Hot Conferences Feature Cool Optics

Source: Hot Chips

The accelerated life cycles that AI is driving woke up a normally sleepy August. Held virtually, Hot Interconnects (HotI 2025) spanned three days with a mix of invited talks, sponsor talks, and tutorials. Some of the brief sponsor talks merely previewed larger disclosures at Hot Chips, which was held at Stanford University the following week. This year, Hot Chips’ agenda included an Optical session that featured three startups plus Nvidia. It also included Networking and Machine Learning (ML) sessions with talks from leading vendors.

Although Nvidia was the marquee name in Hot Chips' Optical session, Gilad Shainer's talk on co-packaged silicon photonics lacked any new technical details on the company's CPO switches. Instead, the company used the event to announce Spectrum-XGS, which extends its Spectrum-X Ethernet solution across data centers. Nvidia calls this "scale-across" networking because it primarily targets data center clusters, but it is effectively the same as DCI. Spectrum-XGS adds new algorithms that account for the propagation delay across longer links and adjust load balancing to minimize latency jitter. Nvidia says that deep-buffer approaches to DCI switching, such as Broadcom's recently announced Jericho4, introduce jitter that is detrimental to AI-workload performance.

Not generating tens of billions of dollars in sales per quarter, the startups are understandably more aggressive in their disclosures. Founded a decade ago, Ayar Labs has developed three generations of its TeraPHY optical chiplets for CPO. Vladimir Stojanovic, co-founder and CTO, presented Ayar's new TeraPHY UCIe optical retimer chiplet. The new chiplet advances Ayar's design in two important aspects: doubling bandwidth to 8Tbps and implementing the UCIe die-to-die interface for integration with XPUs or ASICs.

Lightmatter's Passage M1000 takes a more radical approach to system integration, and Chief Scientist Darius Bunandar revealed many new details at Hot Chips. The M1000 breaks the traditional ASIC-beachfront paradigm by serving as an interposer and using 3D connections to ASICs.

Whereas Ayar and Lightmatter promoted the benefits of micro-ring modulators (MRMs), Celestial AI made the case that electro absorption modulators (EAMs) are the way to go for CPO. CTO Phil Winterbottom presented new details of Celestial's Photonic Fabric Module, which the company claims is the first system-on-a-chip (SoC) with in-die optical I/O. Like Lightmatter, Celestial uses 3D integration, stacking an electrical IC (EIC) on top of a photonic IC (PIC).

Of the startups, Ayar appears closest to achieving high-volume manufacturing, due in part to its modest integration approach. Celestial and Lightmatter represent more radical architectures, which promise higher areal density and greater power efficiency. At the same time, they require customers to codesign their ASICs to work with bespoke PICs/interposers, locking the ASIC design to the vendor's platform. The question, then, is what XPU team will take the enormous risk associated with such a CPO design.

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