Mutual injection-locking and coherent combining of two individual fiber lasers

In this paper, mutual injection-locking and coherent combining are demonstrated with two individual erbium-doped fiber lasers that were coupled by two fiber splitters. Mutual injection-locking theory of two lasers is analyzed. In the free-running state, the far-field beam profile is a simple intensity superposition, as expected of two incoherent beams. Under mutual injection-locking, interference fringes with high contrast ratio are obtained, and the two fiber lasers lase at the same wavelength with a stable output power. We have found that the two fiber lasers are always in out-of-phase mode, which is consistent with theoretical analysis. Coherent beam combining by mutual injection-locking is realized without the need for length or amplitude control. This method can be easily scaled to combine more beams.