In context: AMD was first to market with a 1 GHz processor back in March 2000. Intel beat them to 2 GHz not long after in August 2001 and they also got to 3 GHz first in November 2002. After nine long years AMD hit back with a 4 GHz capable FX-6200 and it took them two more years to cobble together the 5 GHz FX-9590 in 2013. In the six years since things haven’t sped up much from a clock rate point of view.
The rate of progress is slowing, and in the not-too-distant future we’ll need something new. That something new could be chip-scale photonic computing – the integration of light-based hardware to improve performance.
A team of scientists sponsored by the Nippon Telegraph and Telephone Corporation, a Japanese telecom, have made a titanic breakthrough with photonic technology. For the first time, photonics has performance and specs competitive with electronic hardware.
The photonics you might see in the next decade will use light to transfer information and electronics to process it. For example, an electrical signal will reach an Electric to Optic (E-O) device, converting it into light. That light is transferred then hits an Optic to Electric (O-E) device that converts the light into a current, which can be processed or sent to the next E-O device.
The main challenges scientists have faced are the power requirements, which can exceed a thousand times the requirements of electronic processing, and speed, because each time the light is absorbed it must go into a capacitor. That capacitor must fill up and discharge fully to pass the signal on, but up until now, it’s been very challenging to build a capacitor small enough for that to happen quickly.
The research team made leaps and bounds and finally matched silicon hardware in terms of performance and power requirements.
They were able to build an Electro-Optic Modulator (E-O) that runs at 40 Gbps that uses just 42 attojoules per bit, meaning it consumes over an order of magnitude less power than the best of the previous experiments. It outperforms them, too, with about half the capacitance at less than a femtofarad.
They then constructed a photoreceiver (O-E) based on the same technologies, and that was able to run at 10 Gbps using two orders of magnitude less power than other optical systems at just 1.6 femtojoules per bit. It’s also the first to not require an amplifier (which saves power) and have a low capacitance at just a few femtofarads.
Combining the two, they demonstrated the world’s first O-E-O ‘transistor.’ It can function as an all-optical switch, a wavelength converter, and a repeater. The incredible versatility makes it the first device that provides benefits over electronic hardware at chip-scale. The researchers suggest it could be used for inter-core communication and to sustain cache coherency.
The scientists were able to make this breakthrough by developing a new type of photonic crystal (a term meaning a synthetic insulating material that controls light), and it’s a piece of silicon with a bunch of holes drilled in it. The holes are arranged such that if the light goes through them it interferes with itself causing it to cancel out. If a line of holes is blocked, then the light goes follows the path and is funneled into light-absorbing material that converts it into a current. The same system also works in reverse.
It’s hard to understate just how exciting this breakthrough is. Up until now, the only role photonics has played in the data center is long-range communication, targeting distances from 500m to 10km. Recent announcements like Intel’s 400G have shrunk that distance to room-scale, with board-scale known to be in the works. But bringing photonics down to chip-scale makes the technology consumer accessible and has the potential to rewrite the rulebook when it comes to performance. After all, light is faster than electronics.
While photonic technology is only just matching electronic hardware, it's in its infancy and will improve rapidly. That being said, it could be a decade before chip-scale photonic technology gets into the hands of the public, but it will be an exciting day when it does.
Femtofarad optoelectronic integration demonstrating energy-saving signal conversion and nonlinear functions, Nature: Photonics (April 2019)