Practical technique for improving all-fiber coherent combination of multistage high-power ytterbium fiber amplifiers.

We demonstrate coherent combination of two high-gain and high-power single-mode 1 μm fiber amplifiers via direct pump current modulation. Each all-fiber amplifier channel, which is built as a master oscillator-multiple amplifier architecture, can operate either continuous waves or 4 ns, 1 MHz pulses with average power of 50 W and more than 55 dB gain. These two amplifiers are coherently combined by modulating the pump currents of the preamplifiers in a multidither arrangement.