A method and apparatus for femtosecond laser direct writing of single-crystal fiber microstructure arrays

Abstract

This invention relates to the field of laser micro / nano fabrication technology, and discloses a method and apparatus for directly writing a microstructure array of a single-crystal fiber using a femtosecond laser. The method includes: identifying edge regions, irregular regions, and smooth incident windows on the surface of the single-crystal fiber; controlling the fiber rotation to direct the femtosecond laser beam from a direction avoiding the edge regions and irregular regions; shaping the femtosecond laser beam to reduce astigmatism or focal plane separation; focusing the shaped beam into the fiber interior; sequentially writing multiple microstructure units according to preset array parameters to obtain a microstructure array; acquiring images and optical response signals of the fabricated area during or after the writing process; obtaining fabrication quality evaluation results and optical parameter deviations based on the images and signals; and performing closed-loop adjustments to the fiber attitude, beam shaping parameters, and writing parameters of subsequent units. This invention achieves precise fiber attitude control, improved focusing stability, and ensures consistent performance of the microstructure array.

Description

Technical Field

[0001] This invention relates to the field of laser micro-nano fabrication technology, and in particular to a method and apparatus for directly writing single-crystal fiber microstructure arrays using femtosecond lasers. Background Technology

[0002] In existing femtosecond laser direct writing of single-crystal fiber microstructure arrays, the cross-section of single-crystal fibers (such as sapphire fibers) is often non-circular, polygonal, or locally flat, with edges and irregular regions on the surface. When the laser is incident on these regions, scattering and refraction distortion occur, leading to unstable focusing positions. At the same time, the high effective refractive index of single-crystal fibers causes significant astigmatism and focal plane separation after the laser passes through the air-fiber interface, causing the internal modified regions to elongate, break, or become asymmetrical, reducing the reflectivity controllability and repeatability of the microstructure. In addition, the existing fabrication process lacks an online quality feedback mechanism, relying solely on open-loop parameter settings, making it difficult to detect problems such as low reflectivity, abnormal spectral width, structural cracks, or array position deviations in a timely manner, and failing to guarantee the consistency and fabrication success rate among hundreds or even thousands of microstructure units.

[0003] Therefore, existing technologies still need to be improved and developed. Summary of the Invention

[0004] The main objective of this invention is to provide a method and apparatus for directly writing microstructure arrays of single-crystal optical fibers using femtosecond lasers. This invention aims to solve the problems of laser incident distortion caused by the edges and irregular regions on the surface of single-crystal optical fibers, astigmatism and focal plane separation caused by high refractive index and non-circular cross-section, and low consistency and success rate of large-scale microstructure arrays due to the lack of real-time feedback in open-loop processing when directly writing microstructure arrays of single-crystal optical fibers using femtosecond lasers.

[0005] To achieve the above objectives, the present invention provides a method for directly writing a single-crystal fiber microstructure array using a femtosecond laser, the method comprising the following steps: A single-crystal optical fiber is fixed to a clamping device, and a surface morphology image of the single-crystal optical fiber is acquired, wherein the clamping device is configured to drive the single-crystal optical fiber to move axially and rotate about its own axis. The surface morphology image is used to identify the edge regions, irregular regions, and smooth regions on the surface of the single-crystal fiber that can be stably incident by laser. Based on the identification results, the clamping device is controlled to rotate the single-crystal fiber so that the femtosecond laser beam is incident from a direction that avoids the edge regions and / or the irregular regions. Beam shaping is performed on the femtosecond laser beam to reduce astigmatism or focal plane separation generated inside the single-crystal fiber after the femtosecond laser beam is incident on the single-crystal fiber. The femtosecond laser beam, after beam shaping, is focused onto the target writing region inside the single-crystal fiber, and multiple microstructure units are sequentially written according to the preset array design parameters to obtain a microstructure array. During or after writing at least one of the microstructure units, images of the processing area and optical response signals of the microstructure units written are acquired; Based on the image of the processing area and the optical response signal, the processing quality evaluation result and optical parameter deviation are obtained, and based on the processing quality evaluation result and the optical parameter deviation, the fiber attitude, beam shaping parameters and femtosecond laser writing parameters of the subsequent microstructure units are adjusted in a closed loop.

[0006] Furthermore, the step of fixing the single-crystal optical fiber to the clamping device and acquiring a surface morphology image of the single-crystal optical fiber specifically includes: The single-crystal optical fiber is fixed to the clamping device; The contour image and light and dark texture image of the single-crystal optical fiber are acquired using a lateral imaging system or a multi-view imaging system to obtain a surface morphology image containing the contour image and the light and dark texture image.

[0007] Furthermore, the step of identifying the edge regions, irregular regions, and smooth regions suitable for stable laser incidence on the surface of the single-crystal fiber based on the surface morphology image, and controlling the clamping device to rotate the single-crystal fiber based on the identification results, so that the femtosecond laser beam is incident from a direction avoiding the edge regions and / or the irregular regions, specifically includes: Recognition operations are performed on the contour image and light and dark texture image in the surface morphology image to determine the edge position, irregular area position and smooth incident window of the single crystal fiber surface. The recognition operations include edge detection, gray-level gradient analysis, Hough line fitting, curvature calculation, threshold segmentation and learning-based defect recognition operations. Based on the angle between the smooth incident window and the incident direction of the femtosecond laser, a rotation angle correction amount is generated. According to the rotation angle correction amount, the clamping device is controlled to rotate the single-crystal fiber so that the femtosecond laser beam is incident from a direction that avoids the edge region and / or the irregular region.

[0008] Furthermore, the beam shaping of the femtosecond laser beam to reduce astigmatism or focal plane separation generated inside the single-crystal fiber after the femtosecond laser beam is incident on the fiber specifically includes: The diameter, shape profile, effective refractive index, writing depth, and incident angle of the single-crystal fiber are obtained, and a focusing model of the femtosecond laser beam after passing through the single-crystal fiber is established. The amplitude distribution, phase distribution, and numerical aperture of the femtosecond laser beam are adjusted according to the focusing model to reduce the astigmatism or focal plane spacing of the femtosecond laser beam in the first and second lateral directions.

