A method for preparing fused silica microlens array based on combination of ablation rapid removal and melting polishing by CO2 laser

Abstract

The application provides a fused quartz microlens array preparation device and method based on CO2 laser ablation rapid removal and fused quartz microlens array preparation method, and belongs to the field of photochemical processing. In order to solve the problems of high cost, poor thermal stability and difficult to control the machining precision when machining the microlens array of the fused quartz hard and brittle material, and difficult to obtain the low-cost and high-quality microlens array. The coupling model of thermodynamics and fluid mechanics is established, the rough machining structure is simulated to determine the rough machining parameters, the best scanning track is obtained through path planning, the high-power density CO2 laser is used for ablation removal of the fused quartz, and the rapid forming of the microstructure is realized; the low-power density CO2 laser is used for irradiating the microstructure, and the precise fusion polishing is realized. Through the use of a set of CO2 laser for machining the microlens array, the machining cost is reduced, the problem of repeated installation and positioning of the workpiece is avoided, the machining efficiency is further improved, and the high-quality microlens array is prepared with high efficiency and low cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical processing, in particular to a fused quartz microlens array preparation device and method based on CO2 laser ablation rapid removal and melting polishing combination. BACKGROUND

[0002] Micro-lens array is widely used in 3D imaging, moire imaging, compound eye imaging, beam shaping, fiber coupling connection, infrared detection, liquid crystal display and other aspects. In order to meet the requirements of optical elements in high temperature environment on thermal stability, micro-lens array is processed on the surface of fused quartz, which can meet the performance requirements in high temperature environment due to its low thermal expansion coefficient, good thermal stability and chemical stability.

[0003] There are many advanced processing methods for processing micro-lens array, such as ultra-precision cutting method, photoresist hot reflow method, gray scale lithography, reactive ion etching method, etc. can realize the processing of micro-lens array on the surface of various materials. However, ultra-precision cutting, gray scale mask lithography has the problem of high cost, and the photoresist hot reflow method has the problems of poor thermal stability of the prepared micro-lens and difficult shape precision control. Laser processing is a new type of processing method, which has the advantages of fast processing speed, no contact with workpiece and precise micro-machining. At present, femtosecond laser, nanosecond laser, picosecond laser and other short pulse lasers are applied in micro-machining field, but they face the problems of poor laser absorption capacity, high cost of short pulse laser and difficult control of processing precision, which limits the application range of short pulse laser. Therefore, it is urgent to develop a low-cost and high-quality micro-lens array preparation method for hard and brittle materials such as fused quartz. SUMMARY

[0004] The technical problem to be solved by the present application is:

[0005] In order to solve the problem that the cost is high and the processing precision is not easy to control when processing micro-lens array for hard and brittle materials such as fused quartz, and it is difficult to obtain low-cost and high-quality micro-lens array.

[0006] The technical scheme adopted by the present application to solve the above technical problem is:

[0007] The present application provides a fused quartz micro-lens array preparation device based on CO2 laser ablation rapid removal and melting polishing combination, which comprises a moving platform, an industrial computer, a galvanometer system, a CO2 laser and an AOM acousto-optic modulator. The moving platform comprises an X-Y moving platform and a Z-axis moving platform. The X-Y moving platform is used to clamp the workpiece and move the workpiece along the X-axis or Y-axis direction. The galvanometer system is arranged on the Z-axis moving platform and used to adjust the distance between the workpiece and the galvanometer system. The industrial computer is used to control the moving platform and the signal generator, and the signal generator is used to control the CO2 laser and the AOM acousto-optic modulator.

[0008] The CO2 laser is emitted by a CO2 laser, modulated by an AOM acousto-optic modulator, reflected into a 2D galvanometer through a mirror, and then acted on the workpiece surface under the deflection of the galvanometer lens and the focusing of the field lens in the galvanometer system, to complete the laser ablation or polishing process.

[0009] A preparation method of a fused quartz microlens array preparation device using a combination of rapid removal and fusion polishing based on CO2 laser ablation, comprising the following steps:

[0010] Step one, according to the incident laser wavelength, incident spot diameter, exit spot diameter and Fresnel number, the homogenization effect of different parameter microlens array is simulated by Monte Carlo method, and the structure parameters of the target microlens array are optimized and determined, including the microlens aperture L, the microlens height H and the microlens curvature radius R1, by comprehensively considering the machining effect of the microlens array, the Gaussian laser homogenization effect and the energy utilization rate;

[0011] Step two, a coupled model of thermodynamics and fluid mechanics is established, and finite element simulation is performed on the CO2 laser polishing rough machining structure process, and the machining structure parameters of the rough initial microstructure are determined according to the structure and parameters of the target microlens array, including the width and height or width-height ratio of the microcolumn;

[0012] Step three, the CO2 laser parameters and scanning path are obtained by comprehensively analyzing the experimental results under different CO2 laser ablation parameters and the simulation results of the multi-physical field coupling model in the ablation process, and the rough machining structure of fused quartz is obtained by using the obtained CO2 laser parameters and scanning path.

[0013] Step four, the CO2 laser parameters are obtained by comprehensively analyzing the experimental results under different CO2 laser polishing parameters and the simulation results of the multi-physical field coupling model in the polishing process, and the fused quartz microlens array is obtained by using the obtained CO2 laser parameters to melt and polish the rough machining microcolumn array.

[0014] Further, in step two, the determination of the machining structure parameters of the initial microstructure includes the following steps,

[0015] ①, according to the shape and arrangement of the microlenses in the microlens array, an initial microstructure capable of realizing precise polishing of the fused quartz microlens array by CO2 laser is designed;

[0016] ②, based on the theories of thermodynamics and fluid mechanics, a simulation model of CO2 laser fusion polishing of the initial microstructure of fused quartz is established;

[0017] ③ Based on the simulation model established in step two, the temperature field simulation of the initial microstructure of the CO2 laser melting and polishing fused silica microlens array is used to determine the relationship between the stable temperature of the fused silica surface and the laser power and laser radius when CO2 laser irradiates a single point.

[0018] ④ Based on the relationship between the stable temperature of the fused silica surface and the laser power and laser radius determined in step three, as well as the structural parameters of the micropillar, determine the radius and power of CO2 laser melting polishing;

[0019] ⑤ Based on the simulation model established in step two, the CO2 laser melting and polishing process of the initial microstructures with different structural parameters of the fused silica microlens array is simulated to obtain the size and morphology of the microlenses.

