Silicon ring boosts light chips

By Eric Smalley, Technology Research News

Signals 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 devices.

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 communications channel.

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).

Timeline:   unknown
Funding:   Government
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

Page One

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