[0009] Furthermore, the step of focusing the femtosecond laser beam, after beam shaping, onto the target writing region inside the single-crystal fiber, and sequentially writing multiple microstructure units according to preset array design parameters to obtain a microstructure array, specifically includes: The femtosecond laser beam, after beam shaping, is vertically focused onto the target writing region inside the single-crystal fiber through a high numerical aperture focusing objective. According to the preset array design parameters, multiple microstructure units are sequentially written using point-by-point, line-by-line, surface-by-surface, multi-layer bias, or spiral scanning methods to obtain a microstructure array.

[0010] Furthermore, the acquisition of images of the processing area and optical response signals of the microstructured units during or after writing at least one of the microstructured units specifically includes: During or after the writing of at least one of the microstructure units by femtosecond laser, images of the processing area of ​​the target writing region under different illumination and acquisition conditions are acquired, and at least one feature among the length, width, continuity, grayscale contrast, scattering intensity, crack probability, ablation probability, and deviation from the target position of the modified region is extracted from the processing area images. The optical response signal of the microstructure unit is acquired by an online optical frequency domain reflection system. A tapered thermally adiabatic fusion splice mode selection module is provided between the online optical frequency domain reflection system and the single-crystal fiber. The tapered thermally adiabatic fusion splice mode selection module is used to allow the higher-order modes returned by the microstructure array to leak or be converted into radiation modes during transmission.

[0011] Furthermore, the step of obtaining processing quality evaluation results and optical parameter deviations based on the processing area image and the optical response signal, and then performing closed-loop adjustments to the fiber attitude, beam shaping parameters, and femtosecond laser writing parameters of subsequent microstructure units based on the processing quality evaluation results and the optical parameter deviations, specifically includes: The features extracted from the image of the processing area are input into a classification model, a regression model, or an anomaly detection model to obtain the processing quality level, the predicted reflectance value, and the probability of processing defects. The processing quality level, the predicted reflectance value, and the probability of processing defects are used as the processing quality evaluation results. The reflection intensity, center wavelength, spectral width, range domain peak width, or signal-to-noise ratio in the optical response signal are compared with the target value to obtain the optical parameter deviation. Based on the processing quality evaluation result and the optical parameter deviation, the pulse energy, scanning speed, scanning trajectory length, focal position, scanning trajectory spacing, number of repeated scans, slit width, or spatial light modulator compensation phase map of the subsequent microstructure units are adjusted.

[0012] Furthermore, to achieve the above objectives, the present invention also provides an apparatus for directly writing a femtosecond laser onto a single-crystal fiber microstructure array. This apparatus is used to implement the method described above for directly writing a femtosecond laser onto a single-crystal fiber microstructure array. The apparatus comprises: A holding and image acquisition module is used to fix a single-crystal optical fiber to a clamping device and acquire a surface morphology image of the single-crystal optical fiber, wherein the clamping device is configured to drive the single-crystal optical fiber to move along the axial direction and rotate about its own axis. The identification and rotation module is used to identify the edge region, irregular region and smooth region that can be stably incident on the surface of the single crystal fiber according to the surface morphology image, and control the clamping device to rotate the single crystal fiber according to the identification result, so that the femtosecond laser beam is incident from the direction that avoids the edge region and / or the irregular region. A beam shaping module is used to shape the femtosecond laser beam to reduce astigmatism or focal plane separation generated inside the single-crystal fiber after the femtosecond laser beam is incident on the single-crystal fiber. The focusing and writing module is used to focus the femtosecond laser beam after beam shaping onto the target writing region inside the single-crystal fiber, and sequentially write multiple microstructure units according to preset array design parameters to obtain a microstructure array. The acquisition feedback module is used to acquire images of the processing area and optical response signals written to the microstructure unit during or after writing at least one of the microstructure units; The closed-loop control module is used to obtain the processing quality evaluation result and optical parameter deviation based on the processing area image and the optical response signal, and to perform closed-loop adjustment of the fiber attitude, beam shaping parameters and femtosecond laser writing parameters of the subsequent microstructure units based on the processing quality evaluation result and the optical parameter deviation.

[0013] Furthermore, to achieve the above objectives, the present invention also provides a terminal, wherein the terminal includes: a memory, a processor, and a program for a femtosecond laser direct-write single-crystal fiber microstructure array stored in the memory and executable on the processor, wherein when the program for the femtosecond laser direct-write single-crystal fiber microstructure array is executed by the processor, the steps of the method for the femtosecond laser direct-write single-crystal fiber microstructure array as described above are implemented.

[0014] Furthermore, to achieve the above objectives, the present invention also provides a computer-readable storage medium, wherein the computer-readable storage medium stores a program for femtosecond laser direct writing of a single-crystal fiber microstructure array, and when the program for femtosecond laser direct writing of a single-crystal fiber microstructure array is executed by a processor, it implements the steps of the method for femtosecond laser direct writing of a single-crystal fiber microstructure array as described above.

[0015] The beneficial effects of this invention are as follows: By fixing the single-crystal fiber to a clamping device that can move axially and rotate around an axis and acquiring surface morphology images, the fiber attitude can be precisely controlled and surface morphology data can be acquired in real time; by identifying edges, irregular areas, and smoothing the incident window and controlling the fiber rotation to make the laser incident from a direction that avoids the edges, beam scattering and refraction distortion are avoided, ensuring focusing stability; by shaping the femtosecond laser beam to reduce astigmatism and focal plane separation, the focal planes in two orthogonal directions tend to coincide, improving the continuity and symmetry of the modified area; the shaped beam is focused into the fiber and written into microstructure units sequentially according to preset array parameters, accurately forming a microstructure array with consistent optical properties under optimized conditions; during or after writing, images of the processing area and optical response signals are acquired, and the morphological characteristics and optical performance data of the microstructure units are acquired in real time; based on the images and signals, quality evaluation results and optical parameter deviations are obtained, and closed-loop adjustments are made to the fiber attitude, beam shaping parameters, and writing parameters of subsequent units, the processing parameters are dynamically corrected, and the performance consistency of each unit in the large-scale array is guaranteed. Attached Figure Description

