Fused quartz laser polishing method coupled with temperature feedback control system

Abstract

A fused quartz laser polishing method coupled with a temperature feedback control system comprises the following steps that firstly, a thermal imaging instrument is used for conducting real-time temperature measurement on a laser polishing area of fused quartz; then, a real-time temperature signal measured by the thermal imaging instrument is transmitted to the PID controller; and then the PID controller is used for dynamically regulating and controlling the output power of the laser according to the input real-time temperature signal. Through the closed-loop control, the temperature of the laser processing area of the fused quartz is slightly lower than the evaporation temperature of the fused quartz stably, and the dynamic precise adjustment of the laser power is realized. According to the control strategy, uncontrollable evaporation and condensation caused by local overheating or alternate cooling and heating of the fused quartz are prevented, and more importantly, the phenomenon of uneven surface tension change caused by severe temperature fluctuation is eliminated to a great extent, so that the formation of scanning ripples on the surface of the fused quartz is remarkably reduced, and the polishing quality and the process stability are improved.

Description

Technical Field

[0001] This invention relates to the field of laser processing technology for optical components, specifically to a laser polishing process for fused silica components. More precisely, this invention provides a method for suppressing surface scanning ripples during laser polishing using a real-time temperature feedback control system, applicable to the manufacture of high-precision optical components. Background Technology

[0002] Fused silica, due to its high light transmittance, low coefficient of thermal expansion, and excellent laser damage threshold, has become a key material for core components such as windows, lenses, and diffraction gratings in high-power laser devices. Currently, common processing and polishing methods for fused silica optical glass include precision grinding, magnetorheological polishing, and ion beam polishing. While these methods partially meet the processing requirements of components, they still have significant limitations. Precision grinding inevitably causes surface and subsurface damage to the component; the injected polishing powder / liquid easily introduces impurities and defects; and path planning limitations often lead to residual mid-frequency waviness. Magnetorheological polishing is mainly suitable for large-diameter components, but its effectiveness in processing complex, small-diameter components is poor, and it introduces magnetorheological liquid contamination. Ion beam polishing can process complex small-diameter components, but it has low removal efficiency, high cost, and suffers from lattice distortion due to ion implantation. Therefore, there is an urgent need to develop non-contact, pollution-free processing technologies.

[0003] Laser polishing, as a non-contact and pollution-free processing method, can achieve ultra-smooth surfaces through the molten flow of materials. However, this technology suffers from scanning ripple problems, mainly due to drastic temperature changes in the processing area caused by factors such as laser power fluctuations and uneven cooling during the process. When the temperature exceeds the evaporation point, the material undergoes nonlinear evaporation; when it falls below the evaporation point, it condenses and flows back, causing differences in surface tension and ultimately forming periodic scanning ripples with amplitudes typically reaching several micrometers. This significantly degrades surface quality and severely reduces optical performance. Existing contact temperature measurement methods, such as thermocouples, have high hysteresis and cannot meet the requirements for millisecond-level temperature control. Furthermore, insufficient temperature control accuracy leads to alternating evaporation and condensation. While non-contact temperature measurement tools such as thermal imagers can achieve real-time temperature control feedback, existing systems that couple thermal imagers for processing are mostly open-loop controlled. Because they lack closed-loop feedback control of the laser, they cannot respond to thermal disturbances, thus failing to guarantee high-precision fused silica processing.

[0004] Patent document CN109693039A discloses a method for laser polishing of silicon wafer surfaces, which improves surface roughness quality through surface laser polishing. The drawback of this process is that it does not employ a feedback system for optimizing laser polishing process parameters. While this can reduce roughness, it can lead to the generation of scanning ripples.

[0005] Patent document CN109590603A discloses a polishing method for fused silica optical glass, achieving a surface roughness of 0.18 nm through processing, polishing, and annealing. The drawback of this method is that it employs an open-loop control system, relying solely on a thermal imager for temperature measurement without subsequent power feedback control; therefore, the scanning ripple problem remains unresolved.

