Ion beam figuring method for large aperture optical window

Abstract

This invention discloses a method for refining the parallelism of a large-aperture optical window using an ion beam, comprising the following steps: first, the large-aperture optical window sample is sequentially subjected to grinding and polishing, grinding and polishing, and precision polishing; acquiring the tilted transmission wavefront shape data; clamping the sample; using the transmission wavefront shape data to simulate and calculate the dwell time for refining the parallelism, and generating NC machining code; evacuating the ion beam polishing machine, igniting the ion source, and after stabilization, importing the NC machining code to adjust the process parameters, and using a large-aperture ion beam grid for parallelism refinement. The processing method of this invention can ensure high parallelism accuracy without compromising the original surface shape accuracy; it can efficiently and accurately improve problems such as excessive parallelism before surface shape refinement of large-size optical windows using an ion beam; and the processing method involved is convenient and has a short cycle, providing a new approach for the efficient fabrication of large-size optical windows.

Description

Technical Field

[0001] This invention belongs to the field of optical manufacturing technology, specifically relating to a processing method for refining the parallelism of large-aperture optical windows using ion beams. Background Technology

[0002] In the field of optics, optical windows generally need to have a series of excellent properties: ① They must maintain very high transmittance in the ultraviolet, visible and infrared bands; ② They must have good mechanical properties, extremely high thermal shock resistance, very high hardness, and excellent wear resistance; ③ They must have high parallelism accuracy.

[0003] High-precision optical windows must simultaneously possess both surface shape accuracy and parallelism accuracy. To achieve this, the traditional method involves grinding and polishing the optical window to meet these requirements. Traditional polishing methods primarily utilize contact polishing, including mechanical grinding, mechanical polishing, chemical polishing, and chemical-assisted mechanical polishing. However, the high precision requirements for both surface shape and parallelism mean that traditional processing methods, while satisfying surface shape accuracy, cannot meet parallelism accuracy requirements. Conversely, satisfying parallelism accuracy compromises the surface shape. Therefore, simultaneously achieving both surface shape accuracy and parallelism accuracy is challenging, requiring both to be aligned to ensure that both accuracy requirements are met.

[0004] Furthermore, the fabrication of large-aperture optical windows involves numerous uncertainties. Reaching the ion beam processing stage is essentially the final step in optical processing. With ever-increasing demands for processing precision, various factors often lead to excessive flatness differences in large-aperture optical windows, making fine-tuning difficult. This typically necessitates reprocessing by correcting the flatness differences, hindering the rapid and efficient fabrication of large-aperture optical-grade windows. Summary of the Invention

[0005] The main objective of this invention is to address the problems and shortcomings of existing technologies by providing a non-contact ion beam finishing method for obtaining large-aperture optical-grade windows with high surface accuracy and high parallelism accuracy quickly, efficiently, and stably.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for refining the parallelism of a large-aperture optical window using an ion beam includes the following steps:

[0008] 1) Grind and polish the large-diameter optical window sample until the edge thickness difference is <0.01mm;

[0009] 2) Perform grinding and polishing until the surface shape accuracy PV ≤ 15λ (λ = 632.8nm) and parallelism difference < 5″;

[0010] 3) Perform precision polishing until the PV within the light transmission aperture of the large-diameter optical window is ≤2λ, RMS is ≤1.0λ, and parallelism is ≤1.5″;

[0011] 4) The transmitted wavefront shape data of the large-aperture optical window sample with tilt was obtained by measuring with a large-aperture laser interferometer;

[0012] 5) After cleaning and protecting the back of the large-diameter optical window sample in the clean room, place the large-diameter optical window in the customized tooling sink and fix it around the perimeter with pressure strips. After clamping, hang it in the second chamber of the ion beam polishing machine and flip the part by the flipping mechanism so that its working surface is facing down. Finally, drag it to the processing area by the drag bar motor and clamp it.

[0013] 6) The dwell time of the fine-tuning parallelism is obtained by simulation calculation using the inclined transmitted wavefront shape data of the large-aperture optical window sample, and NC machining code is generated.

[0014] 7) Vacuum the ion beam polisher, ignite the ion source, stabilize it, import the NC machining code, adjust the process parameters, and then perform parallel difference fine repair.

