Dark fiber, also known as unlit fiber or black fiber, refers to unused optical fiber that has been laid underground and does not have service or traffic running on it. It is typically used for fiber-optic communication and can be leased from a network service provider. Dark fiber can be used to create a privately operated optical fiber network that is run directly by its operator over dark fiber leased or purchased from another supplier. This is opposed to purchasing bandwidth or leased line capacity on an existing network. Dark fiber networks may be used for private networking, or as Internet access or Internet infrastructure networking.

Dark fiber networks have a high capacity and enable excellent signal strength. Data is transported through the cables using light pulses, as in normal fiber cables, but it is essential that these cables are taken advantage of. Dark fiber networks can operate using the latest optical protocols using wavelength division multiplexing to add capacity where needed and to provide an upgrade path between technologies.

Running a dark fiber network requires purchasing and installing your own transmission equipment, which allows you to have control over the network and provides the assurance of dedicated connectivity. Dark fiber offers near-limitless capacity and provides the ultimate control over the network. Dark fiber networks are generally only available in high-population-density areas where fiber has already been laid, as the civil engineering costs of installing fiber to new locations is often prohibitive.

In summary, dark fiber is unused optical fiber that can be leased or purchased from a network service provider and used to create a privately operated optical fiber network. It provides the maximum level of control for businesses, allowing them to use their preferred protocol and manage their own equipment.