Diode-pumped narrow linewidth multi-kilowatt metalized Yb fiber amplifier.

We investigate high brightness pumping of a multi-kilowatt Yb fiber amplifier in a bi-directional pumping configuration. Each pump outputs 2 kW in a 200 μm, 0.2 NA multimode fiber. Gold-coated specialty gain fibers, with a 17 μm mode field diameter and a 5 dB/m pump absorption, have been developed. The maximum fiber amplifier output power is 3.1 kW, limited by multimode instability, with 90% O-O efficiency and M²<1.15. The fiber amplifier linewidth is 12 GHz.