Yb-doped LMA triple-clad fiber laser

Abstract

The ytterbium-doped large mode area triple-clad fiber design allows for a high concentration of ytterbium in the fiber core which is difficult to achieve with a standard double-clad design. The novelty of the triple-clad fiber design consists in adding to the double-clad fiber design, a first clad next to its core. This first clad offers a better control of the core effective area. With this design a low numerical aperture is achievable (~0.06) for highly rare earth doped large mode area fiber. A 33-μm core ytterbium doped fiber has been fabricated using MCVD and solution doping processes. Selective doping and optimized first clad thickness have been used in the triple-clad design to obtain a nearly bending insensitive and nearly diffraction-limited fiber output. The fiber has been tested in a free-running laser configuration and its slope efficiency is 84% with a laser threshold of 1.4 W. A maximum output power of 26 W at 1070 nm has been achieved for a launched pump power of 34 W at 976 nm. The mode-field diameter has been measured at 18 μm and the output beam M² quality factor is below 1.1. Both output power and beam quality were not significantly affected by fiber bending with loops diameter as small as 2.5 cm. The optical performance of the triple-clad fiber design is robust to mechanical stress and well suited for building very compact high power fiber lasers and amplifier sources.