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September 09, 2010

IBM's light-powered links overcome the greatest speed bump in supercomputing: interconnect bandwidth

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IBM has developed a first-of-its-kind optical data-transfer system, or bus, built right onto the circuit board. IBM will unveil computer systems 100 times as fast as anything available today. Scientists will be able to use those system to be able to visualize things in detail: how the climate will react to man-made greenhouse gases, how neurons organize to form a brain, how to custom design a drug to treat an individual patient.

1000-petaflop (Pflop) or one exaflop systems will require 400 million optical links. Supercomputer processing rates have been going up tenfold every four years. So you can expect 10-Pflop, 100-Pflop, and 1000-Pflop (exaflop) systems to emerge in 2012, 2016, and 2020, respectively. The Blue Waters system—starting at 10 Pflops, and scalable to 16 Pflops—is slightly ahead of the trend, with a scheduled production date of 2011.


The scale of the printed circuit cards and backplanes used in today's supercomputers is about 10 to 200 centimeters. For optical buses to compete with copper interconnects at this scale, the performance and efficiency of optics must become much better than they are. Optical modules with 12 parallel channels now cost several dollars per gigabit per second, offer speeds from 60 to 120 Gb/s with an efficiency of 30 to 100 picojoules per bit, and occupy about two-thirds of a square centimeter on the chip. To be practical for 100-Pflop machines, these modules must be improved on all fronts by a factor of 10 and will have to be scaled commensurately beyond that for exaflop computers.


IBM has made two different optochips, one operating on optical signals at 985 nanometers and the other at 850 nm. They both use vertical-cavity surface-emitting lasers (VCSELs) as the light source. These lasers can be modulated at speeds of up to 40 Gb/s, and they emit light vertically through mirrors, along a path perpendicular to the chip. The lasers are particularly inexpensive to manufacture, in part because they are fabricated in two-dimensional arrays and so can be tested right on the wafer, before it has gone through further processing steps. (VCSELs are so inexpensive—just pennies per device—that they're now found in nearly all optical mice.) The first commercial parallel modules employing VCSELs appeared in the late 1990s and operated at about 1 Gb/s per channel, but the market started to take off only around 2003, when such interconnects began to link racks in computers and routers. The data rates quickly rose, recently reaching 10 Gb/s per channel.

The 985 nm optochip operates at up to 15 Gb/s per channel—the previous record was 10 Gb/s per channel. And all of its 16 channels together provide an aggregate bandwidth of 240 Gb/s while consuming just 2.2 watts of power.

Besides its blazing speed, the optochip can send and receive a single bit with a paltry 9 pJ, an efficiency that's about five times that of the modules now on the market. It's also able to transmit a lot of information per unit of area, achieving 28 Gb/s per square millimeter. That's the highest bandwidth density achieved to date and at least 10 times that of the latest commercial parallel optics. Density is obviously critical in large systems: If you're going to have 100 million optical links, they'd better be small.

A technical motive behind IBMs selection of the 850-nm transceivers. Less light is lost in the polymer waveguides at 850 nm than at 985 nm. That loss matters, particularly at links of up to a meter long—the length we had resolved to support in our Terabus project. On that scale, by working at 850 nm instead of 985 nm you get a whopping six times as much optical power through to the receiver.

IBMs 850-nm design gives half again as many channels as the previous chip did—24 transmitters and 24 receivers, each operating at up to 15 Gb/s per channel. Together they achieve a bidirectional data rate of 360 Gb/s while consuming 2.3 W of power. The power efficiency, at 6.5 pJ per link, beats that of the 985-nm optochips, though the bandwidth density, at about 9 Gb/s/mm2, is only about a third as good. All in all, it's a good bargain, because the 850-nm optochip was specifically designed to use only components that can be mass-produced at low cost, using lasers and photodetectors that are available today.

Our transceiver technology clearly achieves bandwidth and power efficiencies way beyond the capabilities of anything on the market today. Can it go from the laboratory to high-volume manufacturing? We're confident that it can: Our transceivers incorporate CMOS electronics, optoelectronic devices, and microchip packaging techniques that are already in production today. Product development should be straightforward.

Within five years, IBM hopes to connect microprocessors and memory chips right to the optochip, producing the optical analogue to the electrical multichip modules in today's big-iron machines. In these near-future supercomputers, electrical connections will supply only the power, the ground, and the control signals. All the data will shoot through optical interconnects at the speed of light.


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