Nano Switch Routes Light

Using light pulses instead of electrons to send information inside a computer chip may be more attainable as a result of a tiny nanophotonic switch developed at IBM.

With a footprint about 100 times smaller than the cross-section of a human hair, the switch is an important building block to control the flow of information inside future chips and can significantly speed up chip performance while using much less energy, IBM said today.

The company said the advance is one of a several in its quest to develop an on-chip optical network. In 2005, it demonstrated a silicon nanophotonic device that can slow and actively control the speed of light. In 2006, it used an analogous tiny silicon device to demonstrate buffering of more than a byte of information encoded in optical pulses. In December 2007, IBM developed an ultracompact silicon electro-optic modulator, which converts electrical signals into light pulses, a prerequisite for on-chip optical communications.

The silicon broadband optical switch, represented by the black boxes in the figure, performs the key role of 'directing traffic' within the on-chip optical network. Once the electrical signals from each processor core have been converted into pulses of light, the switch devices are set into the necessary positions, as shown by the arrows, for routing the optical messages from the transmitting core to the receiving core. (Image courtesy IBM)

Yurii Vlasov, manager of silicon nanophotonics at IBM's T.J. Watson Research Center, said, "In view of all the progress that this field has seen for the last few years, it looks that our vision for on-chip optical networks is becoming more and more realistic."

He said the device also furthers the company's work to develop high-performance, multicore computer chips that transmit information internally using pulses of light traveling through silicon, instead of electrical signals on copper wires.

In a paper published in the journal Nature Photonics, IBM also unveils a silicon broadband optical switch, another key component required to enable on-chip optical interconnects. Once the electrical signals have been converted into pulses of light, this switching device performs the key role of "directing traffic" within the network, ensuring that optical messages from one processor core can efficiently get to any of the other cores on the chip.

The IBM team said switch has characteristics that make it ideally suited for on-chip applications: It is compact -- as many as 2000 would fit side-by-side in an area of one square millimeter, easily meeting integration requirements for future multicore processors. It can route a huge amount of data, since many different wavelengths, or "colors," of light can be switched simultaneously. With each wavelength carrying data at up to 40 Gb/s, it is possible to switch an aggregate bandwidth exceeding 1 Tb/s -- a requirement for routing large messages between distant cores. The optical switch is also capable of operating within a realistic on-chip environment, in which the temperature of the chip itself can change dramatically in the vicinity of "hot spots," which move around depending on how processors are functioning at any given moment.

"An important trend in the microelectronics industry is to increase the parallelism in computation by multithreading, by building large scale multichip systems and, more recently, by increasing the number of cores on a single chip," IBM said in a statement. For example, the IBM Cell processor that powers Sony's PlayStation 3 gaming console consists of nine "brains," or cores, on a single chip. As users continue to demand greater computing performance, chip designers plan to increase this number to tens or even hundreds of cores.

That approach, however, only makes sense if each core can receive and transmit large messages from all other cores on the chip simultaneously, the company said. "The individual cores located on today's multicore microprocessors communicate with one another over millions of tiny copper wires. However, this copper wiring would simply use up too much power and be incapable of transmitting the enormous amount of information required to enable massively multicore processors," IBM said.

It said the researches are exploring a solution to the problem by connecting cores using pulses of light in an on-chip optical network based on silicon nanophotonic integrated circuits. Like a long-haul fiber-optic network, such an extremely miniature on-chip network will transmit, receive and route messages between individual cores that are encoded as a pulses of light. They envision that by using light instead of wires, as much as 100 times more information can be sent between cores using 10 times less power and generating less heat.

The paper, "High-throughput silicon nanophotonic wavelength-insensitive switch for on-chip optical networks," by Yurii Vlasov, William M. J. Green, and Fengnian Xia of IBM's T.J. Watson Research Center in Yorktown is published in the April 2008 issue of the journal Nature Photonics. The work was partially supported by the Defense Advanced Research Projects Agency (DARPA) through the Defense Sciences Office program Slowing, Storing and Processing Light.

For more information, visit: www.research.ibm.com/photonics