[0006] Patent document CN114149180A discloses a multi-physics field laser polishing method and corresponding equipment for brittle materials. This technology employs ultrasonic vibration-assisted laser polishing to suppress the generation of porosity and cracks in the molten pool. However, the limitation of this method lies in its excessive emphasis on suppressing porosity and cracks, while paying insufficient attention to processing accuracy. Furthermore, although ultrasonic assistance can reduce porosity and cracks to some extent, it cannot effectively eliminate the severe distortion of the workpiece surface caused by thermal stress generated during laser polishing.

[0007] To simultaneously meet the requirements of high surface accuracy and ultra-low roughness for fused silica components, it is urgent to develop a closed-loop laser polishing system with integrated real-time temperature feedback. By dynamically adjusting the laser power and path, the surface tension imbalance caused by temperature fluctuations can be suppressed, thereby fundamentally eliminating scanning ripples. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention proposes a fused silica laser polishing method based on closed-loop temperature feedback control. By establishing a closed-loop temperature control system, the temperature of the processing area is precisely stabilized within a set safe range slightly below the evaporation temperature of the fused silica, thereby eliminating surface ripples caused by temperature fluctuations.

[0009] The technical solution of the present invention is as follows: A laser polishing method for fused silica based on closed-loop temperature feedback control is characterized by the following steps: S1. During the laser polishing process, a thermal imaging instrument is used to continuously scan and monitor the laser processing area of ​​the fused silica workpiece to obtain real-time temperature field data; S2. Transmit the real-time temperature field data obtained in step S1 to the PID controller to form a temperature feedback signal path; S3. The PID controller compares the received real-time temperature signal with the preset sub-evaporation threshold temperature in real time, and dynamically generates a laser power adjustment command based on the proportional-integral-derivative control algorithm; the sub-evaporation threshold temperature is set to a safe temperature range of 10-50℃ lower than the evaporation temperature of fused silica; when the real-time temperature is higher than the set threshold temperature, the PID controller outputs a command to reduce the laser power; when the real-time temperature is higher than the threshold, a power reduction command is generated, and when it is lower than the threshold, a power increase command is generated. S4. Temperature steady-state maintenance and material phase control: The laser executes the power adjustment command generated in step S3 with a power modulation resolution of not less than 0.1% and a response time of less than 10ms; through the closed-loop control formed by S1-S4, the temperature fluctuation of the processing area is stably controlled within ±10℃, so that the molten silica material is always kept in a sub-evaporation melting state, avoiding the alternating phase transition of evaporation and condensation.

[0010] Furthermore, the sub-evaporation threshold temperature for: in, T evap Δ is the evaporation temperature of fused silica. T For safe temperatures, the range is 10 ℃ ≤ Δ T ≤ 50℃.

[0011] Furthermore, the proportional coefficient of the PID controller K p Integral Time T i Differential time T d The control coefficients are pre-calibrated based on the thermal response characteristics of fused silica, with the calibration target being that the temperature overshoot does not exceed 5% and the steady-state settling time does not exceed 500 milliseconds.

[0012] Furthermore, the thermal imaging instrument has a sampling frequency of ≥ 50 Hz, a spatial resolution of ≤ 20 μm, and a temperature resolution of ≤ 1℃, ensuring real-time temperature feedback and accurate local thermal field coverage.

[0013] Furthermore, the steady-state fluctuation range of the temperature control is ≤ ±10℃ to avoid scanning ripples caused by differences in material removal rates.

[0014] Furthermore, the laser is a continuous or pulsed laser with an output wavelength range of 9~11 μm, a power control resolution of not less than 0.1%, and a response time of less than 10 ms.

[0015] Furthermore, the PID controller has an adaptive parameter tuning function, which can adjust the control parameters in real time according to the heat accumulation effect during the processing.

[0016] Furthermore, it also includes: S5. Based on the real-time temperature field distribution data obtained in step S1, and combined with the thermal diffusion characteristic model of fused silica, dynamically optimize the laser scanning path and scanning speed; the optimization objectives include: maximizing the spatial uniformity of the temperature field distribution and minimizing the thermal stress caused by the temperature gradient. S6. The power regulation in step S3 and the path optimization in step S5 are coordinated. When a local temperature anomaly is detected, the laser power and scanning parameters are adjusted simultaneously to achieve dual stability of the temperature field and the material removal rate.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) By deeply coupling non-contact thermal imaging real-time temperature measurement with PID closed-loop control algorithm and applying it to the fused silica laser polishing process, millisecond-level, high-precision dynamic stable control of the temperature of the micron-level processing area is achieved.

