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. The key feature of the scaling scheme is the insertion of a designated second stage preamplifier between the first stage preamplifier and the final power amplifier stage, so as to improve the coherency and to minimize the gain variation of the preamplifier to the power amplifier. Otherwise, overall phase control of the fiber amplifiers with this method is not possible at such a high power level.