[0016] Figure 1 This is a flowchart of a preferred embodiment of the method for directly writing single-crystal fiber microstructure arrays using femtosecond lasers according to the present invention; Figure 2 This is a detailed flowchart illustrating the fabrication process of the single-crystal sapphire fiber microstructure array in the method of this invention. Figure 3 This is a schematic diagram of the conical thermal insulation fusion transition section and OFDR online monitoring feedback in this invention; Figure 4 This is a schematic diagram of the closed-loop control process of the present invention; Figure 5 This is a structural diagram of a preferred embodiment of the device for femtosecond laser direct writing of single-crystal fiber microstructure array of the present invention; Figure 6 This is a schematic diagram of the overall structure of the device of the present invention; Figure 7 This is a structural diagram of a preferred embodiment of the terminal of the present invention. Detailed Implementation

[0017] This application provides a method and apparatus for directly writing single-crystal fiber microstructure arrays using femtosecond lasers. To make the objectives, technical solutions, and effects of this application clearer and more explicit, the following detailed description is provided with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining this application and are not intended to limit this application.

[0018] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless specifically defined as herein.

[0019] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. If the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0020] The method for directly writing single-crystal fiber microstructure arrays using femtosecond lasers according to a preferred embodiment of the present invention, such as... Figure 1 As shown, the method for directly writing a single-crystal fiber microstructure array using a femtosecond laser includes the following steps: Step S10: Fix the single-crystal optical fiber to the clamping device and acquire a surface morphology image of the single-crystal optical fiber, wherein the clamping device is configured to drive the single-crystal optical fiber to move along the axial direction and rotate around its own axis.

[0021] The purpose of this step is to fix the optical fiber with a rotatable and axially movable clamping device and acquire its surface morphology image, providing a data basis for subsequent identification of edges and irregular areas, thereby avoiding femtosecond laser incident from unfavorable directions.

[0022] Further, step S10 specifically includes: Step S101: Fix the single-crystal optical fiber to the clamping device.

[0023] Specifically, the two ends of a sapphire single-crystal fiber (or other single-crystal fibers, such as alumina, lutetium oxide, or zirconia single-crystal fibers) are fixed to a pair of clamping ends of the fiber holding and rotating module, and an appropriate tension is applied to keep the fiber straight. The clamping device includes an axial movement mechanism and a rotation drive mechanism, such as a stepper motor-driven rotary table and a precision linear motor, which can drive the fiber to move axially and rotate around its own axis.

[0024] Step S102: Use a lateral imaging system or a multi-view imaging system to acquire the contour image and the light and dark texture image of the single-crystal optical fiber, and obtain a surface morphology image containing the contour image and the light and dark texture image.

[0025] Specifically, the lateral imaging system includes an illumination source, a microscope objective, and an industrial camera. Illumination methods can employ coaxial illumination, oblique incidence illumination, or dark-field illumination to highlight the edges, cracks, and localized flattened areas of the fiber surface. The multi-view imaging system can acquire images from different angles, and after image stitching, obtain a panoramic view of the fiber's circumferential surface morphology.

[0026] In this embodiment, a sapphire single-crystal fiber with a diameter of approximately 70 to 100 micrometers is clamped in a clamping device, and the lateral imaging subsystem is activated. The camera acquires contour images and light and dark texture images of the fiber at a rate of 30 frames per second. These images clearly show the hexagonal edges, local rough areas, and smooth windows on the fiber surface. The acquired images are transmitted in real time to the closed-loop control module for subsequent processing.

[0027] It should be noted that for long array writing (e.g., requiring the fabrication of hundreds of microstructure units), step S102 can be repeated before writing each microstructure unit, after writing a preset number (e.g., 10) of units, or after the fiber axially moves a preset distance (e.g., 1 mm) to compensate for the change in incident direction caused by the cross-sectional torsion, clamping drift, and gradual change in shape of the fiber along the axial direction.

[0028] Step S20: Identify the edge regions, irregular regions, and smooth regions on the surface of the single-crystal fiber based on the surface morphology image, and control the clamping device to rotate the single-crystal fiber according to the identification results, so that the femtosecond laser beam is incident from a direction that avoids the edge regions and / or the irregular regions.

[0029] The purpose of this step is to actively select the laser incident window to avoid beam scattering and refraction distortion caused by edges and irregular areas, thereby ensuring the quality and stability of the focused spot.

[0030] Further, step S20 specifically includes: Step S201: Perform recognition operations on the contour image and light and dark texture image in the surface morphology image to determine the edge position, irregular area position and smooth incident window of the single crystal fiber surface. The recognition operations include edge detection, gray-level gradient analysis, Hough line fitting, curvature calculation, threshold segmentation and learning-based defect recognition operations.

[0031] For example, the Canny edge detection operator is used to extract the fiber optic profile, and Hough line fitting is used to identify the edge straight lines; local gray-level gradients are calculated, and abrupt changes in gradient magnitude indicate edges or cracks; for complex and irregular regions, pre-trained lightweight convolutional neural networks (such as MobileNet) can be used for pixel-level classification and output smooth region probability maps.

[0032] Step S202: Based on the angle between the smooth incident window and the incident direction of the femtosecond laser, a rotation angle correction amount is generated. According to the rotation angle correction amount, the clamping device is controlled to rotate the single-crystal fiber so that the femtosecond laser beam is incident from a direction that avoids the edge region and / or the irregular region.

[0033] Specifically, assuming the current laser incident direction is fixed (e.g., perpendicular to the fiber axis and along a certain radial direction), the center azimuth angle of the smoothing window is θ. target The azimuth angle corresponding to the current fiber edge is θ. current Then the rotation angle correction Δθ = θ target - θ current The closed-loop control module drives a rotary motor to rotate the optical fiber around its axis by Δθ, and then re-acquires images for verification until the laser incident path completely avoids edges and irregular areas.

