Caroline Ross, the Toyota Professor of Materials Science and Engineering at MIT. Photo: Allegra Boverman |
There
has been enormous progress in recent years toward the development of photonic
chips—devices that use light beams instead of electrons to carry out their
computational tasks. Now, researchers at the Massachusetts Institute of
Technology (MIT) have filled in a crucial piece of the puzzle that could enable
the creation of photonic chips on the standard silicon material that forms the
basis for most of today’s electronics.
In
many of today’s communication systems, data travels via light beams transmitted
through optical fibers. Once the optical signal arrives at its destination, it
is converted to electronic form, processed through electronic circuits and then
converted back to light using a laser. The new device could eliminate those
extra electronic-conversion steps, allowing the light signal to be processed
directly.
The
new component is a “diode for light,” says Caroline Ross, the Toyota Professor
of Materials Science and Engineering at MIT, who is coauthor of a paper
reporting the new device that was published online in Nature Photonics.
It is analogous to an electronic diode, a device that allows an electric
current to flow in one direction but blocks it from going the other way; in this
case, it creates a one-way street for light, rather than electricity.
This
is essential, Ross explains, because without such a device stray reflections
could destabilize the lasers used to produce the optical signals and reduce the
efficiency of the transmission. Currently, a discrete device called an isolator
is used to perform this function, but the new system would allow this function
to be part of the same chip that carries out other signal-processing tasks.
To
develop the device, the researchers had to find a material that is both
transparent and magnetic—two characteristics that rarely occur together. They
ended up using a form of a material called garnet, which is normally difficult
to grow on the silicon wafers used for microchips. Garnet is desirable because
it inherently transmits light differently in one direction than in another: It
has a different index of refraction—the bending of light as it enters the
material—depending on the direction of the beam.
The
researchers were able to deposit a thin film of garnet to cover one half of a
loop connected to a light-transmitting channel on the chip. The result was that
light traveling through the chip in one direction passes freely, while a beam
going the other way gets diverted into the loop.
The
whole system could be made using standard microchip manufacturing machinery,
Ross says. “It simplifies making an all-optical chip,” she says. The design of
the circuit can be produced “just like an integrated-circuit person can design
a whole microprocessor. Now, you can do an integrated optical circuit.”
That
could make it much easier to commercialize than a system based on different
materials, Ross says. “A silicon platform is what you want to use,” she says,
because “there’s a huge infrastructure for silicon processing. Everyone knows
how to process silicon. That means they can set about developing the chip
without having to worry about new fabrication techniques.”
This
technology could greatly boost the speed of data-transmission systems, for two
reasons: First, light travels much faster than electrons. Second, while wires
can only carry a single electronic data stream, optical computing enables
multiple beams of light, carrying separate streams of data, to pass through a
single optical fiber or circuit without interference. “This may be the next
generation in terms of speed” for communications systems, Ross says.
