IBM takes on-chip interconnects to light speed

07 Mar 2008

As IBM's prototype optical interconnect technology hits the headlines, optics.org spoke to IBM researcher Clint Schow to find out what's really under the bonnet.

Researchers at IBM have unveiled an integrated optical module that they claim could transfer information between chips at rates of up to 8 Tbit/s. That kind of bandwidth could be put to work in supercomputing clusters and data centres, and would also allow large datasets to be shared more easily – whether for computer-intensive scientific investigations or for consumers sharing high-definition movies.

"Last year we unveiled an optical transceiver chipset that could transmit high-definition movies in under a second using highly customized optical components and processes," said IBM researcher Clint Schow. "We've now connected those high-speed chips through printed circuit boards with dense integrated optical 'wiring', and also increased the speed of the transceiver."

The optical "wiring" consists of an array of low-loss polymer optical waveguides fabricated on a standard printed circuit board (PCB) to form what IBM calls an "Optocard". "The polymer waveguides are individual 35 x 35 µm 'optical wires' that interconnect each transmitter and receiver pair," explained Schow. "Our Optocard incorporates 48 of these optical wires at a density of 16 channels/mm."

Standard photolithographic processing is used to pattern the individual polymer waveguides on the surface of the PCB, and the light is coupled into/out of the waveguides using microlenses and turning mirrors.

Meanwhile, the new high-speed optical transceiver exploits a parallel arrangement of 24 transmitters and 24 receivers, each operating at 12.5 Gbit/s. This yields a total bidirectional data transfer rate of 300 Gbit/s, almost double that of the company's earlier device. Compared to current commercial optical modules, the transceiver provides 10 times more bandwidth, but in a package that is 10 times smaller.

These and other components are integrated together using a hybrid packaging approach, in which flip-chip packaging is used to assemble chips fabricated in different materials and/or different processes on the same substrate. The end result is a highly integrated 3D module that incorporates 32 optical data links, each one with a capacity of 10 Gbit/s.

Schow says that combining multiple modules together makes it possible to achieve transfer rates of 8 Tbit/s for a total aggregate power consumption of just 100 W. For a typical 100 m data link, IBM's optics-based approach consumes 100 times less power than today's electrical interconnects, and one-tenth of the power of current commercial optical modules.

Roadmap to commercialization
Although the existing module is still a prototype, IBM says that it has made significant progress towards developing a commercial version of the technology. In particular, the 24 x 24 transceiver array now exploits industry-standard 850 nm vertical-cavity surface emitting lasers (VCSELs) rather than customized components.

"The transceiver technology could be commercialized in a relatively short timeframe, on the order of one to two years," said Schow. "The Optocard technology that adds polymer optical waveguides to standard PCBs is less mature and may take up to five years before commercial versions become available."

Integrating the 850 nm VCSELs and photodiodes into the module required a silicon chip carrier to be incorporated in the transceiver package, which allows the transceiver modules to be mounted on the PCB in a similar way to that used for electrical chip carriers. "The silicon carrier is an integral part of the hybrid packaging approach and provides a platform for the compact and dense integration of the CMOS driver and receiver chips with the laser and photodetector chips," commented Schow.

The hybrid packaging approach should also be adaptable to high-volume manufacturing. "Although we have increased the number of components as well as the number of channels, the soldering process and assembly technology is similar to that used for volume production of components and should be readily manufacturable," commented Schow.

The next stage for the work will be to transfer the laser driver and receiver circuits to a more advanced generation of CMOS, which would further improve both the power consumption and the data transfer rate. "We also plan on continuing to use industry-standard 850 nm optical components to facilitate commercialization," said Schow.

IBM has a long history of research into high-bandwidth parallel optics, and the current breakthroughs were achieved as part of a DARPA-funded programme, launched in 2003, to demonstrate high-bandwidth chip-to-chip interconnects through polymer waveguides integrated on a printed circuit board.