[0034] In this embodiment, the closed-loop control module performs edge detection and grayscale gradient analysis on the acquired surface morphology image to identify the positions of the six edges of the sapphire fiber (spaced approximately 60 degrees apart) and two local rough areas. The calculated angle between the current laser incident direction and the nearest smooth window is 15 degrees, thus generating a rotation angle correction of 15 degrees to drive the stepper motor to rotate the fiber. After rotation, the image is acquired again to confirm that the laser incident direction is now aligned with the smooth window.

[0035] It should be noted that for single-crystal optical fibers with non-circular cross-sections (such as approximately hexagonal, polygonal, or locally flat), this step can effectively eliminate focal splitting and energy loss caused by edge incidence, significantly improving the fabrication efficiency and consistency of microstructure arrays.

[0036] Step S30: Perform beam shaping on the femtosecond laser beam to reduce astigmatism or focal plane separation generated inside the single-crystal fiber after the femtosecond laser beam is incident on the single-crystal fiber.

[0037] The purpose of this step is to compensate for the cylindrical lens effect introduced by the high refractive index and non-circular cross section of the optical fiber by actively adjusting the amplitude and phase distribution of the incident beam, so that the focal planes in the two orthogonal directions tend to coincide, thereby improving the continuity and symmetry of the internal modified region.

[0038] Further, step S30 specifically includes: Step S301: Obtain the diameter, shape profile, effective refractive index, writing depth and incident angle of the single-crystal fiber, and establish a focusing model of the femtosecond laser beam after passing through the single-crystal fiber.

[0039] Specifically, a model is established using geometric optics or wave optics methods. For single-mode femtosecond laser incidence, the complex amplitude distribution of the beam after passing through the fiber is calculated based on Fresnel diffraction theory. Parameters such as fiber radius, refractive index, refractive index of the surrounding medium (air is 1), writing depth, and incident angle are considered. For non-circular cross-sections, a thickness function related to the azimuth angle is also introduced. The three-dimensional light intensity distribution in the focal region is calculated using ray tracing or angular spectrum propagation methods, thereby quantifying the focal plane spacing in two orthogonal directions (fiber axis and radial direction).

[0040] Step S302: Adjust the amplitude distribution, phase distribution and numerical aperture of the femtosecond laser beam according to the focusing model, so astigmatism or focal plane spacing of the femtosecond laser beam in the first lateral direction and the second lateral direction is reduced.

[0041] Specific adjustment methods include: (1) Setting an adjustable slit in the optical path, with the slit direction parallel to the fiber axis, to limit the beam width in the radial direction, thereby reducing the effective numerical aperture in that direction and decreasing the focal plane spacing. (2) Using a cylindrical lens group to pre-introduce astigmatism opposite to the fiber cylindrical lens effect. (3) Using a spatial light modulator to load a compensated phase map. The compensated phase map adopts a linear combination of Zernike polynomials, focusing on introducing terms related to astigmatism, such as primary astigmatism terms and higher-order astigmatism terms. The coefficients of each term are determined by inverse optimization based on the astigmatism of the focusing model. For fibers with different diameters, cross-sectional orientations, or writing depths, the corresponding compensation map can be called from the phase map library or updated online by a closed-loop algorithm.

[0042] In this embodiment, the control module first acquires parameters such as the sapphire fiber diameter (85 micrometers), effective refractive index (approximately 1.75), and writing depth (20 micrometers) to establish a focusing model. The model shows that the focal plane spacing in the axial and radial directions is 12 micrometers, which will cause the modified region to elongate. Based on the model calculation results, the slit width is adjusted to 70% of the incident beam diameter, reducing the radial effective numerical aperture to 0.28. Simultaneously, the spatial light modulator is loaded with a pre-calculated astigmatism compensation phase map. The focal spot is remeasured, and the focal overlap in both directions is improved, with the focal spot approaching a circle.

[0043] It should be noted that beam shaping can be achieved by at least one of the following: slit, adjustable aperture, cylindrical lens group, aspherical lens, beam expander / contractor assembly, spatial light modulator, digital micromirror device, and phase plate. This invention is not limited to a single means.

[0044] Step S40: Focus the femtosecond laser beam after beam shaping onto the target writing region inside the single-crystal fiber, and sequentially write multiple microstructure units according to the preset array design parameters to obtain a microstructure array.

[0045] The purpose of this step is to precisely write a microstructure array with preset optical properties under optimized incident conditions and beam quality, ensuring the consistency and repeatability of each unit.

[0046] Further, step S40 specifically includes: Step S401: The femtosecond laser beam after beam shaping is vertically focused onto the target writing region inside the single-crystal fiber through a high numerical aperture focusing objective.

[0047] For example, a high numerical aperture objective lens (e.g., NA = 0.8~1.3, working distance 1~5 mm) is mounted directly above the optical fiber, and its focal position is adjusted by a three-dimensional precision displacement platform. After confirming that the modified linewidth and continuity are within the allowable range through pre-scanning or low-energy trial writing, the writing depth is set (e.g., 10~30 micrometers below the upper surface of the optical fiber).

[0048] Step S402: According to the preset array design parameters, multiple microstructure units are sequentially written using point-by-point, line-by-line, surface-by-surface, multi-layer bias, or spiral scanning methods to obtain a microstructure array.

[0049] Specifically, the array design parameters include the number of microstructure units (e.g., 1000), axial spacing (e.g., 10 mm), target reflectivity (e.g., -50 dB), center wavelength (e.g., 1550 nm), and grating period (e.g., 1.77 μm, corresponding to the fourth Bragg condition at 1550 nm). For weakly reflective grating units, several refractive index modulation regions are periodically written along the fiber axis; for local scattering enhancement lattices, one or more modification points are written at preset positions.

