Dense wavelength division multiplexing (DWDM) is the most effective means for expanding communications capacity in optical networks. The technology places several closely spaced wavelengths of light into one fiber, with each wavelength serving as the communications equivalent of a lane of traffic on a highway. Network providers can easily expand networks by adding wavelengths without having to lay new fibers. The standard wavelength spacing is 50 or 100 GHz, which corresponds to 0.4 or 0.8 nm in the 1550-nm band. To combine (multiplex) or separate (demultiplex) those narrowly spaced signals from the fiber, networks require optical filters with extremely steep spectral isolation characteristics... Meet the author Akira Inoue is involved in research and development at the Yokohama Research Laboratories of Sumitomo Electric Industries Ltd. in Yokohama, Japan. He has a BS from Kyoto University.