Credit: Caltech/Liang Feng |
Stretching
for thousands of miles beneath oceans, optical fibers now connect every
continent except for Antarctica. With less data loss and higher
bandwidth, optical-fiber technology allows information to zip around the
world, bringing pictures, video, and other data from every corner of
the globe to your computer in a split second. But although optical
fibers are increasingly replacing copper wires, carrying information via
photons instead of electrons, today’s computer technology still relies
on electronic chips.
Now,
researchers led by engineers at the California Institute of Technology
(Caltech) are paving the way for the next generation of computer-chip
technology: photonic chips. With integrated circuits that use light
instead of electricity, photonic chips will allow for faster computers
and less data loss when connected to the global fiber-optic network.
“We
want to take everything on an electronic chip and reproduce it on a
photonic chip,” says Liang Feng, a postdoctoral scholar in electrical
engineering and the lead author on a paper to be published in the August
5 issue of the journal Science.
Feng is part of Caltech’s nanofabrication group, led by Axel Scherer,
Bernard A. Neches Professor of Electrical Engineering, Applied Physics,
and Physics, and co-director of the Kavli Nanoscience Institute at
Caltech.
In
that paper, the researchers describe a new technique to isolate light
signals on a silicon chip, solving a longstanding problem in engineering
photonic chips.
An
isolated light signal can only travel in one direction. If light
weren’t isolated, signals sent and received between different components
on a photonic circuit could interfere with one another, causing the
chip to become unstable. In an electrical circuit, a device called a
diode isolates electrical signals by allowing current to travel in one
direction but not the other. The goal, then, is to create the photonic
analog of a diode, a device called an optical isolator. “This is
something scientists have been pursuing for 20 years,” Feng says.
Normally,
a light beam has exactly the same properties when it moves forward as
when it’s reflected backward. “If you can see me, then I can see you,”
he says. In order to isolate light, its properties need to somehow
change when going in the opposite direction. An optical isolator can
then block light that has these changed properties, which allows light
signals to travel only in one direction between devices on a chip.
“We
want to build something where you can see me, but I can’t see you,”
Feng explains. “That means there’s no signal from your side to me. The
device on my side is isolated; it won’t be affected by my surroundings,
so the functionality of my device will be stable.”
To
isolate light, Feng and his colleagues designed a new type of optical
waveguide, a 0.8-micron-wide silicon device that channels light. The
waveguide allows light to go in one direction but changes the mode of
the light when it travels in the opposite direction.
A
light wave’s mode corresponds to the pattern of the electromagnetic
field lines that make up the wave. In the researchers’ new waveguide,
the light travels in a symmetric mode in one direction, but changes to
an asymmetric mode in the other. Because different light modes can’t
interact with one another, the two beams of light thus pass through each
other.
Previously,
there were two main ways to achieve this kind of optical isolation. The
first way—developed almost a century ago—is to use a magnetic field.
The magnetic field changes the polarization of light—the orientation of
the light’s electric-field lines—when it travels in the opposite
direction, so that the light going one way can’t interfere with the
light going the other way. “The problem is, you can’t put a large
magnetic field next to a computer,” Feng says. “It’s not healthy.”
The
second conventional method requires so-called nonlinear optical
materials, which change light’s frequency rather than its polarization.
This technique was developed about 50 years ago, but is problematic
because silicon, the material that’s the basis for the integrated
circuit, is a linear material. If computers were to use optical
isolators made out of nonlinear materials, silicon would have to be
replaced, which would require revamping all of computer technology. But
with their new silicon waveguides, the researchers have become the first
to isolate light with a linear material.
Although
this work is just a proof-of-principle experiment, the researchers are
already building an optical isolator that can be integrated onto a
silicon chip. An optical isolator is essential for building the
integrated, nanoscale photonic devices and components that will enable
future integrated information systems on a chip. Current,
state-of-the-art photonic chips operate at 10 gigabits per second
(Gbps)—hundreds of times the data-transfer rates of today’s personal
computers—with the next generation expected to soon hit 40 Gbps. But
without built-in optical isolators, those chips are much simpler than
their electronic counterparts and are not yet ready for the market.
Optical isolators like those based on the researchers’ designs will
therefore be crucial for commercially viable photonic chips.
In addition to Feng and Scherer, the other authors on the Science
paper, “Non-reciprocal light propagation in a silicon photonic
circuit,” are Jingqing Huang, a Caltech graduate student; Maurice Ayache
of UC San Diego and Yeshaiahu Fainman, Cymer Professor in Advanced
Optical Technologies at UC San Diego; and Ye-Long Xu, Ming-Hui Lu, and
Yan-Feng Chen of the Nanjing National Laboratory of Microstructures in
China. This research was done as part of the Center for Integrated
Access Networks (CIAN), one of the National Science Foundation’s
Engineering Research Centers. Fainman is also the deputy director of
CIAN. Funding was provided by the National Science Foundation, and the
Defense Advanced Research Projects Agency.
Non-reciprocal light propagation in a silicon photonic circuit
