Chip sorts colors
Technology Research News
The simple concept of proportionality is
the key to a significant advance in the emerging field of integrated optics
-- chips that control light rather than electricity.
The field could lead to high-speed, all optical computers, and
smaller, more efficient communications devices.
The photonic crystals that form integrated optics chips contain
regularly spaced gaps that block certain wavelengths of light. The boundaries
between the material and the gaps refract, or bend, light in the same
way that the boundary between air and water produces the familiar bent-drinking-straw
By adding intentional defects, or areas without gaps, researchers
can get photonic crystals to channel light through very small areas. It
has been difficult, however, to make photonic crystals that efficiently
handle several wavelengths, or colors, of light at once, said Susumu Noda,
a professor of electronic science and engineering at Kyoto University.
Researchers at the university have discovered a design principle
that overcomes this problem. They found that using several photonic crystals
and making the size and spacing of the gaps in one crystal proportional
to those in the next permits control of different wavelengths of a single
beam of light. "We can expect to achieve multi-wavelength operation by
simply connecting multiple photonic crystals with proportional unit-cell
sizes," said Noda.
Previous attempts to control multiple wavelengths in photonic
crystals solely through defects of different sizes have resulted in devices
with poor performance, said Noda.
The Kyoto researchers used the method to make an add/drop multiplexer
that is 250,000 times smaller than conventional optical multiplexors,
said Noda. Add/drop multiplexers combine and separate different wavelengths
in the same channel. The devices are widely used in communications networks;
they increase network bandwidth by putting multiple signals over the same
line at the same time.
The researchers' prototype device measures 250 microns by 10 microns;
commercial devices are about 1 square inch, according to Noda. A micron
is one thousandth of a millimeter.
The researchers' add/drop multiplexer consists of a series of
seven photonic crystals, each with a line defect to guide light through
the crystal and a point defect to trap and emit photons, Noda said.
The device's success is due to proportionality: the distance from
the center of one gap to the center of the next gets successively smaller,
from 418.75 nanometers to 411.25 nanometers. Each crystal's gap spacing
is 1.25 nanometers smaller than the previous one. A nanometer is the length
of a row of 10 hydrogen atoms.
The device separates seven slightly different colors, or wavelengths,
of the infrared light used for long distance communications over optical
fibers. The wavelengths differ by five nanometer.
The field of two-dimensional in-plane photonic crystals is "very
exciting right now," and the researchers' method appears to be capable
of producing integrated optical devices, said Eli Yablonovitch, a professor
of electrical engineering at the University of California at Los Angeles.
The method's advantage is simplicity, Yablonovitch said. "The
main difference from previous work is in the simplicity of the design
The researchers are working on a practical add/drop device, said
Noda. The method could also be used for nonlinear optics and nanoscale
biological sensors, he said.
The method could be used to produce practical devices in two to
five years, said Noda.
Noda's research colleagues were Bong-Shik Song and Takashi Asano.
They published the research in the June 6, 2003 issue of Science.
The research was funded by Core Research for Evolution Science and Technology
(CREST), Japan Science and Technology Corporation (JST), and the Japanese
Ministry of Education, Culture, Sport, Science, and Technology.
Timeline: 2-5 years
TRN Categories: Optical Computing, Optoelectronics and
Photonics; Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "Photonic Devices Based
on In-Plain Hetero Photonic Crystals," Science, June 6, 2003
June 18/25, 2003
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