[0018] 2) By stably controlling the processing temperature within the "sub-evaporation" safety range—slightly below the evaporation temperature—the alternation of evaporation and condensation is physically eliminated, which is fundamental to eliminating scanning ripples. In addition to temperature control, temperature distribution data is fed back for dynamic optimization of the laser scanning path, achieving coordinated control of the temperature and motion fields and improving overall process uniformity.

[0019] 3) By adopting a PID controller with adaptive tuning function, the system can cope with nonlinear thermal disturbances during the processing, thereby enhancing the robustness and repeatability of the process. Attached Figure Description

[0020] Figure 1 Schematic diagram of the system of the present invention Figure 2 PID control logic block diagram Figure 3 Results of white light interferometer: (a) Open-loop control; (b) This invention.

[0021] Figure 4 Atomic force microscopy results: (a) Open-loop control; (b) This invention. Detailed Implementation

[0022] The following detailed description of the fused silica laser polishing method for coupled temperature feedback control system, in conjunction with the accompanying drawings and embodiments, should not be construed as limiting the scope of protection of this invention.

[0023] This embodiment is based on Figure 1The system architecture shown is as follows. A 10.6 μm continuous-wave CO2 laser (Diamond GEM-100L, wavelength 10.6 μm, maximum output power 200 W) is used as the heat source. Temperature monitoring is performed by a mid-wave infrared thermal imager (YFD-MW320) with a sampling frequency of 50 Hz, spatial resolution of 15 μm, and temperature resolution <1°C. Real-time radiation calibration is performed using a Mikron M340 blackbody source in the range of 1500-2100°C to ensure temperature measurement accuracy. The PID controller uses FPGA to implement high-speed closed-loop control. The fused silica sample (30 mm × 30 mm × 5 mm) is placed on a three-dimensional displacement platform for polishing.

[0024] Set threshold temperature (T) threshold The temperature was set at 1950°C (based on the evaporation temperature of fused silica, 2000°C, minus a safety margin of 50°C). The step response method was used to tune the PID parameters to obtain the optimal dynamic response. The final determined parameters were: proportional coefficient K... p =5.0, integration time T i =0.1 s, differential time T d =0.1 s. Under this parameter combination, the system temperature overshoot is suppressed to within 5%, and the steady-state settling time is less than 400 milliseconds. The entire PID control logic block diagram is as follows: Figure 2 As shown.

[0025] After the polishing process is initiated, the system enters a closed-loop control state. In open-loop control mode, the temperature in the processing area fluctuates drastically due to factors such as heat accumulation and changes in the scanning path, with records showing a fluctuation range as high as 1750°C to 2150°C (fluctuation ±200°C). This indicates that the fused silica continuously undergoes alternating processes of evaporation and condensation, laying the groundwork for the formation of scanning ripples. Conversely, after applying this invention, by dynamically adjusting the laser power through real-time temperature feedback, the processing temperature is precisely stabilized within the range of 1940°C to 1960°C (fluctuation ±10°C), successfully maintaining the process in an ideal sub-evaporation state and fundamentally avoiding the nonlinear migration of fused silica caused by phase transitions.

[0026] The stability of temperature control is ultimately reflected in the quality of the processed surface. The morphology of the polished surface was characterized within a 1 mm × 1 mm area using a white light interferometer (4D Technology, NanoCam Sq). The surface morphology results are as follows: Figure 3 As shown. Figure 3 (a) The open-loop controlled polished surface exhibits significant, periodic scanning ripples, with a measured ripple PV value of approximately 1912.9 nm. In stark contrast, Figure 3(b) The polished surface obtained by the method of the present invention is smooth and uniform, with a ripple PV value of 138.7 nm, and the periodic ripples are significantly eliminated. The roughness after polishing is characterized by atomic force microscopy (Veeco, Dimension 3100), and the roughness results are as follows. Figure 4 As shown. Figure 4 (a) The roughness RMS value is 0.225 nm under open-loop control. Figure 4 (b) demonstrates that the roughness RMS value is controlled at 0.236 nm under closed-loop control, achieving the processing effect of an ultra-smooth surface.

