Hank Hogan, email@example.com
DAVIS, Calif. – By splitting
up one big, tough problem into many smaller and easier parts, researchers at the
University of California, Davis, may have created the basis for ultrafast optical
communications. Team leader S.J. Ben Yoo noted that the new approach could offer
speeds in the multiple-terabits-per-second line rates, orders of magnitude faster
than is now possible.
“The device can eventually scale up to 10,000 times the
speed of standard electronics at approximately 10 GHz,” Yoo said.
The new technique combines spectral slicing with parallel detection
or generation of an optical signal. Done in one direction, devices employing this
approach could perform real-time optical waveform measurement, useful for investigating
ultrafast optical phenomena or receiving an incoming signal. When run the other
way, those devices could be used for transmission, creating a communication link.
For researchers, the method solves the problem of how to measure
full-field ultrafast optical waveforms. In making such measurements, it’s
not enough to capture intensity changes. Phase must be measured as well. Drawbacks
of existing methods that do both are that they update too slowly to gauge what is
happening on subpico-second timescales, require too much pulse energy or can measure
only for a few nanoseconds’ duration.
Yoo credited the idea for a new solution to graduate student Nicolas
K. Fontaine, lead author of a Nature Photonics paper covering the group’s
work and published online on Feb. 28, 2010.
In it, the researchers described their scheme and demonstrated
it using a silica planar lightwave circuit and balanced photodiodes. The first component
sliced an incoming arbitrary light waveform into separate spectral segments. The
researchers combined those slices with a coherence-ensuring reference signal from
an optical frequency comb, with one reference signal per spectral slice.
silica planar lightwave circuit, researchers divided an incoming arbitrary optical
waveform into more manageable parts. The approach could form the basis for terabits-per-second
line rates. Courtesy of S.J. Ben Yoo, University of California, Davis.
They fed everything into the second part, the photodiodes, using
four-quadrature spectral interferometry for measurement. Finally, they took that
output, sent it through CMOS digital signal processing circuitry and reconstructed
the original incoming signal.
After building the prototype, they tested it, generating an array
of complex and different waveforms in the 1.55-μm telecommunications waveband.
The test waveforms included static, rapidly changing, bright and dark varieties,
Yoo said. “The measurement system works extremely well and captures complex
waveforms across a wide dynamic range.”
In doing this, they demonstrated an instantaneous bandwidth >160
GHz, with record lengths of 2 µs. This length-to-resolution ratio of more than 320,000:1
represents the largest of any single-shot, full-field measurement technique, the
group stated in the paper.
Yoo noted that the technique could be used for a number of applications,
including for ultrahigh-speed or highly secure optical communication, lidar and
multicolor coherent femtosecond spectroscopy. Of these, he said that high data rate
communication is likely to be the most compelling.
One advantage of the new approach, aside from speed, is that it
potentially can be done in a single-silicon photonic integrated circuit. Being able
to achieve this will make the resulting chips more attractive and useful for all
The process of integration will involve two steps, the first being
the merging of the silica planar lightwave circuit and balanced photodiodes into
a hybrid-integrated chip. That intermediate integration, Yoo said, the group could
do relatively soon.
The second step will involve combining the digital signal processing
circuitry with everything on the first intermediate chip. That involves making the
fabrication of the photonic components process fully compatible with large-scale
But it will be worth the additional work, Yoo predicts. “Integration
of both photonic and electronic circuits on the same silicon CMOS platform opens