Deep|Optics: 6D Torus Points to a 4x Uplift in Optical Content per TPU

Key takeaway

Supply-chain rumors suggest Google’s training-oriented TPU 9t might raise the torus dimensionality to 6D while retaining the torus architecture, with deployment targeted for 2027–2028. If realized, wrap-around optical ports per TPU on the scale-up side would rise from 1.5 (3D) to 6, a fourfold increase in OCS ports and optical module content per TPU; combined with the 800G→2.4T Coherent Lite speed and modulation upgrade, the value uplift would exceed 4x. This direction aligns with the two evolution paths we laid out in our earlier report.

I. The Two Evolution Paths from Our Earlier Report

In Deep|AI Infra 2026, we argued that, in the agent era, the system bottleneck is shifting from single-chip compute to system-wide communication efficiency. MoE shifts the communication pattern from All-Reduce toward bisection-bandwidth-hungry All-to-All. Per the GShard paper, as experts scale from 128 to 2,048, All-to-All time for MoE dispatch/combine rises from 16% to 36% of MoE + Transformer layer execution time. On that basis, we expected Google to push along two lines:

1. Scale-out expansion: the number of in-rack TH6 switches rises from one to four–six per rack, DCN bandwidth up 4–6x, the scale-out OCS port ratio moves from 0.2:1 to 1:1, and the optical module ratio increases from 1:1–1:2 to 1:3–1:4;
2. Scale-up dimensionality: a 3D torus’s bisection bandwidth grows only as N^(2/3); assuming the torus architecture is retained, adding dimensions is the main practical route to higher bisection bandwidth, with the OCS Port-to-xPU ratio rising from 1.5:1 toward 2:1–10:1.

II. TPU 8t/8i and Virgo: How the Thesis Is Playing Out, Layer by Layer

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