Visualization of heat accumulation in femtosecond laser machining through quantitative time-resolved imaging

Ultrashort pulse laser processing has attracted considerable interest for its ability to process hard brittle materials like sapphire and glass due to its high precision. To improve processing efficiency, high repetition rate ultrafast laser trains have been employed, but this often leads to damage and large heat-affected zones due to the excessive heat accumulation. In this study, we aim to visualize heat accumulation and its dependency on repetition rate based on quantitative measurement of the ultrafast electron excitation process. Ultrashort laser pulses with fluence above the ablation threshold were applied to a non-alkaline glass sample. We employ pump-probe imaging to visualize ultrafast phenomena on picosecond to nanosecond scales, while a high-speed camera synchronized with the laser’s repetition rate monitors slower internal changes within the material on millisecond timescales, directly revealing subsequent thermal diffusion and heat accumulation processes. Moreover, Mach-Zehnder interferometry is used to quantitatively evaluate heat accumulation through analyzing the phase change of probe light. Through these integrated imaging techniques, we successfully measured the phase distribution and consequently derived the temperature increase. The results indicate that pronounced heat accumulation occurs at a higher repetition rate, which may impact material modification and processing quality. These findings contribute to a deeper understanding of thermal dynamics in femtosecond laser machining, potentially enhancing precision and efficiency in industrial applications.