Energy scaling by stacking of ultrashort pulses in a passive enhancement cavity
New applications of ultrafast laser systems require parameters, which are out of reach for classical single amplifier channels. Coherent pulse combining can help to circumvent the limitations, which state-of-the-art laser systems are facing. For that purpose, passive optical resonators, so-called enhancement cavities, can be used to stack laser pulses, thus, drastically increasing their pulse energy. Extracting such an enhanced pulse from the cavity, could lead to a set of laser parameters, not accessible to other amplifier technologies. Enhancement cavities running at 80 MHz or above have been used as pulse stacking devices before, employing acousto-optic-modulators (AOM) as switches to extract the stacked energy. However, the nonlinear phases, as introduced by self-phase-modulation in the AOM, commonly limit the achievable intra-cavity energy of a so-called stack-and-dump cavity. By lengthen such cavities to 10 MHz, the extractable energies from such a device were improved by three orders of magnitude from 200 nJ to 160 J compared to former experiments. While longer cavities could allow for even higher energies, they are difficult to set-up and are often limited by their complexity and stability related issues. This is why, novel switches would have to be introduced, if the potential of stack-and-dump cavities is to be exhausted. A rotating element in the cavity is identified as one possible solution. It circumvents acceleration-related limitations and could allow for fast extraction of the enhanced pulse without disturbing the enhancement process. Possible switches based on rotation include a chopper wheel with mirror-segments attached to its side-facet and a rotating-cavity-caustic. Both have been investigated and identified to be viable solutions.