[0015] 8) After processing, take out the large-aperture optical window sample and wait for its temperature to stabilize. Then, use a large-aperture laser interferometer to measure its tilted transmission wavefront shape to confirm that it meets the design requirements and obtain a large-aperture optical window sample with high surface accuracy and high parallelism accuracy.

[0016] In the above scheme, the diagonal length of the large-aperture optical window sample is more than 500mm; its material can be common optical glass materials such as sapphire, K9 glass or quartz.

[0017] In the above scheme, a 1.2m copper disc single-axis machine is used in step 1).

[0018] Preferably, the grinding and polishing step includes:

[0019] A. Controlling the process parameters of the copper disc single-axis machine: the spindle speed of the copper disc increases from 0 to 25-30 rpm, the swing speed of the swing shaft increases from 0 to 30-35 rpm, and the pressure of the swing shaft iron pen tip increases from 0 to 0.3-0.35 MPa;

[0020] B. Grind using W28 boron carbide abrasive until the edge thickness difference of the large-diameter optical window is <0.03mm;

[0021] C. Grind using W14 boron carbide abrasive until the edge thickness difference of the large-diameter optical window is <0.01mm.

[0022] In the above scheme, a 1.2m polyurethane single-axis machine is used in step 2).

[0023] Preferably, the polishing step includes:

[0024] A polyurethane disc single-axis machine was used, and the process parameters were controlled as follows: the polyurethane disc spindle speed was increased from 0 to 95-100 rpm, the pendulum speed was increased from 0 to 90-100 rpm, and the pendulum tip pressure was increased from 0 to 0.3-0.4 MPa. Polishing was performed using 5μm diamond solution. A Φ150mm optical template was used to observe the surface shape changes of the large-aperture optical window, and the distance between the large-aperture optical window and the center of the asphalt disc was adjusted based on the surface shape detected by the template. The parallelism of the large-aperture optical window was detected on a large-aperture interferometer. The center of gravity of the pendulum tip was finely adjusted 20-30mm away from the center of the interferogram. After flipping the large-aperture optical window, polishing was continued using 5μm diamond solution. The surface shape and parallelism of the large-aperture optical window were measured until the surface shape accuracy PV of the large-aperture optical window sample was ≤15λ (λ=632.8nm) and the parallelism was <5″.

[0025] In the above scheme, a 2.5m polyurethane disc ring polisher is used in step 3).

[0026] Preferably, the precision polishing step includes:

[0027] A. A polyurethane disc ring polishing machine is used, and the process parameters are controlled as follows: the main polyurethane disc speed is 3-4 rpm, the correction disc speed is 2.5-3 rpm, and the workpiece ring speed is 3.5-4 rpm.

[0028] B. Polish with 3μm diamond liquid until N≤3, ΔN≤0.5 (within Ф150mm diameter), and parallelism difference≤3″;

[0029] C. Polish with 1μm alumina liquid until the light transmission aperture of the large-aperture optical window is PV≤2λ, RMS≤1.0λ (λ=632.8nm), and parallelism ≤1.5″.

[0030] Preferably, step (4) is implemented in the following manner:

[0031] A. Adjust the background fringes of the large-aperture laser interferometer to zero fringes. The mathematical expression for calculating the parallelism difference is:

[0032]

[0033] In the formula, n is the refractive index of the large-aperture optical window; m is the number of interference fringes within the length b; b is the diameter of the large-aperture optical window being measured; and λ is the wavelength of the light source used in the laser interferometer.

[0034] The parallel difference obtained from this mathematical expression is in radians, which can be converted to seconds if needed.

[0035] B. Change the refractive index n of the large-aperture laser interferometer to the refractive index corresponding to the material of the large-aperture optical window;

[0036] C. Fix the large-aperture optical window between the transmission (TF) mirror and the reflection (RF) mirror of the large-aperture laser interferometer;

[0037] D. After masking the large-aperture optical window within its aperture range using the large-aperture laser interferometer testing software, click to measure and obtain the DAT surface shape data of the large-aperture optical window with tilt.

