustment unit and then incident perpendicularly onto the slit base and through each rectangular slit. The laser beam emitted from the slit base is focused onto the target position of the sample to be processed by the focusing unit.

[0021] As one implementation method, the femtosecond laser processing method further includes, The sample to be processed is driven to move along a preset processing trajectory at a preset direct writing speed v1, wherein the direct writing speed v1 satisfies: v1 / (2v2)<d; Where v2 is the rotational speed of the slit base, and d is the focal spot diameter.

[0022] The present invention has at least the following beneficial effects: This invention utilizes a slit base with multiple rectangular slits arranged radially from a central point. When a Gaussian beam passes through these radially spaced slits, it is divided into multiple directional petal-like structures, each petal corresponding to the light-transmitting area of ​​a rectangular slit. This not only achieves a smaller three-dimensional spherical focal spot than existing technologies, but the radially spaced slits also provide multi-directional static pre-shaping, resulting in a more balanced initial light field distribution in all directions, higher tolerance to rotational speed fluctuations, and more stable and reliable processing results. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 A schematic diagram of the femtosecond laser processing apparatus provided in an embodiment of the present invention; Figures 2-4 A schematic diagram of a four-armed, six-armed, and eight-armed radial equal-width slit unit provided in an embodiment of the present invention; Figure 5 A schematic diagram of an integrally formed radial equal-width slit unit provided in an embodiment of the present invention; Figure 6 A schematic diagram of the beam intensity distribution before and after the light is blocked at the center; Figure 7 The images show the Gaussian spot after passing through a static single slit and after passing through a rotating single slit and being focused. Figure 8 The images show the Gaussian spot after passing through a static four-armed radial equal-width slit unit and after passing through a rotating four-armed radial equal-width slit unit and being focused. Figure 9 The images show the Gaussian spot after passing through a static, centrally blocked, four-armed radially equal-width slit unit and a centrally blocked, rotating, four-armed radially equal-width slit unit, and then focusing. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1

[0027] like Figure 1 This invention provides a femtosecond laser processing apparatus based on a radially oriented, equally wide rotating slit, comprising a laser emitting unit 1, a beam adjustment unit, a slit unit 6, and a focusing unit 10 arranged sequentially along the optical path. The slit unit 6 includes a slit base 60, a rotation drive mechanism 7, and a plurality of rectangular slits 62 formed on the slit base 60. Each rectangular slit 62 is of equal width and is radially distributed relative to a radiation center, which is collinear with the optical axis. The rotation drive mechanism 7 is used to drive the slit base 60 to rotate around the optical axis.

[0028] Preferably, the laser emitting unit 1 is used to generate femtosecond laser pulses with a center wavelength in the range of 1030nm~1050nm, a pulse width of no more than 300fs, and a repetition frequency of 100kHz~1MHz.

[0029] Preferably, the beam adjustment unit described above is used to adjust the pulse energy and polarization state of the laser, and to expand the beam diameter to the desired size. In one embodiment, such as Figure 1 The beam adjustment unit includes a half-wave plate 2, a polarization beam splitter cube 3, a quarter-wave plate 4, and a beam expander 5 arranged sequentially along the optical path. The half-wave plate 2 is mainly used to change the polarization angle incident on the polarization beam splitter cube 3, thereby precisely controlling the laser energy that finally enters the processing optical path. The polarization beam splitter cube 3 can split a beam of light into two beams (P-beam and S-beam) with mutually perpendicular polarization states. It works in conjunction with the front-end half-wave plate 2. When the polarization direction of the half-wave plate 2 changes, the energy ratio of the P-beam and S-beam changes. By adjusting the angle, only a small portion (or...) of the required energy can be controlled. Most of the P-wave light is transmitted into the subsequent optical path, thereby achieving high-precision, lossless laser power adjustment; the quarter-wave plate 4 is used to convert linearly polarized light into circularly polarized light. The electric field direction of the circularly polarized light rotates rapidly with time. Under the time averaging effect, its modification effect on the material is uniform in all directions. This helps to cooperate with the above-mentioned radial slits to further improve the roundness and isotropy of the three-dimensional microstructure (such as the optical waveguide section); the beam expander 5 is mainly used to expand the laser beam. After beam expansion, the Gaussian beam can interact more fully with the entire slit structure to achieve precise pre-shaping.

