A type of dispersion-shifted single-mode fiber (SMF) that shifts the optimal dispersion point by adjusting the refractive index profile of the core and the cladding. There are two types of dispersion-shifted fiber (DSF). Zero Dispersion-Shifted Fiber (ZDSF) shifts the point of zero dispersion by increasing material dispersion to the point that it cancels out chromatic dispersion at 1550 nm, rather than 1310 nm. Dense Wavelength Division Multiplexing (DWDM) and Erbium-Doped Fiber Amplifiers (EDFAs) both work in this higher window, which can create yet another noise problem in the form of four-wave mixing (FWM), a phenomenon by which wavelengths interact to create additional wavelengths.The EDFAs amplify those signals, and superimpose them on the DWDM channels. Non Zero Dispersion-Shifted Fiber (NZDF) addresses this issue by shifting the optimal dispersion point slightly above the range in which EDFAs operate. See also chromatic dispersion, cladding, core, dispersion, DSF, DWDM, EDFA, FWM, material dispersion, noise, refractive index, SMF, wavelength, window, and ZDSF.