Energy extraction in a XeCl amplifier is theoretically studied for 0.16 and 3 ps initial laser pulse durations. Intrinsic values of an amplification gain and corresponding saturation energy are defined at each position along the amplification by the degree of matching between a gain spectrum and a propagating laser pulse spectrum. When a laser pulse is amplified by the gain medium with a multilined and inhomogeneously broadened spectrum, the saturation behavior of the amplified laser energy is influenced by the characteristics of change in the laser pulse spectrum during the amplification. Therefore, the saturation energy cannot be simply defined as a unique value as a function of initial laser pulse width. When a laser pulse is amplified to extremely high energy level (>5 mJ/cm2), the laser pulse spectrum, which is filtered by the gain spectrum in the small-signal amplification region, is strongly distorted and tends to shift its peak toward the red spectrum side.
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