Optical fiber only partially lit in a fiber optic transmission system (FOTS) employing wavelength division multiplexing (WDM).WDM technology can support a considerable number of wavelengths running simultaneously over a single optical fiber within a cable comprising perhaps a great number of fibers. A dim fiber is one over which not all available wavelengths have been lit and which, therefore, has excess capacity. See also dark fiber, FOTS, wavelength, and WDM.