[0020] ⑥ Compare the simulated microlens morphology of the initial microstructure with different structural parameters obtained in step five with the morphology and size of the target microlens to determine the critical size of the initial microstructure that can be processed into the target microlens array, i.e., the range of the aspect ratio r of the micropillar.

[0021] Furthermore, the interaction process between CO2 laser and fused silica is simplified. By coupling the temperature field controlled by the heat source equation, heat transfer equation and energy conservation equation, and the melting flow process controlled by the momentum conservation equation and continuity equation, a simulation model of the initial microstructure process of CO2 laser melting polishing of fused silica microlens array is established.

[0022] Furthermore, it also includes determining the material properties of fused silica, selecting boundary conditions, and mesh generation;

[0023] When using a manually controlled method for mesh generation, the fused silica matrix is ​​divided into different domains. A denser mesh is generated near the heat source. By gradually refining the mesh, the maximum mesh size that provides stable calculation results is found and used as the mesh size for formal calculations.

[0024] Furthermore, based on the principle that the micropillar as a whole can melt and flow without ablation, the radius and power of CO2 laser melting polishing were selected and determined, and the surface temperature of the processed surface was between the melting point of fused silica (2273K) and the ablation temperature of fused silica (2973K).

[0025] Furthermore, for the temperature field, boundary 1 needs to consider the laser heat input and external heat convection and heat radiation, while the other boundaries 2 to 4 where the fused silica is in contact with air only need to consider heat convection and heat radiation; for the flow field, the effects of surface tension and Marangoni force need to be considered at boundary 1.

[0026] Furthermore, in step three, the specific steps of the scan path planning method include:

[0027] ①, two different processing methods are used for processing experiments, and the processing method with better processing effect is selected, and the two processing methods include array point processing and array line processing;

[0028] ②, the processing surface height of the micro column on the fused quartz surface is uniform as the standard for path planning, the processing effect of CO2 laser scanning according to the planning scheme is obtained by the finite element simulation method for the processing method with better processing effect selected in step one, the corresponding processing surface quality is analyzed, and the final path scheme is optimized.

[0029] Further, first, the scanning track is preliminarily divided into intersecting tracks and non-intersecting S-shaped tracks, the non-intersecting S-shaped track with a relatively flat processing surface is selected, and the scanning mode of the non-intersecting scanning starting point same side track with a more flat processing surface is selected again.

[0030] Further, when the non-intersecting scanning starting point same side track is scanned, the interval time between two adjacent horizontal tracks and the interval time between two adjacent longitudinal tracks are controlled to be the same, when the non-intersecting scanning starting point same side track is scanned, a small-power ablation short line and a high-power ablation long line can be used to realize that the time interval of the scanning long line and the scanning short line is the same, so that the difference of the heat accumulation effect between the scanning long line and the scanning short line is reduced.

[0031] Compared with the prior art, the beneficial effects of the present application are:

[0032] The present application is a kind of based on CO2 Laser's ablation rapid removal and melting polishing combination's fused quartz microlens array preparation device and preparation method, determine the structure parameter of target micro lens array, establish the coupling model of thermodynamics and fluid mechanics, simulate the rough machining structure and then determine the rough machining parameter, obtain the best scanning track by path planning, use high-power density CO2 Laser irradiation fused quartz, make the surface layer material temperature rise to ablation temperature and evaporate rapidly remove, realize the rapid forming of microstructure;Low-power density CO2 Laser irradiation fused quartz, make the surface layer material temperature gradually rise to the melting point and ablation temperature and occur melting flow, realize the precision melting polishing of microstructure.

[0033] Based on the theory of thermodynamics and fluid mechanics, a simulation model of CO2 laser melting polishing fused quartz initial microstructure for processing micro-lens array is established, which can be used to simulate the temperature field distribution of fused quartz surface under different CO2 laser parameters, establish the relationship between the stable temperature reached by the processing surface during CO2 laser polishing and the laser radius and laser power, and provide guidance for selecting appropriate laser parameters in the actual CO2 laser polishing process; the simulation model of CO2 laser melting polishing fused quartz initial microstructure for processing micro-lens array can be used to simulate the material velocity field distribution of the initial microstructure with different structure parameters at different times during the process of CO2 laser precision polishing, and reveal the change rule of the shape and size of the initial microstructure during the process of CO2 laser precision polishing, which provides guidance for better controlling the shape of micro-lens array; and the problems of large workload and low efficiency of the trial-and-error method of process parameters are avoided.

[0034] The influence of array point and array line two processing methods on the surface morphology is revealed through experiments, the processing process of CO2 laser ablation rapid material removal along different trajectories is simulated, the influence of scanning along different trajectories on the surface morphology is revealed, and a theoretical basis for better controlling the surface quality is provided; the path trajectory is gradually optimized by combining the method of finite element simulation, and finally the scheme of non-intersection point scanning starting point same side trajectory which can improve the flatness of the processing surface is planned, so that the problems of excessive or insufficient surface ablation of the initial microstructure of the target micro-lens array are solved.

[0035] The present application is a kind of based on CO2 laser's ablation rapid removal and melting polishing combination's fused quartz micro-lens array preparation device and preparation method, by using a set of CO2 laser to process micro-lens array, reduce the processing cost, avoid the problem of repeated installation positioning workpiece, further improve the processing efficiency, realize high efficiency and low cost preparation high quality micro-lens array. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 It is a structure schematic view of a kind of fused quartz micro-lens array preparation device based on CO2 laser's ablation rapid removal and melting polishing combination in the embodiment of the present application;

[0037] Figure 2 It is a flow chart of a kind of fused quartz micro-lens array preparation method based on CO2 laser's ablation rapid removal and melting polishing combination in the embodiment of the present application;

[0038] Figure 3 It is the flow chart of design method in step two in the embodiment of the present application;

[0039] Figure 4 It is the variation rule diagram of fused quartz parameter with temperature in the embodiment of the present application;

[0040] Figure 5 Flowing diagram for laser heating fused quartz to make it melt in the embodiment of the present application;

[0041] Figure 6 Temperature distribution comparison chart for the embodiment of the present application when the spot radius R=500μm;

