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piece of the future internet has surfaced in a lab in Japan: a memory chip that stores bits of light.
Researchers at Japanese telecom giant NTT have built an optical random access memory (o-RAM) chip — a conceptual cousin to the electronic memory in your computer. The goal is not to make a light-speed replacement for DRAM. That’s out of the realm of possibilities for the foreseeable future. Rather, the idea is to make fast, efficient storage buffers for internet routers and the communications switches that connect thousands of servers in data centers.
The NTT researchers built a 4-bit prototype that operates at 40 gigabits per second. If the technology were scaled up, a 1 megabit device would take up a square centimeter and consume less than 100 milliwatts. “Our RAM is just a 4-bit memory. We need to increase the scale of integration,” says NTT researcher Masaya Notomi.
NTT is targeting 10-kilobit to 1-megabit memory chips for future all-optical routers. According to Notomi, the prototype shows that these goals are reasonable in terms of size and power consumption. Getting to that scale will take time. The company expects to reach 10 kilobits by around 2020, and 1 megabit by around 2025.
Optical RAM doesn’t have to hold very much to be useful for networking. Buffers on future optical routers could be very small compared to the electronic memory used today, says Nick McKeown, an electrical engineering and computer science professor at Stanford. McKeown calls a device that can hold 500 kilobits and read and write data at 100 gigabits per second “very interesting.”
Each cell in NTT’s o-RAM is a nanoscale photonic crystal — a type of material that channels light in very small spaces. Varying the intensity of the light entering the chip switches the material between transparent and opaque, two states that can represent 1s and 0s in digital signals.
The NTT researchers’ advance was to embed a tiny piece of indium gallium arsenide phosphate inside the photonic crystal, creating a nanocavity. The nanocavity is very efficient at the light switching activity, which makes the device very energy-efficient. Storing a bit takes 30 nanowatts, which is 300 times lower than the next most efficient optical memory.
The photonic crystal is made of indium phosphide, which isn’t as efficient as indium gallium arsenide phosphate at light switching but is very good at dissipating heat. This greatly increases the amount of time the device can store a bit. The NTT device increased storage time over previous attempts by more than seven orders of magnitude, from 250 nanoseconds to 10 seconds.
Much of the Internet runs over optical fiber today, but that doesn’t mean the ‘Net runs at the speed of light. The internet’s traffic cops — routers — slow things down considerably. Data enters and leaves most routers on beams of light. But inside those boxes, where packets containing your diced up Facebook updates and Google searches are shunted toward their destinations, the traffic hits a speed bump. Routers convert optical signals to electronic signals, which are sorted on specialized computer chips. This conversion slows the whole process.
All-optical routers are faster and more energy-efficient than today’s electronic devices. The challenge is making all-optical routers that are compact enough and inexpensive enough to be commercially viable. A key ingredient is optical storage on a chip: o-RAM.
NTT’s bi-stable switch is one of several horses in the race to develop optical memory chips, says Daniel Blumenthal, an electrical and computer engineering professor at the University of California, Santa Barbara. Other researchers are developing holographic memory, light trapping, ring resonators and delay lines. Blumenthal is a lead investigator for a DARPA-funded project to build delay lines in chips. Delay lines are long, tightly coiled waveguides that send light pulses the long way around to delay them.
It’s also not clear what role all-optical routers will play in the future Internet, says Blumenthal. “It depends on what the future Internet is going to look like.”
The key question is how much the future Internet will use routers that move packets as quickly as possible and how much it will use routers that make decisions based on the content of the traffic. “This re-architecting of the Internet is really going to dictate where all-optical buffers and routing can take a place. And that still remains to be seen,” says Blumenthal.
This uncertainty isn’t going to slow down the drive to develop optical memory, however. Internet routing isn’t even the technology’s main driving force. That distinction belongs to the explosive growth of data centers, which use similar technology to connect the thousands of servers that store and process the internet’s data. The growth of data centers and the convergence of telecommunications and data center communications has driven up demand for photonic devices, says Blumenthal. “The router world will have to keep up with the data center world.”
Another driving force is the future of Internet servers and supercomputers: many-core chips. These chips require fast communications between processor cores, leading to a network-on-a-chip architecture. “Optical RAM will be implemented inside ultrafast many-core CPU chips with a photonic network design,” says NTT’s Notomi. “This strategy will [ultimately] need to introduce optical routing in a chip, and then optical RAM will be called for.”