[0050] In this embodiment, the femtosecond laser has a center wavelength of 515 nm, a pulse width of 150 femtoseconds, and a repetition frequency of 1 MHz. After beam shaping, the laser is vertically focused through an objective lens with NA=1.25 to a depth of 20 micrometers inside the sapphire fiber. The preset array parameters are: 1000 weakly reflective grating units are written at equal intervals within a 10-meter axial length of the fiber. Each grating consists of more than 200 periods, and each period contains a modification point with a diameter of ~1 μm. A point-by-point processing method is used, with a scanning speed of 100 μm / s and a point spacing of 0.8 μm. During the writing process, a three-dimensional displacement platform moves stepwise along the fiber axis, triggering the laser to write the next grating after each grating length movement. The final result is a microstructure array composed of 1000 weakly reflective gratings.

[0051] It should be noted that the femtosecond laser writing parameters include pulse energy, repetition frequency, processing speed, processing trajectory spacing, focal depth, defocusing amount, number of repeated scans, polarization direction, beam shaping parameters, etc., which can be initially set according to the mapping relationship calibrated in the pre-experiment.

[0052] Step S50: During or after writing at least one of the microstructure units, acquire an image of the processing area and an optical response signal of the microstructure unit that has been written.

[0053] Further, step S50 specifically includes: Step S501: During or after the writing of at least one of the microstructure units by femtosecond laser, images of the processing area of ​​the target writing area under different illumination and acquisition conditions are acquired, and at least one feature among the length, width, continuity, grayscale contrast, scattering intensity, crack probability, ablation probability, and deviation from the target position is extracted from the processing area images.

[0054] The image feedback module can employ bright-field, dark-field, scattering, fluorescence, phase-contrast, or polarized illumination. Dark-field illumination is particularly suitable for detecting cracks and ablation defects. The image processing unit extracts the geometric and textural features of the modified area in real time.

[0055] Step S502: Acquire the optical response signal of the microstructure unit written in the online optical frequency domain reflection system. The online optical frequency domain reflection system and the single crystal fiber are provided with a tapered thermal insulation fusion splice mode selection module. The tapered thermal insulation fusion splice mode selection module is used to allow the higher-order mode returned by the microstructure array to leak or be converted into a radiation mode during transmission.

[0056] It should be noted that the Optical Frequency Domain Reflectometry (OFDR) system emits swept probe light (wavelength scanning range 1525nm~1610nm, scanning rate 100nm / s), which is coupled to the single-crystal fiber via a circulator and a tapered adiabatic fusion splice mode selection module. The reflected signal from the microstructure unit returns along the original path, is received by the OFDR system, and demodulated into a distance domain reflection distribution. The mode selection module consists of a standard single-mode fiber and a tapered transition section, whose tapered length, waist diameter, and tapered angle are designed to ensure that the transmission loss of higher-order modes is higher than that of the fundamental mode.

[0057] In this embodiment, after each microstructure unit is written, the processing laser is turned off (by blocking it with an optical shutter), triggering an OFDR to acquire a reflection signal. The acquisition time is approximately 1 second. The OFDR system first injects a swept-frequency probe light, receives the returned signal, and performs an inverse Fourier transform to obtain the range-domain reflection curve. Within a range window (100 micrometers wide) corresponding to the theoretical position of the current microstructure unit, parameters such as peak reflection intensity, peak width (full width at half maximum), and signal-to-noise ratio are extracted. Simultaneously, the processing image feedback module acquires a dark-field image during the writing process, extracting the continuity and scattering intensity of the modified region. If the image shows discontinuities or cracks in the modified region, the defect characteristics are immediately recorded.

[0058] It should be noted that the specific structure of the tapered thermal fusion splice mode selection module includes a standard single-mode fiber (SMF-28), a transition multimode fiber (graded-index multimode fiber, 5mm in length), a tapered transition section (tapered section length greater than 5mm, waist section diameter 10 micrometers), and a glass sleeve fixing section. This module makes the loss of higher-order modes (such as LP11, LP21) about 15dB higher than that of the fundamental mode (LP01), so that the fundamental mode accounts for more than 95% of the signal received by the OFDR, and the distance domain reflection peak is narrower than 50 micrometers.

[0059] Step S60: Based on the processing area image and the optical response signal, obtain the processing quality evaluation result and optical parameter deviation, and based on the processing quality evaluation result and the optical parameter deviation, perform closed-loop adjustment of the fiber attitude, beam shaping parameters and femtosecond laser writing parameters of the subsequent microstructure unit.

[0060] The purpose of this step is to dynamically adjust the processing parameters using image and optical feedback information to achieve adaptive array fabrication and ensure consistent performance of each microstructure unit.

[0061] Further, step S60 specifically includes: Step S601: Input the features extracted from the image of the processing area into a classification model, regression model or anomaly detection model to obtain the processing quality level, the predicted reflectance value and the probability of processing defects, and use the processing quality level, the predicted reflectance value and the probability of processing defects as the processing quality evaluation results.

[0062] For example, a random forest classification model can be used, with input features including the length and width of the modified region, grayscale contrast, and scattering intensity, outputting four quality levels: "Excellent," "Good," "Medium," and "Poor." Regression models (such as support vector regression) can predict reflectivity. If the defect probability exceeds a threshold (e.g., 0.7), it is judged as unqualified.

[0063] Step S602: Compare the reflection intensity, center wavelength, spectral width, range domain peak width, or signal-to-noise ratio in the optical response signal with the target value to obtain the optical parameter deviation. Based on the processing quality evaluation result and the optical parameter deviation, adjust the pulse energy, scanning speed, scanning trajectory length, focal position, scanning trajectory spacing, number of repeated scans, slit width, or spatial light modulator compensation phase map of the subsequent microstructure units.

[0064] For example, the target reflection intensity is -50dB, the measured value is -53dB, and the deviation is -3dB. A closed-loop control algorithm (such as a PID controller) calculates the pulse energy increment, increasing the pulse energy of subsequent units from 200nJ to 230nJ. If image evaluation shows discontinuities in the modified area, the scanning speed is reduced or the number of repeated scans is increased.

