Hybrid Bubble Removal for High-Efficiency Underwater Pulse Laser Machining: Effects of Water Flow and Vibration

Underwater laser micromachining (ULMM) offers a promising alternative to conventional in‑air laser processing by markedly reducing thermal damage. Its industrial adoption, however, has been hindered by the adverse effects of laser‑induced cavitation bubbles and ejected debris, which cause severe optical shielding, process instability, and degraded machining quality. Here, we introduce and experimentally validate a hybrid bubble mitigation strategy designed to overcome these limitations by combining macroscopic advection through forced fluid flow with microscopic agitation by high‑frequency vibration. Using a 1064 nm pulsed fiber laser to machine single‑crystal silicon, we evaluate the performance of this approach. Experiments demonstrate a significant enhancement and clear synergy in the hybrid configuration, achieving meaningful improvements in groove depth and material removal efficiency compared with the baseline air machining condition. While negligible material removal was observed in still water and vibration‑only conditions, the forced flow and hybrid conditions yielded substantially greater removal. The hybrid method simultaneously broadened the kerf, enhancing material removal, while reducing the heat-affected zone (HAZ), thereby demonstrating improved surface quality. We attribute this performance to a macro-micro multiscale mechanism in which bulk flow establishes a stable, optically transparent processing environment and high‑frequency acoustic agitation executes targeted, high‑energy cleaning at the laser‑material interface. The proposed hybrid technique therefore enables high‑efficiency, low‑damage micromachining of silicon and may be useful for materials other than silicon.