Weak coupling fibers for MIMO-free mode division multiplexing systems

As a potential technique to further increase the transmission capacity in a single fiber, few mode fibers based mode division multiplexing (MDM) is attracting increasing interest in the worldwide. However, multiple input multiple output (MIMO) digital signal processing (DSP) is required to mitigate the mode crosstalk induced by mode coupling during the transmission, which increase the cost and complexity of the whole system. An effective approach to obtain negligible modal crosstalk in FMFs, so as to eliminate MIMO, is to lift the degeneracy between the adjacent eigenmodes and achieve effective index difference, Δneff values larger than 10⁻⁴. Elliptical core fiber (ECF) and elliptical ring core fiber (ERCF) are designed to achieve a high index difference between adjacent eigenmodes. However, since most of the commercial mode selective multiplexers/demultiplexers (MUX/DEMUX), such as photonic lanterns, are designed for circle core fiber, when connecting ECF/ERCF with commercial mode selective MUX/DEMUX, the mode field mismatch between the ECF/ERCF and the circle core fiber (CCF) leads to large coupling loss and mode crosstalk. In this work, we design and fabricate an asymmetric circle core fiber (ACCF) for MIMO-free MDM systems. Compared with ECF, the proposed ACCF can connect well with the commercial photonics lanterns with reduced the coupling loss and mode crosstalk.