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1 July 2004 Effect of FEL micropulse duration on ablation characteristics at 6.45 μm
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Pulsed mid-infrared (6.45 μm) radiation has been shown to cut soft tissue with minimal collateral damage (<40 mm); however, the mechanism of ablation has not been elucidated to date. The goal of this research was to examine the role of the unique pulse structure of the Vanderbilt Mark-III FEL and its role in the efficient ablation of soft tissue with minimal collateral damage. The pulse structure consists of a 2.865 GHz train of one picosecond micropulses within a 4-5 μs macropulse envelope operated between 2 and 30 Hz. The effect of the picosecond micropulses was examined by running the native FEL pulse structure through a pulse stretcher in order to increase the micropulse length from 1 picosecond up to 100 picoseconds. This allowed us to determine whether or not the picosecond train of micropulses played any role in the ablation process. The pulse stretcher was varied between 1, 30, 60, and 100 picoseconds. The ablation threshold was determined for water and mouse dermis for each micropulse length using PROBIT analysis of 100 individual observations of the macropulse. The results of the analysis showed no statistical difference between 1 and 100 picoseconds. The ablation efficiency was also measured on 90% w/w gelatin and mouse dermis for the different micropulse lengths. Multiple ablation craters were made by varying the number of pulses delivered between 5 and 500. The ablated crater depth was measured using OCT. No significant difference was observed between 1 and 60 picoseconds; however, the 100 picosecond micropulse did show a reduction in the efficiency of ablation. We have shown that the effect of micropulse duration of the FEL on the ablation process is negligible between 1 and 100 picoseconds. Further analysis is needed beyond 100 picoseconds.
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Mark A. Mackanos, John A. Kozub, and E. Duco Jansen "Effect of FEL micropulse duration on ablation characteristics at 6.45 μm", Proc. SPIE 5319, Laser Interaction with Tissue and Cells XV, (1 July 2004);

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