UC Boulder microprocessor talks to its memory with light

2015 has been a big year for photonics. In May, IBM announced a silicon photonics transceiver fabricated using standard silicon fabrication processes.  Now, researchers at the University of Colorado Boulder have unveiled a microprocessor that can talk to its main memory with light. This processor uses traditional electronic circuits in conjunction with 850 optical input/output components to demonstrate the advantages of light-based signaling. It's also worth noting that this chip was produced using traditional silicon manufacturing processes.

Before we go too much further, let's dive into exactly what this chip is doing with light. According to the full journal article in Nature, the brain of the chip is a dual-core, RISC-V CPU linked to a 1MB block of on-die SRAM. The research team says the chip can use its built-in optical transmitters and receivers to allow the CPU and the SRAM to communicate both with one another and with "off-chip components" without the need for separate optical devices. Despite the optical tech on board, the team says the chip is fabricated on a "commercial high-performance 45-nm" CMOS SOI process.

One advantage of using light to carry signals to this chip's memory is an improvement in energy efficiency. According to the research team, light can be transmitted much farther than electricity can for a given amount of power. Another potential advantage is in bandwidth. The researchers claim that they were able to densely pack the chip's communication ports onto its die, thanks to the short wavelength of light the chip uses. That packing results in an impressive-sounding 300Gbps of "bandwidth density per square millimeter," all on a 3-mm by 6-mm die. For perspective, the team claims that this measure is 10 to 50 times greater than what today's microprocessors can achieve.

In a more concrete sense, though, this chip isn't very fast at the moment. In the Nature article above, the researchers say that the CPU runs at "1/80th of the aggregate bit rate" of the optical processor-to-memory (and memory-to-processor) interconnects when it's talking to the SRAM with light, or at about 31MHz. Each of those links can move about 2.5Gbps in that mode.

The researchers behind this project are understandably excited about the applications of the chip. Miloš Popović, one of the study’s authors, claims that "light-based integrated circuits could lead to radical changes in computing and network chip architecture in applications ranging from smartphones to supercomputers to large data centers.” It might take a while for this technology to make it into your desktop computer or phone, but the possibilities are certainly intriguing.

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