Experimental characterization of a micro-hole drilling process with short micro-second pulses by a CW single-mode fiber laser

• High aspect ratio microholes can be drilled with short microsecond pulses by modulating a CW single-mode fiber laser.
• These modulated laser pulses contain an initial spike whose power is five times of its CW power.
• A hole of 167 μ deep was produced in a stainless substrate with one single pulse at 1 μs.
• With a single pulse from 1 μs to 5 μs, aspect ratios of the holes range from 6.7 to 8.8.
• The main hole drilling mechanisms are found to be adiabatic evaporation and ejection of fine droplets based on experiments.

Laser ablation with pulse durations in a few microseconds is a viable solution for micro-hole drilling applications which require large material removal rate (MRR) with moderate hole quality. However, the body of work regarding short microsecond laser drilling/ablation is small. The objective of this paper is to experimentally characterize this short micro-second laser micro-hole drilling technique using a 300 W, CW, single-mode fiber laser. This CW fiber laser is controlled to produce modulated pulses from 1 μs to 8 μs and these modulated laser pulses have a unique profile which contains an initial spike with a peak power of 1500 W for 1 μs, followed by the steady state power of 300 W. Because of its excellent beam quality, the laser beam produced by this fiber laser can be focused to a small spot size of 10 μm to achieve very high power density up to 1.9 GW/cm2. With one single laser pulse at approximately 1 μs, a blind hole of 167 μm in depth and 23 μm in opening diameter can be created in a stainless substrate. The experimental characterization of this micro-hole drilling process includes laser control, laser beam characterization, hole formation, photodiode measurements of the vapor intensity, high-speed photography of vapor/plasma formation, and spectroscopic measurements of plasma. The results show that, due to very high irradiance of the fiber laser beam, the absorbed energy not only is sufficient to melt and vaporize the material, but also is able to dissociate vapor into intense plasma at temperatures over 16,000 K. The hole drilling mechanism by this short microsecond laser ablation is due to a combination of adiabatic evaporation and ejection of fine droplets.