[0038] In the above scheme, the cleaning step 5) includes wiping the front and back of the large-diameter optical window with alcohol in the clean bench to make its surface free of watermarks, fingerprints and other stains.

[0039] The back protection steps include: applying high-temperature resistant adhesive tape to the unprocessed surface of the large-diameter optical window for back protection;

[0040] The clamping steps include: placing the large-diameter optical window in the customized tooling sink and fixing it around its perimeter with pressure strips; after clamping, hanging it in the second chamber of the ion beam polishing machine and flipping the part with the flipping mechanism so that its working surface faces down; and finally dragging it to the processing area and clamping it with the drag bar motor.

[0041] Preferably, step (6) is implemented in the following manner:

[0042] A. Import the DAT surface shape data file of the large-aperture optical window with tilted transmission wavefront obtained by the large-aperture laser interferometer into Metro Pro software, perform flipping and aperture calibration processing, and save it as an XYZ surface shape data file.

[0043] B. Using the XYZ surface data file as the initial surface shape, the dwell time and NC machining code for the fine parallelism of the ion beam polishing machine are obtained through simulation using MATLAB software.

[0044] Preferably, step (7) is implemented in the following manner:

[0045] A. Once the vacuum level in the main chamber is less than 20 Pa, replace it with a molecular pump for fine vacuum evacuation;

[0046] B. After the vacuum level of the main chamber is less than 20 MPa, turn on the RF switch of the ion source to perform power matching. After successful matching, stabilize for 5 to 10 minutes.

[0047] C. Turn on and set the argon flow rate to 12-15 sccm to ignite the ion source. Stabilize the ion source for 5-10 minutes after ignition.

[0048] D. Turn on the beam voltage, accelerating voltage, and neutralizer. After the ion source has stabilized for more than 30 minutes, import the NC code, adjust the process parameters, and then perform parallel difference finishing: use FWHM with a 32mm ion beam grid for processing. Its process parameters are: Gas 12~15sccm, RF 200~250W, Beam 1300~1500eV, Accelerator 220~250eV, Neutralizer 120~150mA.

[0049] Preferably, step (8) is implemented in the following manner:

[0050] Within the large-aperture optical window, the transmitted wavefront PV of the sample was measured to be ≤1.0λ. Within any Ф150mm range, the transmitted wavefront PV was ≤λ / 10, RMS was ≤λ / 50 (λ=632.8nm), and the parallelism (Parallel_Theta) was ≤0.5″.

[0051] If the design requirements are not met, continue to repeat steps (4) to (8).

[0052] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0053] This invention addresses the parallelism error problems commonly found in large-aperture optical windows. It employs a combination of large-aperture grids and optimized ion beam polishing processes to quantitatively correct parallelism errors without compromising the original surface accuracy. This ensures high surface accuracy and parallelism error precision within a shorter processing cycle. It breaks away from the traditional approach in optical processing that relies solely on strict control of parallelism errors in early stages of production. This invention efficiently and accurately corrects parallelism error problems before ion beam finishing.

[0054] 1) This invention employs optimized grinding and polishing, shovel polishing, and precision polishing processes. For grinding and polishing, the traditional cast iron disc is replaced with a copper disc, effectively addressing the problems of severe grinding and poor cleaning that easily cause scratches on the optical window surface when using a cast iron disc. For shovel polishing, the traditional asphalt disc is replaced with a polyurethane disc, effectively addressing the problems of easy wear, reduced grinding efficiency, and poor cleaning that easily cause scratches on the optical window surface when using a traditional asphalt disc. For precision polishing, the replacement of the traditional asphalt disc with a polyurethane disc, while ensuring grinding efficiency and reducing surface scratches, facilitates pressure treatment based on the surface shape distribution of the large-diameter optical window during precision polishing. This process often easily causes large-area collapse of the asphalt disc, requiring the recasting of the asphalt disc and grooving.

