LaserComm's dispersion compensator passes lab tests

By Susan Fogarty, On-line Editor
LaserComm Inc. (Richardson, TX) announced successful results from a series of laboratory tests of its Hi-Mode Dispersion Management Device (DMD). The Hi-Mode DMD was designed to overcome problems caused by chromatic dispersion and dispersion slope to facilitate the migration from 2.5 Gbits/sec to 10 Gbits/sec in dense wavelength-division multiplexing (DWDM) systems. The technology behind the Hi-Mode DMD is based on the use of high-order modes (HOMs) that can be configured to offer compensation for many values of chromatic dispersion and dispersion slope. The product is a combination of specialized mode transformers and a proprietary HOM dispersion-compensated fiber.

The improved optical budget from HOM-based compensators enables optical add/drop capability for higher channel count, 10-Gbit/sec DWDM systems. The slope compensation and lower noise means more channels can be accommodated with longer spans between regenerators. Says Yochay Danziger, chief technical officer at LaserComm, "With these tests, we have proven the physics and mathematics behind our radical approach to the problem of chromatic dispersion. We also have shown the Hi-Mode can be developed to meet customer specifications."

Chromatic dispersion is the outcome of the differences in velocity of lightwaves as they travel through an optical fiber. Dispersion leads to loss of signal, requiring expensive regeneration stations along the network. LaserComm's new technology will allow network owners to use fewer stations spaced further apart, driving down the cost of high-bandwidth networks.

As 10-Gbit/sec transport systems became available, Danziger explains, fiber vendors developed non-zero dispersion-shifted fiber to reduce the effects of chromatic dispersion. To permit operation on existing singlemode fiber, special fiber was developed and packaged in modules that were integrated into the 10-Gbit/sec systems. Neither of these solutions has been ideal, because there is still significant dispersion difference between the long- and short-wavelength DWDM channels. This "second-order-effect dispersion" limits the number of channels that can be transported long distances with current dispersion-compensation techniques.

Dispersion management balances positive and negative dispersion over the length of the fiber network so that, as the signals travel through the fiber, they always experience some chromatic dispersion. When the signals reach the receiver, however, the total dispersion is near zero, or within an acceptable limit. LaserComm has incorporated its Spatial Mode Transformation technology to produce a passive optical device that is capable of continuous broadband management of chromatic dispersion and dispersion slope. "The HOM dispersion compensators also have a much higher level of optic power tolerance before nonlinear effects begin to occur," Danziger adds.

Beta trials of the Hi-Mode DMD, with both vendors and carriers, will be conducted this fall. Field trials are scheduled for the beginning of 2000, with product availability around March of 2000.