Space-division multiplexing (SDM)

A single-mode fiber carries information in amplitude, phase, two polarizations, and many wavelengths, and coherent transmission with wavelength-division multiplexing already uses all of them. What remains is space: more than one spatial path through the same glass. Space-division multiplexing is the umbrella term for the ways of doing that, and it is motivated by the nonlinear limit on what one core can carry: raising the launch power stops helping once fiber nonlinearity degrades the signal as fast as the power improves it, so further capacity must come from more paths rather than more power per path.

There are three families. Uncoupled multi-core fiber places several single-mode cores far enough apart that each behaves as its own fiber, and the governing specification becomes the crosstalk between cores. Few-mode and multimode fibers instead carry separate signals on different spatial modes of one larger core; the modes exchange power as they propagate, so the channels are mixed and must be separated at the receiver by multiple-input multiple-output (MIMO) equalization, the same kind of processing a coherent DSP applies to the two polarizations, extended to many more signals. Coupled-core fibers sit between the two: cores placed close enough to couple strongly, handled as a set of supermodes with MIMO, but with a smaller spread of propagation delays than a multimode core. A fourth option, simply more fibers in the cable, is SDM in the broad sense and remains the one most widely deployed.

The families trade in different currencies. In uncoupled fiber the crosstalk accumulates with length, and the receiver has no way to undo it. Adding crosstalk in power, a fiber specified at −50 dB/km reaches −20 dB after 1000 km, a level that begins to cost margin for high-order formats. In coupled and few-mode systems crosstalk is part of the channel and is removed by the DSP, but the equalizer must span the differential delay between the fastest and slowest paths, a form of modal dispersion, so fiber designs minimize it and the MIMO complexity grows with both the mode count and the distance.

The economic case is sharing. Seven cores behind one cladding do not by themselves cost less per bit than seven fibers; the saving comes when amplifiers, lasers, and the packaging around them are shared. Cladding-pumped or multi-core erbium-doped amplifiers that serve every core from one pump, and transceivers that address several cores through one fan-in, are the components that turn the extra paths into a lower cost per bit. Submarine cable, where the capacity per cable cross-section and the electrical power fed from shore are both fixed, is where that case has been strongest.

References: D. J. Richardson, J. M. Fini, L. E. Nelson, Nat. Photonics 7, 354 (2013); B. J. Puttnam, G. Rademacher, R. S. Luís, Optica 8, 1186 (2021).