Yb-doped large mode area tapered fiber with depressed cladding and dopant confinement

A polarization-maintaining Yb-doped large mode area fiber with depressed-index inner cladding layer and confinement of rare-earth dopants has been drawn as a long tapered fiber. The larger end features a core/clad diameter of 56/400 mu m and core NA similar to 0.07, thus leading to an effective mode area over 1000 mu m(2). The fiber was tested up to 100 W average power, with near diffraction-limited output as the beam quality M-2 was measured < 1.2. As effective single-mode guidance is enforced in the first section due to enhanced bending loss, subsequent adiabatic transition of the mode field in the taper section preserves single-mode amplification towards the larger end of the fiber.