205.8-Tb/s Weakly-Coupled 2-Mode 7-Core Transmission Over 1170-km FM-MCF Only Using 2×2 MIMO-DSP

Space-division multiplexing (SDM) technology by exploring fiber cores and linearly-polarized (LP) modes in few-mode multicore fibers (FM-MCF) as spatial channels is highly expected to break the capacity bottleneck of long-haul optical fiber transmission systems. However, the utility of LP modes is seriously impeded by huge computation complexity induced by inter-modal multiple-input multiple-output digital signal processing (MIMO-DSP) when the modal crosstalk in the fiber link is not strictly suppressed. In this paper, we propose a sparse mode-division multiplexing (MDM) scheme for weakly-coupled FM-MCF long-haul transmission, in which only a set of non-adjacent LP modes in each fiber core are selected to be active channels to significantly suppress inter-modal crosstalk. A multiple-ring-core 6-LP-mode 7-core fiber with low crosstalk is first designed and fabricated. The all-fiber spatial multiplexer and demultiplexer matched with the weakly-coupled FM-MCF are realized, which achieve low insertion loss and low crosstalk among all the spatial channels. Then, experimental recirculating-loop FM-MCF transmission system is established to verify the feasibility of the proposed sparse-MDM scheme. 205.8-Tb/s transmission over 1170-km 2-mode (LP₀₁/LP₀₂) 7-core weakly-coupled FM-MCF is experimentally demonstrated only utilizing 2×2 MIMO-DSP. 14 SDM × 180 wavelength-division multiplexing (WDM) channels across C-band bearing 24.5-Gbaud dual-polarization quadrature phase shift keying (DP-QPSK) signals are adopted. The proposed scheme could achieve high compatibility with conventional single-mode optical transceivers, and may pave the way for near-term long-haul SDM transmission applications.