Silicon photonics adds off-chip comms to a RISC-V processor

A group of researchers have developed a microprocessor that uses silicon photonics-based optics to send and receive data.

"For the first time a system - a microprocessor - has been able to communicate with the external world using something other than electronics," says Vladimir Stojanovic, associate professor of electrical engineering and computer science at the University of California, Berkeley. 

Vladimir Stojanovic

The microprocessor is the result of work that started at MIT nearly a decade ago as part of a project sponsored by the US Defense Advanced Research Projects Agency (DARPA) to investigate the integration of photonics and electronics for off-chip and even intra-chip communications.     

The chip features a dual-core 1.65GHz RISC-V open instruction set processor and 1 megabyte of static RAM and integrates 70 million transistors and 850 optical components.

The work is also notable in that the optical components were developed without making any changes to an IBM 45nm CMOS process used to fabricate the processor. The researchers have demonstrated two of the processors communicating optically, with the RISC core on one chip reading and writing to the memory of the second device and executing programs such as image rendering.

This CMOS process approach to silicon photonics, dubbed 'zero-change' by the researchers, differs from that of the optical industry. So far silicon photonics players have customised CMOS processes to improve the optical components' performance. Many companies also develop the silicon photonics separately, using a trailing-edge 130nm or 90nm CMOS process while implementing the driver electronics on a separate chip using more advanced CMOS. That is because photonic devices such as a Mach-Zehnder modulator are relatively large and waste expensive silicon real-estate if implemented using a leading-edge process.  

IBM is one player that has developed the electronics and optics on one chip using a 90nm CMOS process. However, the company says that the electronics use feature sizes closer to 65nm to achieve electrical speeds of 25 gigabit-per-second (Gbps), and being a custom process, it will only be possible to implement 50-gigabit rates using 4-level pulse amplitude modulation (PAM-4).

We are now reaping the benefits of this very precise process which others cannot do because they are operating at larger process nodes

"Our approach is that photonics is sort of like a second-class citizen to transistors but it is still good enough," says Stojanovic. This way, photonics can be part of an advanced CMOS process.

Pursuing a zero-change process was first met with skepticism and involved significant work by the researchers to develop. "People thought that making no changes to the process would be super-restrictive and lead to very poor [optical] device performance," says Stojanovic. Indeed, the first designs produced didn't work. "We didn't understand the IBM process and the masks enough, or it [the etching] would strip off certain stuff we'd put on to block certain steps." 

But the team slowly mastered the process, making simple optical devices before moving on to more complex designs. Now the team believes its building-block components such as its vertical grating couplers have leading-edge performance while its ring-resonator modulator is close to matching the optical performance of designs using custom CMOS processes. 

"We are now reaping the benefits of this very precise process which others cannot do because they are operating at larger process nodes," says Stojanovic.     

Silicon photonics design

The researchers use a micro ring-resonator for its modulator design. The ring-resonator is much smaller than a Mach-Zehnder design and is 10 microns in diameter. Stojanovic says the dimensions of its vertical grating couplers are 10 to 20 microns while its silicon waveguides are 0.5 microns. 

Photonic components are big relative to transistors, but for the links, it is the transistors that occupy more area than the photonics. "You can pack a lot of utilisation in a very small chip area," he says.

A key challenge with a micro ring-resonator is ensuring its stability. As the name implies, modulation of light occurs when the device is in resonance but this drifts with temperature, greatly impairing its performance. 

Stojanovic cites how even the bit sequence can affect the modulator's temperature. "Given the microprocessor data is uncoded, you can have random bursts of zeros," he says. "When it [the modulator] drops the light, it self-heats: if it is modulating a [binary] zero it gets heated more than letting a one go through." 

The researchers have had to develop circuitry that senses the bit-sequence pattern and counteracts the ring's self-heating. But the example also illustrates the advantage of combining photonics and electronics. "If you have a lot of transistors next to the modulator, it is much easier to tune it and make it work," says Stojanovic.  

A prototype set-up of the chip-to-chip interconnect using silicon photonics. Source: Vladimir Stojanovic

Demonstration

The team used two microprocessors - one CPU talking to the memory of the second chip 4m away. Two chips were used rather than one - going off-chip before returning - to prove that the communication was indeed optical since there is also an internal electrical bus on-chip linking the CPU and memory. "We wanted to demonstrate chip-to-chip because that is where we think the biggest bang for the buck is," says Stojanovic.

In the demonstration, a single laser operating at 1,183nm feeds the two paths linking the memory and processor. Each link is 2.5Gbps for a total bandwidth of 5Gbps. However the microprocessor was clocked at one-eightieth of its 1.65GHz clock speed because only one wavelength was used to carry data. The microprocessor design can support 11 wavelengths for a total bandwidth of 55Gbit/s while the silicon photonics technology itself will support between 16 and 32 wavelengths overall. 

The group is already lab-testing a new iteration of the chip that promises to run the processor at full speed. The latest chip also features improved optical functions. "It has better devices all over the place: better modulators, photo-detectors and gratings; it keeps evolving," says Stojanovic.

We can ship that kind of bandwidth [3.2 terabits] from a single chip

Ayar Labs

Ayar Labs is a start-up still in stealth mode that has been established to use the zero-change silicon photonics to make interconnect chips for platforms in the data centre. 

Stojanovic says the microprocessor demonstrator is an example of a product that is two generations beyond existing pluggable modules. Ayar Labs will focus on on-board optics, what he describes as the next generation of product. On-board optics sit on a card, close to the chip. Optics integrated within the chip will eventually be needed, he says, but only once applications require greater bandwidth and denser interfaces.

"One of the nice things is that this technology is malleable; it can be put in various form factors to satisfy different connectivity applications," says Stojanovic. 

What Ayar Labs aims to do is replace the QSFP pluggable modules on the face plate of a switch with one chip next to the switch silicon that can have a capacity of 3.2 terabits. "We can ship that kind of bandwidth from a single chip," says Stojanovic.

Such a chip promises cost reduction given how a large part of the cost in optical design is in the packaging. Here, packaging 32, 100 Gigabit Ethernet QSFP modules can be replaced with a single optical module using the chip. "That cost reduction is the key to enabling deeper penetration of photonics, and has been a barrier for silicon photonics [volumes] to ramp," says Stojanovic.

There is also the issue of how to couple the laser to the silicon photonics chip. Stojanovic says such high-bandwidth interface ICs require multiple lasers: "You definitely don't want hundreds of lasers flip-chipped on top [of the optical chip], you have to have a different approach".  

Ayar Labs has not detailed what it is doing but Stojanovic says that its approach is more radical than simply sharing one laser across a few links, "Think about the laser as the power supply to the box, or maybe a few racks," he says.

The start-up is also exploring using standard polycrystalline silicon rather than the more specialist silicon-on-isolator wafers. 

"Poly-silicon is much more lossy, so we have had to do special tricks in that process to make it less so," says Stojanovic. The result is that changes are needed to be made to the process; this will not be a zero-change process. But Stojanovic says the changes are few in number and relatively simple, and that it has already been shown to work. 

Having such a process available would allow photonics to be added to transistors made using the most advanced CMOS processes - 16nm and even 7nm. "Then silicon-on-insulator becomes redundant; that is our end goal,” says Stojanovic.    

Further information

Single-chip microprocessor that communicates directly using light, Nature, Volume 528, 24-31 December 2015

Ayar Labs website