[0042] Figure 7 Chart for the relationship between the stable temperature of the fused quartz processing surface and the laser power and radius when CO2 single-point irradiation is used in the embodiment of the present application;

[0043] Figure 8 Fused quartz micro-column melting flow rule in the embodiment of the present application Figure 1 ;

[0044] Figure 9 Fused quartz micro-column melting flow rule in the embodiment of the present application Figure 2 ;

[0045] Figure 10 CO2 laser ablation rapid removal processing mode in the embodiment of the present application Figure 1 ;

[0046] Figure 11 CO2 laser ablation rapid removal processing mode in the embodiment of the present application Figure 2 ;

[0047] Figure 12 Micro-lens and focusing lens structure in the embodiment of the present application

[0048] Figure 13 CO2 laser ablation rapid removal of rough machining micro-lens array initial microstructure in the embodiment of the present application

[0049] Figure 14 CO2 laser ablation rapid removal micro-column array in the embodiment of the present application

[0050] Figure 15 Scanning diagram for CO2 laser ablation rapid removal of rough machining in the embodiment of the present application

[0051] Figure 16 Micro-lens array imaging and focusing effect diagram in the embodiment of the present application

[0052] Figure 17 Micro-lens array homogenization laser effect comparison diagram in the embodiment of the present application. DETAILED DESCRIPTION

[0053] In the description of the present application, it should be noted that the terms such as "upper", "lower", "front", "rear", "left", "right" and the like in the embodiments indicate the position relationship based on the position relationship of the drawings for the purpose of simplifying the description, and do not represent that the elements and devices referred to must be operated according to the specific position and limited operation and method, structure in the description, and such position terms do not constitute a limitation on the present application.

[0054] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application will be described in detail below with reference to the drawings.

[0055] Specific implementation scheme one: combining Figure 1 As shown in the figure, the present application provides a fused quartz microlens array preparation device combined with ablation and melting polishing based on CO2 laser, which comprises a signal generator, a moving platform, an industrial computer, a galvanometer system, a CO2 laser and an AOM acousto-optic modulator, the moving platform comprises an X-Y moving platform and a Z-axis moving platform, the X-Y moving platform is used for clamping a workpiece and moving the workpiece along the X-axis or Y-axis direction, the galvanometer system is arranged on the Z-axis moving platform and used for adjusting the distance between the workpiece and the galvanometer system, the industrial computer is used for controlling the moving platform and the signal generator, and the signal generator is used for controlling the CO2 laser and the AOM acousto-optic modulator.

[0056] The CO2 laser emitted by the CO2 laser is reflected into the 2D galvanometer in the galvanometer system through the AOM acousto-optic modulator, and then acts on the surface of the workpiece under the deflection action of the galvanometer lens and the focusing action of the field lens in the galvanometer system, so as to complete the laser ablation or polishing process. When preparing the fused quartz microlens array, only the preparation device is needed to sequentially complete the laser ablation and polishing process, and the fused quartz does not need to be removed after laser ablation and installed on the polishing device, so that the process of reinstallation and positioning of the workpiece is avoided, and the operation is more simple and efficient.

[0057] Specific implementation scheme two: combining Figure 2 As shown in the figure, the present application provides a fused quartz microlens array preparation method combined with ablation and melting polishing based on CO2 laser, which comprises the following steps:

[0058] Step one, according to the incident laser wavelength, incident spot diameter, exit spot diameter and Fresnel number, the homogenization effect of the microlens array with different parameters is simulated by Monte Carlo method, and the structure parameters of the target microlens array are optimized and determined by comprehensively considering the machining effect of the microlens array, the Gaussian laser homogenization effect and the energy utilization rate, including the microlens aperture L, the microlens height H and the microlens curvature radius R1;

[0059] Step two, the coupling model of thermodynamics and fluid mechanics is established to simulate the process of CO2 laser polishing the rough structure, and the parameters of the rough structure are determined according to the target microlens array structure and parameters, including the width, height or width-height ratio of the micro column;

[0060] In combination Figures 3 to 9 As shown in the drawings, specifically comprising:

[0061] ①, according to the shape and arrangement of the microlens in the microlens array, the initial microstructure of the CO2 laser precision polishing fused quartz microlens array is designed;

[0062] Among them, the initial microstructure refers to the micro column array similar to the arrangement of the microlens array, which is processed by removing the excess material through CO2 laser ablation, including a plurality of uniformly distributed micro columns, and the subsequent CO2 laser fusion polishing of the microstructure makes it melt and flow to obtain the target microlens array; the micro column refers to a square column with a certain height and a square horizontal section;

[0063] ②, based on the theory of thermodynamics and fluid mechanics, a simulation model of CO2 laser fusion polishing of the initial microstructure of fused quartz is established;

[0064] The interaction process of CO2 laser and fused quartz is simplified, and the simulation model of the process of CO2 laser fusion polishing of the initial microstructure of the fused quartz microlens array is established by coupling the temperature field controlled by the heat source equation, heat transfer equation and energy conservation equation and the melting flow process controlled by the momentum conservation equation and continuity equation;

[0065] The temperature field in the laser polishing process is controlled by the energy conservation equation, as follows:

[0066] (1)

[0067] In the formula, ρ is the density of the material; Cp is the specific heat capacity of the material, Cp is the equivalent specific heat capacity considering the latent heat of melting in the simulation process; is the rate of change of the thermodynamic temperature T of the material with time t; is the melt velocity; q is the heat conduction term, which is controlled by the Fourier equation; q0 is the value of heat exchange with the outside world, which is specifically controlled by the boundary condition; k is the thermal conductivity; ▽ is the gradient operator;

[0068] The volume heat source considers that part of the laser is absorbed by the surface layer and part of the laser is transmitted into the material to directly heat the material matrix. In order to be closer to the real physical situation, the volume heat source is used to express the laser heat source, and its expression is as follows:

[0069] (2)

[0070] where, is the heat of the body; a is the absorption coefficient of fused quartz to 10.6 pm laser; R is the Fresnel reflection coefficient; x0and y0are the coordinates of the laser spot position along the x-axis and y-axis, respectively; x, y, and z are the coordinates of the material along the x-axis, y-axis, and z-axis, respectively; a is the radius of the laser at 1 / e of the light intensity;

[0071] The heat transfer caused by thermal convection and thermal radiation is considered in the model. There is natural thermal convection at the interface between the fused quartz and the air, and its formula is as follows:

