Pulse trap makes optical switch

By Kimberly Patch, Technology Research News

Scientists who work with light pulses so short that one trillion of them pass by in a second are laying the groundwork for higher bandwidth communications and blazingly-fast, all-optical computer chips.

But making the most of these ridiculously short pulses requires that they be switched on and off using other light pulses rather than electronic devices. Researchers have been working on all-optical switching for decades; the challenge is finding a fast, efficient method that uses little power.

Researchers from Nagoya University in Japan have found a way to capture an ultrashort pulse by using a second pulse to filter out the first one as it travels through an optical fiber. Selectively knocking out pulses from a string, or train, of pulses makes for a sequence of pulses and gaps that can represent the ones and zeros of digital information. Using light pulses to do this means that, like electronic circuits, the output of one logic unit can control another.

The method takes advantage of a phenomenon known as optical pulse trapping -- meaning one optical pulse can overlap with and travel with another. "The trapped pulse spatially overlaps with [a] control pulse and they copropagate along the fiber," said Norihiko Nishizawa, an assistant professor of quantum engineering at Nagoya University.

The researchers demonstrated the method by sending a train of signal pulses down an optical fiber, then picking off one of the pulses in the train by making a control pulse overlap the targeted signal pulse. "We can trap [a single] pulse from high-repetition-rate pulse trains," said Nishizawa. The method is nearly 100 percent efficient, he said.

As the pulses propagate down a fiber optic line, the waveform of the trapped pulse is compressed into a shorter wavelength. The control pulse is a soliton, a type of wave that doesn't normally spread out as it travels. Solitons can be made to shift to a longer wavelength, and when a soliton that is trapping another pulse shifts, the trapped pulse is forced to shift to a shorter wavelength to compensate.

The shift makes it possible to identify and pick off only the trapped pulses, said Nishizawa. "Since the wavelength of the trapped pulse is distinctly shifted and separated from the other untrapped pulses, we can pick off only the trapped pulse easily using [a] wavelength filter," he said.

The researchers used an ultra-fast optical pulse device that they demonstrated in 1999 to provide light pulses separated by only 1.5 picoseconds, or trillionths of a second. A trillionth of a second is to a second as a second is to 31,709 years.

This is equivalent to a communications rate of .67 terahertz, or trillion bits per second, according to Nishizawa. The method could make it possible to use pulses separated by one picosecond to provide a communications speed of one terahertz, he said. Today's high-speed communications equipment uses tenth of a nanosecond pulses to provide top communications speeds of 10 billion bits per second per channel.

The researchers modified a crosscorrelation frequency resolved optical grating (X-FROG) system to measure the ultrashort optical pulses so they could prove that the system worked. The test system provided the researchers with spectrograms that showed the wavelength of light passing through at any given trillionth of a second. "We have developed a highly sensitive X-FROG system so that we could directly observe the ultrafast all-optical switching," said Nishizawa.

The pulse trapping method could eventually be used in ultrafast optical communications, and optical information processing, said Nishizawa.

The idea of using soliton trapping gates is not new, but the researchers are using a different physical effect -- the compensating shift of the trapped pulse -- to filter out pulses, said Curtis Menyuk, a computer science and electrical engineering professor at the University of Maryland Baltimore County.

The method is one of a large number of proposed all-optical switching schemes, said Menyuk. The Nagoya method is relatively intricate and delicate, however. Its drawbacks are that the range of allowed powers and frequencies is small, and the approach doesn't make it possible to cascade more than one device, he said.

The researchers' next step is demonstrating the method at the 1.55 micron wavelength region used for long distance optical communications.

Nishizawa's research colleague was Toshio Goto. The work appeared in the February 24, 2003 issue of Optics Express. The research was funded by the Japanese Ministry of Education, Science, Sports and Culture.

Timeline:   5 years
Funding:   Government
TRN Categories:  Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "Ultrafast All Optical Switching by Use of Pulse Trapping across Zero-Dispersion Wavelength," Optics Express, February 24, 2003.


March 24/31, 2004

Page One

Molecular logic proposed

System susses out silent speech

Virtual people look realistically

Pulse trap makes optical switch

Irregular layout sharpens light
Bacteria make clean power
Curve widens 3D display
Triangles form one-way channels
DNA has nano building in hand
Nanowires span silicon contacts


Research News Roundup
Research Watch blog

View from the High Ground Q&A
How It Works

RSS Feeds:
News  | Blog  | Books 

Ad links:
Buy an ad link


Ad links: Clear History

Buy an ad link

Home     Archive     Resources    Feeds     Offline Publications     Glossary
TRN Finder     Research Dir.    Events Dir.      Researchers     Bookshelf
   Contribute      Under Development     T-shirts etc.     Classifieds
Forum    Comments    Feedback     About TRN

© Copyright Technology Research News, LLC 2000-2006. All rights reserved.