Dark fibre
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Dark fibre or unlit fibre (or fiber) is the name given to fibre optic cables which have yet to be used. They are hence not yet connected to any device, and are only there for future usage.
The term was originally used when talking about the potential network capacity of telecommunication infrastructure, but now also refers to a form of telecommunication network product that is purchased by network operators from fibre providers.
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Dark fibre for capacity expansion
The reason that dark fibre exists in well-planned networks is that much of the cost of installing cables is in so called civils - the civil engineering work required in order to get the cables installed. This includes planning and routing, obtaining permissions, creating ducts and channels for the cables, and finally installation and connection. This work accounts for more than 60% of the cost of developing fibre networks, with only a relatively small proportion actually being invested in the optical fibre cable and high-tech networking infrastructure.
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.
Dark fibre overcapacity
In the dot-com boom, a large number of telcos 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.
Unfortunately for them, the collapse of the dot-com boom left fibre supply greatly exceeding even the most optimistically forecast fibre demand by a factor of up to thirty in many areas. The availablity of wavelength division multiplexing further reduced the demand for fibre by increasing the capacity that could be placed on a single fibre by a factor of up to 100. As a result, the wholesale price of data traffic collapsed. A number of these companies filed for bankruptcy protection, or went bankrupt, as a result.
Just as with the railway boom, the misfortune of one sector of the market became the good fortune of another, and this overcapacity created a whole new telecommunications market sector.
The dark fibre market
Until recently, no telco would sell dark fiber, as it regarded selling access to this core asset as commercial suicide. However, this attitude has changed due to the enormous overcapacity installed in the ground, and dark fiber is now available for sale on the wholesale market for both metro and wide area links at prices previously associated with leased line rental.
This has resulted in a market segmentation between fibre providers and network operators, which were previously the same entities.
The creation of a market in dark fiber has also encouraged telcos to swap fiber capacity with one another, thus increasing the reach of their networks in places where their competitor has a presence, in exchange for provision of fiber capacity on places where that competitor has no presence.
Dark fiber capacity is typically used by network operators to build wavelength-division multiplexed networks, usually involving meshes of self-healing rings.
Managed dark fibre is a form of wavelength-division multiplexed access to otherwise dark fibre where a simple "pilot" signal is injected into the fibre by the fibre provider for management purposes.
Wavelength multiplexing allows a service provider to offer virtual dark fiber to customers, offering individual wavelengths or lambdas as individual dark fibers.
There is also a market in "colours" where access to a dark narrowband WDM optical channel is provided over a wavelength division multiplexed fibre network that is managed at the physical level, but unlit by the network provider.