Incorporating news from O plus E magazine, Tokyo
TOKYO—Researchers at the NTT Science and Core Technology Laboratory Group have developed a high-density dense-wavelength-division-multiplexing (DWDM) light source—a supercontinuum (SC) light source—that can be used for WDM applications involving more than 1000 channels. Supercontinuum light sources generate broadband light as high-output optical pulses go through optically nonlinear media. The group used a silica glass optical fiber specially designed for SC light sources, and a modelocked semiconductor laser (12.5-GHz repetition rate) for the source of pulses that were used to seed the setup.
The seed pulse is amplified before entering the SC light-generating optical fiber. The spectrum of the seed pulse has four or five discrete channels before entering the fiber. Upon exiting the fiber, the spectrum has more than 1000 channels spread over the 1500- to 1600-nm range. The distance between adjacent channels is exactly the same as the repetition rate of the seed-pulse light source, namely 12.5 GHz. The noise of each wavelength was measured using an arrayed-waveguide-grating filter developed for high-density WDM communications. The quality has been deemed sufficient for communications purposes. This is the first time a single communications light source has simultaneously produced more than 1000 channels.
The fiber has been designed so that the wavelength-dispersion characteristics are in the form of a convex function, with the dispersion values decreasing as the light travels from the input end to the output end (see figure). As seed-pulse light travels along the fiber, the spectrum spreads due to nonlinear effects, resulting in a short, flat spectrum at the output end. The convex wavelength-dispersion properties lead to the flat spectrum.
When the seed pulse has a discrete spectrum, as in the case of a modelocked semiconductor laser, the output SC spectrum also is discrete. Thus, the different channels can be separated using a narrow-range wavelength filter. In order to maintain communications-level quality in each of the separated channels, a modelocked semiconductor must be used as the seed pulse source to ensure wavelength reliability and stability, and a polarized-wave-preserving optical fiber especially designed for SC light generation must be used in order to produce stable SC light. It is anticipated that the SC light source will reduce the cost of building fiberoptic networks.
Courtesy O plus E magazine, Tokyo