Mode-Division Multiplexing Over 96 km of Few-Mode Fiber Using Coherent 6$\,\times\,$6 MIMO Processing

We report simultaneous transmission of six spatial and polarization modes, each carrying 40 Gb/s quadrature-phase-shift-keyed channels over 96 km of a low-differential group delay few-mode fiber. The channels are successfully recovered by offline DSP based on coherent detection and multiple-input multiple-output processing. A penalty of ${<}1.2$ dB is achieved by using 6 $\,\times\,$ 6 feed-forward equalizers with 120 taps each. The 6 $\,\times\,$ 6 impulse-response matrix fully characterizing the few-mode fiber is presented, revealing the coupling characteristics between the modes. The results are obtained using mode multiplexers based on phase plates with a mode selectivity of ${>}28$ dB.