[0055] 2) Based on the above-mentioned improved grinding and polishing-grinding-precision polishing process, for ion beam finishing, the existing RF40 small-aperture ion source and its processing conditions are replaced with an RF80 large-aperture ion source combined with higher ion beam processing parameters: ① The RF80 ion source has a larger grid aperture, and the volume removal rate per unit time is 3 to 5 times higher than that of the RF40 ion source; ② The RF80 ion source has a larger grid aperture, and the energy distribution is more dispersed than that of the RF40 ion source, making it less likely to experience instantaneous energy accumulation in the RF40 ion source area, which would result in a large-aperture optical window. The risk of breakage (for large-aperture optical windows, if energy is concentrated in a point area at an instant, uneven heating may cause breakage); ③ Large-aperture optical windows are processed at the nanoscale, which requires a certain level of smoothness and roughness. Long-term ion beam processing can easily cause a large number of bright spots on the surface of the optical window or even create a rough surface; Using the RF80 ion source, due to its "fat and short" beam shape, the processing cycle of large-aperture optical windows is short. Combined with higher ion beam processing parameters, this phenomenon is less likely to occur, which can effectively improve the problem of bright spots or even rough surfaces on the product surface. Attached Figure Description

[0056] Figure 1 Report on the surface shape inspection of large-aperture optical windows before ion beam processing

[0057] Figure 2 Report on the Tilted (TLT) Surface Shape Inspection of Large-Aperture Optical Windows Before Ion Beam Processing

[0058] Figure 3 This is a report on the surface shape inspection of a large-aperture optical window after ion beam processing.

[0059] Figures 4-7 This is a report on the local surface shape inspection of four Ф150mm ranges selected for the large-aperture optical window after ion beam processing. Detailed Implementation

[0060] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. These descriptions are only for further illustrating the features and advantages of the present invention, and are not intended to limit the scope of the claims of the present invention.

[0061] The large-aperture optical windows in the following specific examples are made of quartz material and were provided by Chengdu Guangming Co., Ltd., with specific dimensions of 525mm×400mm×12mm.

[0062] Example 1

[0063] A method for refining the parallelism of a large-aperture optical window (quartz) using an ion beam includes the following steps:

[0064] 1) A large-diameter optical window sample (525mm×400mm×12mm) was ground and polished using a 1.2m cast iron disc single-axis machine, specifically through the following method:

[0065] A. Controlling the process parameters of the 1.2m copper disc single-axis machine: the spindle speed of the copper disc is increased from 0 to 30 rpm, the swing speed of the swing shaft is increased from 0 to 35 rpm, and the pressure of the swing shaft iron pen tip is increased from 0 to 0.35 MPa;

[0066] B. Grind using W28 boron carbide abrasive until the edge thickness difference of the large-diameter optical window is <0.03mm;

[0067] C. Grind using W14 boron carbide abrasive until the edge thickness difference of the large-diameter optical window is <0.01mm;

[0068] 2) A 1.2m polyurethane disc single-axis polishing machine was used to grind and polish the large-aperture optical window sample, specifically through the following method:

[0069] A. Controlling the process parameters of the 1.2m polyurethane disc single-axis machine: the polyurethane disc spindle speed increases from 0 to 100 rpm, the swing shaft speed increases from 0 to 100 rpm, and the swing shaft iron pen head pressure increases from 0 to 0.4 MPa;

[0070] B. Polish using 5μm diamond liquid;

[0071] C. Use a Φ150mm optical template to observe the surface shape changes of the large-aperture optical window, and adjust the position of the large-aperture optical window away from the center of the polyurethane disk according to the surface shape detected by the template. Detect the parallelism difference of the large-aperture optical window on a large-aperture interferometer, and finely adjust the center of gravity of the pen tip of the pendulum axis away from the center of the interferogram. After flipping the large-aperture optical window, continue polishing with 5μm diamond solution.

[0072] D. Measure the surface shape and parallelism of the large-aperture optical window. When the surface shape accuracy PV ≤ 15λ (λ = 632.8nm) and the parallelism < 5″, proceed to the next process.

[0073] 3) A 2.5m polyurethane disc ring polisher is used to perform precision polishing on large-diameter optical window samples. Specifically, this is achieved as follows: A. Control the process parameters of the 2.5m polyurethane disc ring polisher: main polyurethane disc speed: 2 rpm, correction disc speed: 3 rpm, workpiece ring speed: 4.0 rpm;

[0074] B. Polish with 3μm diamond liquid until N≤3, ΔN≤0.5 (within Ф150mm diameter), and parallelism difference≤3″;

[0075] C. Polish with 1μm alumina liquid until the light transmission aperture of the large-aperture optical window is PV≤2λ, RMS≤1.0λ (λ=632.8nm), and parallelism ≤1.5″.