[0030] Optionally, the focusing unit 10 employs an objective lens with a numerical aperture of 0.4 to 0.6. The focusing unit 10 is used to focus the laser beam emitted from the slit base 60 onto the target position of the sample 11 to be processed, for example, to focus the laser beam modulated by the slit unit 6 onto the interior of the glass sample.

[0031] More preferably, such as Figure 1 The femtosecond laser processing apparatus also includes a motion control unit 12, which carries the sample 11 to be processed and drives the sample 11 to move along a preset trajectory. In one embodiment, the motion control unit 12 employs a three-dimensional electric displacement platform to achieve three-dimensional scanning processing.

[0032] More preferably, such as Figure 1 The device also includes an illumination source 13, a dichroic mirror 9, and a CCD camera 14 for real-time observation of the processing. The light emitted from the illumination source 13 is directed onto the sample via the dichroic mirror 9 and the focusing unit 10. The reflected light returns and passes through the dichroic mirror 9 before entering the CCD camera 14. The laser emitted from the slit unit 6 is reflected by the reflecting mirror 8 and the dichroic mirror 9, and then focused by the focusing unit 10 onto the target position on the sample.

[0033] Furthermore, the femtosecond laser processing device also includes a computer control system 15, which is electrically connected to the laser emitting unit 1, the rotary drive mechanism 7, the motion control unit 12, and the CCD camera 14 to achieve coordinated control.

[0034] Understandably, the length direction of the rectangular slit 62 is parallel to its radiation direction, while the width direction is perpendicular to its length direction. Furthermore, the rectangular slit 62 penetrates both sides of the slit base 60 to ensure the laser beam can pass through. In one embodiment, the width of the rectangular slit 62 is in the range of 300~600μm.

[0035] The rectangular slits 62 are evenly spaced, meaning that the included angle between any two adjacent rectangular slits 62 is equal. Figure 2 In the illustrated scheme, the slit base 60 is provided with four rectangular slits 62, and the included angle between any two adjacent rectangular slits 62 is 90°; Figure 3 In the illustrated scheme, the slit base 60 has six rectangular slits 62, and the included angle between any two adjacent rectangular slits 62 is 60°; Figure 4 and Figure 5 In the illustrated scheme, the slit base 60 is provided with eight rectangular slits 62, and the included angle between any two adjacent rectangular slits 62 is 45°.

[0036] In one embodiment, the aforementioned rotary drive mechanism 7 includes a hollow rotating shaft and a rotary power structure. A slit base 60 is fixedly mounted on one end of the hollow rotating shaft, the axis of which is collinear with the optical axis. The rotary power structure drives the hollow rotating shaft to rotate, including but not limited to a drive motor and a rotary transmission assembly. The rotary transmission assembly can be a gear drive, belt drive, sprocket drive, etc. By using a hollow rotating shaft to drive the slit base 60 to rotate, the hollow inner cavity of the hollow rotating shaft allows the laser beam to pass through, thus preventing interference with the optical path.

[0037] Preferably, the rotary drive mechanism 7 is used to drive the slit base 60 to rotate at a speed of 10 to 30 revolutions / s.

[0038] Based on the above scheme, by setting multiple rectangular slits 62 arranged radially around the slit base 60, when a Gaussian beam passes through the radially equal-width slits, the beam is divided into multiple directional petal structures, each petal corresponding to the light-transmitting area of ​​a rectangular slit 62. In the spatial frequency domain, this multi-directional structure corresponds to spectral components in multiple directions, and its distribution is more uniform than that of a single slit. Specifically, the spectrum of a single slit is mainly concentrated on an axis perpendicular to the slit direction, while the spectrum of a radially equal-width slit is distributed on multiple axes, more closely resembling a circularly symmetrical distribution. This spectral distribution characteristic makes the initial contributions of each direction more balanced during subsequent rotational superposition, requiring fewer rotations to achieve a spherical focal field and lowering the requirements for rotational speed.

[0039] In one embodiment, such as Figures 1-5 A light-blocking part 61 is provided at the radiation center, and the light-blocking part 61 is connected to the slit base 60 via a connecting bridge 63. The light-blocking part 61 is preferably circular.

