All-fiber control-free coherent beam combining amplification of noiselike pulse toward high power supercontinuum light source

Fiber laser amplification usually uses amplifiers in a single core, such as a master oscillator power amplifier (MOPA). However, nonlinear optical effects resulting from the high peak power of short pulse fiber lasers limit their output [Chen et al., Opt. Express 17(26), 24008–24012 (2009)]. Coherent beam combining (CBC) systems can overcome this output limitation and achieve higher powers [Fan, IEEE J. Sel. Top. Quantum Electronics 11(3), 567–577 (2005); Yoshida, Jpn. J. Opt. 40(3), 122–128 (2011)]. On the other hand, with all-fiber CBC systems, it is difficult to efficiently amplify short pulses, which have short coherence lengths and whose phase and polarization can change due to external factors such as temperature changes and mechanical vibrations. Conventional CBC systems require precise control of both phase and polarization. To solve this problem, a novel CBC system that does not need phase or polarization control has been developed, and the successful amplification of femtosecond pulses has been reported [Kambayashi, IEEJ Trans. Electron. Inf. Syst. 136(1), 70–75 (2016)]. We attempted amplification using a novel passive CBC system, employing noiselike pulses (NLPs) as the seed light. NLPs contain numerous sub-picosecond and picosecond pulses within a nanosecond envelope. In the work reported in this paper, we achieved a highly efficient and stable combined output, demonstrating a combining efficiency of 98.7% and output fluctuations of less than 0.9% for 2 h.