[0076] 4) The transmissive wavefront shape data file of the large-aperture optical window sample with tilt was obtained by measuring with a large-aperture (450mm) laser interferometer. Specifically, this was achieved in the following way:

[0077] A. Adjust the background fringes of the large-aperture laser interferometer to zero fringes. The mathematical expression for calculating the parallelism difference is:

[0078]

[0079] In the formula, n ~ refractive index of the large-aperture optical window, m ~ number of interference fringes within the length b, b ~ the large-aperture optical window under test, and λ ~ wavelength of the light source used in the laser interferometer;

[0080] The parallel difference obtained from this mathematical expression is in radians, which can be converted to seconds if needed.

[0081] B. Change the refractive index n of the large-aperture laser interferometer to the refractive index corresponding to the material of the large-aperture optical window;

[0082] C. Fix the large-aperture optical window between the TF mirror and the RF mirror of the large-aperture laser interferometer;

[0083] D. After masking the large-aperture optical window within its aperture range using the large-aperture laser interferometer testing software, click to measure and obtain the DAT surface shape data file of the large-aperture optical window with tilted transmission wavefront.

[0084] 5) Clean, protect the back of, and clamp the large-aperture optical window sample in the cleanroom. Then, hang the large-aperture optical window sample on the sample fixture in the secondary chamber of the ion beam polishing machine and close the secondary chamber. Finally, drag the large-aperture optical window sample to the designated position in the main chamber using a magnetic connecting bar and clamp it. This is achieved in the following way:

[0085] A. Wipe the front and back of the large-diameter optical window with alcohol inside the clean bench to remove watermarks, fingerprints and other stains from its surface.

[0086] B. High-temperature resistant adhesive tape is applied to the unprocessed surface of the large-diameter optical window for back protection.

[0087] C. The large-aperture optical window is clamped onto the ion beam polishing machine parts clamping fixture, then the clamping fixture is hung on the transport trolley, and then dragged to the designated position in the main cavity via the magnetic connecting bar. Finally, the parts clamping fixture is locked by the locking device.

[0088] 6) The dwell time of the fine-tuning parallelism is obtained by simulation calculation using the data file of the inclined transmitted wavefront shape of the large-aperture optical window sample, and NC machining code is generated. This is specifically achieved in the following way:

[0089] A. Import the DAT surface shape data file of the large-aperture optical window with tilted transmission wavefront obtained by the large-aperture laser interferometer into Metro Pro software, perform flipping and aperture calibration processing, and save it as an XYZ surface shape data file.

[0090] B. Using the XYZ surface data file as the initial surface shape, the dwell time and NC machining code for the fine parallelism of the ion beam polishing machine are obtained through simulation using MATLAB software.

[0091] 7) Start the mechanical pump of the ion beam polisher to perform a rough vacuum evacuation. After the vacuum level in the chamber drops below 20 Pa, switch to a molecular pump for fine evacuation. After the vacuum level drops below 20 MPa, ignite the ion source. After the ion source stabilizes for 30 minutes, import the NC machining code for parallel difference fine finishing. This is achieved in the following way:

[0092] A. Turn on the mechanical pump of the ion beam polisher to perform a rough vacuum pumping of the cavity. After the vacuum level of the main cavity is less than 20 Pa, switch to the molecular pump to perform a fine vacuum pumping.

[0093] B. After the vacuum level of the main chamber is less than 20 MPa, turn on the RF switch of the ion source to perform power matching. After successful matching, stabilize for 5 to 10 minutes.

[0094] C. Turn on and set the argon flow rate to 12 sccm to ignite the ion source. Stabilize the ion source for 5-10 minutes after ignition.

[0095] D. Turn on the beam voltage, acceleration voltage, and neutralizer. After the ion source has stabilized for more than 30 minutes, import the NC code to adjust the process parameters for processing. Among them, FWHM with a 32mm ion beam grid is used for processing, and its process parameters are: Gas = 12sccm, RF = 200W, Beam = 1500eV, Accelerator = 220eV, Neutralizer = 120mA.

