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The small-signal absorption coefficients of 193- and 213-nm nanosecond laser pulses in bovine corneal tissue have been studied. The absolute reflectance of a planar quartz-cornea interface was measured at various angles of incidence for low-intensity laser irradiation (i.e., pulse fluences 3 orders of magnitude below the ablation threshold). The reflectance-versus-angle data were analyzed by the use of Fresnel theory to estimate the effective complex index of refraction of the tissue. This analysis indicated corneal absorption coefficients of 39,900 ± 9800 cm(-1) at 193 nm and 21,400 ± 4900 cm(-1) at 213 nm.

A theoretical description of the ultraviolet laser etching process is developed. The threshold for laser ablation is reached when the density of absorbed photons is approximately equal to the density of chromophores in the material. Saturation of the absorption coefficient, absorption by the plume of ablated products, and multiphoton effects are considered. Agreement with all available experimental etch data, including femtosecond ultraviolet laser ablation, is found. The description is based on an analysis of the radiation transport at high intensities and is independent of the question as to whether ultraviolet laser ablation is photochemical or thermal.

The energetics of 308-nm excimer laser irradiation of human aorta were studied. The heat generation that occurred during laser irradiation of atherosclerotic aorta equaled the absorbed laser energy minus the fraction of energy for escaping fluorescence (0.8-1.6%) and photochemical decomposition (2%). The absorbed laser energy is equal to the total delivered light energy minus the energy lost as specular reflectance (2.4%, air/tissue) and diffuse reflectance (11.5-15.5%). Overall, about 79-83.5% of the delivered light energy was converted to heat. We conclude that the mechanism of XeCl laser ablation of soft tissue involves thermal overheating of the irradiated volume with subsequent explosive vaporization. The optical properties of normal wall of human aorta and fibrous plaque, both native and denatured were determined. The light scattering was significant and sufficient to cause a subsurface fluence (J/cm2) in native aorta that equaled 1.8 times the broad-beam radiant exposure, phi o (2.7 phi o for denatured aorta). An optical fiber must have a diameter of at least 800 microns to achieve a maximum light penetration (approximately 200 microns for phi o/e) in the aorta along the central axis of the beam.