KDP晶体激光无损镜面加工机理及关键技术研究

Both cutting of KDP crystal blanks and ultra-precision polishing process of KDP crystal have been one of the key problems which plagued the completion ICF device in our country. The project of laser nondestructive mirror processing for KDP crystal is first proposed an innovative technology, which combines the " cold" effect of ultrafast laser processing with the "hot" effect of common laser processing to induce a micro stress line breakage atomic or molecular bonds so that both separation and super finishing of KDP crystal can be completed at the same time. The use of high peak power ultrafast laser pulses produces an nonlinear absorption effects with the occurrence of two-photon ionization or multiphoton ionization or impact ionization to prefabricate sub-micron laser induced phase transition line inside KDP crystal phase near surface.Then,a traditional main laser beam with higher transmittance, suitable energy and polarization direction is modified by a special optical system to obtain a suitable geometric dimension beam, which is adopted to scan the laser modified sub-micron line. Due to the absorption of the laser modified region is increased and the unmodified region remains relatively low, the difference in laser body absorption and thermal effects between the modified and unmodified region will produce a greater temperature gradient to form a specific internal stress and optical field distribution region.Since the modified region of atomic and molecular bond breaking or weakening, the thermal stress generated by the laser will break the bond along the atoms or molecules to form a nanometric crack. Further,a low power laser beam power at the front of the main laser beam alongthe laser modified line preheats the region to be processed in order to control and guide nano slit expansion trend. Meanwhile, the main laser beam produces a uniform heating along the thickness direction of slit side wall to separate and superfinish KDP crystal quickly due to heat expansion effect and atomic or molecular bond breakage. Some researches on the crack tip transmission, absorption characteristics,dynamic temperature and stress distribution model will be done to investigate the formation mechanism of micro tensile stress and stress rupture chemical bond theory as well as principles of micro-and nano crack, draw a complete micro-instant heat stress breaking chemical bond theory, master micro-and nano crack propagation direction control key technologies, research and develop corresponding laser beam shaping optical system to form a quasi-parallel laser line of focus characteristics. The researches of simulating laser nondestructive mirror finishing the best technical parameters and experiment optimization will be also carried out to achieve a new technology theory of effective laser nondestructive mirror processing to complete instant KDP crystal superfinishing nondestructive mirror separation processing.

KDP晶体坯件的切割和超精密抛光加工一直是困扰我国完成ICF装置的关键问题之一。本项目首次提出KDP晶体激光无损镜面加工，将超快激光的"冷"加工效应和常规激光的"热"加工效应有机相结合，诱导微区应力线断裂原子或分子键，实现一次完成晶体分离和超精加工的一种创新技术。利用超快激光在KDP晶体近表层诱导相变、预制亚微米改性线和常规激光体吸收加热效应，在改性区形成纳米裂纹。通过研究裂缝和裂缝尖端的光传输、吸收特性和动态温度、应力场分布模型，探讨微区拉应力形成机理和应力断裂化学键理论以及微纳米裂纹扩展原理，得出一套完整的微区瞬间热应力断裂化学键理论。研发相应的激光压束整形光学系统，在切缝尖端形成特定的光场和温度场分布，诱导微纳裂纹的扩展，掌握其扩展方向控制的关键技术。对激光无损加工KDP晶体过程进行数值模拟，计算最优加工技术参数，同时进行实验优化研究，获得一套完整的KDP晶体无损镜面加工理论技术。

由于激光惯性约束核聚变（ICF）成为未来最有前途的新能源技术之一，因而备受到了世界各国的广泛关注。大口径磷酸二氢钾光学单晶体(KDP)制成的倍频器和普克尔斯盒是 ICF 装置中其关键器件。我国已经成功地生长出了 ICF所需的 KDP 光学晶体坯件，但晶体坯件切割和超精密抛光加工的成功率和效率一直是阻碍我国“神光计划”项目的关键问题之一。由于西方发达国家对我国进行技术封锁和禁运，因而本项目提出了具有自主知识产权的KDP晶体激光无损镜面加工技术，将超快激光的"冷"加工效应和常规激光的"热"加工效应有机相结合，诱导微区应力线断裂原子或分子键，实现一次完成晶体分离和超精加工的一种创新技术。通过研究超快激光在KDP晶体近表层诱导相变、预制亚微米改性线；研究裂缝和裂缝尖端的光传输、吸收特性和动态温度、应力场分布模型；探讨微区拉应力形成机理和应力断裂化学键理论以及微纳米裂纹扩展原理，得出一套完整的微区瞬间热应力断裂化学键理论。通过研发相应的激光压束整形光学系统，掌握切缝尖端形成诱导微纳裂纹扩展方向控制的关键技术，并对激光无损加工KDP晶体过程进行数值模拟和实验优化研究，获得一套完整的KDP晶体无损镜面加工理论技术。利用该理论技术实现了成品KDP晶体（厚度 20mm）的抛光级无损快速分离应用。相应的分离速度可高达2mm/s，约为传统机械分离技术的300-400余倍，分离侧壁粗糙度可达4.7nm（p-v）和2.1nm（RMS），分离侧壁的平面度可达5.433μm，分离角精度可达0.06°，均远远高于传统机械分离方法。同时，利用该技术可从晶体母材上直接分离出厚度小于1.5mm的无损镜面机晶体薄片。该项目共获得中国发明专利5项、实用新型专利5项，发表论文11篇（其中顶级SCI期刊4篇、SCI论文4篇，中国核心期刊3篇），国际会议报告3篇，正在审稿SCI论文1篇，正在撰写SCI论文1篇，国际荣誉一项，已经超额完成本项目预期任务指标。由本团队在国内外首次提出的KDP晶体无损镜面加工机理及关键技术具备极强的实际应用价值，并具有着重要的指导意义。