Highly efficient cladding-pumped single-mode three-level Yb all-solid photonic bandgap fiber lasers

Efficient cladding-pumped three-level Yb fiber lasers are difficult to achieve due to the competing four-level system and necessary high inversions. We demonstrate an efficiency of $\sim 62.7\%$ versus a coupled pump, a record for cladding-pumped fiber lasers with a single-pass pump. 84 W at $\sim 978\mathrm{nm}$ with $\sim 1.12M^2$ was achieved, a record power for flexible fibers. Amplified spontaneous emission was suppressed by $>40\mathrm{dB}$. The efficiency is quantum-limited $\sim 94\%$ versus an absorbed pump. This is made possible by the use of a photo-darkening-free Yb phosphosilicate core and recent progress in single-mode large-core all-solid photonic bandgap fiber designs, which provide the necessary large core-to-cladding ratio and suppression of the four-level system.