Mirror, mirror, on the laser

Although it drastically expands network capacity, wave division multiplexing brings new challenges. One is the ability to home in on the multiple wavelengths within a fiber, and in the future of fully optical networks, how to add, drop and cross-connect individual wavelengths.

Several companies are developing techniques that will allow them to tune lasers, which will eventually enable better optical network management and the promulgation of optical cross-connects. Lucent Technologies has been quietly working on tunable lasers, but it hasn't stated development plans. Nortel Networks' thermal tunable laser moves the laser across the color spectrum via heat.

Many proposed tunable laser solutions use an "edge emitting" approach, meaning the laser comes from the edge of a semiconductor.

Those are valid but costly solutions, said Parviz Tayebati, CEO of CoreTek Inc., a component developer. "Our [signal] is vertically emanated," he said. CoreTek created MEM-SEL, based on microelectromechanical (MEM) techniques. CoreTek's system integrates a tiny mirror with a vertical cavity surface emitting laser, or VCSEL, chip. The mirror hangs over the laser, which is on top of the semiconductor chip. The tuning is based on how far the laser is from the mirror, and the mirror is adjusted by applying minute amounts of voltage.

"It's a much simpler tuning control mechanism," Tayebati said. "It requires [minimal] power, it is microseconds fast and it allows continuous tuning. You don't miss any wavelengths in between."

Because of their flexibility, tunable technologies will push forward WDM and optical networking, particularly optical cross-connects.

Tunable lasers aren't the only solution. AT&T Labs is focusing on the optical cross-connect aspect but isn't using tunable lasers, said Lih Lin, a senior technical staff member in the lightwave networks research department. AT&T Labs is using a free-space MEMs switching technology that uses microscopic mirrors to permit or deny transmission of a laser beam signal.

The mirrors reside on a chip and when the mirror flips down, the signal is allowed to pass through to the output fiber. If the mirror is up, it reflects the signal to the light source (see figure). A tiny mirror corresponds to each intersection point in the fiber-collimator array. If a fiber cut occurs, the respective mirrors adjust to prohibit transmission on the damaged fiber. Other mirrors adjust to cross-connect the signal to a backup link, Lin said.

AT&T's technology provides optical add/drop capabilities without demultiplexing the signal in an electrical switching fabric. Although it is a new technology, Lin argued that its optical cross-connect capability is superior to today's electrically based offerings.

"The only technology that exists is an electrical cross-connect. The problem is that they cannot handle signals with a very fast bit-rate," she said. "Electronic switch fabrics [force signals] to be demultiplexed into a slower rate and multiplexed again. It makes the switching fabric big, expensive and hard to manage."

AT&T's technology isn't susceptible to polarization of light, as are wave-guiding technologies, and it has low loss and is bit-rate independent, Lin said.

As WDM deployments continue to grow, carriers need more effective management systems to accommodate the increase in Internet and video traffic.

As Lin pointed out, "Application requirements for the whole network will increase rapidly. There will be hundreds of wavelengths being transmitted around, and there has to be some kind of network element that can [optically] switch the signal to the output route." Once that's in place, carriers can realize the full potential of optical networking.

AT&T USES CISCO FOR QOS AT&T is using Cisco Systems' MPLS-based IP+ATM solutions to deliver enhanced QOS within an IP VPN and other new services to customers. The system will interpolate with AT&T's existing Cisco frame relay network.

OPEN SYSTEM ROUTING Nbase-Xyplex has announced the OSR8000 Linux switch router. The 18-slot chassis has a 40 Gb/s routing fabric and supports up to 26 million packets per second.