Ablation with femtosecond pulses: The effect of temporal contrast

An inherent property of the ultrashort pulse lasers used presently for materials processing is that the main pulse is accompanied by an amplified spontaneous emission background. The temporal quality of the pulses is characterized by the temporal contrast. Due to the 5-6 orders of magnitude difference in the duration of the main pulse and the pedestal, high intensity contrast does not mean necessarily high enough difference in the respective energies. In materials processing applications consideration of the energy contrast is advisable. The importance of the contrast in determining the ablation process is exemplified by the dependence of the ablation rate of boron carbide on the cleanness of the pulses of a high brightness hybrid dye/excimer laser system. It is shown that serious consideration of the effects of the nanosecond background is mandatory when evaluating the results.