[0040] Furthermore, the aforementioned light-blocking part 61, rectangular slit 62, and slit base 60 are integrally formed. Specifically, the light-blocking part 61 and the connecting bridge 63 are pre-formed when processing the plurality of rectangular slits 62 on the slit base 60, i.e., they are manufactured using an integral molding process. This includes, but is not limited to, processing in the following manner: using a thin metal plate with a thickness of 0.10mm to 0.30mm as the slit base 60; cleaning the slit base 60 and then processing it using a laser cutting device; completely cutting away the slit area to form a light-transmitting gap; leaving the light-blocking part 61 and the connecting bridge 63 uncut to form an integral structure. Preferably, there are multiple connecting bridges 63 (more preferably three or more), with a width W of 100μm to 300μm, evenly distributed in the area between adjacent rectangular slits 62, used to connect the central light-blocking part 61 and the slit base 60.

[0041] From the perspective of Fourier optics, the central portion of an incident Gaussian beam corresponds to the low-frequency component of the spatial spectrum, while the edge portion corresponds to the high-frequency component. In this embodiment, a light-blocking part 61 is provided in the central region of the radially shaped equal-width slit structure to block the low-frequency component at the center of the beam, which is equivalent to applying a high-pass filter to the beam. After the light is blocked, the relative proportion of the high-frequency component in the spatial spectrum of the transmitted beam increases. Based on the imaging theory of diffraction-limited optical systems, the increase in high-frequency components helps to reduce the focal spot size. At the same time, since the low-frequency background is weakened, the intensity of the side lobes around the focal spot is also reduced, thus significantly improving the laser processing effect.

[0042] In the single-slit rotation scheme, the low-frequency background persists throughout the rotation process and is quite intense, making focal spot compression difficult. In this embodiment, however, the radial, equal-width slit structure itself already segments the beam in multiple directions, which to some extent reduces the proportion of the central low-frequency component. Furthermore, applying a central light block further eliminates the residual low-frequency background. The combined effect of these two methods results in a final focal field that not only possesses spherical symmetry but also has a smaller size and lower side lobes.

[0043] Furthermore, since the radial equal-width slit structure provides an initial distribution in multiple directions, the transient non-uniformity during the rotation superposition process is effectively smoothed, and the conditions for generating periodic stripes become more stringent, thereby expanding the processing parameter window for aperiodic structures.

[0044] Furthermore, a mathematical model is used to describe the formation process of the focal field in this embodiment: Let the electric field distribution of the incident Gaussian beam be: ; Where ω0 is the radius of the beam waist spot, and x and y are the spatial coordinates in the transverse plane perpendicular to the optical axis.

[0045] The transmission function R(x,y) of slit unit 6 is derived from the transmission functions R of multiple rectangular slits 62. i The (x,y) are superimposed, and each rectangular slit 62 corresponds to a rectangular window in a specific direction with a width of L.

[0046] The central blocking function C(x,y) is defined as follows: when r < D / 2, C = 0 (within the blocking area); when r ≥ D / 2, C = 1 (outside the blocking area). D is the characteristic dimension of the light-blocking part 61 (e.g., the diameter of the circular light-blocking part 61).

[0047] Therefore, the total transmission function after passing through the slit is: N is the number of rectangular slits 62.

[0048] The beam modulated by the radial slit structure of equal width is then rotated, and the rotation angle φ changes with time as φ=ωt, where ω=2πv2, v2 is the rotation speed, and t is time.

[0049] The transmission function after rotation is: T rot (x,y,t)=T(xcosωt+ysinωt,−xsinωt+ycosωt); According to the Fresnel diffraction integral formula, the electric field distribution E near the focal plane is... focus (x, y, z) can be obtained by performing a Fourier transform on the field distribution on the exit pupil surface. Considering the time-averaging effect of rotation, the effective intensity distribution actually acting on the material is: dt.

[0050] When the rotation speed is sufficiently high, the time averaging effect causes the focal field distribution to tend towards circular symmetry. Compared with the single-slit rotation scheme, due to the introduction of the radial equal-width slit structure and the central light blocking, the lateral and axial dimensions of the effective focal field are significantly reduced, and the sidelobe intensity is greatly reduced.

