Target-in-the-loop coherent beam combining based on an all-fiber structure.

Coherent beam combining (CBC) is a promising technique to realize high-brightness laser output. As a key point to implement CBC, an appropriate phase control feedback structure should be established. With the advantages of a compact structure and no requirement for mirrors to sample, the all-fiber phase control feedback structure has been widely studied. However, the structure faces the challenge of π phase ambiguity. We propose to introduce the target-in-the-loop (TIL) technique into the all-fiber phase-locking structure. By adding measurement laser channels outside the main laser channels, the high-intensity phase noise in amplifiers and low-intensity phase shifts induced by atmospheric turbulences could be decoupled. The π phase ambiguity could be compensated together with the low-intensity phase shifts. In this paper, the principle of the all-fiber phase-locking structure is demonstrated and corresponding numerical simulations are carried out. A TIL CBC system with six laser channels is built to verify the technique. Additionally, the all-fiber structure is easy to scale to large arrays and compatible with high-power amplifiers, which can provide a significant reference for the system design of the TIL CBC system.