Delay line coherent pulse stacking.

Ultrafast fiber amplifiers are usually limited in terms of pulse energy and peak power due to fiber damage and non-linearities. On the other hand, fiber amplifiers are very energy efficient. To take advantage of this efficiency, but still scale the pulse energy, coherent combining and pulse stacking have become the method of choice. Here we show how to directly and symmetrically stack up the pulses from an oscillator chain, using delay lines and phase preshaping of a 48 MHz, 1.5 mu m master oscillator. This method avoids the need for high-power low-loss dumpers as in passive stack and dump cavities and, compared to divided pulse amplification (DPA), it eliminates the need for splitting pulses and pulse picking and, thus, optimally utilizes the available seed power. Furthermore, it avoids the asymmetrical gain of the pulse bursts, which typically limits the efficiency in DPA. (C) 2017 Optical Society of America