Silicon ring boosts light chips
Technology Research News
travel at the speed of light through optical fibers, but overall, optical
communications is much slower than light speed because signals are switched
on and off and routed through networks using electronic rather than light-based
Scientists have been working on ways of speeding optical networks
that involve using a second light beam to switch an information-carrying
light beam, but these devices have either been made from expensive materials
or are too large to fit on a computer chip.
Researchers at Cornell University have developed an all-optical
switch that is made from silicon and is small enough to be made by the thousands
on computer chips. "We have demonstrated a device that allows one low-powered
beam of light to switch another on and off, on silicon," said Michal Lipson,
an assistant professor at Cornell University.
The researchers' all-optical switch turns on and then off in 450
picoseconds, which is about 70 times faster than emerging silicon electromechanical
optical switches, according to Lipson. The all-optical switch has the potential
to be as fast as a few picoseconds, she said. A picosecond is a trillionth
of a second; a picosecond is to a second as a second is to 31,709 years.
The first practical application of the switch is likely to be in
devices that route signals in fiber-optic communications networks, said
Lipson. Today, optical signals must be converted to electrical signals that
can be processed in conventional electronic chips and often converted back
to optical signals for retransmission, an extremely slow process, she said.
An all-optical switch would remove the need to convert signals between optical
and electrical, and so would dramatically speed the system.
Other silicon all-optical switch prototypes are either too large
to fit on a chip or require high-powered lasers, said Lipson. The researchers'
ring resonator switch is 20 microns long and can operate with as little
as 25 picojoules of energy, which makes it suitable for use in optical chips,
said Lipson. A picojoule is a trillionth of a joule, or 1,000 times the
energy of a single ultraviolet photon. A double A battery contains about
1,000 joules of energy.
The Cornell all-optical switch consists of a straight waveguide,
or light channel, connected to a circular waveguide, or ring resonator,
both 450 nanometers wide, in the configuration of a circle touching the
middle of a line. Light ordinarily travels from the waveguide into the ring.
If the circumference of the ring resonator is a multiple of the wavelength
of the light used, however, the ring is resonant, or in tune, with the light.
This blocks the light from entering the ring.
Shining a second beam of light of a slightly different wavelength
into the device blocks the first beam by altering the ring's index of refraction.
Refraction, which is responsible for the bent drinking straw illusion, refers
to lightwaves changing direction as they pass from one material to another.
Light from this second, switching, beam is absorbed by the silicon
in the ring resonator, which generates electrons in the ring. The concentration
of electrons slightly alters the ring's index of refraction, which in turn
changes its resonant frequency. This brings the ring out of tune with the
signal beam, blocking the beam from entering the ring.
Configuring the device with waveguide on both sides of the ring
makes it possible to route signals from one waveguide to the other. Turning
the device on with the switching beam allows the signal beam to pass from
one waveguide through the ring to the other.
Because the device works with specific wavelengths, a series of
the devices could be used to make an all-optical add-drop multiplexer, which
channels multiple signals of different wavelengths into and out of a single
The researchers are working on reducing the amount of light lost
in the waveguide by making the edges of the rings smoother, said Lipson.
They are also making the switch smaller, which will reduce the its power
needs and increase its speed, she said.
Lipson's research colleagues were Vilson R. Almeida, Carlos A. Barrios
and Roberto R. Panepucci. The work appeared in the October 28, 2004 issue
of Nature. The research was funded by the Defense Advanced Research
Projects Agency (DARPA), the Air Force Office of Scientific Research (AFOSR),
and the National Science Foundation (NSF).
TRN Categories: Optical Computing, Optoelectronics and Photonics;
Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "All-Optical Control of
Light on a Silicon Chip," Nature October 28, 2004
December 15/22, 2004
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