Stable $4\times 10\ \text{Gb}/\mathrm{s}$ MIMO-Free IM/DD MDM Transmission Enabled by Degenerate-Mode-Selective Couplers

We propose and fabricate non-circularly-symmetric degenerate-mode-selective couplers for LP
₁₁
and LP
₂₁
modes, based on which we demonstrate stable 4-mode MIMO-free MDM transmission over 10-km of weakly-coupled FMF with 10-Gb/s OOK and simple direct detection.