[0096] 8) After processing, take out the large-aperture optical window sample and wait for its temperature to stabilize. Then, use a large-aperture laser interferometer to measure its tilted transmission wavefront shape. If it meets the requirements of the drawing, a large-aperture optical window sample with high parallelism difference is obtained. If it does not meet the requirements of the drawing, repeat steps (4) to (8). Specifically, this is achieved in the following way:

[0097] Within the aperture of the large-aperture optical window, the parallelism difference of the sample is ≤0.5″;

[0098] If the requirements of the drawing are not met, continue to repeat steps (4) to (8).

[0099] Figure 1 This is a report on the surface shape inspection of a large-aperture optical window before ion beam processing. Figure 2 The report for the large-aperture optical window with a tilted (TLT) surface shape shows a parallelism error (Parallel_Theta) of 1.17063 seconds, which falls short of the requirement of less than 0.5 seconds (indicating an out-of-tolerance issue). Furthermore, the highest point of the parallelism error is located in the lower left region of the large-aperture optical window. In this case, returning to the previous process to correct the parallelism error would compromise the original surface shape accuracy. However, using a large-aperture ion beam grid for quantitative fine-tuning of the parallelism error can complete the correction while maintaining the original surface shape accuracy. Figure 3 The surface shape inspection report for the large-aperture optical window after ion beam processing shows that the parallelism (Parallel_Theta) is 0.46931 seconds, which meets the parallelism requirement of less than 0.5 seconds. Furthermore, this invention is specifically for the aforementioned 525mm×400mm×12mm. Figures 4-7 This report presents local surface shape inspection results for four Ф150mm ranges selected for large-aperture optical windows after ion beam processing. The results show that the large-aperture optical-grade window sample obtained by this invention has a transmission wavefront PV ≤ 1.0λ, and within any Ф150mm range, the transmission wavefront PV ≤ λ / 10 and RMS ≤ λ / 50 (λ = 632.8nm), exhibiting high surface shape accuracy and high parallelism accuracy. Furthermore, the entire processing cycle is approximately 45 days, demonstrating high processing efficiency for large-aperture optical windows and making it suitable for widespread application.

[0100] This invention is not limited to the embodiments described above. Those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A method for refining the parallelism of a large-aperture optical window using an ion beam, characterized in that, Includes the following steps: 1) The large-diameter optical window sample was ground and polished using a copper disk until the edge thickness difference was <0.01mm; 2) Use a polyurethane disc to grind and polish until the surface shape accuracy PV≤15λ, λ=632.8nm, and parallelism <5″; 3) Use a polyurethane disc for precision polishing until the PV within the large-diameter optical window is ≤2λ, RMS is ≤1.0λ, and parallelism is ≤1.5″. 4) The transmitted wavefront shape data of the large-aperture optical window sample with tilt was obtained by measuring with a large-aperture laser interferometer; 5) After cleaning and protecting the back of the large-aperture optical window sample, place the large-aperture optical window sample in the tooling sink and fix it around the perimeter with pressure strips. After clamping, hang it in the second chamber of the ion beam polishing machine and flip the part with the flipping mechanism so that its working surface is facing down. Finally, drag it to the processing area and clamp it with the drag bar motor. 6) The dwell time of the fine-tuning parallelism is obtained by simulation calculation using the inclined transmitted wavefront shape data of the large-aperture optical window sample, and NC machining code is generated. 7) Vacuum the ion beam polisher, ignite the ion source, stabilize it, import the NC machining code to adjust the process parameters, and use a large-diameter ion beam grid for parallel difference fine finishing. 8) After processing, take out the large-aperture optical window sample and wait for its temperature to stabilize. Then, use a large-aperture laser interferometer to measure its tilted transmission wavefront shape to confirm that it meets the design requirements and obtain a large-aperture optical window sample with high surface accuracy and high parallelism accuracy. The diagonal length of the large-aperture optical window sample is over 500 mm; The large-aperture ion beam grid mentioned in step 7) has an FWHM of 32mm, and the parallel difference finishing process parameters used include: Gas 12~15sccm, RF 200~250W, Beam 1300~1500eV, Accelerator 220~250eV, Neutralizer 120~150mA.

