There is much disagreement on the timing of the transition from copper interconnects to optical interconnects, but almost everyone agrees we are moving to all-optical interconnects over the next five years. Optical interconnects already dominate scale-out. Optical interconnects will next move into scale-up by connecting racks in very large pods of hundreds to thousands of XPUs (AI compute units), then optical scale-up will replace copper within the rack. Today, optical is dominated by pluggable transceivers mostly using single wavelengths (DR) with 2 or 4 fibers. Increasing performance using pluggables would require power-hungry higher-speed PHYs (physical layer interfaces) and/or more fibers, which will drive up costs because connectors cost more than fibers. To reduce power and cost while increasing bandwidth, the industry consensus is shifting toward multi-wavelengths per fiber with bidirectionality, using NPO (near-packaged optics) then CPO (co-packaged optics). Alternatives may develop, but the emerging consensus for the future of optical interconnect appears to be OCI-MSA. The optical super highway for AI: OCI-MSA OCI-MSA (Optical Compute Interconnect Multi-Source Agreement) is an industry consortium recently formed for an open, interoperable optical interconnect spec for AI scale-up. Founding members include AMD, Broadcom, Meta, Microsoft, Nvidia, and OpenAI. In that mix are both GPU giants, the largest maker of custom AI compute, two of the hyperscalers, and one of the two Frontier Model companies. This is a big bandwagon. Of these, AMD, Broadcom, Meta, and Microsoft have made presentations on their plans for OCI-MSA at recent industry conferences. The stated objectives for the OCI-MSA are: To create an open, interoperable specification for AI scale-up that breaks the constraints of proprietary interconnects to ensure seamless cross-vendor compatibility for hyperscale AI clusters. To provide architectural guidance to transition the industry to low-power optical SerDes, delivering the latency, bandwidth, and energy efficiency required for next-generation AI. To secure a scalable, multi-vendor AI supply chain, uniting hyperscale end-users and silicon innovators to reduce integration risk, accelerate development, and ensure long-term scale deployments without vendor lock-in. Drew Alduino (Meta’s director of optical infrastructure), Near Margalit (VP & GM of Broadcom’s Optical Systems Division) & Matthew Streshinsky (AMD senior director for photonics) were on a panel in September at the AI Infra Summit. “We want AMD GPUs talking to Broadcom switches,” Alduino said. “We want all of our compute and switches to be interoperable.” Alduino went on to say: Meta wants to standardize as little as possible to achieve interoperability while making it possible for everyone to rapidly innovate. Multi-wavelength means we don’t ever need faster SerDes for faster optical. Meta wants OCI to look like Ethernet today. Gen 1 OCI has 4 wavelengths of 50Gbps in each direction. Gen 2 is in definition and will double the bandwidth, while Gen 3 will double bandwidth again. 200G × 16 wavelengths has already been shown to be “viable,” which is 16 × the bandwidth of Gen 1. As Near Margalit noted, what matters for interoperability is what appears on the fiber. OCI-MSA Gen 1 The spec for Gen 1 of OCI-MSA can be found at oci-msa.org . OCI is workable for pluggables, NPO, CPO in the package, or CPO on the interposer. Fig. 1: OCI-MSA Gen 1 has 8 channels per fiber, 4 in each direction. Source: oci-msa.org The OCI-MSA spec is 24 pages. The Optical specification provides for 8 wavelengths per fiber, with 4 in each direction. Each wavelength is 53.125Gbps NRZ. The wavelengths in the groups are spaced slightly more than 2nm apart. The reference optical fiber link model is based on 500m SMF-28 fiber with 2.5 dB total insertion loss, mainly driven by the connector losses in the link between transceivers. (The effective data rate is 50Gbps per wavelength after