[0051] The ratio D / L between the feature size of the light-blocking part 61 and the width of the rectangular slit 62 is one of the key parameters affecting the shaping effect. When D / L < 0.3, the light-blocking part 61 is insufficient in blocking the low-frequency components of the beam center, the focal spot size compression effect is not obvious, and the side lobes are still visible. When D / L > 0.5, although the focal spot is further compressed and the side lobes are completely suppressed, excessive energy loss leads to a decrease in the contrast of the inscribed lines, affecting the processing quality. Therefore, in this embodiment, controlling D / L within the range of 0.3 to 0.5 can effectively compress the focal spot size, basically eliminate the side lobes, and ensure sufficient beam energy transmittance, achieving the best balance between focal spot compression, side lobe suppression, and energy efficiency.

[0052] Figure 6 The diagram shows a comparison of the beam intensity distribution before and after the center light block: In the intensity distribution diagram of the incident Gaussian beam (top image), a circular spot is shown, with high brightness at the center and low brightness at the edges. The Gaussian distribution is represented by a grayscale gradient, with the horizontal axis representing the radial distance r and the vertical axis representing the normalized intensity I / I0. In the intensity distribution diagram of the beam after the center light block, an annular spot is shown, with the central area blocked to form a dark area, while the outer perimeter maintains the Gaussian distribution outline. The horizontal axis represents the radial distance r, and the vertical axis represents the normalized intensity I / I0. The intensity corresponding to the blocked area is zero.

[0053] from Figures 7-9 Comparison shows that: Figure 7 The single-slit rotation scheme shown forms an elliptical focal field with a large focal spot size and obvious side lobes. Figure 8 The four-arm radial slit rotation scheme shown can obtain a spherical focal field, but there is still a certain side lobe phenomenon. Figure 9 The four-arm radial slit rotation scheme shown, with the addition of a central light blocker, completely suppresses the side lobes, resulting in the purest focal field.

[0054] Example 2

[0055] This invention provides a femtosecond laser processing method based on a radially oriented, uniformly wide rotating slit. This femtosecond laser processing method is implemented based on the femtosecond laser processing apparatus provided in Embodiment 1 above.

[0056] The femtosecond laser processing method includes: Start laser emitting unit 1 to generate femtosecond laser; The slit base 60 is rotated by the rotary drive mechanism 7; The femtosecond laser beam is expanded by the beam adjustment unit and the expanded femtosecond laser beam is incident perpendicularly onto the slit base 60 and passes through each rectangular slit 62. The laser beam emitted from the slit base 60 is focused onto the target position of the sample 11 to be processed by the focusing unit 10.

[0057] The relevant content has been described in the above embodiment one, and will not be repeated here.

[0058] Preferably, when the sample to be processed 11 is a glass sample, after the laser beam is modulated by the slit unit 6, the focusing unit 10 focuses it to a depth of 100~500μm inside the glass sample.

[0059] More preferably, the femtosecond laser processing method further includes: The sample 11 to be processed is driven to move along a preset processing trajectory at a preset direct writing speed v1, wherein the direct writing speed v1 satisfies: v1 / (2v2)<d; Where v1 is in μm / s; v2 is the rotational speed of the slit base 60 in revolutions / s; and d is the focal spot diameter in μm. Although v1 / (2v2) is in μm / revolution in terms of dimensions, the numerical value is compared in the process criteria. Specifically, a processing unit is defined as half a revolution of slit rotation. Under this definition, the sample displacement corresponding to each processing unit is v1 / (2v2) (numerically equivalent to a spatial distance in μm). To ensure that the working areas of adjacent processing units overlap to form continuous processing lines, the sample displacement must be less than the focal spot diameter d.

[0060] Based on the above scheme, by strictly controlling the ratio between the laser direct writing speed and the slit rotation speed, periodic stripe structures can be avoided, thus achieving a better processing effect with uniformity and continuity.

[0061] Example 3

[0062] This embodiment uses a quartz glass sample as an example to illustrate the femtosecond laser processing apparatus and method. Quartz glass has excellent optical properties and thermal stability, and is a commonly used substrate material for optical waveguide devices.

