Fig. 1. Simulated bend losses of LP01 FM (blue) and LP11 HOM (red) for a 30 μm step-index fiber modeled with 2×10−4 index fluctuations (as fabrication tolerance) in the core and cladding. The solid lines are the predicted performance based on the measured RIP of the fiber reported in this work [see Fig. 2(a)].

Fig. 2. Characterization results of the 30/200 low NA double-clad YDF that consists of a Ge-doped cladding and Yb/Al/P-doped cores. (a) 2D RIP, (b) 1D RIP, (c) cladding absorption, and (d) EDX dopant concentration profiles (SEM image of fiber cross section in inset).

Fig. 3. (a) All-fiber tandem-pump laser setup (top) with its free-space beam diagnostics layout (bottom). The red solid arrow indicates direction of the laser output beam. (b) Optical performance of the fabricated low NA YDF. Top, the output power and efficiency of the amplified signal at 1065 nm, plotted with respect to the pump power at 1018 nm; middle, the spectra of the seed (left) and the amplified signal (right); bottom, M2 of the laser output before (left) and after (right) amplification.