[0027] In summary, this embodiment combines Figure 3 and Figure 4 The data strongly demonstrates that by introducing closed-loop temperature feedback control, this invention achieves millisecond-level precise control of the thermal field during laser polishing. This not only suppresses temperature fluctuations within the range of ±10°C, but also eliminates the problem of uneven surface tension caused by drastic temperature changes from a mechanistic perspective. As a result, a high-quality polishing effect with no ripples and ultra-smooth surface is achieved on the fused silica element, significantly improving the reliability and repeatability of the process.

Claims

1. A method for laser polishing fused silica based on closed-loop temperature feedback control, characterized in that, Includes the following steps: S1. During the laser polishing process, a thermal imaging instrument is used to continuously scan and monitor the laser processing area of ​​the fused silica workpiece to obtain real-time temperature field data; S2. Transmit the real-time temperature field data obtained in step S1 to the PID controller to form a temperature feedback signal path; S3. The PID controller compares the received real-time temperature signal with the preset sub-evaporation threshold temperature in real time, and dynamically generates a laser power adjustment command based on the proportional-integral-derivative control algorithm; the sub-evaporation threshold temperature is set to a safe temperature range of 10-50℃ lower than the evaporation temperature of fused silica; when the real-time temperature is higher than the set threshold temperature, the PID controller outputs a command to reduce the laser power; when the real-time temperature is higher than the threshold, a power reduction command is generated, and when it is lower than the threshold, a power increase command is generated. S4. Temperature steady-state maintenance and material phase control: The laser executes the power adjustment command generated in step S3 with a power modulation resolution of not less than 0.1% and a response time of less than 10ms; through the closed-loop control formed by S1-S4, the temperature fluctuation of the processing area is stably controlled within ±10℃, so that the molten silica material is always kept in a sub-evaporation melting state, avoiding the alternating phase transition of evaporation and condensation.

2. The fused silica laser polishing method based on closed-loop temperature feedback control according to claim 1, characterized in that: The sub-evaporation threshold temperature for: in, T evap Δ is the evaporation temperature of fused silica. T For safe temperatures, the range is 10 ℃ ≤ Δ T ≤ 50℃.

3. The fused silica laser polishing method based on closed-loop temperature feedback control according to claim 1, characterized in that: The proportional coefficient of the PID controller K p Integral Time T i Differential time T d The control coefficients are pre-calibrated based on the thermal response characteristics of fused silica, with the calibration target being that the temperature overshoot does not exceed 5% and the steady-state settling time does not exceed 500 milliseconds.

4. The fused silica laser polishing method based on closed-loop temperature feedback control according to claim 1, characterized in that: The thermal imaging instrument has a sampling frequency of ≥ 50 Hz, a spatial resolution of ≤ 20 μm, and a temperature resolution of ≤ 1℃, ensuring real-time temperature feedback and accurate local thermal field coverage.

5. The fused silica laser polishing method based on closed-loop temperature feedback control according to claim 1, characterized in that: The steady-state fluctuation range of the temperature control is ≤ ±10℃ to avoid scanning ripples caused by differences in material removal rates.

6. The fused silica laser polishing method based on closed-loop temperature feedback control according to claim 1, characterized in that: The laser is a continuous or pulsed laser with an output wavelength range of 9~11 μm, a power control resolution of not less than 0.1%, and a response time of less than 10 ms.

7. The fused silica laser polishing method based on closed-loop temperature feedback control according to claim 1, characterized in that, Its features are: The PID controller has an adaptive parameter tuning function, which can adjust the control parameters in real time according to the heat accumulation effect during the processing.

8. The fused silica laser polishing method based on closed-loop temperature feedback control according to claim 1, characterized in that... Also includes: S5. Based on the real-time temperature field distribution data obtained in step S1, and combined with the thermal diffusion characteristic model of fused silica, dynamically optimize the laser scanning path and scanning speed; the optimization objectives include: maximizing the spatial uniformity of the temperature field distribution and minimizing the thermal stress caused by the temperature gradient. S6. The power regulation in step S3 and the path optimization in step S5 are coordinated. When a local temperature anomaly is detected, the laser power and scanning parameters are adjusted simultaneously to achieve dual stability of the temperature field and the material removal rate.

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