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An OCS is a switch with no transistors. Rotating MEMS mirrors physically redirect beams of light from input fibres to output fibres. The result: the same physical cabling can be re-wired into any topology you want, on the order of milliseconds. Google's TPU v4+ pods and the Jupiter data-centre fabric are the canonical deployments. Toggle between modes; the cyan ring at the centre is the OCS.
Two reasons: power and flexibility. Photons don't care what bit-rate you push through them, so an OCS doesn't need to be upgraded when the line cards do — the same mirrors handle 100G or 800G. And because the switching is physical (no opto-electrical conversion), the per-port power cost is roughly an order of magnitude below an electrical packet switch.
The flexibility lets the operator build a different fabric for different workloads. A TPU pod doing all-reduce-heavy training wants a torus shape; a pod running parameter-server inference wants a hub. The OCS reconfigures inter-block links to match — same hardware, different logical topology.
OCS is circuit-switched, not packet-switched. Once a beam path is set, the connection is point-to-point: no statistical multiplexing, no per-packet routing decisions, no congestion-based rerouting. Reconfiguration takes ~10 ms — fine when topology decisions happen at the granularity of a training run, not per packet. So OCS lives between blocks of packet switches; it doesn't replace them. A TPU v5p pod has packet-switched ICI inside each cube and OCS-switched fibres between cubes.
That hybrid is the move. Packet switches handle micro-bursts; OCS handles topology. NVIDIA's NVL72 takes the opposite bet (all-electrical NVLink Switch — see the fat-tree explorer). It's an architectural disagreement, not a technology gap.
Google has run OCS in production since TPU v4 (Palomar OCS), making the inter-cube fabric in TPU v4/v5p/Trillium pods reconfigurable. The Jupiter fabric extended the same pattern to the data-centre WAN. NVIDIA, AMD, and the major hyperscalers are all evaluating optical fabrics; expect more launches across 2026-2027 as silicon photonics co-packages with switch ASICs.
Compare with the 3D torus, dragonfly, and fat-tree — those are the static shapes OCS can be reconfigured into.