Abstract

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.

© 2017 IEEE