Lightmatter Connects Chiplets Optically

Lightmatter’s Passage substrate is an active photonic interposer for interconnecting chiplets. All the photonic components and supporting electrical circuits reside in a single multi-reticle piece of silicon.

Bryon Moyer

Lightmatter’s photonic interconnect platform enables dense high-speed photonic links between processor chiplets for high-performance computing (HPC). Disclosed at Hot Chips, the Passage technology operates as an active photonic interposer, allowing high-bandwidth chiplet assemblies while abstracting away the photonic details. It’s a single piece of silicon spanning multiple reticles.

In its computing work, the company found that the interconnect between processors was a bottleneck impeding additional processor gains, so it turned its attention to balancing performance. By creating a silicon-photonics-based interposer, it makes those connections optically using silicon waveguides for higher bandwidth. Conversion between the electrical and optical domains occurs in the interposer, making it look like a standard electrical interposer. Passage has active components, however, allowing it to reroute connections during operation.

Lightmatter is a Boston-based startup whose primary focus has been photonic computing for HPC and AI. After launching the Envise computing platform, it completed an $80 million Series B funding round last year, bringing its total to $113 million.

Passage increases bandwidth—normally limited to 3.2Tbps per fiber array—to 768Tbps per chiplet site. Signals can route anywhere with a single hop, and 2ns maximum latency makes retiming unnecessary. The initial panel runs at 21W, less than 10% of a typical HPC chip’s power.

Lightmatter implements Passage designs on a custom basis; GlobalFoundries fabricates them. It may establish an off-the-shelf version, given market demand. The company withheld pricing.