All-fiber mode converter based on LP11 coupled-mode for mode division multiplexing systems

•Two mode converters based on MSTMF and CSIF are proposed.•Both mode converters were modeled by laterally misaligning two identical fibers.•Coupling efficiency of 94% and 69% were achieved for MSTMF and CSIF, respectively.•Mode purity of 85% and 58.8% were achieved, for MSTMF and CSIF, respectively.•The proposed systems have a potential to be used in mode division multiplexing system. In this paper, two LP11 mode converters were reported and compared; one, employing two-mode conventional step index fiber (CSIF) and the other, using two identical multi-step two-mode fiber (MSTMF). In each case, the two fibers constituting the proposed devices were laterally misaligned. With the appropriate adjustments of the lateral displacements (h) and separation distance (d) between the axis of the identical CSIF and MSTMF, all-fiber converters with the coupling efficiencies of 69% and 94% were achieved, respectively. The performance of the two devices is compared in terms of the coupling efficiency (CE) and mode purity (MP) at the outputs. The results based on the coupling power indicate that MSTMF-based device offers higher CE and batter MP than the CSIF-converter. An MP of 84% and 58.8% were respectively determined for the MSTMF and CSIF converters. Therefore, the MSTMF device can be more suitable in mode conversion during mode division multiplexing (MDM) systems due to its high CE and MP.