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Dark fiber is unused optical fiber that has been laid but is not currently being used in fiber-optic communications. Because fiber-optic cable transmits information in the form of light pulses, a "dark" cable refers to one through which light pulses are not being transmitted.
There are thousands of miles of dark fiber across the U.S., as companies have installed additional fiber optic systems. These companies assume that they will be able to lease those dark fibers to cable TV, telephone or other companies looking to expand their reach in the future. The fibers are neither controlled nor connected to the phone company, but the phone company provides the necessary functional components.
Dark fiber services are provided by local exchange carriers (LECs) to maintain optical fiber transmission capacity between customer locations where light for fibers is provided by the customers.
Dark fiber is also known as unlit fiber.
Managed dark fiber is a form of wavelength division multiplexed to dark fibers where pilot signals are beamed into fiber by the fiber provider for management purposes using transponders tuned to an assigned wavelength. Central management is preferred for a dense wavelength division multiplexed system. This is because the closely spaced wavelength may be disrupted by signals that do not have tightly controlled parameters when amplification is needed for signal transmissions of more than 100 km.
Virtual dark fiber that uses wavelength multiplexing permits service providers to offer individual wavelengths where access to narrowband wavelength division multiplexing optical channels is provided over a wavelength division multiplexed fiber network. The network is managed at the physical level and is left unlit by the network providers. Virtual dark fiber is provided using coarse bandwidth division multiplexing with spacing as wide as 20 nanometers, making the system less susceptible to interference.