An original architecture of an active fiber allowing a nearly diffraction-limited beam to be produced is demonstrated. The active medium is a double-clad large-mode-area photonic-bandgap fiber consisting of a $\mathrm{10,000}\mathrm{ppm}$ by weight ${\mathrm{Yb}}^{3+}$ -doped core surrounded by an alternation of high- and low-index layers constituting a cylindrical photonic crystal. The periodic cladding allows the robust propagation of a $\sim 200\mathrm{\mu }{\mathrm{m}}^2$ fundamental mode and efficiently discriminates against the high-order modes. The $M^2$ parameter was measured to be 1.17. A high-power cw laser was built exhibiting 80% slope efficiency above threshold. The robust propagation allows the fiber to be tightly bent. Weak incidence on the slope efficiency was observed with wounding radii as small as $6\mathrm{cm}$ .