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For the XeCl-excimer (308 nm, 115 ns) laser the contribution of photo-thermal and photo- mechanical processes was studied during the penetration of aortic tissue by bare fibers in dependence of fiber diameter, exerted force, pulse repetition rate, and fluence. Though, theoretically, Monte-Carlo simulation shows that the light penetration depth is diameter dependent for fibers up to 550 micrometers , experimentally, tissue penetration was not affected when the fiber diameter increased from 300 to 950 micrometers . Also the change in optical properties of tissue due to denaturation did not affect the penetration behavior significantly. Fiber tissue penetration increased when the exerted force increased, but it started only after a series of 5 - 20 initial pulses. The penetration per pulse became only slightly larger increasing the pulse repetition rate from 2 to 60 Hz while the tissue temperature rise near the fiber was up to 60 degrees. Increasing the temperature of the surrounding tissue itself prior to laser exposure only slightly affected tissue penetration in the case of both normal and denatured tissue. Delivery of laser energy in successive pulse trains, accelerated penetration after the first train. Close-up, high speed video recording showed the presence of rapidly expanding, short-life (50 - 150 microsecond(s) ) vapor bubbles, in the first instance on top of the tissue surface and later in the tissue itself while the fiber was penetrating the tissue. From our measurements and observations it is inferred that the mechanical effect of the bubble is especially important for the penetration of the fiber into the tissue. Theory suggests that the energy is deposited in a 100 - 200 micrometers layer in front of the fiber where the temperature is instantly increased to above boiling temperature inducing a rapid expanding vapor bubble. The mechanical force of the bubble breaks down tissue structures to small pieces creating a channel for the fiber to penetrate the tissue. So the formation of water vapor seems to be the dominant mechanism for tissue ablation.
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