[0063] (1) Device configuration

[0064] Laser emitting unit 1 uses a femtosecond laser with a center wavelength of 1030 nm, a pulse width of 270 fs, and a repetition frequency of 200 kHz. The laser beam emitted from the laser is expanded to a diameter of 6 mm by a beam expander 5. A half-wave plate 2 and a polarization beam splitter cube 3 are placed in the optical path before the beam expander 5 to adjust the pulse energy, and a zero-order quarter-wave plate 4 is placed to convert linearly polarized light into circularly polarized light.

[0065] The core component of this device is a four-armed radial slit unit 6 of equal width, manufactured using a one-piece molding process. Specifically, a 0.15mm thick stainless steel sheet is used as the base material, and it is processed using ultraviolet laser cutting equipment. Each of the four rectangular slits 62 has a width of 600μm, with an included angle of 90° between adjacent arms. Each arm extends outward from the center with an equal width and a total length of 2mm. At the intersection center of the four arms, a circular light-blocking part 61 with a diameter of 240μm is formed using a one-piece molding process. This light-blocking part 61 is connected to the outer slit base 60 via four small connecting bridges 63, forming an integral structure.

[0066] The rotary drive mechanism 7 employs a power element capable of providing a stable rotational speed, ranging from 6 revolutions per second to 55 revolutions per second. A slit base 60, equipped with a rectangular slit 62 and a light-blocking portion 61, is precisely mounted on the hollow shaft of the rotary drive mechanism 7, aligning the radiation center with the optical axis. In this embodiment, the rotational speed of the rotary drive mechanism 7 is set to 18 revolutions per second.

[0067] The focusing objective is a plan apochromatic objective with a numerical aperture of 0.5, which has good transmittance and aberration correction performance in the near-infrared band. In this embodiment, the objective lens has a magnification of 20x and a working distance of 2.1mm.

[0068] The quartz glass sample to be processed measures 10mm × 10mm × 5mm, and all six sides are optically polished. The sample is fixed on a three-dimensional electric displacement platform with a positioning accuracy of 0.5 micrometers. The sample position is adjusted so that the laser focus is located at a depth of 200μm below the sample surface.

[0069] (2) Processing parameters

[0070] In this embodiment, the processing parameters are set as follows: pulse energy is 120 nJ per pulse, slit rotation speed is 18 revolutions per second, and laser direct writing speed is 40 μm / s.

[0071] (3) Processing results

[0072] Under the aforementioned parameters, the laser beam passes sequentially through the rotating four-armed radial equal-width slit unit 6 and the focusing objective lens, forming a spherical focal field inside the glass. The displacement platform is controlled to move linearly by 10 mm in the horizontal direction, thus etching a linear optical waveguide.

[0073] The glass sample with the optical waveguide inscribed on it was polished at the end face, and the morphology of the waveguide cross-section was observed using an optical microscope. The observation results showed that the waveguide cross-section was perfectly circular with a diameter of approximately 7.2 μm. Compared with the approximately 9.3 μm circular cross-section obtained by the traditional single-slit rotation technique, the focal spot size of this embodiment was reduced by approximately 23%.

[0074] Optical coupling tests were conducted using a 1550 nm semiconductor laser. The laser was coupled into the waveguide input via a single-mode fiber, and near-field spot images were captured at the output using an infrared camera. Test results showed that the waveguide supports stable single-mode transmission, with a circularly symmetrical output spot and a mode field diameter of approximately 7.5 μm. Input and output power were measured using a power meter, and the insertion loss of the device was calculated to be approximately 1.35 dB.

[0075] Example 4

[0076] This embodiment uses the fabrication of an optical waveguide with a curved section in Eagle glass as an example for illustration.

[0077] (1) Device configuration

[0078] In this embodiment, the radially oriented equal-width slit unit 6 adopts a six-arm structure (i.e., six rectangular slits 62) and is manufactured using the same integrated molding process as in Embodiment 3. Each rectangular slit 62 has a width of 300 μm. The central light-blocking portion 61 is circular with a diameter of 120 μm, and the ratio of the diameter of the light-blocking portion 61 to the width of the rectangular slit 62 is 0.4.

[0079] The focusing objective lens is a plan apochromatic objective lens with a numerical aperture of 0.5. The Eagle glass sample to be processed has dimensions of 10mm × 10mm × 5mm.

