Observation of laser-generated ultrasound for in-process control of laser processing

Ultrasound is generated when a femtosecond laser pulse is focused on a material surface. The ultrasound provides helpful information about laser-irradiation conditions, laser-matter interactions, and fabricated structures. The ultrasound was measured by a microphone and an interferometer. The interferometer implemented with grating beam splitters has high stability against external disturbance due to its inline arrangement. Using the microphone, a comparative study was conducted between the processing sound and the structure of the machined holes. In contrast, the interferometer was used to investigate the feasibility of practical implementation.