201.6-Bit/s/Hz Spectral-Efficiency Transmission over a 180-μm Cladding-Diameter Seven Ring Core Fiber Using Low-Complexity 4×4 MIMO Equalization

Space-division multiplexing (SDM) has been widely studied to increase the per-fiber capacity/spectral efficiency (SE) and break the nonlinear Shannon limit of the single-mode fiber (SMF). In spite of the achieved dramatic progress, the high SE of reported SDM systems is realized generally at the cost of a large fiber cladding diameter or high-complexity multiple-input multiple-output (MIMO) processing. In this paper, SDM transmission with a net spectral efficiency over 200 bit/s/Hz is experimentally demonstrated utilizing a low-complexity modular MIMO equalization with a dimension of 4×4 and a time-domain length of 15. A 14-km seven ring-core fiber (SRCF) is employed in the SDM systems, supporting 84 orbital angular momentum (OAM) mode channels (7 cores × 6 OAM modes × 2 polarizations) within a cladding diameter of 180μm, each of which carries 40 wavelengths. With a 12-Gbaud 8-quadrature amplitude modulated (8QAM) signal carried in each data channel, a raw (net) SE of 241.92 (201.6) bit/s/Hz and capacity of 120.96 (100.8) Tbit/s can be achieved, where the bit-error rate (BER) of all channels is below the 20% soft-decision forward error correction (FEC) threshold of 2.4×10 ^-2 .