The Scaling Limits of Beam-Splitter-Based Coherent Beam Combining

Ultrafast lasers allow deploying energy into tiniest volumes and shortest timescales, which enables to probe and manipulate matter with unprecedented spatial and temporal resolution. Today, the combination of chirped-pulse amplification and advanced laser geometries routinely allows for the simultaneous generation of GW-level peak power and kW-level average power. Such lasers are widely applied, e.g. in materials processing and in the generation of extreme ultraviolet radiation. While most applications can benefit from higher power, some fields stringently require power scaling of sizable order. Two examples are space-debris removal and next generation laser-driven particle colliders that demand for 100 kW-level average power with diffraction limited beam quality and, in the latter case, even high-contrast ultrafast operation. Today’s laser technology cannot supply these parameters due to limitations imposed by the laser active medium. The key technology to circumvent this problem is coherent beam combination, which synthesizes a much more powerful laser beam from many individual emitters. Out of the many options of building such systems, plain beam splitters offer a way to highly efficient and compact systems. Following this approach, the worlds most average-powerful ultrafast lasers are built and the fundamental scaling limits are determined in this work. In the first part, two lasers based on coherent polarization beam combination of rod-type fiber amplifiers are presented. From the later system, an average power of 1.9 kW and multi-GW peak power is obtained. Both lasers are used to drive a series of scientific experiments, giving proof of their practical applicability. Nonlinear absorption in ambient air is found a technical challenge to be overcome with a protective atmosphere or a vacuum environment. At the same time, heating of the final beam ombining element is observed, which indicates that thermo-optical effects are about to become relevant. In the second part, two more laser systems are built based on intensity beam splitters and industrial-grade 20/400-fiber amplifiers as a means to even higher average power. The first system already delivered a record-breaking average power of 3.5 kW and showed no signs of thermal limitation. Technical shortcomings are identified, explained, and resolved in the subsequent system, from which an output average power of 10.4 kW is demonstrated at otherwise close-to-ideal laser parameters. Both systems feature an automatic spatio-temporal interferometer alignment that enables a close-tounity efficiency. In the third part of this work, the fundamental power limits in filled-aperture coherent beam combining are analyzed theoretically. It is found that the Kerr effects remains negligible even for Joule-level pulse energy and typical chirped-pulse durations while linear absorption in the combining elements is more critical. Still, an average power significantly above 100 kW can be reached when using fused-silica-based intensity beam splitters. Technical challenges associated with the beam steering and the linear compression of 100 kW-level average power are discussed and found to be surmountable. Furthermore, simplification of the setups can be reached drawing on multi-core fibers, which is demonstrated experimentally. Finally, nonlinear post-compression of the developed lasers in multipass cells is investigated for its compatibility with the developed laser sources. The design criteria for such cells are derived and the compression of 1mJ-energy pulses to 31 fs pulse duration is demonstrated in an experiment at 1 kW average power, representing an average power record for sub-100-femtosecond sources. A numerical propagation model is developed that accurately reproduces the experiment and which is used to evaluate the average power scalability of multipass cell compression revealing compatibility with 100 kW-level average power. All in all, this thesis demonstrates that filled-aperture coherent beam combination based on beam splitters is a highly efficient method to overcome the fundamental peak and average power limits intrinsic to laser amplifiers. The average power scaling can continue for at least one order of magnitude beyond the demonstrated record value of 10.4 kW and a pulse energy in the Joule range is attainable. Thus, this technology certainly is one way to enable some of those applications that otherwise would have to remain visionary.