于让尘

AI optical interconnects are in a supercycle driven by a 10x gap: compute has grown roughly 300x since 2022, while total optical-connection bandwidth has grown only about 30x. TeraHop Vice President 于让尘 compares compute with gray matter and connectivity with white matter: the two occupy nearly a 1:1 ratio in the human brain, while the white-matter-to-gray-matter ratio in cats and dogs is roughly 10%-20%; “we may still be at the mouse-and-kitten stage.” Optical interconnects currently account for only a single-digit percentage of data-center capex, but he argues that connectivity investment must grow faster than compute: “this gap has to be filled, and the entire industry has to work on it together.”

Scale up is the industry’s biggest structural opportunity: bandwidth is roughly 10x scale out, while copper is nearing its physical limits. Nvidia’s all-copper NVL72 rack “is already constraining the brain’s development”; the industry is moving from the 576-chip systems now coming online, and 1,000-chip systems, toward Google’s interconnection of 9,000 TPUs. Copper can currently run only 1.5-2 meters; in the 400G/lane era, “even 1 meter will be a stretch,” pointing to a long-term shift toward optical replacing copper.

The economics need to be calculated backwards: compare output per token, not the cost of optics versus copper. 于让尘 cites the industry rule to “use copper when you can, use optics when you must,” but stresses that end customers care about returns per unit of compute and per token. GPUs cost $30K-$50K, versus several thousand dollars for an optical module; “if you save on interconnect, you are sacrificing your compute.”

The technology roadmap is not a religious either-or: “Internet companies are practical; they do not make religious choices.” A single company may choose pluggable optics, NPO and CPO at the same time—“adults don’t do multiple-choice questions” (“成人不做选择题”). Google has said publicly that “CPO is always two years away, and every year it is still two years away.” The major cloud companies are all developing proprietary chips—TPU, MTIA, Trainium, Maia, OpenAI Titan and xAI—which makes them receptive to an open optical-interconnect ecosystem. CPO’s failure to reach scaled commercial deployment is partly because customers dislike vertical integration and suppliers gain too much bargaining power, while the technical bottlenecks remain formidable.

TeraHop is positioning itself around open standards: its 12.8T XPO and Open CPO-X multi-source protocols can deliver 4-8x bandwidth growth, with about 100 companies already participating. The case for the technology rests on silicon-photonics maturity: more than 1,000 eight-channel products are already in use, making 16, 32 and 64 channels a natural progression. Nvidia’s $4B investment in Lumentum and Coherent in early March was interpreted by 于让尘 as concern that the supply chain could not keep up with 10x growth—“locking down the supply chain with real money.”

OCS is a greenfield market Google began developing roughly 10 years ago, while other major players remain at zero or very low adoption—but electrical switching is not going away. Optical switching cuts power, latency and cost, but it switches slowly and is suited only to predictable workflows. 于让尘 is explicit in his caution: it “cannot completely replace” nanosecond-scale electrical switching over the next 5 years; “never say never,” and the two technologies will coexist over the long term. OCS’s 2-3dB loss makes coherent light a good pairing, and Google’s strategic investment arm has backed the coherent-light startup Celero.

The industry’s biggest bottleneck is that semiconductor integration remains early-stage, while the value chain will soon become a trillion-dollar question. Lasers are still made on 3-inch wafers and are working toward 6-inch, versus 12-inch wafers for silicon photonics; quantum-dot lasers could eventually put the light source directly on silicon. Over the next 5 and 10 years, the direction is semiconductor-based optical interconnects and deep optoelectronic integration, including TSMC’s COUPE platform. Interconnects should be treated as part of compute itself and ultimately become an equally important component.

🔗 Original source & video: Silicon Valley Coordinates x Innolight’s 于让尘: The AI Optical Interconnect Supercycle