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Investigation of Corneal Ablation Efficiency Using Ultraviolet 213-nm Solid State Laser Pulses

From the Lions Eye Institute, Center for Ophthalmology and Visual Science, University of Western Australia, Nedlands.

PURPOSE. To determine the threshold and efficiency of corneal ablation for various values of laser fluence at the ultraviolet wavelength of 213 nm.

METHODS. A commercial Q-switched Nd:YAG laser was used to produce the fifth harmonic wavelength of 213 nm. Ablation trials were carried out on porcine corneas. Slit ablations of dimensions 0.5 x 2.5 mm were performed using seven values of laser fluence to obtain the most efficient fluence for ablation. The morphology of each ablation was obtained using a computer-automated confocal profiling system. These profiles were then analyzed to determine the ablation depth for the range of fluence values used.

RESULTS. A fluence in the region of 200 mJ/cm² was found to be the most efficient for ablation. The efficiency in this region was approximately 0.35 mm³/J, and the ablation rate was found to be 0.6 µm/pulse. The ablation threshold was found to occur at a fluence of 50 mJ/cm². In the region of highest efficiency, the peak varied slightly in the fluence range between 150 and 250 mJ/cm².

CONCLUSIONS. This study confirms that the corneal ablation properties at 213 nm are comparable with those at the 193-nm excimer laser wavelength. Increased pulse energy was obtained for the fifth harmonic of Nd:YAG lasers at 213 nm through the use of new nonlinear optical crystals to perform the frequency conversion. A solid state laser is feasible to replace the excimer gas laser for performing refractive surgery procedures. For the first time, the increased energy at 213 nm allows large-beam ablations to be performed at this wavelength.

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