Inert gas beam delivery for ultrafast laser micromachining at ambient pressure

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, bef ore reaching the workpiece. At the very high laser intensities associated w ith ultrafast lasers, the gas can ionize, resulting in a rapid increase in free electron (plasma) density, which decreases the gas refractive index, r esulting in plasma defocusing and self-phase modulation. Plasma-induced eff ects distort the temporal and spatial profile of the laser beam, which degr ade feature quality and repeatability for ultrafast laser micromachining. I n addition, plasma absorption reduces the energy available for materials pr ocessing, resulting in a decreased material removal rate. To avoid these ef fects, processing has traditionally been performed in a vacuum chamber, how ever this makes real-time processing on a large scale impractical. This art icle presents a beam delivery technique that uses inert gas as the beam pro pagation environment instead of air or a vacuum chamber. Plasma defocusing, self-phase modulation, and shielding effects are minimized due to the high er ionization potential of inert gas and thus less plasma forms along the b eam path. Experiments were performed by delivering Ti:Sapphire femtosecond laser pulses in four different environmental gases: air, nitrogen, neon, an d helium, to machine holes through a copper plate, with the best feature qu ality and machining efficiency obtained in helium and the worst in air. Thi s technique shows potential as an innovative method to maintain high beam q uality without the need for a vacuum chamber, which significantly improves processing throughput in practical ultrafast laser applications. (C) 2001 A merican Institute of Physics.