[0072] (3)

[0073] where, h is the convective heat transfer coefficient, h = 15 W / (m 2 *K); T ext is the ambient temperature;

[0074] The energy carried away by thermal radiation is related to the temperature of the object, and its specific formula is as follows:

[0075] (4)

[0076] where, is the thermal radiation coefficient, for fused quartz, take = 0.8;

[0077] The surface tension of the two-phase flow balances the total normal stress on both sides of the interface, which is described by the following equation:

[0078] (5)

[0079] (6)

[0080] where, is the dynamic viscosity of air, is the dynamic viscosity of the melt; u1is the velocity of air, u2is the velocity of the melt; n is the normal direction; t is the tangent direction; P is the pressure; σ is the surface tension; ▽ s is the surface gradient operator; i represents the i-th node of the grid element after the grid division of the micro-column physical model;

[0081] Since the viscosity of air is much lower than the viscosity of the melt, the viscosity term in the air can be ignored, and since no material ablation occurs during laser polishing, the back pressure of the gas on the liquid at the interface can be ignored, i.e. ; P1is the pressure of air on the interface, and P2is the pressure of the melt on the interface;

[0082] The expression of the Marangoni force acting on the surface of the molten pool is as follows:

[0083] (7)

[0084] In the formula, γ is the surface tension coefficient of fused silica; For the normal component of the fused silica surface; The tangential component of the fused silica surface; ▽ s For surface gradient operators;

[0085] The melting and flow of materials during CO2 laser melting and polishing involves fluid dynamics, governed by the momentum conservation equation and the continuity equation:

[0086] Assuming the melting flow of fused silica is an incompressible laminar flow, the Navier-Stokes equations (NS equations) are used to solve it, as shown in equation (8):

[0087] (8)

[0088] In the formula, I is the identity matrix; p is the pressure inside the melt; this process takes into account the effect of gravity, and g is the gravitational acceleration (m / s²). 2 );

[0089] During the molten flow process, the laser power is controlled to keep the temperature below the ablation temperature of the material. Therefore, the mass of the molten material is conserved. Since it belongs to the continuous medium model, the mass conservation can be expressed as a continuity equation, as shown in formula (9). Since the density of molten quartz is constant in this model and the material is isotropic, the continuity equation can be simplified to formula (10).

[0090] (9)

[0091] (10)

[0092] The above process also requires addressing issues such as determining the properties of fused silica material, selecting boundary conditions, mesh generation, and verifying the accuracy of the model.

[0093] Determining the properties of fused silica materials includes obtaining viscosity, specific heat capacity, liquid phase ratio, surface tension coefficient, Fresnel reflectance, emissivity, convective heat transfer coefficient, and the law of density variation with temperature.

[0094] During laser polishing, fused silica undergoes a non-isothermal process involving temperature rise, phase change, melting and flow, and cooling and solidification. During this process, the material properties change due to temperature. Therefore, it is necessary to obtain the variation law of material properties with temperature.

[0095] Material viscosity The expression for the change with temperature is as follows, and the curve for the change with temperature is as follows. Figure 4(a) shown,

[0096] (11)

[0097] Specific heat capacity of fused quartz The expression of the variation with temperature is as follows:

[0098] (12)

[0099] Assuming that a solidus and a liquidus exist, the specific heat capacity between the solid-liquidus temperature is replaced by an equivalent specific heat capacity as shown in equation (13), where the liquidus proportion f L is determined according to equation (14),

[0100] (13)

[0101] (14)

[0102] where L is the latent heat of fusion, 2 MJ / (Kg.K); f L is the liquidus proportion; and are the solidus and liquidus temperatures of the material (K), respectively;

[0103] The curve of the thermal conductivity of fused quartz as a function of temperature is shown in Figure 4 (b);

[0104] The expression of the variation with temperature of the surface tension coefficient of fused quartz γ is as follows:

[0105] (15)

[0106] where is the surface tension coefficient of fused quartz at temperature T0 (2273.15 K), 0.38 N / m; the derivative of the surface tension with respect to temperature θ = 6 x 10 5 N / (m.K);

[0107] Other properties are shown in Table 1,

[0108] Table 1. Physical property constants of fused quartz

[0109]

[0110] At Figure 5The middle boundary 1 is the laser action boundary, for the temperature field, the boundary 1 needs to consider the laser heat input and the heat convection and heat radiation to the outside, the other fused quartz and air contact boundary 2 to boundary 4 only need to consider the heat convection and heat radiation, the boundary 5 to the boundary 7 does not consider heat exchange, because the model volume is larger than the material volume heated by the heat source, the heat exchange of the boundary 5 to the boundary 7 has little effect on the temperature field, and is not considered;

[0111] Figure 5 Table 2 is the corresponding boundary condition setting;

[0112] For the flow field, the surface tension and the Marangoni force need to be considered at the boundary 1, and the other boundaries do not need to be considered; because the model volume is large, the laser heating can only make the material near the boundary 1 melt and flow, and the other boundaries are still in the solid state, so only the surface tension and the Marangoni force are considered at the boundary 1;

[0113] Because the depth of the molten pool is less than 100 μm during laser polishing, and considering that the heat transfer from the molten pool to the substrate will affect the overall temperature distribution of the substrate, the simulation substrate size can be 1200 μm × 1200 μm × 300 μm; The experiment is carried out at normal temperature and pressure, so the initial temperature of the material and air domain is 293 K, and the environmental pressure is 1 atm.

[0114] Table 2 Boundary condition setting of the simulation model of laser-induced fused quartz melting and flowing

[0115]

[0116] The grid is divided by artificial control method, the fused quartz substrate is divided into different domains, dense grid is divided in the place close to the heat source, through the method of gradually encrypting the grid, the maximum grid size of the stable calculation result is found, as the grid size of the formal calculation;

[0117] In order to verify the accuracy of the heat transfer module, the simulation results are compared with the experimental results measured by Steven T. Yang et al. (Yang S T, Matthews M J, Elhadj S, et al. Thermal transport in CO2 laser irradiated fused silica: In situ measurements and analysis [J]. Journal of Applied Physics, 2009, 106(10): 103106.) of Lawrence Livermore Laboratory, this experiment is selected as the comparison experiment because it is one of the most authoritative experiments in the field.