[0065] In this embodiment, after the first microstructure unit is written, image feature extraction shows that the continuity score of the modified area is 0.6 (threshold 0.8), the crack probability is 0.3 (threshold 0.2), and the processing quality is rated as "medium". OFDR measured the reflection peak intensity as -53dB (target -50dB) and the peak width as 80μm (target 50μm). Based on the deviation, the closed-loop control module increases the pulse energy of subsequent units from 200nJ to 220nJ, decreases the scanning speed from 100μm / s to 80μm / s, and adds one additional scan. From the third unit onwards, the image quality rating improves to "excellent", the reflection intensity stabilizes between -49dB and -51dB, and the peak width narrows to 52μm. Subsequent units maintain the corrected parameters. Due to the large transmission loss and mode field mismatch loss of single-crystal optical fibers, while ensuring a sufficient signal-to-noise ratio, the reasonable reflection intensity threshold of the microstructure along the fiber length needs to be gradually decreased to ultimately complete the fabrication of all 1000 microstructure units.

[0066] It should be noted that the closed-loop adjustment also includes: when the image evaluation indicates that the modified area deviates from the target position by more than a threshold (e.g., 2μm), the displacement platform coordinates or fiber rotation angle are corrected; when the OFDR detects that the signal-to-noise ratio of the distance domain peak is lower than the threshold, the coupling state of the mode selection module and the fiber incident attitude are checked first, and the rotation angle is adjusted accordingly.

[0067] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0068] Figure 2 This is a detailed flowchart illustrating the fabrication process of the single-crystal sapphire fiber microstructure array in the method of this invention. Figure 2 As shown, the detailed process includes the following steps: First, the single-crystal sapphire fiber is clamped and initially positioned (step S1). Then, the fiber edges and irregular areas are identified using a lateral imaging system, and the fiber's rotational attitude is determined (step S2). Next, the rotation module is controlled to orient the surface to be processed toward the incident laser, establishing stable focusing conditions (step S3). The single-crystal sapphire fiber has no independent cladding; its equivalent light-guiding boundary is the sapphire-air interface. Afterward, femtosecond laser parameters are set, and beam shaping and aberration compensation are performed using a slit, cylindrical lens, or spatial light modulator (step S4). Subsequently, microstructural units are written into the sapphire fiber by vertical focusing from above using a microscope objective (step S5). During or after writing, images of the processed area are acquired, and image quality is evaluated or machine learning analysis is performed (step S6). Simultaneously, the reflection spectrum or distance domain signal is monitored using a tapered adiabatic fusion splicing mode selection module and an online optical frequency domain reflectance (OFDR) system, extracting features such as peak value, peak position, and spectral width (step S7). Finally, the extracted features are compared with the target parameters, and the pulse energy, scanning speed, focal position, writing trajectory, number of repetitions and rotation angle are corrected. Then the next unit is written, and the cycle is repeated until a complete microstructure array is formed (step S8), thus completing the fabrication of the microstructure array.

[0069] Figure 3 This is a schematic diagram of the conical thermal insulation fusion transition section and OFDR online monitoring feedback in this invention. Figure 3 The structure of a single-crystal sapphire fiber microstructure array (with cells 1 to n already written and cell n+1 to be written) connected to an OFDR system via a tapered, insulated fusion splice transition section and a quartz single-mode fiber is demonstrated. The OFDR system includes a swept-frequency laser source, a circulator, a reference arm, a measurement arm, and an interferometric detection and range-domain demodulation unit. Figure 3 The diagram also shows the feature extraction module (extracting reflection peak value, peak position, spectral width, distance domain peak shape, and signal-to-noise ratio), the target parameter comparison module, the processing parameter correction module (correcting pulse energy, scanning speed, focal position, writing trajectory length, and number of repetitions), and the process of feeding back to the femtosecond laser processing system.

[0070] Figure 4 This is a schematic diagram of the closed-loop control process of the present invention. Figure 4 As shown, the closed-loop control process includes: setting target microstructure parameters and target spectral parameters; executing the current unit write; acquiring images of the processing area and OFDR online monitoring data; performing image quality evaluation and feature extraction; comparing with target parameters or thresholds; if the requirements are met, proceeding to the next unit write; if the requirements are not met, calculating the deviation and correcting the processing parameters (pulse energy, scanning speed, focal position, rotation angle, write trajectory length, number of repetitions) through a closed-loop control algorithm (such as PID control or iterative optimization), and then re-executing the current unit write or continuing to the next unit; looping until the array is completed.

[0071] Furthermore, such as Figure 5 As shown, based on the above-described method for directly writing single-crystal fiber microstructure arrays using femtosecond lasers, this invention also provides a corresponding apparatus for directly writing single-crystal fiber microstructure arrays using femtosecond lasers. The apparatus for directly writing single-crystal fiber microstructure arrays includes: The image acquisition module 51 is used to fix the single-crystal optical fiber to the clamping device and acquire the surface morphology image of the single-crystal optical fiber, wherein the clamping device is configured to drive the single-crystal optical fiber to move along the axial direction and rotate about its own axis. The identification and rotation module 52 is used to identify the edge region, irregular region and smooth region that can be stably incident on the surface of the single crystal fiber according to the surface morphology image, and to control the clamping device to rotate the single crystal fiber according to the identification result so that the femtosecond laser beam is incident from the direction that avoids the edge region and / or the irregular region. The beam shaping module 53 is used to shape the femtosecond laser beam to reduce astigmatism or focal plane separation generated inside the single-crystal fiber after the femtosecond laser beam is incident on the single-crystal fiber. The focusing and writing module 54 is used to focus the femtosecond laser beam after beam shaping onto the target writing region inside the single crystal fiber, and sequentially write multiple microstructure units according to the preset array design parameters to obtain a microstructure array. The acquisition feedback module 55 is used to acquire images of the processing area and optical response signals written to the microstructure unit during or after writing at least one of the microstructure units. The closed-loop control module 56 is used to obtain the processing quality evaluation result and optical parameter deviation based on the processing area image and the optical response signal, and to perform closed-loop adjustment of the fiber attitude, beam shaping parameters and femtosecond laser writing parameters of the subsequent microstructure unit based on the processing quality evaluation result and the optical parameter deviation.

