Experimental Investigation of Optical Breakdown Thresholds in Ocular Media under Single Pulse Irradiation with Different Pulse Durations

Curves of probability of occurrence of optical breakdown have been plotted in respect to measurements carried out in liquids used as models for ocular media. These studies also included exposures in calf vitreous. The Nd:YAG lasers used were able to deliver single pulses at the fundamental wavelength with three different pulse durations, i.e., 7 ns.220 and 30 ps respectively, in three different focusing conditions. The results show that breakdown threshold intensities increase substantially with decreasing the pulse duration, an observation similar to that found in solids. In addition, breakdown curves obtained with picosecond pulses are steeper than those obtained with nanosecond pulses, thus showing that with shorter pulses the process of onset of breakdown is more deterministic. Impurities are shown to play a decreasing role in the initiation of the process of optical breakdown as the pulse duration is decreased. Finally it has been found that, with tight focusing, the breakdown threshold intensity increases much more with picosecond pulses than with nanosecond pulses. The importance of these observations both for the characterization of different laser disruptors and for the clinical use of the Nd:YAG laser in ophthalmic microsurgery will be discussed