Mechanisms and strategies for residual stress inhibition in laser-polished fused silica

Laser polishing technology can significantly improve the laser-induced damage threshold of optics. However, large residual stress induced by surface densification and material modification seriously affects surface accuracy and service life. To avoid these negative effects, this work proposes an in-situ and efficient inhibition strategies of residual stress. A 3D multi-physics coupling model considering temperature evolution, plastic deformation, stress-strain formation and structural relaxation is innovatively established. The formation process of residual stress including stress initiation, stress increases as the structure transitions into non-equilibrium state, and volume shrinkage after solidification is analyzed. The 3D distribution of residual stress induced by laser polishing is first explored. The maximum stress position and stress-induced optical path difference are obtained through crack experiments and birefringence measurement system, thereby verifying the accuracy of the model established. The results show that large residual tensile stress around the polishing boundary, where the fictive temperature (T_f) gradient is maximum, is the reason for crack formation after polishing. Then, the effect of laser process parameters on residual stress distribution is investigated systematically. Using slow speed and high substrate temperature can reduce the cooling rate of the material, then achieve low residual stress. Additionally, the inhibition mechanism of residual stress through laser in-situ annealing is innovatively revealed. Three annealing stress states following laser annealing are proposed based on the T_f distribution inside the optic. The residual stress can be effectively inhibited by reducing T_f inside the heat-affected zones (HAZ) with no depth increment of HAZ, which is defined as full annealing state. After that, the critical factors required to achieve optimal annealing results are identified. By measuring the molecular vibration states through Raman spectrometer, the residual stress and T_f after parameter optimization and laser in-situ annealing are characterized. The increment of T_f is reduced by 43%, residual stress is reduced by 46 %, and critical crack size for material fracture induced by stress is increased by 245 %, which means that laser damage will be very difficult to induce cracks and destroy the optics. Finally, the measured result of surface profile shows that the inhibition strategies customized in this work also helps mitigate the surface densification induced by laser polishing. This work reveals the mechanism of residual stress formation involved in laser polishing and provides effective and efficient inhibition strategies of residual stress, laying a theoretical foundation for the high-performance manufacturing of optics.