A laser system includes a master oscillator, which emits a train of optical seed pulses with variable intervals between the pulses. An optical power amplifier includes an optical gain medium, which receives and amplifies the optical seed pulses from the master oscillator, and a pump, which applies pump radiation to the optical gain medium. A pulse generator applies a control input to the master oscillator, which causes the intervals between the optical seed pulses to vary by at least 50% at a rate of change that is greater than a response frequency of the optical gain medium. A control unit drives the pump responsively to predicted intervals between the optical seed pulses, at a variable pump power selected so that the pulse amplitudes of the output pulses vary by no more than 20% irrespective of the varying intervals between the optical seed pulses.