PCI Express explores PC tech that will run on light

Anyone who’s bought or researched buying a new SSD for their PC understands their limitations: they’re often constrained by the PCI Express bus to the rest of the PC, and they generate tons of heat. The architects of PCI Express are thinking about a solution: replacing PCIe’s electrical interconnect with one that uses light instead.

The PCI Special Interest Group (PCI-SIG) is forming a workgroup to develop a standardized optical interconnect that could replace the electrical bus found in PCs. Replacing electrons with photons could save power and heat by reducing resistance, as well as improving the overall performance of the PC itself.

PCI Express is possibly the most important bus technology within the PC, connecting its most important components: the processor, the external GPU, and the SSD. Any changes to the PCI Express specification dramatically improve the PC’s performance. A shift from electricity to light, however, would be revolutionary.

Don’t expect optical PCIe to arrive any time soon, however, either as a specification or as an actual product. For one thing, the PCI Express 7 specification is currently scheduled to be released in 2025, offering a whopping 128 gigatransfers per second. But as we bemoaned in our roundup of the best internal SSDs, the state of the art is a PCI Express 4 SSD — a standard, incidentally, that was finalized in 2017.

It’s also unclear what technology the SIG will adopt, either. In its statement, the SIG described its goals in brief terms: “this new optical workgroup will work to make the PCIe architecture more optical-friendly,” it said. Traditionally, the SIG’s role is to simply publish a standard, ensuring interconnectivity and allowing manufacturers to work to a common standard.

Why would PCI Express use light?

Using optics to transmit data has been in the works for over a decade. Intel, for example, first talked about a technology called Light Peak as an optical replacement for USB, a specific goal it later abandoned. But by 2010 Intel had begun showing off optical interconnects inside PCs that could hit 50Gbits per second. The technology, known as silicon photonics, could form the basis for an optical PCIe interconnect.

Why use light instead of electrons? As one paper summed up, the thought that Moore’s Law was coming to an end prompted new approaches. Moore’s Law — which postulates that transistor density on a chip doubles every 12 to 18 months, and which Intel chief executive Pat Gelsinger has vowed to continue and even beat — forced chip designers to consider multi-core chips instead. But connecting all of those cores requires a sophisticated interconnect, and that interconnection scheme can actually consume more than 80 percent of a chip’s power. That justifies a hard look at an optical replacement for that electrical interconnect.

It’s likely that PCI-SIG’s work will benefit servers and cloud computing first, which can shoulder the high costs of early hardware. But if the PCI SIG’s work comes to fruition, it’s possible that someday our PCs will herd photons as well as electrons inside them.