Bending insensitive, highly Yb-doped LMA triple-clad fiber for nearly diffraction-limited laser output

The new highly rare-earth doped triple-clad fiber design comprises a first clad next to the core of the well-known double-clad design. The added clad allows to reduce and to better control the core effective numerical aperture for achieving a highly doped large mode area amplifying fiber with a very low numerical aperture (similar to 0.07). The triple-clad design is optimized to obtain a nearly bending insensitive fiber output while keeping excellent beam quality through proper ytterbium doping. The high ytterbium concentration allows for very high gain from a short (similar to 1 m) fiber length which, in many applications, is required to prevent the onset of nonlinear effects such as stimulated Brillouin scattering. A polarization-maintaining 22-mu m core Yb-doped triple-clad fiber was first tested. A laser slope efficiency of up to 86% with a polarization extinction ratio exceeding 24 dB and a M-2 output beam quality factor below 1.1, for both laser and amplifier configurations, have been measured. Moreover, beam quality and output power were not significantly affected when coiling the fiber down to a 1.2 cm diameter, thus showing the optical robustness of the triple clad fiber design and offering the opportunity to build very compact high power fiber amplifiers and laser sources.