Impact of cross-sectional characteristics on the performance of all-fiber signal combiners.

Beam combining based on all-fiber signal combiners has achieved remarkable progress in both output power scaling and beam quality improvement. In this paper, we numerically investigate the impact of cross-sectional characteristics on the performance of typical all-fiber signal combiners with 3, 4, 7, and 19 input ports. Three main types of cross sections are considered: non-collapsed model, fully collapsed model, and circular-shaped model. The simulation results demonstrate that a non-ideal cross section alters the mode field distribution within the tapered fiber bundle, impairs mode field matching with the output multimode fiber, and consequently degrades both transmission efficiency and beam quality. For combiners with few input ports, the degradation predominantly manifests in beam quality, whereas for those with many input ports, it mainly affects transmission efficiency. In general, the non-collapsed model exhibits more severe degradation, while the difference between the fully collapsed model and the circular-shaped model is comparatively small. These quantitative findings provide a clear theoretical basis for the tolerance design of cross-sectional deformation and process optimization aimed at achieving high beam quality and low insertion loss in all-fiber signal combiners.