optical circuit switch

An optical circuit switch (OCS) is a network switch that routes light directly from an input fiber to an output fiber without ever converting it to electrical signals. Instead of reading packets and forwarding them, it physically steers whole optical beams — classically with tiny tilting MEMS mirrors — to form dedicated light paths, or circuits, between ports. In AI datacenters it provides a reconfigurable optical fabric that can rewire which machines connect to which, complementing the packet network that carries ordinary traffic.\n\n**It eliminates the optical-electrical-optical conversion.** A conventional packet switch is electrical: every arriving beam is converted to electrons by a photodetector, buffered and routed by a switch ASIC, then turned back into light by a laser — the O-E-O cycle. That conversion burns power and adds latency at every hop, and the port speed is capped by the switching silicon. An OCS keeps the signal in the optical domain end to end, so it consumes far less power per port and imposes almost no per-bit latency.\n\n**Because it just bends light, it is transparent — but slow to switch.** Steering a beam with a mirror does not care what bit-rate or modulation format the light carries, so an OCS needs no upgrade when link speeds rise; the same switch passes faster optics unchanged. The catch is that mechanically repositioning mirrors takes on the order of milliseconds, versus nanoseconds for an electrical switch. So an OCS cannot make per-packet decisions; it establishes circuits that persist, and you reconfigure the topology occasionally to match shifting traffic patterns rather than routing each packet.\n\n| | Electrical packet switch | Optical circuit switch |\n|---|---|---|\n| Signal path | O-E-O each hop | stays optical |\n| Decision | per packet | per circuit |\n| Switch speed | ~nanoseconds | ~milliseconds |\n| Bit-rate/format | tied to ASIC | transparent |\n| Power / latency | high per hop | low per hop |\n| Role | general routing | reconfigurable topology |\n\n```svg\n\n```\n\n**In AI clusters it reshapes the fabric to the job.** Large training runs have structured, slowly changing communication patterns, so an OCS can wire the network into the topology a given job wants — a rail-optimized layout for all-reduce, say — and rewire between jobs, delivering high bandwidth without paying O-E-O power on every link. This is the datacenter-scale sibling of on-package optics: silicon photonics and co-packaged optics move light onto the chip's edge, while an OCS switches those optical links between machines. Together they push more of the network into the optical domain where bandwidth is cheap and conversion is the cost.\n\nRead the optical circuit switch through a quant lens rather than a 'fancy fiber switch' lens: it trades switching granularity (millisecond circuits, not nanosecond packets) for near-zero per-hop power and latency and full bit-rate transparency. The design question is how stable your traffic is — if the communication pattern holds long enough to amortize a millisecond reconfiguration, an OCS delivers packet-switch bandwidth without the O-E-O tax; if traffic is bursty and unpredictable, you still need the electrical packet network. It is a power-and-bandwidth optimization that pays off exactly when the topology can be planned.