Long-Haul WDM/SDM Transmission Over Coupled 4-Core Fibers Installed in Submarine Cable

We demonstrated long-haul transmissions over coupled four-core fibers (C4CFs) installed in a 15-km-long submarine cable. A small propagation-loss coefficient of 0.16 dB/km and spatial-mode dispersion coefficient of 5.1 ps/
${\sqrt{\mathbf{km}}}$
were confirmed after cabling. A recirculating loop consisting of two spans of a 60-km C4CF were constructed using eight C4CFs installed in the submarine cable. 16-channel wavelength-division multiplexed (WDM) and 4-core space-division multiplexed (SDM) 32-Gbaud polarization-division-multiplexed quadrature phase shift keying signals were transmitted. By using adaptive multi-layer filters that compensate for mode coupling as well as in-phase and quadrature impairments in transmitters and receivers, we confirmed that error-free transmission after forward error correction was obtained up to 6000 km. We also evaluated the transmission performance during long-term measurement and the characteristics of mode dependent loss (MDL). The root-mean square MDL averaged over frequency within a WDM channel was estimated to be 0.8 dB per two-span loop and temporally fluctuated after long-haul transmission. These results indicate that C4CFs are viable for long-haul submarine application and that MDL should be further suppressed for a longer distance and more stable performance. We also evaluated a WDM/SDM transmission of probabilistic-constellation-shaped 16-quadrature amplitude modulation signals. Depending on the information rate from 3.2 to 1.2 b per symbol per mode, error-free transmission was achieved up to 2160 and 6600 km.