PHOTOABLATION OF GELATIN WITH THE FREE-ELECTRON LASER BETWEEN 2.7 MU-M AND 6.7 MU-M

BACKGROUND: Photoablation in the infrared (IR) is an option for future refractive and corneal surgery; its basic principles have not yet bee n investigated systematically. For the first time, the free electron l aser allows the dynamic study of photoablation over a wide range of wa velengths with variable combinations of pulselength and energy. The go al of this study is to use the free electron laser as a tool to descri be photoablation in the IR quantitatively. We studied the function of wavelength as it is related to target material spectroscopy and the ef fects of corneal hydration and the pulse repetition rate. METHODS: Sur face absorption spectroscopy of the human cornea and of gelatin as a p roven model of the cornea was performed between 2.7 and 6.7 mum. Gelat in probes of well-defined thickness (140 +/- 5 mum) and controlled hyd ration (wet/dry weight 1 to 4.5) served as target material. Photoablat ion was performed with the Vanderbilt University free electron laser ( Nashville, Tenn) in September 1992 at a fluence of 1.27 J/cm2, and a m acropulse of 4 mus, composed of 2 ps micropulses at a 2.9 GHz pulse re petition rate. Wavelength was tunable between 2.7 and 6.7 mum at stabl e beam profiles. Ablation experiments were performed as a function of energy, hydration, and pulse repetition rate. Ablation rates were asse ssed by a) perforation experiments, and b) direct measurements using c onfocal laser topometry (UBM, Ettlingen, FRG). RESULTS: Ablation rate, assessed by perforation experiments and topometry, correlated well wi th the corresponding measured absorbencies of the target material: max imal ablation rate at maximal target absorption, around the 3- and 6-m icrometer water absorption bands. The ablation threshold at 6.2 mum wa s 0.7 +/- 0.05 J/cm2 (perforation) and 0.55 +/- 0.08 J/CM2 for depth m easurements. Ablation rate as a function of hydration increased to 2.3 (wet/dry weight) with a decrease for higher hydrations. Ablation rate as a function of the pulse repetition rate showed an increase of up t o 20 Hz, where it was found to be 60% higher. CONCLUSION: The first sy stematic use of free electron laser technology positively correlated a blation efficiency with target material absorption, thus identifying a ''new'' promising wavelength at around 6.2 mum for materials with a h igh water content such as corneal tissue.