Optical Interconnects: Lightmatter’s Optical Interposers Could Start Speeding Up AI in 2025

Fiber-optic cables are creeping closer to processors in high-performance computers, replacing copper connections with glass. Technology companies hope to speed up AI and lower its energy cost by moving optical connections from outside the server onto the motherboard and then having them sidle up alongside the processor. Now tech firms are poised to go even further in the quest to multiply the processor’s potential—by slipping the connections underneath it.

That’s the approach taken by
Lightmatter, which claims to lead the pack with an interposer configured to make light-speed connections, not just from processor to processor but also between parts of the processor. The technology’s proponents claim it has the potential to decrease the amount of power used in complex computing significantly, an essential requirement for today’s AI technology to progress.

Lightmatter’s innovations have attracted
the attention of investors, who have seen enough potential in the technology to raise US $850 million for the company, launching it well ahead of its competitors to a multi-unicorn valuation of $4.4 billion. Now Lightmatter is poised to get its technology, called Passage, running. The company plans to have the production version of the technology installed and running in lead-customer systems by the end of 2025.

Passage, an optical interconnect system, could be a crucial step to increasing computation speeds of high-performance processors beyond the limits of Moore’s Law. The technology heralds a future where separate processors can pool their resources and work in synchrony on the huge computations required by artificial intelligence, according to CEO Nick Harris.

“Progress in computing from now on is going to come from linking multiple chips together,” he says.

An Optical Interposer

Fundamentally, Passage is an interposer, a slice of glass or silicon upon which smaller silicon dies, often called chiplets, are attached and interconnected within the same package. Many top server CPUs and GPUs these days are composed of multiple silicon dies on interposers. The scheme allows designers to connect dies made with different manufacturing technologies and to increase the amount of processing and memory beyond what’s possible with a single chip.

Today, the interconnects that link chiplets on interposers are strictly electrical. They are high-speed and low-energy links compared with, say, those on a motherboard. But they can’t compare with the impedance-free flow of photons through glass fibers.

Passage is cut from a 300-millimeter wafer of silicon containing a thin layer of silicon dioxide just below the surface. A multiband, external laser chip provides the light Passage uses. The interposer contains technology that can receive an electric signal from a chip’s standard I/O system, called a serializer/deserializer, or SerDes. As such, Passage is compatible with out-of-the-box silicon processor chips and requires no…