Efficient passive phasing of an array of 20 ring fiber lasers

Abstract : A 2D array of fiber lasers has been passively coupled by a phase-contrast imaging technique in a ring cavity. Efficient coherent operation was demonstrated for 3 × 3, 4 × 4 and finally 4 × 5 ytterbium doped fiber lasers. A phase locking level of 78% was measured with 20 lasers in agreement with simulations. Passive synchronization was stable in time despite the unprotected environment. The measured performances clearly surpass the initial limitation found for passive fiber laser coupling. The improved efficiency and stability are believed to be connected with the contribution of gain to the phase accumulated on a cavity round-trip together with the cavity design.