[0072] Figure 6This is a schematic diagram of the overall structure of the device of the present invention. Figure 6 The exhibits include a femtosecond laser, a power / polarization control module, a beam shaping module (including a slit, cylindrical lens, spatial light modulator, mirror, and beam expander), a microscope objective, a lateral imaging subsystem (including coaxial illumination or ring light source, microscope imaging objective, and CMOS camera), a fiber clamping and rotation module, a three-dimensional precision displacement platform, a tapered thermally adiabatic fusion transition section, a quartz single-mode fiber, a circulator, an OFDR online spectral monitoring module, and a control and feedback unit. Figure 3 The non-circular / near-hexagonal cross-sectional structure of single-crystal sapphire fiber is also shown, as well as its characteristics of having no independent cladding and an equivalent light-guiding boundary of sapphire-air interface, which are compared with the core-cladding structure of traditional quartz fiber.

[0073] Furthermore, such as Figure 7 As shown, based on the above-mentioned method and apparatus for direct writing single-crystal fiber microstructure arrays using femtosecond lasers, the present invention also provides a terminal, which includes a processor 10, a memory 20, and a display 30. Figure 7 Only some of the terminal components are shown; however, it should be understood that it is not required to implement all of the components shown, and more or fewer components may be implemented instead.

[0074] In some embodiments, the memory 20 may be an internal storage unit of the terminal, such as a hard disk or memory. In other embodiments, the memory 20 may be an external storage device of the terminal, such as a plug-in hard disk, smart media card (SMC), secure digital card (SD), flash card, etc., equipped on the terminal. Further, the memory 20 may include both internal and external storage units of the terminal. The memory 20 is used to store application software and various types of data installed on the terminal, such as the program code installed on the terminal. The memory 20 can also be used to temporarily store data that has been output or will be output. In one embodiment, the memory 20 stores a program 40 for femtosecond laser direct-writing of a single-crystal fiber microstructure array, which can be executed by the processor 10 to implement the method for femtosecond laser direct-writing of a single-crystal fiber microstructure array in this application.

[0075] In some embodiments, the processor 10 may be a central processing unit (CPU), a microprocessor, or other data processing chip, used to run program code stored in the memory 20 or process data, such as executing the method of femtosecond laser direct writing single-crystal fiber microstructure array.

[0076] In some embodiments, the display 30 may be an LED display, a liquid crystal display, a touch-sensitive liquid crystal display, or an OLED (Organic Light-Emitting Diode) touchscreen. The display 30 is used to display information on the terminal and to display a visual user interface. The components of the terminal communicate with each other via a device bus.

[0077] The present invention also provides a computer-readable storage medium, wherein the computer-readable storage medium stores a program for femtosecond laser direct writing of a single-crystal fiber microstructure array, and when the program for femtosecond laser direct writing of a single-crystal fiber microstructure array is executed by a processor, it implements the steps of the method for femtosecond laser direct writing of a single-crystal fiber microstructure array as described above.

[0078] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or terminal that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or terminal. Unless otherwise specified, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or terminal that includes that element.

[0079] Of course, those skilled in the art will understand that all or part of the processes in the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium, and when executed, it can include the processes of the embodiments of the above methods. Any references to memory, storage, databases, or other media used in the embodiments provided by this invention can include non-volatile and / or volatile memory. Non-volatile memory can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), or flash memory. Volatile memory can include random access memory (RAM) or external cache memory. By way of illustration and not limitation, RAM is available in various forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), dual data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link DRAM (SLDRAM), Rambus direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM), etc.

[0080] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method for directly writing single-crystal fiber microstructure arrays using femtosecond lasers, characterized in that, The method for directly writing a femtosecond laser-based single-crystal fiber microstructure array includes the following steps: A single-crystal optical fiber is fixed to a clamping device, and a surface morphology image of the single-crystal optical fiber is acquired, wherein the clamping device is configured to drive the single-crystal optical fiber to move axially and rotate about its own axis. The surface morphology image is used to identify the edge regions, irregular regions, and smooth regions on the surface of the single-crystal fiber that can be stably incident by laser. Based on the identification results, the clamping device is controlled to rotate the single-crystal fiber so that the femtosecond laser beam is incident from a direction that avoids the edge regions and / or the irregular regions. Beam shaping is performed on the femtosecond laser beam to reduce astigmatism or focal plane separation generated inside the single-crystal fiber after the femtosecond laser beam is incident on the single-crystal fiber. The femtosecond laser beam, after beam shaping, is focused onto the target writing region inside the single-crystal fiber, and multiple microstructure units are sequentially written according to the preset array design parameters to obtain a microstructure array. During or after writing at least one of the microstructure units, images of the processing area and optical response signals of the microstructure units written are acquired; Based on the image of the processing area and the optical response signal, the processing quality evaluation result and optical parameter deviation are obtained, and based on the processing quality evaluation result and the optical parameter deviation, the fiber attitude, beam shaping parameters and femtosecond laser writing parameters of the subsequent microstructure units are adjusted in a closed loop. The step of identifying the edge regions, irregular regions, and smooth regions suitable for stable laser incidence on the surface of the single-crystal fiber based on the surface morphology image, and controlling the clamping device to rotate the single-crystal fiber based on the identification results, so that the femtosecond laser beam is incident from a direction avoiding the edge regions and / or the irregular regions, specifically includes: Recognition operations are performed on the contour image and light and dark texture image in the surface morphology image to determine the edge position, irregular area position and smooth incident window of the single crystal fiber surface. The recognition operations include edge detection, gray-level gradient analysis, Hough line fitting, curvature calculation, threshold segmentation and learning-based defect recognition operations. Based on the angle between the smooth incident window and the incident direction of the femtosecond laser, a rotation angle correction amount is generated. According to the rotation angle correction amount, the clamping device is controlled to rotate the single-crystal fiber so that the femtosecond laser beam is incident from a direction that avoids the edge region and / or the irregular region. The acquisition of images of the processing area and optical response signals of the microstructure units during or after writing at least one of the microstructure units specifically includes: During or after the writing of at least one of the microstructure units by femtosecond laser, images of the processing area of ​​the target writing region under different illumination and acquisition conditions are acquired, and at least one feature among the length, width, continuity, grayscale contrast, scattering intensity, crack probability, ablation probability, and deviation from the target position of the modified region is extracted from the processing area images. The optical response signal written into the microstructure unit is acquired by an online optical frequency domain reflection system. A tapered thermally adiabatic fusion splice mode selection module is provided between the online optical frequency domain reflection system and the single-crystal fiber. The tapered thermally adiabatic fusion splice mode selection module is used to allow the higher-order modes returned by the microstructure array to leak or be converted into radiation modes during transmission.

