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One of the most intriguing chips presented at HOT CHIPS earlier this summer was Lightmatter's Passage. I covered it briefly in my summary post about the conference HOT CHIPS Day 1: Hot Chiplets. Steve Klinger, Lightmatter's VP of product, presented an update recently at EDPS.
One of the big challenges with silicon photonics is that optical fibers are massive compared to on-chip nanophotonic waveguides. See the image above. That's not the only issue. Optical fiber attachments are expensive and have low throughput. Since fibers are big, the maximum fiber count that can be attached is a couple of hundred.
The current state-of-the-art is to use chiplets and co-packaged optics, but this doesn't really scale and rapidly becomes a rats' nest.
Lightmatter has created an alternative approach that doesn't require the fiber. Instead of linking up the chiplets using co-packaged optics, they mount the chiplets (customer ASICs) on Passage. This is a sort of optical interposer complete with photonics, electronics, and lasers. The chiplets, instead of requiring fiber to be attached, communicate in the vertical dimension, and photonics is used to configure the interposer, with just a few fibers to be attached.
Passage is actually manufactured on an SoI 300mm wafer. This is then diced up into 48 tiles. There is 768Tbps within Passage and 128Tbps per fiber attach. The user can then attach up to 8 chiplets using standard chip-on-wafer packaging, with SerDes (today) or UCIe (2023).
Above is a cross-section. On the left is the fiber array. On top are two ASICs and two HBM memory stacks. One signal is shown, coming down from the ASIC with SerDes, then optically going across Passage, and then back up into the second ASIC. Up to 700W of power can be delivered per tile through TSVs.
He didn't say much about the lasers, but generally, you can't build lasers out of silicon, usually, it is Indium Phosphate (InP). And it's working, as you can see from the photograph below, the world's first photonic wafer-scale interconnect.
This has nothing directly to do with Lightmatter...but it certainly has to do with silicon photonics. Cadence's annual photonics event and workshop, Photonics: Riding the Waves Along the Spectrum, will be on January 11th and 12th, 2023. Details, including a link for registration (already open), are on the event page.
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