Above the ablation threshold the removal of tissue is characterized by a luminous plasma, followed by a plume of non-luminous debris. Both the plasma and the plume are capable of shielding the ablation site, attenuating the beam and decreasing the ablation rate significantly at high numbers of pulses (n) and high fluence. The ablation of several biological tissues by a XeCl excimer laser at 308 nm has been studied. The laser pulse length is 200 ns, around a factor of 10 longer than previously reported studies. In order to study the plume's effect on the ablation rate is has been captured using an Imacon 468 camera capable of 1x10⁸ frames per second. We have calculated the evolutionary speed and the extent of the plasma and ensuing debris with respect to pulse repetition rate (PRR), n and energy for a range of tissues. Probe beam experiments have also been carried out to confirm these results. With this data we can determine the amount of time that the tissue is shielded on the time scale of the incoming pulses and use the results to help explain the ablation rate measurements. A maximum velocity of 2.58x10⁴ ms/s was found for dentine and the tissue was found to be shielded for a maximum of 120 microsecond(s) by the luminous plasma and 10 ms by the non-luminous plume.

Date Published: 9 July 2001
PDF: 13 pages
Proc. SPIE 4257, Laser-Tissue Interaction XII: Photochemical, Photothermal, and Photomechanical, (9 July 2001); doi: 10.1117/12.434710

Published in SPIE Proceedings Vol. 4257:
Laser-Tissue Interaction XII: Photochemical, Photothermal, and Photomechanical
Donald Dean Duncan; Donald Dean Duncan; Steven L. Jacques; Peter C. Johnson; Peter C. Johnson, Editor(s)