Active Phase Locking of Four Yb-doped Fiber Amplifiers with a Multi-mode Fiber Combiner

To the best of our knowledge, we report an experimental demonstration on active phase-locking of four coherent combined all-fiber Yb-doped amplifiers with a fiber combiner for the first time. Four 100 mW power-level single mode fiber amplifiers are combined in a fiber combiner with the core diameter of 50 μm. Without active phasing, the far field intensity pattern of the combined beam fluctuates randomly in the experiment. With active phasing the whole system by single frequency dithering technique, the far field intensity pattern of output beam is locked to be quite stable. Furthermore, with the active phasing technique, the typical beam quality factor M2 of the combined beam improves from 33 to about 19.