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Sumitomo 2C Z-PLUS ULL Fiber

2C Z-PLUS ULL Fiber is Sumitomo Electric Industries' submarine 2-core (dual-core) ultra-low-loss optical fiber, developed and manufactured as the fiber for Meta's Petal cable — the first subsea system to deploy multi-core fiber at scale. Sumitomo is "a pioneer with nearly four decades of experience in ultra-low loss submarine fiber manufacturing." (Source: sources/2026-09-21-meta-inside-petal-building-the-worlds-first-petabit-class-transoceanic-subsea-cable)

The two engineering problems

Packing two light-carrying cores into a single fiber strand — while doubling capacity per strand — hinges on solving two problems:

  1. Low attenuation at the standard 125 µm diameter. The fiber must keep the 125 µm outer width (about the diameter of a human hair) so it drops into the existing subsea cable/repeater envelope. Sumitomo achieves low loss using ultra-pure synthetic silica in the preform. A preform "2–3 m long and 20 cm wide" is drawn to "1000s of kilometers long and 125 µm wide."

  2. Minimal inter-core crosstalk. The two cores must not leak into each other. Sumitomo controls for high refractive indexes in the cores against lower indexes in the surrounding medium and counter-propagates the optical signals in the two cores — resulting in "nearly immeasurable crosstalk" and optical performance "nearly identical to single-core fiber."

Counter-propagation is the multi-core analogue of the single-core industry practice of using a pair of fibers to send traffic in opposite directions; Petal's 2-core fiber "combines this capacity into one fiber strand."

Role in Petal

Sumitomo supplies the fiber to NEC (Petal's turnkey system supplier). At the repeaters, each 2-core fiber is split back into two single-core fibers for amplification via a Fan-In/Fan-Out (FIFO) interface (see systems/nec-sdm-repeater), so the low-loss/low-crosstalk 2-core fiber pairs with efficient single-core amplification end to end.

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