What Is Dark Fiber?
Dark fiber is unused optical fiber cabling leased to a customer, who attaches their own optics to light it and controls capacity, protocol, and equipment.
Dark fiber is optical fiber cable that has been installed and spliced but has no active electronics, lasers, or transceivers attached, so no light signal is carried on it. Because it is unlit, the fiber itself is simply glass and physical infrastructure. The customer who leases or owns the strands supplies the optics, choosing bandwidth capacity and protocol themselves.
Dark Fiber vs. Lit Services
With a lit service, a carrier installs and manages the optics, then sells bandwidth as a defined service, such as a 10 Gbps Ethernet circuit. The carrier controls the equipment, the protocol, and how the fiber is upgraded. With dark fiber, the carrier or fiber owner delivers unlit glass strands between two points, and the customer installs their own optical transceivers or DWDM systems to light it. That gives the customer direct control over capacity, protocol, and future upgrades, since adding bandwidth is a matter of swapping electronics at each end rather than renegotiating a service contract. Many hyperscalers, carriers, and large enterprises choose dark fiber for this control.
Why Organizations Lease or Build Dark Fiber
Enterprises, carriers, and data center operators lease or build dark fiber when they want a private physical path between two sites without depending on a carrier's shared network. Because there is no active equipment in the path besides what the customer installs, the connection is a direct point to point route between two locations, which supports low latency and simplifies security review. Dark fiber is also common in data center interconnect (DCI) and campus or metro builds, where an operator needs a dedicated path it can upgrade to higher speeds or new protocols over time as its own equipment evolves, without a new contract.
How FCC Supports Dark Fiber Projects
FCC builds the physical dark fiber path itself: aerial and underground construction, cable placement, splicing, and testing to hand off strands that meet loss and continuity specs. We also support engineering and permitting for new dark fiber routes, including data center interconnect and meet-me-room builds. Lighting the fiber, choosing optics, and configuring protocol are decisions made by the network owner or customer after construction and testing are complete.
Dark Fiber, answered
What Is Dark Fiber?
Dark fiber is optical fiber cable that has been installed and spliced but has no active electronics, lasers, or transceivers attached, so no light signal is carried on it. Because it is unlit, the fiber itself is simply glass and physical infrastructure. The customer who leases or owns the strands supplies the optics, choosing bandwidth capacity and protocol themselves.
Does dark fiber have a bandwidth cap?
No. Dark fiber has no active electronics attached, so it carries no signal and has no built-in bandwidth limit. Capacity depends entirely on the optics and protocol the customer installs. Upgrading speed later means changing the transceivers or DWDM equipment at each end, not renegotiating a carrier's service tier.
How is dark fiber different from a lit circuit?
A lit circuit is a finished service: the carrier installs the optics, manages the equipment, and sells you bandwidth. Dark fiber is unlit glass only, delivered as physical strands. The customer supplies and manages their own optics, so they control capacity, protocol, and upgrade timing.
Who is responsible for lighting dark fiber?
The customer or network owner lights the fiber, not the fiber construction contractor. Lighting means installing transceivers or DWDM/CWDM systems at each end of the route. FCC constructs, splices, and tests the physical path so it is ready to accept that equipment once handed off.