Yb-doped large-pitch fibres: effective single-mode operation based on higher-order mode delocalisation

Rare earth-doped fibres are a diode-pumped, solid-state laser architecture that is highly scalable in average power. The performance of pulsed fibre laser systems is restricted due to nonlinear effects. Hence, fibre designs that allow for very large mode areas at high average powers with diffraction-limited beam quality are of enormous interest. Ytterbium-doped, rod-type, large-pitch fibres (LPF) enable extreme fibre dimensions, i.e., effective single-mode fibres with mode sizes exceeding 100 times the wavelength of the guided radiation, by exploiting the novel concept of delocalisation of higher-order transverse modes. The non-resonant nature of the operating principle makes LPF suitable for high power extraction. This design allows for an unparalleled level of performance in pulsed fibre lasers. A new design of optical fibre could allow fibre lasers to reach unprecedented output powers while maintaining excellent beam quality. The design, developed by Jens Limpert and co-workers from Friedrich-Schiller Universität, Helmholtz Institute Jena and Fraunhofer Institute for Applied Optics and Precision Engineering in Jena, Germany, uses a large-pitch photonic-crystal fibre doped with ytterbium to provide gain. The key to the fibre's performance is the delocalisation of higher order modes due to the transversal arrangement of air-holes. The concept ensures that the fibre operates with a large fundamental mode that has a high-quality beam profile and good power handling characteristics, while suppressing unwanted higher order modes. A pulsed fibre laser based on this design emitted diffraction-limited pulses containing 26 mJ of energy with an average power of 130 W.