Cabling · Networking
Dark Fiber
Installed fiber optic cable that carries no active traffic — either spare capacity in a cable bundle or leased raw fiber that an organization equips with its own transceivers.
The term "dark" means no light is being transmitted — the fiber is physically present but not in service. It comes in two contexts:
Internal dark fiber — spare fibers in a cable run that were installed for future capacity. When running backbone cabling through conduit, installing a larger cable bundle than currently needed is standard practice — the labor cost is in the run, not the additional fibers.
Leased dark fiber — carriers own extensive underground fiber networks, much of which isn't being actively used. Organizations with high bandwidth requirements — ISPs, universities, healthcare systems, large enterprises — can lease raw fiber strands between two locations and equip them with their own DWDM or active equipment, effectively owning the capacity of that link rather than paying per-bit for a managed circuit.
Leasing dark fiber gives an organization full control of the link's capacity and technology. You bring your own transceivers, choose your own wavelengths, and can upgrade the bandwidth by changing the optics without renegotiating the fiber contract. The trade-off: you're responsible for the active equipment at each end, and fiber cuts or physical infrastructure issues are still the carrier's problem to repair.