CHANNEL PROPAGATION IN WATER AND GELATIN BY A FREE-RUNNING ERBIUM LASER

Precise cutting of biological tissue is possible with the Er:YAG laser because of the strong absorption of radiation exhibited by water containing media at a wavelength of 2.94 mum. To achieve control of the depth of drilled channels a thorough knowledge of the channel propagation mechanism is required. The channel propagation process of pulsed erbium laser radiation in liquid water, and in gelatin with a high water content, as substitutes for biological tissue is investigated experimentally and modeled theoretically. We explain the propagation process with a-hydrodynamic model, which describes the channel propagation process in terms of energy, mass, and momentum balance equations, which influence the evaporation pressure at the phase boundary. As the key feature, the theory takes into account the deformability of cold material below the zone of absorption. We show that by modeling this hydrodynamic effect with the Bernoulli equation we can explain the channel propagation velocity in water and gelatin as a function of laser intensity over three orders of magnitude with appreciable accuracy. The comparison with the experimental data suggests that the channel propagation velocity for intensities below 0.1 MW/cm2, and the threshold intensity of 12 kW/cm2 for channel propagation, are dependent on the surface tension and the liquid viscosity. At intensities above 0.1 MW/cm2, we can even predict a small difference between the propagation velocities found in these materials by considering the effect of the different elastic properties on the pressure in the propagating channel.