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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.