[0118] As Figure 6 (a) is the temperature distribution along a line through the center of the spot after 5 s of continuous laser irradiation at powers of 1.29 W, 2.48 W, 3.5 W, 5.75 W, and a laser radius of 500 μm. As Figure 6 (b) shows the temperature distribution at various times for R = 500 μm and P = 3.5 W. It can be seen that the experimental results agree well with the simulation results when the laser power is less than 3.5 W, but when the power is higher and the surface temperature of the fused quartz is higher, the simulation results are slightly lower than the experimental results. This verifies the accuracy of the temperature module of the simulation model and provides a solid foundation for the subsequent analysis of the mechanism of the molten flow.

[0119] It should be noted that since Steven used an infrared detector to measure the surface temperature of the fused quartz by measuring the Planck radiation at λ = 9 μm, and the highest temperature calibrated was 1473 K, there was a 3% measurement error at high temperatures, so the 8% difference between the experimental results and the simulation results includes the experimental measurement error and the simulation error, which is within an acceptable range.

[0120] ③. Based on the simulation model established in step two, the temperature field of the initial microstructure of the CO2 laser fused and polished fused quartz microlens array is simulated again to determine the relationship between the stable temperature of the fused quartz surface and the laser power and laser radius when the CO2 laser is irradiated by a single point;

[0121] Since the initial microstructure of the CO2 laser fused and polished fused quartz microlens array is to make the initial microstructure undergo molten flow to become a microlens, rather than to remove material, it is necessary to determine the CO2 laser power parameter that only causes molten flow on the processed surface of the fused quartz without ablation and can reach a stable temperature;

[0122] From the simulation results Figure 7 It can be seen that it is appropriate for the processing surface temperature to be stable between the melting point of 2273 K and the ablation temperature of 2973 K, and to make the fused quartz more easily molten, the processing surface temperature is expected to be closer to the ablation temperature;

[0123] Based on the determination of the processing surface temperature, since the structure size of the microcolumn determines the spot diameter during CO2 laser polishing, for a microcolumn with a larger width, a laser with a larger radius and higher power is selected to heat the fused quartz microcolumn over a larger range to make it molten and flow as a whole to become a microlens;

[0124] ④. According to the relationship between the stable temperature of the fused quartz surface and the laser power and laser radius determined in step three, the radius and power of the CO2 laser for fused and polished are selected and determined based on the principle that the microcolumn as a whole can undergo molten flow without ablation;

[0125] V. According to the simulation model established in step ii, the CO2 laser melting polishing process of the initial microstructure of the fused quartz microlens array with different structural parameters is simulated to obtain the size and morphology of the microlens;

[0126] The specific process includes selecting microcolumns with different widths, calculating the height and width-height ratio of the microcolumn according to the equal volume of the microcolumn and the microlens, and performing finite element simulation on the CO2 laser melting polishing of the microcolumns with different widths. The final microlens simulation morphology is obtained according to the velocity field of the material at different times during the polishing process.

[0127] Taking the velocity field change of a microcolumn with a width of 100 μm and a height of 40 μm during the melting flow process as an example, as shown in FIGS. 8 and 9, the melting flow rule of the fused quartz microcolumn is obtained as follows: Figure 8 Figure 9 At the beginning, the material flows at a relatively high speed, and the highest speed occurs at the position where the microcolumn is connected to the substrate, and the Z-direction velocity component is downward. Subsequently, upward velocity components appear at the top of the microcolumn and other positions, so that the height of the microcolumn is higher than the initial height. With the increase of the melting flow time of the microcolumn, the highest speed of the fused quartz flow begins to decrease, and at the same time, the microcolumn gradually becomes a microlens, and the upward velocity component at the top gradually disappears. The Z-direction velocity component at the position where the microlens is connected to the substrate still exists, so that the fused quartz microlens is flattened, and the height gradually decreases, and finally a spherical cap microlens with a larger curvature radius is processed.

[0128] V. By comparing the simulation morphologies of the microlenses corresponding to the initial microstructures with different structural parameters obtained in step v with the target microlens morphology, the critical size of the initial microstructure capable of processing the target microlens array, i.e., the range of the width-height ratio r of the microcolumn, is determined.

[0129] Through comparison, it is found that the standard deviation of the processed microlens decreases with the increase of the width-height ratio r, i.e., the larger the width-height ratio r, the easier it is to form a shape close to a spherical cap. However, considering the height of the target microlens, the width-height ratio should be limited within a certain range. This is because when the width-height ratio r is too large, the microcolumn is too flat, and under the action of surface tension, the height of the microlens will slightly increase, but it still cannot meet the height range required by the target microlens. When the width-height ratio r is too small, the microcolumn is too high, and the difference between the shape of the obtained microlens and the required profile of the target microlens is large, which is specifically manifested in the concave on both sides of the microlens simulation result. Therefore, the width-height ratio cannot be too small.

[0130] Step iii: obtaining the CO2 laser parameters and scanning path by comprehensively analyzing the experimental results under different CO2 laser ablation parameters and the simulation results of the multi-physical field coupling model of the ablation process, and quickly removing the fused quartz to obtain a rough machining structure by using the obtained CO2 laser parameters and scanning path.

[0131] ​The CO2 laser parameters include laser power P, laser frequency f, laser spot diameter d, scanning speed v, scanning interval, scanning line number k and scanning number N;

[0132] Determination of laser frequency and laser spot diameter:

[0133] The laser frequency is determined by single factor experiment and is selected as 10 kHz; in order to concentrate laser energy and remove material faster, the CO2 laser ablation spot diameter is adjusted as much as possible to the focal spot diameter, which is 120 μm;

[0134] Determination of laser power, scanning speed and scanning interval:

[0135] The influence law of single line scanning depth of laser parameters is studied by single factor experiment, and the laser power P and scanning speed v are determined according to the experimental results; the scanning interval is selected as 30 μm by comparing the experimental effects; a higher scanning speed can be used to reduce the influence of heat accumulation effect on the consistency of ablation depth, and the target ablation depth is achieved by multiple scanning ablation; in order to prevent the machined surface from being rough and uneven due to too large spot overlap rate or reducing the machining efficiency due to too small spot overlap rate, the scanning interval is determined by experiment;

[0136] Determination of scanning line number k and scanning number N:

[0137] According to the results of single line scanning, simulation experiment of multi-line scanning ablation of wider groove is carried out, and the scanning line number k and scanning number N are determined by process experiment (still single factor simulation experiment) of different scanning line number k and different scanning number N, combined with the simulation results of different scanning number N and different scanning line number k, i.e. cross section profile; since the width of the material to be removed between the micro column array is wide, single line scanning cannot process the target micro column width, and multi-line scanning mode with scanning line number k is adopted; the scanning line number k is determined according to the simulation results of multi-physical field coupling model in the ablation process;

[0138] The parameters for rough machining can be set as: CO2 laser power P = 6 W, laser frequency f = 10 kHz, spot diameter d = 120 μm, scanning speed v ≥ 125 mm / s;

[0139] According to the determination method of the scanning path shown in Figure 10 and Figure 11 , the determination method of the scanning path comprises:

[0140] ①, two different machining methods are used for machining experiment respectively, and the machining method with better machining effect is selected, and the two machining methods include array point machining and array line machining;

[0141] According to the width of the removed material required for processing the micro-column array, single-point scanning, single-line scanning, multi-point scanning or multi-line scanning is selected, and the single-line scanning or multi-line scanning can be selected according to the scanning pitch, when the scanning pitch is narrow and the target micro-column interval width meeting the size and topography requirements can be processed by one scanning, the single-line scanning can be selected; when the scanning pitch is wide and the target micro-column interval width meeting the size and topography requirements cannot be processed by one scanning, the multi-line scanning can be selected;

[0142] The present application is suitable for multi-line scanning, because single-point or single-line scanning cannot process the required width, and multi-point or multi-line scanning is required to ablate a wider groove;

[0143] As can be seen from the comparison between multi-point scanning and multi-line scanning, the processing texture between points is very obvious in multi-point scanning, and becomes more obvious with the increase of processing depth, resulting in a higher roughness of the processed surface, while although the processing texture also appears in multi-line scanning, such texture does not become more obvious with the increase of processing depth, and considering that the subsequent molten flow can remove such texture to a certain extent, therefore, the processing effect of multi-line processing can meet the process requirements of the target microlens array;

[0144] Among them, array point processing is to control the laser to act on the designed to be processed position for a period of time, and then process the next point to remove the excess material; array line processing is to control the laser to move along a straight line on the surface to be processed at a certain speed to remove the excess material;

[0145] ②, the processing surface height of the micro-column interval on the fused quartz surface is uniform as the standard for path planning, the processing effect of the CO2 laser scanning according to the planning scheme is obtained by the finite element simulation method for the processing mode selected in step 1 with better processing effect, and the corresponding processing surface quality is analyzed;

[0146] Firstly, the scanning trajectory is divided, including cross trajectory and non-cross S-shaped trajectory, combined with Figure 11 As shown in the figure, the cross trajectory has intersection between the horizontal and vertical trajectories, along Figure 11 (a) in the trajectory, scanning in the direction of the arrow, ablation quickly removes the material to process the micro-column array; the horizontal and vertical trajectories of the non-cross S-shaped trajectory do not have intersection, along Figure 11 (b) in the trajectory, scanning in the direction of the arrow, ablation quickly removes the material, and it is feasible to scan horizontally or vertically first when determining the scanning order;

[0147] In the scanning process, the horizontal and vertical trajectories of the non-cross scanning starting point on the same side of the trajectory do not have intersection, and the scanning starting points of the horizontal and vertical trajectories are on the same side of the material surface, along Figure 11(c) the middle track is scanned in the arrow direction, when scanning the transverse track, after scanning the same row, the next row of transverse track with a specific scanning interval is scanned, and the longitudinal scanning is the same as the transverse scanning;

[0148] wherein the scanning interval can be 30 μm, but the scanning interval is not a constant value, and different experimental parameters can obtain different scanning intervals, which can be obtained by single factor experiment to study the influence law of laser parameters on single line scanning depth, and obtained by comparison experiment;

[0149] The processing mode of multi-line scanning selected in ① is used to scan the cross track and non-cross S-shaped track, and it can be known by comparing the finite element simulation that the cross position of the micro column array processing surface processed along the cross track is ablated excessively due to the heat accumulation effect, and the concave pits appear on the processing surface, while the micro column array processing surface processed along the non-cross S-shaped track is not ablated excessively, but the high protrusions are easily formed on the longitudinal ablation surface due to the insufficient ablation, resulting in the rough and uneven processing surface;

[0150] ③, the scanning method of the same side track of the non-cross scanning starting point is obtained by optimizing the scanning method selected in ② with the best processing effect, the scanning sequence is controlled, and the small power ablation short line and the large power ablation long line are used, here, the large power and the small power are selected under the premise of ablation removal, so that the interval time between the two adjacent transverse tracks and the interval time between the two adjacent longitudinal tracks are close or the same, and then the influence of the heat accumulation effect on the ablation depth of the transverse and longitudinal processing surfaces is close or the same, and the processing surface with the same height is obtained as much as possible;

[0151] Step four, the CO2 laser parameters and scanning path are obtained by comprehensively analyzing the experimental results under different CO2 laser ablation parameters and the simulation results of the multi-physical field coupling model in the ablation process, and the fused quartz micro lens array is obtained by using the obtained CO2 laser parameters and scanning path to melt and polish the rough micro column array;

[0152] The parameter determination and path planning method are the same as step three, and the process parameters also include laser power P, laser spot diameter d, scanning speed v, scanning interval, scanning line number k and scanning number N;

[0153] The CO2 laser frequency f is preferably 10 kHz.

[0154] Specific implementation scheme two: different from the specific implementation scheme one, the processing quality of the processed micro lens array is evaluated from three aspects of fused quartz micro lens array imaging effect, focusing effect and laser energy homogenization effect.

[0155] The other combinations and connection relationships of the embodiment are the same as those of the specific implementation scheme one.

[0156] Embodiment

[0157] This embodiment is used to design a diffractive microlens array, and the basic design parameters are shown in Table 3. According to the basic parameters in Table 1 and the corresponding formula, the microlens system parameters of different apertures (200 μm~900 μm) are calculated, and the specific parameters are shown in Figure 12 .

