IBM is developing the means to use CMOS transmitter macros - typically employed for driving a 50O digital line - for directly modulating laser light sources in card-to-card optical links.
The technology, already demonstrated driving vertical cavity surface emitting lasers (VCSELs) in a 5Gbit/s serial connection, is intended to reduce the power, area, and cost of optical comms, and make them viable for high-density datacomms applications.
"On the link topology there are things that need to be looked at," said Dr Christoph Berger, who leads the work on optical interconnects at IBM’s Zurich Research Lab. "One of the things we are actively looking at is if we can drive the VCSELs right out of the CMOS process."
VCSELs offer a number of advantages for the optical source, alongside being driven straight off the chip. For example, they emit at around 850nm, which is a good window for the acrylate materials used to build embedded optical waveguides in PCBs, and, unlike telecoms edge emitters, they can also be probed on the wafer, reducing manufacturing costs.
Last year the Zurich lab reported a 10Gbit/s CMOS-driven VCSEL link that consumed 2.5mW/Gbit/s, or one-tenth of the SiGe equivalent.
"For terabit class data flows, for the big machines, you simply have to assume there are hundreds of channels in such systems, because no single device is fast enough for you just to use a few of them," said Berger. "That brings us down to having many, many lasers to operate, and that’s the reason we’re looking at VCSELs, and not at telecom-class edge emitters. We need arrays."
A key problem in using large numbers of densely distributed optical channels is ensuring accurate alignment of the waveguide with the source. Unlike long-haul telecoms links, hand assembly of active components is clearly not an option. While tolerances in standard PCB manufacture are 100-200µm, for boards with integrated optical waveguides they need to be 5-10µm. Using large diameter waveguides to try to relax alignment tolerances affects modal distortion, and therefore link length.
Berger also said some circuit functions are duplicated unnecessarily. For example, chips feature clock data recovery, but commercial optoelectronic modules also do. The bottom line, as always, is cost.
"If it can be done in CMOS it will be done in CMOS," said Berger. "If it can be done in electronics it will be done in electronics."