熔石英损伤修复坑下游光场调制的数值模拟与实验研究

Study of downstream light intensity modulation induced by mitigated damage pits of fused silica using numerical simulation and experimental measurements

系统研究了熔石英激光损伤修复后的形貌特征,根据测量数据建立了典型的损伤修复坑三维模型,利用标量衍射理论并结合快速傅里叶变换算法研究了修复坑在351 nm激光辐照下游光场调制的分布规律.研究表明,修复坑引起的光场调制会使得下游不同距离位置处出现环形光场增强区和轴上位置光场增强点;环形光场增强区位置距离修复元件较近,其环形调制极大值主要受修复坑深度的影响,且随修复坑深度的增大而逐渐增加;轴上位置光场增强点位置距离修复元件较远,其轴上调制极大值主要受修复坑边缘凸起高度的影响,且随凸起高度的增大而快速增加;环形调制极大值或轴上调制极大值增大的同时,其分布位置与修复元件之间的距离均会逐渐减小.实验验证表明,利用三维修复坑模型得到的下游光场调制数值模拟结果与实验测量结果具有较好的一致性.本研究结果对控制熔石英元件损伤修复形貌特征以抑制调制增强效应给出了具体的控制方向,对修复工艺的改进与完善提供了非常有意义的参考.

For high-power UV laser facilities, one of the key problems limiting the maximum light influence and sa^e routine operation is that the UV laser induces damage to fused silica optics. The most effective mitigation protocol of the damaged optics is the CO_2 laser processing that leads to make locally melt or evaporate the damage. While the mitigated damage sites possess particular morphology, which may modulate the passing laser beam and induce the downstream intensification that will ruin the neighbor optics. In this work, the morphology features of the mitigated damage pits of fused silica optics are systematically investigated. According to the measured morphology features, a 3D grid model of mitigated pit is built, and the downstream light intensity distribution of the mitigated pit model under incident 351 nm laser is studied by scalar diffraction theory and fast fourier transform （FFT） methods. Results indicate that there are two kinds of downstream intensification： off-axis and on-axis intensifications. In the former intensification, the maximum intensity is located near the output surface of the optics and comes mainly from the depth of the mitigated pit; it increases with the depth. In the alter intensification, the maximum intensity is located far from the output surface of the optics and is mainly dependent on the height of the rim structure at the fringe of the mitigated damage pit; so it increase with increasing height. In addition, it is found that the location of the maximum off-axis or on-axis intensity can approach the output surface of the optics with increasing maximum intensity. For comparison, experimental measurements of downstream intensification induced by the mitigated pits are carried out, and the experimental results are almost consistent with the numerical simulation, implying the validity of the numerical simulation of the mitigated pit model. Results of this research indicate that the downstream intensification of mitigated pits can be suppressed by controlling the morphology features of mitigated pits. this is significant for the development and improvement of the mitigated techniques of damage optics.