PORTLAND, Ore. Optical interconnections on silicon herald a future in which photons will replace electrons to shuttle high-speed data streams between multiple microprocessor cores.
A key component is an electro-optical modulator that permits one core's electrical output to modulate a silicon laser beam into a coded stream of pulses that can be routed to the input of any other core.
IBM's T.J. Watson Research Center (Yorktown, N. Y.) said Thursday (Dec. 6) it has succeeded in shrinking a 10 Gbit/sec Mach-Zehnder electro-optic modulator down to 100 microns, at power levels comparable to today's discrete optical devices.
"We are reporting two important advances. The first is a factor of 1,000 smaller footprint on a CMOS die, and the second is the lowest reported RF power for a silicon electro-optical modulator," said William Green, an IBM research scientist. "For on-chip applications, this scaling is now really comparable with existing optical devices used in telecommunications.
Green added that "we also plan to take this technology--our ability to control optical signals electrically--forward to the many other kinds of electrically controllable devices we need for on-chip optical networks, such as silicon optical switches."
Electro-optical modulators allow an electrical data stream to control the opening and closing of an optical shutter, thereby encoding digital electrical information onto a laser by chopping it into a series of light pulses. There are numerous ways of implementing a silicon electro-optical modulator. For instance, IBM and others have fabricated ring resonator-based modulators five times smaller than the one IBM reported on Thursday. But ring modulators cannot be as easily manufactured.
"We chose the Mach-Zehnder because it is based on an interferometer that is broadband, rather than a resonator like a ring modulator," said Green. "Mach-Zehnder modulators are insensitive to environmental fluctuations like temperature, or even fabrication variations in size, which makes it ideal for a manufacturing environment where you have to have some reasonable tolerances for size and temperature variations."
Previously reported Mach-Zehnder modulators have been up to a centimeter in length, primarily because the injection of carriers that control the refractive index were inefficient when interacting with the light. By going to a deeply scaled nanophotonic waveguide structure, IBM was able to make that interaction much more efficient, resulting in the shrinking of the device's length to as little as 100 microns.
"Our key challenge was shrinking it down and showing that it was possible to make at a size that approaches the size of a ring modulator. Now we are about five times bigger, but we think we can someday make Mach-Zehnder modulators of comparable size to ring modulators," said Green.
Next, the IBM team plans to optimize the architecture in an attempt to lower RF power consumption from milliamps to microamps. That way, current discrete devices can be integrated onto complete optical networks on a chip.
"We need to continue to reduce the power consumption by making the electrical control more efficient so that we can put hundreds of modulators, switches and other optical devices on the same CMOS chip," said Green. "We also need to explore various architectures for incorporating CMOS devices into optical networks with interfaces appropriate for what the chip designers want."