Large-mode-area fused-fiber combiners, with nearly lowest-mode brightness conservation

We show the feasibility of producing a low-mode all-fiber combiner fabricated from a large core and extremely small NA fibers. Although these fibers support multiple modes, the combiner that we produce can be operated nearly at the single mode regime while preserving the brightness of the combined beam almost perfectly with respect to the inputs. The M-square parameter of the combined beam was 2.3 and the power transfer efficiency was close to 100%. Such an all-fiber beam combining device is a rugged solution for high-brightness, high-efficiency beam delivery.