2-Fold inner cladding symmetry design for actualizing 100μm-core class double-clad large-pitch fibers with the preferential fundamental mode amplification property

Silica/air large-pitch photonic crystal fibers, known simply as large-pitch fibers (LPFs) due to their hole-to-hole spacing 10 times larger than the wavelength, performing mode-field diameters (MFDs) about 50 μm along with nearly diffraction-limited output beam quality provide a versatile platform for realizing compact and stable multi-kW average power and multi-mJ pulse energy ultrashort pulse fiber laser systems [1]. Major efforts have been devoted to improving mode delocalization phenomena in Yb-doped double-cladding LPFs with a view to reduce the spatial overlap of higher-order modes (HOMs) with the doped area [2]. However, the thermo-optic effect which changes the refractive index profile and reconfines a group of the most competitive HOMs (TE01, HE21, TM01) in the gain region jeopardizes the acceptable conditions for the practical fundamental mode (FM) operation and consequently power scaling capabilities of such fibers [3]. For this reason, development of new double-clad LPF designs providing extended HOMs delocalization properties under the extreme heat load is an urgent challenge in the field of fiber laser technologies.