Cornell
researchers have created a broadband light amplifier
on a silicon chip, a major breakthrough in the quest
to create photonic microchips. In such microchips,
beams of light traveling through microscopic waveguides
will replace electric currents traveling through
microscopic wires.
A team of researchers working with Alexander Gaeta,
Cornell professor of applied and engineering physics,
and Michal Lipson, assistant professor of electrical
and computer engineering, used the Cornell NanoScale
Facility to make the devices. They reported their
results in the June 22 issue of the journal Nature.
The amplifier uses a phenomenon known as four-wave
mixing, in which a signal to be amplified is "pumped" by
another light source inside a very narrow waveguide.
The waveguide is a channel only 300 x 550 nanometers
(nm = a billionth of a meter, about the length of
three atoms in a row) wide, smaller than the wavelength
of the infrared light traveling through it. The photons
of light in the pump and signal beams are tightly
confined, allowing for transfer of energy between
the two beams.
The advantage this scheme offers over previous methods
of light amplification is that it works over a fairly
broad range of wavelengths. Photonic circuits are
expected to find their first applications as repeaters
and routers for fiber-optic communications, where
several different wavelengths are sent over a single
fiber at the same time. The new broadband device
makes it possible to amplify the multiplexed traffic
all at once.
The process also creates a duplicate signal at a
different wavelength, so the devices could be used
to convert a signal from one wavelength to another.
Although four-wave mixing amplifiers have been made
with optical fibers, such devices are tens of meters
long. Researchers are working to create photonic
circuits on silicon because silicon devices can be
manufactured cheaply, and photonics on silicon can
easily be combined with electronics on the same chip.
"A number of groups are trying to develop optical
amplifiers that are silicon compatible," Gaeta said. "One
of the reasons we were successful is that Michal
Lipson's group has a lot of experience in making
photonic devices on silicon." That experience, plus
the manufacturing tools available at the Cornell
NanoScale Facility, made it possible to create waveguides
with the precise dimensions needed. The waveguides
are silicon channels surrounded by silicon dioxide.
Computer simulations by the Cornell team predicted
that a waveguide with a cross section of 300 x 600
nm would support four-wave mixing, while neither
a slightly smaller one -- 200 x 400 nm -- nor a larger
one -- 1,000 x 1,500 nm -- would. When Lipson's Cornell
Nanophotonics Group built the devices, those numbers
checked out, with best results obtained with a channel
measuring 300 x 550 nm.
The devices were tested with infrared light at wavelengths
near 1,555 nm, the light used in most fiber-optic
communications. Amplification took place over a range
of wavelengths 28 nm wide, from 1,512 to 1,535 nm.
Longer waveguides gave greater amplification in a
range from 1,525 to 1,540 nm. The researchers predict
that even better performance can be obtained by refining
the process.
They also predict that other applications of four-wave
mixing already demonstrated in optical fibers will
now be possible in silicon, including all-optical
switching, optical signal regeneration and optical
sources for quantum computing.
The work was supported by the Cornell Center for
Nanoscale Systems. The Center for Nanoscale Systems
and the Cornell NanoScale Facility are funded by
the National Science Foundation and the New York
State Office for Science, Technology and Academic
Research (NYSTAR).
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