[0158] Table 3 Basic parameters of the designed microlens array

[0159]

[0160] (1) Step one: According to the incident laser wavelength, incident spot diameter, exit spot diameter and Fresnel number, the uniformization effect of different parameter microlens array simulated by Monte Carlo method is determined to determine the target microlens array structure parameters, including microlens aperture L, microlens height H and microlens curvature radius R1;

[0161] Considering the uniformization effect of different structure parameters of microlens on Gaussian laser beam and the utilization rate of laser, combined with the processing conditions, in order to explore the influence of laser parameters and initial microstructure parameters obtained by CO2 laser ablation rapid removal on the processing target microlens, several microlens array structure parameters with different apertures and good uniformization effect on laser are determined, as shown in Table 4.

[0162] By comparing the processing quality of the microlens array obtained by subsequent processing, the microlens structure parameters with better processing quality are selected. This embodiment only gives the processing effect of micropillar with period of 200 μm and 400 μm.

[0163] Table 4 Size optimization design results of microlens array (2)

[0165] (3) Step two: According to the structure of microlens array, the rough machining structure removed by CO2 laser ablation is micropillar array, as shown in Figure 13 . Through finite element simulation of CO2 laser polishing fused quartz to process microlens array, the micropillar array parameters required to be obtained by CO2 laser ablation removal are designed, as shown in Table 5,

[0166] Table 5 Target micropillar array structure parameters

[0167]

[0168] (4) Step three: melt quartz components clamped on the X-Y axis moving platform, the galvanometer system is set in the Z-axis moving platform, by adjusting the vertical plane in the workpiece position, and the horizontal direction between the galvanometer system and the component distance to adjust the defocus distance. The design parameters are, CO2 laser power P = 6W, laser frequency f = 10kHz, spot diameter d = 120μm, scanning speed v ≥ 125mm / s, using the design of processing method along the set scanning trajectory to remove the excess material, processing micro column array, such as Figure 14 shown,

[0169] CO2 laser ablation processing parameters are shown in Table 6, CO2 laser power P = 6W, frequency f = 10kHz, the distance between the surface to be processed and the focal point is changed to obtain a laser spot with a diameter d = 120μm. A higher scanning speed v (≥ 125mm / s) is used to reduce the influence of heat accumulation on the consistency of ablation depth, and the target ablation depth is achieved by multiple ablation. In order to prevent the processing surface from being rough due to excessive spot overlap rate or reducing the processing efficiency due to small spot overlap rate, the scanning pitch is determined to be 30μm through experiments. The scanning pitch d y as shown below Figure 15 is the distance between the centers of two adjacent longitudinal spots.

[0170] Table 6 CO2 laser ablation rapid removal of rough machining parameters

[0171]

[0172] Through measurement, the processing results are shown in Table 7, the difference between the height of No. 1 sample with a period of 200μm and the target height of 40μm and the height of No. 7 sample with a period of 400μm is within 2μm, which meets the requirements of processing accuracy, and the groove has good consistency and the groove bottom is relatively flat, which meets the processing requirements.

[0173] Table 7 CO2 laser ablation rapid rough machining of micro column array structure parameters

[0174]

[0175] (4) Step four: CO2 laser scanning melt quartz micro column array to make it melt and flow, using a large spot with low speed to polish the micro column array, and by controlling the scanning times to obtain a micro lens with target curvature. The processing parameters are shown in Table 8. No. 1 micro lens array with a period of 200μm is polished twice by CO2 laser, laser power P = 8.44W, spot diameter d = 1.4mm (1 / e 2 ), scanning pitch 0.15mm, scanning speed v = 0.1mm / s, processing area 2×2mm 2square region. The 1st microlens array with a period of 400 μm was polished twice by CO2 laser with laser power P = 14.22 W, spot diameter d = 2 mm (1 / e 2 ), scanning pitch 0.3 mm, scanning speed v = 0.15 mm / s, and the processing region was 3 × 3 mm 2 square region.

[0176] Table 8 Processing parameters of CO2 laser melting polishing for processing micropost array

[0177]

[0178] The top of the processed microlens was fitted. The top of the micropost (1st sample) with a period of 200 μm was spherical, and the span was about 80 μm. Three 57 × 50 μm 2 square regions at the top of the sphere were selected to measure the roughness, and the average roughness Ra was 32.56 nm. The top of the micropost (7th sample) with a period of 400 μm was also spherical, and the span was about 300 μm. Three 125 × 147 μm 2 square regions at the top of the sphere were selected to measure the roughness, and the average roughness Ra was 44.49 nm.

[0179] (5) Step five: evaluating the processing quality of microlens

[0180] The processing effect of the microlens array was evaluated from three aspects of imaging, focusing characteristics and laser energy homogenization effect. The device used to observe the imaging effect of the microlens array is shown in Figure 16 (a). The imaging effects of the 1st microlens array with a period of 200 μm and the 7th microlens array with a period of 400 μm are shown in Figure 16 (b) and 16(d), respectively. It can be seen that the imaging effects of the two arrays are good, and there is no distortion. When the object is close to the microlens, the imaging is small, which is consistent with the action of ordinary convex lens. At the same time, the size consistency of the letter-shaped imaging is good, that is, the focusing consistency of the lens is good, and the polishing effect is relatively uniform. The focusing effect of the microlens array is shown in Figure 16 (c) and 16(e), respectively. There is a bright spot at the top of the microlens, which has a good converging effect on light.

[0181] As shown in Figure 17 (a) and 17(b), respectively, are the shapes of laser energy distribution before and after homogenization. It can be seen that the laser distribution in the x direction is relatively uniform, and the laser energy homogenization effect is good. However, the laser in the y direction is still Gaussian distribution, which is almost the same as the energy distribution before the spot homogenization. Therefore, the processing method combining ablation rapid removal and melting polishing based on CO2 laser can prepare a fused quartz microlens array with good imaging and light focusing effect, good focusing consistency and good energy homogenization effect.

[0182] Although the present application has been disclosed with reference to the above examples, the scope of the present application is not limited to the above examples. Those skilled in the art to which the present application pertains will be able to make various changes and modifications without departing from the spirit and scope of the present application, and such changes and modifications will fall within the scope of the present application.

