Photonics & optoelectronics

Optical waveguides and silicon nanophotonics pave the way to energy-efficient supercomputers


The Photonics group at IBM Research – Zurich has developed an integrated optical interconnect technology to reduce drastically the energy consumption of future IT systems. The high-speed, low-loss features of optical interconnects will allow significantly greater bandwidths. We have taken the electrical signals from processors and converted them into optical light signals that are distributed via optical waveguides onto printed circuit boards. This is paving the way for integrating optical functions and components directly into processors.

Supercomputers comprise increasing numbers of high-performance processors to handle an ever-growing data processing workload. In 2012, IBM is planning the release of its Blue Gene/Q supercomputer “Mira” featuring 750,000 processor cores. In view of this trend, electrical interconnects are becoming insufficient to transmit the enormous amount of bits in system-internal data streams. This means that losses increase and that the high-performance electronic components needed to compensate for these losses generate a considerable amount of heat, which in turn requires energy-intensive cooling. In addition, if conventional copper lines are placed too densely, electromagnetic interference distorts the transmitted signals. Optical data communication is the solution to this bottleneck. By converting electrical signals into light pulses via optoelectronics, optical data transmission provides the necessary signal quality even at extremely high data rates.

We approached this problem first by intercepting the processor’s electrical signals from the edge of the circuit board and rerouting them through a converter. Then we positioned optoelectronic components directly next to the processor and connected them with optical fibers. Now we are working on integrating the optical components into the processor package and printing optical waveguides onto the circuit board.

Our optical module features extremely short connections, which enable efficient signal transmission. The principle is as follows: Electrical signals from the processor are routed to a chip adjacent to the processor, where they are transformed into optical signals using an array of tiny lasers. The light pulses generated in this way are directed by minute mirrors into the optical waveguide. Analogously, detectors receive the optical signals and transform them back into electrical signals for the processor.

The optical module reaches input and output rates of 160 gigabits per second between two processors with 32 integrated waveguides. This corresponds to twice the data transmission rate between computers generally available today. This system can be scaled by simply placing several optical modules around a processor. In practice, optical chip-to-chip connections will first be applied to high-end computing. We are optimistic that this technology could be ready for use in commercial systems within a few years.