Measurement Of Small–signal Absorption Coefficient And Absorption Cross–section Of Collagen For 193–nm Excimer Laser Light

Abstract

Abstract: : Purpose:Clinical excimer laser refractive surgery systems are continually evolving to improve accuracy and precision. However, a complete understanding of the physics and mechanisms of corneal ablation and the role of tissue constituents such as collagen remains a topic of research. A better understanding of the ablation process may lead to enhanced refractive procedures Methods: 193–nm ArF excimer laser transmission measurements were recorded at subablative fluence through varying solution concentrations of Type III collagen (0.06–0.24 mg/mL in 0.5 N acetic acid). The collagen absorption coefficient and absorption cross–section were calculated using the Beer–Lambert law and baseline subtraction for the acetic acid absorption coefficient. In addition, dry collagen films were prepared and ablation rate data were recorded as a function of laser fluence. Results:The measured absorption cross–section of collagen was 1.18x10^–17 cm² corresponding to the polypeptide bond as the primary chromophore. This translates to an equivalent corneal tissue absorption coefficient of about 16,050 cm^–1. Based on a blow–off model using Beer–Lambert absorption and a measured ablation threshold, the ablation rate data for the dry collagen films yielded an absorption coefficient of about 91,000 cm^–1. This was in excellent agreement with the predicted absorption coefficient based on the measured collagen absorption cross–section and the dry film collagen bond density. Conclusions: Collagen alone is highly absorbing at 193 nm, and the present data support a significant corneal tissue absorption coefficient based on direct measurement of the collagen absorption cross–section. The experiments suggest that collagen–based absorption coefficients alone are consistent with predicted corneal tissue ablation rates. Although dynamic changes in corneal tissue absorption properties are likely still important in the overall ablation mechanism, these effects need not be order of magnitude. Overall, the current study helps to provide a clearer understanding of the role of collagen in 193 nm photochemistry and ablation.

Keywords: laser • refractive surgery: other technologies • cornea: basic science