A regenerative amplifier includes a gain-medium that is optically pumped by CW radiation. The amplifier has primary resonator for amplifying pulses. The primary resonator has an optical switch for opening and closing the primary resonator. The amplifier has a secondary resonator that includes the gain-medium and a selectively variable attenuator, but not the optical switch. The optical switch and the variable attenuator are cooperatively controlled in response to repeated trigger-signals such that the amplifier delivers corresponding amplified pulses each having about the same energy independent of repetition frequency of the signals below some predetermined value.