Very-large-mode-area fibers for high-power laser operation

In recent decades, ultrashort-pulse laser systems have become a key enabling technology for both science and industry. Fiber lasers are a thriving representative of this class of lasers due to their excellent beam quality, their high efficiency and their outstanding thermal handling capability resulting in highest average output powers. However, in the rapid average-power development continuous-wave fiber lasers have always been ahead of pulsed fi ber lasers. The reason is that pulsed laser systems operate with signifi cantly higher intensities inside the core, which require an extra effort to mitigate the onset of detrimental nonlinear effects e. g. by chirped-pulse amplification, short fiber lengths and large mode areas. In this thesis a novel fiber concept suitable for mode-area scaling of high-power, effective single-mode fibers to mode- field diameters beyond 100 µm is explored. The underlying working principle is the delocalization of higher-order modes. The scalability of higher-order-mode delocalization is investigated numerically and experimentally in terms of practical influences on the guiding mechanism, including index matching and thermal gradients. Large-pitch fibers are introduced as the first representatives of this novel fiber concept, reaching effective single-mode mode-field diameters beyond 100 µm and, therefore, enabling the current records for pulse energy and peak power of ultrashort-pulse fiber-laser systems.