Inert gas beam delivery for ultrafast laser micromachining at ambient pressure

Abstract

Ultrafast laser micromachining is realized by focusing a femtosecond laser beam to a small spot, where very high optical intensity is achieved at the workpiece. Often, however, the beam must pass through a gas, e.g., air, before reaching the workpiece. At the very high laser intensities associated with ultrafast lasers, the gas can ionize, resulting in a rapid increase in free electron (plasma) density, which decreases the gas refractive index, resulting in plasma defocusing and self-phase modulation. Plasma-induced effects distort the temporal and spatial profile of the laser beam, which degrade feature quality and repeatability for ultrafast laser micromachining. In addition, plasma absorption reduces the energy available for materials processing, resulting in a decreased material removal rate. To avoid these effects, processing has traditionally been performed in a vacuum chamber, however this makes real-time processing on a large scale impractical. This article presents a beam delivery technique that uses inert gas as the beam propagation environment instead of air or a vacuum chamber. Plasma defocusing, self-phase modulation, and shielding effects are minimized due to the higher ionization potential of inert gas and thus less plasma forms along the beam path. Experiments were performed by delivering Ti:Sapphire femtosecond laser pulses in four different environmental gases: air, nitrogen, neon, and helium, to machine holes through a copper plate, with the best feature quality and machining efficiency obtained in helium and the worst in air. This technique shows potential as an innovative method to maintain high beam quality without the need for a vacuum chamber, which significantly improves processing throughput in practical ultrafast laser applications.