Claims

1. A method for fabricating fused silica microlens array by a combination of CO2 laser based ablation rapid removal and melting polishing, characterized in that, The method comprises the following steps: Step one, according to the wavelength of incident laser, the diameter of incident spot, the diameter of exit spot and the Fresnel number, the homogenization effect of different parameter micro-lens array is simulated by Monte Carlo method, the structure parameters of target micro-lens array are determined by comprehensively considering the machining effect of micro-lens array, the homogenization effect of Gaussian laser and the energy utilization rate, including the micro-lens aperture L, the micro-lens height H and the micro-lens curvature radius R1; Step two, a coupled model of thermodynamics and fluid mechanics is established, the finite element simulation of the CO2 laser polishing rough machining structure process is carried out, the machining structure parameters of the rough initial microstructure are determined according to the structure and parameters of the target micro-lens array, including the width and height of the micro-column or the width-height ratio; Step three, the CO2 laser parameters and scanning path are obtained by comprehensively considering the experimental results under different CO2 laser ablation parameters and the simulation results of the multi-physical field coupling model in the ablation process, and the rough machining structure is quickly removed from the fused quartz by using the obtained CO2 laser parameters and scanning path; Step four, the CO2 laser parameters are obtained by comprehensively considering the experimental results under different CO2 laser polishing parameters and the simulation results of the multi-physical field coupling model in the polishing process, and the fused quartz micro-lens array is obtained by using the obtained CO2 laser parameters to melt and polish the rough machining micro-column array; The device for preparing the fused quartz micro-lens array comprises a signal generator, a moving platform, an industrial computer, a galvanometer system, a CO2 laser and an AOM acousto-optic modulator, the moving platform comprises an X-Y moving platform and a Z-axis moving platform, the X-Y moving platform is used for clamping a workpiece and moving the workpiece along the X-axis or Y-axis direction, the galvanometer system is arranged on the Z-axis moving platform and is used for adjusting the distance between the workpiece and the galvanometer system, and the industrial computer is used for controlling the moving platform and the signal generator, and the signal generator is used for controlling the CO2 laser and the AOM acousto-optic modulator; The CO2 laser emitted by the CO2 laser is modulated by the AOM acousto-optic modulator, is reflected into the 2D galvanometer in the galvanometer system through the reflecting mirror, and then is acted on the surface of the workpiece under the deflection of the galvanometer lens and the focusing of the field lens in the galvanometer system, so that the laser ablation or polishing process is completed.

2. The method for fabricating fused silica microlens array by combined ablation and melting polishing with CO2 laser according to claim 1, wherein: The determination of the machining structure parameters of the initial microstructure comprises the following steps, ①, according to the shape and arrangement mode of the micro-lens in the micro-lens array, an initial microstructure capable of realizing the precise polishing of the fused quartz micro-lens array by the CO2 laser is designed; ②, based on the theory of thermodynamics and fluid mechanics, a simulation model of the CO2 laser melt-polishing of the initial microstructure of the fused quartz is established; ③, on the basis of the simulation model established in step two, the temperature field of the initial microstructure of the CO2 laser melt-polishing of the fused quartz micro-lens array is simulated, which is used to determine the relationship between the stable temperature of the fused quartz surface and the laser power and the laser radius when the CO2 laser single-point irradiation; ④, according to the relationship between the stable temperature of the fused quartz surface and the laser power and the laser radius determined in step three, and the structure parameters of the micro-column, the radius and power of the CO2 laser melt-polishing are determined; ⑤ Based on the simulation model established in step two, the CO2 laser melting and polishing process of the initial microstructures with different structural parameters of the fused silica microlens array is simulated to obtain the size and morphology of the microlenses. ⑥ Compare the simulated microlens morphology of the initial microstructure with different structural parameters obtained in step five with the morphology and size of the target microlens to determine the critical size of the initial microstructure that can be processed into the target microlens array, i.e., the range of the aspect ratio r of the micropillar.

3. The method for fabricating fused silica microlens array by combined ablation and melting polishing with CO2 laser according to claim 2, wherein: The interaction process between CO2 laser and fused silica is simplified. By coupling the temperature field controlled by the heat source equation, heat transfer equation and energy conservation equation, and the melting flow process controlled by the momentum conservation equation and continuity equation, a simulation model of the initial microstructure process of CO2 laser melting polishing of fused silica microlens array is established.

4. The method for fabricating fused silica microlens array by combined ablation and melting polishing with CO2 laser according to claim 3, wherein: It also includes determining the material properties of fused silica, selecting boundary conditions, and mesh generation; When using a manually controlled method for mesh generation, the fused silica matrix is ​​divided into different domains. A denser mesh is generated near the heat source. By gradually refining the mesh, the maximum mesh size that provides stable calculation results is found and used as the mesh size for formal calculations.

5. The method for fabricating fused silica microlens array by combined ablation and melting polishing with CO2 laser according to claim 4, wherein: Based on the principle that the micropillar can melt and flow without ablation, the radius and power of CO2 laser melting polishing were selected and determined, and the surface temperature was between the melting point of fused silica (2273K) and the ablation temperature of fused silica (2973K).

6. The method for fabricating fused silica microlens array by combined ablation and melting polishing with CO2 laser according to claim 5, wherein: In step three, the specific steps of the scan path planning method include: ① Conduct processing experiments using two different processing methods to select the processing method with better processing effect. The two processing methods include array point processing and array line processing. ② Using the uniformity of the processing surface height between the micropillars on the fused silica surface as the standard, path planning is carried out. For the processing method with better processing effect selected in step one, the processing effect of CO2 laser scanning according to the planned scheme is obtained by finite element simulation method. The corresponding processing surface quality is analyzed, and the final path scheme is optimized and obtained.

7. The method for fabricating fused silica microlens array by combined ablation and melting polishing with CO2 laser according to claim 6, wherein: First, the scanning trajectory is initially divided into intersecting trajectory and non-intersecting S-shaped trajectory. The non-intersecting S-shaped trajectory with a relatively flat processing surface is selected. Then, it is optimized again to select the scanning method of the non-intersecting scanning starting point on the same side as the processing surface with a flatter processing surface.

8. The method for fabricating fused silica microlens array by combined ablation and melting polishing with CO2 laser according to claim 7, wherein: When scanning trajectories on the same side of the non-intersecting scanning starting point, the interval between two adjacent transverse trajectories is controlled to be the same as the interval between two adjacent longitudinal trajectories. When scanning trajectories on the same side of the non-intersecting scanning starting point, low power can be used to ablate short lines and high power to ablate long lines to achieve the same time interval between scanning long lines and scanning short lines, so as to reduce the difference in thermal accumulation effect between scanning long lines and scanning short lines.

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