2. The processing method according to claim 1, characterized in that, The grinding and polishing steps include: A. A single-axis copper disc machine is used, and the process parameters are controlled as follows: copper disc spindle speed 25~30 rpm, swing shaft speed 30~35 rpm, and swing shaft iron pen tip pressure 0.3~0.35 MPa; B. Grind using W28 boron carbide abrasive until the edge thickness difference of the large-diameter optical window is <0.03mm; C. Grind using W14 boron carbide abrasive until the edge thickness difference of the large-diameter optical window is <0.01mm.

3. The processing method according to claim 1, characterized in that, The polishing steps include: using a polyurethane disc single-axis machine, and controlling the process parameters including: polyurethane disc spindle speed 95~100rpm, pendulum speed 90~100rpm, pendulum tip pressure 0.3~0.4MPa; polishing with 5μm diamond solution; using an optical template to check the surface shape change of the large-aperture optical window, and adjusting the distance between the large-aperture optical window and the center of the polyurethane disc according to the surface shape detected by the template; measuring the surface shape and parallelism of the large-aperture optical window until the surface shape accuracy PV of the large-aperture optical window sample is ≤15λ, λ=632.8nm, and the parallelism is <5″.

4. The processing method according to claim 1, characterized in that, The precision polishing step includes: A. A polyurethane disc ring polishing machine is used, and the process parameters are controlled as follows: the main polyurethane disc speed is 3~4 rpm, the correction disc speed is 2.5~3 rpm, and the workpiece ring speed is 3.5~4 rpm; B. Polish with 3μm diamond solution until N≤3, ΔN≤0.5, and parallelism difference≤3″; C. Polish with 1μm alumina liquid until the light transmission aperture of the large-diameter optical window has PV≤2λ, RMS≤1.0λ, and parallelism ≤1.5″.

5. The processing method according to claim 1, characterized in that, Step 4) is achieved in the following way: A. Adjust the background fringes of the large-aperture laser interferometer to zero fringes. The mathematical expression for calculating the parallelism difference is: In the formula, n ~ refractive index of the large-aperture optical window; m ~ number of interference fringes within the length b; b ~ diameter of the large-aperture optical window being measured; λ ~ wavelength of the light source used in the laser interferometer; the unit of the obtained parallelism difference is radians; B. Change the refractive index n of the large-aperture laser interferometer to the refractive index corresponding to the material of the large-aperture optical window; C. Fix the large-aperture optical window between the transmission mirror and the reflection mirror of the large-aperture laser interferometer; D. After masking the large-aperture optical window within its aperture range on the large-aperture laser interferometer testing software, click to measure and obtain the DAT surface shape data of the large-aperture optical window with tilt.

6. The processing method according to claim 1, characterized in that, Step 6) is achieved as follows: A. Import the DAT surface shape data file of the large-aperture optical window with tilted transmission wavefront obtained by the large-aperture laser interferometer into Metro Pro software, perform flipping and aperture calibration processing, and save it as an XYZ surface shape data file. B. Using the XYZ surface data file as the initial surface shape, the dwell time and NC machining code for the fine parallelism of the ion beam polishing machine are obtained through simulation using MATLAB software.

7. The processing method according to claim 1, characterized in that, Step 7) is achieved as follows: A. Once the vacuum level in the main chamber is less than 20 Pa, replace it with a molecular pump for precise vacuum evacuation; B. After the vacuum level of the main chamber is less than 20MPa, turn on the RF switch of the ion source to perform power matching. After successful matching, stabilize for 5~10 minutes. C. Turn on and set the argon flow rate to between 12 and 15 sccm, and start the arc ignition of the ion source. Stabilize for 5 to 10 minutes after ignition. D. Turn on the beam voltage, acceleration voltage, and neutralizer. After the ion source has stabilized for more than 30 minutes, import the NC machining code, adjust the process parameters, and then perform parallel difference fine-tuning.

8. The processing method according to claim 1, characterized in that, In step 8), the parallelism of the obtained large-aperture optical window sample is ≤0.5″.

Images