Virtually everyone who has ever gone online has been caught in a Web traffic jam. Half-loaded pages and frozen computers litter the shoulders on the information superhighway, and the traffic that engenders the slow-downs is only increasing as consumer demand for high-bandwidth services--graphic-rich Internet, audio and video streaming, and online gaming--continues to intensify.

The ubiquitous optical fibers that line the walls and floors of our buildings and snake beneath our roads and under the oceans are the primary highway for these services. They transmit information as light pulses along glass or plastic wire or fiber, and according to Imran Hayee, associate professor of electrical and computer engineering at the University of Minnesota, Duluth, they are capable of meeting the increasing demands. "Relatively speaking, optical fiber is unlimited in its capacity, if it can be optimized," said Hayee. "There is a limit, but it is above our current and near-future needs."

Those needs include helping to transmit signals from wireless devices like cell phones and PDAs. Wireless networks currently carry signals for a relatively short distance before switching the signals over to optical fiber networks, which are quicker and less expensive. Ultimately, the explosion of wireless devices, along with the growth of voice over internet protocol (VoIP) and video on demand (VOD), will further add to the traffic on fiber-optic lines.

Five years ago optical fibers could carry 2.5 gigabits of information per second (Gb/s) on each of many wavelengths (colors) of light. Today they carry 10Gb/s on each wavelength with a few unused wavelengths to spare. But the current 10Gb/s system is like a crowded highway in a booming metropolis with car pool lanes providing the potential for additional capacity. Ultimately, a new system will need to be built--imagine a new highway around the expanding suburbs and a commuter rail line through the densest areas. Unfortunately, that upgrade is many years and billions of dollars in the future. "To fully upgrade today's telecommunication infrastructure it will be necessary to replace thousands of kilometers of existing fiber and amplifiers in the ground with new fiber and new amplifiers," said Hayee.

Until then, Hayee's research aims to find a way to coax the existing fiber-optic system to carry 40Gb/s on those few spare wavelengths of light. If he is successful, this system should be able to handle traffic for years to come and meet the incredible expectations of consumers. "Ultimately, we want our computers to be like a crystal ball," he said. "We want to look into our monitor and see in time and space [information from past to present and from servers everywhere on the planet]. Optical fiber networks can do this."

Keeping a finger on the pulse distortions

Until the late 1980s, optical fibers transmitted data on a single optical channel using transmitters and receivers to send and receive the signals and amplifiers to boost the signals as they deteriorated en route. But improved transmitters, better amplifiers, and the introduction of devices capable of discerning different wavelengths, allowed the system to handle more capacity. Wavelength-selective devices use laser-light wavelengths as signature addresses of origin that can be routed and recovered out of the chaos of multiple signals allowing for the transmission of many independent signals simultaneously on one fiber.

Hayee's research focuses on the use of wavelength dimension multiplexing (WDM), a technique in which each of the multiple wavelengths of light carry the same amount of information in parallel on a single hair-width fiber without interfering with each other. There are other ways to expand bandwidth in the current system, but WDM has the ability to use more of the latent potential in the existing fiber.

According to Hayee, the major hurdles to reaching 40Gb/s WDM transmission on optical fibers are fiber dispersion and nonlinearity, both of which cause signal degradation. Fiber dispersion is the spread of optical laser pulses in the fiber over time, causing pulses to interfere with neighboring pulses of the same wavelength. Fiber nonlinearity manifests itself as severe pulse distortion by causing multiple wavelengths of light to interact with each other, thereby deteriorating the information bits on multiple channels.

Hayee's team is proposing to combat these difficulties through a technique (nodal dispersion mapping technique) where a specific amount of predispersion compensation is added at the transmitter end and a specific amount of post dispersion compensation is added at the receiver end--like buoys in no-wake zones guiding ships out of and into ports. This allows for the control of expansion and contraction of 40Gb/s pulses, minimizing nonlinear distortion and preventing the pulses from interfering with each other.

"The initial research will use computer modeling and simulations," said Hayee. The next step will be to build a transmission test bed in the laboratory to confirm the results of the computer simulations. Fortunately Hayee has significant experience with this type of effort, having worked in private industry for five and a half years developing telecommunications products and techniques to improve the performance of trans-oceanic optical fiber communications systems. He holds more than a dozen U.S. patents (issued and pending), and he is continuing along the same research path in his UMD lab as he pursued in the optical telecommunications industry.

If all goes well, this research can bridge the gap between network capacity and burgeoning demand until a new network is installed, helping ease the traffic that frustrates us all.

WRITTEN BY BRIAN LIEB