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UC Berkeley scientists develop processor with photonic I/O

Engineers at the University of California, Berkeley have fabricated a microprocessor with integrated photonic interconnects, a first according to the researchers. The 3 x 6mm chip contains two processor cores with 70 million transistors and 850 photonic components.

The microprocessor was fabricated with mass production equipment, showing it can be scaled up easily. The research was described in a paper published in Nature.

‘This is a milestone. It’s the first processor that can use light to communicate with the external world,’ said Vladimir Stojanovic, an associate professor of electrical engineering and computer sciences at UC Berkeley, who led the development of the chip. ‘No other processor has the photonic I/O in the chip.’

In demonstrating the chip, the engineers showed that it had a bandwidth density of 300 gigabits per second per square millimetre, about 10 to 50 times greater than packaged electrical-only microprocessors currently on the market.

‘This is the first time we’ve put a system together at such scale, and have it actually do something useful, like run a program,’ said UC Berkeley professor Krste Asanovic, who helped develop the free and open architecture called RISC-V (reduced instruction set computer), used by the processor.

The photonic I/O on the chip is also energy efficient, using 1.3 picojoules per bit, equivalent to consuming 1.3W of power to transmit a terabit of data per second. In the experiments, the data was sent to a receiver 10 metres away and back.

‘The advantage with optical is that with the same amount of power, you can go a few centimetres, a few metres or a few kilometres,’ said study co-lead author Chen Sun, a recent UC Berkeley PhD graduate from Stojanovic’s lab at the Berkeley Wireless Research Center. ‘For high-speed electrical links, one metre is about the limit before you need repeaters to regenerate the electrical signal, and that quickly increases the amount of power needed. For an electrical signal to travel 1km, you’d need thousands of picojoules for each bit.’

One potential application of this technology is to make data centres more energy efficient. According to the Natural Resources Defense Council, data centres consumed about 91 billion kilowatt-hours of electricity in 2013, about 2 per cent of the total electricity consumed in the United States.

The research has already spun off two startups this year with applications in data centres in mind. SiFive is commercialising the RISC-V processors, while Ayar Labs is focusing on photonic interconnects. Earlier this year, Ayar Labs – under its previous company name of OptiBit – was awarded the MIT Clean Energy Prize. Ayar Labs is getting further traction through the Citris Foundry start-up incubator at UC Berkeley.

The photonic I/O components included a silicon ring modulator, a silicon-germanium photodetector, and a vertical grating coupler to connect the chip to the outside world.

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