In fiber-optic communications, dark fiber or unlit fiber (or fibre) refers to unused fibers, available for use.
The term was originally used when talking about the potential network capacity of telecommunication infrastructure, but now also refers to the increasingly common practice of leasing fiber optic cables from a network service provider.
The dark fiber market
For many years incumbent local exchange carriers would not sell dark fiber to end users, because they believed selling access to this core asset would cannibalize their other, more lucrative services. Incumbent carriers in the US were required to sell dark fiber to competitive local exchange carriers as Unbundled Network Elements (UNE), but they have successfully lobbied to reduce these provisions for existing fiber, and eliminated it completely for new fiber placed for fiber to the premises (FTTP) deployments.
Competitive local carriers were not required to sell dark fiber, and many do not, although fiber swaps between competitive carriers are quite common. This increases 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. This is a practice known in the industry as "coopetition".
Meanwhile, other companies arose specializing as dark fiber providers. Dark fiber became more available when there was enormous overcapacity after the boom years of the late 1990's through 2001. The market for dark fiber tightened up with the return of capital investment to light up existing fiber, and with mergers and acquisitions resulting in consolidation of dark fiber providers.
In the U.S. Level(3) Communications acquired a number of dark fiber providers in 2005–2006, and took their dark fiber off the market. AboveNet still markets dark fiber and is one of the only national providers left.
Dark fiber has been, and still is, available for sale on the wholesale market for both metro and wide area links, but it may not be available in all markets or city pairs. Prices for dark fiber may sometimes be lower than the price of a high speed leased line rental. In the UK there are few dark fiber providers, with the majority of carriers providing leased lines and network services over fibre. AboveNet owns and operates a London-based dark fiber network with lit services also offered. In addition, FibreSpan sell dark fiber metro and wide area network links, becoming a dominant player due to its microduct technology.
Dark fiber capacity is typically used by network operators to build SONET and dense wavelength division multiplexing (DWDM) networks, usually involving meshes of self-healing rings. Now, it is also used by end-user enterprises to expand Ethernet local area networks, especially since the adoption of IEEE standards for Gigabit Ethernet and 10 gigabit Ethernet over single-mode fiber. Running Ethernet networks between geographically separated buildings is a practice known as "WAN elimination".
-- from Wikipedia
"Dark fiber" refers to fiber-optic cable that's already been laid, but is not yet in use. Thousands of miles of dark fiber are available in the United States, but there have been few takers because of the high costs of making it operational.
A handful of dark-fiber projects have been gaining momentum recently, mostly involving large consortia of private companies, universities and medical facilities, sometimes with heavy government backing. Best-known is the National LambdaRail (NLR) , which has acquired more than a third of the 28,000 route miles of dark fiber so far snapped up by the research community, according to Steve Corbato, Internet2's director of network initiatives and an NLR board member.
-- By Evan Hansen, Staff Writer, CNET News.com