In the project multicore fibers suitable for coherent combination were successfully manufactured. Leveraging state-of-the art process technology, first a flexible fiber with 16 Ytterbium-doped cores in a 4x4 configuration was realized. A compact 1:16 beam splitter and combiner was designed and acquired for the beam splitting and combination. Finally, a piezo driven array of 16 mirrors that can control the optical phase of each beam individually was realized. With a novel algorithm, the phase of each beam is controlled that all beams recombine into a single output beam at the beam combiner. In a first experiment, picosecond pulses were successfully combined with a high efficiency of 80%. A femtosecond chirped-pulse laser system incorporating this amplifier configuration was also realized and, an average power of 170 W and a pulse duration of 260 fs after compression could be achieved. The coherent combination of the beams could be realized with a high efficiency of 80%.
As the next step, the design of rod-type multicore fibers was pursued that can offer higher average power capabilities due to its all glass structure. It contains an integrated octagonal cladding for guiding the pump and also contains 16 signal cores. The drawn fiber shows well-functioning amplification up to 160 W of average power in a first test and a homogenous, near single-mode output profile. Significant energy extraction in the multi-mJ range could also be observed. For future channel count scaling, we have realized beam splitting and combination elements for a 7x7 beam array. In order to stabilize such a high beam count interferometer, a phase array based on novel MEMS technology is employed. To lower complexity of the optical elements for high channel counts, simplified splitter and combiner designs have also been investigated.
Finally, electro-optically controlled divided-pulse amplification that presents a compact implementation of temporal pulse combination was successfully realized with up to 12 parallel main amplifier channels. The spatio-temporal combination of the total 96 pulse replicas resulted in an average power of 674 W at a repetition rate of 25 kHz and a pulse energy of 23 mJ in the main pulse feature. The pulses were compressible to a pulse duration of 235 fs. So far, this is the highest pulse energy ever achieved with a fiber CPA system.
