Switch promises optical chips
Technology Research News
One way to make tomorrow's communications
equipment considerably faster and cheaper is to make computer chips from
optical switches that are tripped with the same type of light pulses that
carry information over fiber-optic lines. Optical switches could also
eventually find use as circuits in fast, all-optical computer chips.
Marrying optical switches and computer chips also promises to
address a looming problem with electronic circuits: as electronic circuits
get faster, the time it takes to shunt a signal from one part of a chip
to another becomes a limiting factor. Using optical circuits to transmit
signals between various parts of an otherwise electronic chip would sidestep
The challenges to putting optical switches on computer chips are
making the switches small enough and finding ways to control them with
light. Today's optical switches are controlled electrically.
Researchers from Stanford University and the Massachusetts Institute
of Technology have taken a large step in this direction with designs for
a simple optical switch and an optical transistor made from photonic crystal.
The devices would be small, switched by light signals, would require little
power, and could be fabricated in the same facilities used to make today's
The researchers' transistor design is a few microns wide, and
the switch design is smaller than a micron -- "far smaller than any conventional
optical switching element [and] comparable to current electronic devices
that perform the same functionality," said Mehmet Fatih Yanik, a researcher
in the department of applied physics at Stanford University.
Key to both types of switches is the use of photonic crystal,
a material made from tiny arrangements of rods or a solid material with
patterns of holes. The boundaries between the medium and gaps refract,
or bend, light. This is the same effect that produces the familiar illusion
of a drinking straw bending at the air-liquid boundary.
The researchers' switches could be used as optical interconnects
in future computer chips. They could also be used to construct all-optical
circuits for use in a fast, inexpensive generation of optical communications
equipment like routers and high-speed switches that control telecommunications
The devices could be switched using only a few thousandths of
a watt of power, allowing them to directly switch the low-power optical
pulses that travel over conventional fiber-optic cable, said Yanik.
The switches' low-power requirements and small size make it possible
to put many optical switching elements on a single chip. This would dramatically
cut the cost of optical switching equipment and at the same time improve
the equipment's information processing capabilities, said Yanick.
The researchers' simple switch design calls for a semiconductor-rod-based
photonic crystal arranged to contain a waveguide situated near a tiny
optical cavity. The waveguide is simply a row without rods. The optical
cavity is an elliptical rod that is half the width and a little more than
twice the length of a normal rod and is oriented perpendicular to the
The optical cavity briefly stores light energy; this allows the
device to act as a switch because the stored energy changes the angle
of refraction, blocking light from moving through the adjacent waveguide.
When the energy level in the cavity is low, it has little effect on the
waveguide and a beam of light passes through unhindered.
Sending a pulse through the light beam in the waveguide increases
the energy level in the cavity, turning the switch off. "The output of
this device switches between near-perfect transmission to zero transmission
digitally, like an electronic flip-flop [switch]," said Yanik.
Key to the design is that the structure of the photonic crystal
enables a large amount of energy to build up in a very small space, even
when the input power is low.
The researchers' transistor design consists of a pair of waveguides
that intersect at right angles. A rod at the intersection of the waveguides
produces the switching effect. In the same manner as an electronic transistor,
a signal through one waveguide is blocked at the intersection until a
signal passes through the other waveguide, which alters the optical properties
of the intersection, allowing the first signal through.
The switch and transistor can be made in existing microchip fabrication
facilities, said Yanik. It "does not require any complex processing...
it can be readily fabricated with the same processing steps used in microchip
fabrication," he said.
The researchers are now working on fabricating the transistor,
said Yanik. "It might take about a year or so to experimentally demonstrate
the first prototype," he said.
The method could be used to make practical on-chip all-optical
information processing devices in two to five years, according to Yanik.
Yanik's research colleagues were Shanhui Fan of Stanford University
and Marin Soljacic of MIT. The work appeared in the October 6, 2003 issue
of Applied Physics Letters and the December 15, 2003 issue of Optics
Letters. The research was funded by the National Science Foundation
(NSF) and the Defense Advanced Research Projects Agency (DARPA).
Timeline: 2-5 years
TRN Categories: Optical Computing, Optoelectronics and Photonics
Story Type: News
Related Elements: Technical papers, "High-Contrast All-Optical
Bistable Switching in Photonic Crystal Microcavities," Applied Physics
Letters, October 6, 2003; "All-Optical Transistor Action with Bistable
Switching in a Photonic Crystal Cross-Waveguide Geometry," Optics Letters,
December 15, 2003.
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