Optical fiber compensates for soliton fluctuations

Optical fiber compensates for soliton fluctuations

lynn haber

Early testing of soliton transmission over a newly developed optical fiber reveals the potential to increase current lightwave data rates by as much as five times while reducing overall costs.

Engineers from Corning Inc. in Corning, NY, and the University of Rochester in Rochester, NY, have built and tested an ultra-fast soliton/optical fiber transmission system. The system holds the promise of boosting data rates beyond 100 gigabits and of making soliton technology commercially viable. The work is funded by Corning, the New York State Science and Technology Foundation and the U.S. Army.

To date, the use of solitons--pulses or lightwaves that retain their shape as they travel under controlled conditions--in transmission systems has been experimental, according to Alan Evans, Corning senior research scientist.

As solitons travel along the length of the fiber, their intensity level changes the fiber`s index of refraction and keeps the solitons intact. However, solitons can run into dispersion problems because their energy level ultimately decreases as the solitons speed through long lengths of fiber. But boosting energy levels too high can cause the solitons to become unstable. Corning therefore built a special fiber to compensate for energy fluctuations by matching the solitons` decreasing energy level with a fiber whose dispersion decreases proportionally.

To that end, the use of a soliton transmission system--developed by Andrew Stentz, a graduate student at the University of Rochester, working closely with Robert Boyd, a professor at the university`s Institute of Optics--combined with Corning`s distinctive dispersion-decreasing fiber, helps keep the light pulses intact. "This is a first step to see if these fibers can be used to improve soliton transmission," says Stentz.

"Changing the properties of the optical fiber improves the transmission properties of the solitons. The solitons tend to keep their shape, which means little data loss," adds Evans.

Faster data rates

Because optical fiber is increasingly being installed in commercial and residential communications systems for carrying voice, data and video information, researchers are continually exploring ways to increase transmission rates. The challenge of faster data rates in fiber networks is keeping the signals intact by overcoming attenuation losses as the signals are transmitted and amplified through the fiber.

Using optical solitons in conventional and constant-dispersion fiber to increase data rates does not work well. The solitons or light pulses eventually lose energy and require amplification. Over very long cable distances, such as those involved in submarine ocean systems, the pulses eventually destabilize and fall apart. An indication of soliton degradation is an expanded and flattened pulse width.

In many transmission studies, researchers have overlooked changing the properties of the fiber along its cable length. For example, constant-dispersion fiber has been modified in composition, but its shape has remained constant. That is, the cross-section of the fiber maintains the same dimension throughout the length of the cable. In this manner, constant-dispersion fiber maintains a path-averaged balance along the length of the fiber.

On the other hand, the cross-section of Corning`s dispersion-decreasing fiber is tapered, causing the dispersion properties of the fiber to change as a function of length. Consequently, as the pulse energy decreases and nonlinear effects diminish, the dispersion proportionally affects decrement. All these interactions enable the pulses to propagate without changing shape.

To test Corning`s fiber, Stent¥built a soliton laser system based on erbium-doped optical amplifiers. The laser generated 1-picosecond pulses, which are many times faster than those used in today`s fiber systems. Stent¥transmitted the light pulses through a 40-kilometer spool of dispersion-decreasing fiber in a laboratory environment. The solitons emerged intact.

In addition to faster data rates and lower losses, dispersion-decreasing fiber permits network optical amplifiers to be set farther apart. Consequently, fewer optical amplifiers are needed for a given segment of fiber, thereby reducing network costs. Amplifiers cost approximately $20,000.

Amplifiers for submarine systems

In submarine systems, amplifiers are needed every 25 to 30 kilometers, or 15 to 18 miles. With the new system, submarine distances for amplification could potentially extend more than 100 kilometers, according to Evans. Such systems that use the special compensating fiber would need fewer amplifiers, resulting in substantial savings.

The joint experiment by Corning and the University of Rochester should also help eliminate the attenuation issues associated with the use of solitons in transmission systems. Other developments, such as filtering schemes addressed by AT&T Bell Laboratories, Norcross, GA, to reduce noise in optical amplifiers, could clear the way for future uses of solitons in practical transmission systems.

In another soliton investigation, the Centre National d`Etudes des Telecommunications research laboratories of France Telecom in Paris have achieved 1 million kilometers of error-free, 10-gigabit-per-second transmissions supported by optical amplifiers spaced at 70-km intervals (see Lightwave, October 1994, page 8).

Nevertheless, more work is needed. "This experiment demonstrated the capability of the special fiber," says Evans. Next, researchers will have to send encoded data through the fiber and measure the bit-error rate of the output.

Even after additional experimentation is completed, issues related to the feasibility and cost associated with manufacturing the new optical fiber will have to be determined. Consequently, commercial products based on this experiment appear to be several years away. q

Lynn Haber is a freelance writer based in Boston.

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