2. The method for directly writing single-crystal fiber microstructure arrays using femtosecond lasers according to claim 1, characterized in that, The process of fixing the single-crystal optical fiber to the clamping device and acquiring a surface morphology image of the single-crystal optical fiber specifically includes: The single-crystal optical fiber is fixed to the clamping device; The contour image and light and dark texture image of the single-crystal optical fiber are acquired using a lateral imaging system or a multi-view imaging system to obtain a surface morphology image containing the contour image and the light and dark texture image.

3. The method for directly writing single-crystal fiber microstructure arrays using femtosecond lasers according to claim 1, characterized in that, The beam shaping of the femtosecond laser beam to reduce astigmatism or focal plane separation generated inside the single-crystal fiber after the femtosecond laser beam is incident on the fiber specifically includes: The diameter, shape profile, effective refractive index, writing depth, and incident angle of the single-crystal fiber are obtained, and a focusing model of the femtosecond laser beam after passing through the single-crystal fiber is established. The amplitude distribution, phase distribution, and numerical aperture of the femtosecond laser beam are adjusted according to the focusing model to reduce the astigmatism or focal plane spacing of the femtosecond laser beam in the first and second lateral directions.

4. The method for directly writing single-crystal fiber microstructure arrays using femtosecond lasers according to claim 1, characterized in that, The process of focusing the femtosecond laser beam, after beam shaping, onto the target writing region inside the single-crystal fiber, and sequentially writing multiple microstructure units according to preset array design parameters to obtain a microstructure array, specifically includes: The femtosecond laser beam, after beam shaping, is vertically focused onto the target writing region inside the single-crystal fiber through a high numerical aperture focusing objective. According to the preset array design parameters, multiple microstructure units are sequentially written using point-by-point, line-by-line, surface-by-surface, multi-layer bias, or spiral scanning methods to obtain a microstructure array.

5. The method for directly writing single-crystal fiber microstructure arrays using femtosecond lasers according to claim 1, characterized in that, The process involves obtaining processing quality evaluation results and optical parameter deviations based on the processed area image and the optical response signal, and then performing closed-loop adjustments to the fiber attitude, beam shaping parameters, and femtosecond laser writing parameters of subsequent microstructure units based on these results and optical parameter deviations. Specifically, this includes: The features extracted from the image of the processing area are input into a classification model, a regression model, or an anomaly detection model to obtain the processing quality level, the predicted reflectance value, and the probability of processing defects. The processing quality level, the predicted reflectance value, and the probability of processing defects are used as the processing quality evaluation results. The reflection intensity, center wavelength, spectral width, range domain peak width, or signal-to-noise ratio in the optical response signal are compared with the target value to obtain the optical parameter deviation. Based on the processing quality evaluation result and the optical parameter deviation, the pulse energy, scanning speed, scanning trajectory length, focal position, scanning trajectory spacing, number of repeated scans, slit width, or spatial light modulator compensation phase map of the subsequent microstructure units are adjusted.

6. A device for directly writing a single-crystal fiber microstructure array using a femtosecond laser, characterized in that, The device for direct-writing single-crystal fiber microstructure arrays using femtosecond lasers is used to implement the method for direct-writing single-crystal fiber microstructure arrays as described in any one of claims 1-5, wherein the device comprises: A holding and image acquisition module is used to fix a single-crystal optical fiber to a clamping device and acquire a surface morphology image of the single-crystal optical fiber, wherein the clamping device is configured to drive the single-crystal optical fiber to move along the axial direction and rotate about its own axis. The identification and rotation module is used to identify the edge region, irregular region and smooth region that can be stably incident on the surface of the single crystal fiber according to the surface morphology image, and control the clamping device to rotate the single crystal fiber according to the identification result, so that the femtosecond laser beam is incident from the direction that avoids the edge region and / or the irregular region. A beam shaping module is used to shape the femtosecond laser beam to reduce astigmatism or focal plane separation generated inside the single-crystal fiber after the femtosecond laser beam is incident on the single-crystal fiber. The focusing and writing module is used to focus the femtosecond laser beam after beam shaping onto the target writing region inside the single-crystal fiber, and sequentially write multiple microstructure units according to preset array design parameters to obtain a microstructure array. The acquisition feedback module is used to acquire images of the processing area and optical response signals written to the microstructure unit during or after writing at least one of the microstructure units; The closed-loop control module is used to obtain the processing quality evaluation result and optical parameter deviation based on the processing area image and the optical response signal, and to perform closed-loop adjustment of the fiber attitude, beam shaping parameters and femtosecond laser writing parameters of the subsequent microstructure units based on the processing quality evaluation result and the optical parameter deviation.

7. A terminal, characterized in that, The terminal includes: a memory, a processor, and a program for a femtosecond laser direct-write single-crystal fiber microstructure array stored in the memory and executable on the processor. When the program for the femtosecond laser direct-write single-crystal fiber microstructure array is executed by the processor, it implements the steps of the method for a femtosecond laser direct-write single-crystal fiber microstructure array as described in any one of claims 1-5.

8. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a program for a femtosecond laser direct-write single-crystal fiber microstructure array, which, when executed by a processor, implements the steps of the method for a femtosecond laser direct-write single-crystal fiber microstructure array as described in any one of claims 1-5.

Images