Multi-core fiber (MCF)

A multi-core fiber packs several single-mode cores into one cladding, most practically four cores in the standard 125 µm diameter, so existing cabling, coating, and much of the handling ecosystem carries over while capacity per fiber multiplies. It is the first commercially serious form of space-division multiplexing, the one axis of capacity scaling left after amplitude, phase, polarization, and wavelength have been spent, and submarine systems are its beachhead: seabed cable is voracious for capacity per cross-section, and trans-oceanic four-core systems entered deployment in the mid-2020s.

The governing specification is inter-core crosstalk, quoted in decibels per length, typically better than about -50 dB/km for uncoupled designs at standard pitch. Crosstalk accumulates statistically with distance and bends, worsens toward longer wavelengths, and sets the reach over which cores can be treated as independent channels. The design levers are core pitch, index trenches around each core, and deliberate propagation-constant mismatch between neighbors. A separate branch accepts coupling on purpose: coupled-core fibers let the cores mix and undo the mixing with MIMO processing in the coherent receiver, buying lower loss and better bend behavior at the price of joint detection.

The unglamorous hard parts are at the ends. Every MCF link needs fan-in fan-out devices to map cores to standard fibers, each adding its own loss and crosstalk, and splicing requires rotational alignment so cores land on cores, which standard cleave-and-fuse tooling only partly automates. Measurement practice adds a matrix habit: loss and crosstalk are per-core and per-pair quantities, so a four-core span is characterized as a 4x4 picture rather than a single number, with a marker-core convention to keep orientation unambiguous.

References: B. J. Puttnam, G. Rademacher, R. S. Luís, Optica 8, 1186 (2021); T. Hayashi et al., J. Lightw. Technol. multi-core design and crosstalk literature.