Investigation on laser beam figuring of fused silica using microsecond pulsed CO2 laser radiation

Highlights: • High precision ablation with ablation depths of ~0.1 nm per laser pulse was achieved. • Characteristic regimes and related mechanisms in laser ablation were identified. • Increase in fictive temperature results in densification of fused silica. • Feasibility of laser beam figuring of fused silica was successfully demonstrated. • Effective reduction of surface waviness was achieved by iterative laser beam figuring. A laser-based process chain is a possible solution to increase precision and reduce production time in optics manufacturing of components with complex shaped surfaces. A new high-precision laser ablation process was investigated using CO₂ laser radiation and microsecond laser pulses. An average ablation depth of approx. 0.1 nm per laser pulse indicates that material ablation on an atomic/molecular scale was achieved. Four characteristic laser processing regimes and their related dominant physical process mechanisms were identified. The effect of crucial process parameters such as pulse energy, pulse duration and fluence on ablation depth and spatial resolution was empirically investigated. Ablation depths smaller than 5 nm per layer and a spatial resolution of less than 50 μm were achieved in areal laser ablation. The potential use of this high-precision laser ablation process was studied for an iterative process cycle (laser beam figuring - LBF) consisting of contactless surface measurement and subsequent laser ablation. The feasibility of LBF for form correction of fused silica components was successfully demonstrated, while a reduction in average surface roughness from Sa = 10.3 nm to 3.5 nm was achieved. High stability in pulse energy is the key requirement for high precision in ablation depth.