Heat accumulation and surface roughness evolution in CO2 nanosecond laser ablation of quartz glass

Abstract

Quartz glass is frequently used for optical components, but traditional machining methods, particularly for aspherical optics, are often associated with long process times and high costs. One possible approach to achieve higher efficiencies and higher speeds in contactless processing is laser ablation using CO2 laser radiation. This study investigates the evolution of surface topography during laser ablation of quartz glass using nanosecond pulses from a Q-switched CO2 laser beam source. The special focus of a broad empirical study lay on ablation depth, ablation rate, ablation efficiency and resulting surface roughness for an ablation process using 300 ns laser pulses at a maximum laser power of 115 W and pulse frequencies up to 150 kHz. Ablation efficiency shows a characteristic logarithmic dependency on laser fluence. It was revealed that local heat accumulation significantly affects the optical penetration depth, which reduces the threshold fluence for multi-pulse laser ablation down to approx. 0.37 J/cm2. Ablation rates of up to 1.48 mm3 min−1 W−1 were achieved, which exceeds results achieved for laser ablation using cw laser radiation or ultra-short pulses. An ablation rate of up to 170 mm3/min is a particularly noteworthy result. Therein, a maximum ablation efficiency of approx. 64% shows that the largest part of the incident laser energy was used for material ablation. Finally, based on a simplified model, numerical calculations and experimental results, pulse stability was identified to have a decisive impact on the evolution of the resulting surface roughness. The resulting surface roughness Sa is typically increased in pulsed laser processing and depends linearly on standard deviation of ablation depth and the square root of the number of ablation layers.