Optical Fiber installed without a Transmitter or receiver. These fibers are the un-used fibers in a cable. Some fibers are held in reserve for future requirements. Others are often leased to different carriers (or large customers) for use in their individual systems.

Dark fiber is fiber optic Cable that has not been used yet and is not connected to any device. Dark fiber is usually installed to avoid the expense of having to go back and lay fiber again later. The abundance of dark fiber in the U.S. occured in the 1990s when there was a large upswing in the telecom and computer industries. Companies with installed dark fiber often lease it out to other companies and/or providers.

fiber that has been installed but is reserved for future use. Carrying no light.