The development of thermal damage during pulsed Holmium:YAG ((lambda) equals 2.12 micrometer, (tau) _p equals 250 microseconds) irradiation of albumen was analyzed experimentally and numerically with the intent of validating the Arrhenius integral and investigating the influence of beam profile and dynamic changes in absorption. Fast flash videography was performed to document the transient development of coagulation during and after the laser pulse. A two-dimensional dynamic optical-thermal model was developed. The optical component involved a discretized Beer's law method in which absorption is a function of temperature. The thermal component was comprised of (1) a cylindrical coordinate (r,z) finite difference formulation of the heat conduction equation in which the output from the optical model was used as the source term and (2) a routine for calculating thermal damage using the Arrhenius relation. At the end of each time step, the temperature distribution was used to recalculate the absorption coefficient array, which in turn is used in the optical component. The model showed good agreement with experimental data and the literature. The Arrhenius integral was shown to be valid for the sub-vaporization pulsed laser regime. Experimental and numerical results indicated that spatial beam profile has a significant impact on thermal damage. Simulations predicted that at higher radiant exposures dynamic changes in absorption become more significant, accounting for a 10% increase in coagulation depth.

Date Published: 13 May 1998
PDF: 11 pages
Proc. SPIE 3254, Laser-Tissue Interaction IX, (13 May 1998);

Published in SPIE Proceedings Vol. 3254:
Laser-Tissue Interaction IX
Steven L. Jacques; Jeff Lotz, Editor(s)