High precision surface ablation of soft tissue using the fifth harmonic of a Nd:YLF-picosecond laser at 211 nm

Three nonlinear beta-barium berate (BBO) crystals were used to generate the fourth and fifth harmonic of a Nd:YLF picosecond laser with a fundamental wavelength of 1053 nm. The UV laser pulses were used to study the surface ablation of bovine soft tissue. Whereas the general study was concentrated on brain tissue, first results were also obtained with cornea tissue. The ablation efficiency as a function of pulse energy and the ablation threshold were determined for both UV wavelengths. Histology and SEM-microscopy of the performed laser lesions are presented. Furthermore, time-resolved flash photographs were recorded by illuminating the ablation area with green picosecond laser pulses and detecting the probe beam with a CCD chip. The photographs yielded informations about the physical properties of the ablation process. The emission of a shock wave into the air environment as well as the ejection of dissected ablation fragments was observed for pulse energies down to 10 mu J. Both UV wavelengths can be combined with an ophthalmic application system, which comprises an xyz-scanning unit for realizing precalculated ablation patterns and a z-tracker for detecting the current surface position of the cornea. The information of the z-tracker can be used to compensate the axial eye-movement with the xyz-scanning unit.