  The performance of fiber laser systems has drastically increased over recent
decades, which has opened up new industrial and scientific applications for
this technology. However, currently a number of physical effects prevents
further power scaling. Coherent combination of beams from multiple emitters has
been established as a power scaling technique beyond these limitations. It is
possible to increase the average power and, for pulsed laser systems, also
parameters such as the pulse energy and the peak power. To realize such laser
systems, various aspects have to be taken into account which include beam
combination elements, stabilization systems and the output parameters of the
individual amplifiers. After an introduction to the topic, various ways of
implementing coherent beam combination for ultrashort pulses are explored.
Besides the spatial combination of beams, the combination of pulses in time
will also be discussed. Recent experimental results will be presented,
including multi-dimensional (i.e. spatial and temporal) combination. Finally,
an outlook on possible further developments is given, focused on scaling the
number of combinable beams and pulses.
