Optical fiber left unlit in a fiber optic cable. Fiber optic cables often contain a great many fibers, some of which are lit, and others of which are left unlit, or dark. The dark fibers can be spares for backup purposes, can be held in reserve to accommodate future demand, or can be available for lease or sale to other carriers or user organizations with private line requirements. See also dim fiber and dry copper pair.
(1) Also known as "unlit fiber," it is optical fiber that is laid in the ground, but not used. During the late 1990s, so much fiber had been installed worldwide in anticipation of Internet demand that there is still a huge amount of fiber that has never been lit. The projected demand has also been curbed by the improvements in WDM technology, which allows more channels to be transmitted in a single fiber strand. Contrast with lit fiber.
(2) Optical fibers that are installed and sold to carriers and large businesses without any optical or electronic signaling in their path. The customer is responsible for adding the transmission system at both ends. See TONS.