Beam quality output of a few-modes fiber seeded by an off-center single-mode fiber source

We study the beam quality at the output of a multimode step-index optical fiber with a small number of modes following injection from a single-mode input fiber. It is shown that the output beam quality is optimized when the injecting fiber is offset from the center of the receiving core. Theoretical results demonstrating this phenomenon are in good agreement with an experimental demonstration. These results may have practical consequences in beam-coupling schemes and in applications emphasizing the importance of beam quality.