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Spatial division multiplexing

Definition

Spatial division multiplexing (SDM) is the optical-networking strategy of scaling a fiber-optic link's total capacity by increasing the number of parallel spatial paths carrying light — rather than squeezing more bits into a single path. In subsea cables this historically meant packing more fiber pairs into one cable; the frontier extends it to multiple cores within a single fiber strand (multi-core fiber).

SDM is the industry's answer to the Shannon limit: for decades, subsea capacity grew by improving per-fiber spectral efficiency (coherent optical transmission, dispersion-uncompensated cable designs drove "10x and more" gains through the 2010s), but that per-fiber ceiling — set by information theory — was approached and "finally pushed back." Once you can't reliably send more bits down one path, the remaining lever is more paths: more fibers, or more cores per fiber. (Source: sources/2026-09-21-meta-inside-petal-building-the-worlds-first-petabit-class-transoceanic-subsea-cable)

Why it matters

  • It moves the scaling axis from spectral efficiency to physical parallelism. When a single fiber is near its Shannon ceiling, doubling capacity means doubling the count of independent optical paths — a fundamentally different (and more manufacturable) engineering problem than chasing marginal bits-per-Hertz.
  • The constraint shifts from bandwidth to power and physical space. More fibers/cores need more optical amplification, and subsea repeaters are power-limited (see concepts/submarine-cable). SDM designs live or die on capacity per watt, not just raw capacity — the central trade-off in Meta's Petal design.

SDM scaling ladder (Meta subsea cables)

Meta's transatlantic cables are a clean illustration of SDM scaling by fiber count, then by core count:

Cable SDM approach Capacity
Marea 8 fiber pairs (first Meta transatlantic)
Amitié 16 fiber pairs —
Anjana 24 fiber pairs 0.5 Pbps (first 0.5 Pbps transatlantic)
Petal 24 pairs of 2-core fiber (= 48-pair equivalent) 1 Pbps

Three ways to double from Anjana's 0.5 Pbps were on the table (Source: sources/2026-09-21-meta-inside-petal-building-the-worlds-first-petabit-class-transoceanic-subsea-cable):

  1. More fibers — go to 48 fiber pairs (conventional SDM).
  2. Wider optical band — use the L-band in addition to the C-band (24-pair C+L transmission, as deployed on the PLCN Pacific cable).
  3. Multi-core fiber — 2-core fiber in a 24-pair system, doubling capacity per fiber strand without proportional power/infrastructure growth.

Petal chose option 3, making it "the first subsea cable system to deploy multi-core fiber technology at scale."

Multi-core fiber as the SDM frontier

Multi-core fiber packs two (or more) light-carrying cores into a single 125 µm strand — the standard outer diameter — so capacity roughly doubles without changing the cable's physical envelope. The two hard problems (see systems/sumitomo-2c-zplus-ull-fiber):

  • Attenuation must stay low while holding the 125 µm diameter → ultra-pure synthetic silica preforms.
  • Inter-core crosstalk must be minimized → high-index cores in a lower-index medium plus counter-propagating signals, yielding "nearly immeasurable crosstalk."

To keep amplification efficient, repeaters split multi-core fiber back into single-core fibers via a Fan-In/Fan-Out (FIFO) interface, amplify with a pump-sharing architecture, then recombine (see systems/nec-sdm-repeater).

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