[0080] (2) Processing parameters

[0081] The laser processing parameters are set as follows: pulse energy is 50 nJ per pulse, slit rotation speed is 25 revolutions per second, laser direct writing speed is 80 μm / s, and processing depth is 200 μm below the glass surface.

[0082] The curved optical waveguide design employs a circular arc bending structure with bending radii of 30mm, 40mm, and 50mm. The total waveguide length is 20mm, with the curved section located in the middle. A computer-controlled displacement platform scans along a preset bending trajectory to complete the characterization of the waveguide structure.

[0083] (3) Processing results

[0084] The cross-sectional morphology of various parts of the curved waveguide was observed using an optical microscope. The results showed that the waveguide cross-section was circular in both the straight and curved sections, with a uniform diameter of approximately 4.5 μm.

[0085] The insertion loss of the waveguide was tested using a 1550nm wavelength laser. For a waveguide with a bending radius of 50mm, the insertion loss was approximately 2.2 dB; for a waveguide with a bending radius of 40mm, the insertion loss was approximately 3.5 dB; and for a waveguide with a bending radius of 30mm, the insertion loss was approximately 5.6 dB. In comparison, using a single-slit rotation technique with the same parameters to fabricate a waveguide with a bending radius of 50mm, the insertion loss was approximately 4.9 dB, and effective light guiding could not be achieved when the bending radius was less than 50mm.

[0086] Example 5

[0087] This embodiment uses the fabrication of hollow microchannels in Foturan photosensitive glass, which can be used for microfluidic chips, as an example for illustration.

[0088] (1) Device configuration

[0089] In this embodiment, the radial equal-width slit unit 6 adopts an eight-arm structure, and the width of each rectangular slit 62 is 450μm. The central light-blocking part 61 is circular with a diameter of 180μm, and the ratio of the diameter of the light-blocking part 61 to the width of the rectangular slit 62 is 0.4.

[0090] The focusing objective is a plan apochromatic objective with a numerical aperture of 0.5.

[0091] (2) Processing parameters

[0092] The laser processing parameters are as follows: pulse energy 25nJ per pulse, slit rotation speed 22 revolutions per second, laser direct writing speed 120μm per second, and processing depth 150μm below the glass surface.

[0093] (3) Post-processing technology

[0094] After the predetermined pattern is inscribed in the Foturan glass, the sample needs to undergo heat treatment to develop the modified areas. The heat treatment process is as follows: the sample is placed in a muffle furnace and heated from room temperature to 500°C at a heating rate of 5°C per minute, and held at this temperature for 1 hour; then heated to 560°C at a heating rate of 1°C per minute, and held again for 1 hour; finally, it is slowly cooled to room temperature at a cooling rate of 3°C per minute. After heat treatment, the areas irradiated by the femtosecond laser turn brownish-black due to the precipitation of silver nanoparticles, while the unirradiated areas remain transparent.

[0095] After annealing, the sample is immersed in a 10% hydrofluoric acid solution and chemically etched with the aid of ultrasonic cleaning for about 15 minutes.

[0096] (4) Processing results

[0097] After etching, the sample was rinsed with deionized water and dried. Observations showed that the microchannel cross-section was circular with a diameter of approximately 7.5 μm. The cross-sectional shape and size of the etched channels were consistent along different directions.

[0098] To demonstrate the three-dimensional processing capabilities of this femtosecond laser processing device, a three-dimensional helical structure was fabricated in Foturan glass. The helix extends vertically with a pitch of 100 μm and a total height of 300 μm. The cross-section of the helix is ​​circular at any position, with a uniform diameter.

[0099] Example 6

[0100] This embodiment aims to analyze the process parameter window of the femtosecond laser processing method to provide guidance for practical applications.

[0101] (1) The effect of direct writing speed on machining quality

[0102] The pulse energy was fixed at 80 nJ per pulse, the slit rotation speed was 25 revolutions per second, and the laser direct writing speed varied within the range of 20 μm / s to 300 μm / s. The cross-sectional morphology and uniformity of the written lines were observed.

[0103] When the direct writing speed is below 50 μm / s, the line cross-sectional dimensions are large and uniform. When the direct writing speed is in the range of 50 μm / s to 150 μm / s, the lines are uniform, and the cross-sectional dimensions decrease with increasing speed. When the direct writing speed exceeds 150 μm / s, periodic microstructures begin to appear inside the lines, and the spacing increases with increasing speed.

