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."