Performance scaling of ultrafast laser systems by coherent addition of femtosecond pulses

In the last decades, ultrafast lasers and amplifiers have achieved an extraordinary power increase and have enabled a plethora of scientific, medical or industrial applications. However, especially in recent years, it has become more and more challenging to keep up with this pace since intrinsic physical limitations are becoming difficult to avoid. A promising way to get around this problem is the technique of spatially and/or temporally separated amplification and subsequent coherent addition of ultrashort pulses. It turns out that fiber amplifiers are perfect candidates for this concept due to their outstanding average-power capability and their simple single-pass setups, which can be easily parallelized. Herein we provide an overview of the most important experimental implementations of this concept and recent results. We discuss the ability of these approaches to generate laser parameters that, only a few years ago, seemed impossible to achieve.