[0104] The spacing of periodic microstructures is equal to the laser direct writing speed divided by twice the slit rotation speed. To achieve uniform, non-periodic lines, the spacing needs to be smaller than the focal spot diameter, meaning the ratio of the direct writing speed to twice the rotation speed must be less than the focal spot diameter.

[0105] (2) Effect of rotation speed on focal spot size

[0106] The direct writing speed was fixed at 50 μm per second, while the rotational speed varied from 10 to 34 revolutions per second. Measurements of the cross-sectional diameter of the inscribed lines revealed that the diameter remained essentially constant throughout the testing range. This indicates that as long as the rotational speed is high enough to ensure uniform stacking, the focal spot size is determined by the pulse energy and slit width, and not by the rotational speed.

[0107] (3) The influence of the center light-blocking size

[0108] A fixed six-arm radial slit unit of equal width is used. The pulse energy is 50 nJ per pulse, the rotation speed is 25 revolutions per second, and the direct writing speed is 80 μm / s. The ratio of the diameter of the light-blocking part 61 to the slit width is varied, ranging from 0.2 to 0.6. When the ratio is 0.2, the light-blocking effect is not obvious, the focal spot size is approximately 5.0 μm, and the side lobes are still visible. When the ratio is 0.4, the focal spot size shrinks to approximately 4.5 μm, and the side lobes almost disappear. When the ratio is 0.6, the focal spot size further shrinks to approximately 4.2 μm, but due to significant energy loss, the contrast of the written lines decreases. Considering all factors, a ratio range of 0.3 to 0.5 is the preferred option.

[0109] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A femtosecond laser processing apparatus based on a radially oriented, uniformly wide rotating slit, comprising a laser emitting unit, a beam adjustment unit, a slit unit, and a focusing unit arranged sequentially along the optical path, characterized in that: The slit unit includes a slit base, a rotary drive mechanism, and a plurality of rectangular slits formed on the slit base. Each rectangular slit is of equal width and is distributed radially relative to a radiation center. The radiation center is collinear with the optical axis. The rotary drive mechanism is used to drive the slit base to rotate around the optical axis. A light-blocking part is provided at the radiation center, and the light-blocking part is connected to the slit base through a connecting bridge.

2. The femtosecond laser processing apparatus as described in claim 1, characterized in that: The light-blocking part has a circular structure.

3. The femtosecond laser processing apparatus as described in claim 1, characterized in that: Both the light-blocking part and the connecting bridge are pre-formed when the plurality of rectangular slits are machined on the slit base.

4. The femtosecond laser processing apparatus as described in claim 1, characterized in that: The ratio of the feature dimension of the light-blocking part to the width of the rectangular slit is controlled within the range of 0.3 to 0.

5.

5. The femtosecond laser processing apparatus as described in claim 1, characterized in that: The width of the rectangular slit is in the range of 300~600μm.

6. The femtosecond laser processing apparatus as described in claim 1, characterized in that: The rotary drive mechanism is used to drive the slit base to rotate at a speed of 10~30 revolutions / s.

7. The femtosecond laser processing apparatus as described in claim 1, characterized in that: It also includes a motion control unit, which is used to carry the sample to be processed and drive the sample to be processed to move along a preset trajectory.

8. A femtosecond laser processing method based on a radially oriented, uniformly wide rotating slit, characterized in that, Implemented based on the femtosecond laser processing apparatus according to any one of claims 1 to 7; The femtosecond laser processing method includes: The laser emitting unit is activated to generate a femtosecond laser. The slit base is driven to rotate by a rotary drive mechanism; The femtosecond laser beam is expanded by the beam adjustment unit and then incident perpendicularly onto the slit base and through each rectangular slit. The laser beam emitted from the slit base is focused onto the target position of the sample to be processed by the focusing unit.

9. The femtosecond laser processing method as described in claim 8, characterized in that, The femtosecond laser processing method also includes The sample to be processed is driven to move along a preset processing trajectory at a preset direct writing speed v1, wherein the direct writing speed v1 satisfies: v1 / (2v2)<d Where v2 is the rotational speed of the slit base, and d is the focal spot diameter.

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