Pulsed laser-induced material ablation and its clinical applications
Removing portions of a material, using a laser is referred to as laserinduced material ablation. The general goal of laser ablation is the efficient removal of material with minimum damage to the surrounding region. The appropriate selection of laser parameters, which determine the ablation mechanism, is essential to achieving a successful outcome. The research described in this dissertation was designed to evaluate the ablation mechanisms associated with pulsed lasers operating with different pulse durations and their medical applications. The role of a transparent liquid layer during a laser ablation process was studied. In comparison with a dry ablation process, the liquid-assisted ablation process resulted in augmented ablation efficiency and reduced ablation threshold. The results indicate that increased photon energy conversion to mechanical energy is responsible for the enhanced material ablation. Transparent targets were exposed to the one picosecond mid-IR pulse in order to investigate the origin of laser-induced surface damage. The results indicate that the surface damage was initiated by the laser-induced plasma created through the optical breakdown process. The retropulsive momentum of calculus during the Ho:YAG laser lithotripsy was measured using a high speed camera. The correlation between laser-induced crater shape, the trajectory of debris, and the retropulsive momentum is discussed. Due to the strong interaction between the laser pulse and calculus, the endoscopic delivery fiber may be subjected to damage resulting in diminished fragmentation efficiency. Deterioration of the delivery fiber during lithotripsy was quantified in terms of transmission loss and change in exit beam profile. To test the feasibility of Er:YAG laser lithotripsy, the fragmentation efficiency of Ho:YAG and Er:YAG lasers was compared. The results suggest that although the Er:YAG laser produced more precise drilling it did not create more fragmentation of calculus than the Ho:YAG laser for multiple pulse processes.