Amplification of short pulse KrF lasers is theoretically studied by using both coherent and incoherent interaction models, which consist of five vibrational levels in the upper laser state and a single repulsive lower laser state. The multilevel models predict a significant decrease in the saturation energies for short applied laser pulses. When the laser pulse width is extremely short (≤1 ps), the higher vibrational levels in the upper KrF laser state are repumped from the lower laser state instead of being extracted by the laser pulse. Since the small-signal gain also decreases as the applied pulse width decreases, due to mismatch between the laser pulse and the KrF gain spectrum, the energy extraction efficiencies for subpicosecond laser pulses deteriorate significantly. For longer pulse widths (≥10 ps), population repumping from the KrF(C) state increases the effective saturation energy of the KrF(B) state. A steady-state population of ≊28% relative to the B-state population is estimated for the C state from recent experimental results of KrF gain recovery.
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