Optical Circuit Switching (OCS) in AI Data Centers: Cabling Implications
An optical circuit switch (OCS) connects fibers to fibers with moving mirrors, without converting light to electrical signals. It can rewire a fabric in software, but every path through it adds optical loss and possible reflections. For cabling, that means single-mode fiber, a tighter loss budget, high return loss on every connector, and careful labeling. One published hyperscale deployment reports up to 2 dB insertion loss per pass through its switch.
What an Optical Circuit Switch Does
A packet switch reads every packet. An OCS does not: it points light from an input fiber to an output fiber and holds that path until it is told to change it.
| Electrical packet switch | Optical circuit switch | |
|---|---|---|
| Works on | Packets | Light paths (circuits) |
| Conversion | Optical → electrical → optical | None, passive path |
| Data rate | Fixed by the switch | Independent of rate and format |
| Reconfiguration | Per packet | Slow, milliseconds (published deployment) |
| Main cabling effect | Transceiver at each port | Adds loss and reflections to the link |
In the published deployment, the switch uses 3D MEMS mirrors with 136 × 136 ports (maker/operator data). It sits where a fixed patch panel or shuffle box would sit, between groups of switches, so the fabric can be rewired without touching cables.
Why Single-Mode and Bidirectional Links
- Single-mode fiber. MEMS mirror switches are built for single-mode light. Plan OS2 trunks to and from the OCS. See OS2 vs OM3 vs OM4 vs OM5.
- Circulators. The published deployment adds an optical circulator at each transceiver, so transmit and receive share one fiber. This halves the OCS ports and fibers needed (operator data). Cabling then counts single fibers per link, not pairs.
- Reflections matter. With both directions on one fiber, reflected light reaches the receiver. The operator reports typical OCS return loss of −46 dB and a specification better than −38 dB, and notes that reflections degrade PAM4 signals. Every connector on the path needs high return loss; single-mode APC is the usual choice. See APC vs UPC.
Loss Budget With an OCS
Example at specification limits for a 500 m single-mode link with four mated connections:
| Item | Without OCS | With OCS |
|---|---|---|
| 4 connections × 0.35 dB (Huawellux limit) | 1.40 dB | 1.40 dB |
| 0.5 km OS2 × 1.0 dB/km (TIA-568.3-D inside-plant limit) | 0.50 dB | 0.50 dB |
| OCS, worst case (published deployment) | — | 2.00 dB |
| Total (derived) | 1.90 dB | 3.90 dB |
The OCS roughly doubles the passive loss in this example. Compare the total with the transceiver's channel limit, and cut connections where possible. Use the insertion loss budget guide.
Cabling Checklist for OCS Fabrics
- Port count. Size trunks to the OCS port count and the number of switch groups it connects.
- Fiber count per link. With circulators, one fiber per link; without, two. Confirm before ordering trunks.
- Connector loss. Every 0.1 dB saved per connection helps when the OCS takes 2 dB.
- Return loss. Specify single-mode APC with high return loss on every connection in the path.
- Labels. The software changes the paths, so physical labels must name ports, not destinations. See fiber cable labeling.
- Testing. Test each trunk for loss and return loss before connecting it to the OCS.
For the wider picture, see AI cluster fiber cabling and co-packaged optics.
Frequently Asked Questions
What is an optical circuit switch?
A switch that connects input fibers to output fibers with moving mirrors or similar optics, without converting the signal to electrical form. Paths change in milliseconds in the published deployment.
Does an OCS replace packet switches?
No. It reconfigures the connections between packet switches. Packet switches still route the traffic.
How much loss does an OCS add?
One published deployment reports up to 2 dB insertion loss per pass, typically below 2 dB. Add it to the link budget with connectors and fiber.
Does an OCS need single-mode fiber?
MEMS mirror switches are built for single-mode light, so plan OS2 cabling to and from the switch.
Why is return loss important with an OCS?
When links are bidirectional on one fiber, reflections reach the receiver and degrade PAM4 signals. Use connectors with high return loss.
Huawellux Single-Mode Trunks for OCS Fabrics
Huawellux builds OS2 MPO and LC trunks and patch cords to length, with APC or UPC end-faces, port labels and polarity to your drawing. Huawellux can also supply bend-insensitive fiber. Send the port count, fiber count per link, connector type and lengths, and we confirm the build in the quotation.
Huawellux's specification for MPO and MTP® assemblies (identical for both) is insertion loss ≤0.35 dB for single-mode and multimode, with return loss >60 dB single-mode (OS2, APC) and >30 dB multimode (OM3/OM4/OM5). LC and SC patch cords are listed at insertion loss ≤0.35 dB and return loss ≥50 dB (single-mode UPC), ≥60 dB (single-mode APC) and >30 dB (multimode). Every Huawellux product receives end-face visual inspection under a 400x video microscope and is tested for insertion loss (IL) and return loss (RL) before it ships. Interferometric end-face geometry (fiber height, coplanarity, end-face angle and radius) is checked by sample-based testing in standard production, and 100% interferometer testing of a batch is available on request. Dust caps are fitted on every connector, and a formal test report is available on request.
Huawellux replies to inquiries within 24 hours. Custom orders typically take 1–5 business days to complete production, with final timing confirmed case by case in the quotation. For special/custom requirements, MOQ is 2 km of fiber length. Standard listed patch cords have no formal MOQ. The sample fee is credited against the bulk order.
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