High-efficiency interface between multi-mode and single-mode fibers

Multi-mode fibers (MMFs) and single-mode fibers (SMFs) are widely used in optical communication networks. MMFs are the practical choice in terms of cost in applications that require short distances. Beyond that, SMFs are necessary because of the modal dispersion in MMFs. Here, we present a method capable of interfacing an MMF with an SMF using a re-programmable multi-plane light conversion scheme (MPLC). We demonstrate that only three phase modulations are necessary to achieve MMF–SMF coupling efficiencies from 30% to 70%, i.e., an insertion loss from 5 dB to 1.5 dB, for MMFs with core diameters up to 200 µm. We show how the obtained coupling efficiency can be recovered if the output field of the MMF changes entirely, e.g., through strong deformation of the fiber, by simple monitoring of the field. Furthermore, we test the influence of the resolution of both essential devices (field reconstruction and MPLC) on coupling efficiencies. We find that commercially available devices with increased speed and efficiency, such as wavefront sensors and deformable mirrors, are sufficient for establishing an MMF–SMF interface that auto-corrects any decoupling in the kilohertz regime.