The Mark-III Free Electron Laser (FEL), tuned to 6.45 microns in wavelength has been demonstrated to provide for efficient ablation in ocular, neural, and dermal tissues with minimal collateral damage. To date, the role of the unique pulse structure of the FEL on the ablation mechanism has not been determined. In this study, the native pulse structure of the FEL, a 2.85 gigahertz repetition of picosecond pulses within a five microsecond macropulse envelope, was changed using a pulse stretcher. This device changes the duration of the micropulse from its native one picosecond to 30-200 picoseconds in length, thus reducing the peak intensity of the micropulse down to 1/200th of the original intensity, while the macropulse energy remains unchanged.
Two basic metrics were studied: the ablation threshold on water and mouse dermis and the ablation crater depth on gelatin and mouse dermis. These metrics were employed at 6.45 and 6.1 microns in wavelength for 1, 100, and 200 picoseconds in micropulse duration. In addition, bright-field imaging was used to compare the ablation dynamic between 1 ps and 200 ps micropulses on water at 6.1 and 6.45 microns. The effect of changing the micropulse duration was also studied on the ablation of mouse dermis for histological analysis. Craters (500 micron diameter) were created with 25 pulses at three times the ablation threshold as determined for mouse dermis within 8 hours of removal. Three rows of twenty craters were created on each piece of mouse dermis for a given parameter set. The native one picosecond micropulse and 200 picosecond stretched micropulse were compared at 6.1 and 6.45 microns in wavelength. There was no difference seen between the native 1 ps micropulse and the stretched micropulse durations with respect to the ablation threshold, efficiency, dynamics, and thermal damage.