Breaking the limit of maximum effective area for robust single-mode propagation in optical fibers.

We propose and demonstrate a novel approach in optical fiber design in which the optical waveguide is formed by a ring of large air holes surrounding a solid silica core. With an appropriate choice of the geometrical configuration, robust single-transverse-mode propagation with a record effective area of 1417 microm2, verified by various methods, was demonstrated. A breakthrough was made toward the development of practical ultra-high-power fiber lasers as we observed negligible loss of the fiber at bending diameters as small as 15 cm.