Dark fiber

A dark fibre (or dark fiber) or unlit fibre is an unused optical fibre, available for use in fibre-optic communication.

The term dark fibre was originally used when referring to the potential network capacity of telecommunication infrastructure, but now also refers to the increasingly common practice of leasing fibre optic cables from a network service provider, or, generally, to the fibre installations not owned or controlled by traditional carriers. In common vernacular, dark fibre may sometimes still be called "dark" if it has been lit by a fibre and not the cable's owner.

A dark fibre network or simply dark network is a privately operated optical fiber network that is run directly by its operator over dark fibre leased or purchased from another supplier. This is in contrast to purchasing bandwidth or leased line capacity on an existing network. Dark fibre networks may be used for private networking, or as Internet access or infrastructure.

Much of the cost of installing cables is in the civil engineering work required. This includes planning and routing, obtaining permissions, creating ducts and channels for the cables, and finally installation and connection. This work usually accounts for more than 60% of the cost of developing fibre networks. For example, in Amsterdam's city-wide installation of a fibre network, roughly 80% of the costs involved were labour, with only 10% being fibre. It therefore makes sense to plan for, and install, significantly more fibre than is needed for current demand, to provide for future expansion and provide for network redundancy in case any of the cables fail.

Many fibre optic cable owners such as railroads or power utilities have always added additional fibres for lease to other carriers.

During the dot-com bubble, a large number of telephone companies built optical fibre networks, each with the business plan of cornering the market in telecommunications by providing a network with sufficient capacity to take all existing and forecast traffic for the entire region served. This was based on the assumption that telecoms traffic, particularly data traffic, would continue to grow exponentially for the foreseeable future.

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